Decline effects are rare in ecology: a meta-meta-analysis

The scientific evidence base on any given topic changes over time as more
studies are published. Currently, there is widespread concern about
non-random, directional changes over time in the scientific evidence base
associated with many topics. In particular, if studies finding large
effects (e.g., large differences between treatment and control means) tend
to get published quickly, while small effect sizes tend to get published
slowly, the net result will be a decrease over time in the estimated
magnitude of the mean effect size, known as a “decline effect”. If decline
effects are common, then the published scientific literature will provide
a biased and misleading guide to management decisions, and to the
allocation of future research effort. We compiled data from 466
meta-analyses in ecology to look for evidence of decline effects. We found
that decline effects are rare. Only ~5% of ecological meta-analyses truly
exhibit a directional change in mean effect size over time arising for
some reason other than random chance, usually but not always in the
direction of decline. Most apparent directional changes in mean effect
size over time are attributable to regression to the mean, consistent with
primary studies being published in random order with respect to the effect
sizes they report. Our results are good news: decline effects are the
exception to the rule in ecology. Identifying and rectifying rare cases of
true decline effects remains an important task, but ecologists should not
overgeneralize from anecdotal reports of decline effects.