Resource pulses, species interactions, and diversity maintenance in arid and semi-arid environments.

Section of Evolution and Ecology, University of California, Davis 95616, USA.
Oecologia (Impact Factor: 3.25). 11/2004; 141(2):236-53. DOI: 10.1007/s00442-004-1551-1
Source: PubMed

ABSTRACT Arid environments are characterized by limited and variable rainfall that supplies resources in pulses. Resource pulsing is a special form of environmental variation, and the general theory of coexistence in variable environments suggests specific mechanisms by which rainfall variability might contribute to the maintenance of high species diversity in arid ecosystems. In this review, we discuss physiological, morphological, and life-history traits that facilitate plant survival and growth in strongly water-limited variable environments, outlining how species differences in these traits may promote diversity. Our analysis emphasizes that the variability of pulsed environments does not reduce the importance of species interactions in structuring communities, but instead provides axes of ecological differentiation between species that facilitate their coexistence. Pulses of rainfall also influence higher trophic levels and entire food webs. Better understanding of how rainfall affects the diversity, species composition, and dynamics of arid environments can contribute to solving environmental problems stemming from land use and global climate change.

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    ABSTRACT: QuestionsHow do inter-annual fluctuations in water availability affect the functional trait patterns along spatial gradients of resource availability and disturbance?LocationMediterranean grasslands in central Spain, near Madrid.Methods We surveyed plant communities from 66 sites under different grazing regimes (from heavy grazing to grazing abandonment) in productive and unproductive habitats (corresponding to upper and lower topographic zones) in 2 yr with contrasting rainfall conditions. Community weighted mean (CWM) and Rao quadratic entropy for three key plant ecological strategy traits (specific leaf area, height and seed mass) were calculated for each community. We used null models to estimate functional richness (FR) and functional divergence (FD), the two components of functional diversity with the highest power to detect changes in community assembly processes across environmental gradients.ResultsThe patterns of CWM remained rather constant across years, with the only exception being seed mass, which experienced considerable temporal changes that suggested that heavy-seeded species are favoured under stressful conditions. Patterns for FR were consistent across years. They revealed both trait convergence and divergence depending on the niche axis and context. Convergence was observed for vegetative traits in unproductive habitats and seed mass in productive ones, and divergence for seed mass in unproductive habitats and vegetative traits in productive ones. In contrast, the patterns of FD of the vegetative traits changed considerably between years, as shown by increased divergence during the wet year in unproductive habitats and decreased divergence in grazing-abandoned productive habitats.Conclusions Temporal changes in Mediterranean grassland composition depend on complex interactions between species traits, resource availability and disturbance. Increased rainfall appeared to have contrasting effects on assembly processes in stressful and productive habitats. In stressful habitats we found evidence that increased rainfall promoted niche complementarity, while in productive habitats, especially in the absence of disturbance, it increased trait convergence.
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    ABSTRACT: Arid and semiarid savannas are characterized by the coexistence of trees and grasses in water limited conditions. As in all drylands, also in these savannas rainfall is highly intermittent. In this work we develop and use a simple implicit-space model to conceptually explore how precipitation intermittency influences tree-grass competition and savanna occurrence. The model explicitly includes soil moisture dynamics, and life-stage structure of the trees. Assuming that water availability affects the ability of both plant functional types to colonize new space and that grasses outcompete tree seedlings, the model is able to predict the expected sequence of grassland, savanna and forest along a range of mean annual rainfall. In addition, rainfall intermittency allows for tree-grass coexistence at lower mean annual rainfall values than for constant precipitation. Comparison with observations indicate that the model, albeit very simple, is able to capture some of the essential dynamical processes of natural savannas. The results suggest that precipitation intermittency affects savanna occurrence and structure, indicating a new point of view for reanalyzing observational data from the literature. This article is protected by copyright. All rights reserved.
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    ABSTRACT: We tested whether community structure of insect seed predators is influenced by spatiotemporal variation in mast availability on host-tree species. Specifically, mast production and acorn weevil (Curculio L., 1758) occupancy were estimated annually from 2006 to 2008 for individual host trees in a sample of 74 northern red oaks (Quercus rubra L.), 100 white oaks (Quercus alba L.), and 81 shagbark hickories (Carya ovata (Mill.) K. Koch) in west-central Indiana, USA. Occupancy and vital rates of nine Curculio species on their primary host-tree species were derived using multispecies, multiseason (MSMS) models within a Bayesian framework, accounting for imperfect detection. Mast production of host trees had a strong positive effect on community-level occupancy and survival of Curculio. Mast production varied considerably between years and generally was spatially autocorrelated only at distances Curculio created a spatial storage effect that, when coupled with a temporal storage effect induced by prolonged diapause common among Curculio, facilitated species coexistence. Methodologically, increased precision of parameter estimates from MSMS models makes it generally more useful than single-species models in studies of community dynamics.
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