Trophic cascades and detrital subsidies in montane temperate grasslands
Ecologists have used many types of gradients to study the responses of species and communities to abiotic conditions, but gradients may also be used to test how changing abiotic conditions influence food web structure and trophic cascades. We are now beginning to understand how resource abundances impact trophic interactions at particular locations, within particular ecosystems types, or with regard to species richness and abundance. A better understanding of how abiotic conditions influence trophic cascades is essential if we are to predict how ecosystems respond to climate change. Here, we present preliminary results of a 4-year project that: describes changes in a grassland food web structure along a gradient of resource availability; examines changes in food web structure in response to the removal of vertebrate secondary consumers (i.e. song birds and small mammals); and documents how the doubling or removal of dead plant material (i.e. litter) impacts plant and insect communities.
Our data show that the response of entire food webs depends on the resource availability of the site (i.e., position along the gradient), and that losses of upper trophic levels (i.e., song birds and small mammals) or changes in the physical structure of the plant community (i.e., reduced plant litter) produce significant changes to both the diversity and abundance of organisms in these ecosystems. Points on the gradient with higher resource availability (i.e., more water, lower temperatures) can either buffer or alter the direction of trophic cascades as well as alter the abundances of particular species. Generally, food web complexity is reduced with low resource availability making ecosystems prone to stronger trophic cascades. By incorporating both observational and experimental methods to examine trophic structure in similar ecosystems across a range of abiotic conditions, ecologists may be able to predict how abiotic conditions will influence trophic cascades within ecosystems. The indirect effects of climate manifest through trophic pathways is another important means by which climate change impacts both organisms and ecosystems. Further research is required on trophic cascades within and between ecosystems to determine how changes in resources availability transmit through food webs.