Challenges With Inferring How Land-Use Affects Terrestrial Biodiversity: Study Design, Time, Space and Synthesis

Land use has already reshaped local biodiversity on Earth, with effects expected to increase as human populations continue to grow in both numbers and prosperity. An accurate depiction of the state of biodiversity on our planet, combined with identifying the mechanisms driving local biodiversity change, underpins our ability to predict how different societal priorities and actions will influence biodiversity trajectories. Quantitative syntheses provide a fundamental tool by taking information from multiple sources to identify generalisable patterns. However, syntheses, by definition, combine data sources that have fundamentally different purposes, contexts, designs and sources of error and bias; they may thus provide contradictory results, not because of the biological phenomena of interest, but due instead to combining diverse data. While much attention has been focussed on the use of space-for-time substitution methods to estimate the impact of land-use change on terrestrial biodiversity, we show that the most common study designs all face challenges-either conceptual or logistical-that may lead to faulty inferences and ultimately mislead quantitative syntheses. We outline these study designs along with their advantages and difficulties, and how quantitative syntheses can combine the strengths of each class of design.