Proximity to reef influences density of small predatory fishes, while type of seagrass influences intensity of their predation on crabs

Abstract

The magnitude of the influence that predators exert on their prey is partly determined by factors that influence the density and behaviour of predators, and partly by factors that influence the survival of prey in the presence of predators. We tested whether the presence of reefs influences the density of small predatory fishes inhabiting seagrass meadows, and the intensity of predation by these fishes on tethered crabs, by taking measurements at increasing distances from 10 rocky reefs at 2 locations separated by several hundred km. We also tested whether these variables were influenced by type of seagrass by comparing patterns yielded in meadows dominated by Amphibolis spp. and Posidonia sinuosa, 2 seagrasses with different morphological and architectural characteristics. The densities of small predatory fishes declined rapidly with increasing distance from reefs, with significantly lower densities 30 m from reefs than immediately adjacent to reefs. Densities > 300 m from reefs were generally not significantly different to densities 30 m from reefs. However, the survival of tethered crabs showed no consistent trend associated with distance from reefs. Type of seagrass was not a significant influence on the density of small predatory fishes, but did influence the survival of tethered crabs: those in Amphibolis spp. meadows were 3.1× more likely to be eaten than those in P. sinuosa meadows. These patterns were consistent between locations. Our results suggest that over landscape scales (hundreds of metres) reefs are an important influence on abundance of small predatory fishes, but not on the intensity of predation on crabs. Differences in the intensity of predation between the 2 seagrass species are likely due to their dissimilar morphological and architectural characteristics.

Recovery from the impact of light reduction on the seagrass Amphibolis griffithii, insights for dredging management

“…Consequently, there is a higher biomass and diversity of algae and fauna living on A. griffithii compared to other seagrass species (Borowitzka et al, 1990;Edgar, 1990;Gartner et al, 2010), including a unique assemblage of fish characterised by different species and larger fish than those found in other large seagrass, such as Posidonia (Hyndes et al, 2003). Greater predation rates have also been observed in A. griffithii meadows compared to Posidonia meadows (Vanderklift et al, 2007).…”

Seascape ecology of coastal biogenic habitats: advances, gaps, and challenges

“…Acosta & Robertson (2002) found 30% more edge-habitat fish species on small (700 m 2 ) reefs exhibiting a higher perimeter:area ratio than large (2300 m 2 ) reef patches, and fish species composition was markedly different when comparing the bottom edges of patch reefs with the reef top. Vanderklift et al (2007) examined fish communities at varying distances (0 to 1100 m) from rocky reefs in Western Australia and observed an abrupt decline of small predatory fish within the first 30 m from the reef, indicative of a negative edge effect. Similarly, Dorenbosch et al (2005) in the western Indian Ocean observed an edge effect for coral reef-associated species and generalist species, where fish densities and species richness decreased significantly within 30 m of the patch reef boundary.…”

Patterns of scale-dependency and the influence of map resolution on the seascape ecology of reef fish

“…The scale of fish-seascape interactions can be identified by evaluating a local fish assemblage in relation to the surrounding mosaic of habitat patches (Irlandi & Crawford 1997, Pittman et al 2004, Kendall 2005, Grober-Dunsmore et al 2007, Vanderklift et al 2007). The spatial extent of elements to include in such analysis is critical, and analyses should seek to systematically vary the spatial scale and distances over which fish and seascape associations are measured (Addicott et al 1987, Wiens 1989, Riitters et al 1997, Sale 1998, Kendall 2005.…”

“…Small or rare patches of habitat can be subsumed into larger features as map resolution is coarsened. Many species use edges or ecotones between habitat patches (Shulman 1985, Sweatman & Robertson 1994, Dorenbosch et al 2005, Pittman et al 2007, Vanderklift et al 2007, and such boundaries can be greatly simplified or even removed depending on map characteristics (Kendall & Miller 2008).Seascape composition can affect fish ecology at several levels of biological organization. At the broadest level, species diversity, richness, and total abundance of fish have been partly explained by seascape variables (Kendall 2005, Grober-Dunsmore et al 2007, Pittman et al 2007.…”