Seed dispersal by cattle
Student report
P.G. Vos
D71 IJ
Seed dispersal by cattle
The importance of endozoochory by cattle for the establishment of heathiand and oligotrophic grassland species.
Student report P.G. Vos
Supervisors:
Drs. H.M.C. Verhagen Drs. A.M. Mouissie
Prof. dr. J.P. Bakker
Lab. v. Plantenoecologie RUG, Haren, 2001
Pijksijniversitejt
Grorqcr
Bibliotheek Biologisch Cenr'ji Kerklaan 30 — Poshus 1 4
9750 AA HAPEN
Photo front page:
Scottish Highland cattle, resting under oak trees.
© A.M. Mouissie
1
—
Summary
This report discusses the endozoochorous seed dispersal by cattle (Scottish Highland cattle) in two study sites. The study sites consist of oligotrophic grassland, heathiand and former arable
fields. EndozoochorOUs seeddispersal may play an important role in the development of oligotrophic grassland and heath on these former arable fields.
The study sites have been divided in different area parts. Ineach area part plots of 20 by 20 m were created. In these plots vegetation was recorded, seed supply was determined and the number of dung pats was counted. Seed that survived the passage throughthe cattle was determined in the greenhouse. Also germination on field dung pats wasdetermined.
Target species form a small part of the seed supply in both study sites.Selectivity of cattle in terrain use determined the seed intake. The species dominant in seed intake could be
determined by hand clipping in the grazing area. High number of dung pats wasfound on the mesotrophic grassland in both study sites and on the heathland in Delleburen. Cattle avoided the oligotrophic grassland in Delleburen. In Hullenzand theMolinia-stands and the heathland were avoided.
Many species of heathland and oligotrophic grassland are dispersed by cattle dung. The species dispersed most are Poa trivialis, P. annua, P. pratensis, CerastiumfontaflUm and Juncus species. In the field less seedlings germinated than in the greenhouse. A number of
target species germinated on field dung pats. Most species appearing on a field dung pat were locally new.
Sterilising by heating of cattle dung killed almost all seeds but also alteredthe structure of the dung. Sterilised dung pats were used to examine the amount of seeds blown in by the wind.
With sterilised dung pats it was shown that Rumex acetosella was blown in.
EndozoochorouS dispersal by cattle is of little importance for the dispersal of plant species of heathland and oligotrophic grassland communities. It is discussed thatgermination conditions are very important in determining the importance ofendozoochorous dispersal for
establishment.
2
Contents
Summary 2
Introduction 4
Dispersal of plant seeds 4
Endozoochorous seed dispersal 4
The importance of endozoochory 7
Methods 8
Study sites 8
Selection area partS 9
Terrain use of cattle and distribution of cattle dung 10
Potential seed intake 11
Potential dispersal 11
Colonisation of cattle dung 11
Blow in control 11
Data analyses 12
Results 13
Seed supply 13
Distribution of cattle and dung 15
Colonisation of dung pats in the field 15
Blow in control 18
Dispersal of target species 20
Discussion 22
Seed intake 22
Terrain use and distribution of dung 22
Germination 23
Target species 25
Conclusion 26
References 27
Appendices 30
1 Target species 30
2 Vegetation relevés July Delleburen and Hullenzand 31
3 Vegetation relevés September Delleburen 35
4 Mean number of seeds per inflorescence 37
5 Mean number of inflorescences 37
6 Number of seeds in the hand clippings 40
7 Mean number of seeds per liter dung 41
8 Mean number of dung pats per count date 42
3
Introduction
Dispersal of plant seeds
Seed plants are sessile, and have a limited ability for self-ppulsion. Successful colonisation of new sites is therefor dependent upon arrival of seeds. There are significant advantages for plants bearing traits that increase the probability of successful dispersal. Seeds falling beneath the parent plant are faced with competition for resources with their parent, such as higher levels of density dependent seed predation or higher densities of competing siblings with the associated epidemiological problems associated with high densities such as fungal or viral transmission among individuals (Stiles 1992). Seed dispersal is the movement of seeds away from the parent plant. The number of seeds dispersed is first determined by the seed output of a plant species (Nathan & Muller Landau 2000). Dispersal is a spatial process, but survival of seeds in a soil seed bank can be entitled as dispersal in time (Willson 1992).
The spatial distribution of seeds around their source is called a 'seed shadow' (Janzen 1971). The term is most commonly used in reference to the post-dispersal distribution of seeds around the maternal parent, but it can also be used to refer to the distribution of seeds around a source composed of multiple parents. The unit of dispersal may be technically a fruit or a group of fruits and the generic label should therefore be diaspore or propagule (Willson
1992). The term of seed shadow may also be used to include all diaspores ofany morphological and genetic derivation (Willson 1992) and it is also entitled as a seed- dispersion pattern (Nathan & Muller-Landau 2000).
Plant seeds can be dispersed in many ways. Wind dispersal occurs in almost all species. Water, man, machines and animals are dispersal agents too (Bakker et al. 1996). For many plant species, animals provide the means for the critical mobile stage in the plant's life history (Stiles 1992). Specific feeding habits, territorial and migration habits as well as the possibility of epi-, endo and dysozoochory complicate the possible seed-dispersion patterns by zoochory (Harper 1977). Exo- or epizoochory means animals transport seeds in their fur or hoofs. Diaspores can also be constituent of the diet and they are dispersed endo- or
dysozoochorically. Dysozoochory means that seeds are transported to particular places to be eaten or to be stored (Bonn & Poschlod 1998). Endozoochory means that seeds are taken in with the food and excreted with the faeces. In most cases animals are predators and dispersers at the same time. Losses of seed and germination ability should be seen as the price for endo- and dysozoochory (Janzen 1971). Janzen (1984) postulated the Foliage-as-Fruit-hypothesis:
seed dispersal through consumption of the seeds by large herbivores while they are eating the foliage of the parents plants is a normal and selected dispersal mode for a number of species of small-seeded herbaceous plants. This means that this group of plant species has been adapted to seed dispersal by large herbivores.
Endozoochorous seed dispersal
This study focuses on endozoochorous seed dispersal. The potential range of spreading is determined by the seed supply, the feeding habits of the animal species and the survival of seeds through the gut. Also dispersal distance depends on retention time and range of action of a particular animal species (Bonn & Poschlod 1998). Malo & Suárez (1995b) studied endozoochory over a whole season. They found that endozoochory seems to be mainly determined by seed production of the plant community. It was not dependent on herbivore species. This may be true for the whole season, while much variation is levelled out. But selectivity of cattle in seed intake may be expected from their specific feeding habits. Plant communities differ in structure and species composition and plant palatability and may therefore differ in attractiveness to herbivores. Wallis De Vries (1994) found that cattle discriminate between and within different plant communities. Cattle preferred riverine grassland above heathland. Within plant communities cattle preferred short and leafy patches above tall and stemmy patches. Moreover cattle took more bites at leafy patches than patches with much heath. Van Rees (1984) found that cattle were highly selective in their dietary
preferences and concentrated on a relatively small number of species. Diet selection in grassland and heathiand communities was very similar (WallisDe Vries 1994).
