Weathers dramatic effects

A long-term study of several species of Darwin's finches on the Galapagos Islands provided a particularly compelling - and very dramatic - demonstration of how this link of weather-plant-herbivore-carnivore operates.

These finches breed during the short, hot, wet season, although often, on these relatively arid islands, there is little rain and no birds will breed at all. Some species eat mostly insects, some, mostly young soft seeds. All eat insects in the breeding season and feed them to their young. They have no predators.

The study was conducted on some of the smaller islands (Daphne, for example, is only 40 hectares) so that every one of the birds present was identified and counted, and all their nests were found and observed. The investigation had been running for about 10 years when the extreme 1982-83 El Niño hit the islands.

The wet season started much earlier and finished much later than usual, with 10 times more rainfall than the previously recorded maximum. Many more plants grew than in most years, and they were larger and more lush, and flowered and fruited continuously. They produced 11 times more seed than the previous season, and the proportion of soft new seeds (the preferred food of the finches) rose from 25 to 80 per cent.

There was an equally dramatic rise in the abundance of insects living on the plants, especially of the caterpillars the finches preferentially feed to their young. These were six times as abundant as usual, and present for four times longer.

The finches responded predictably to this explosion of their plant and animal food. They bred continuously and for twice as long as the usual four-month breeding season. They produced four times as many clutches, laid five times as many eggs and fledged four times as many young as in a normal year.

Figure 7.1 During very high El Nino-generated rainfall on the Galapagos Islands plants grow more luxuriantly, set much more seed, and very many caterpillars grow on them. Galapagos finches, confronted with this sudden super-abundance of food, breed for longer, lay many more eggs and fledge many more young than in a normal year. They quickly increase to 'outbreak' numbers but just as quickly die off once the rain ceases and the food supply dries up. Photo courtesy of Sonia Kleindorfer.

Figure 7.1 During very high El Nino-generated rainfall on the Galapagos Islands plants grow more luxuriantly, set much more seed, and very many caterpillars grow on them. Galapagos finches, confronted with this sudden super-abundance of food, breed for longer, lay many more eggs and fledge many more young than in a normal year. They quickly increase to 'outbreak' numbers but just as quickly die off once the rain ceases and the food supply dries up. Photo courtesy of Sonia Kleindorfer.

Many more females than usual bred, new territories were established in previously unoccupied areas and nests were built in places where they would not normally be built. Some species bred on islands where they had not previously been recorded doing so. Young birds, which normally would not breed until they were two years old, bred before the end of the season.

However, because of the incessant stormy weather with heavy rain and strong winds, many nests were deserted and large numbers of nestlings died. Not withstanding this great increase in mortality, by the end of the season numbers of finches were exceptionally high and most of them were young birds - a veritable outbreak of finches!

At the same time numbers of other birds, in particular the Galapagos mockingbird and cuckoo, also increased markedly. But, one man's meat ...! While the birds' numbers increased so dramatically, more than 60 per cent of their near neighbours, the Galapagos marine iguanids, starved to death. Rising El Niño sea temperatures around the archipelago had killed most of their algal food growing on the rocks. And it was two years before any of the surviving females bred again.

Meantime the party was soon over for the finches. Dry years followed the El Niño deluge and high mortality of both adult and juvenile birds soon reduced the population to pre-El Niño proportions.

This dramatic explosion and collapse in numbers - this outbreak - gives a strong clue as to why there are not nearly so many finches in 'normal' times. They have the capacity to increase, but not the wherewithal to do so - they have access to much less food. There are many similar examples, with more being described all the time. And, increasingly, the role of major changes in the weather like the only recently understood El Niño-Southern Oscillation (ENSO), are being recognised as the driving force behind these fluctuations in abundance of animals, and how this flush of food travels up the food chains, each level dependent upon changes in the one below.

Such is now the case with outbreaks of a dozen or so species of small rodents in the drier parts of South America. There are records of these outbreaks dating back over 450 years to the middle of the 16th century from Chile, Argentina, Brazil and Peru. All of these, it is now emerging, have been generated by El Niño events producing peaks of exceptional rains where usually there is little rain. And with each peak there is a great flush of growth and seeding of grasses, especially several species of bamboo, and ephemeral herbs - staple food for small rodents. In semi-arid northern Chile rains associated with the 1991-92 El Niño produced a three-fold increase in these seeds and herbs, and some 11 species of rats and mice which eat them exploded to more than 20 times their usual levels of abundance. The major predators of these rodents - hawks, owls and foxes - showed a delayed response to this flush of their food, more than doubling in frequency 12 months later.

