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5. Discussion
Biodiversity
Both artificial reefs have a higher species richness compared to the natural reef and the total abundance for Reef 1 is more than twice as high as for the Reef 2. This was mainly due to the high amount of Littorinidae on the first reef which were occasionally counted as more than a hundred individuals in a single quadrant.
The higher species richness on the artificial reefs might be because of their considerably larger size compared to the natural reef. This provides a larger and denser structure and thereby probably a calmer shelter area because it is able to reduce the strength of the water flow more. (Fritz, 2013) It is also possible that the conditions at the location of the natural reef limit the amount of species for example turbidity or water depth. Even small changes on the specific location can determine the survival of some species. (Thomsen, et al., 2007)
The evenness on the natural reef was highest out of all reefs. The species were more equally divided over the community compared to the other reefs. However the Shannon-Wiener Index was highest on the second artificial reef due to the higher species richness.
In this case the Shannon-Wiener index is the most important tool to assess the biodiversity on the reefs because it takes multiple things into account. If the number of species is low they also experience less competition. This might result into a reef with a higher evenness, but if that is combined with a low species richness the index will still be low. It is clear that artificial reef 2 shows the highest biodiversity. This might be due to several characteristics. To start off it is the biggest reef out of all reefs found in the area. As said before this makes it able to change the dynamics of the system like flow velocity more helping species to settle. Besides that the reef is placed in a different position more in line with the flow direction compared to Reef 1. Also it has a more dense structure than artificial reef 1 because Reef 1 has the loose shells which are washed from one side to the next depending on the tides. All these things combined provide a more stable habitat helping species to settle. (Fritz, 2013) (Harwell, et al., 2011) Finally as said about the natural reef it might just be
because it is at a better location in general when comparing factors as flow velocity, light or nutrients (Thomsen, et al., 2007). Besides these characteristics it can also be the case that there are more species on the artificial reefs because they have been there for a shorter amount of time. The species may not have had time to outcompete each other yet which still gives some species a shot at being there at the moment. This situating is sometimes called the ‘boom and bust’ strategy. (Harwell, et al., 2011)
If the Shannon Index is shown over time for the two artificial reefs it is consistently higher for Reef 2.
Day one has an almost equal evenness, but still a higher Shannon Index because the species richness was consistently higher on Reef 2 compared to reef one. For day 2 and 3 the difference is more extreme between the reefs due to the higher evenness for the second reef on those days.
Out of all species the exotic species on Reef 1 take up close to a quarter (26 % average) of the total species where as it is just over a third (36% average) for Reef 2. Of the organisms on the natural reef,
34 30 % were exotic species. If the total number of exotic species is compared per day between reefs, Reef 2 has more than twice as many exotic species compared to the other reefs. Every exotic species found on the natural reef and Reef 1 also occurs on Reef 2. This can be explained because the total species richness of reef 2 is much higher in general. Even though the species richness differed greatly between reefs, the percentages are not that far apart and appear to behave the same.
A remarkable difference however, was the amount of the colonial ascidian Didemnum sp. present at the natural reef covering 53% of the reef on average compared to a maximum of 15% on reef 2. This type or organism is known to be able to grow rapidly overgrowing almost all other kinds of sessile organisms, suffocating them. This makes it an invasive species with a high impact on the ecosystem.
(Gittenberger, et al., 2010)
Another species which should be mentioned is the Japanese Oyster drill, Ocinebrellus inornatus. As the name already suggests it feeds on oysters, but also mussels. This makes it a threat to the oyster reefs and the aquaculture in the Eastern Scheldt. Only one individual was found which suggests they have not truly establishes themselves on the reefs yet, but the find itself is proof that they are getting to this part of the Eastern Scheldt. It is unclear on when they were introduced, but they did start to become more abundant after 2007 (Lützen, et al., 2012).
Hemigrapsus takanoi was the only exotic crab species. It is believed these crabs were introduced into Europe during the early 1990’s by the transport of Asian oyster or hull fouling or ballast water. Since then they have spread out along the European Coast at a high rate (100 km a year). They are able to spread that fast because their young reproduction age and because of their planktonic larvae (Noël et. al. 1997) which they may have up to six times a year. (Dumoulin 2004)
Around the turn of the century Hemigrapsus takanoi was unintentionally introduced into the Dutch delta via shellfish transport for aquaculture purposes. While the number of Carcinus maenas had declined over the past 20 years the decline already started before the introduction of Hemigrapsus takanoi but C. maenas is now clearly outnumbered by H. takanoi. The decrease in the C. maenas population was not started by H. takanoi but it is possible that H. takanoi is taking advantage of these decreasing numbers and so contributing to the decline of C. maenas (Van den Brink, 2012).
Van den Brink et al. (2012) hypothesized that this outnumbering of C. maenas by H. takanoi was due to adult H. takanoi out-competing juvenile C. maenas smaller or similar size to adult H. takanoi for shelter.
As part of the monitoring survey all crab species were counted and their size and gender were noted.
This gives an overview of the population density and distribution of the species on the artificial reefs.
Combined with the behavioral experiments this can give an indication of the effect exotic species may have on the community composition.