Photo 1. Cattle resting in a Molinia stand in the shadow of trees. © Drs. H.M.C. Verhagen, and also the remaining photos.
Behaviour is not only determined by the presence of food. It is also strongly influenced by the time of the day. Numerous studies have shown an intensive grazing period starting around sunrise, followed by a resting period around noon, followed by another intensive grazing period at late afternoon, which lasts till just after sunset. Cattle can graze during night when they have consumed little food during daytime (Van Rees 1984).
The potential range of seed dispersal by endozoochory is also determined by the resistance of particular species against the process of chewing and gut passage (Russi et a!.
1992, Gardener et a!. 1993). The influence that a particular animal species may have on this process is by means of for example the chewing habits, the way of digestion and the retention time. Some diaspore characteristics are related with endozoochory, for example size and hardness of the seed coat. Survival of legume seeds, both in the digestive tract and outside in the dung was largely dependent on the fraction of hard or impermeable seed in the sample (Gardener eta!. 1993). Gardener eta!. (1993) showed that soft seeds swelled on imbibition, the seed coat ruptured, and seeds became fragmented especially after 70 h in the digestive tract. Legume seeds survive passage through the digestive tract better than grass seeds (Gardener eta!. 1993). However digestion by animals may also have a positive effect by enhancing germination by scarifying seeds in the gut (Howe & Smallwood 1982). Ingestion reduced hardseededness, so that a greater proportion of the seeds was capable of germinating after ingestion (Russi eta!. 1992). Gut passage is a selective process because species differ for the properties mentioned above.
An important aspect of endozoochorous seed dispersal, influencing post-dispersal processes may be the gap formation by dung deposition. A 350 kg cow voids ca 34 kg of faeces (5-6 kg dry weight) and covers ca 0.75 m2 of ground each day (MacLusky, 1960 in Harper 1977). A large faecal dropping represents a disaster for the plants beneath and around
it. The effects are at least fourfold: (1) smothering and exclusion of light from the plants; (2) a local disturbance of the nutrient relations of the pasture which may extend beyond the faecal pat; (3) changes in the pattern of grazing around the pat which animals tend to avoid; and (4) the creation of an island for colonisation (Harper 1977) (Photo 2-5). Dung pat gaps must be recolonised from propagules already present in the soil, in the manure, or from propagules that arrive subsequently. The success of plants and seeds situated underneath the dung is related to their resistance to toxic effects from the fresh faeces and the ability to penetrate the pats (Welch 1985). Weeda (1967 in Van Rees 1984) found in his study site that formation of
5
a hard crust on the cow pat, and consistency of the dung influenced the rate of disappearance of cow pats. The relatively fibrous nature of alpine vegetation ensures that cattle faeces are deposited as solid masses, which because of the thy atmosphere, rapidly dry out with the formation of a thick hard crust (Van Rees 1984). On the salt marsh of Schiermonnikoog decaying of dung pats varied from 3 weeks to more than 30 weeks (De Boer 1974). The photos 2-5 showdifferent stadia from the decay of cattle dung pats in Hullenzand.
Dispersed seeds defecated in a viable state by the herbivore can play an important role in the recolonisation of these gaps, since they are closer to the surface in the manure than seeds in the soil seed bank (Malo & Suárez 1995a). Pakeman eta!. (1999)found that three of the four most common species in rabbit pellets established better under field circumstances than most other species that were found in the pellets.
Photo 2. Freshly deposited cattle dung.
Photo 3. A. Cattle dung pat with many holes made by little dung beetles or other insects. B. Cattle dung pat with a hard crust and tracks of dung beetle activities on the right.
Photo 4 A, B, C. Old, trampled, dried out dung pats.
The importance of endozoochory
Nowadays many agricultural soils become available for nature development. In many of these sites topsoil is removed to create the appropriate abiotic conditions for restoration of
heathland and species-rich grassland. However with topsoil removal, the vegetation is removed and also a large part of the seeds in the soil are often removed too. This means that these sites must be colonised by seed dispersal. The impact of wind is probably small.
Windtunnel experiments with 22 species of pasture and heath made clear that wind dispersal occurs on a scale of a few meters (Zijlstra 1992). Strykstra eta!. (1998) showed this with a windtunnel experiment in particular for the rare wind-dispersed Arnica montana. Klooker et a!. (1999) found with seed traps that 90% of the seeds originated from within 2 meters of the seed trap. Therefore it has been hypothesised that invasion of target species can be stimulated by fencing the nature development site together with source sites and graze them by different types of herbivores (Klooker eta!. 1999). De Smidt & Heil (2000) showed in a comparison of grazed with ungrazed sites a positive effect of grazing by cattle on spread and establishment of characteristic heathland species. These species were mostly mosses and epizoochory may be responsible for this.
Dispersal is a very important process for the colonisation of nature development sites.
Cattle ingest numerous seeds and many of these seeds survive the digestive tract. Hansen (1911, in Harper 1977) recorded that a cow grazing on a weedy field consumed in one day 89.000 seeds of Planrago spp. and 564.000 seeds of Matricaria chamomilla of which 85.000 and 198.000 respectively were voided at 58% and 27% germination capacity. In many nature conservation and restoration sites grazing is used as a management tool. Cattle can therefor act as dispersal agents in a nature development site when this site is fenced in with source sites. Selectivity in terrain use, feeding habits and survival of plant seeds in the digestive tract affect the success of dispersal by cattle for nature development.
In the present study the importance of endozoochory by cattle for the establishment of plant species of heathland and oligotrophic grassland plant communities is investigated.
Therefore (1) a comparison has been made between the potential intake of seeds and the potential dispersal. The potential intake is measured as the number of seeds that is present in the area. The potential dispersal is the number of seeds that survive the digestive tract.
Therefor dung has been sampled from cattle, horses and sheep and brought in a greenhouse.
(2) Terrain use of cattle has been determined by measuring the number of dung pats in different area parts. (3) Colonisation of dung pats by vegetative shoots and seedlings in the field has been recorded. Special attention has been paid to the dispersal and establishment of characteristic species of heathland and oligotrophic grassland vegetation on the nature development sites, further referred to as target species (Appendix 1).
7
Photo 5 A, B. Old dung pats, probably under moister conditions, become vegetated.
Methods
Study sites
The study is carried out in two sites, Delleburen and Hullenzand. Delleburen and Hullenzand have been selected because part of the sites is nature development site, part harbours
oligotrophic and mesotrophic plant communities and the sites are both grazed by cattle.
