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2020 Oct 12;101:17016. doi: 10.1038/s41598-020-73979-0.
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Size-specific recolonization success by coral-dwelling damselfishes moderates resilience to habitat loss.
Increasing degradation of coral reef ecosystems and specifically, loss of corals is causing significant and widespread declines in the abundance of coral reef fishes, but the proximate cause(s) of these declines are largely unknown. Here, we examine specific responses to host coral mortality for three species of coral-dwelling damselfishes (Dascyllus aruanus, D. reticulatus, and Pomacentrus moluccensis), explicitly testing whether these fishes can successfully move and recolonize nearby coral hosts. Responses of fishes to localized coral loss was studied during population irruptions of coral feeding crown-of-thorns starfish, where starfish consumed 29 (34%) out of 85 coral colonies, of which 25 (86%) were occupied by coral-dwelling damselfishes. Damselfishes were not tagged or individually recognizable, but changes in the colonization of different coral hosts was assessed by carefully assessing the number and size of fishes on every available coral colony. Most damselfishes (> 90%) vacated dead coral hosts within 5 days, and either disappeared entirely (presumed dead) or relocated to nearby coral hosts. Displaced fishes only ever colonized corals already occupied by other coral-dwelling damselfishes (mostly conspecifics) and colonization success was strongly size-dependent. Despite movement of damselfishes to surviving corals, the local abundance of coral-dependent damselfishes declined in approximate accordance with the proportional loss of coral habitat. These results suggest that even if alternative coral hosts are locally abundant, there are significant biological constraints on movement of coral-dwelling damselfishes and recolonization of alternative coral habitats, such that localized persistence of habitat patches during moderate or patchy disturbances do not necessarily provide resilience against overall habitat loss.
Figure 1. Changes in availability and use of different branching corals across the four (10 × 10 m) experimental plots from day 0 (orange) to day 16 (grey). Habitat availability (A) is calculated based on the sum of estimated projected areas of live tissue for every colony of each coral taxa (species or morphotype). χ2 statistics (Pearson statistic with Yate’s correction) were used to compare the proportion of corals that were consumed by CoTS relative to occupation by damselfishes for all corals combined (χ2All) and then for P. cf. damicornis separately (χ2Pdam). Changes in habitat use for Dascyllas aruanus (B), Dascyllus reticulatus (C), and Pomacentrus moluccensis (D) are shown based on the total number of fishes recorded across all colonies of each coral taxa. Here, χ2i compares patterns of coral use by each damselfish species on day 0 with proportional availability of the different corals, and χ2ii tests for changes in patterns of habitat use between day 0 and day 16.
Figure 2. Size-dependent responses of (A,B) Dascyllas aruanus, (C,D) Dascyllus reticulatus, and (E,F) Pomacentrus moluccensis to mortality of host coral colonies following the introduction of crown-of-thorns starfish to experimental plots. Persistence (A,C,E) on dead coral hosts (consumed by crown-of-thorns starfish) is measured based on the mean (± SE) time in days that individual fishes remained on coral hosts devoid of any live tissue, which was analyzed using ANOVA (Table 1). The fate of fishes that ultimately vacated dead coral hosts (B,D,F) is expressed as the ratio of fishes that relocated and recolonized alternative corals hosts within the 100 m2 of experimental plots (expressed as a percentage of the total number of fishes displaced from coral colonies that died). χ2 statistics (log-linear statistic) compared frequencies of fish that moved (grey bars) versus disappeared (white bars) within each size class for each damselfish species separately.
Figure 3. Temporal variation in (A,B) areal extent of live tissue across all branching corals and C–H) mean number of fishes per colony (± SE). Data is presented separately (in A,C,E,G) for colonies that died (consumed; red lines) versus survived (green lines) in experimental plots where starfish persisted and consumed at least some of the corals, and for the Lagoon plot (control; blue lines) where there was no mortality of branching corals throughout the study. All data is then aggregated (in B,D,F,H) to test for overall changes in habitat availability and density of fishes, as per separate analyses in Table 2.