Functional tradeoffs determine species coexistence via the storage effect

Abstract

How biological diversity is generated and maintained is a fundamental question in ecology. Ecologists have delineated many mechanisms that can, in principle, favor species coexistence and hence maintain biodiversity. Most such coexistence mechanisms require or imply tradeoffs between different aspects of species performance. However, it remains unknown whether simple functional tradeoffs underlie coexistence mechanisms in diverse natural systems. We show that functional tradeoffs explain species differences in long-term population dynamics that are associated with recovery from low density (and hence coexistence) for a community of winter annual plants in the Sonoran Desert. We develop a new general framework for quantifying the magnitude of coexistence via the storage effect and use this framework to assess the strength of the storage effect in the winter annual community. We then combine a 25-year record of vital rates with morphological and physiological measurements to identify functional differences between species in the growth and reproductive phase of the life cycle that promote storage-effect coexistence. Separation of species along a tradeoff between growth capacity and low-resource tolerance corresponds to differences in demographic responses to environmental variation across years. Growing season precipitation is one critical environmental variable underlying the demographic decoupling of species. These results demonstrate how partially decoupled population dynamics that promote local biodiversity are associated with physiological differences in resource uptake and allocation between species. These results for a relatively simple system demonstrate how long-term community dynamics relate to functional biology, a linkage scientists have long sought for more complex systems.biodiversity ͉ coexistence mechanism ͉ functional trait ͉ population dynamics ͉ specific leaf area H ow competing species stably coexist is a long-standing ecological problem. All niche-based mechanisms for stable coexistence rely on ecological differences that enable each species to recover when perturbed to low density and thus remain in the community. Some of these coexistence mechanisms, such as differential exploitation of multiple limiting resources (1, 2) and frequency-dependent predation (3), operate independently of fluctuations in the environment. Other stable coexistence mechanisms depend critically upon environmental fluctuations that allow species to recover from low density (4). These include competition/ colonization tradeoffs in a disturbance matrix (5, 6), relative nonlinearity of competition (7) and the storage effect (8, 9). The storage effect combines species-specific responses to the environment and population-dynamic buffering by persistent life history stages in a way that results in a positive average low-density growth rate for each species. It is perhaps the dominant fluctuationdependent mechanism for organisms in variable environments. Its role has been explored for diverse groups ranging from freshwater z...

Within-host competitive interactions as a mechanism for the maintenance of parasite diversity

“…through a storage effect [41]). Empirical demonstrations of these mechanisms are less well established than niche partitioning or competition-colonization trade-offs; nevertheless, they suggest that variation in how species experience competition is an important mechanism of coexistence [42,43]…”

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“…The theory of temporal niche dynamics (TND) suggests that temporal variation in niche availability allows the storage effect to operate, buffering species against extinction 4,5 . Increased species coexistence in fluctuating environments has been documented for both natural [6][7][8] and experimental 9,10 communities, and has, in a few cases, been explicitly linked to TND 6,11 . In parallel, diversification under fluctuating selection has been of much interest for evolutionary biologists [12][13][14] .…”