Delleburen
The Dellebuursterheide (200 ha) is situated in east-Friesland (20 km east of Heerenveen) (Amersfoort coordinates 205/206, 552/553),and is a remnant of extensive heathlands along the Tjonger. The study site consists of the Delleboeren (heathland, bog and forest), the Hoorn (river dunes), former arable field of which topsoil is removed (25 ha) and several pastures (Klooker eta!. 1999). Almost the whole site (180 ha) is grazed year-round since 1979 by livestock. Numbers fluctuate (Jager 1999). During the study period the site was grazed by 13 Scottish Highland cattle, 18 Exmoor pony's and about 30 Drentse-Heide-sheep (this number became about 60 in November).
Fig. 1. The study site Delleburen. The left part is the oligotrophic grassland (0), the middle part is the heathiand and on the right lies the nature development site (N). Further F= forest, M= mesotrophic grassland, W= water. The north is at the top of the chart.
Hullenzand
Hullenzand (80 ha) is situated in south Drenthe (10 km south east of Beilen) (Amersfoort coordinates 235, 533). It is a remnant of extensive heathlands. It harbours dry and wet heathiand communities with some oligotrophic grassland and drift sand communities and agricultural fields that lie fallow from which 1.5 ha topsoil is removed (Klooker eta!. 1999).
Ten (7 adults, 3 calves) Scottish Highlander cattle graze the site year-round. The site has been enlarged with a large (60-70 ha) former agricultural field from which topsoil is removed in
1998 and 1999. This part has almost no vegetation cover. Cattle were sometimes seen resting there.
Selection of area parts
Each site is classified in different area parts. The following criteria were used to select the area parts: the proportion of the total site, relevance for nature development, difference in structure and plant composition, and not too many diverse area parts. Table I shows the classification for Delleburen and Hullenzand. These figures are used as base for further calculation in this study. In some area parts no dung plots were placed. In the Molinia stands of Delleburen for example no plots were established, as these parts are not easily accessible because of the many tussocks and therefor also not attractive to cattle. Further dung could not be counted in this area part because of shallow water.
9
Fig. 2. The study site Hullenzand. The left and top part of the chart are former arable field (X). Further Z=sanddunes, C=Calluna-vegetation, K=Empetrum-vegetation, M=Molinia-stand, D=Deschampsia- vegetation, O=young trees, B=Wood, T=deforested sanddunes, W=water. The north is at the top of the chart.
Tabel 1. Surface area in hectares and the number of dung plots per area part, for Delleburen (A) and Hullenzand (B).
A) Delleburen
Area part Area (ha) N plots
Nature development site (1) 25* 10
Molinia/bog community 20** 0
Oligotrophic grassland (4) 18** 11
Mesotrophic grassland Hoorn (2) 36** 0
Other mesotrophic grassland 9** 5
Heathland (3) 20** 10
Forest 34** 0
Remaining (mainly). effususstand) 18 0
Total grazing site
I 80"
36*Klookeretaj 1999
**Computedfrom chart Jager 1999 Jager 1999
B) Het Hullenzand:
Area part Area (ha) N plots
Grassland (2) 7.5 7
Juncus effusus stand 18* 0
Forest 6* 0
Fen community (1) 1.5** I
Moliniastand (5) 7* 7
Heathland (3) 2.5* 7
Deforested sand dunes (4) 12* 7
Sand dunes / heath 25.5* 0
Total site 80** 29
* Computed from chart Natuurmonumenten 1994
** Klookeret al. 1999, also derived from Wolters-Noordhoff Atlasprodukties (1987) Grote topografische atlas van Nederland deel 2 Groningen
Terrain use and distribution of cattle dung
The distribution of cattle in the study site's was determined in two ways: by direct
observation and indirect by counting dung pats in the different area parts. The indirect method means that distribution of dung was measured using 20 by 20 m. plots, further referred to as dung plots. These dung plots were laid out at random in each area part. Bamboo sticks in every edge defined a plot. In Delleburen 36 plots were established and in Hullenzand 29 plots.
Counting occurred once every three to four weeks. Dung pats were counted 7 times in Delleburen and 6 times in Hullenzand from July till December. Double counting was
prevented with paint-marking. At different count dates different colours of paint were used, so dung pats of different ages could be identified later. Results of the first count date are not used for calculation, because densities may be biased by different decay rates. These are for example greater in pasture than in heathiand (Bakker et al. 1983). Direct observations were done mainly when the site was visited. Direct observation means that location and activity of the cattle in the site was noted when fieldwork was done. This occurred 2 to 3 times on a field day. These observations were done to control/correct the results of the indirect method.
Additional information could be obtained from the position of fresh dung, tracks etc. Two days before sampling of cattle dung, cattle were observed to know the foraging site to make it
possible to relate this knowledge to the seed content of the dung. These observations have only taken place for Hullenzand in July, for the other sample dates this was not possible.
Potential seed intake
Seed supply was measured once in July and once in October. Seed supply was determined in plots of 0.25 m2 of patches that differed in structure and plant composition. All stems bearing seeds were counted within these plots. Three of these relevés were done per dung plot (a plot of 20 by 20 m, see under terrain use. For almost all species with seed, seed content of the stems was determined. This was done once in July and once in October in both study sites.
From these data the absolute seed supply of the dung plots could be computed.
Vegetation relevés were made in July per dung plot to correct for species that were not found with the seed supply estimation. With this method it was noted if a species was dominant (d), if it had seed (z) and if it was flowering (b). In September, for all species abundancy was noted according to the Tansley scale (Tansley 1946) : D= dominant A=
abundant F= frequent 0= occasional R= rare.
The absolute seed supply may be no good estimator for seed intake by cattle. When bites are simulated this may be a better estimator for the seed intake by cattle. Therefor seed intake per bite was estimated by plucking vegetation low to the ground by hand, according to the method described by Wallis De Vries (1994). 25 bites were simulated for each area part (five sample plots with five bites per sample plot). This was only done two days before dung collection. In July it was done only once in the Hullenzand study site and in October it was done once in Delleburen. In combination with direct observations this may be a good estimation of seed intake.
Potential dispersal
To detect which species can be dispersed by endozoochory, seed content of dung can be measured. Seeds that have been swallowed and survived the chewing and gut passage can be determined by germinating them in the greenhouse. Twelve to fifteen freshly deposited cattle dung samples (total 10 1.) from different dung pats was collected once in July and once in October in each study site. After collection dung was stratified two weeks at four degrees Celsius. After stratification dung was homogenised by washing on a sieve and spread on trays with sterile sand and soil. Seedlings occurring in the dung were identified and alter that removed. Plants becoming so large as to obscure or prevent germination of other seeds were removed; if not positively identified they were grown separately until identification was possible. July dung was determined till 10 January 2001 (five months). Only July dung samples and seed supply is used in the analyses presented in this report, because data of cattle dung collected in October are presently incomplete.
