What is biodiversity?
The concept of biodiversity refers to the variety of life on three different levels: genetic diversity; species diversity; and ecosystem diversity.
Genetic diversity refers to the amount of genetic variability between individuals of a given species. Genetic variability may cause differences in behaviour, appearance or physiology that make some individuals of a species less susceptible to a disturbance. Therefore, high levels of genetic variability buffer species against environmental change by increasing the likelihood that at least some individuals will survive. Different populations of species must be protected to conserve genetic diversity.
Species diversityp takes into account the number of different species (e.g. the species richness) and the ‘evenness’ (or equitability) with which the different species are distributed. For example, in two hypothetical communities, each with 100 individuals made up of two species (A and B), a community with 50 individuals of species A and 50 individuals of species B is more diverse than a community with 99 individuals of species A and 1 individual of species B.
Ecosystem diversity refers to the variety of different ecosystems. An ecosystem consists of communities of organisms interacting with each other, and with the physical and chemical features of the environment.
The biodiversity of estuaries may be affected by a variety of factors including size of the estuary, nutrient supply and retention, the complexity of water circulation, the variety and extent of sediment types on the bottom, the range of habitats available and tidal range. As estuaries receive freshwater inflows from land as well as saline waters from the sea, there are few species that are well adapted to cope with all the habitats present in an estuary. The minimum number of species occurs wherever the variation in water salinity is greatest. Species which inhabit the seafloor (benthic forms) are largely controlled by the nature of sea floor sediment present (mud, silt, sand, gravel, rock) and the dissolved oxygen status of bottom waters.
As estuaries receive nutrients from land drainage they may have high levels of productivity and hence increased biodiversity. Where nutrient loads are periodically increased or remain excessive, an increase in biomass and reduction in biodiversity may take place. This results in proliferation in lower levels of the food chain and a reduction in higher levels, thus decreasing overall species diversity while some lower forms proliferate in number.
Biodiversity levels in estuaries are a sensitive measure of their health and balance. Their proximity to human settlements make them particularly vulnerable to the impacts of vegetation clearing in adjacent catchments, urban runoff and the influx of stormwater and wastewater. Each of these flows may contain excessive nutrient loadings that may trigger changes in the distribution and structure of estuarine communities, usually by reducing their complexity.
The densities of animals may be very high in some parts of an estuary e.g. oysters and other encrusting animals at the intertidal zone. This reflects the adaptation of these creatures to a specific habitat and not necessarily the overall level of biodiversity. If weed growth proliferated in place of the oysters or other growths it may be indicative of excessive nutrient levels unsuited to oyster habitation and hence a reduction in biodiversity.
Why is it important to conserve Australia’s biodiversity?
There are several arguments for conserving biodiversity and they involve one of more ethical, aesthetic, utilitarian and economic reasons, including maintenance of fundamental biological processes. Significant reasons for conserving estuarine biodiversity in Australia include:
- maintaining the health and wellbeing of our waterways;
- ensuring that valuable nutrients are retained an recycled for maximum biological use; and
- improving the level of natural biological filtration at the land/sea interface.
- Biodiversity can also enhance the value of international tourism to Australia’s economy.
Our unique and fascinating native plants and animals and the natural Australian landscape play a central role in attracting tourist dollars from other countries. Another economic argument is the potential for plants and animals to provide us with useful compounds and chemicals for medicinal purposes, commercial sources of food and fibre.
It has also been suggested that the conservation of biodiversity need no justification or human benefit, and that humans have no right to cause the extinction of any other species. Another argument is that we don’t know what the environmental repercussions are of losing elements of biodiversity.
What happens if we change biodiversity?
Biodiversity reflects both the variety of species as well as their interactions between them. Hence, any impacts upon some part of the connected web of species will impact upon the remainder. In estuaries, such impacts may comprise loss of habitat due to the spread of human settlement or activities, excessive exploitation of some species for food or recreation and the flow of high nutrient loads as a direct, or indirect, consequence of human activities.
As a simple analogy, imagine a large pyramid shaped pile of oranges just like you see at the fruit markets. Each orange represents a species. The one on top relies on all the other oranges below in order to stay in place (or, for the purposes of our analogy, in order to survive). If you were able to magically remove a particular orange from the middle of the pile what effect would it have? Which other oranges would topple down? It’s very hard to know – until after you remove the orange, after the species has gone and the effects have been felt. You can’t put the orange back after the pyramid has been destroyed.
The take home message is the importance of any one species is very hard to know until it is lost.
