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. 2020 Mar;13(2):115-126.
doi: 10.1111/icad.12412. Epub 2020 Mar 4.

Are Insects Declining and at What Rate? An Analysis of Standardised, Systematic Catches of Aphid and Moth Abundances Across Great Britain

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Free PMC article

Are Insects Declining and at What Rate? An Analysis of Standardised, Systematic Catches of Aphid and Moth Abundances Across Great Britain

James R Bell et al. Insect Conserv Divers. .
Free PMC article

Abstract

Although we have known anecdotally that insects have been declining in Great Britain for more than 100 years, insect declines have only been statistically estimated over the last 20 years. Estimation of the rate of those declines is still hotly debated, fuelled by a lack of standardised, systematically collected data.More than 24 million individual moths and aphids collected from 112 light traps and 25 12.2 m suction-traps, respectively, were analysed using mixed models. Our objective was to estimate the long-term trends in both groups based on annual totals recorded every year between 1969 and 2016.The models showed that two paradigms existed: Over 47 years, long-term linear trends showed that moths had declined significantly by -31%, but short-term trends indicated that there were periods of significant decline and recovery in most decades since the 1960s. Conversely, despite aphid annual totals fluctuating widely, this group was in a steady state over the long-term, with a non-significant decline of -7.6%. Sensitivity analysis revealed that moth trends were not driven by a group of abundant species, but the sign of the overall aphid trends may have been driven by three of the most abundant species.The spatial extent of moth trends suggests that they are extremely heterogeneous. Uniquely, moth declines were different among several habitat types, with robust significant declines found in coastal, urban and woodland habitats, but notably not in agricultural, parkland and scrubland habitats. Conversely, aphid trends showed spatial synchrony extending to 338 km, albeit with local variation.

Keywords: Aphids; insect conservation; light traps; mgcv; moths; poptrend; suction traps.

Figures

Figure 1
Figure 1
Map of the United Kingdom showing the location of the Rothamsted Insect Survey traps used in this study. Icons show light traps (filled circles) and suction traps (crossed boxes)
Figure 2
Figure 2
Population index for aphids with random effects (dots and whiskers), indicating yearly mean and variances, and 95% confidence intervals (blue). Two models were run (a) a log‐linear GLMM (b) a very flexible non‐linear GAMM model with d.f. fixed to k = 46 that follows the interannual variation over time. Neither the short‐ or long‐term trends were significant. [Color figure can be viewed at http://wileyonlinelibrary.com]
Figure 3
Figure 3
Population index for moths with random effects (dots and whiskers), indicating yearly mean and variances, and 95% confidence intervals (blue). Two models were run (a) a log‐linear GLMM (b) a very flexible non‐linear GAMM model with d.f. fixed to k = 46 that follows the interannual variation over time. Significantly different increasing or decreasing short term trends at the 5% level are coloured green and orange, respectively. These short‐term trends are imposed on top of the long‐term trend, which is coloured black. The shape of change (i.e. second derivatives) is indicated along the x axis, illustrating downturning ( − orange bars) or upturning ( – green bars) curvatures in the trend. [Color figure can be viewed at http://wileyonlinelibrary.com]
Figure 4
Figure 4
Effect of habitat on non‐linear moth declines from the model featured in Fig. 3b. The estimated smoothed terms on the y axis are a transformed function of habitat which on the axis is centred on zero (red line). For each habitat plot, a Wald zero‐effect test indicates if the smoother is equal to zero. Significant P‐values show that smooths have significantly departed from zero. A formal test is provided in Table 1 and significant trends are asterisked * accordingly. In only coastal, urban and woodland could the trends be described as significantly declining. Both mixed and moorland habitats fluctuate widely, and we are cautious about the significance attached to those trends. [Color figure can be viewed at http://wileyonlinelibrary.com]
Figure 5
Figure 5
Multivariate spline correlograms with bootstrapped 95% confidence intervals of the annual catch of across Great Britain (a) Aphids. The y intercept indicates the local covariance function (0.23), the x, the spatial extent (0–800 km) and the x intercept is an estimate of the correlation length (338 km) (b) Moths. The y intercept indicates the local covariance function (0), the x, the spatial extent (0–800 km) and the x intercept is an estimate of the correlation length (0 km).

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