Colonisation of cattle dung
A number of fresh dung pats in the Delleburen area were marked in July (14-07-00). They were marked with flags in the nature development site (n=8) and in the mesotrophic grassland (n=7). These dung pats were examined in August and December. Dung pats that were marked with paint when counting dung (from the first three count dates) were examined in Delleburen in September. Observed dung pats were located in the nature development site (n=17),
heathland (n=16) and oligotrophic grassland (n=5). The age of these dung pats ranged from 1 to 3.5 months after dung voiding. The number of colonisers (germination and vegetative in growth) and amount of bare dung were noted on these dung pats.
Blow in control
Seedlings that germinate on cattle dung do not necessarily originate from that cattle dung.
They may be blown in by wind or otherwise have been dispersed to the dung pat. To
determine if seedlings are originating from the dung two methods were used. The first method assumes that only seeds of the surrounding vegetation blow in. Most seeds fall within 30 cm from the parent plants (Klooker eta!. 1999). Species than can be assumed to be dispersed from elsewhere through cattle, if they are found on the dung and not in the vegetation 30 cm around the dung pat. Therefor vegetation in a circle of 30 cm around the dung pats was determined.
11
The second method is more direct. When seedless dung pats are put next to normal dung pats seed blow in can be determined directly and distinguished from dispersal by cattle dung. The biggest problem with making dung seedless is to prevent loss of dung structure. Freezing till 80 degrees is effective in killing seeds, but the structure of cattle dung is lost and pats
decompose faster in the field (pers. comm. J.P. Bakker). In the present study heating dung till 80 degrees was used to kill the seeds inside.
An extra sample of fresh cattle dung (ill) was collected for sterilisation of dung in Delleburen in July (14-07-00). This dung from different dung pats was diluted to 12 liters, mixed and homogenised. Mixing was done by hand in a bucket. 3 x 1 liter dung were
stratified for two weeks, washed and then spread on sterile soil in the greenhouse to determine seed content. 6 liters of dung were sterilised by drying at 80 °C for three days. 3 x 1 liter of sterilised dung was spread on soil in the greenhouse to determine the effectiveness of killing seeds by drying. 3 x 1 liter of sterilised dung and 3 x 1 liter that were neither sterilised nor stratified were spread within a 15.1-cm diameter PVC ring on the nature development site in Delleburen. This resulted in a pat that had the same shape and thickness as natural dung pats.
Table 2: Overview of the blow in control experiment. Eleven liters dung were sampled and mixed. To enable mixing, the dung was diluted with water to twelve liters. The mixed dung was divided in twelve portions of 1 liter. Three portions were stratified, three portions were stored, and six portions were sterilised. three sterilised portions were brought to the field, and 3 sterilised Dortions in the greenhouse.
Sampling Mixing 21-7 Treatment Purpose
14-7
Dung -Dung mixed, 3 x 1 I. stratified 25-7 till 14-8, to Determine seed content sampled -diluted to 12
liters,
-divided in 12 portions
greenhouse
3 x 11. stored till 1-8, to field
6 x 1 I. sterilised 3 x 1 I. 1-8, to 29-7 till 31-7 field
3 x I I. cooled till 14/8, to greenhouse
Determine germination in the field
Determine blow in of seed in the field Control for sterilisation success
Data analyses
The potential intake of seeds has been compared with potential seed dispersal in two ways.
Seed supply in July was compared with seed germination in the greenhouse dung of July by means of a Spearman's rank correlation. Because seed supply and seed germination in the greenhouse were not identical samples, data could not be tested for species separately. Data for species are shown qualitatively. Direct observations in combination with hand clippings were compared with greenhouse germination with a Spearman's rank correlation, also.
Terrain use of cattle was tested for randomness with correction for the different sizes of the area parts with a Chi-square test. Actual numbers of dung per dung plot over the whole season were tested against an equal number of dung pats per dung plot. With a Tukey HSD test differences between area parts were tested.
Germination of seedlings in the greenhouse and in the field was compared with a Spearman's rank correlation. Germination on cattle dung was compared between dung pats of different ages with a Spearman's rank correlation. Blow in of seeds was not tested but shown qualitatively. With a T-test was tested if displacing of dung within the sterilisation experiment had an effect in comparison with untreated field dung. Differences between species were tested with a Kruskall-Wallis-Test. All analyses were carried out with the package SPSS 9.0
Results
Seed supply
In Table 3 the percentages of different species in cattle dung and in the field are shown. The seed presence (seed supply) in the field and germination from dung pats in the greenhouse are not correlated. This means that the species producing the highest number of seeds in the field are not the dominant species germinating in the dung in the greenhouse. For Delleburen seeds in the cattle dung are mainly of Poa trivialis, Alopecurus genicularus, Cerastiumfontanum, Juncus bulbosus, Ranunculus repens (Table 3A). In the field seed supply is dominated by Erica tetralix, Agrostis capillaris, Juncus articulatus, Juncus effusus (Table 3A). For Hullenzand seeds in cattle dung are mainly of Juncus bufonius, Juncus effusus, Poa trivialis, Cerastiumfontanum, Juncus bulbosus (Table 3B). In the field seed supply is dominated by Rumexacetosella, Holcus lanatus, Gnaphalium uliginosum and Erica tetralix (Table 3B).
Table 3: Percentage of dung seed content in the greenhouse and seed supply in the field of different plant species. Target species are bold. No value means a percentage <1. Betula sp. has probably blown in in the cattle dung.
Dung (green-
house)
Seed supply
Poa trivialis 72
Alopecurus geniculatus 7
Cerastiumfontanum vulgare 5
Juncus bulbosus 5
Ranunculus repens 3
Holcus lanatus I— 1
Poa pratensis I
Juncus bufonius
Veronica serpyllifolia — —
Poa annua Lolium perenne
Sagina procunzbens 1
Leontodon autumnalis 1
Rumex acetosella 2
Erica tetralir 10
Agrostis capillaris 11
Juncus articulatus 21
Dung (green-
house)
Seed supply
Juncusbufonius 19
Juncus effusus 18
Poa trivialis 13
Cerastiumfontanum 13 2
Juncusbulbosus 10
Holcus lanatus 7 21
Trifolium repens 4
Poa pratensis 3
Rumex acetosella 2 59
Epilobium palustre 2 2
Festuca ovina 2
Poaannua 2
Betulasp —I
Sagina procumbens I
Ranunculus repens I
Agrostis capillaris I
Hypochaeris radicata 1
Erigeron canadensis 1
Deschampsia flexuosa 3
Erica tetra lix 4
Gnaphalium uliginosum 5
For Delleburen it is unknown where the cattle were foraging two days before sampling.
