Declining abundance of beetles, moths and caddisflies in the Netherlands

Basset

Collins

et al. 2020

303

355

Many insect species are under threat from the anthropogenic drivers of global change. There have been numerous well‐documented examples of insect population declines and extinctions in the scientific literature, but recent weaker studies making extreme claims of a global crisis have drawn widespread media coverage and brought unprecedented public attention. This spotlight might be a double‐edged sword if the veracity of alarmist insect decline statements do not stand up to close scrutiny. We identify seven key challenges in drawing robust inference about insect population declines: establishment of the historical baseline, representativeness of site selection, robustness of time series trend estimation, mitigation of detection bias effects, and ability to account for potential artefacts of density dependence, phenological shifts and scale‐dependence in extrapolation from sample abundance to population‐level inference. Insect population fluctuations are complex. Greater care is needed when evaluating evidence for population trends and in identifying drivers of those trends. We present guidelines for best‐practise approaches that avoid methodological errors, mitigate potential biases and produce more robust analyses of time series trends. Despite many existing challenges and pitfalls, we present a forward‐looking prospectus for the future of insect population monitoring, highlighting opportunities for more creative exploitation of existing baseline data, technological advances in sampling and novel computational approaches. Entomologists cannot tackle these challenges alone, and it is only through collaboration with citizen scientists, other research scientists in many disciplines, and data analysts that the next generation of researchers will bridge the gap between little bugs and big data.

Section: Introductionmentioning

confidence: 99%

Interpreting insect declines: seven challenges and a way forward

Basset

Collins

et al. 2020

303

355

“…For example, in Germany, there has been a 75% decline in total flying insect biomass since 1989 (Hallman et al, 2017), while reductions of ca. 61% for macro moths and at least 42% for ground beetles were observed in the Netherlands from 1985 to 2006 (Hallmann et al, 2019). In England, a 58% reduction in the total abundance of 17 widespread butterfly species was recorded from 1984 to 2012 (Gilburn et al, 2015), while in southern Britain, long-term declines in flying insect abundance (particularly for Diptera) have been recorded (Shortall et al, 2009), with changing agricultural practices being a probable factor in the declines (but see McGregor et al, 2019).…”

Section: Introductionmentioning

confidence: 99%

Glowing, glowing, gone? Monitoring long‐term trends in glow‐worm numbers in south‐east England

Gardiner

2020

The glow‐worm Lampyris noctiluca (Linnaeus, 1767) (Coleoptera: Lampyridae) is thought to be declining in the United Kingdom. Yet, much of the evidence for this is anecdotal, with a shortage of standardised long‐term data to investigate temporal changes in abundance. We present an 18‐year time series of standardised transect surveys for glowing adult females at 19 sites within south‐east England (Essex) from 2001 to 2018. We used generalised additive mixed models (GAMMs) to control for varying sampling effort, temporal autocorrelation, non‐stationarity of seasonal phenology and non‐linearity of temporal trajectories across sites. We found a significant long‐term reduction in counts of glowing female glow‐worms, after accounting for a significant shift in seasonal phenology across years, and a negative effect of warmer climatic conditions on glow‐worm abundance. Average glowing counts in south‐east England declined by ca. −3.5% per annum from 2001 to 2018, and this result held true even after a range of sensitivity tests to account for potential methodological artefacts in citizen science data collection. Temporal trajectories in abundance were strikingly out of phase across the 19 sites, suggesting that local‐scale factors in addition to climate are driving greater reduction in numbers at some sites than others. These standardised surveys present the first quantitative evidence that numbers of glow‐worms could well be declining in the United Kingdom. There is a clear signal of climate warming and drying effects on glow‐worm numbers, but a substantially greater proportion of variation in glowing female counts is explained by local‐scale site factors, such as unmanaged scrub encroachment. Conservation strategies that can mitigate local population losses could be an essential buffer against climate‐driven declines in south‐east England.

“…Even in those groups showing declines, not all species were affected equally. Moth species associated with grass or herb host plants were more severely affected, as were ground beetle species that were closely associated with xerophytic habitats (Hallmann et al ., ). This would tend to suggest that land‐use and land management changes were important drivers of population trends, but causal mechanisms could not be specifically identified.…”

mentioning

confidence: 97%

“…In the Netherlands, Hallmann et al . () monitored populations of a number of different terrestrial and aquatic insect taxa using light trapping and pitfall trapping at up to 48 sites over time series of up to 26 years and detected declines in abundance and biomass (of up to 40–60%) for moths, beetles and caddisflies, while true bugs, mayflies and lacewings did not exhibit similar changes. Even in those groups showing declines, not all species were affected equally.…”

mentioning

confidence: 99%

Spotlight on insects: trends, threats and conservation challenges

Collins

Forister

et al. 2020

There is mounting concern over the conservation status and long‐term trends in insect populations. Many insect populations have been reported to be falling and many species are threatened with extinction. While this is true, the evidence does not support unqualified statements of ‘global insect decline’. Global environmental change does not affect all species equally, and there are clear winners as well as losers from anthropogenic impacts. In this special issue of Insect Conservation and Diversity, we draw together articles that (i) identify key challenges in robust inference about insect population trends, (ii) present new empirical evidence for declines (and increases) in insect populations, spanning whole communities down to single species, in both aquatic and terrestrial ecosystems, and (iii) address the interacting drivers of population change, from empirical studies of environmental correlates, to experimental manipulation of driving mechanisms. We argue that the way forward for insect conservation includes more nuanced language and approaches when communicating ecological evidence to peer and public audiences, beyond just a simplistic focus on the insect decline narrative. This will require an expanded portfolio of approaches to promote the value of insects to society, which in turn, should reinforce the social licence to prioritise insect conservation research. This should help us to deliver the rigorous science necessary to document ongoing trends and understand the drivers and mechanisms of population change. Only then will we be able to mitigate or reverse declining populations.