Human actives that can change biodiversity
Any disturbance that changes critical habitat areas or species numbers has the potential to alter biodiversity. Such disturbances may include or be associated with habitat modification or destruction, excessive use and predation for recreation or food, habitat destruction and fragmentation, wastewater disposal, oxygen depletion and accumulation of human wastes and compounds. It is worth mentioning however, that the Intermediate Disturbance Hypothesis proposes that biodiversity is highest when disturbance is neither too frequent nor too rare((Connell, J.H. 1978. Diversity in tropical rain forests and coral reefs. Science 199, 1302-1310.)), implying that low levels of diversity can be a natural state. This is true for example, in the near-pristine estuaries located on the west coast of Tasmania((Edgar, G.J., Barrett, N.S., and Last, P.R. 1999b. The distribution of macroinvertebrates and fishes in Tasmanian estuaries. Journal of Biogeography 26, 1169-1189.))((Edgar, G.J. and Barrett, N.S. 2002. Benthic macrofauna in Tasmanian estuaries: scales of distribution and relationships with environmental variables. Journal of Experimental Biology and Ecology 270, 1-24.)). These estuaries have low numbers of species due to a paucity of food((Edgar, G.J., Barrett, N.S., and Last, P.R. 1999b. The distribution of macroinvertebrates and fishes in Tasmanian estuaries. Journal of Biogeography 26, 1169-1189.)) ((Edgar, G.J. and Barrett, N.S. 2002. Benthic macrofauna in Tasmanian estuaries: scales of distribution and relationships with environmental variables. Journal of Experimental Biology and Ecology 270, 1-24.)).
The concept of surrogacy
There is no practical way of measuring Australia’s actual biodiversity. To do this would mean classifying and counting every single organism. This is simply impossible with current resources. Australia’s biodiversity can only be estimated using easily measurable factors that have known and predicable associations with the biota. Such surrogates can be other organisms (biotic) or part of the physical environment (abiotic). Because many abiotic factors are relatively easy to measure (especially spatially), there have been many studies that have linked the spatial abundance and distribution of organisms to these factors, and a distinct set of surrogates is beginning to emerge. For example, in terrestrial ecology latitude, altitude, rainfall, slope and rock type can be used to reliably predict which of 20-30 vegetation types (i.e., grassland, tundra, etc) is likely to occur at any site((Longhurst, A.R. 1998. Ecological Geography of the Sea. Elsevier, NY. 398pp.)). From this we can then predict other organisms that are expected to occur at these sites based on their known associations with that environment. Much less is known about Australia’s marine environment (especially the deep ocean) and a simple set of surrogates does not yet exist. Although the influence of several abiotic variables (e.g., latitude, depth, current speed, substrate type, seabed hardness, rugosity) on seabed marine biodiversity is clear and well documented((Anderson, T.J. and Yoklavich, M.M. 2007. Multiscale habitat associations of deepwater demersal fishes off central California. Fisheries Bulletin 105, 168-179.))((Snelgrove, P.V.R. and Butman, C.A.. 1994. Animal-sediment relationships revisited: cause versus effect. Oceanography and Marine Biology: An annual review 32, 111-177.))((Williams, A. and Bax, N. 2001. Delineating fish-habitat associations for spatially-based management: and example from the south-eastern Australian continental shelf. Marine and Freshwater Research 52, 513-536.)). Further research is being undertaken to derive a simple set of surrogates that can be used to estimate Australia’s marine biodiversity((Post, A.L., Wassenberg, T.J. and Passlow, V. 2006. Physical surrogates for macrofaunal distributions and abundance in a tropical gulf. Marine and Freshwater Research 57, 469-483.)).
A comprehensive suite of indicators (pressures on biodiversity, condition of biodiversity and responses to pressures on biodiversity and condition of biodiversity) were developed as part of biodiversity theme for national State of the Environment reporting.
- a reduction in the areas of seagrasses, mangroves, salt marshes, intertidal mudflat/sand flats and beaches and dunes. Table 1 in the Habitat Variability Index fact sheet shows that mangroves and saltmarshes should occur in tide-dominated coastal waterways ( e.g. tide-dominated deltas, tide-dominated estuaries, and tidal creeks). The absence of mangroves and salt marshes from these waterway types signifies a major disturbance and a probable loss of biodiversity;
- increased frequency of fish kills, algal blooms, or anoxic & hypoxic events;
- a reduction in the diversity of benthic invertebrates, intertidal invertebrates or fish assemblages;
- water column pH values <6 or >~9;
- water column turbidity, chlorophyll a, toxicant or heavy metal levels in excess of ANZECC/ARMCANZ trigger values;
- sediment heavy metal or toxicant levels in excess of ANZECC/ARMCANZ trigger values;
- widely fluctuating diurnal ranges of pH or dissolved oxygen;
- widely fluctuating seasonal ranges of pH, dissolved oxygen and salinity; and
- denitrification efficiencies lower then 57%, benthic carbon dioxide fluxes in excess of 12 mmol m -2 d -1 or sediment TOC:TS ratios < 5.
For more information of biodiversity consult the Biodiversity website at the Department of Environment, Water, Heritage and the Arts.
Some important links with biodiversity content can also be found at the BiologyBrowser web site.
More information on biota removal/disturbance
Donna Hazell, Center for Resource and Environmental Studies, Australian National University
Ian Lavering, Geoscience Australia
Andrew Heap, Geoscience Australia