Review: Scientists’ warnings on climate change, droughts and their effects on insects.
Biodiversity loss is one of the most concerning trends that we are currently facing since it can result in the breakdown of ecosystems and the loss of associated services. The most prominent drivers of biodiversity loss are overexploitation, habitat degradation and climate change.
Climate change can have a profound effect on species and communities, altering abundances, interactions and distributions, sometimes leading to extinction.
Insect populations are declining across the globe, in what some have referred to as an “insect apocalypse”. These losses are great enough to merit significant concern since the many ecosystem services they provide, such as pollination and nutrient recycling, are also being lost.
An academic review published in the journal Ecological Society of America investigated the potential impacts of climate change on insects, including short-term, unpredictable events like drought.
Types of drought
In some places, drought events — insufficient rainfall over long periods — can be long-lasting, often occurring for months at a time. The intensity and duration of these droughts are increasing and are often associated with other weather events such as elevated temperatures and fires.
Pulsed droughts differ in that, although they can also be long-lasting, they are briefly interrupted by short spells of heavy rainfall.
Both types can have direct negative physiological impacts on insect populations or lead to impacts on plant communities, which insects depend on for food and shelter, affecting the entire food chain.
A matter of diet
The impact of drought stress on insects is complex. Insects that feed on trees may have a significantly different response to drought events compared with insects that feed on smaller plants, such as grasses and forbs.
Since these smaller plants are much more prone to water stress, the insect populations that rely on them will decline faster, particularly during the summer season.
(A Cinnabar moth (Tyria jacobaeae) caterpillar feeding on ragwort, Taschii/Pixabay)
This scarcity of food will also alter other interspecific interactions within the community. One consequence of water loss from plant tissue is an increase in competition for the remaining resources further up the food chain.
Conversely, insects that feed on trees are largely shielded from drought, as the larger root and shoot biomass of trees greatly improves their ability to withstand intense periods of drought compared to herbaceous species.
Drought stress alters root and shoot concentrations of primary metabolites (e.g., nutrients like sugars) and secondary metabolites (e.g., defensive chemicals), which in turn can affect the growth and development of the herbivorous insects that feed on these plants.
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Studies have shown that different insect herbivore guilds are generally negatively impacted by drought and have different but predictable responses, though there are exceptions.
In an experimental drought study conducted in an Australian rainforest, wood borer beetles were found to respond favourably to drought. Researchers found that there was significantly more wood borer activity, as indicated by tree damage, in the droughted area than in the control part of the forest.
Other insects are changing their diets in response to drought. Many ant species that live in the Australian rainforest feed on nectar they acquire directly from plants, while other species rely on honeydew, a waste product produced when aphids — small sap-sucking insects — pierce plant tissues to feed. However, as these food sources become increasingly limited in times of drought, many ant species are becoming more predatory, altering the rainforest’s food webs.
(Ants feeding on honeydew alongside aphids, MelaniMarfeld/Pixabay)
Drought impacts on insect populations
Population dynamics have been noted to correlate strongly with climate parameters. For example, a study investigating how climate change is impacting a species of grasshopper found that sudden changes in climate are a major driver of population decline. Reproduction was negatively correlated with drought since the egg stage is dependent on a high soil moisture content for development.
Droughts can also interrupt plant signalling and the quality of pollen production, reducing pollinator attraction and plant reproduction.
Even episodic droughts can greatly increase the extinction risk of a species and cause a reduction in genetic diversity.
In the UK, a drought in 1995 resulted in an overall increase in butterfly abundance, though significant changes in community composition also occurred, especially in northern habitats. More vulnerable specialist species were lost, while generalists thrived in their absence. Communities had not recovered the following year, a trend reflected by butterfly populations in parts of the United States.
Winners and losers
Ultimately, the physiological and ecological mechanisms of drought are complex and poorly understood in many cases. However, the consequences are now becoming more evident.
A recent severe drought in North America had catastrophic and long-lasting impacts on the populations of montane butterflies, comparable to the impacts of years of habitat degradation.
Some insects that are endemic to regions that have historically been subjected to periodic droughts may be more tolerant of more severe droughts in the future. In South Africa, for example, adult dragonflies have been known to stay close to the margins of their ponds even when they have all but dried up, while resident water beetles typically leave when droughts continue for extended periods.
Insects that have evolved in wet or humid habitats, such as rain forests, are more likely to be challenged by drought events.
Drought events will create winning and losing species and populations among insects. These will largely be based on changes in plant quality and availability. However, the review found that when the impacts of drought events are considered alongside the impacts of other anthropogenic changes and stresses, almost all insects will be losers in the long run.