Ecologists have long known that complex habitats often provide prey with refuges from predation. This is true for a wide variety of habitat types in terrestrial, freshwater and marine systems. Despite the recognized importance of structural habitat complexity, ecologists have defined and measured complexity in many different ways. We propose 2 new indices of structural habitat complexity that are dimensionless, that can be applied across various habitat types and scales, and that directly measure how structural complexity interferes with a predator's foraging ability. These indices are: the total area of cover within a habitat divided by the total area of the habitat (C t /A t ) and the average inter-structural space size divided by the size of the predator (Sp/Pr). C t /A t measures the amount of cover available within a habitat that interferes with a predator's ability to see, or otherwise sense, prey within the habitat. Sp/Pr measures the extent to which the structure interferes with a predator's ability to move through the habitat in search of, or while pursuing, prey. We predicted that prey survivorship should increase hyperbolically with increasing C t /A t , and that survivorship should decrease sigmoidally with increasing Sp/Pr. We also predict that both C t /A t and Sp/Pr can influence survivorship independently, and that they form a survivorship plane. We tested our model in 3 laboratory experiments with the fish Fundulus heteroclitus as predator and amphipods as prey, and in 1 field experiment. The results of our laboratory experiments support our model for Sp/Pr, but are only suggestive for C t /A t . The results of the field experiment are consistent with our laboratory results, and our model.
Excessive input of nitrogen to estuaries and coastal waters leads to eutrophication; the resulting organic matter over-enrichment of sediments and seasonal hypoxia of bottom water have significant deleterious effects on benthic community biodiversity, abundance, and biomass. Our goal was to better understand how these losses carry through to impairment of key ecosystem functions of benthic communities. Recent management efforts to address eutrophication have reduced nitrogen loading to several estuaries of the Virginian Biogeographic Province (northeast United States). How the ecosystems will respond remains to be seen. Using Narragansett Bay as an example estuary within this Province, we compared measures of community structure and function from stations in seasonally hypoxic areas with stations in normoxic areas. We analyzed a benthic data set spanning 20 years and 155 stations, along with ancillary data from other sources. Hypoxic areas had half the species richness, many fewer rare species, lower biomass, and lower secondary production. Benthic communities in the hypoxic areas had a significantly different abundance structure, were at an earlier successional stage, and bioturbated the sediments to a depth about one-fifth that of the normoxic areas. On average, sediments in the hypoxic areas took up more oxygen-used for aerobic metabolism and oxidation of reduced compounds from anaerobic metabolism. Sediments in hypoxic areas released into the overlying water two to three times more ammonium and phosphate. Mean flux of dissolved oxygen into the sediments of hypoxic areas and mean net flux of nitrogen gas (from sediment denitrification) out were slightly higher. Eutrophication-driven over-enrichment of organic matter, along with seasonal hypoxia in the northern part of the Bay have led to degradation of benthic community structure and function, which have serious implications for sustainable provision of ecosystem services. We quantified fifteen stressor-response relationships that can help understand how, following a reduction in nitrogen inputs, a recovery of benthic ecosystem functions in hypoxic areas could proceed.
Many types of anthropogenic stress to estuaries lead to destruction and conversion of habitats, thus altering habitat landscapes and changing the "arena" in which the life history interactions of native fauna take place. This can lead to decreased populations of valued fauna and other negative consequences. The Tampa Bay Estuary Program (TBEP) pioneered a system-wide management framework that develops estuarine habitat restoration and protection goals based on supporting estuarine-dependent species and the habitat landscapes they require (for example, the extent of seagrass beds, mangrove forests, oyster reefs, or oligohaline marshes) within an estuary. We describe this framework and provide related statistics as methods to help managers set system-wide ecological goals using larger conceptual approaches that are easily communicated to stakeholders and the public; we also discuss applications of the approach to existing and evolving paradigms of estuarine management. The TBEP and partners used this framework to combine a simple and unifying vision with a diverse and complex set of management tools, resulting in greatly improved environmental conditions within Tampa Bay.
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