Patchy habitats are good for fish

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Current wisdom contends that habitat fragmentation leads to a reduced species diversity, but research involving the painstaking construction of artificial reefs with bare hands is providing evidence to suggest otherwise.

The effects of habitat loss and habitat fragmentation on biodiversity are often confounded because disturbances often cause both the loss and fragmentation of habitat.

The study by Mary Bonin, Glenn Almany and Geoffrey Jones, which is published in a recent issue of the journal Ecology, seeks to experimentally separate the effects of habitat fragmentation and loss and examine the independent and interactive effects of these processes on survival, abundance, and species richness of recruitment-stage, coral-associated fishes.

The authors carried out their study in a shallow, sandy area of Kimbe Bay in Papua New Guinea (that had no naturally-occurring coral reef), where they constructed 30 artificial reefs from coral rubble moved from shore in boats to which live colonies of the common bottlebrush coral Acropora subglabra were attached. Each reef was cleared of existing fishes and stocked with 20 Goldtail demoiselles (Chrysiptera parasema).

The reefs were then subjected to one of five habitat manipulation treatments (with six replicates per treatment):

1. control (left undisturbed throughout the study);
2. disturbance control (habitat removed, shaken by divers for 2 minutes, and replaced);
3. fragmentation (reef divided into three equal-sized fragments arranged in a triangular pattern with fragments separated from each other by 1 m);
4. 75% loss (live coral area reduced from 1 m2 to 0.25 m2), and
5. 75% loss plus fragmentation (live coral area reduced from 1 m2 to 0.25 m2 and reef divided into three equal-sized fragments arranged in a triangular pattern with fragments separated from each other by 1 m).

Monitoring and recording the species richness of fish over a period of 16 weeks, the authors found that the species richness was the lowest in the reefs subjected to 75% loss, which was as expected.

More surprisingly, they discovered that the fragmented reefs had greater abundance and species abundance of fishes compared to the control (non-fragmented) reefs over the study period.

The authors hypothesise that fragmented habitats reduce competition among fishes, allowing a greater chance for competitively weaker species to settle and seek refuge in areas that would otherwise be settled by competitively stronger species.

The management implications of this study are evident: the greater diversity brought about by fragmentation allows for reef managers to restore badly damaged coral communities by mimicking its effects. 

"The fact that habitat patchiness can have a positive effect on fish diversity is really exciting because it means that even if it isn’t possible for managers to restore an entire coral reef, it will still be highly beneficial to restore small patches of habitat," according to author Mary Bonin.

For more information, see the paper: Bonin, MC, GR Almany and GP Jones (2011) Contrasting effects of habitat loss and fragmentation on coral-associated reef fishes. Ecology 92, pp. 1503–1512.

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