Biodiversity from the bottom up: causes and consequences of resource species diversity.




Narwani, Anita

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Species diversity may simultaneously be a cause and a consequence of variability in population, community and ecosystem properties. Ecology has traditionally focused on elucidating the causes of biodiversity. However, in the last decade and a half ecologists have asked the opposite question: What are the consequences of species diversity? The majority of these studies elucidated the effects of species diversity within single trophic levels. Incorporating trophic complexity is the next step in this research program. In this dissertation I investigated the causes of resource species diversity, as well as the impacts that resource diversity has on rates of consumption and the stability of population, community and ecosystem properties over time in planktonic food webs. The high diversity of phytoplankton found in nature appears to defy the competitive exclusion principle, and elucidating the mechanisms which maintain this diversity continues to be a challenge. In general, variability in limiting factors is required to maintain non-neutral species diversity, but this variability can be generated by forces outside of the competitive community (i.e. exogenous), or may be the outcome of competitive interactions themselves (i.e. endogenous). Using microcosm experiments, I showed that endogenously generated variability in limiting factors was more effective at maintaining phytoplankton species diversity over the long-term, although the strength of this effect depended on the composition of the phytoplankton community. Existing resource diversity has been proposed to generally weaken consumer-resource interaction strengths and limit consumer control of resource biomass. This is because more diverse resource communities are more likely to contain inedible, unpalatable, toxic or non-nutritious species. However, when resource communities contain multiple palatable species, diversity may also accelerate consumption. Using grazing experiments with multiple zooplankton consumer species, I found that the mechanism, direction and magnitude of modulation of consumption depended on the feeding selectivity of the consumer and the composition of the resource community. By altering consumer-resource interaction strengths in the short-term, resource species diversity may impact the stability of consumer-resource dynamics in the long-term. In separate microcosm experiments, I investigated the influence of resource species diversity, community composition and consumer feeding selectivity on population, community, and ecosystem properties over time. Diversity had positive effects on phytoplankton population biomass, resource community biomass, the rate of photosynthesis, the standing stock of particulate nutrients, and the generalist consumer’s population density. It also stabilized resource community biomass and the stocks of particulate nutrients over time. Unexpectedly, diversity did not stabilize either of the consumer populations, regardless of feeding selectivity. This suggests that effects of diversity on resource community properties do not impact consumer dynamics linearly. Resource community composition was generally more important than resource species diversity in determining food web properties. The importance of community composition in determining both the causes and consequences of resource diversity in these experiments points to the importance of species’ traits and the outcomes of their interactions. I suggest that the use of complex adaptive systems theory and trait-based approaches in the future will allow a consideration of the feedbacks between the causes and consequences of species diversity in food webs.



biodiversity, competition, coexistence, food webs, stability, phytoplankton, zooplankton, nutrients, photosynthesis, microcosms, ecosystem functioning, consumer resource interaction, consumption, community composition, species' traits