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Chapter 4 – Evolution, Biological Communities, and Species Interactions
Key Terms
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abundance
climax community
coevolution
commensalism
competition
complexity
convergent evolution
divergent evolution
diversity
ecological niche
ecological succession
ecotone
edge effect
environmental indicators
evolution
exotic species
fire climaxed communities
fundamental niche
generalist
habitat
interspecific competition
intraspecific competition
keystone species
natural selection
mutualism
parasitism
pathogen
pioneer species
predation
primary productivity
primary succession
principle of tolerance
realized niche
resilience
resource partitioning
secondary succession
specialist
stability
symbiosis
territorality
tolerance limits
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Skills
1. Relate tolerance limits to the survival of an organism.
2. Examine the concept of natural selection and understand how adaptation plays a role in evolution.
3. Distinguish between niche and habitat. Compare and contrast the ecological, fundamental, and realized niches an organism may occupy. Explain the following species interactions and give examples of each: predation, parasitism, mutualism, commensalism, and coevolution.
4. Define keystone species and give examples.
5. Differentiate between Batesian and Müllerian mimicry.
6. Relate competition to the survival of individual organisms and to species survival. Characterize resource partitioning as a way to avoid competition.
7. Relate the primary productivity of an ecosystem to its diversity.
8. Characterize an ecotone. Relate their significance in species diversity.
9. Summarize ecological succession. Differentiate between primary and secondary succession.
10. Assess the potential damage to an ecosystem and indigenous species when an invasive exotic species is introduced.
Law of Tolerance
The law of tolerance states that the factor in lowest supply in an environment will be the factor that determines the survival of an organism. Tolerance limits are the maximum and minimum values that encompass an organism’s ability to survive. Occasionally a single factor determines the tolerance limit for a species; more often, several factors function together to determine survival. A tolerance curve illustrates the range for survival of a species. The optimal range is the set of conditions that result in abundant numbers of the species. In the zone of physiological stress, the old, sick, and young are culled from the population. The zone of intolerance is the range of conditions that will result in death of even the most hearty of the species.
Environmental Indicators
Environmental indicators are species whose presence indicates a certain environmental parameter is present in a community. Environmental indicators are a direct, visible key to determine the condition of the environment. The presence of cattails in Florida’s Everglades is a sign of high levels of phosphates in the water. Cattails are therefore indicative of phosphate pollutants and serve as an environmental indicator of the pollution. In studies of stream benthic macroinvertebrates, mayfly, dobsonfly, and caddisfly larvae are indicative of excellent water quality, whereas the presence of blood worms, leeches, and pouch snails are indicative of very poor water quality.
Natural Selection and Evolution
Charles Darwin did not devise the theory of evolution; he proposed a mechanism by which natural selection occurs. Organisms more suited to their environment survive to reproduce. Those organisms not as suited do not survive, therefore, they do not reproduce. To a biologist, the term fitness refers to an organism’s ability to reproduce successfully. Therefore, survival of the fittest means survival and reproduction of those most suited to the environment. In this fashion, favorable adaptations are passed to subsequent generations and unfavorable adaptations are phased out of the population of organisms. An individual organism cannot evolve; only populations can evolve. Natural selection can be attributed to the alteration of the gene pool in these populations. The ultimate source of variation in genes is mutation. If a mutation is favorable, the organism becomes more suited to the environment. If the mutation is unfavorable, the trait will be unlikely to enhance the survivability of the species. The environmental conditions that result in natural selection cause selective pressure to be placed upon organisms. The pressure selects for those most suited to survive in the environment. Some selective pressures include physiological stress, predation, competition, and chance. For example, the Galapagos finches underwent adaptive radiation. When an ancestral finch landed on the islands from the mainland of South America, there were numerous niches to fill. Over time, the finches developed adaptations to allow them to fill the available niches. The adaptations allowed the finches to radiate.
Divergent evolution is when organisms become less alike over time but share a common ancestry. An example is the arm of a human and the forelimb of a dog. The bone structure is similar but each limb is adapted for its particular function. In convergent evolution, selective pressures cause two organisms lacking a recent common ancestor to become more similar. An example is the wing of a bird and the wing of a butterfly. Because both fly, they require wings; however, the structure of the two types of wings is dissimilar.
Niches
A habitat is the place where an organism lives out its life. The ecological niche is the role it fulfills in an ecosystem. The niche is controlled by the abiotic and biotic conditions of the ecosystem. An organism can occupy its fundamental niche if it is not experiencing competition, a situation highly unlikely in nature. The fundamental niche is the full range of resources or habitat it could use in the absence of competition. The realized niche is the range of resources and habitat an organism occupies due to the constraints of competition. For example, the arrival of the exotic lizard species, the Cuban brown anole, has displaced the native green Carolina anole in parts of Florida. The Cuban anole resides close to the ground, and the green anole lives in the upper layers of vegetation instead of the entire vegetated area that they would occupy in the absence of the Cuban anoles.
Species that can eat a wide variety of foods and live in a great variety of habitats are called generalists. Examples of generalists include rabbits and raccoons. A species that occupies a very narrow niche is called a specialist. Koalas are specialists because they eat only eucalyptus.
