Insect population trends and the IUCN Red List process

Ian Middlebrook,Richard Fox,Chris R Shortall,Colin A Harrower,Robert J Wilson,James R Bell

doi:10.1007/s10841-018-0117-1

Abstract

Reliable assessment of extinction risk is a key factor in the preparation of Red Lists and in prioritizing biodiversity conservation. Temporal population trends can provide important evidence for such assessments, but imperfect sampling (observation errors) and short-term stochastic variation in population levels caused by environmental variability (process errors) can reduce the reliability of trends and lead to incorrect quantification of extinction risk. The assessment of insect taxa is likely to be particularly prone to these problems, due to the highly dynamic nature of many insect populations, driven by short life-cycles and sensitivity to environmental factors such as the weather. Using long-term United Kingdom monitoring data for 54 butterfly and 431 macro-moth species, we demonstrate the impact of insect population variability on the assessment of extinction risk using the International Union for Conservation of Nature (IUCN) Red List Criterion A (reduction in population size over the last 10 years). For both taxa, varying the start year of the 10-year population trend had a substantial effect on whether particular species met Red List thresholds and on the overall number of species assessed as threatened. We conclude that for these insect taxa strict application of the 10-year rule produces Red List classifications that are unacceptably biased by the start year. Use of long-term trends with adjustment based on species performance over the last decade may offer a pragmatic solution to this problem. We call for further IUCN guidance for practitioners undertaking Red List assessments of taxa with populations that have high temporal variability.

Highlights

Biodiversity conservation practitioners rely on robust assessments of extinction risk to prioritise the use of limited resources
We consider two case studies using United Kingdom (UK) insect population data for butterflies and macro-moths derived from long-term (40 + years) monitoring schemes to assess the impact of species’ population variability on Red Listing using International Union for Conservation of Nature (IUCN) Criterion A
We considered the impact of arbitrary start date on the Red List outcomes for UK butterflies based on published 10-year population trends derived by linear regression from the UK Butterfly Monitoring Scheme (UKBMS) over 6 consecutive years

Summary

Introduction

Biodiversity conservation practitioners rely on robust assessments of extinction risk (at global, regional, national and even local scales) to prioritise the use of limited resources. The Red List process developed by the International Union for Conservation of Nature (IUCN) plays an important role both as the global standard for extinction risk assessment (Miller et al 2007; Mace et al 2008) and, indirectly, in catalysing conservation activity. Widespread and common species, with large population sizes and ranges, can qualify as being threatened with extinction on Red Lists if they are undergoing rapid decline. Criterion A is justified because even large populations would eventually be driven to extinction by continuing decline (Mace et al 2008), especially as other negative feedback loops may come into play at low population densities (e.g. Allee effects, genetic inbreeding), and because the reduction in abundance of common and widespread species may be of particular significance to ecosystem structure and functioning (Gaston and Fuller 2008; Winfree et al 2015)

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