Abstract
Coupling experiments at small spatial scales with large-scale surveys can help to gen- eralize experimental results across large spatial scales. The goal of the present study was to evaluate patterns of crab herbivory within and, at a larger scale, between many southwestern (SW) Atlantic salt marshes. Hence, we conducted experiments in an Argentinean salt marsh to elucidate the effects of crab Chasmagnathus granulatus grazing on the cordgrass Spartina densiflora and the factors that can affect this interaction, and then examined the potential generality of these results across SW Atlantic salt marshes from Brazil to Argentina (15 marshes, range ≈ 2000 km). Experimental exami- nation of the effects of crabs (control and exclusion) on marsh grass transplants, and factors that can affect them, i.e. the presence or absence of plant neighbors and marsh height (middle and low), revealed that crab herbivory decreased plant biomass and increased stem mortality; however, her- bivory effects were significantly diminished in the presence of plant neighbors and with increasing marsh elevation. Our geographical survey showed that crab herbivory is common in SW Atlantic salt marshes, with more than 20% of leaves damaged in most marshes and with greater consumption at marshes with higher crab densities. In addition, plants at the lower edge of marshes were generally the most consumed (max. >60% leaves consumed) and crabs preferred S. alterniflora over S. densi- flora. Over a regional spatial scale, our results suggest that herbivory may affect plant production at some marshes and can also play a role in limiting the lower tidal elevation limit of low-marsh plants.
Highlights
Plant growth can be controlled from the bottom-up by nutrients and/or physical factors, and from the topdown by grazers and their predators
We examine small- (10s of m) and large-scale (100s of km) spatial variation in herbivore pressure by Chasmagnathus granulatus in SW Atlantic salt marshes by: (1) experimentally evaluating the extent to which crab herbivory controls the growth of marsh plants across the intertidal, and (2) quantifying the intensity of crab herbivory on Spartina densiflora and S. alterniflora along 2000 km of SW Atlantic coastline, comprising most of the distribution range of both plant species coexisting with this crab in eastern South America
Crab herbivory in the low marsh at Mar Chiquita was twice as high as in the middle marsh, and more than 3 times higher when transplants were deployed in bare patches with crabs present (Tables 1 & 2)
Summary
Plant growth can be controlled from the bottom-up by nutrients and/or physical factors, and from the topdown by grazers and their predators. Examples of top-down regulation (Halpern et al 2006), bottom-up effects (Nixon & Buckley 2002) and simultaneous control (Nielsen 2003) are numerous and can be found in aquatic (Williams et al 2002) and terrestrial systems (Chase et al 2000). In many cases, both top-down and bottom-up forces are important (Nielsen 2003). Both top-down and bottom-up forces are important (Nielsen 2003) Many of these studies, have been conducted on local scales, and there is little information on how these factors vary across large spatial scales.
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