Habitat degradation affects the strength of interspecific competition in coral dwelling damselfishes

Background/Question/Methods

A thorough understanding of the processes that regulate and structure ecological communities facilitate predictions about how communities may respond to natural and anthropogenic stresses. As global ecosystems are facing an increase in the frequency, severity and range of disturbances, conservation and management efforts need to be augmented by an increased understanding of how natural ecosystems are disrupted. Shallow coastal marine environments are particularly vulnerable to anthropogenic disturbances due to the close proximity to urban areas. These disturbances have led to a worldwide decline in live coral habitat, with an estimated 30 percent of reefs already seriously degraded. Habitat loss can diminish critical resources and natural biological interactions such as competition may become a dominant force in eliminating species in degrading environments. Although habitat degradation may exacerbate competition for critical resources, the relationship between habitat quality and competitive interactions is poorly understood. In this study we used a manipulative experiment to test the effects of habitat degradation on competition between two planktivorous, coral-dwelling damselfishes; Chrysiptera parasema and Dascyllus melanurus. Patch reefs were constructed with either healthy (100% live) or degraded (10% live) Acropora coral,stocked with varying densities of the two fish species, and monitored after two months. 

Results/Conclusions

Mortality of C. parasema was influenced by a significant interaction between habitat degradation and competition. On healthy reefs, the presence of the competitor D. melanurus increased mortality of C. parasema by 50% compared with reefs where D. melanurus was absent. In contrast, mortality of C. parasema was uniformly high on degraded reefs, regardless of D. melanurus presence. Additionally, C. parasema mortality increased with conspecific density on healthy reefs, but not on degraded reefs. Mortality of D. melanurus was higher on degraded reefs than on healthy reefs, but was not influenced by the presence of C. parasema in either habitat treatment. Behavioural observations revealed that per capita aggressive interactions for both species increased with density on degraded habitat, but not on healthy habitat. These results demonstrate that habitat degradation can alter the outcome of competitive interactions between coral reef fishes, and that instead of exacerbating competition for limited resources it may cause existing competitive hierarchies to collapse. This highlights the profound effect modifications and destruction of habitat has on the processes that shape coral reef fish communities.