Intertidal areas (e.g. mangroves, salt marshes and sub-tidal sands and mudflats) provide habitat for a myriad of invertebrates (animals without backbones) including insects, worms, molluscs and crustaceans. Changes in the abundance, diversity, biomass and species composition of intertidal invertebrates can indicate important changes in the coastal environments of which they are a part, and can have effects that cascade to other trophic levels. Changes in the species composition of invertebrates in mangroves, salt marshes and sub-tidal sands and mudflats are suggested indicators for State of the Environment reporting (e.g. Indicator's 2.7, 2.8 & 2.6 respectively in the Estuaries and the Sea volume) .
Photo 1. This species of crab, Helograpsus haswellianus, is referred to as a grapsid or shore crab. Their burrowing activities are very important for the maintenance of shore sediments as the 40 cm deep burrows enable saltwater infiltration, thus increasing the surface area of the shore available for chemical exchange by many times. The crabs grow to a maximum carapace width of about 30 mm and can travel within a home range of around 20 m from their main burrow. These crabs are highly adapted to a terrestrial life and are one of the most successful macroinvertebrate groups on the marsh (photo by Mark Breitfuss, Australian School of Environmental Studies, Griffith University).
Intertidal invertebrates are susceptible to the range of pressures that affect the quality and extent of mangrove, saltmarsh and sub-tidal sand and mudflat habitats. In fact, shifts in species composition of groups of organisms are often more sensitive indicators of ecosystem perturbation than changes in ecosystem function (e.g. production, decomposition and nutrient cycling) . Some specific anthropogenic threats to intertidal invertebrates include sediment contaminants such as oil pollution , heavy metals and other toxicants , low-salinity stormwater (see hermit crabs) and low-pH runoff from acid sulfate soils (with high dissolved aluminum and iron loads) .
Invertebrates are integral in the structure, health and functioning of intertidal habitats. For example, some intertidal invertebrates hold important positions in detrital food chains . In processing detritus, they also play a role in carbon and nutrient cycling and in the transfer of energy to higher trophic levels . Their faeces can also support coprophagous (faeces-based) food chains that may extend to coastal waters . In addition, burrowing by intertidal invertebrates locally aerates the soil, and creates conduits for water and nutrient exchange . These effects, and grazing on propagules, wood and leaves, play an important role in the succession of mangrove plant species , and in nutrient recycling and habitat productivity .
Changes in intertidal invertebrate abundances can be assessed from time series analysis of species counts obtained from regular quadrat sampling . Monitoring for change is reasonably straightforward. The real problem lies in interpreting the CAUSES of such changes, particularly when the monitoring has been for CHANGE. The detection of cause is an experimental design issue and cannot be reached via simple monitoring. There are some very well documented approaches that now allow temporal and spatial variation to be incorporated into the experimental designs so that impacts caused by humans can be detected [16-20], in a context of a naturally variable world [21-22]. These have been tested in a number of situations and work. Moreover, examining the entire assemblage/community tends to provide a more powerful test of whether there has been a human impact (still making use of appropriate experimental designs) than monitoring a single population, especially when the population may have been chosen because it was believed to be an "indicator". The excellent work done in Europe (Plymouth Marine Laboratories) has shown this very clearly and even developed new statistical techniques that can be used to analyse the community data.
Crabs are conspicuous and dynamic invertebrates in intertidal habitats, particularly in mangroves. Under some circumstances, counts of crab burrows offer a simple, rapid and non-destructive sampling technique [12,13,14]. For example, significant relationships between burrow numbers and crab abundance, and between burrow diameter and carapace width have been shown for some species [12,13]. However, crab hole counts do not equate with absolute crab populations so calibration is required. Moreover, burrow counts are only valid for some species (e.g. Heloecius) and cannot be generically applied. Crab burrow counts are also used to calculate soil aeration indices in community monitoring programs (e.g. Waterwatch) .
Museums, universities and other research institutions and state governments have information on intertidal invertebrate faunas. However, there are virtually no field guides and taxonomic keys available for the greater bulk of marine invertebrates in Australia. The Queensland Museum has released their guide to Moreton Bay and this is a good start but unfortunately only covers the larger more obvious animals for one region.
More information on biota removal/disturbance.
Whether shifts in the species composition of invertebrate assemblages occur prior to the loss of critical habitat areas is worthy of exploration.