An Empirical Evaluation of the Insurance Hypothesis in Diversity–stability Models

Abstract

An important stabilizing mechanism in most diversity stability models is the insurance hypothesis, which involves correlation/covariance relationships among species. These models require that species do not fluctuate synchronously over time: that is, the correlation between pairs of species does not equal 1.0. However, the strength of this stabilizing mechanism increases as correlations decline away from 1.0, especially as they become more negative and also as the summed covariance across all species pairs becomes more negative. We evaluated the importance of the insurance hypothesis as a stabilizing mechanism by examining a variety of terrestrial assemblages using long-term data from the Global Population Dynamics Database, the Breeding Bird Survey, and a long-term site in southeastern Arizona, USA. We identified co-occurring assemblages of species and calculated the Spearman rank correlations of all pairs of species and the summed covariance of the entire assemblage. We found that, in most assemblages, positive correlations were two to three times more common than negative and that the magnitude of the positive correlations tended to be stronger than the negative correlations. For all but three assemblages, the summed covariance was positive. Data from larger spatial scales tended to exhibit more positive correlations, but even at the smallest spatial scales, positive correlations outnumbered negative. We suggest that species often covary positively because coexisting species respond similarly to fluctuations in their resource base driven by climatic fluctuations. As such, our review suggests that the insurance hypothesis may not be a strong mechanism stabilizing fluctuations in natural terrestrial communities.

Resilience and stability in bird guilds across tropical countryside

“…Using yearly variation in total detections and mean detections as proxies for abundance, we found scaling parameters above one In contrast, we found that assemblages with higher species richness did not exhibit more negative covariance (frugivores: β < 0.01, R 2 < 0.01, P = 0.90; granivores: β < 0.01, R 2 < 0.01, P = 0.78; insectivores: β < 0.01, R 2 < 0.01, P = 0.63; nectarivores: β = 0.06, R 2 = 0.07, P = 0.10). In fact, we found no evidence that populations exhibit negative covariance in population size over time (17)(18)(19)29), although positive correlations were sometimes present (Fig. 3).…”

“…It may confer resilience, because if species react differently to disturbances, then there is a higher likelihood that guild abundance will remain constant after ecosystems change (7,17). Second, negative covariation in abundance between species in the same guild can confer resilience by causing the total abundance of a guild to remain constant through disturbances (17)(18)(19). For example, the decline of one insect-eating bird after a disturbance could be offset by an increase in abundance of another species within the same guild.…”

“…For example, the decline of one insect-eating bird after a disturbance could be offset by an increase in abundance of another species within the same guild. Such negative correlations may arise either through competition (referred to as density compensation) (17)(18)(19) or differing responses to abiotic conditions. Determinants of community stability are better understood.…”

Biotic mechanisms of community stability shift along a precipitation gradient

“…Previous synthesis studies that have calculated the variance ratio at multiple sites for several taxa reported more positive than negative covariance (Houlahan et al 2007, Valone andBarber 2008). These patterns contrast with experimental findings, which often provide evidence of compensatory dynamics (Klug et al 2000, Hector et al 2010.…”

Species traits and species diversity affect community stability in a multiple stressor framework

“…by the insurance model (Yachi & Loreau 1999), which bases diversity effects on the fact that diverse communities are more likely to show compensatory dynamics than communities of low biodiversity. Although compensatory dynamics in response to environmental fluctuation has been analysed in natural systems (Vasseur & Gaedke 2007, Winfree & Kremen 2009) and experiments (Gonzalez & De s campsJulien 2004, Downing 2008, there is considerable debate as to whether compensatory dynamics generally prevail in ecosystems (Houlahan et al 2007, Valone & Barber 2008.…”