Multivariate drivers of primary consumer diversity regulation in a changing world
Primary consumers (herbivores) continue to undergo substantial changes associated with global environment change (GEC), potentially affecting diversity, structure, and function of biological systems given their importance for these processes. This is especially true for insects – invertebrate primary consumers have substantial ecological and evolutionary impacts on plants, including diversity and biomass – these impacts stand to be transformed depending on how human disturbances on contemporary landscapes affect insect consumers. As with all primary consumer groups, theoretical models on the regulation of insect herbivores emphasize interactions between resource-based bottom-up processes and predatory top-down influences. These processes are likely to be affected by GEC thru eutrophication and habitat fragmentation causing the loss of apex insect predators, or changes in plant diversity. How these changes affect primary consumers, however, is unclear because of potentially co-occurring or confounded effects. Here, we examine these issues with a large-scale field experiment, testing co-occurring impacts of environmental changes on primary consumer communities using structual equation modeling. We work in grasslands, focusing on insect primary consumers on plants. We quantify responses to several treatments – eutrophication, habitat loss, and grassland perturbation – at three levels: plants (food source for primary consumers), primary consumers, and predators of primary consumers.
We found an interactive response to the multiple stressors with changes in primary consumer diversity mainly driven by plant response. We also found that the interactive effect of stressors can change across habitat sizes with bigger patches protecting communities against producer spatial homogenization thus supporting more consumers. Our results suggest an important context-dependence nature of the effect of environmental stressors and that, within grassland ecosystems, understanding the resource-based mechanisms by which global change affects producer communities at local and regional scales might be the key to predict future changes across the whole ecosystem.