Species interactions and a chain of indirect effects driven by reduced precipitation

Abstract

Climate change can affect species directly and indirectly by altering interactions between species within communities. These indirect effects can ramify through a community and affect many species, including some that may not have been directly affected by the perturbation. Identifying these chains of indirect effects is difficult, and most studies only follow indirect effects across two or three species. Here, we use a factorial field experiment to demonstrate that precipitation affects spotted aphids through a complex chain of indirect interactions that are mediated by other herbivores and a generalist predator. We experimentally simulated drought, which reduced water content in alfalfa plants. While water stress in alfalfa had no direct effect on spotted aphids, it lowered the population growth rate of pea aphids, another common alfalfa pest. Because ladybeetle predators were attracted to high pea aphid densities, predator densities were lower in drought treatments. Consequently, spotted aphid densities were released from top-down control (apparent competition) in drought treatments and reached densities three times higher than spotted aphids in ambient treatments with high pea aphid densities. Thus, drought affected spotted aphids in the interaction chain: drought --> alfalfa --> pea aphids --> predators --> spotted aphids. This result illustrates the lengthy path that indirect effects of climate change may take through a community, as well as the importance of community-level experiments in determining the net effect of climate change.

Reduced wind strengthens top‐down control of an insect herbivore

“…2). These findings are consistent with other examples of how wind alters predation by asymmetrically affecting predators and their prey (e.g., Quinn and Cresswell 2004), as well as a broader literature showing that species interactions can propagate indirect effects of an abiotic factor throughout a community (Tylianakis et al 2008, Barton andIves 2014a).…”

“…This design emphasized the role of predator immigration over predator reproduction, a point reinforced by my failure to detect any larval ladybeetles during the surveys. However, this is a reasonable emphasis because ladybeetles are mobile generalist predators and consequently their population dynamics are not strongly tied to aphid density within a single field (Forbes and Gratton 2011, Ragsdale et al 2011, Barton and Ives 2014a. So, what would happen if the entire landscape slowed homogenously?…”

“…Experiments often require relatively large treatment effects in order to detect the direct and indirect effects of an abiotic manipulation within reasonable time scales and replication (Newman et al 2011, Barton andIves 2014a). It was not my goal in this study to specifically predict how wind will affect ladybeetle and aphid interactions in a future, less windy environment.…”

“…Instead, I offer an experimental demonstration that wind can affect predator-prey interactions and encourage ecologists to consider this changing component of climate in future studies. Although I have emphasized the effects of wind in this community, it is essential to remember that these species can be simultaneously affected by other components of climate change, such as temperature (Harmon et al 2009, Barton andIves 2014b) and precipitation (Barton and Ives 2014a). The task may seem daunting, but integrating multiple abiotic factors, including wind, into future studies is essential if we are to understand the net effect of climate change on species and their interactions within ecosystems.…”

Direct and indirect effects of warming on aphids, their predators, and ant mutualists

“…This task is particularly important for endangered species, species with economic value, and pests (e.g., Bale et al 2002, Carpenter et al 2008, Stirling and Derocher 2012, Barton and Ives 2014. Climate change can affect species both directly through physiology and/or behavior, and indirectly by affecting other species with which the focal species interacts.…”