Therefore no comparison can be made between the seed intake of and germination in the greenhouse. For Hullenzand some observations of grazing cattle have been done and some hand clippings have been done in this vegetation, but on different places then where the cattle have grazed. Table 4 shows the mean number of seedlings per liter dung and the number of seeds simulated by means of the hand clipping. For Spearman's rank correlation all sample places got an equal weight because the exact grazing time on these places was not known.
There is a significant correlation between the seed content of the dung pats in the greenhouse and the seed supply found by means of hand clippings (p<O.O5) if all species in the hand clippings and/or in the cattle dung are included in the analysis. For the species that are present in both hand clippings and cattle dung there is no correlation when the other species are
A) Delleburen. B) Hullenzand.
Juncus effusus 54
13
(
omitted. This means that the species found most in the dung in the greenhouse had the highest number of seeds found by the hand clippings. Species in the hand clippings are the dominant species in the dung in the greenhouse (Table 4), except for Juncus bufonius. This species was not found by means of hand clipping.
Photo 6. It was not possible to observe the grazing behaviour of cattle of Hullenzand in high vegetation from a distance, because they were shy.
Table 4: Mean number of seedlings per liter dung in the greenhouse and the number of seeds/40 bites simulated by means of the hand clipping. The second column is the sum of the last four columns and is used for the test for correlation. The last four columns show the four sample plots where cattle have been grazing and are shown to indicate variation among the samples. N is the number of samples. The Poa sp. is Poa pratensis or Poa trivia us. Target species are bold.
seeds/I - seeds!
40 bites
seed/lO bites
total 2 3 4 5
N 15 4 5 3 2 1
Juncus bufonius 50
Juncuseffusus 46 160 160
Poasp. 42 28 28
Cerastiumfontanum 33 0.3 0.3
Juncus bulbosus 26 470 62 39 370
Holcus lanatus 19 8.3 8.3
Trifolium repens 10 33 33
Rumexacetosella 6.3 99 51 48
Epilobium palustre 5.7
Festuca ovina 4.2
Poa annua 4.2 0.9 0.9
Betula sp 2.7
Sagina procumbens 2.2
Ranunculus repens 1.5
Lolium perenne 1.2 1.1 1.1
Geranium sp. 0.8
Veronica ari'ensis 0.7
Veronica serpyllifolia 0.6
Plantago lanceolaza 0.3
Chamerion angustifolium 0.2
Plantago major 0.2
Sonchus asper 0.2
Cardamine pratensis 0.1
Carexnigra 0.1
Hypochaeris radicata 0.
Juncusacutiflorus 0.
Luzula sp. 0.
1.0 1.0
Distribution of cattle and dung
Distribution of dung differs from randomness for different plots (x2test,p<O.OO2). Table 5 shows the numbers of dung pats in different area parts in Delleburen and Hullenzand. In Delleburen dung density in oligotrophic grassland is significantly smaller than the dung density in the heathland and mesotrophic grassland. In Hullenzand the dung density in the Molinia stand and in the heathland is significantly smaller than in the deforested sand dunes (Table 5).
Table 5: Mean numbers of dung pats per count date in different area parts in Delleburen (A) and Hullenzand (B). These results have been obtained with a Tukey HSD (a0.05) for the number of dung pats in different area parts over all data. Different letters indicate significant differences.
A) Delleburen
Area part N Mean ±SE
Oligotrophic grassland 77 0.91 ±0.40a
Nature development site 70 1.53 ±0.42ab
Heath 70 2.77 ±0.42 b
Mesotrophic grassland 35 2.80 ±0.60b
B) Hullenzand
Area part N Mean ± SE
Molinia vegetation 35 0.17 ± 1.61a
Heath 35 2.34 ±0.61 a
Mesotrophic grassland 35 2.97 ±0.61 ab
Fen 5 3.40± 0.61 ab
Deforested sanddunes 35 6.00 ±0.61 b
Colonisation of dung pats in the field
The identified seedlings on dung pats in the field have been divided into monocotyles and dicotyles. The proportion of monocotyles: dicotyles is 7:3 for all ages of field dung pats. The mean number of seedlings does not significantly differ between dung pats of different ages (Kruskall-Wallis). Table 6 shows the species composition for dung pats of three different ages. Poa annua, P. trivialis and Agrostis capillaris are the most frequent species germinating on the cattle dung in the field.
15
Table 6: Mean number of seedlings found on dung pats per species in Delleburen, September 2000.
Percentages are given between brackets. The colour of the paint that was used with counting and marking of counted dung could be used to determine roughly the age of the cattle dung pats. The first two categories do overlap because marking was done on two days the first time with about a week in between. Examining dune Dats in Sentember was done in a one-week period. Target species are bold.
age >9 wks 7-11 wks 3-7 wks
N 13 14 11
Poa annua 4.3 (32) 1.3 (10) 0.73 (8)
Poa trivialis 3.0(22) 4.0 (31) 1.9 (21)
Agrostis capilaris 1.4 (10) 3.0 (23) 0.55 (6)
Rumex acetosella 0.68 (5) 0.52 (4) 0.46(5)
Ranunculus repens 0.41 (3) 2.34 (18) 0.55 (6)
Gnaphalium uliginosum 0.27 (2) 0.13 (1) 0.27 (3)
Cerastiumfontanum 0.14(1) 0.26 (2) 0.27 (3)
Molinia caerulea 0.14 (1) 0.52 (4)
Stellaria palustris 1.5 (11)
Hypochaeris radicata 0.68 (5)
Agrostis stolonifera 0.54 (4)
Agrostis canina 0.14 (1)
Deschampsia flexuosa 0.14 (1)
Lysimachia vulgaris 0.14 (1)
Taraxacum sp. 0.27 (2) 0.09(1)
Alopecurus geniculatus 0.14 (1) 0.09 (1)
Plantago lanceolata 0.78 (6) 0.36 (4)
Holcus lanatus 0.26 (2) 0.36 (4)
Lolium perenne 0.27 (3)
Festuca pratensis 0.27 (1)
Luzulasp. 0.27(1)
Scirpus cespitosa 0.27 (1)
Steliaria media 0.27 (1)
unidentified monocotyledons 2.0 (22)
unidentified dicotyledons 0.82 (9)
Mean number of seedlings per dung pat 13.5 13 9.1
There is only one significant correlation between dung of 7- 11 and 3-7 weeks old (Table 7).
The species composition and abundance is partly similar on these dung pats. No significant correlation is found with dung of
more than 9 weeks old (Table 7), although this partly overlaps with 7-11 weeks.
Table 7: Comparison of the number of seedlings per plant species compared for field dung of different ages.
The correlation coefficients (Spearman's rho) are shown.
N=23.
7-11 wks 3-7wks
>9 wks ns ns
7-llwks
**Correlation issignificant at the .01
0.776**
level (2-tailed).
