![]() (credit a: modification of work by Linda Tanner credit b: modification of work by Frank Vassen) (a) The tropical walking stick and (b) the chameleon use their body shape and/or coloration to prevent detection by predators. In another example, the chameleon can change its color to match its surroundings (Figure 3b). The tropical walking stick is an insect with the coloration and body shape of a twig, which makes it very hard to see when it is stationary against a background of real twigs (Figure 3a). Many species use their body shape and coloration to avoid being detected by predators. (credit a: modification of work by Huw Williams credit b: modification of work by Philip Jägenstedt) The (a) honey locust tree uses thorns, a mechanical defense, against herbivores, while the (b) foxglove uses a chemical defense: toxins produces by the plant can cause nausea, vomiting, hallucinations, convulsions, or death when consumed. (Biomedical scientists have repurposed the chemical produced by foxglove as a heart medication, which has saved lives for many decades.) Figure 2. For example, the foxglove produces several compounds, including digitalis, that are extremely toxic when eaten (Figure 2b). Many plant species produce secondary plant compounds that serve no function for the plant except that they are toxic to animals and discourage consumption. Many animals produce or obtain chemical defenses from plants and store them to prevent predation. Mechanical defenses, such as the presence of armor in animals or thorns in plants, discourage predation and herbivory by discouraging physical contact (Figure 2a). Defenses may be mechanical, chemical, physical, or behavioral. Species have evolved numerous mechanisms to escape predation (including herbivory, the consumption of plants for food). Such ecological relationships between specific populations lead to adaptations that are driven by reciprocal evolutionary responses in those populations. Likewise, traits that allow a predator to more efficiently locate and capture its prey will lead to a greater number of offspring and an increase in the commonness of the trait within the population. Any heritable character that allows an individual of a prey population to better evade its predators will be represented in greater numbers in later generations. Predation and predator avoidance are strong influenced by natural selection. Defense Mechanisms against Predation and Herbivory The cycling of snowshoe hare and lynx populations in Northern Ontario is an example of predator-prey dynamics. ![]() When the lynx population is low, the hare population size begins to increase due, in part, to low predation pressure, starting the cycle anew. When the lynx population grows to a threshold level, however, they kill so many hares that hare numbers begin to decline, followed by a decline in the lynx population because of scarcity of food. ![]() An apparent explanation for this pattern is that as the hare numbers increase, there is more food available for the lynx, allowing the lynx population to increase as well. This cycling of predator and prey population sizes has a period of approximately ten years, with the predator population lagging one to two years behind the prey population. The most often cited example of predator-prey population dynamics is seen in the cycling of the lynx (predator) and the snowshoe hare (prey), using 100 years of trapping data from North America (Figure 1). Population sizes of predators and prey in a community are not constant over time, and they may vary in cycles that appear to be related. The narrowest definition of predation describes individuals of one population that kill and then consume the individuals of another population. Perhaps the classical example of species interaction is the predator-prey relationship. Scientists study ecology at the community level to understand how species interact with each other and compete for the same resources. Areas with low species diversity, such as the glaciers of Antarctica, still contain a wide variety of living organisms, whereas the diversity of tropical rainforests is so great that it cannot be accurately assessed. The number of species occupying the same habitat and their relative abundance is known as the diversity of the community. Populations that interact within a given habitat form a community. Populations typically do not live in isolation from other species. ![]()
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