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45.5B: Predation, Herbivory, and the Competitive Exclusion Principle - Biology

45.5B: Predation, Herbivory, and the Competitive Exclusion Principle - Biology


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Predation and herbivory are two methods animals use to obtain energy; many species have developed defenses against them.

Learning Objectives

  • Distinguish between predation and herbivory and describe defense mechanisms against each

Key Points

  • Predation, the hunting and consuming of animals by other animals, often shows cyclical patterns of predator/prey population sizes; predators increase in numbers when prey species are plentiful.
  • Herbivory is the eating of plant material for energy and can assist the plants with seed distribution.
  • Plants have evolved spines and toxins to defend against being eaten by herbivores.
  • Animals use bright colors to advertise that they are toxic; mimicry to hide from predators; or have spines, shells, and scales to protect themselves.
  • Batesian mimicry is when a non-toxic species looks similar to a poisonous one, which deters predator attacks.

Key Terms

  • camouflage: resemblance of an organism to its surroundings for avoiding detection
  • herbivory: the consumption of living plant tissue by animals
  • Batesian mimicry: the resemblance of one or more non-poisonous species to a poisonous species, for example, the scarlet king snake and the coral snake

Predation and Herbivory

Most animals fall into one of two major categories when it comes to obtaining the energy they need to survive in the environment: predation or herbivory. An animal that hunts, kills, and eats other animals is called a predator. Examples of predators include tigers, snakes, and hawks. Herbivory, on the other hand, refers to animals that eat plant matter. Deer, mice, and most song birds are examples. To protect themselves against these feeding mechanisms, many organisms have developed methods that keep them from being eaten.

Predation is the hunting of prey by its predator. 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. The most-often-cited example of predator-prey dynamics is seen in the cycling of the lynx (predator) and the snowshoe hare (prey), which is based on nearly 200-year-old trapping data from North American forests. This cycle of predator and prey lasts approximately 10 years, with the predator population lagging 1–2 years behind that of the prey population. As the hare numbers increase, there is more food available for the lynx, allowing the lynx population to increase as well. When the lynx population grows to a threshold level, they kill so many hares that the hare population begins to decline. This is followed by a decline in the lynx population because of scarcity of food. 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.

Herbivory describes the consumption of plants by insects and other animals. Unlike animals, plants cannot outrun predators or use mimicry to hide from hungry animals. Some plants have developed mechanisms to defend against herbivory. Other species have developed mutualistic relationships; for example, herbivory provides a mechanism of seed distribution that aids in plant reproduction.

Defense Mechanisms against Predation and Herbivory

The study of communities must consider evolutionary forces that act on the members of the various populations contained within it. Species are not static, but slowly changing and adapting to their environment by natural selection and other evolutionary forces. Species have evolved numerous mechanisms to escape predation and herbivory. 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.

Many species use their body shape and coloration to avoid being detected by predators. The tropical walking stick is an insect with the coloration and body shape of a twig, which makes it very hard to see when stationary against a background of real twigs. In another example, the chameleon can change its color to match its surroundings. Both of these are examples of camouflage: avoiding detection by blending in with the background.

Some species use coloration as a way of warning predators they are not good to eat. For example, the cinnabar moth caterpillar, the fire-bellied toad, and many species of beetle have bright colors that warn of a foul taste, the presence of toxic chemical, and/or the ability to sting or bite, respectively. Predators that ignore this coloration and eat the organisms will experience their unpleasant taste or presence of toxic chemicals and learn not to eat them in the future. This type of defensive mechanism is called aposematic coloration, or warning coloration.

While some predators learn to avoid eating certain potential prey because of their coloration, other species have evolved mechanisms to mimic this coloration to avoid being eaten, even though they themselves may not be unpleasant to eat or contain toxic chemicals. In Batesian mimicry, a harmless species imitates the warning coloration of a harmful one. Assuming they share the same predators, this coloration then protects the harmless ones, even though they do not have the same level of physical or chemical defenses against predation as the organism they mimic. Many insect species mimic the coloration of wasps or bees, which are stinging, venomous insects, thereby discouraging predation.


45.5B: Predation, Herbivory, and the Competitive Exclusion Principle - Biology

The Landauer-Büttiker formalism has been very successful in describing electronic transport in mesoscopic systems. However, unanswered questions remain regarding the role of the exclusion principle, especially if we allow inelastic processes to occur within the device. In this paper we try to answer these questions by starting from the Keldysh formalism and obtaining explicit expressions for the transmission, in terms of microscopic parameters, in the linear-response regime. The transmission is shown to obey reciprocity. The only assumption in this derivation is that the phase-breaking scatterers are assumed to remain in equilibrium and are treated within the self-consistent Born approximation.

©1992 American Physical Society

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Predation and Herbivory

Perhaps the classical example of species interaction is predation: the hunting of prey by its predator. Nature shows on television highlight the drama of one living organism killing another. 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. 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 (see the figure below). This cycle of predator and prey lasts approximately 10 years, with the predator population lagging 1–2 years behind that of the prey population. As the hare numbers increase, there is more food available for the lynx, allowing the lynx population to increase as well. 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. 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.

The cycling of lynx and snowshoe hare populations in Northern Ontario is an example of predator-prey dynamics.

The idea that the population cycling of the two species is entirely controlled by predation models has come under question. More recent studies have pointed to undefined density-dependent factors as being important in the cycling, in addition to predation. 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. The hare cycling would then induce the cycling of the lynx because it is the lynxes’ major food source. The more we study communities, the more complexities we find, allowing ecologists to derive more accurate and sophisticated models of population dynamics.

