All communities occur in habitat patches that vary in size. Larger patches always contain more species, but individual species vary in their sensitivity to habitat patch size. Some species occur in large habitats, caused either by low relative abundance (uncommon species are disproportionately present in larger patches) or by intolerance of an abiotic stress in smaller patches. Other species may occur in small patches because strong biotic interactions exclude them from large patches (where predators and competitors are more dense). However, it is not clear what evolutionary relationships distinguish small from large-habitat specialists.
Taxonomically related species may respond to habitat size gradients in either very similar or very divergent ways. Similar responses may occur because related species share the same sensitivity to abiotic or biotic stress. Divergent responses may occur because related species may exclude one another from patches where both are present.
To determine whether taxonomically related species respond similarly or differently to habitat size, we applied a combination of null models, experimental manipulation and demographic modelling to field data from bromeliad communities. Bromeliads are neotropical plants that collect water in their leaves, forming an aquatic habitat which varies widely in size and contains a diverse assemblage of macroinvertebrates.
Results/Conclusions
We used observational data to measure the effect of taxonomic relatedness on species response to habitat size. We found that macroinvertebrates from the same genus have more divergent habitat size preferences than expected by chance. That is, congenerics are often divided into two groups: habitat size generalist (present in plants of all size) and habitat size specialist (present only in large plants).
We chose a pair of congeneric chironomid species (Polypedilum spp.) and performed a field experiment to find out what causes these related species to have such different responses to habitat size. We transplanted larvae between large and small patches (to measure response to abiotic conditions), and manipulated the presence of the congener (to measure competition) and of a damselfly larvae (to measure predation). We found that the habitat generalist was negatively affected by predators, but not the environment, while the large-patch specialist was negatively affected by the environment, not by predators. Demographic modelling revealed that predators induced faster development in the habitat size specialist, allowing them to survive while a predator was present.
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