Predation & herbivory (article) | Ecology | Khan Academy
The snowshoe hare and the Canadian lynx in the boreal forests of North America .. During the – hare cycle, lynx were a major predator at Kluane. The effect of climate on the hare-lynx cycle has been largely ignored. By using partial cross the cycles of snowshoe hares and lynx is still not fully. understood. spatial correlation between hare and lynx numbers, but the correlation between hare and their prey populations remain a central issue problem is especially challenging because e of snowshoe hare (Lepus americanus) deaths re- densit.
During a cycle, their density may increase by as much as fold and then drop precipitously. It was once thought that the rapid declines were mainly the result of predation by lynx, but studies of snowshoe hare populations in places where lynx are not very abundant or are absent altogether—places such as Jacquot Island in southwestern Yukon and Prince Edward Island in eastern Canada—revealed that island snowshoe hares, like their mainland counterparts, also experience cyclic fluctuations.
A snowshoe hare displaying its brown-colored summer coat. In The Conservation of the Wild Life of CanadaHewitt graphed the data from the records for a period extending from into the first decades of the s.
His graphs emphasized the close relationship in population density between snowshoe hares and Canada lynx. Proposed causes have ranged from disease to predation to constraints in food supply. The most significant factor driving fluctuations in snowshoe hare populations, however, appears to be simply exposure to stress, whether in the form of predation, disease, or scarcity of food.
Population dynamics of predators and prey Populations of predators and prey in a community are not always constant over time. Instead, in many cases, they vary in cycles that appear to be related. The most frequently cited example of predator-prey dynamics is seen in the cycling of the lynx, a predator, and the snowshoe hare, its prey.
Strikingly, this cycling can be seen in nearly year-old data based on the number of animal pelts recovered by trappers in North American forests. The population cycles of lynx and hare repeat themselves approximately every 10 years, with the lynx population lagging one to two years behind the hare population. The classic explanation is this: As hare numbers increase, there is more food available for the lynx, allowing the lynx population to increase as well.
BBC - GCSE Bitesize: Predators and their prey
When the lynx population grows to a threshold level, however, it kills so many hares that the hare population begins to decline. This is followed by a decline in the lynx population due to scarcity of food.
When the lynx population is low, the hare population begins to increase—due, at least in part, to low predation pressure—starting the cycle anew. Today, ecologists no longer think that the cycling of the two populations is entirely controlled by predation.
Predation & herbivory
For instance, it appears that availability of plant foods eaten by the hares—which decreases when hares become too abundant, due to competition—may also be a factor in the cycle. Defense mechanisms against predation When we study a community, we must consider the evolutionary forces that have acted—and continue to act! Species are not static but, rather, change over generations and can adapt to their environment through natural selection.
Predator and prey species both have adaptations—beneficial features arising by natural selection—that help them perform better in their role.
For instance, prey species have defense adaptations that help them escape predation. These defenses may be mechanical, chemical, physical, or behavioral. Mechanical defenses, such as the presence of thorns on plants or the hard shell on turtles, discourage animal predation and herbivory by causing physical pain to the predator or by physically preventing the predator from being able to eat the prey.
Chemical defenses are produced by many animals as well as plants, such as the foxglove, which is extremely toxic when eaten. Both reproduction and survival rates continue to fall for 2 to 3 years after the peak of the cycle, and over the low phase they start to recover to high values. Causes of the cycle What causes these changes in reproduction and survival?
There are three main factors that seem most likely to cause hare cycles: In addition to these single-factor explanations, two multifactor explanations have been suggested, one involving food and predation, and the other—the most complex hypothesis—involving all three factors. Many other factors might affect snowshoe hare cycles, but these seem more likely to be modifying influences than primary causes.
Disease and parasitism are two ecological factors that might affect hare populations but do not seem to be an essential cause of cycles. Parasite loads, for example, might cause hares to be in poor condition and therefore more susceptible to predators.
Lloyd Keith and his students surveyed hare parasite loads for many years in Alberta and concluded that none of the many parasites of the hares caused much direct mortality Keith et al. Experimental work with antihelminthics in field populations of hares either had no measurable impact on reproduction and survival Sovell and Holmes or produced minimal effects Murray et al.
The conclusion is that disease and parasites may affect some hare populations sporadically see reports in Chitty, for examplebut they cannot be an essential cause of cycles. The food hypothesis is attractive because it can explain both why reproduction changes over the cycle and why survival might change as well. There are two variants of the food hypothesis. First, hares may run out of food and starve, or, second, the quality of the food may decline. Because hares eat a variety of green plants in the summer, no one has considered food shortage in summer to be an important factor.
