Climate change and the potential for transformation of food web connectedness

Background/Question/Methods   Food web analyses typically aim to develop general theory for system structure and dynamics based on emergent patterns in species interconnections that are observed among systems with widely varying species compositions. This approach necessarily assumes that the interconnections remain intact in the face of environmental changes. I present experimental evidence for research on an old-field grassland food web that leads to questioning this assumption. This experiment simultaneously tested new theory for multiple predator effects on food webs by using warming to alter predator habitat use and hence direct and indirect interactions among two dominant spider predator species, a dominant grasshopper herbivore and species of grass and herb plants.

Results/Conclusions   Under ambient environmental conditions, spiders used habitat in spatially complementary locations. Consistent with theory, the multiple predator effect on grasshoppers and on plants was the average of the individual predator effects. Warming strengthened the single predator effects. It also caused the spider species to overlap lower in the vegetation canopy. Consistent with theory the food web became entirely transformed into an intraguild predation system with the consequent extinction of one spider species. Global climate change may thus transform not only the interconnectedness among species but changing connectedness may in turn precipitate subsequent species extinctions. This calls for food web analyses to incorporate more dynamic perspectives when aiming to understand pattern and process.