Determination of the species occuring in a radius of 30 cm around the dung pats shows that 60-90 % of the seedlings on a dung pat is not found in a circle with a radius of 30 cm of the dung pat (Table 8). When unidentified species are omitted, then about 60% of the seedlings in the nature development site is new on that pat, for heathland and oligotrophic grassland it is even about 90%. The nature development site is species rich compared to the other area parts.
Table 8: Percentage of all seedlings germinating on cattle dung in the field in three area parts. The first column shows the mean number of seedlings, in the second column the number of seedlings is corrected for potential seed blown in. For correction, relevés have been made of the surrounding vegetation in a circle with a radius of 30 cm. For species that were present within this circle seedlings were considered as been blown in from the local vegetation and omitted in the second column. . means this species was also found in cattle dung in the greenhouse. Target species are bold. Values in bold are values that differ between measured and corrected values.
Heathland Nature development site
Oligotrophic grassland total corrected total corrected total corrected
N 17 16 5
Ranunculus repens • 3.2 3.2 14.2 9.3 25 25
Rumexacetosella • 0.5 0.5 6.4 4.9 19 19
Poa trivialis • 30 30 25 15
Poaannua' 30 30 7.4 7.4
Agrostis capillaris • 14.9 6.8 14.2 5.9 9.4 0
Plantago lanceolata. 1.4 1.4 5.9 5.9
Cerastiumfontanum • 2.3 2.3 1.5 1.5
Alopecurus geniculatus • 0.5 0.5 0.5 0.5
Hypochaeris radicata 4.4 4.4
Agrostis canina 1 1
Festuca pratensis • 0.5 0.5
Stellaria palustris 9 9
Gnaphalium uliginosum • 3.6 3.6
Taraxacumsp. 1.8 1.8
Lysimachia vulgaris 0.5 0.5
Scirpus cespitosa 0.5 0.5
Loliu,n perenne • 9.4 9.4
Deschampsiaflexuosa 3.1 3.1
Luzulasp.• 3.1 3.1
Stellaria media. 3.1 3.1
Agrostisstolon(fera 3.4 0
Holcus lanatus • 3.4 0
Molinia caerulea 2.3 0 2 0
Unidentified species 11 28
Mean number of seedlings 13.1 12.8 6.4
Newly introduced species as a % of total
number of seedlings 90 56 63
17
Blow in control
Sterilising by means of heating method killed almost all seeds in dung (Table 9). Seedlings of Erica tetralix, Asteraceae arid Sagina procumbens germinated on the sterilized dung. Sonchus asper, Chamerion angustfo1ium, Epilobium palustre and Betula sp. germinated on sterilized, stratified and control and thus are probably blown in.
Table 9: sterilized dung compared with stratified dung in the greenhouse. Seedlings in the control tray show blow in of seeds in the greenhouse. For each species number of seedlings is given and percentages are given between brackets. Target species are bold.
sterilized Stratified - control
Poa trivialis 99 (37.8)
Juncus bufonius 86 (32.8)
Ranunculus repens 18 (6.7)
Carexovalis 11(4.3)
Cerastiumfontanum 7.4 (2.8)
Juncus bulbosus 6.6 (2.5)
Juncus acutiflorus 6.6 (2.5)
Alopecurus geniculatus 5.0 (1.9)
Juncus effusus 3.9 (1.5)
Poa pratensis 2.9(1.1)
Holcus lanatus 2.1 (0.8)
Poa annua 1.6 (0.6)
Veronica serpyllifolia 1.3 (0.5)
Trifolium repens 1.1(0.4)
Epilobium ciliatum 0.3 (0.1)
Gnaphalium uliginosum 0.3 (0.1)
Juncus spec 0.3 (0.1)
Luzula sp. 0.3 (0.1)
Polygonum sp. 0.3 (0.1)
Rumex acetosella 0.3 (0.1)
Sonchus asper 0.3 (0.1)
Glyceriafluitans 0.3 (0.1)
Lolium perenne 0.3 (0.1)
Erica tetralix 0.33 (3.7)
Sonchus asper 0.33 (3.7)
Asteracea 0.33 (3.7)
Chamerion angustzfolium 0.67 (7.4)
unidentified dicotyledons 0.67 (7.4) (0.1)
Sagina procumbens 1.7 (18.5) (1.6)
Epilobiuni palustre 1(11.1) (0.6) 0.34 (20)
Betula sp 4 (44.4) (0.1) 1.36 (80)
mean number of seedlings/sample 9 263 1.7
Rumex acetosella and Lythrum portula are both found on sterilised pats in the field, but not in the greenhouse dung (Table 10). These species were also in the surrounding of the dung pat and had set seed. The number of seedlings does not differ significantly between displaced (not sterilised and stenlised field dung pats.
Table 10: The number of seedlings for different treatments of the blow in control experiment (total 12 x II.). Results are shown for Lythrum portula and Rumex acetosella, totalmono- and dicotyledons and total seedlings.
Greenhouse Field
sterilised stratified Blank Nature development Not sterilised
sterilised
sample 1 2 3 1 2 3 1 1 2 3 4 1 2 3 1 2
Lythrum portula
Rurnex acetosella 10 7 5
Total
monocotyledons
153 36
106 2
—
176—
42
1
1 1
9
—
10 15 2
Total dicotyledons
—
2 23 27 5 12 7 5 10
Total seedlings: 11 10 6 358 180 251 5 31 2 2 10 5 22 22 5 12
There is no significant difference between field dung pats and dung pats that have been stored for two weeks for the germination of monocots, dicots or total number of seedlings (Figure 3).
There is however a significant difference for Cerastiumfontanum. Dicots germinate later than monocots and in the stored samples there have germinated almost no dicots on the first date, while on the undisturbed field dung pats dicots have already germinated than. On the second date (November) the difference has disappeared. A same thing can be seen for the monocots:
there are slightly more seedlings on the undisturbed field dung pats than on the displaced dung pats on the first date, while on the second date, most monocots on the ondisturbed dung pats have died. Although the differences are not significant, it suggests that there is a delay in germination. Monocots and dicots may germinate later on displaced dung pats. Monocots
show a high mortality.
Figure 3: Delleburen: number of seedlings of monocots and dicots on untreated dung pats and dung pats that were stored for two weeks.
19
O Untreated dung patches Nature Development Area
•Treated but unsterilized dung patches
Monocots Monocots
16-8-00 28-11-00
Dispersal of target species
In Hullenzand 62% of the seed supply in the area parts measured are seeds of target species (Fig 4). Almost the whole seed supply from target species is seed from Rumex acetosella, and if Rumex acetosella is omitted only 3% of the seed supply is formed by target species (Table 11). In the greenhouse dung the target species form only 14.3% of the seed supply. Rumex acetosella is present in a much smaller proportion in the greenhouse dung. In Delleburen target species form 13.7% of the seed supply in the field and 5.5% of the species in the greenhouse dung (Fig 4). The percentage of seedlings of target species on field dung pats is almost equal with the percentage of target species in the seed supply, but the species composition differs. There is only small overlap in target species in the seed supply, dung seed content and field germination. In Hullenzand seven out of 25 target species in the seed supply were found in the greenhouse dung (Table 11). In Delleburen eleven out of 30 target species in the seed supply were found in the greenhouse dung and additionally five target species were found germinating on field dung pats.