Winter food plants are the small terminal twigs of willow, birch, and small trees, as well as other shrubs; most studies have concentrated on the possibility that winter foods are limiting to hares Keith et al. Like all herbivores, snowshoe hares have preferred winter foods and may browse a large fraction of these preferred plants at the peak of the cycle. There is little evidence from our Yukon studies that overall food quantity is limiting at any time. We measured food abundance over the cycle by quantifying edible forage and we assessed consumption rates of marked twigs.
Consumption increased markedly during the peak Smith et al. Alternatively, food quality could change over the cycle.
Predators and their prey
John Bryant at the University of Alaska suggested one attractive qualitative food hypothesis based on secondary chemicals: Shrubs and small trees can fight back against browsers by increasing their content of secondary chemicals such as tannins and resins, which deter digestion in herbivores Bryant Indeed, experimental browsing of shrubs in Alaska has shown that the plants can respond to damage by increasing their secondary chemical defenses Bryant et al. The key question is whether these plant changes can influence the hare cycle.
To answer this question, we conducted five food-addition experiments during two hare cycles in the southwestern Yukon. In four we provided high-quality rabbit chow without limit to hares; in the fifth experiment we added high-quality natural food to a declining hare population Krebs et al. The response of hares to rabbit chow is classic: Hares move into the food-addition areas and their density increases approximately two- to threefold in comparison with control areas.
But once the density increases on the food-addition areas, the hare cycle continues unchanged. Hares decline in number at the same time and at the same rate on the food areas as on unmanipulated controls. Artificial food-addition experiments have been criticized because the added food is high quality and not natural. We tried to address this criticism by supplying natural food to one declining hare population. Tony Sinclair and Jamie Smith had shown that snowshoe hares largely avoided small white spruce trees because the needles contain camphor Sinclair and SmithRodgers and Sinclair Consequently, small spruce seedlings were the least preferred food in cafeteria trials with hares.
But foliage from large white spruce trees with branches beyond the reach of hares contain no camphor, and these branches become highly preferred food when supplied in a cafeteria trial. This observation was dramatically verified when a large white spruce tree was blown over by a windstorm: Hares devoured the fallen branches.
We therefore decided to feed a population of hares through a decline by cutting down white spruce trees and thus providing natural, highly preferred food to a collapsing hare population. Stan Boutin and Scott Gilbert did this experiment over three winters, with the results shown in Figure 5.
The extra natural food produced no detectable effect on the rate of population collapse. The failure of this extra food to affect the hare population decline was shown clearly on five areas in two cycles in the southwest Yukon. Such results imply that food shortage by itself is not the explanation for the hare cycle.
Whatever secondary chemical changes occur in winter food plants, they are at most a contributing factor, not the primary cause of the hare collapse. Another way of approaching the possible role of food in hare cycles is to improve the quality of the vegetation by adding nutrients in fertilizer.
We ran this experiment from to on two areas in the southwestern Yukon, each 1 km2. On each area we added NPK fertilizer each spring to increase the availability of soil nutrients. Boreal forest soils are typically impoverished in nutrients, particularly nitrogen, and nutrients added through fertilizer are immediately taken up by plants. We measured plant responses to nutrient additions and found large increases in individual plant growth in grasses, shrubs, and trees Turkington et al.
None of this plant improvement resulted in more snowshoe hares on the fertilized areas, relative to the controls; we therefore concluded that the dynamics of the hare cycle cannot be changed by nutrient additions to the ecosystem. The message seemed to be repeated: The hare cycle is not driven primarily by plant—herbivore interactions. The predation hypothesis is the next most likely explanation for the hare cycle. Few hares died with signs of malnutrition, and those that did starve were found more often in the increase and peak phases rather than in the decline Hodges a.
In contrast to adult hares, leverets are killed by a variety of small raptors, such as boreal owls, red-tailed hawks, kestrels, and hawk owls, and by small mammals, particularly red squirrels and ground squirrels. These observations of natural history support the contention that predation by a variety of birds and mammals plays an important role in the hare cycle.
For mammalian predators in winter, we used snow tracking to monitor density changes and kill rates of lynx and coyotes.
All hare predators showed strong numerical changes that lagged behind the hare cycle 1—2 years Boutin et al. In addition, both lynx and coyotes killed more hares per day in the peak and decline phases than during the increase. These kill rates were well above previous estimates and well in excess of energy demands.