Relationships among depth distributions, habitat specialisation and demography in coral reef fish communities

Jankowski, Matthew William (2015) Relationships among depth distributions, habitat specialisation and demography in coral reef fish communities. PhD thesis, James Cook University.

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Patterns in the distribution and abundance of organisms in nature vary along environmental gradients, such as altitude and depth. These patterns can be determined by the degree of habitat specialisation and environmental constraints of demographic rates. On coral reefs, despite many studies identifying depth as a major source of variation in coral reef fish, few studies have solely focused on the determinants of depth distributions and their relationships with the degree of habitat specialisation and key demographic parameters. Understanding these relationships is becoming critically important in assessing long-term responses to declining coral cover in shallow water and the potential for deep reefs to offer a refuge. The aim of this thesis was to investigate patterns in distribution, habitat use and specialisation with depth in coral reef fishes and to consider how these patterns might affect the depth refuge hypothesis.

Coral reef structure varies with both depth and reef profile, from vertical walls to flat shelf habitats. The differing roles of depth and reef profile in reef fish distribution patterns have received little attention. This was addressed in Chapter 2 by surveying the reef fish community and benthic habitat in quadrats on 3 profiles at 2 depths. Depth and profile showed strong correlations with both the reef fish community and the benthic habitat present. Wall habitats were most distinct from other profiles in terms of community structure, whereas differences between communities on slopes and shelves were driven by depth differences. Depth and profile had similar effects on the benthic habitat structure. A significant correlation between changes in the fish community and changes in the benthic habitat was best explained by differences in the abundance of sand, branching coral, encrusting coral, massive coral and rubble. Depth and profile had a significant interaction suggesting both are clearly important factors affecting reef fish communities. Benthic habitat however appears to only partially explain these patterns.

The ecology of reef fish communities is likely to change along depth gradients. Trends in diversity, community structure and coral reliance have seldom been described. Deeper reef environments could potentially provide refuge for reef fish from shallow water disturbances. In Chapter 3, fish communities were surveyed along a depth gradient (0-20m) down to the bottom of the reef. Communities had species with shallow water distributions as well as species with much broader depth distributions. Diversity showed linear decline with increasing depth. Depth ranges of species were largest for species with mean depths of occurrence around the midpoint of the gradient and species at the extremes of the gradient showed restricted depth ranges. Niche breadth decreased with depth, suggesting deeper species were more specialised. Unexpectedly, there was a higher association with branching corals in the deepest depth strata showing a great reliance on coral in the patchy reef edge habitat. Clearly, there are dramatic changes in the ecology of reef fishes and their habitat over a depth gradient of 0-20m, and a variety of physical and biological processes are likely to be important. Although deep reef coral-specialists may occupy a refuge from shallow water disturbances, the narrow distributions of species at the bottom of the reef and high reliance on corals are unlikely to contribute to long-term resilience in relation to widespread reef degradation.

Patterns in distribution and abundance along depth gradients may be dependent on changes to habitat structure or other environmental factors. In Chapter 4, the separate effects of depth and habitat changes were isolated experimentally by quantifying the community of reef fish that developed on patch reefs involving 4 habitat types, over 3 different depths and at 2 different sites. Depth was strongly correlated with the communities present. Site was also strongly correlated with fish communities. Habitat had a significant correlation with the fish communities present, although this was weaker than depth or site. Depth had an effect on the proportions of different feeding guilds of fish within communities, with herbivores and planktivores more prevalent on shallow patch reefs. Diversity and species richness were highest on shallow patch reefs at site 1 and on the deep and mid patch reefs at site 2. Communities on Mid depth patch reefs at site 1 and deep patch reefs at site 2 showed the highest evenness. This study further demonstrates the important effects depth can have on reef fish communities independently of habitat changes, which may also suggest, in terms of community structure, that habitat changes alone, may not explain depth distribution. The complex nature of depth gradients means there are many other physical and biological factors that could be driving these patterns and further study would be needed to assess the importance of these factors.

Coral reef fishes may be able to move into deeper water in response to declining coral cover on shallow reefs. If so, species should be able to shift distributions downward, while maintaining abundance on disturbed reefs. In addition, there should be no adverse costs to being in deeper water in terms of increased mortality, slower growth and reduced condition. Chapter 5 tested these predictions using two different approaches. First, surveys were carried out to compare distribution and abundance of species on reefs with high and low coral cover in shallow water. Secondly, an experiment in which individuals of three damselfish species were transplanted to three different depths was carried out to test the effects of being within and below normal depth ranges on demographic rates and condition. Some species exploited new space and showed higher abundances on the lower coral cover reefs. Only one species (Chromis ternatensis) showed a change in its distribution from being abundant in shallow water to having a deeper distribution. The transplant experiment showed no consistent depth-related patterns in mortality. Overall body condition decreased with depth and growth was higher on deepest reefs. This study suggests a limited capacity to move into deeper water following shallow water disturbances for most species. The underlying mechanisms for higher growth in deep water are unknown, but the results suggest that environmental pressures that cause changes in depth distributions will likely have unexpected demographic consequences.

Item ID: 46281
Item Type: Thesis (PhD)
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 2: Jankowski, M.W., Gardiner, N.R., and Jones, G.P. (2015) Depth and reef profile: effects on the distribution and abundance of coral reef fishes. Environmental Biology of Fishes, 98 (5). pp. 1373-1386.

Chapter 3: Jankowski, M.W., Graham, N.A.J., and Jones, G.P. (2015) Depth gradients in diversity, distribution and habitat specialisation in coral reef fishes: implications for the depth-refuge hypothesis. Marine Ecology Progress Series, 540. pp. 203-215.

Date Deposited: 09 Nov 2016 00:01
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060202 Community Ecology (excl Invasive Species Ecology) @ 50%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060299 Ecology not elsewhere classified @ 50%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 100%
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