% 70
60 50 40 30 20 10
Hz seed supply
Figure4: Percentage of target species in Hullenzand and Delleburen in seed supply, dung samples and germination on dung patches in the field.
Ob seed Db Db field
greenhouse supply greenhouse germination
germination germination
Table 11: Overview of the percentage of target species in the seed supply (per surface area), seedlings in the greenhouse and in the field dung for Delleburen (A) and for Hullenzand (B). nm= not measured.
0 means it was determined the species had no seed in July, field germination was measured in September.
A) Delleburen:
_______ ______
B)Hullenzand seed
supply
seed in dung
field
Erica tetralix 10
—
Rumex acetosella 2 <1 2
Leontodon autumnalis 1 <1
—
Juncus squarrosus <1 Galiu,n saxatile <1
Festucaovina <1 <1
Carexovalis <1 <1
Carexnigra <1 <1
—
Ranunculusfiammula <1 <1
Hypochaeris radicata <1 1
Juncus acutiflorus <1 <1
—
Potentilla erecta <1
Veronica scutellata <1 <1
Juncusbulbosus nm 5
Anthoxanthum odoratum nm <1
<1 Luzula campestris/multifora. nm <1
Stellaria palustris nm 2
Agrost is canina nm <1
Scirpus cespitosa nm <1
Carex panicea nm
—
Carexpiluhfera nm
—
Drosera intermedia nm
Drosera rotundifolia nm Empet rum nigrum nm
—
Juncus con glomeratus nm
Potentilla anserina nm
Spergularia rubra nm
—
Stellaria graminea nm
Viola canina nm
—
Viola palustris nm
Molinia caerulea 0 <1
Achillea millefolium 0
Calluna vulgaris 0
Festuca rubra 0
seed supply
seed in dung
Rumex acetosella 59 2
Erica tel ralix 4
Hypochaeris radicata 1 <1
Juncusbulbosus nm 10
Festuca ovina nm 2
Luzula campestris/multiflora nm <1
Carexnigra nm <1
Juncusacutiflorus nm <1
Agrostis canina nm
Anthoxanthum odoratum nm
Carexovalis nm
Carex panicea nm
Carexpilulifera nm
Drosera intermedia nm
Drosera rotundifolia nm
Empetrum nigrum nm
Gentiana pneumonanthe nm
Galium saxatile nm
Juncus squarrosus nm
Leontodon autumnalis nm
Nardus stricta nm
Potentilla anserina nm
Potentilla erecta nm
Ranunculusflam,nula nm
Corynephorus canescens nm
Achillea millefolium 0
Calluna vulgaris 0
Festuca rubra 0
Molinia caerulea 0
Calamagrostis epigejos 0
21
Discussion
Seed intake
The method used for measuring seed supply was very time consuming. Per area part density of inflorescences was estimated and the number of seeds per inflorescence was determined per species. It was assumed that all seeds in the inflorescences were ripe. The seed supply was calculated by multiplying these numbers with the areas. A number of species present in the
field were not found in the dung. However it is not known if this is due to the fact that these species are not eaten, or do not survive the digestive tract of the cattle. The seeds of Agrostis capillaris were probably not ripe yet on the date of dung sampling. There are some species that have been blown in such as Betula sp. that were found in the control tray. Species that were not in the control tray but have probably been blown in are Chamerion angustfo1ium and Sonchus asper. Chamerion angustifolium was present in both study sites but had no seed at the when dung was sampled and Sonchus asper may be a good wind disperser like Sonchus oleracea that has a terminal velocity of 0.27 rn/s (Bonn & Poschlod 1998).
When seed content of dung is compared with seed supply in the field for different periods, it was found by Malo & Suarez (1995b) that seed content of dung is correlated with seed supply in the field. In the present study this could not be shown because there was only one sample date. In the present study there was no correlation found between seed content in the dung and seed supply in the field. So seed supply is not the only important factor
determining the seed content of dung. Selectivity by cattle is therefore also an important factor. The study sites in this research are not homogeneous and are too large to graze as a whole in a short time period. Herbivores have to decide where to graze. Therefore at least in the short term seed intake is a selective process. A method to deal with this problem in future research is to take dung samples from different dates within a short period. Grazing over the
whole site may then be represented in the dung samples. Then a better correlation between seed supply and seed intake may be expected.
Another method to deal with this problem is to observe the cattle two or three days before sampling as most small seeds are retained in cattle for 2-3 days. The precise duration for gut passage is not known (Squires 1981 in Gardener et al. 1993). In Hullenzand direct observations have been done two days before sampling. The cattle could only be observed from a large distance, because cattle were very shy, probably due to the presence of calves. At four sites where the cattle grazed, hand clippings have been taken. In the analysis it was assumed that grazing on these sites was equal, to make a comparison with dung seed content possible. A significant correlation between dung seed content and seeds in the hand clippings was found. Species in the hand clippings are the dominant species in the greenhouse dung.
The only exception is .1. bufonius. Probably too few samples have been taken to detect this species that may have a clustered distribution. Wallis De Vries (1994) also used the method of hand clipping. Correlation between dung seed content and hand clippings makes it likely that this is a good method to measure seed intake.
Terrain use and distribution of dung
It is useful to distinguish between occupancy and foraging. Occupancy includes implications of treading and of dunging and urinating. Foraging is used for grazing denoting consumption (De Leeuw & Bakker 1986). Occupancy can easily be measured from the amount and dispersion of voided dung (Welch 1984a). A relationship between foraging intensity and terrain use is sometimes found (Van den Bos & Bakker 1990) but not always. Although the main activities of the animals are foraging and resting, direct observation by De Leeuw &
Bakker (1986) revealed that these activities did not take place evenly over their study site.
Foraging patterns could therefore not be quantified from occupancy patterns. Time spent resting and foraging varies among plant communities (Bakker eta!. 1983). If the amount of dung voided is a measure of foraging activities, the time spent on foraging and resting should be proportional in various pats of the vegetation. In their study site no resting was observed in sections whereas foraging did take place. In other sections six times more resting than
foraging was found to be the case. Hence, the amount of dung provides information about the occupancy but not about foraging. Thus the recording of the dung pats give good indication of occupancy but not of grazing. The method of counting dung pats is an accurate and direct way to determine the distribution of endozoochorous dispersed seeds.
