Life on the edge: Coral reef fishes exhibit strong responses to a habitat boundary

Abstract

Habitat edges commonly support discrete communities compared to adjoining habitats in response to unique boundary conditions. Coral reefs are often adjacent to other habitats, e.g. sand or seagrass meadows, but little is known about how reef-associated organisms respond to the presence of edges. Here, we examined fish communities and benthic assemblages at varying distance from a coral reef−sand edge. At 25 sites, 30 m transects were placed at 5 locations: 1 along the edge and at 5 m and 10 m away from the edge into reef and sand habitats. Counts were made of 51 fish species and benthic composition surveyed. The fish community along the edge was substantially different to communities only 5 m into the sand and reef habitats. Overall, 61% of the 51 fish species appeared to respond to the edge, with 43% significantly higher or lower in abundance along the edge compared to other locations. Twelve species showed positive edge responses, including the wrasse Thalassoma lunare, which was 30 times more abundant along the edge compared to 10 m into the reef. Ten species exhibited negative edge responses, including the damselfish Pomacentrus moluccensis, which was 7 times more abundant 10 m into the reef compared to the edge. Feeding ecology appears to influence edge-related distribution patterns, with carnivores and omnivores dominating the edges and planktivores more abundant into the reef. These findings suggest that habitat edges may play an important role in structuring coral reef fish communities. Future research should focus on experimental approaches to identify the underlying mechanisms driving edge responses.

Damselfishes alleviate the impacts of sediments on host corals

“…This notion is supported by previous results which have highlighted that the positive impacts of aggregating damselfishes on coral growth are highest in sand patches and reef slope/base areas [50,83]. Moreover, D. aruanus, P. moluccensis and other coral-inhabiting damselfishes are most commonly found on corals located in sheltered (flow less than 21.2 cm s −1 , see [84,85]), reef/sand edge environments [83,86]. Sheltered sites with low hydrodynamic energy facilitate the settlement of finer sediments suspended in the water column [37,55], maximizing sedimentation rates that can lead to the smothering of corals, a common phenomenon on many inshore reefs [31].…”

“…Consequently, the positive net effect of hosting damselfishes on corals [2] is likely to be heavily context-dependent and may be particularly important for specific coral colonies on sheltered, inshore reefs, where negative impacts of nutrientladen terrigenous sediments are the most pervasive [81,82]. This notion is supported by previous results which have highlighted that the positive impacts of aggregating damselfishes on coral growth are highest in sand patches and reef slope/base areas [50,83]. Moreover, D. aruanus, P. moluccensis and other coral-inhabiting damselfishes are most commonly found on corals located in sheltered (flow less than 21.2 cm s −1 , see [84,85]), reef/sand edge environments [83,86].…”

Historical Maps provide insight into a century and a half of habitat change in Fijian coasts

“…Increased habitat fragmentation has been shown to increase immigration and increase coral recruit survival (Bonin et al., 2011 ). Edge habitat supports fish species that prefer sandy habitat and can be more complex than interior reef habitat (Ault & Johnson, 1998 ; Friedlander & Parrish, 1998 ) although edge habitat has been observed to have lower coral cover and more soft coral (Sambrook et al., 2016 ). Increased habitat heterogeneity and patchiness may result in increased diversity at fine scales, although the limit of this phenomenon is not known and some of this increased richness may come at the cost of interior‐adapted species like damselfish and other planktivores (Sambrook et al., 2016 ).…”

“…Edge habitat supports fish species that prefer sandy habitat and can be more complex than interior reef habitat (Ault & Johnson, 1998 ; Friedlander & Parrish, 1998 ) although edge habitat has been observed to have lower coral cover and more soft coral (Sambrook et al., 2016 ). Increased habitat heterogeneity and patchiness may result in increased diversity at fine scales, although the limit of this phenomenon is not known and some of this increased richness may come at the cost of interior‐adapted species like damselfish and other planktivores (Sambrook et al., 2016 ). Historical estimates of coral habitat loss can be used to assess site vulnerability, identify causes of loss, and develop effective conservation efforts (e.g., Bromberg & Bertness, 2005 ).…”

Differential Occupation of Available Coral Hosts by Coral-Dwelling Damselfish (Pomacentridae) on Australia’s Great Barrier Reef

“…Many damselfish species exhibit 'clumped' or 'patchy' distributions, leading to increased fish-coral interactions with increased fish abundance [47]. Sand patch and slope/base habitats, often categorized as edge habitats [43] with lower coral cover, host more fish-coral interactions and allow for more 'open' colonies, rather than nested corals along continuous reefs [39,93]. The isolation and spacing of colonies occupied may allow for: (a) continual use and residency by fish (i.e., distance to nearest available habitat is beyond the fish's home range); (b) increased impacts of association defense and reduction of fish predation [9,[94][95][96]; (c) access to plankton resources and reduced competition [9,[97][98][99]; and (d) larger borders with sandy substrates as an alternative foraging substrate [100].…”