Herbivory describes the consumption of plants by insects and other animals, and it is another interspecific relationship that affects populations. Unlike animals, most plants cannot outrun predators or use mimicry to hide from hungry animals. Some plants have developed mechanisms to defend against herbivory. Other species have developed mutualistic relationships for example, herbivory provides a mechanism of seed distribution that aids in plant reproduction.


Competitive Exclusion Principle

Resources are often limited within a habitat and multiple species may compete to obtain them. All species have an ecological niche in the ecosystem, which describes how they acquire the resources they need and how they interact with other species in the community. The competitive exclusion principle states that two species cannot occupy the same niche in a habitat. In other words, different species cannot coexist in a community if they are competing for all the same resources. An example of this principle is shown in the figure below, with two protozoan species, Paramecium aurelia and Paramecium caudatum. When grown individually in the laboratory, they both thrive. But when they are placed together in the same test tube (habitat), P. aurelia outcompetes P. caudatum for food, leading to the latter’s eventual extinction.

Paramecium aurelia and Paramecium caudatum grow well individually, but when they compete for the same resources, the P. aurelia outcompetes the P. caudatum.

This exclusion may be avoided if a population evolves to make use of a different resource, a different area of the habitat, or feeds during a different time of day, called resource partitioning. The two organisms are then said to occupy different microniches. These organisms coexist by minimizing direct competition.


45.5B: Predation, Herbivory, and the Competitive Exclusion Principle - Biology

The Niche Every species has its own tolerance, or a range of conditions under which it can grow and reproduce. A species’ tolerance determines its habitat, the place where it lives.

  • A niche consists of all the physical and biological conditions in which a species lives and the way the species obtains what it needs to survive and reproduce.
  • An organism’s niche must contain all of the resources an organism needs to survive. A resource is any necessity of life, such as water, nutrients, light, food, or space.

Competition Competition occurs when organisms try to use the same limited resources.

  • Direct competition between species often results in one species dying out. This is the basis of the competitive exclusion principle. This principle states that no two species can occupy exactly the same niche in exactly the same habitat at the same time.
  • Competition helps to determine the number and type of species in a community.

Predation, Herbivory, and Keystone Species Predator-prey and herbivore-plant interactions help shape communities.

  • Predation occurs when one organism (the predator) captures and eats another (the prey).
  • Herbivory is an interaction that occurs when an animal (the herbivore) feeds on producers (such as plants).
  • Sometimes changes in the population of a single species, often called a keystone species, can cause dramatic changes in the structure of a community.

Symbioses Symbiosis occurs when two species live closely together in one of three ways: mutualism, commensalism, or parasitism.


53.5 Contrasting views of community structure are the subject of continuing debate

Integrated and Individualistic Hypotheses- The integrated hypothesis states that the species within a community are locked into particular biotic interactions. The individualistic hypothesis proposes that communities are loosely of independently distributed species with the same abiotic requirements

Rivet and Redundancy Models - The rivet model suggests that all species in a community are linked together in a tight web of interactions, so that the loss of even a single species has strong repercussions for the community. The redundancy model proposes that if a species is lost from a community, other species will fill the gap.


Defense Mechanisms against Predation and Herbivory

Predation and predator avoidance are strong selective agents. 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. 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. Such ecological relationships between specific populations lead to adaptations that are driven by reciprocal evolutionary responses in those populations. Species have evolved numerous mechanisms to escape predation and herbivory (the consumption of plants for food). Defenses may be mechanical, chemical, physical, or behavioral.

Mechanical defenses, such as the presence of armor in animals or thorns in plants, discourage predation and herbivory by discouraging physical contact (Figure 19.14). Many animals produce or obtain chemical defenses from plants and store them to prevent predation. Many plant species produce secondary plant compounds that serve no function for the plant except that they are toxic to animals and discourage consumption. For example, the foxglove produces several compounds, including digitalis, that are extremely toxic when eaten (Figure 19.14). (Biomedical scientists have purposed the chemical produced by foxglove as a heart medication, which has saved lives for many decades.)

Many species use their body shape and coloration to avoid being detected by predators. 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 19.15). In another example, the chameleon can change its color to match its surroundings (Figure 19.15).

Some species use coloration as a way of warning predators that they are distasteful or poisonous. For example, the monarch butterfly caterpillar sequesters poisons from its food (plants and milkweeds) to make itself poisonous or distasteful to potential predators. The caterpillar is bright yellow and black to advertise its toxicity. The caterpillar is also able to pass the sequestered toxins on to the adult monarch, which is also dramatically colored black and red as a warning to potential predators. Fire-bellied toads produce toxins that make them distasteful to their potential predators. They have bright red or orange coloration on their bellies, which they display to a potential predator to advertise their poisonous nature and discourage an attack. These are only two examples of warning coloration, which is a relatively common adaptation. Warning coloration only works if a predator uses eyesight to locate prey and can learn—a naïve predator must experience the negative consequences of eating one before it will avoid other similarly colored individuals (Figure 19.16).

While some predators learn to avoid eating certain potential prey because of their coloration, other species have evolved mechanisms to mimic this coloration to avoid being eaten, even though they themselves may not be unpleasant to eat or contain toxic chemicals. In some cases of mimicry, a harmless species imitates the warning coloration of a harmful species. Assuming they share the same predators, this coloration then protects the harmless ones. Many insect species mimic the coloration of wasps, which are stinging, venomous insects, thereby discouraging predation (Figure 19.17).

In other cases of mimicry, multiple species share the same warning coloration, but all of them actually have defenses. The commonness of the signal improves the compliance of all the potential predators. Figure 19.18 shows a variety of foul-tasting butterflies with similar coloration.