Directional trends in species composition over time can lead to a widespread overemphasis of year‐to‐year asynchrony

Abstract

Questions: Compensatory dynamics are described as one of the main mechanisms that increase community stability, e.g., where decreases of some species on a yearto-year basis are offset by an increase in others. Deviations from perfect synchrony between species (asynchrony) have therefore been advocated as an important mechanism underlying biodiversity effects on stability. However, it is unclear to what extent existing measures of synchrony actually capture the signal of year-to-year species fluctuations in the presence of long-term directional trends in both species abundance and composition (species directional trends hereafter). Such directional trends may lead to a misinterpretation of indices commonly used to reflect year-toyear synchrony. Methods: An approach based on three-term local quadrat variance (T3) which assesses population variability in a three-year moving window, was used to overcome species directional trend effects. This "detrending" approach was applied to common indices of synchrony across a worldwide collection of 77 temporal plant community datasets comprising almost 7,800 individual plots sampled for at least six years. Plots included were either maintained under constant "control" conditions over time or were subjected to different management or disturbance treatments. Results: Accounting for directional trends increased the detection of year-to-year synchronous patterns in all synchrony indices considered. Specifically, synchrony values increased significantly in ~40% of the datasets with the T3 detrending approach while in ~10% synchrony decreased. For the 38 studies with both control and manipulated conditions, the increase in synchrony values was stronger for longer time series, particularly following experimental manipulation. Conclusions: Species' long-term directional trends can affect synchrony and stability measures potentially masking the ecological mechanism causing year-to-year This article is a part of the Special Feature

Synchrony matters more than species richness in plant community stability at a global scale

“…It should be noted that nutrient addition and/or grazing pressure could promote directional changes in species composition, with some species increasing over the years and others decreasing (28). This could cause a decrease in synchrony values for indices studied here (29), with the indices reflecting not only year-to-year fluctuations due to compensatory dynamics but also, these long-term trends. More research is certainly needed in the future to account for the effect of directional trends on the interplay of biotic and abiotic effects on stability.…”

Species abundance fluctuations over 31 years are associated with plant–soil feedback in a species‐rich mountain meadow

“…Field data on plant species abundances are available from the LOng Term Vegetation Sampling (LOTVS) database of Valencia et al, (2020). Experimental plant-soil feedback data are available via…”

Why westillneed permanent plots for vegetation science

“…In fact, the presence of species with different responses to environmental conditions within a community is a key mechanism promoting overall temporal stability of communities, i.e., compensatory dynamics (McCann, 2000) — see also below and Valencia et al . (2020, this issue). Fischer et al .…”

“…Using a newly assembled LOTVS dataset comprising a global collection of 77 temporal plant‐community datasets, with almost 7,800 individual plots sampled over at least six years, Valencia et al . (2020, this issue) disentangle drivers of temporal vegetation stability due to directional trends and year‐to‐year fluctuations. By using a novel method (Lepš et al ., 2019), they analyse common measures of species’ temporal synchrony to understand compensatory mechanisms between species (McCann, 2000).…”

“…(2020) and Valencia et al . (2020; both this issue) it will thus be important to decompose the effect of fluctuations due to directional trends in vegetation from the effect of year‐to‐year fluctuations.…”