338 Intertidal Ecosystems Site I 40 m Site 40 m Site 40 m Site Figure Variability in the numbers and types of animals living in a mangrove forest in New South Wales, Australia The animals were sampled in four sites along the bank of a river Each site was sampled using four quadrats, placed 1–2 m apart Each point on this figure represents the types and numbers of animals in each quadrat The relative distance between two points indicates how similar the animals were in those two quadrats Points close together represent quadrats containing very similar types of animals Points far apart indicate that the quadrats contained very different numbers and types of animals Figure 3(a) shows that when the animals were sampled the first time, the diversity of animals was highly variable from one quadrat to another Quadrats in the same site (meters apart) showed similar amounts of variability in these measures of diversity as shown by quadrats in different sites Figure 3(b) shows what the patterns looked like only three months later Animals were more similar at the scale of meters (among quadrats in each site), but the sites appeared to represent a gradient along the river This relatively large change in the pattern of diversity was not seasonal, predictable, or found in other nearby sites It was mainly due to changes in the relative numbers of numerous small crustaceans Sandy beaches and mudflats have very few large plants, although heaps of decaying plant material, commonly known as beach wrack, can be found on the strandline This material – decaying seagrasses, seaweeds, and kelps, which have been washed up by the waves – is rapidly decomposed by bacteria, fungi, and small animals, many of which are dependent on wrack for food This activity ultimately releases nutrients back into the coastal waters (Griffiths et al., 1983) Sandy beaches and mudflats appear at first glance to resemble deserts – there are often few animals to be seen, other than birds and a few crabs The birds are generally visitors to these shores, feeding along the shoreline or over the extensive flats during low tide, but spending the rest of their time elsewhere There are relatively few animals high on the shore, mainly semiterrestrial species, such as ghost crabs that spend the day in burrows above the level of high tide, coming out at night to feed Nevertheless, despite first appearances, there is diverse life lower on the shore As is the case in mangrove forests, however, most of the animals live under the sediments Very small animals live in the tiny spaces among the grains themselves These include unicellular and multicellular animals from many different phyla Many of these very small animals resemble juveniles of larger species Many are also extremely simple For example, although multicellular, they may only consist of a few cells and lack structures – such as limbs, kidneys, or a circulatory system – relying on exchange of substances through the body wall to acquire oxygen or eliminate waste (Brown and McLachlan, 1990)