In general only Hemigrapsus takanoi and Carcinus maenas were found, however on day 2, four Porcellana platycheles individuals were found on Reef 1. The total number of crabs cannot be compared between the reefs because of the use of a different collection method (chapter 3.1).
35 When the results of both reefs are combined it can be observed for all three sampling days that individuals of both H. takanoi and C. maenas of the same size are present on the reefs. The fact that there are crabs of both species within the same size range on the reef supports the hypothesis that the H. takanoi is competing with juvenile C. maenas.
Where on day 1 there are still some individuals of C. maenas that are larger than the largest found H.
takanoi they are none left on day 3. This is explained by the fact that larger C. maenas move to deeper water when it gets colder (Audet , et al., 2008).
Behavior experiments on the competition for food and shelter can give more insight to support the hypothesis even further.
Behavioral experiments
During the behavior experiment for food. The presence of a competitor did not make any significant difference (p= 0, 72) for the behavior of which crab was first to eat meaning the crabs behaved the same either with, or without a crab from the other species present.
Besides that competition did affect the number of times eaten, the total time eaten and the average eating time. In general the crabs without a competitor ate less often, but for a longer amount of time on average and in total.
Between the species C. maenas was significantly the first to eat (p=3E -20) and both species ate more often but less time on average and in total.
C. maenas seemed to be better at getting the mussel open to feed on it. All mussels were drilled to Anker them and to trigger an extra feeding response and the crabs were always trying to open that hole further. The slimmer chelae of C. maenas managed to chip of pieces easier than H. takanoi did.
All these results were expected and are similar to the results from (Jensen , et al., 2002) who did similar behavioral experiment between C. maenas and H. sanguineus and C.maenas and H.
oregonensis. This also accounts for the confrontation results.
The interaction results show H.takanoi was significantly more succesful in both attack (p=1,53E -07) and defense (p=1,25E -08) at the moment of a confrontation (table 6). When the total successful encounters are put into percentages per species H.takanoi had a successful outcome for 88% of the time and C. maenas only 12% of the time. Although H. sanguineus gets larger than the closely related H. takanoi their behavior is very similar. The results found by (Jensen , et al., 2002) showed an even more extreme difference with 95% successful encounters for H. sanguineus compared to 5%
for C. maenas.
It was observed that H. takanoi often acted as a ‘bully’ meaning they would displace C. maenas from the food but afterwards hardly paid attention to the food. This could be named an act of territorial display.
The shelter experiment consisted of a more straight forward measurement than the behavioral experiment. For the shelter experiment the tides were simulated twice within 48. Half a mussel shell
36 was provided as shelter. Instead of videotaping it, the position of the crabs was noted at the start of the experiment (high tide), during the first low tide, the second high tide and at the second low tide.
A crab was considered to use the shelter if at least all his legs on one side are covered by the shell, because this would enable him to retreat quickly if necessary.
In general it seemed H. takanoi had a better tactic to get under the shell because the C. maenas often struggled not to flip the shell over. This happened because C. maenas grabbed the edge of the shell with its chelae instead of just using it as a lever like H. takanoi did.
Both species showed no significant difference (p=0,18) for when competition was present or not.
Meaning competition had no effect on the amount of times shelter was used. There was however a significant difference (p= 0,02) between the behavior comparing the two species. Meaning H. takanoi used the shelter significantly more than C. maenas. When put in percentages C. maenas used the shelter 25% and H. takanoi 75% out of the total times the shelter was used. This is again similar to the results of C. maenas (6,6%) against H. sanguineus (93,3%) from (Jensen , et al., 2002).
These results cannot be compared to the natural behavior of the crab species in the wild because the crabs in this setting were forced to interact due to the confined space. Normally they would have the ability to flee or take the food with them to an easier to defend spot. However it does shed some light on the interaction between an exotic and native species. H. takanoi was more successful in both experiments. Even though they interacted in a different in an experimental set up, we have seen both species occur on the reefs within the same size range and are therefore competing for both food and shelter. This means H. takanoi is very likely to negatively influence the population of C maenas.
It can be expected the artificial reefs will develop into natural reefs if more oyster spat settles onto the already existing shells. One of the conditions however is that the oyster shells are packed dense enough to prevent the shells from shifting too much as seen on artificial reef 1. The biodiversity on reef 2 may decrease eventually when some species outcompete each other. Besides that both Didemnum sp. and O. inornatus do pose a risk for the reefs and aquaculture in the area. The reefs provide the perfect habitat for them and they hardly have any competition from the other species.
This risk should be taken seriously and must be taken into account when building more reefs.
Also it could be wise to change the shape of the reef. Suggested by (Fritz, 2013) was to give the reefs a more natural rounded shape. Besides being more appealing this shape helps to curve the incoming water flow making it easier for the shells to stay in place and for the organisms on the edge to stay put while still benefitting from the water flow on the outside of the reef. Even deliberately add open spaces in between the structure creating some larger patches can be recommended. Some species prefer to live on the edge and this increases their chances. These can also act as refuge for some species of fish (Harwell, et al., 2011).
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