Three Classic Studies of Competition Paramecium: a laboratory "bottle" experiment - Paramecium is a single-celled, ciliated protozoan that feeds on bacteria. It can be raised in a test-tube, in a broth of bacteria, which is its food supply. In fact, there are a number of species of paramecium, which can be raised in this fashion. What will happen if two species are raised separately, and then together?
As Figure 1 shows, both species thrive when raised alone, reaching some limit set by the volume of the test-tube and the supply of bacteria. When raised together, the growth rate is slowed for both, until one species declines, and the other then grows to the capacity of the test-tube. Evidently these two ciliates are competing for a limiting resource, share the same niche, and competition determines that only one persists.
This study, and others like it, were influential in establishing the fact that competition could occur, and could eliminate one species from a shared environment. Does the same species always win out?
Can two species ever coexist? How well can we extrapolate from these laboratory systems to nature? Many such "bottle" studies have now been performed, with flour beetles, Drosophila, yeast, various plants. In general, one species wins, and one species is eliminated. These experiments gave rise to the competitive exclusion principle: No two species of similar requirements can long occupy the same niche coexist.
In the case of paramecium, the same species always won. Studies with flour beetles, in contrast, found that changes in micro-environment temperature, moisture could shift the outcome in favor of one or the other species.
The simplicity of such experiments remains as a nagging concern: do real populations, living in a complex, variable world, also experience competitive exclusion? Warblers in a spruce forest: a field observational study - Warblers are migratory, spending the winter in the tropics, and flying to northern forests in early spring to set up territories, mate, and rear their young. An important study was made of five species of warblers in the genus Dendroica. These warblers are similar in body size, beak size, and all feed on insects.
Moreover, they were studied in a spruce forest in northern Maine, almost as uniform a natural habitat as one could find. Surely if competition occurred in nature, this was the place to show it. Cape May Warbler A careful study of the foraging behavior of these five warblers found many differences in the way they made use of the seemingly homogeneous habitat. These species differed in what part of a spruce tree they frequented, whether they captured insects on the wing, from needles, or under bark, and so on.
The conclusion was that each species occupied a distinct niche. Rather than showing that competition was important in nature, this study seemed to establish the opposite.
Even superficially similar species really are different, upon close inspection. This study helped to establish the idea that each species occupies its own distinct niche. The niche of a species includes all aspects of its habitat, how it makes a living, and the physical environment in which it is found. All rights reserved. Apparent competition occurs when two individuals that do not directly compete for resources affect each other indirectly by being prey for the same predator Hatcher et al.
Consider a hawk predator, see below that preys both on squirrels and mice. In this relationship, if the squirrel population increases, then the mouse population may be positively affected since more squirrels will be available as prey for the hawks. However, an increased squirrel population may eventually lead to a higher population of hawks requiring more prey, thus, negatively affecting the mice through increased predation pressure as the squirrel population declines.
The opposite effect could also occur through a decrease in food resources for the predator. If the squirrel population decreases, it can indirectly lead to a reduction in the mouse population since they will be the more abundant food source for the hawks. Apparent competition can be difficult to identify in nature, often because of the complexity of indirect interactions that involve multiple species and changing environmental conditions.
Predation requires one individual, the predator, to kill and eat another individual, the prey Figure 3. In most examples of this relationship, the predator and prey are both animals; however, protozoans are known to prey on bacteria and other protozoans and some plants are known to trap and digest insects for example, pitcher plant Figure 4.
Typically, this interaction occurs between species inter-specific ; but when it occurs within a species intra-specific it is cannibalism. Cannibalism is actually quite common in both aquatic and terrestrial food webs Huss et al. It often occurs when food resources are scarce, forcing organisms of the same species to feed on each other. Surprisingly, this can actually benefit the species though not the prey as a whole by sustaining the population through times of limited resources while simultaneously allowing the scarce resources to rebound through reduced feeding pressure Huss et al.
The predator-prey relationship can be complex through sophisticated adaptations by both predators and prey, in what has been called an "evolutionary arms race. Prey species have evolved a variety of defenses including behavioral, morphological, physiological, mechanical, life-history synchrony and chemical defenses to avoid being preyed upon Aaron, Farnsworth et al. Figure 3: Crocodiles are some of the evolutionarily oldest and dangerous predators. Figure 4: A carnivorous pitcher plant.
