Climate change-mediated temperature extremes and insects: From outbreaks to breakdowns

Abstract

Insects are among the most diverse and widespread animals across the biosphere and are well-known for their contributions to ecosystem functioning and services. Recent increases in the frequency and magnitude of climatic extremes (CE), in particular temperature extremes (TE) owing to anthropogenic climate change, are exposing insect populations and communities to unprecedented stresses. However, a major problem in understanding insect responses to TE is that they are still highly unpredictable both spatially and temporally, which reduces frequency- or direction-dependent selective responses by insects. Moreover, how species interactions and community structure may change in response to stresses imposed by TE is still poorly understood. Here we provide an overview of how terrestrial insects respond to TE by integrating their organismal physiology, multitrophic, and community-level interactions, and building that up to explore scenarios for population explosions and crashes that have ecosystem-level consequences. We argue that TE can push insect herbivores and their natural enemies to and even beyond their adaptive limits, which may differ among species intimately involved in trophic interactions, leading to phenological disruptions and the structural reorganization of food webs. TE may ultimately lead to outbreak-breakdown cycles in insect communities with detrimental consequences for ecosystem functioning and resilience. Lastly, we suggest new research lines that will help achieve a better understanding of insect and community responses to a wide range of CE.

Keywords: anthropogenic climate change; biodiversity; climatic extremes; heatwaves; herbivory; insect physiology; multitrophic interactions; parasitoids; predators.

FIGURE 1

Overview of insect responses to temperature extremes (TE). In (a) species attributes are given that are potentially affected by TE. Color coding, from green (positive) to yellow (negative) or gray (undefined) depicts the hypothesized effect size of exposure to TE. In (b) an example is given of a simple insect food web with their interactions (dashed lines). Evidence suggests that species higher in the food chain are more negatively affected by TE than species lower in the food chain (depicted by the different color gradients). In (c) examples are given of insect population outbreak and breakdown scenarios in response to single or sequential TE events. In the left panel, we show three coexisting insect species (indicated by three line colors) exhibiting seasonal fluctuations in their population dynamics. The middle panel presents an outbreak event of an insect species (red line) followed by its breakdown in response to a single TE event (red triangle). Outbreaks occur when a population reaches or crosses an upper threshold (considerably above the carrying capacity, which approximates the upper threshold) and subsequently leads to its breakdown. A key assumption here is that the outbreaking insect overexploits its main food resource leading to its population crash (see main text for details). In response to a single TE event, breakdowns may also occur in insect species (blue line) when its population size becomes very small, and close to its lower critical threshold. The insect species that is less affected (orange line) by a single TE event could still face an outbreak‐to‐breakdown scenario or a direct breakdown (shown by dashed lines) when another TE event takes place (second red triangle). Such scenarios (right panel) are likely to depend on the abiotic characteristics of the sequential TE event, and potential maladaptation in the (orange line) insect species during the first TE event

FIGURE 2

Across the biosphere, insects are responding, both positively and negatively, to temperature extremes (TE). (a) Outbreaks of the desert locust, Schistocerca gregaria, in eastern Africa, are driven by a combination of high temperatures and extreme precipitation events. These outbreaks are leading to significant crop losses in several countries. (b) The Colorado potato beetle, Leptinotarsa decemlineata, originates from warm regions of the southwestern United States and Mexico. Climate warming has enabled the species to invade many temperate regions globally where it is a major pest of potato crops. It thrives when exposed to high temperatures. (c) In many temperate biomes, bumblebee (Bombus sp., depicted Bombus terrestris) species are declining rapidly. Although several anthropogenic stresses are driving these declines, exposure to TE during heatwaves has recently been shown to be a major factor. (d) Many butterflies (Lepidoptera), including species that were once numerous, are declining rapidly in North America and Eurasia. Grassland‐dependent species, such as the small skipper, Thymelicus sylvestris, respond poorly to heat stress, owing to the negative effects of TE on host plant quality and abundance. Copyright information: Schistocerca gregaria swarm: ©ChriKo, 2014, Wikimedia Commons License CC‐BY‐SA‐4.0; Schistocerca gregaria pair: ©Adam Matan, 2013, Wikimedia Commons License CC‐BY‐SA‐3.0; Leptinotarsa decemlineata: ©Tavo Romann, 2013, Wikimedia Commons License CC‐BY‐SA‐4.0; Bombus terrestris: © Alvesgaspar. 2007, Wikimedia Commons License CC‐BY‐SA‐3.0; Thymelicus sylvestris: ©Bernard DuPont, 2014, Wikimedia Commons License CC‐BY‐SA‐2.0