Habitat association, disturbance dynamics, and the role of spatial scale in structuring coral reef fish assemblages

Syms, Craig (1998) Habitat association, disturbance dynamics, and the role of spatial scale in structuring coral reef fish assemblages. PhD thesis, James Cook University.

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Habitat association, disturbance dynamics, and the role of spatial scale in structuring coral reef fish assemblages. Understanding how patterns and processes at one scale are related to those at other scales is of central importance in developing ecological theory. However, in order for scaling rules to be useful to empirical ecologists, they must have a rational, measurable, and critically examinable basis. In this study I consider the role spatial scale plays in structuring coral reef fish assemblages and how habitat structure may mediate scaling rules for the assemblage.

The relationship between a population's mean and variance provide a measure of whether that population is indeed scale dependent. I counted fish and measured habitat variables in 701 transects, allocated across 12 habitat zones. Slopes of the power plots for most species lay between 1 and 2 which indicated that the variance, as a proportion of the mean, of small samples was lower than in large samples and hence scale-dependent. 28% of the variation of the data set was explained by habitat variables which indicated that a large percentage of the scale dependence could be modelled by habitat variables alone.

Peaks of variability associated with changes in scale are indicators of the scales at which organisms are spatially structured. It has been hypothesised that coincident variance peaks are indicators of common scales of organisation and thus should also correspond to the scale of maximal correlation. I tested this idea by quantifying fish-habitat associations at different scales on contiguous coral reefs. I mapped fish and habitat to a 3x3m resolution in 24 30x30m grids and then progressively aggregated adjacent squares and recalculated the correlation between fish and benthic cover, physical reef structure, and locality over 9 spatial scales ranging from 9-225m ². Both fish and habitat variables were patchy at the smallest scale, yet maximal correlation occurred at larger scales (>54m ²). A complex suite of responses were found among fish taxa, with some species associated simply with benthic cover and locality, while others were associated with complex interactions between different types of habitat measures. The scale of maximal correlation was not indicative of the scale at which fishes responded to their environment. Maximal correlation was found when the likelihood of the occurrence of a particular fish species and the likelihood of the occurrence of preferred habitat type were symmetrised. In other words, the scale of measurable fish-habitat association was a measurement of the optimal scale at which predictability of fish given habitat type, and predictability of habitat type given fish were maximised.

Studies carried out on small patch reefs have provided the basic information from which much ecological theory of coral reef fishes has been derived. However no published studies have attempted to document what scaling effects exist in coral reef systems, and whether we can extrapolate or interpolate between studies carried out on different scales. I mapped randomly selected patch reefs, ranging in size from 0.26m ² to 63.5m ², and censused the resident fish fauna. I then partitioned variation amongst reef area, reef shape and patchiness, and benthic cover. Species responded in a variety of ways to reef parameters. Some species were strongly area-dependent, others were well predicted by reef shape and patchiness, and a considerable number of species were well predicted by the benthic cover of the reef. Further groups of species were associated with combinations of these factors. In order to measure the effect of scaling up or down, I divided the data set into small, medium, and large reefs, recalculated regression equations and measured the predictive ability of each equation. Surprisingly equations derived from the smaller reefs were better predictors of larger reefs than vice versa. As a consequence, the lessons drawn from experiments carried out on small reefs can, in the light of prior information, be cautiously, and with strong caveats, applied to large reefs. Central to these scaling rules is the incorporation of habitat as an explanatory variable.

To establish the bounds within which habitat may influence fish assemblage structure, I carried out two experiments. First, I experimentally reduced coral cover in 10x10m quadrats on contiguous reef from 55% to 47%, 43%, and 34% and monitored the assemblage over two years. Contrary to what might be expected from many correlative studies, all fish species considered were resistant, at this scale and level, to habitat disturbance. However, a large portion of variation in the fish assemblage was explainable by spatial and temporal variables. It is hypothesised that spatial-temporal structure at the landscape level may moderate local disturbance to habitat structure on contiguous reef.

The second disturbance experiment was carried out on small patch reefs. To re-evaluate the current models of reef fish assemblage organisation, I implemented a factorial combination of direct (by fish removal) and indirect disturbance (by habitat alteration) and monitored the experiment over two years. Habitat disturbance generated strong, predictable changes in the fish assemblage which explained almost half the variation in the data set. In contrast, direct disturbance generated a lesser and shorter-term effect. The results from this experiment supported a model of reef fish assemblages as deterministic (within broad bounds), yet weakly interacting systems, the determinism of which was mediated by habitat.

This study supports the initial premise that scaling rules for coral reef fish assemblages are mediated by habitat. As a consequence, habitat structure must be included into a general theory of coral reef fish ecology. An important precursor to the successful incorporation will be the parameterisation of the spatio-temporal dynamics of habitat structure, and the scales and forms of responses to habitat disturbance that fishes can be expected to make. Scale, far from being a black-box within which incongruous results are filed, can exert rational, mechanistic effects which can be incorporated both into the theoretical and empirical development of coral reef fish ecology.

Item ID: 24128
Item Type: Thesis (PhD)
Keywords: coral reefs; ecology; fishes; habitats; Lizard Island; populations; space; structure
Date Deposited: 13 Dec 2012 02:06
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 100%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 100%
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