In both study sites cattle use all area parts but terrain use is not random. Avoidance of oligotrophic grassland was found in Delleburen and this may be caused by the presence of sheep that mainly graze there. Avoidance of Molinia-stands and in heath was found in Hullenzand. Bokdam & Gleichman (2000) found similar results in a heathland site. In their study Deschampsia heath was preferred. In Hullenzand, deforested sand dunes were preferred. Also much Deschampsia was present in that area part.
Cattle have other preferences for plant communities when resting than when grazing (Van Rees 1984). In Delleburen and Hullenzand cattle often rest under trees or stand in or near water. Bokdam & Gleichman (2000) found that forest was intensively used as a shelter during the night. The high values of dung pats in heathland in Delleburen could be explained with preference of cattle for that area parts for resting. Sheep also prefer heathland to rest (Bakkeretal. 1983).
Welch (1984) found that the factors most influencing occupancy were nearness to improved grasslands or swards containing many attractive graminoids, and the role of each moorland tract in the management of the farm to which it belonged. Spatial arrangement may play a role. The high values of dung pats in heathland in Delleburen could also be explained with spatial arrangement, because in Delleburen, heathland is situated in the center of the site and may be visited therefor frequently. In Hullenzand, nearness of wood may play a role for the shy cattle to hide and could therefore influence terrain use.
It is assumed that dung plots represent the different area parts. However the dung density measured can be influenced by different factors. First, differential search abilities can make a recovery of dung to be not equally easy in all communities (De Leeuw & Bakker
1986). In our sites this is not the case because in all dung plots dung pats could be easily counted. Second spatial arrangement and edge effects may play a role. Edge effects may influence the value for mesotrophic grassland, because all dung plots in that type were situated at the east side of the study site in Delleburen, while a large part of mesotrophic grassland was situated at the south side. Finally in October large parts of nature development site and mesotrophic grassland became flooded. It must be assumed that also here dung plots represent the area part well. To correct for the inaccuracies of the indirect method, direct observations have been done. Results are not shown because the number of observations was small. An indirect observation represents a 3- to 4-week period, a direct observation only one day. To determine the distribution over different area parts many more observations should be done, because observation on one day usually gives information of just that particular day.
Germination
Many seeds germinated from dung in the greenhouse. Species present in dung have been eaten by cattle, have survived chewing and gut passage followed by germination. The proportions of plant species in dung are affected by differences in intake, survival and germination. In the field significant less seedlings were found on the dung pats than in the greenhouse. There are many possible explanations for that. Germination in the field was recorded in September but greenhouse seedlings were counted until January. Surface site/
volume ratio in the greenhouse is larger than in the field. Estimation of surface site and volume of field dung is very difficult. When field dung was compared with greenhouse dung this was done with and without correction for the estimated volume. The correction gave no difference in the outcome of the statistical tests. Vegetation cover, growth rate of the vegetation and height and wetness of the soil on may influence colonisation of dung pats.
Malo & Suárez (l995a) found in a Mediterranean pasture that cattle dung pats are mainly colonised by seeds transported within the faeces, in contrast to the predominance of
vegetative colonisation of dung pats in Scottish heathland (Welch 1985). These factors were not measured in the present study.
23
In many studies the same species or genera of species are found to be transported by dung. In many studies Poa species were found the main species dispersed by dung (Van Rees 1984, Welch 1985, Malo & Suárez 1996, Bokdam & Gleichman 2000, Mitlacher 2000). In the present study Poa trivialis was the most dispersed species and Poa annua and Poa pratensis were found to be dispersed very well by dung. Cerastium species are also often found (Bokdam & Gleichman 2000, Welch 1985) as in the present study. Also Juncus species are often present in the dung. In the present study they were not found on dung pats in the field, but Welch (1985) found that 13% of the seedlings in the field were Juncus species. Bokdam
& Gleichman (2000) related germination on field dung to Ellenberg values: species on dung pats have an nitrogen value IN? 6. Nitrophilous species may have an advantage by
endozoochorous dispersal since they can germinate under eutrophic conditions in the dung.
Dung deposition and decomposition have impact on the distribution of plant species.
Welch (1985) states that with cattle dung several transmitted species attained greater cover than in the previously existing vegetation. Malo & Suarez (1995) concluded that
endozoochorous seeds were the main source of recovery in gaps generated by dung pats. A small scale spatial pattern in which gaps were dominated by endozoochorous species was the result. The impact of dung deposition and decomposition on the formation of pats of plant species could proceed in three ways (Dai 2000). One way of dung impact is by influencing the deposition of seeds in the dung. This is described in the present study. A second way is by changing the relative abundance of species in the soil seed bank under dung. Seeds that are viable in soil for <1 year (transient) vanish from the soil during decomposition of dung. On the long term when dung composes soil seed bank is altered in this way. A third way is the intensification of the growth of some species through nutrient release. Seed arrival is no guarantee for establishment.
Post-dispersal processes have to be considered to understand the importance of dispersal (Nathan & Muller-Landau 2000). Dung deposition and decomposing determine an important part of the success of dispersed seeds. Dung pats can function as open gaps in the vegetation. Seeds that germinate in dung may not only result from endozoochorous dispersal, but germination can also result from seeds that are blown in by the wind or are otherwise dispersed. When is assumed that only seeds of species within a circle with a radius of 30 cm are blown in, than most species germinating on cattle dung appear to be locally new and are probably transported by cattle dung. The time scale of this research is too small to conclude that the species can establish in a new gap. For the long term further research has to reveal the role of germination of seeds on dung pats for recovery of the gaps created by dung deposition and establishment of dung-dispersed species in the vegetation.
The mortality of the monocots in the field shows that differences between species in the breaking of dormancy are important for survival. Dormancy appeared to be important in preventing losses in the faeces (Gardener et al. 1993). Dormancy is a delaying mechanism, which prevents germination under conditions, which might prove to be unsuitable for establishment. The breaking of dormancy does not in itself trigger germination, but is a necessary prerequisite of it. Thus a seed may need to experience some environmental conditions which act as trigger for germination, but which would be quite unsuitable for germination as such (Fenner 1985). Dormancy can be broken by passage through the
digestive tract and defecated or regurgitated seeds germinate to higher percentages than those that have not been ingested (Baskin & Baskin 1998). The way in which a digestive system breaks physical dormancy is unknown, but it is assumed to be via acid and/or mechanical scarification. In evaluating germination data of seeds with physical dormancy separated from faecal or regurgitated material, attention should be paid to the amount of time between deposition of the material and the retrieval of seeds. Fermentation of faecal material could increase temperature; consequently, seeds would receive a wet-heat treatment after they are deposited. Also, because waste materials are dropped on the soil surface, seeds are exposed to the extremes of daily temperature fluctuations that occur in the habitat. These daily
temperature changes may be high enough to break physical dormancy (Baskin & Baskin 1998).
