ABSTRACT

Most benthic ecological survey techniques are based on a two-dimensional (planar) unit of measurement On substrates that are highly spatially complex, such as coral reefs, this approach might lead to an under-estimation of species diversity due to the existence of understorey and cryptic species. Planimetric measurements will be biased towards species with flat (e.g. encrusting) morphologies and against those with upright (e.g. columnar) morphologies. In this study a comparison was made between coral community structure assessments using three commonly-used survey techniques Two of these techniques, chain transects and visual estimates of 1 m2 quadrants, were aimed at determining the relative surface cover of three-dimensional surfaces The thud, digitalization of photoquandrats, assessed planar or two-dimensional surface area cover, a widely used measure in benthic surveys While the three techniques ranked sites in the same order of percent total coral cover, species diversity rankings were dependent both on the survey technique and the type of index or species abundance distribution used Disparity between the techniques was greatest at a spatially complex site and least at a low-relief site. Since changes in spatial complexity are likely to accompany changes in community structure over time, or between different sites, none of these methods can be viewed as ideal for monitoring coral reef community structure Novel techniques are discussed which will allow true 3-D surface cover measurements to be made from underwater photographs Certain applications of these techniques can be realised now, whereas further developments are needed before community structure can be routinely assessed

INTRODUCTION

Both local and region-wide deterioration of coral reefs continue to be widely reported (Rogers, 1985, Brown & Suharsono, 1990, Smith & Ogden, 1993, Smith et al 1997) Although we cannot draw any definite conclusions about a global effect related to Greenhouse warming (D'Ella et al, 1991), the need for comprehensive long-term monitoring of what may be particularly sensitive ecosystems is now clearly recognized throughout the scientific and resource management community

Researchers on coral reefs have relied largely on methods used for the analysis of plant communities, for example point intersect (Rutzler, 1978; Kinzie & Snider, 1978, Dodge et a 1982), line intersect (Loya & Slobodlun, 1971, Loya, 1972), area coverage (Pearson, 1974; Weinberg, 1978; Bouchon, 1981) or "plotless" methods (Loya, 1978). However, coral reef communities present some special problems for assessing diversity and community structure. Not least, the environment is underwater and access time for field work is strictly limited Much emphasis has therefore been placed on the relative efficiency of different assessment techniques. information gamed versus time or effort expended (Weinberg, 1981, Dodge et al1982; English et al 1994) Weinberg (1981) compared various methods on a reef plot that had previously been completely mapped and analyzed for species abundance distributions, and concluded that visual estimation of percent cover within 1 m2 quadratns was the most accurate technique. These methods all estimate the relative planar (two-dimensional) surface of reef occupied by different species Reefs are extremely topographically complex three-dimensional structures. This structure can be changed by human activities such as chronic oil pollution (Bak 1987).

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