In many parts of inland Australia there is usually little rain. So for much of the time this land is a dry, arid place, where the only plants that can survive are deep-rooted and drought-resistant perennials. Nevertheless, a great many species of animals still manage to live in this habitat. The most obvious of these are several species of large kangaroos. Usually they persist at very low levels of abundance, spread thinly over great tracts of land. A few adults will find sufficient food to survive, so that the population does not go entirely extinct within the greater habitat. But they do not breed, except for a chance few that are lucky enough to find a rare place, such as a natural waterhole (and since European settlement artificially created ones), where there is sufficient green feed growing to allow some young to be produced. At the same time the kangaroos' predator, the dingo, ekes out an equally pitiful existence. Small numbers of them persist by scavenging on the odd carcase and catching any prey they can - usually only insects; now and then a small rodent. Nor do they breed - or if they occasionally do, their pups have slim hopes of surviving long beyond their birth.

But now and again a great deal of rain can fall in quite a short time. Then the desert blooms. All manner of plants, evolved to remain dormant for long periods without rain, sprout and grow rapidly, producing enormous quantities of green growth, flowers and seeds. And kangaroos eating this lush, nutritious food start to breed. They are exquisitely adapted to respond very quickly to any such sudden flush of good food: and to keep on breeding so long as it lasts. Females during dry times carry embryos in their uteri, but these do not develop. As soon as there is green feed for the females to eat, however, the embryos develop, are born within days, and climb into the pouch where they start suckling. Immediately after birth each mother is again impregnated so that as soon as the young joey in her pouch is big enough to leave, the next young one is born.

And so it goes. So long as there is green feed kangaroos will keep on breeding, and their numbers will rapidly explode. And, of course, the dingoes, suddenly presented with a great supply of young, easily captured kangaroos to eat, produce numerous and large litters of pups nearly all of which will quickly mature and start breeding.

Inevitably, however, this scenario cannot last because there will be no more rain; maybe not for years. So the plants set seed and die. Then hundreds - thousands - of kangaroos, most of them not yet fully mature, starve and die as their food disappears. For a short time this further boosts the supply of easily caught prey for dingoes. But soon, too, their turn comes. Life returns to its usual state. Once more few kangaroos - or dingoes - can live in this harsh environment.

Many ecologists, in spite of the evidence of these binge and bust events, still believe that predation by dingoes, rather than the amount of food, is what is restricting the numbers of kangaroos in arid Australia. They often quote as evidence a study which showed there were many kangaroos on the side of the famous dingo fence where the dogs are shot out, but few if any on the other side where they roam unmolested. A recent re-evaluation at the same study site, however, revealed a different story. It so happened that here the fence coincided with a natural boundary. On one side is a large basin into which streams, flowing after rain, drain and terminate. On the other there is dry, sand dune country which does not receive any of this runoff. Green herbage grows in abundance in the basin, and lasts for some time after rains, and kangaroos gather there and breed. Further south of the basin there is one stream which sometimes, when there is a lot of rain, flows through the fence into the dingo country. There it creates a small flood-out; and that is the only place on that side of the fence where green grass grows. Over four years of observation this was the only place where kangaroos were found on the dingo side of the fence.

Reinforcing the conclusion that it is the availability of green feed, not the absence of dingoes, which decides where kangaroos will gather and breed, there is an area some 600 km further away from this site where the landscape is well watered on both sides of the fence. Here there is no difference in the numbers of kangaroos on either side.

In this self-same environment the introduced European rabbit and its predators, the feral red fox and domestic cat, have become established. And,

like the kangaroos and dingoes, the numbers of rabbits and their introduced predators fluctuate just as violently in response to weather-driven changes in the amount of green feed. So, too, incidentally, do the numbers of the native wedge-tailed eagles, which have found rabbits to be an ideal and easily caught food.

Another example from the dry centre of Australia is that of locusts. As happens in many parts of the world, with many different species of locusts, the Australian plague locust now and then appears in huge numbers out of the dry interior and swarms across pasture and crop alike, destroying all before it. During the usual times of drought in the interior a few small populations of locusts manage to find enough plants on which they can survive and breed. But mostly these plants, although still green, have finished growing and are less than ideal food for growing young locust hoppers: 90 per cent of them are dead soon after hatching. Once more, however, infrequent and unpredictable good rains generate vast areas of new green growth in the dry interior. Then hoppers hatching in this actively growing grass fare much better: only about 60 per cent of them die. This is, however, a huge, fourfold increase in their survival. When this happens over thousands of square kilometres, and for two or more generations of locusts, it is enough to generate a massive explosion in numbers which soon start the march out into farmland. And they are nearly impossible to stop, even by killing untold millions of them with insecticide. Today, knowing and understanding how and where these plagues are generated, we can monitor the weather and, when the rains first start, move to quickly kill the relatively few early hoppers until the habitat again dries up.

Two recently published examples arising from long-term ecological studies, while not finding such sudden or dramatic changes in numbers as the previous examples, nevertheless again reveal how the weather-driven supply of the new growth of plants determines how abundant an animal will be.

