The metacommunity structure of salt marsh arthropods: Experimental tests of local and spatial mechanisms

Background/Question/Methods

Metacommunity theory is concerned with the relative contribution of local (within a resource patch) vs. spatial (among resource patches) processes in structuring multi-species communities.  However, despite much theory, experimental tests of metacommunity predictions are scarce.  Using observations and experiments, we investigated how local and/or spatial factors influence community composition .  We studied a terrestrial arthropod community occupying fragmented salt marsh habitats to observe community patterns, analyze species’ traits underlying these patterns, and test the mechanisms structuring the community.  This system comprises plant patches that vary in nutrient quality, plant density, and structural complexity (local factors) as well as size and connectivity (spatial factors).  The diverse suite of arthropods exploiting these patches vary in key ecological traits, such as habitat and resource specificity, mobility, and trophic position.  Using repeated surveys of arthropods from 60 host-plant patches, we asked whether overall community composition is driven by local or spatial factors and whether component parts of the community, such as particular guilds or trophic levels, differ in their responses to habitat heterogeneity.  Next, we experimentally tested whether dispersal limitation, patch quality, or both factors structure this community.  We defaunated a series of small resource patches that varied in nutrient quality and manipulated local interactions through predator additions and removals. 

Results/Conclusions

We found that community structure depended on both local and spatial factors, but across species the relative importance of these influences depended on species’ traits.  As predicted, specialists and poor dispersers were more influenced by spatial structure than were generalists and highly mobile species; dispersal-limited species showed strong declines in density with increasing patch isolation and decreasing patch size.  We also explored the effects of predation on community structure, since the major predators in the system are generalist hunting spiders (Lycosidae) that abound in small patches where prey species are uncommon.  Experimental manipulation of predator density showed that spiders negatively impacted mesopredator densities, but herbivore species showed mixed responses.  Although predation may influence the spatial distribution of certain prey species, it does not explain the widespread decline of common herbivores observed on small and isolated resource patches.  Instead, resource quality and dispersal limitation appear to jointly influence the structure of the arthropod community in this fragmented system.