What are Intertidal Invertebrates?
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)((Ward, T., Butler, E. and Hill, B. 1998. Environmental Indicators for State of the Environment Reporting, Commonwealth of Australia, pp. 81.)).
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).
What causes Intertidal Invertebrates to change?
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)((Weis, J.S. and Weis, P. 2002. Contamination of saltmarsh sediments and biota by CCA treated wodd walkways. Marine Pollution Bulletin 44, 504-510.)). Some specific anthropogenic threats to intertidal invertebrates include sediment contaminants such as oil pollution((Duke, N.C. 1992. Mangrove floristics and biogeography. In Tropical Mangrove Ecosystems. Coastal and Estuarine Studies No. 41., (Eds. Robertson, A.I. and Alongi, D.M.), American Geophysical Union.)), heavy metals and other toxicants((MacFarlane, G.R., Booth, D.J., and Brown, K.R. 2000. The Semaphore crab, Heloecius cordiformis: bioindication potential for heavy metals in estuarine systems. Aquatic Toxicology 50, 153-166.)), low-salinity stormwater (see hermit crabs) and low-pH runoff from acid sulfate soils (with high dissolved aluminum and iron loads)((Sammut, J., Melville, M.D., Callinan, R.B. and Fraser, G.C. 1995. Estuarine acidification: Impacts on aquatic biota of draining acid sulphate soils. Australian Geographical Studies 33(1), 89-100.)).
Significance of Intertidal Invertebrates
Invertebrates are integral in the structure, health and functioning of intertidal habitats. For example, some intertidal invertebrates hold important positions in detrital food chains((MacFarlane, G.R., Booth, D.J., and Brown, K.R. 2000. The Semaphore crab, Heloecius cordiformis: bioindication potential for heavy metals in estuarine systems. Aquatic Toxicology 50, 153-166.)). In processing detritus, they also play a role in carbon and nutrient cycling and in the transfer of energy to higher trophic levels((Camilleri, J.C. 1992. Leaf-litter processing by invertebrates in a mangrove forest in Queensland. Marine Biology 114, 139-145.)). Their faeces can also support coprophagous (faeces-based) food chains that may extend to coastal waters((Lee, S.Y. 1997. Potential trophic importance of the faecal material of the mangrove sesarmine crab Sesaerma mess. Marine Ecology Progress Series 159, 275-284.)). In addition, burrowing by intertidal invertebrates locally aerates the soil, and creates conduits for water and nutrient exchange((Ridd, P.V. 1996. Flow through animal burrows in mangrove creeks. Estuarine, Coastal and Shelf Science 43, 617-625.)). These effects, and grazing on propagules, wood and leaves, play an important role in the succession of mangrove plant species((Smith, T.J. III 1992. Forest structure. In Tropical Mangrove Ecosystems. Coastal and Estuarine Studies No. 41 (Eds Robertson, A.I. and Alongi, D.M.), American Geophysical Union, pp 101-137.)), and in nutrient recycling and habitat productivity((Thomas J. Smith III, Kevin G. Boto, Stewart, D. Frusher, and Raymond L. Giddins 1991. Keystone species and mangrove forest dynamics: the influence of burrowing by crabs on soil nutrient status and forest productivity. Estuarine and Coastal Shelf Science 33, 419-432.)).
Considerations for measurement and interpretation
Changes in intertidal invertebrate abundances can be assessed from time series analysis of species counts obtained from regular quadrat sampling((Ward, T., Butler, E. and Hill, B. 1998. Environmental Indicators for State of the Environment Reporting, Commonwealth of Australia, pp. 81.)). 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((Underwood, A.J. 1991. Beyond BACI: Experimental designs for detecting human environmental impacts on temporal variations in natural populations. Australian Journal or Marine and Freshwater Research 42, 569-588.))((Underwood, A.J. 1992. Beyond BACI: the detection of environmental impacts on populations in the real, but variable world. J. Exp. Mar. Biol. Ecol. 161, 145-178.))((Underwood, A.J. 1993. The mechanics of spatially replicated sampling programmes to detect environmental impacts in a variable world. Aust. J. Ecol. 18, 99-116.))((Glasby, T.M. 1997. Analysing data from post-impact studies using asymmetrical analyses of variance: a case study of epibiota on marinas. Aust. J. Ecol. 22, 448-459.))((Underwood, A.J. 1989. The analysis of stress in natural populations. Biol. J. Linn. Soc. 37, 51-78.)), in a context of a naturally variable world((Underwood, A.J. 1995. Detection and measurement of environmental impacts. In Underwood, A.J. and Chapman, M.G. (eds), Coastal Marine Ecology of Temperate Australia, University of New South Wales Press, Sydney, Australia. pp. 311-324.))((Skilleter, G.A. 1995. Environmental Disturbances. In Underwood, A.J. and Chapman, M.G. (eds), Coastal Marine Ecology of Temperate Australia, University of New South Wales Press, Sydney, Australia. pp. 263-276.)). 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((Warren, J.H. 1990. The use of open burrows to estimate abundances of intertidal estuarine crabs. Australian Journal of Ecology 15, 277-280.))((MacFarlane 2002. Non-destructive sampling techniques for the rapid assessment of population parameters in estuarine shore crabs. Wetlands 20(2), 49-54.))((Barros, F. 2001. Ghost crabs as a tool for rapid assessment of human impacts on exposed sandy beaches. Biological Conservation 97, 399-404.)). For example, significant relationships between burrow numbers and crab abundance, and between burrow diameter and carapace width have been shown for some species((Warren, J.H. 1990. The use of open burrows to estimate abundances of intertidal estuarine crabs. Australian Journal of Ecology 15, 277-280.))((MacFarlane 2002. Non-destructive sampling techniques for the rapid assessment of population parameters in estuarine shore crabs. Wetlands 20(2), 49-54.)). 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)((Department of Natural Resources and Mines. 2002, Waterwatch Queensland Estuarine Monitoring Module (draft), Department of Natural Resources and Mines, Brisbane.)).
Existing information and data
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.
Mark Breitfuss, Australian School of Environmental Studies, Griffith University
Greg Skilleter, Marine and Estuarine Ecology Unit, University of Queensland