Unravelling insect declines: can space replace time?

Abstract

Temporal trends in insect numbers vary across studies and habitats, but drivers are poorly understood. Suitable long-term data are scant and biased, and interpretations of trends remain controversial. By contrast, there is substantial quantitative evidence for drivers of spatial variation. From observational and experimental studies, we have gained a profound understanding of where insect abundance and diversity is higher—and identified underlying environmental conditions, resource change and disturbances. We thus propose an increased consideration of spatial evidence in studying the causes of insect decline. This is because for most time series available today, the number of sites and thus statistical power strongly exceed the number of years studied. Comparisons across sites allow quantifying insect population risks, impacts of land use, habitat destruction, restoration or management, and stressors such as chemical and light pollution, pesticides, mowing or harvesting, climatic extremes or biological invasions. Notably, drivers may not have to change in intensity to have long-term effects on populations, e.g. annually repeated disturbances or mortality risks such as those arising from agricultural practices. Space-for-time substitution has been controversially debated. However, evidence from well-replicated spatial data can inform on urgent actions required to halt or reverse declines—to be implemented in space.

The Morphological Diversity of Antlion Larvae and Their Closest Relatives over 100 Million Years

“…Human society has begun to recognise a major challenge: human-induced climate change, loss of habitats, excessive use of pesticides, and general human interference together have caused a tremendous loss in biodiversity in the last 50 years [ 1 ]. Estimates in the last decade indicate a 41% decline in populations of Insecta; about a third of sampled species of this group have been given the status “threatened” [ 2 ], overall popularising the expression “insect decline” (e.g., [ 3 , 4 , 5 , 6 , 7 , 8 ]). This situation affects many ecosystems, for example, due to a loss of key species such as native pollinators, which reduces agricultural output and is hence also an economic issue [ 9 , 10 ].…”

Different roles of concurring climate and regional land-use changes in past 40 years’ insect trends

“…Recent spatial analyses suggest that insect communities may be shaped by both additive and interactive effects of climate, land use and changes therein [16][17][18] . Such analyses based on space-for-time substitution provide valuable insights into insect declines in the absence of timeseries data 19 , but to understand past developments of insect populations, time series are key 20 . In particular, for linking the combined effects of climate and land-use changes to insect trends over relevant temporal scales (i.e., decades 19 ), data on insects in combination with data on these drivers over large temporal and spatial scales is crucial, but rarely available.…”

“…Such analyses based on space-for-time substitution provide valuable insights into insect declines in the absence of timeseries data 19 , but to understand past developments of insect populations, time series are key 20 . In particular, for linking the combined effects of climate and land-use changes to insect trends over relevant temporal scales (i.e., decades 19 ), data on insects in combination with data on these drivers over large temporal and spatial scales is crucial, but rarely available. The few studies that simultaneously addressed climate and land-use effects on insect trends highlight the importance of both drivers for observed changes 11,[21][22][23] .…”

Quantifying the benefits of reducing synthetic nitrogen application policy on ecosystem carbon sequestration and biodiversity

“…To estimate biodiversity responses to human impacts across such a global and heterogeneous dataset, linear mixed-effects models are used; random intercepts account for differences in biogeographic factors, sampling methodology and taxonomic focus, and the spatial layout of sites within studies. Using the PREDICTS database to assess the impact of human pressures on biodiversity assumes that space-for-time substitution is valid 44 ; it assumes that the sites have reached equilibrium and so the impact of pressures on biodiversity over time can be observed across space and that the relationship between biodiversity and drivers do not vary over time. SR is calculated as the number of species at each site; it is a widely used measure of biodiversity and is both simple and intuitive.…”