Functional tradeoffs determine species coexistence via the storage effect. Proc Natl Acad Sci USA

Department of Biology, Colorado State University, Fort Collins, CO 80523, USA.
Proceedings of the National Academy of Sciences (Impact Factor: 9.67). 08/2009; 106(28):11641-5. DOI: 10.1073/pnas.0904512106
Source: PubMed


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.

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    • "These results suggest that, at least in the dehesa community we studied, spatial niche segregation on soil moisture gradients may not be an important mechanism of co-existence in the species-rich sub-community of annuals. Given that rainfall in the area is highly variable from year-to-year (Ceballos et al. 2013), temporal niche segregation (the storage effect), as found among Sonoran desert annuals (Angert et al. 2009), is an alternative possibility. "
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    ABSTRACT: Questions: Are communities structured on a hydrological (soil moisture) gradient? Is there spatial segregation into hydrological niches? What is the shape of the hydrological niches of individual species? Controlling for spatial autocorrelation, how much of the spatial structure in the community is due to variation in hydrology? Do annuals and perennials behave alike with respect to the above questions? Locations: La Mina in Moscosa Farm, Salamanca, western Spain (dehesa community) and Laguna Larga in the Urbión Peaks, Soria, central-northern Spain (alpine grassland). Methods: The presence of plant species was sampled in two contrasting field sites, for which we also built hydrological models. First, we reduced the dimensionality of the plant distribution data (non-metric multidimensional scaling) and measured the correlation between the resulting ordination and the hydrological gradient. Then we defined hydrological niches and tested niche segregation of plant species against null models (Pianka metrics). Finally, we characterized the hydrological niche of each species using generalised additive mixed models and partitioned the species distribution variance into (1) a hydrological component, (2) a linear trend component and (3) and a spatial component, defined through sets of spatial variables (Moran's eigenvector maps). Results: Both plant communities were primarily structured along hydrological gradients, and spatial segregation into hydrological niches occurred among perennial species, although not among annuals in the dehesa community. Dehesa annuals were spatially aggregated in the driest niches. Hydrological variation shaped the responses of 60% of the annual and about 70% of the perennial species in both the dehesa meadow and the alpine community. Most responses were either monotonic or hump-shaped. Finally, spatially structured hydrological variation proved to be the main driver of spatially structured species composition in all cases. Conclusions: Linearly (gradient of slope) and topographically (at a fine scale) structured variation in hydrology is the main driver of spatially structured species composition in both communities. Our results support the ecological hypothesis that spatial niche segregation on soil moisture gradients is an important mechanism of co-existence for perennials in both test communities, although not for the species-rich sub-community of annuals in the dehesa meadow. How do plant species co-exist? A dehesa meadow and an alpine pasture in Spain were used to investigate species segregation into hydrological niches. Our results support the ecological hypothesis that spatial niche segregation on soil-moisture gradients is an important mechanism of coexistence for perennials in both test communities, though not for the species-rich sub-community of annuals in the dehesa meadow.
    Full-text · Article · Nov 2015 · Journal of Vegetation Science
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    • "ect ( Chesson , 2000 ) , when inter - annual variation in climate or resource availability favors alternatively one group of species over the others ( e . g . , Zavaleta et al . , 2003 ) . Not only inter - annual but also seasonal variability contributes to fluctuating resources that increase the number of coexisting species in different systems ( Angert et al . , 2009 ; Shimadzu et al . , 2013 ) . Oscillations at the population level"
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    ABSTRACT: The consequences of global change for the maintenance of species diversity will depend on the sum of each species responses to the environment and on the interactions among them. A wide ecological literature supports that these species-specific responses can arise from factors related to life strategies, evolutionary history and intraspecific variation, and also from environmental variation in space and time. In the light of recent advances from coexistence theory combined with mechanistic explanations of diversity maintenance, we discuss how global change drivers can influence species coexistence. We revise the importance of both competition and facilitation for understanding coexistence in different ecosystems, address the influence of phylogenetic relatedness, functional traits, phenotypic plasticity and intraspecific variability, and discuss lessons learnt from invasion ecology. While most previous studies have focused their efforts on disentangling the mechanisms that maintain the biological diversity in species-rich ecosystems such as tropical forests, grasslands and coral reefs, we argue that much can be learnt from pauci-specific communities where functional variability within each species, together with demographic and stochastic processes becomes key to understand species interactions and eventually community responses to global change.
    Full-text · Article · Nov 2015 · Frontiers in Plant Science
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    • "The environment experienced by individuals in these communities routinely fluctuates because resource accessibility is dynamic in space and time. Such environmental fluctuations can facilitate competitor coexistence if functional trade-offs cause species to vary in their competitive superiority (e.g., Yu and Wilson 2001; Angert et al. 2009; Holt and Chesson 2014). We previously showed how fluctuating patch accessibility leads to coexistence among ephemeral patch competitors, given a trade-off between fecundity and dispersal ability (Duthie et al. 2014). "
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    ABSTRACT: The ecological principle of competitive exclusion states that species competing for identical resources cannot coexist, but this principle is paradoxical because ecologically similar competitors are regularly observed. Coexistence is possible under some conditions if a fluctuating environment changes the competitive dominance of species. This change in competitive dominance implies the existence of trade-offs underlying species’ competitive abilities in different environments. Theory shows that fluctuating distance between resource patches can facilitate coexistence in ephemeral patch competitors, given a functional trade-off between species dispersal ability and fecundity. We find evidence supporting this trade-off in a guild of five ecologically similar nonpollinating fig wasps and subsequently predict local among-patch species densities. We also introduce a novel colonization index to estimate relative dispersal ability among ephemeral patch competitors. We suggest that a dispersal ability–fecundity trade-off and spatiotemporally fluctuating resource availability commonly co-occur to drive population dynamics and facilitate coexistence in ephemeral patch communities.
    Full-text · Article · May 2015 · The American Naturalist
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