What do animals do when they are frightened? They increase vigilance, scanning their surroundings with all available senses. Some ‘freeze’ in hopes that immobility renders them invisible. Some hide, in the best available cover. And some run. Any of those responses interferes with other necessary activities. In addition, the bodies of frightened animals respond to fear by increasing the production of stress hormones, and that increases the heart rate, the metabolic rate, and thus the expenditure of energy. Prolonged fear adds negative impacts on the immune system and reproductive physiology. All of these negative effects have consequences not only for individual animals but for their populations.
It doesn’t much matter if the cause of fear is a real threat or something merely perceived to be threatening. Failure to evade a threat bears a high cost and is often lethal, so even a perceived threat generally necessitates a reaction, just in case it is real. Play it safe, so you have a chance to play another day!
The importance of a perceived risk of predation was shown experimentally in a population of song sparrows in British Columbia. The perceived risk of predation was manipulated by using playbacks of predator calls, every few minutes all day and night long, four days on followed by four days off, throughout the entire nesting season. Some nests were exposed to various kinds of predator calls, such as hawk, owl, crow, and raccoon; as a control for the generation of extra noises, other nests were exposed to calls of non-predators (loon, goose, seal). Playbacks began well before female sparrows built their nests.
Nests were located before any eggs were laid. Direct predation on nests was prevented by netting and electric fencing, and video cameras recorded the entire nesting cycle.
The results for song sparrow reproduction were striking. Female sparrows exposed to predator calls built their nests in denser vegetation than those exposed to non-predator calls. Nests exposed to predator playbacks contained fewer eggs and hatching success was lower. Females with nests exposed to predator calls were jumpier, left the nest more often and stayed away longer, with the result that their chicks got chilled. Chicks in nests exposed to predator calls were fed less often by their parents, so they weighed less. As a consequence of chilling and fewer food deliveries, chick mortality was higher than in nests exposed to non-predator calls. The reproductive output of the population of sparrows with the perceived risk of predation was reduced by forty percent, compared that of the population without the perceived risk. It is likely that the ultimate difference between the two populations was even greater than forty percent, because deprivation during growth usually has continuing negative effects into adulthood.
Other studies have shown similar effects of perceived predation risk on the biology of other species, including elk and snowshoe hares. When hares were regularly exposed to the mere presence of a dog (a potential predator), their stress hormones increased. Females then produced smaller litters and the young hares (leverets) were unusually small and thin.
In addition, the risk of predation commonly affects where animals live and so can limit the availability of suitable habitat. For example, marmots avoid habitats where the detection of predators is impaired. Juvenile salmon and several other freshwater fishes avoid habitats pervaded by alarm cues from the bodies of dead or injured companions (although risk taking is likely to increase if the animals are not well fed). In the presence of tiger sharks near Australia, dugongs became wary and tended to move into different areas. Bears become less active in daytime and more active at night when close to roads and development. Wildlife abundance and activity is known to be lower near trails frequented by dogs.
By constraining habitat use, predation or risk of predation also affects foraging opportunities. For example, brown bears are dominant over black bears, which may avoid salmon streams visited frequently by brown bears; in such areas, black bears consume fewer salmon than where brown bears are scarce. Female bears, both black and brown, with cubs often avoid salmon-spawning areas frequented by male bears, to reduce the risk of infanticide; they have reduced intake of salmon and lighter-weight cubs as a result. For bears, fat mamas tend have bigger cubs and to be more successful in cub-rearing than thinner females, so risk avoidance has a cost (presumably a lower cost than with infanticide, however).
It is clear that predation risk and the perception of risk affect not only the behavior of individuals but also have probable consequences for animal populations, by affecting reproductive output. Furthermore, the consequences of fear may extend beyond the species that is directly exposed to the risk of predation, with cascading effects through the network on interacting species, although the magnitude of such effects probably varies greatly.
An example is found in invertebrates: when grasshoppers are stressed by the risk of predation by spiders, their body composition changes. Then, when the grasshoppers die and decompose, their altered chemistry slows the subsequent decomposition of the leaf litter. So materials are recycled more slowly, with other consequences still to be recorded.
A far more dramatic example was seen when elk in Yellowstone changed their patterns of habitat use to avoid wolves, moving uphill and losing much of the lush foraging near the streams. However, many other components of that ecosystem changed for the better. Vegetation near streams was no longer over-browsed; willows, aspens, cottonwoods recovered, which helped stabilize stream banks and improve fish habitat. When their major food plants (just mentioned) recovered, beavers moved back in, creating ponds that support fish, amphibians, and lots of insects on which other animals feed. Berry bushes also rebounded and were again able to produce good crops of berries, which feed bears, birds, and other animals. Good shrub cover in the streamside zones provides important nesting and foraging sites for songbirds, including Neotropical migrants. In short, dozens of species and the entire ecosystem benefit from a reduction of elk browsing in this area.