UBC Theses and Dissertations
The influence of spatial processes at multiple scales on local community structure and function LeCraw, Robin Marie
Spatial patterns, at multiple scales, can influence the functioning of local communities, but studies of community interactions are often conducted at a local scale. To investigate how spatial complexity at local, regional, and geographic scales influences a local aquatic macroinvertebrate community, I designed experiments using the natural mesocosm of bromeliad phytotelmata, quantifying community structure (functional diversity and trophic structure) and function (decomposition) of an entire food web in Puerto Rico, Costa Rica, and Brazil. At the local scale I studied the effects of habitat patch size on trophic structure and decomposition rates at all three sites. The relationship between decomposition and bromeliad size varied by site. In Costa Rica, size-sensitive top predators drove strong top-down control of leaf decomposition in larger bromeliads causing a negative relationship between decomposition and bromeliad size. In Puerto Rico this relationship was positive, as the lack of a large top predator allowed reduced top-down control in larger bromeliads. In Brazil, climatic conditions potentially shifted decomposition processes to microbial control, and no relationship between bromeliad size and decomposition rate existed. At the regional scale, by enclosing varying numbers of bromeliads I determined how functional traits of species determined extinctions within the metacommunity. Small metacommunities became less diverse and local extinctions were more stochastic than in large ones. Large-bodied predators and invertebrates requiring resources in the terrestrial matrix were most sensitive to metacommunity size. At the geographic scale, I combined experimental replicates in each site and community-analogues in common sites to identify mechanisms of context-dependency of community structure and function relationships among study sites. Top-down control of detritivore communities by predators was influenced by geographic differences in species traits, whereas cascading effects of predators on decomposition were influenced by geographic variation in environmental conditions. With these studies I was able to identify relationships between community functions and spatial processes operating at a hierarchy of spatial scales. I identified how biogeographical shifts in climate and large-scale connectivity among study sites can influence traits in the species pool which can affect both metacommunity dynamics and sensitivity of species to local extinctions due to habitat patch size.
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