![]() Chemical defenses are produced by many animals as well as plants, such as the foxglove which is extremely toxic when eaten. 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. These defenses may be mechanical, chemical, physical, or behavioral. Species have evolved numerous mechanisms to escape predation and herbivory. Species are not static, but slowly changing and adapting to their environment by natural selection and other evolutionary forces. The study of communities must consider evolutionary forces that act on the members of the various populations contained within it. Defense Mechanisms against Predation and Herbivory Other species have developed mutualistic relationships for example, herbivory provides a mechanism of seed distribution that aids in plant reproduction. Some plants have developed mechanisms to defend against herbivory. Unlike animals, most plants cannot outrun predators or use mimicry to hide from hungry animals. Herbivory describes the consumption of plants by insects and other animals, and it is another interspecific relationship that affects populations. The more we study communities, the more complexities we find, allowing ecologists to derive more accurate and sophisticated models of population dynamics. The hare cycling would then induce the cycling of the lynx because it is the lynxes’ major food source. One possibility is that the cycling is inherent in the hare population due to density-dependent effects such as lower fecundity (maternal stress) caused by crowding when the hare population gets too dense. More recent studies have pointed to undefined density-dependent factors as being important in the cycling, in addition to predation. Some researchers question the idea that predation models entirely control the population cycling of the two species. When the lynx population is low, the hare population size begins to increase due, at least in part, to low predation pressure, starting the cycle anew.įigure 45.18 The cycling of lynx and snowshoe hare populations in Northern Ontario is an example of predator-prey dynamics. When the lynx population grows to a threshold level, however, they kill so many hares that hare population begins to decline, followed by a decline in the lynx population because of scarcity of food. As the hare numbers increase, there is more food available for the lynx, allowing the lynx population to increase as well. This cycle of predator and prey lasts approximately 10 years, with the predator population lagging 1–2 years behind that of the prey population. The most often cited example of predator-prey dynamics is seen in the cycling of the lynx (predator) and the snowshoe hare (prey), using nearly 200 year-old trapping data from North American forests ( Figure 45.18). Populations of predators and prey in a community are not constant over time: in most cases, they vary in cycles that appear to be related. ![]() Nature shows on television highlight the drama of one living organism killing another. Perhaps the classical example of species interaction is predation: the consumption of prey by its predator. ![]() Ecology is studied at the community level to understand how species interact with each other and compete for the same resources. Areas with low diversity, such as the glaciers of Antarctica, still contain a wide variety of living things, whereas the diversity of tropical rainforests is so great that it cannot be counted. ![]() ![]() The number of species occupying the same habitat and their relative abundance is known as species diversity. All populations occupying the same habitat form a community: populations inhabiting a specific area at the same time. The interactions between these populations play a major role in regulating population growth and abundance. In most cases, numerous species share a habitat. Populations rarely, if ever, live in isolation from populations of other species.
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