Species abundances in 2,124 BioTIME 2.0 local assemblages vary together at 2- to 10-year timescales; compensatory dynamics are not more frequent than the null rate

Question: in local ecological assemblages, do species fluctuate in opposite directions (compensatory dynamics) or in the same direction (synchrony), and does the answer change with timescale? Data: BioTIME 2.0 (doi:10.5281/zenodo.15222193). We built 2,124 annual series of local assemblages (one study at one location) from 196 studies, with 10 to 97 consecutive years and 5 to 1,456 species. Method: for each assemblage we calculated the variance ratio (VR) and divided it with a Fourier method into a long band (periods of 4 years or more) and a short band (periods below 4 years). We tested each value against 999 independent circular shifts of the species series. These shifts keep the autocorrelation of each species and remove covariance between species. Each study had equal weight, and 95% intervals come from a bootstrap of studies. Results: 50.5% (44.3-56.7%) of assemblages were significantly synchronous (one-sided p < 0.025). Only 2.3% (0.8-4.0%) were significantly compensatory. This is in the false-positive range of 2.5-5.0% that we measured on real species series that are independent by design. The long band (41.2% synchronous, 1.9% compensatory) and the short band (42.6%, 2.8%) gave the same pattern. The frequency-resolved excess of total variance over the null was positive at all periods from approximately 14 to 2 years. The small long-band excess was not significant after we removed linear trends. Removal of rare species, a Poisson sampling-noise correction, a subset with a constant number of samples per year, and all realms and large taxon groups gave the same result. The variance-weighted mean correlation between species pairs was 0.16 (0.14-0.18), and it decreased slightly with richness. Limitation: the VR cannot separate shared environmental forcing from shared changes in sampling effort or detectability. Thus the synchrony is an upper limit for environmental synchrony, and it does not show that competition has no effect.

community synchrony · compensatory dynamics · variance ratio · timescale-specific variance ratio · BioTIME · biodiversity time series · community stability · cyclic-shift null model · spectral decomposition · macroecology