Food supply is the key

Finally, back with vertebrate animals, the story of the Australian pelican is another good illustration of how populations of predators are limited, just like herbivores, by a shortage of food. They, too, have the in-built capacity to explode in numbers immediately food becomes plentiful: but just as quickly die off again when the supply disappears.

Most of the time pelicans live on the coast, breeding only occasionally if the fishing is good enough. However, when the dry salt-flat that is Lake Eyre South, 650 km inland, fills with water flowing from heavy rains in Queensland (a fairly rare and unpredictable event), pelicans quickly migrate from as far afield as the coasts of South Australia and Papua New Guinea, to feed and breed on the enormous number of small fish teeming in the lake. Their numbers explode in response to this sudden superabundance of good food. During one such event in 1989-90 there were estimated to be 200 000 birds there - about 80 per cent of Australia's total population of pelicans! But before long evaporation caused the lake to rapidly dry out, becoming eight times as salty as the sea and killing the fish. Then thousands of unfledged chicks died, and adult birds deserted nests with unhatched eggs, and departed

Figure 6.2 Pelicans breed on Australia's Lake Eyre on the rare occasions when it is filled with water and teeming with fish. They rapidly increase to outbreak numbers, but once the water starts to dry up, and the fish die, the pelicans too die in their thousands. Photo courtesy of Tony Lewis.

for the coast. Any fledged young accompanied them, but any bird unable to fly was doomed to stay and starve. But in this short time of superabundant food a great many new pelicans had been produced.

The pelicans' story is repeated for their cousins around the world. The brown pelican of the Californian Gulf eats practically nothing but anchovies. In a good year when these are reasonably abundant, adult birds can get enough to maintain themselves. At such times there are enough anchovies in the Gulf to feed all the young that they could possibly produce. However, these fish are very patchily distributed. So the adults often cannot find plentiful shoals within foraging range of their breeding colonies. Then their young are malnourished or starved simply because their parents are unable to gather enough anchovies, fast enough, to fuel their rapid growth.

But in El NiƱo years real disaster strikes. The warm current flowing towards the American west coast suppresses the upwelling of the cold nutrient-rich waters. This sudden blocking of the flow of nutrients passes rapidly up the food chain, via plankton, invertebrates and fish; ending in widespread starvation for many species of seabirds. For the brown pelicans the disappearance of the anchovies means that many nestlings starve, fewer are fledged and many nests are deserted. In especially poor years whole nesting colonies abandon their nests and even adult birds starve.

Far away in the Rift Valley of Africa small populations of the great white pelican persist wherever there is permanent water containing enough fish to permit some breeding. However, infrequently and unpredictably, heavy rains will fill vast areas of what are usually dry salt pans in the valley with fresh water. Then there is a huge bloom of algae and invertebrates in the water. In turn the fish breed in millions. Just as happens in Lake Eyre, so do the pelicans, feeding on the fish. Then, just as happens there, once the water evaporates the fish die, and so do the pelicans. First the young as the adults abandon the whole colony, leaving the chicks to starve, but then adults, too, as the supply of fish continues to shrink.

This same species of pelican also breeds on Arel Island in Mauritania. There it undergoes the same boom and bust fluctuation in its fortunes, but annually, and driven by ocean currents, not rainfall. Each year in July there is an inflow of warm water from the Gulf of Guinea and this brings hordes of pelicans that feed on the abundant small fish and crustaceans in this water. They lay eggs in successive waves through until the end of November. But early each December an upwelling of cold water displaces the warm water to the south and with it their prey living near the surface. The cold water is almost devoid of surface-dwelling fish, so there is an abrupt end to the food supply for the breeding pelicans. Adults and any young that can fly quickly depart, leaving deserted nests and thousands of unfledged chicks that soon starve.

So we see that three species of pelican, in widely separated parts of the world, are responding in the same way as the rabbits do to a sudden and great increase of their food: and they collapse in just as spectacular a manner when the supply of food dries up. Not only do these stories (and the others in this chapter) clearly illustrate that populations of carnivores - just like the herbivores - are limited by their food, they make it quite clear that carnivores do not regulate the abundance of their food. Furthermore, in the case of the pelicans (and big cats and wolves), because, unlike rabbits, they do not have any predators attacking them, there is nothing to invoke as a 'regulator' of their abundance.

There is one exception to all this. That is where humans have introduced predators to places where they never had been, and where they encounter prey which have never evolved ways of avoiding being caught and eaten by a predator. In these sorts of situations - and there are many of them around the world - the introduced predator has the capacity to eliminate some species -and has often done so.

One good example of this is the plight of the Australian malleefowl. This bird has evolved a very special way of raising its young. The adult birds build large mounds of dead leaves that generate heat as they decay. Once this process is under way the hens lay their eggs in the mound. By carefully monitoring the temperature and adjusting the depth of leaves in the mound, the adult birds can keep the temperature constant and at a level for successful incubation of the eggs. Once the chicks hatch they dig their way out of the mound and run off into the bush where they must fend for themselves. Introduced European foxes have become adept at waiting by a mound and catching these young birds when they first surface. They have little chance of escape and studies have shown that, where foxes are sufficiently abundant, they will repeatedly kill all young malleefowl emerging from a mound, season after season. Even for these long-lived birds, extinction is assured should this situation persist.

One attempt in NSW to prevent this slaughter consisted of repeatedly spreading poison baits for the foxes. But this proved to have little effect. While very many foxes were killed, there remained a hard core of'bait-shy' individuals. They would never take a bait, and maintained sufficient vigil alongside mounds to continue to clean up most of the young chicks. It was only when it was realised that the foxes could maintain this pressure because they were not dependent on the young birds for food that success was achieved. Rabbits are the natural prey of the fox, and provide its staple diet. So as long as there were plenty of rabbits the malleefowl chicks were simply more easily caught 'icing on the cake' for the foxes. Once the considerable local population of rabbits had been removed, and vigorous baiting of the foxes was maintained, their numbers were reduced sufficiently to allow a significant numbers of chicks to survive.

Across the Tasman Sea a similar situation exists. European stoats were long ago introduced to New Zealand (along with ferrets and weasels!) in the mistaken belief that they would control the burgeoning populations of introduced rabbits. They have since spread and colonised native forests on both the North and South Island where they are a serious threat to the continued existence of several native birds, especially flightless ones like the kiwi, attacking their newly hatched young. There are no rabbits in these forests, but the stoats' staple diet is made up of two other introduced animals, the house mouse and ship rat, both also everywhere adapted to life in New Zealand's forests. So, once more, an introduced predator has a base diet to sustain it while it puts constant pressure on a much more easily caught alternative prey - young, defenceless native birds. In some forests recruitment of juvenile kiwis stays in continual decline until the numbers of stoats can be reduced by 80 per cent. Such reduction is virtually unsustainable in the exceedingly rough country where these forests grow. And it is impossible when the occasional outbreaks of rats and mice allow the stoats to breed up to large numbers. Then, when the outbreaks collapse, many now-starving stoats put even greater pressure on the kiwis. Careful experimental manipulations of the numbers of stoats in these forests have also revealed that the stoats have little influence on the changes in abundance of the rats and mice. This is generated by changes in abundance of the food of the rodents; southern beech seed masts and the associated increased numbers of herbivorous insects.

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