No net insect abundance and diversity declines across US Long Term Ecological Research sites

Abstract

Recent reports of dramatic declines in insect abundance suggest grave consequences for global ecosystems and human society. Most evidence comes from Europe, however, leaving uncertainty about insect population trends worldwide. We used >5,300 time series for insects and other arthropods, collected over 4–36 years at monitoring sites representing 68 different natural and managed areas, to search for evidence of declines across the United States. Some taxa and sites showed decreases in abundance and diversity while others increased or were unchanged, yielding net abundance and biodiversity trends generally indistinguishable from zero. This lack of overall increase or decline was consistent across arthropod feeding groups and was similar for heavily disturbed versus relatively natural sites. The apparent robustness of US arthropod populations is reassuring. Yet, this result does not diminish the need for continued monitoring and could mask subtler changes in species composition that nonetheless endanger insect-provided ecosystem services.

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Fig. 1: Map of LTER sites.
Fig. 2: Time trends in arthropod abundance among LTERs.
Fig. 3: Time trends in arthropod diversity among LTERs.
Fig. 4: Change in relative abundance of taxa over time.
Fig. 5: Comparison of species rank abundance and community composition.

Data availability

The data supporting the findings of this study (curated arthropod abundances and estimated time trends) are available at the Dryad Data Repository (https://doi.org/10.5061/dryad.cc2fqz645).

Code availability

The R code used to curate and analyse data are available at the Dryad Data Repository (https://doi.org/10.5061/dryad.cc2fqz645).

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Acknowledgements

A. R. Ives (University of Wisconsin-Madison) provided invaluable advice on our analyses, and M. R. Strand (University of Georgia) and W. F. Fagan (University of Maryland) made suggestions to improve the paper. We acknowledge funding from USDA-NIFA-OREI 2015-51300-24155 and USDA-NIFA-SCRI 2015-51181-24292 to W.E.S.

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M.S.C., A.R.M., W.E.S. and M.D.M. conceived of the idea for the paper, and M.S.C. and A.R.M. conducted analyses; M.S.C., A.R.M., W.E.S., M.D.M., E.M.B., D.L.-K., G.L.H., L.L.B., L.C.C., D.H.N., K.P. and S.V. assisted with data collection and curation; M.S.C., W.E.S. and M.D.M. primarily wrote the paper, although all authors contributed to the final manuscript.

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Correspondence to Michael S. Crossley.

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Extended data

Extended Data Fig. 1 Time trends in abundance of arthropod feeding groups among LTERs.

(a) herbivores, (b) carnivores, (c) omnivores, (d) detritivores, (e) parasites, and (f) parasitoids. Right panels depict average change in diversity metrics and 95% confidence intervals among LTERs. Blue shading and font indicate LTER sites reporting aquatic taxa.

Extended Data Fig. 2 Time trends in insectivorous bird (a) and fish (b) abundance among LTERs.

Boxplots depict medians (thick line), 25th and 75th percentiles (box edges), 95th percentiles (whiskers), and outliers (circles).

Extended Data Fig. 3 Sensitivity analysis on stringency of time series quality filtering.

Abundance trends of all taxa under (a) moderate vs. relaxed time series filtering criteria and (b) strict vs. moderate filtering criteria. (c) Boxplots of abundance trends under relaxed, moderate, and strict timer series filtering criteria. Relaxed criteria required at least four years of counts, one of which had to be non-zero (n = 5,328 out of 6,501 trends remained). Moderate criteria required at least eight years of counts, of which four had to be non-zero (n = 2,266 trends remained). Strict criteria required at least 15 years of counts, of which 10 had to be non-zero, and that temporal autocorrelation be < 1 (n = 308 trends remained).

Extended Data Fig. 4 Explanatory variables overlaid on (sorted) time trends in arthropod abundance among LTERs.

(a) Start year of LTER site sampling. (b) Human Footprint Index associated with LTER site. The average HFI value for locations within the US is 7; LTER sites ranged from 1 to 38. (c) Mean annual temperature at LTER sites. (d) Mean cumulative annual precipitation at LTER sites.

Extended Data Fig. 5 Importance of explanatory variables in predicting time trends of arthropod abundance.

Contribution of each variable to the accuracy of the Random Forests classifier, defined as the percent increase in Mean Square Error (decrease in accuracy) when the variable was excluded from decision trees.

Extended Data Fig. 6 Time trends in arthropod abundance, average among studies with similar start years.

Abundance trends are averaged among LTERs where sampling start years were earlier than 1990, spanned 1990–2000, spanned 2000–2010, or were after 2010. Results were the same when trends were grouped according to final sampling years (except that no final sampling years predated 1990).

Extended Data Fig. 7 Relationships among temporal trends in α diversity metrics.

Dots represent the change over time of a diversity metric at an LTER site. Species evenness was calculated as Pielou’s Evenness Index, and dominance represents the proportional frequency of the most abundant taxon. Light gray lines divide each plot into quadrants to help visualize sites where the sign of change in diversity metrics was similar (top right, bottom left) or opposite (top left, bottom right). Black dashes denote the line of best fit. Slopes are significant at the α = 5% level, R2 = 0.36 for evenness vs. richness, and R2 = 0.68 for evenness vs. dominance.

Extended Data Fig. 8 Time trends in Midwest Farmland aphid abundance 2006–2019.

Left panel depicts abundance trends separated by ecoregion level I. Right panel depicts abundance trends separated by ecoregion level II. Boxplots depict quantiles among LTER sites. Boxplots depict trends among insects as medians (thick line), 25th and 75th percentiles (box edges), 95th percentiles (whiskers), and outliers (circles).

Extended Data Fig. 9

Human Footprint Index values in the USA (left panel) and among LTER sites (right panel).

Extended Data Fig. 10 Relationships among temporal trends in β diversity metrics.

Dots represent the change over time of a diversity metric at an LTER site. The grey dashed line denotes the 1:1 line.

Supplementary information

Supplementary Information

Supplementary Tables 1–3.

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Supplementary Table 1

Attributes of LTER sites included in analyses.

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Crossley, M.S., Meier, A.R., Baldwin, E.M. et al. No net insect abundance and diversity declines across US Long Term Ecological Research sites. Nat Ecol Evol (2020). https://doi.org/10.1038/s41559-020-1269-4

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