Biological Invasions: the role of biodiversity in determining community susceptibility to invasion
Shucksmith, R. (2007). Biological Invasions: the role of biodiversity in determining community susceptibility to invasion. PhD Thesis. University of Aberdeen: Aberdeen.
Invasion by non-native species has the potential to change native biodiversity, community structure, food webs and interactions between species.. Small scale manipUlative experiments :using sessile organisms have shown that increasing biodiversity reduces community susceptibility to invasion by non-native species. However, large scale observational studies have shown that some ofthe most diverse systems are the most invaded. The search fora generalised mechanism that may make a resident community resistant to invasion by a non-native species has proven difficUlt and has provided contrasting resUlts. Small scale experiments have been criticised for their dinllnptive spatial scale and -rdatively homogeneous environmental conditions. _ The contrasting results between the experimental and observational studies may be due to the heterogeneous nature ofmost environments which iilllY promote invasion as well as coexistence. In this thesis I study the relationship between invasion and competition in heterogeneous en'ir0nments using the North-East Asian amphipod Caprella mutica. Caprellids have been shown to live on marine sessile species such as, algae, hydroids, tunicates, mussels, bryozoans and sponges which attach to a hard substratum and form '. . erect structures that project into the wat~r column. Species-rich epifaunal comm.unities . can create a structurally diverse habitat. In choice experiments using different structural patches and structure types in the laboratory, there was no effect ofthe number of different types ofstructures in a patch on the number of C. mutica attached to a patch, but structure type was critically important. Caprella mutica preferred the filament.ous structures long t¢and short turf .over mussel mimics. In field experiments, densities were 7 times greater than the laboratory experiments.. There were twice as many caprellids on the structurally heterogeneous patches than on the structurally homogeneous patches. However, invasion patterns in the field did follow the same structure type as the laboratory experiments. Intraspecific competition appeared to be stronger on the homogeneous patches, whereas there were more refuges and microhabitats on the heterogeneous patches and this may have reduced the aggressive interactions between conspecifics. Artificial and live habitats were placed together in laboratory experiments and there was .t'. no difference in the number of caprellids attached to the artificial and live habitats. Lack ofdifferences between live biota and the artificial mimics showed that C. mutica habitat choice is most likely due to the physical characteristics ofthe habitat. Furthermore, choice oftype ofstructure did not differ betWeen the artificial and live patches. I In competition experiments between C. mutica and two native cap~ellids Caprella linearis and Pseudoprotella.phasma, C. mutica successfully displaced both species from homogeneous artificial habitat patches after 24 hours. Patches that had a refuge attached reduced the number of C. linearis being displaced but only when C. mutica was at a low density. Further experiments, using 3 different levels of structural heterogeneity with C. linearis .and C. mutica, showed that the two species could occupy the structurally heterogeneous patches at similar densities after the 24 hour experimenta~ period. On structurally homogeneous patches C. mutica significantly displaced C. linearis. Aggressive interactions were likely to have been more intense on the homogeneous than structurally heterogeneous patches and this study suggests that C. mutica displaces C. linearis through agonistic interactions. The same processes that reduced intraspecific competition in the first set ofexperiments also promoted coexistence between the two ecologically similar species. Consequently, structurally diverse habitats have the potential to promote invasion while at the same time reducing the impact of an invader on native species. Kelp communities consisting ofLaminaria hyperborea and Saccharina latissima in close proximity to known populations of C. mutica were surveyed and showed that C. mutica did disperse to the kelp communities but was not found on any ofthe kelp plants in the upper zone of the kelp bed. This study was limited to only sampling kelp plants to a depth of 1 to 2 m on spring tides and it is possible that C. mutica may inhabit kelp ' . plants that are deeper. However, laborat~ experiments showed Carcinus maenas predated on C. mutica and may provide biotic resistance to benthic emjronments from 'the invasion by C. mutica. Even at small spatial scales where environmental conditions were similar for both the laboratory and field experiments, the probability of invasion cannot be predicted ' reliably from aggregate community 'measures such as species richness (analogous to the number ofdifferent types ofstructures). Individual characteristics ofthe invading ,' species and positive'associations with structure types, even at small spatial scales, may increase the probability ofinvasion regardless of species richness. Thus, invasion by C. mutica is likely if an appropriate structure type is prese.nt and the success ofinvasion increases with an increasing richness of structure-forming epibiota. For native fauna, which have a similar ecological requirement to the invader, structural heterogeneity appears to reduce the impact ofthe invading species. Habitat heterogeneity may be essential for co-existence between a superior and inferior competitor, as structurally complex habitats offer a variety of different microhabitats and niches. The invasibility of a community, therefore, depends on the diversity and composition of the community being invaded, and the invasive potential of a species depends not only on the community properties, but also on the characteristics of the invader.
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