Ecology of Sulawesi. Tony Whitten
Читать онлайн книгу.
crabs, insects, snails and even other mudskippers (Burhanuddin and Martosewojo 1978; Mcintosh 1979; Martosewojo et al. 1982).
Most mudskippers occupy deep burrows and some species have burrows with turretted tops (fig. 2.47). All their activities centre on these burrows which are entered briefly throughout the day to moisten the skin and to replace water in the gill pouches. They are usually evenly spaced on the mud thereby reducing the likelihood of conflicts (Stebbins and Kalk 1961), but fights within and between and species are avoided by complex behavioural mechanisms (MacNae 1968). Males dig the burrows and make courtship displays to nearby females by jumping, and by erecting their dorsal fin in the hope of being chosen as a mate. After pairing, eggs are laid on the sides of the burrow which is then defended by the male. Territorial defence is rare outside the breeding season (Nursall 1981; Hutomo and Naamin 1982).
The other main representatives of the epifauna are fiddler crabs Uca which can be observed by waiting patiently on the beach. Population densities of the often colourful crabs can reach 50 per m2. Male fiddler crabs have one huge claw which is useless for feeding because it cannot reach the mouth, and they therefore have to pick organic deposits off the sediment surface at half the rate of the females which have two normal-sized claws. Presumably the males have to eat for twice as long. The large claw is used for warning away males and for attracting females.
Males compete with other males for females rather than for breeding sites because food resources are so rich and potential sites so abundant. Mating occurs near the burrow defended by the female, but the male-female association is very brief. Preliminary observations suggest that large and small, resident and non-resident males all have the same likelihood of breeding, but smaller males produce fewer viable larvae. Since food resources are not limiting and do not have to be defended, promiscuity is possible (Christy and Salmon 1984).
On coarser-grained beaches near the high water level, burrows about 8 cm across can be found with small piles of sand around them. Their occupants are adult, beige-coloured sand crabs Ocypode (fig. 2.27) which are rarely seen during the day. It is extremely difficult to dig these crabs out of their burrows or to catch them as they run across the sand. Young Ocypode are very numerous, and can be seen on the sand surface both by day and night. Ocypode feed mainly on organic material in the sand, but are sometimes predatory on small crustaceans.
On wider beaches the small ghost crabs Dotilla may occur in thousands with densities of over 100/m2 (Mclntyre 1968; Hails and Yaziz 1982). Although some of the larger individuals are coloured light blue with pinkish legs, the majority are sand-coloured. As the tide rises and covers the beach, each ghost crab builds a shelter of wet sand pellets over its back. Air becomes trapped beneath the crab, and as it burrows down so the air pocket is carried down with it. The crabs emerge as the tide falls, and they are followed by a stream of small bubbles.
Isopod crustaceans can be found by careful examination of the sand and the organic material washed ashore onto the upper shore, and wading birds can sometimes be seen feeding on these and other small animals.
Lower down the beach a variety of molluscs occur but are rarely seen because they burrow beneath the surface. Examples of the bivalves are the white and pinkish Tellina, the large Pinna, and the economically important edible cockle Anadara granosa. This bivalve mollusc spawns seasonally, and breeding seems to be triggered by a drop in water salinity at the start of the wet season. Since plankton and algae on mud always seem to be available, the reason for synchronous breeding is probably to ensure maximum fertilization of the eggs, and to reduce the chance of any one larva being eaten by swamping the potential predators (Broom 1982).
Shorebirds
In addition to four species of resident shorebirds, at least 34 migratory species visit Sulawesi's coasts twice each year. They can be seen between February and April and between September and November, on their way to and from their breeding grounds in northeastern and eastern Asia and their wintering grounds, possibly in northwestern Australia (White 1975). One species, the Australian courser Stiltia isabella, migrates from the south between February and April, and returns between September and November (table 2.5; fig. 2.28). These birds would most often be encountered on muddy rather than sandy shores (fig. 2.29).
Figure 2.27. An Ocypode crab, a common member of the beach epifauna.
Very little is known about the movements of these birds within Indonesia and the basic questions posed thirty years ago have barely begun to be answered. That is: What are the normal migration routes? How many birds are there (Coomans de Ruiter 1954)? EoS teams had the opportunity to work with an ornithologist from Interwader, an international shore-bird study programme, during the first part of 1986, and two areas of mudflat were visited: the north of Bone Bay, and the coast north and south of Watampone. The northern site had extensive mangroves but the mud was rather sandy and, therefore, not especially suitable for waders. One exception was the muddy estuary of the Balease River where at least 18 species were seen, four of which constituted about half of the total number of birds seen. The coasts around Watampone were found to have less sand than in the north, and the shorebirds were consequently more common though of fewer species (Uttley 1986).
* Indicates resident species
White and Bruce 1986
Figure 2.28. Some waders that visit Sulawesi shores, a - broad-billed sandpiper Limicola falcinellus; b - grey-tailed tattler Tringa breviceps; c - rufous-necked stint Calidris ruficollis.
After Beadle 1985
Figure 2.29. Areas of mudflats, the habitat most often visited by shorebirds (indicated in black).
After Salm and Halim 1984
From the above it is clear that the physical composition of the sediment influences the numbers of shorebirds feeding upon it, but within a suitable area of mud it is still not known precisely what attracts birds to one part of a beach and not to another. There are clues, however. Small Ocypode crabs, prawns, fish larvae, polychaete worms and small bivalves are among the most important foods for shorebirds and the distribution of these foods between beaches is very uneven. Differences in the fauna in mudflats can really only be determined by direct investigation (Swennen and Marteijn 1985). Where suitable prey is present, density is the most important factor, followed by prey size, prey depth and the penetrability of the substrate (Myers et al. 1980).
Tidal state, wind and disturbance all affect the density and availability of prey, and this is why certain beaches are only used by the waders at certain times (Evans 1976; Grant 1984). Casts of mud thrown up by suspension feeders and swimming movements of small crustaceans are visual clues for the birds, showing them where to feed (Pienkowski 1983), but some birds use tactile rather visual clues and have sensitive beak tips which can sense prey underground. Sandpipers, one group of partially tactile feeders, may avoid sandy mud because the sand grains are very similar in size to the polychaete and oligochaete worms upon which they feed (0.5-1 mm) (Quammen 1982).
The penetrability of a beach sediment depends on its water content (p. 111). This may be the reason that some shorebirds can be seen running along the water's edge on the ebbing tide pushing their bills into the thixotropic (fluid) sand. A careful examination of bill marks made in tidally formed sand ripples by dowitchers, a wading bird similar to godwits, showed that more marks were found on the crests than in the water-logged troughs. Neither the distribution of prey nor sediment grain size showed any difference between crests and troughs, but penetrating the crests required only 50%-70% of the force required to penetrate the troughs. Thus, concentrating effort on the crests reduced energy expenditure. Ripple crests are sites of active sediment transport and the arrangement of the grains is relatively unstable. This larger volume of pore space allows a higher water content and offers less resistance to penetration. Although the differences