In population ecology, there has been a fundamental controversy about the relative importance of competition-driven (densitydependent) population regulation vs. abiotic influences such as temperature and precipitation. The same issue arises at the community level; are population sizes driven primarily by changes in the abundances of cooccurring competitors (i.e., compensatory dynamics), or do most species have a common response to environmental factors? Competitive interactions have had a central place in ecological theory, dating back to Gleason, Volterra, Hutchison and MacArthur, and, more recently, Hubbell's influential unified neutral theory of biodiversity and biogeography. If competitive interactions are important in driving year-to-year fluctuations in abundance, then changes in the abundance of one species should generally be accompanied by compensatory changes in the abundances of others. Thus, one necessary consequence of strong compensatory forces is that, on average, species within communities will covary negatively. Here we use measures of community covariance to assess the prevalence of negative covariance in 41 natural communities comprising different taxa at a range of spatial scales. We found that species in natural communities tended to covary positively rather than negatively, the opposite of what would be expected if compensatory dynamics were important. These findings suggest that abiotic factors such as temperature and precipitation are more important than competitive interactions in driving year-to-year fluctuations in species abundance within communities.biological interactions ͉ community dynamics ͉ negative covariance ͉ neutral models ͉ zero-sum

The structure of ecological communities reflects a tension among forces that alter populations. Marine ecologists previously emphasized control by locally operating forces (predation, competition, and disturbance), but newer studies suggest that inputs from large-scale oceanographically modulated subsidies (nutrients, particulates, and propagules) can strongly influence community structure and dynamics. On New Zealand rocky shores, the magnitude of such subsidies differs profoundly between contrasting oceanographic regimes. Community structure, and particularly the pace of community dynamics, differ dramatically between intermittent upwelling regimes compared with relatively persistent downwelling regimes. We suggest that subsidy rates are a key determinant of the intensity of species interactions, and thus of structure in marine systems, and perhaps also nonmarine communities. Many ecological processes determine the structure and dynamics of communities and ecosystems. Theoretical and experimental advances have led to a growing awareness that different processes operate at different spatial and temporal scales (1, 2). Capturing the full richness of ecosystem dynamics thus requires studies ranging across a wide range of scales and allowing effective evaluation of the relative impact of the relevant factors. Doing so can be challenging. The logistics are daunting, especially in using the power of experimentation over large spatial scales. One solution, the ''comparative-experimental'' approach (3), involves replicated local-scale experimentation at multiple sites spanning larger scales, coupled with local-scale repeated sampling of factors that vary at characteristically larger scales.In its early development, marine community ecology focused on the dynamic consequences of primarily local-scale processes such as species interactions and physical disturbance (4). More recently, marine and nonmarine ecologists alike have documented the influence on communities of larger-scale phenomena including subsidies of materials and propagules transferred between adjacent ecosystems (5-8). Although evidence for the importance of subsidies is growing, questions remain about their impact, generality, magnitude, and interdependence and the physical and biotic mechanisms that underlie them.Here we use the comparative-experimental approach to address the role of large-scale oceanographic phenomena in structuring communities on New Zealand rocky shores. Based on earlier results (6, 9), we predicted that intertidal community structure and dynamics would reflect the coastal oceanographic regime. We hypothesized that the influences of oceanographically modulated subsidies (propagules, as a cause of increases in population density of benthic species, and the concentration of phytoplankton and detritus, as food for filter feeders) would be high with upwelling and low with downwelling. Study SystemPrevious research on the west and east coasts of the South Island of New Zealand (pairs of sites 100-500 m apart on each coast) revealed strik...

Our understanding of the relative influence of different ecological drivers on the number of species in a place remains limited. Assessing the relative influence of local ecological interactions versus regional species pools on local species richness should help bridge this conceptual gap. Plots of local species richness versus regional species pools have been used to address this question, yet after an active quarter-century of research on the relative influence of local interactions versus regional species pools, consensus remains elusive. We propose a conceptual framework that incorporates spatial scale and ecological interaction strength to reconcile current disparities. We then test this framework using a survey of marine rocky intertidal algal and invertebrate communities from the northeast Pacific. We reach two main conclusions. First, these data show that the power of regional species pools to predict local richness disintegrates at small spatial scales coincident with the scale of biological interactions, when studying ecologically interactive groups of species, and in generally more abiotically stressful habitats (e.g., the high intertidal). Second, conclusions of past studies asserting that the regional species pool is the primary driver of local species richness may be artifacts of large spatial scales or ecologically noninteractive groups of species.

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