Species relationships in the extremes and their influence on community stability

Abstract

Synchrony among population fluctuations of multiple coexisting species has a major impact on community stability, i.e. on the relative temporal constancy of aggregate properties such as total community biomass. However, synchrony and its impacts are usually measured using covariance methods, which do not account for whether species abundances may be more correlated when species are relatively common than when they are scarce, or vice versa. Recent work showed that species commonly exhibit such ‘asymmetric tail associations’. We here consider the influence of asymmetric tail associations on community stability. We develop a ‘skewness ratio’ which quantifies how much species relationships and tail associations modify stability. The skewness ratio complements the classic variance ratio and related metrics. Using multi-decadal grassland datasets, we show that accounting for tail associations gives new viewpoints on synchrony and stability; e.g. species associations can alter community stability differentially for community crashes or explosions to high values, a fact not previously detectable. Species associations can mitigate explosions of community abundance to high values, increasing one aspect of stability, while simultaneously exacerbating crashes to low values, decreasing another aspect of stability; or vice versa. Our work initiates a new, more flexible paradigm for exploring species relationships and community stability. This article is part of the theme issue ‘Synchrony and rhythm interaction: from the brain to behavioural ecology’.

Tail‐dependent spatial synchrony arises from nonlinear driver–response relationships

“…What we call tail dependence in this manuscript has been called 'asymmetric tail association' or simply 'tail association' in other studies (Ghosh et al, 2020a,b,c;. The terminology 'lower-tail dependence' or 'upper-tail dependence' was used in early work (Ghosh et al, 2020a) to describe associations between variables in the corresponding tails, but this terminology was later abandoned in favour of 'lower-tail association' and 'upper-tail association' (Ghosh et al, 2020b(Ghosh et al, , 2021, because the earlier terminology could be construed to imply a direct causal dependence between the variables, which need not be the case. We found utility in distinguishing between tail association, which as used here does not necessarily imply difference or asymmetry between the tails, and tail dependence, which does; we preferred the brevity of tail dependence to asymmetric tail association, but note that these refer to the same concept.…”

“…We assessed tail dependence in giant kelp spatial synchrony and in the relationship between kelp biomass and environmental drivers using the partial Spearman correlation (Ghosh et al, 2020a,b,c; 2021). Given two bounds ( stands for `lower’ and for ‘upper’), for two positively correlated variables, the partial Spearman correlation is the portion of the standard Spearman rank correlation which arises because of the range of quantiles in the two variables bounded by and (see Supplementary Materials for mathematical details).…”

“…Here, 'tails' refers to the upper or lower portions of a variable's distribution. Tail-dependent associations between ecological and environmental variables are common (Ghosh et al, 2020a), including in the context of synchrony among interacting species in a community (Ghosh et al, 2020b(Ghosh et al, , 2021. However, tail dependence in spatial synchrony has been little studied, so its prevalence, mechanisms, and consequences in empirical populations are not yet well understood.…”

Synchrony and rhythm interaction: from the brain to behavioural ecology

“…Ecologists are interested in such interspecific synchrony because it may de-stabilize a community by making extremely low populations and extinctions more likely. Ghosh et al [22] pursue this issue, focusing on the statistical indices used to measure community synchrony, and the inability of traditional indices to contend with the different ways in which a given degree of synchrony can be achieved. In particular, they show that a situation in which several species are highly synchronous only when rare will exert a different influence on stability than a situation where several species are synchronous when common, and they introduce modified indices to accommodate these distinctions.…”

“…leaders and followers in animal choruses [19,20], and it is the asynchronous imperfection that can have profound consequences for the individuals or species. Ghosh et al [22] point out how asynchrony in the temporal alignment of species forming a biological community bears resemblance to events in the business cycle, highlighting further external connections, in this case to the field of human economic affairs.…”