The first is a 40-year study of a population of wildebeest on the tropical African Serengeti. This is a very large, but essentially closed population (i.e. they do not migrate in or out of it) and the animals are grass-feeders. Most animals die before they are a year old, and the greatest mortality occurs in newborn calves. There are five large carnivores which prey upon wildebeest, lions and hyenas being the principal ones. Nevertheless their combined efforts play only a minor role in limiting the wildebeest population. It is food supply that is the primary cause of mortality of wildebeests - 75 per cent of deaths are due to malnutrition. And variations in the supply of food - green grass - is directly caused by variations from year to year of the rainfall. Parallel long-term studies of the wildebeests' two major predators, lions and hyenas, have revealed that changes in their numbers are determined, via changes in the survival of their young, by these weather-driven changes in the availability of their prey.

The second study revealed the same story for very different animals in a very different part of the world - birds of the grouse family which live in the cold of northern Europe. Their populations have been extensively and intensively studied in several countries over many years. Probably the best known of these is the red grouse, managed for centuries as a sporting bird in Scotland. Although grouse chicks eat insects in the first few weeks of life, they are otherwise strict herbivores, feeding on flush new growth of their food plants (in the case of red grouse this is exclusively heather). These birds go through cycles of abundance, and the conclusion from these studies is that the main cause of changes in their breeding success - and hence their numbers - is the variation in the recruitment of young birds into the population from year to year. And the level of this recruitment is due primarily to the quality and quantity of the diet of the hens and their chicks. This, in turn, varies in response to variations in the weather influencing the flush of new growth of heather and the abundance of insects.

All these examples are fairly straightforward cases of animals responding to an increase in plants driven by an increase in the amount of rain. But there are other, and often less direct ways in which changes in the weather drive changes in the abundance of animals by changing not just the amount of their food, but its quality. One such is the case of the Australian zebra finch. As I described in Chapter 2, these small birds rely on a supply of newly ripening grass seeds to be able to breed and raise their nestlings. They live in mobile populations which move over large home ranges in the arid interior, and are opportunistic breeders. At any time of year when there is sufficient rain for grasses to germinate and grow they will start to breed, and the heavier and more prolonged the rain, the more intensely and longer they will breed. Two months after rain - a month or so longer in the winter when the grasses grow more slowly - they start to nest. The first clutches of eggs hatch just as the first ripening seeds become available. Nestlings are fed exclusively on these. If there are follow-up rains they will continue with surges of breeding activity, each new hatching corresponding precisely with the onset of a new flush of ripening seeds. And they may continue to do so for long periods (in a particularly wet spell in Alice Springs they were recorded breeding continuously for nearly 11 months). On the other hand they will not breed at all over long periods when there is no rain, or too little rain.

Not surprisingly weather-driven changes in the availability of protein food extends beyond herbivores to carnivores, and their success can just as surely depend on these changes.

Some of the infrequent but exceptional rains in the dry Australian interior cause floodwaters to flow into Australia's inland Lake Eyre which is just a dry salt pan for most of the time. When the flooding is great, the lake is filled with nutrient-rich water. In this water bacteria and invertebrates eating them proliferate, as do the fish feeding on the invertebrates. Then, as related in Chapter 6, Australian pelicans flock from far and wide to eat the fish and breed in enormous numbers. Soon, however, the water evaporates and becomes highly saline. This kills the fish and most of the newly bred young birds starve.

Exactly the same sequence of events happens with the great white pelican on the large lakes of the great Rift Valley of Africa. There, too, infrequent but great rains cause flooding of huge areas of usually dry lake beds. However, as happens in Lake Eyre, the water evaporates, the food supply dies and so do the pelicans that have briefly enjoyed a period of rapid expansion.

Then think of the example of the Peruvian condors in Chapter 5. On the coast they ceased to breed when their food decreased, and in the desert their food supply boosted their breeding when it increased; both responses generated at the same time by the same El Niño affecting the amount of food for them to eat - but in different ways.

And back to pelicans again. The Californian brown pelican lives and breeds in many places along the American west coast feeding on the abundant schools of anchovies that live and breed in the nutrient-rich cold water upwelling from the deep ocean. But in times of El Niño this all changes. Warm water flowing from the west stops the cold upwelling, so phyto- and zoo-plankton don't grow, the fish don't breed and thousands of pelicans starve to death.

Ironically the reverse happens for the sub-population of the great white African pelican which breeds on Arel Island off the coast of Mauritania, and it is an annual event. Yet, as I related in Chapter 6, the outcome is the same. Cold water, devoid of small surface fish, wells up each year and drives out the warm water which is rich in small fish. The pelicans' breeding collapses, thousands of eggs are abandoned and immature chicks are left to starve.

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