Studies of highly mobile marine animals, such as fish and decapod crustaceans have been rarely undertaken at scales appropriate to the way the animals use their environment. A major limitation of many studies is that they have focused on: 1) a single scale for animals that respond to their environment at multiple scales; or 2) a single habitat type for animals that use multiple habitat types. As a result, very little is known about the multi-scale environmental variables that influence animal distribution, abundance and assemblage composition, including the structure of the marine landscape mosaic. This study utilises landscape ecology concepts and methods to address these limitations.
The outcomes of the study indicate that, for some species found using mangroves at high tide, the composition of substratum adjacent to the mangroves was relatively more important than the composition and spatial arrangement of plants and sediments within the mangroves. At the assemblage level, density and number of species was influenced most by the spatial arrangement and core area of patches of mangrove plants. For species using vegetated intertidal flats at high tide, the proportion of seagrass cover in the mosaic, particularly the amount of long-leaved seagrasses was most important. Overall, inshore areas with a mosaic of mangroves and adjacent seagrasses supported more species and higher densities. Examination of the density of the most abundant species and the proportion of seagrasses within the surrounding mosaic revealed that abrupt changes did not occur at approximately 60% suitable habitat cover as proposed by Percolation Theory. However, distinct cut-off points did exist (mostly <20%) below which, seagrasses ceased to function as habitat for many animals.
During a cyanobacterial (Lyngbya majusculd) bloom that occurred during the study, animal density, live mass and number of species were significantly lower and this was particularly marked in continuously vegetated inshore areas. Small-bodied, epibenthic resident species were most susceptible, with greatest during-bloom declines and lowest post-bloom recovery. However, whilst some species declined, others increased in density. Complex interactions between the bloom and nekton at all stages of its development appeared to determine the observed changes.
The approach for this study involved the development of a conceptual framework that critically evaluated the suitability of several of the most fundamental concepts in animal ecology. Spatially-explicit concepts drawn from hierarchy theory and landscape ecology were then applied to develop an alternative organism-based perspective of animal-environment relations, which deals with the problem of scale and environmental heterogeneity. Animal movement patterns (that is, the extent of the tidal excursion) are considered important in the selection of appropriate scales for measuring functional heterogeneity in the environment.
The study area within Moreton Bay, southeast Queensland, Australia encompasses a landscape mosaic with seagrasses, mangroves and unvegetated sand and mud varying in composition and spatial arrangement. Field sampling of fish and prawns integrates conventional sampling techniques of marine ecology with spatial stratification to quantify species density, biomass and diversity across the study area using a combination of buoyant pop-netting and beam trawling techniques. Horizontal and vertical substratum structure was quantified using a combination of ground surveys, underwater videography, aerial photography and satellite Thematic Mapper data. All data were spatially and temporally referenced, allowing integration within a Geographic Information System. The spatial pattern of substratum structure was quantified using a unique process termed -ecological mapping' This process incorporates key animalenvironment relations at the digitisation and classification stages of map production, followed by the application of landscape metrics to measure mosaic pattern at the level of the benthic habitat class and the whole mosaic. This is the first study to apply such metrics in order to examine the importance of mosaic structure to marine animals.
The research outcomes are important because they indicate that the composition and spatial patterning of the surrounding landscape mosaic influence marine nekton and that species respond differently to structure in the environment, and at their own unique set of spatial scales. However, different species are also likely to respond similarly to a single variable or set of variables at a particular scale. For instance, fine-scale variables of the substratum, such as leaf length, are more significant for resident epibenthic species than for transient demersal species. Therefore, life-history strategy and anatomical, physiological and behavioural responses will interact to determine the linkages between an animal and its environment. The study concludes that environmental variables (abiotic and biotic) are important at a range of spatial scales, highlighting the need for a hierarchical landscape approach to the ecological investigation of species-environment relations and for the development of effective and appropriately scaled conservation strategies. Further studies that adopt a similar multiscale approach are urgently needed to determine the most important focal scales for conservation and resource management strategies.