Quantitative and experimental study of biodiversity on intertidal shores near and away from urban environments
Professor Tony Underwood, Professor Gee Chapman and Dr Vicky Cole
There is no simple unarguable definition of biodiversity, although we know that true biological diversity is more than simply a list of species. A useful working measure includes the numbers of individuals and types of animals and plants in an area or habitat. Very little is known about biodiversity in marine habitats, even though we know a lot about the ecology of some animals.
Intertidal animals and plants are very patchy in distribution and abundance. There are large numbers in some patches of habitat and small numbers in others. The differences found between patches of shore only centimetres or metres apart can be as great as differences from shore to shore.
Because numbers of individuals vary from place to place at small spatial scales, we can predict that biodiversity (a measure of all of the species) in these habitats will also be most variable at these scales. Knowing the scales at which to measure biodiversity is an important start to being able to describe biodiversity and to understand the factors that influence it.
Nevertheless, there are no quantitative data about variation in biodiversity at different spatial scales in coastal habitats in NSW.
This project is developing robust methods for measuring biodiversity on intertidal rocky shores around Sydney. These measures must include variation in the numbers and the types of animals and plants. These methods are being used to test hypotheses about the scales at which we expect diversity to vary, by making comparisons between patches different distances apart on the same shore and among shores.
There is concern that built structures around cities fragment natural habitat into smaller patches. This may change biodiversity. We have started testing hypotheses about biodiversity and size of habitat by making comparisons among shores of different sizes. We are also testing hypotheses about effects of intervening built structures on biodiversity.
Our methods for measuring biodiversity in these variable and complex habitats can then be used in other coastal habitats, e.g. mangrove forests, subtidal reefs, etc., to test for their general usefulness.
Many different processes could explain differences in biodiversity from place to place. Which are important and which are not can only be determined by appropriate experiments. On rocky shores, it has been possible to do the complex experiments needed to unravel the many processes that might influence biodiversity. This is not possible in most habitats. We are now developing and experimentally testing different models to explain patterns of biodiversity on intertidal rocky shores. The experimental designs and information from these experiments are proving useful for examining biodiversity in other complex habitats.
Some of our initial analyses have shown interesting complexity between patterns in these organisms and the ways in which they are measured. For example, the number of species can be estimated per sampling unit, per site or per shore; each of these incorporates variability at different spatial scales within a shore. Furthermore, the season of sampling also influences estimates of the number of species in an area. No method is "correct"; different methods measure different things.
The presence of a positive trend in the number of species along the coast depended on the spatial scale (particularly, the size of the unit) and the season. Models about differences in the densities of organisms or their distribution in space may explain why such observations were made. Tests of specific predictions derived from these types of models have shown that trends in the number of species along the coast at different spatial scales are influenced by differences in the distribution of invertebrate fauna rather than algae. Moreover, differences in the densities of organisms did not account for the trend. Coastal trends in the number of species may also be influenced by the existence of separate biotopes consisting of different types and numbers of species. Current work is investigating this theory.