Pratchett Morgan S.

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Morgan S.

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  • Preprint
    The use of specialisation indices to predict vulnerability of coral-feeding butterflyfishes to environmental change
    ( 2011-07) Lawton, Rebecca J. ; Pratchett, Morgan S. ; Berumen, Michael L.
    In the absence of detailed assessments of extinction risk, ecological specialisation is often used as a proxy of vulnerability to environmental disturbances and extinction risk. Numerous indices can be used to estimate specialisation; however, the utility of these different indices to predict vulnerability to future environmental change is unknown. Here we compare the performance of specialisation indices using coral-feeding butterflyfishes as a model group. Our aims were to (i) quantify the dietary preferences of 3 butterflyfish species across habitats with differing levels of resource availability; (ii) investigate how estimates of dietary specialisation vary with the use of different specialisation indices; (iii) determine which specialisation indices best inform predictions of vulnerability to environmental change; and (iv) assess the utility of resource selection functions to inform predictions of vulnerability to environmental change. The relative level of dietary specialisation estimated for all three species varied when different specialisation indices were used, indicating that the choice of index can have a considerable impact upon estimates of specialisation. Specialisation indices that do not consider resource abundance may fail to distinguish species that primarily use common resources from species that actively target resources disproportionately more than they are available. Resource selection functions provided the greatest insights into the potential response of species to changes in resource availability. Examination of resource selection functions, in addition to specialisation indices, indicated that Chaetodon trifascialis was the most specialised feeder, with highly conserved dietary preferences across all sites, suggesting that this species is highly vulnerable to the impacts of climate-induced coral loss on reefs. Our results indicate that vulnerability assessments based on some specialisation indices may be misleading and the best estimates of dietary specialisation will be provided by indices which incorporate resource availability measures, as well as assessing responses of species to changes in resource availability.
  • Preprint
    Trade-offs associated with dietary specialization in corallivorous butterflyfishes (Chaetodontidae: Chaetodon )
    ( 2007-10-25) Berumen, Michael L. ; Pratchett, Morgan S.
    Increasing dietary specialisation is an inherently risky strategy because it increases a species’ vulnerability to resource depletion. However, risks associated with dietary specialisation may be offset by increased performance when feeding on preferred prey. Though rarely demonstrated, highly specialised species are expected to outperform generalists when feeding on their preferred prey, whereas generalists are predicted to have more similar performance across a range of different prey. To test this theory, we compared growth rates of two obligate coral-feeding butterflyfishes (Chaetodon trifascialis and C. plebeius) maintained on exclusive diets of preferred versus non-preferred prey. In the field, C. trifascialis was the most specialised species, feeding almost exclusively on just one coral species, Acropora hyacinthus. Chaetodon plebeius meanwhile, was much less specialised, but fed predominantly on Pocillopora damicornis. During growth experiments, C. trifascialis grew fastest when feeding on A. hyacinthus and did not grow at all when feeding on less preferred prey (P. damicornis and Porites cylindrica). Chaetodon plebeius performed equally well on both A. hyacinthus and P. damicornis (its preferred prey), but performed poorly when feeding on P. cylindrica. Both butterflyfishes select coral species that maximise juvenile growth, but contrary to expectations, the more specialised species (C. trifascialis) did not outperform the generalist (C. plebeius) when both consumed their preferred prey. Increased dietary specialisation, therefore, appears to be a questionable strategy as there was no evidence of any increased benefits to offset increases in susceptibility to disturbance.
  • Preprint
    Relative gut lengths of coral reef butterflyfishes (Pisces: Chaetodontidae)
    ( 2011-05-14) Berumen, Michael L. ; Pratchett, Morgan S. ; Goodman, B. A.
    Variation in gut length of closely related animals is known to generally be a good predictor of dietary habits. We examined gut length in 28 species of butterflyfishes (Chaetodontidae), which encompass a wide range of dietary types (planktivores, omnivores, corallivores). We found general dietary patterns to be a good predictor of relative gut length, although we found high variation among groups and covariance with body size. The longest gut lengths are found in species that exclusively feed on the living tissue of corals, while the shortest gut length is found in a planktivorous species. Although we tried to control for phylogeny, corallivory has arisen multiple times in this family, confounding our analyses. The butterflyfishes, a speciose family with a wide range of dietary habits, may nonetheless provide an ideal system for future work studying gut physiology associated with specialisation and foraging behaviours.
  • Preprint
    Habitat associations of juvenile versus adult butterflyfishes
    ( 2007-10) Pratchett, Morgan S. ; Berumen, Michael L. ; Marnane, M. J. ; Eagle, J. V. ; Pratchett, D. J.
    Many coral reef fishes exhibit distinct ontogenetic shifts in habitat use while some species settle directly in adult habitats, but there is not any general explanation to account for these differences in settlement strategies among coral reef fishes. This study compared distribution patterns and habitat associations of juvenile (young of the year) butterflyfishes to those of adult conspecifics. Three species, Chaetodon auriga, Chaetodon melannotus, and Chaetodon vagabundus, all of which have limited reliance on coral for food, exhibited marked differences in habitat association of juvenile versus adult individuals. Juveniles of these species were consistently found in shallow-water habitats, whereas adult conspecifics were widely distributed throughout a range of habitats. Juveniles of seven other species (Chaetodon aureofasciatus, Chaetodon baronessa, Chaetodon citrinellus, Chaetodon lunulatus, Chaetodon plebeius, Chaetodon rainfordi, and Chaetodon trifascialis), all of which feed predominantly on live corals, settled directly into habitat occupied by adult conspecifics. Butterflyfishes with strong reliance on corals appear to be constrained to settle in habitats that provide access to essential prey resources, precluding their use of distinct juvenile habitats. More generalist butterflyfishes, however, appear to utilise distinct juvenile habitats and exhibit marked differences in the distribution of juveniles versus adults.
  • Preprint
    Differences in demographic traits of four butterflyfish species between two reefs of the Great Barrier Reef separated by 1,200 km
    ( 2011-10) Berumen, Michael L. ; Trip, E. D. ; Pratchett, Morgan S. ; Choat, J. Howard
    Many species demonstrate variation in life history attributes in response to gradients in environmental conditions. For fishes, major drivers of life history variation are changes in temperature and food availability. This study examined large-scale variation in the demography of four species of butterflyfishes (Chaetodon citrinellus, C. lunulatus, C. melannotus, and C. trifascialis) between two locations on Australia’s Great Barrier Reef (Lizard Island and One Tree Island, separated by approximately 1200km). Variation in age-based demographic parameters was assessed using the re-parameterised von Bertalanffy growth function. All species displayed measurable differences in body size between locations, with individuals achieving a larger adult size at the higher latitude site (One Tree Island) for three of the four species examined. Resources and abundances of the study species were also measured, revealing some significant differences between locations. For example, for C. trifascialis, there was no difference in its preferred resource or in abundance between locations, yet it achieved a larger body size at the higher latitude location, suggesting a response to temperature. For some species, resources and abundances did vary between locations, limiting the ability to distinguish between a demographic response to temperature as opposed to a response to food or competition. Future studies of life histories and demographics at large spatial scales will need to consider the potentially confounding roles of temperature, resource usage and availability, and abundance / competition in order to disentangle the effects of these environmental variables.