Predator-Prey Cycles

① How do predators affect populations of the prey animals? The answer is not as simple as might be thought. Moose reached Isle Royale in Lake Superior by crossing over winter ice and multiplied freely there in isolation without predators. When wolves later reached the island, naturalists widely assumed that the wolves would play a key role in controlling the moose population. Careful studies have demonstrated, however, that this is not the case. The wolves eat mostly old or diseased animals that would not survive long anyway. In general, the moose population is controlled by food availability, disease and other factors rather than by wolves.

捕食者影响被捕食者的数量这个说法没那么简单：比如驼鹿和狼，驼鹿的数量受到食物可利用性、疾病和其它因素影响而不是狼的数量。

② When experimental populations are set up under simple laboratory conditions, the predator often exterminates its prey and then becomes extinct itself, having nothing left to eat. However, if safe areas like those prey animals have in the wild are provided, the prey population drops to low level but not extinction. Low prey population levels then provide inadequate food for the predators, causing the predator population to decrease. When this occurs, the prey population can rebound. In this situation the predator and prey population may continue in this cyclical pattern for some time.

在实验环境下被捕食者被消灭，捕食者也灭绝；在野生环境下，被捕食者减少（但不灭绝），从而捕食者减少，再导致被捕食者数量回弹，形成循环模式。

③ Population cycles are characteristic of small mammals, and they sometimes appear to be brought about by predators. Ecologists studying hare populations have found that the North American snow shoe hare follows a roughly ten-year cycle. Its numbers fall tenfold to thirty in a typical cycle, and a hundredfold change can occur. Two factors appear to be generating the cycle: food plants and predators.

数量周期是小型哺乳动物的典型特征（北美雪鞋野兔10年循环：降到10倍或者30倍甚至100倍），两个因素：可食用植物、捕食者。

④ The preferred foods of snowshoe hares are willow and birch twigs. As hare density increases, the quantity of these twigs decreases, forcing the hares to feed on low-quality high-fiber food. Lower birth rates, low juvenile survivorship, and low growth rates follow, so there is a corresponding decline in hare abundance. Once the hare population has declined, it takes two to three year for the quantity of twigs to recover.

野兔密度增加>>嫩枝减少>>野兔低出生率和生存率、低生长率>>野兔减少>>2-3年，嫩枝恢复。

⑤ A key predator of the snowshoe hare is the Canada lynx. The Canada lynx shows a ten-year cycle of abundance that parallels the abundance cycle of hares. As hare numbers fall, so do lynx numbers, as their food supply depleted.

加拿大猞猁相应变化。

⑥ What causes the predator-prey oscillations? Do increasing number of hares lead to overharvesting of plants, which in turn results in reduced hare populations, or do increasing numbers of lynx lead to overharvesting hares? Field experiments carried out by Charles Krebs and coworkers in 1992 provide an answer. Krebs investigated experimental plots in Canada's Yukon territory that contained hare populations. When food was added to those plots (no food effect) and predators were excluded (no predator effect) from an experimental area, hare numbers increased tenfold and stayed there — the cycle was lost. However, the cycle was retained if either of the factors was allowed to operate alone: if predators were excluded but food was not added (food effect alone), or if food was added in the presence of predators (predator effect alone). Thus both factors can affect the cycle, which, in practice, seems to be generated by conjunction of the two factors.

食物和捕食者两者互相作用，导致predator-pray波动。

⑦ Predators are an essential factor in maintaining communities that are rich and diverse in species. Without predators, the species that is the best competitor for food, shelter, nesting sites, and other environmental resources tends to dominate and exclude the species with which it competes. This phenomenon is known as "competitor exclusion". However, if the community contains a predator of the strongest competitor species, then the population of that competitor is controlled. Thus even the less competitive species are able to survive. For example, sea stars prey on a variety of bivalve mollusks and prevent these bivalves from monopolizing habitats on the sea floor. This opens up space for many other organisms. When sea stars are removed, species diversity falls sharply. Therefore, from the stand point of diversity, it is usually a mistake to eliminate a major predator from a community.

捕食者是维持群落物种丰富多样的重要因素：例子，海星捕食双壳贝类。