Life stage and species identity affect whether habitat subsidies enhance or simply redistribute consumer biomass.

Quantifying the response of mobile consumers to changes in habitat availability is essential for determining the degree to which population-level productivity is habitat limited rather than regulated by other, potentially density-independent factors. Over landscape scales, this can be explored by monitoring changes in density and foraging as habitat availability varies. As habitat availability increases, densities may: (1) decrease (unit-area production decreases; weak habitat limitation); (2) remain stable (unit-area production remains stable; habitat limitation) or (3) increase (unit-area production increases; strong habitat limitation). We tested the response of mobile estuarine consumers over 5 months to changes in habitat availability in situ by comparing densities and feeding rates on artificial reefs that were or were not adjacent to neighbouring artificial reefs or nearby natural reefs. Using either constructed or natural reefs to manipulate habitat availability, we documented threefold density decreases among juvenile stone crabs as habitat increased (i.e. weak habitat imitation). However, for adult stone crabs, density remained stable across treatments, demonstrating that habitat limitation presents a bottleneck in this species' later life history. Oyster toadfish densities also did not change with increasing habitat availability (i.e. habitat limitation), but densities of other cryptic fishes decreased as habitat availability increased (i.e. weak limitation). Feeding and abundance data suggested that some mobile fishes experience habitat limitation, or, potentially in one case, strong limitation across our habitat manipulations. These findings of significant, community-level habitat limitation provide insight into how global declines in structurally complex estuarine habitats may have reduced the fishery production of coastal ecosystems.

Threshold effects of habitat fragmentation on fish diversity at landscapes scales.

Habitat fragmentation involves habitat loss concomitant with changes in spatial configuration, confounding mechanistic drivers of biodiversity change associated with habitat disturbance. Studies attempting to isolate the effects of altered habitat configuration on associated communities have reported variable results. This variability may be explained in part by the fragmentation threshold hypothesis, which predicts that the effects of habitat configuration may only manifest at low levels of remnant habitat area. To separate the effects of habitat area and configuration on biodiversity, we surveyed fish communities in seagrass landscapes spanning a range of total seagrass area (2-74% cover within 16 000-m2 landscapes) and spatial configurations (1-75 discrete patches). We also measured variation in fine-scale seagrass variables, which are known to affect faunal community composition and may covary with landscape-scale features. We found that species richness decreased and the community structure shifted with increasing patch number within the landscape, but only when seagrass area was low (<25% cover). This pattern was driven by an absence of epibenthic species in low-seagrass-area, highly patchy landscapes. Additional tests corroborated that low movement rates among patches may underlie loss of vulnerable taxa. Fine-scale seagrass biomass was generally unimportant in predicting fish community composition. As such, we present empirical support for the fragmentation threshold hypothesis and we suggest that poor matrix quality and low dispersal ability for sensitive taxa in our system may explain why our results support the hypothesis, while previous empirical work has largely failed to match predictions.

Fiddler crabs facilitate Spartina alterniflora growth, mitigating periwinkle overgrazing of marsh habitat.

Ecologists have long been interested in identifying and testing factors that drive top-down or bottom-up regulation of communities. Most studies have focused on factors that directly exert top-down (e.g., grazing) or bottom-up (e.g., nutrient availability) control on primary production. For example, recent studies in salt marshes have demonstrated that fronts of Littoraria irrorata periwinkles can overgraze Spartina alterniflora and convert marsh to mudflat. The importance of indirect, bottom-up effects, particularly facilitation, in enhancing primary production has also recently been explored. Previous field studies separately revealed that fiddler crabs, which burrow to depths of more than 30 cm, can oxygenate marsh sediments and redistribute nutrients, thereby relieving the stress of anoxia and enhancing S. alterniflora growth. However, to our knowledge, no studies to date have explored how nontrophic facilitators can mediate top-down effects (i.e., grazing) on primary-producer biomass. We conducted a field study testing whether fiddler crabs can facilitate S. alterniflora growth sufficiently to mitigate overgrazing by periwinkles and thus sustain S. alterniflora marsh. As inferred from contrasts to experimental plots lacking periwinkles and fiddler crabs, periwinkles alone exerted top-down control of total aboveground biomass and net growth of S. alterniflora. When fiddler crabs were included, they counteracted the effects of periwinkles on net S. alterniflora growth. Sediment oxygen levels were greater and S. alterniflora belowground biomass was lower where fiddler crabs were present, implying that fiddler crab burrowing enhanced S. alterniflora growth. Consequently, in the stressful interior S. alterniflora marsh, where subsurface soil anoxia is widespread, fiddler crab facilitation can mitigate top-down control by periwinkles and can limit and possibly prevent loss of biogenically structured marsh habitat and its ecosystem services.