Gradients and the Structure of Neotropical Metacommunities: Effects of Disturbance, Elevation, Landscape Structure, and Biogeography

Abstract

A metacommunity perspective provides ecological insight into spatiotemporal dynamics because it explicitly considers the structure and organization of communities along environmental gradients, and seeks to understand the local (e.g., biotic interactions, environmental tolerances, habitat preferences) and regional (e.g., dispersal, habitat fragmentation, landscape structure) processes that generate these patterns. More specifically, a metacommunity is a network of communities or sites that are potentially connected to each other via dispersal of individuals among constituent communities or sites. Local emigration and immigration, when coupled with other spatially explicit ecological processes such as species sorting, habitat filtering, priority effects, or interspecific competition, imbue the network with an emergent structure that corresponds to underlying environmental gradients. High biodiversity, a complex biogeographical history, and a heterogeneous topography associated with orogenic events combine to make the Neotropics an ideal test bed for theories related to ecological gradients and the assembly of communities.Metacommunity structure is an emergent property defined by the relationships among species distributions along particular environmental gradients. These patterns are based on three characteristics of species ranges called elements of metacommunity structure: coherence, range turnover, and range boundary clumping. Coherence (species distributions that are molded by a common gradient) characterizes many structures that have been described in ecology, with the extent and locations of species distributions along this gradient determining metacommunity structure. Nested structures are coherent structures that exhibit less range turnover than expected by chance. In contrast, multiple distinct coherent structures are characterized by more turnover than expected by chance, and these structures are distinguished by range boundaries that are clumped (Clementsian structure), randomly distributed (Gleasonian structure), or hyperdispersed (evenly spaced structure). These idealized structures form the framework of a continuum of possible structures, from those with high species turnover (e.g., Clementsian, Gleasonian) to those with low species turnover (e.g., nested subsets), and from those with coincident range boundaries (i.e., Clementsian) to those with hyper-dispersed range boundaries (i.e., evenly spaced). Although elements of metacommunity structure can identify particular structures based on the distributions of species, supporting (e.g., canonical correspondence analysis, hierarchical partitioning of biodiversity, general linear models), and complementary (e.g., variation partitioning) analyses are required to determine the nature of environmental gradients along which the metacommunity is structured, the number and locations of compartments, or the relative influence of potential structuring mechanisms.Metacommunity approaches have revealed structures associated with many types of environmental gradients in the Neotropics, have done so from small to large spatial scales, and in a hierarchical fashion. Neotropical metacommunities are structured by gradients associated with abiotic variation, biotic interactions, edaphic features, habitat type, disturbance, contamination, land use, or legacies of historical factors. By simultaneously evaluating mechanisms that affect patterns at different time scales (e.g., biogeographical history versus modern land use change), metacommunity approaches have generated and tested hypotheses associated with a wide range of ecological, biogeographical, evolutionary, and conservation questions. Large-scale metacommunity structures are typically associated with historical factors (evolutionary histories, biogeography), whereas smaller scale patterns arise in response to variation in local factors (e.g., habitat type, disturbance history) or anthropogenic activities (e.g., habitat loss and fragmentation, pollution and contamination). Finally, ecological differences between taxa have resulted in taxon-specific structure along the same gradients, confirming that the study of multiple metacommunities in the same system may be required to understand the relative influence of historical and contemporary mechanisms.Two aspects of biodiversity monitoring in the Neotropics can improve the understanding of metacommunity dynamics. First, long-term studies that repeatedly sample at regular intervals to quantify changes in metacommunities along gradients subject to combinations of press (e.g., eutrophication, contamination, pollution, climate change) and pulse (e.g., extreme weather events, logging) disturbances are necessary to understand spatiotemporal dynamics. Second, comparative studies of multiple taxa (e.g., bats, rodents, birds, amphibians, arthropods, trees, fungi, microbes) along the same spatial or environmental gradients are needed to determine which groups are particularly vulnerable to changing environmental conditions, as well as to identify emerging conservation concerns. This is particularly urgent as the tempo and mode of human-induced disturbances associated with the Anthropocene will likely have great impact on the structure and functioning of vital ecological systems in the New World tropics, and beyond.