A carnivorous pitcher plant that preys upon insects by luring them into the elongated tube where the insects get trapped, die and are then digested. Another interaction that is much like predation is herbivory , which is when an individual feeds on all or part of a photosynthetic organism plant or algae , possibly killing it Gurevitch et al.
An important difference between herbivory and predation is that herbivory does not always lead to the death of the individual. Herbivory is often the foundation of food webs since it involves the consumption of primary producers organisms that convert light energy to chemical energy through photosynthesis.
Herbivores are classified based on the part of the plant consumed. Granivores eat seeds; grazers eat grasses and low shrubs; browsers eat leaves from trees or shrubs; and frugivores eat fruits. Plants, like prey, also have evolved adaptations to herbivory.
Tolerance is the ability to minimize negative effects resulting from herbivory, while resistance means that plants use defenses to avoid being consumed. Physical for example, thorns, tough material, sticky substances and chemical adaptations for example, irritating toxins on piercing structures, and bad-tasting chemicals in leaves are two common types of plant defenses Gurevitch et al.
Figure 5: Sharp thorns on the branch of a tree, used as anti-herbivory defense. Symbiosis is an interaction characterized by two or more species living purposefully in direct contact with each other. The term "symbiosis" includes a broad range of species interactions but typically refers to three major types: mutualism, commensalism and parasitism. Mutualism is a symbiotic interaction where both or all individuals benefit from the relationship.
Mutualism can be considered obligate or facultative. Be aware that sometimes the term "symbiosis" is used specifically to mean mutualism. Species involved in obligate mutualism cannot survive without the relationship, while facultative mutualistic species can survive individually when separated but often not as well Aaron et al. For example, leafcutter ants and certain fungi have an obligate mutualistic relationship.
The ant larvae eat only one kind of fungi, and the fungi cannot survive without the constant care of the ants. As a result, the colonies activities revolve around cultivating the fungi. They provide it with digested leaf material, can sense if a leaf species is harmful to the fungi, and keep it free from pests Figure 6. A good example of a facultative mutualistic relationship is found between mycorrhizal fungi and plant roots.
Yet the relationship can turn parasitic when the environment of the fungi is nutrient rich, because the plant no longer provides a benefit Johnson et al. Thus, the nature of the interactions between two species is often relative to the abiotic conditions and not always easily identified in nature. Figure 6: Leaf cutter ants. Leaf cutter ants carrying pieces of leaves back to the colony where the leaves will be used to grow a fungus that is then used as food. The ants will make "trails" to an acceptable leaf source to harvest it rapidly.
Commensalism is an interaction in which one individual benefits while the other is neither helped nor harmed. For example, orchids examples of epiphytes found in tropical rainforests grow on the branches of trees in order to access light, but the presence of the orchids does not affect the trees Figure 7.
Commensalism can be difficult to identify because the individual that benefits may have indirect effects on the other individual that are not readily noticeable or detectable. If the orchid from the previous example grew too large and broke off the branch or shaded the tree, then the relationship would become parasitic. Figure 7: Epiphytic bromeliads that grow on the limbs of large tropical rainforest trees.
The bromeliads benefit by occupying space on the limb receiving rain and sunlight, but do not harm the tree. Parasitism occurs when one individual, the parasite, benefits from another individual, the host, while harming the host in the process. Parasites feed on host tissue or fluids and can be found within endoparasites or outside ectoparasites of the host body Holomuzki et al. For example, different species of ticks are common ectoparasites on animals and humans.
Parasitism is a good example of how species interactions are integrated. Parasites typically do not kill their hosts, but can significantly weaken them; indirectly causing the host to die via illness, effects on metabolism, lower overall health and increased predation potential Holomuzki et al. How to Make Science Films.
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America's Most Common Tree Frog. Iceland: Home of Clearest Freshwater on Earth! This type of competition is a basic factor in natural selection. It leads to the evolution of better adaptations within a species. Interspecific competition occurs between members of different species. For example, predators of different species might compete for the same prey.
Interspecific Competition and Extinction Interspecific competition often leads to extinction. Interspecific Competition and Specialization Instead of extinction, interspecific competition may lead to greater specialization.
Watch the beginning of the following video to learn more about competition.
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