Climate variability has a stabilizing effect on the coexistence of prairie grasses

Abstract

How expected increases in climate variability will affect species diversity depends on the role of such variability in regulating the coexistence of competing species. Despite theory linking temporal environmental fluctuations with the maintenance of diversity, the importance of climate variability for stabilizing coexistence remains unknown because of a lack of appropriate long-term observations. Here, we analyze three decades of demographic data from a Kansas prairie to demonstrate that interannual climate variability promotes the coexistence of three common grass species. Specifically, we show that (i) the dynamics of the three species satisfy all requirements of "storage effect" theory based on recruitment variability with overlapping generations, (ii) climate variables are correlated with interannual variation in species performance, and (iii) temporal variability increases low-density growth rates, buffering these species against competitive exclusion. Given that environmental fluctuations are ubiquitous in natural systems, our results suggest that coexistence based on the storage effect may be underappreciated and could provide an important alternative to recent neutral theories of diversity. Field evidence for positive effects of variability on coexistence also emphasizes the need to consider changes in both climate means and variances when forecasting the effects of global change on species diversity.

Fig. 1.

Observed basal cover of the little bluestem community at Hays, Kansas, 1937–1968. Shown are means for the perennial grasses B. curtipendula, B. hirsuta, and S. scoparium, along with other species from a group of four 1-m² quadrats located on shallow limestone soils within a livestock exclosure. Abundances were low early in the time series because of the Great Drought of the 1930s. The three focal grasses continue to co-occur in the permanent quadrats, with S. scoparium still the most abundant (P.B.A., unpublished data). The vertical bars show deviation from the mean annual precipitation of 580 mm. This period includes both the wettest water year on record at Hays (1,122 mm in 1951) and the driest (226 mm in 1956).

Fig. 2.

Evidence for the three conditions of the storage effect. (A–C) B. curtipendula (A), B. hirsuta (B), and S. scoparium (C) have the potential for long lifespans, buffering population growth as required by condition 1. (D–F) Comparisons of exponential yearly intrinsic growth rates for each pair of the three species in each of 29 years show considerable scatter (ρ = 0.17, 0.17, and 0.44, respectively), evidence that the species differ in their response to interannual variability (condition 2). (G–I) For B. curtipendula (ρ = −0.49, P = 0.009) (G), B. hirsuta (ρ = –0.78, P < 0.0001) (H), and S. scoparium (ρ = –0.67, P < 0.001) (I), competition had stronger negative effects on growth in years of high intrinsic growth rates (more favorable years), satisfying condition 3. Positive values, indicating that crowding caused relative increases in growth rates, occurred in years of low intrinsic growth rates.

Fig. 3.

Estimated long-term low-density growth rates in constant and variable environments. The potential for each species [B. curtipendula (A), B. hirsuta (B), and S. scoparium (C)] to invade a resident community, measured by the species’ long-term low-density exponential growth rate, was much higher in simulations incorporating observed interannual variability in survival and colonization than in simulations based on constant mean survival and colonization.