The Law of Competitive Exclusion states that two organisms cannot occupy the same niche for an indefinite period without one eventually migrating, dying, or undergoing resource partitioning. The classic example is growing two species of paramecium together in culture. One species was able to use the resources more efficiently and rapidly multiply and the other species died out, as shown in figure 4.2. Occasionally some species appear to occupy the same niche with both surviving, but resource partitioning is usually occurring. Resource partitioning is the use of the resource in a slightly different way or at a different time to allow both species to use the same resource. For example, warblers use different layers of the forest, and yet all use the same forest resources.
Population
density
Species Interactions
Predators are organisms that feed on other living organisms. The predator feeds upon a prey species. For example an osprey is a predator that preys upon small fish. Predators rely on a strong sense of smell, speed, or stealth to catch their prey. Prey have evolved a variety of mechanisms to avoid predation, including camouflage, noxious taste, spines or thorns, and speed. Some species have evolved camouflage by mimicking vegetation, larger animals, or even eyes. Batesian mimicry is observed when a species evolves to mimic a harmful or noxious species for protection. The Viceroy butterfly is tasty to birds, whereas the Monarch butterfly is noxious. That the mimic resembles the toxic model is protective for the Viceroy butterfly. Müllerian mimicry is based upon the fact that two species may resemble one another when both are toxic. Both species are protected because predators will avoid interacting with them. An example of Müllerian mimicry is that most wasps have yellow and black stripes to demonstrate to predators that they are dangerous.
A special type of predation is parasitism. A parasite is an organism that benefits from a relationship, and the host is the organism harmed by the relationship. Parasites can live on or in the host species. Examples of parasites include ticks, mosquitoes, tapeworms, and flukes. Pathogens, disease causing microbes such as bacteria or fungi, may also be considered parasitic.
Symbiosis is the relationship between two organisms. Parasitism is an example of a symbiotic relationship. Two other types of symbiosis are mutualism and commensalism. Mutualism is an interaction where both species benefit. An example would be lichens or mycorrhizae. Lichens are an interaction between a fungus and an algae or cyanobacteria. The fungus provides shelter and moisture to the algae, and the algae provides sugars to the fungus. Mycorrhizae are interactions between a plant’s roots and a fungus. The plant gains surface area for absorption, and the fungus receives sugars in return. Commensalism is when one species is benefited and the other is neither harmed nor benefited. An example is barnacles on a whale. The whale is not damaged or helped by the presence of the barnacle, but the barnacle is given more feeding opportunities by traveling on the body of the whale instead of remaining attached to a stationary surface.
Coevolution is evolution occurring because species are exerting selective pressures on each other. An example of coevolution is that moths pollinate flowers that are strong smelling, light in color, and open at night because most moths are nocturnal species. The animal gains a food resource in the nectar and the plant benefits from having its pollen spread to other plants. This is also an example of mutualism.
Keystone Species
A keystone species is a species whose relative abundance may not reflect its importance to an ecosystem. Many keystones are top predators. Gopher tortoises dig burrows in soft sand. Numerous other species use the burrows, including rattlesnakes, foxes, burrowing owls, armadillos, frogs, and insects. Gopher tortoises are therefore extremely important in the biological community, and their removal can interfere with the survival of the other species that depend upon their burrows for protection.
Competition
Organisms compete for food, water, shelter, space, mates, and any other component they require for life. Competition between organisms of the same species is known as intraspecfic competition, and competition between different species is known as interspecific competition. Territoriality is a form of intraspecific competition where organisms maintain their own area for mating, food sources, and even nesting sites. Territoriality has the additional benefit of decreasing environmental degradation because species are usually spread out.
Exotic Species
Exotic species are organisms that are not native to an area but are introduced. They may out compete endemic (native) species, lack predators, or damage ecosystems. If they are not held in check in the new environment, they are known as invasive exotic species. These species can cause tremendous amounts of environmental damage, as well as be expensive to eradicate once they establish successful populations. Characteristics of areas that make them more susceptible to invasion by exotics include tropical/semi-tropical climate and high levels of tourism. Examples of exotic species include water hyacinth, zebra mussels, and fire ants.
Community Properties
Primary productivity is the rate at which biomass is accumulated in an ecosystem. The energy left over after cellular respiration is the net primary productivity (NPP). Areas that have high NPP include marshes, coral reefs, and tropical rain forests. Areas with low NPP include deserts, the tundra, and open ocean.
The more complex an ecosystem is, the more likely it is able to withstand damage. Ecosystem diversity is the number of different species occupying different trophic levels. An ecosystem that has great diversity is more stable and therefore more resilient in the event of a disruption.
Boundaries between adjacent ecosystems have tremendous diversity because the two ecosytems transition into each other in an area known as an ecotone. Therefore, species from each ecosystem may survive in the ecotone, resulting in tremendous diversity known as the edge effect. A good example of an ecotone is an estuary. Organisms from both fresh- and saltwater systems can survive in the estuary, thus making it a diverse ecosystem with high NPP.
Succession
Succession is the orderly change of species in an ecosystem over time. Primary succession occurs when an area is newly exposed and has previously not had a biological community. The area lacks soil, and soil must be formed prior to invasion by plant species. Examples of primary succession would be a boulder falling on a mountainside or volcanic flow hardening into bare rock. Pioneer species are the first to grow in an area. Lichens can grow on bare rock and even facilitate the formation of soil by releasing carbonic acid, which breaks down rock into soil. They are the pioneer species in primary succession.
The process by which an early species makes subsequent species more likely to be successful is facilitation. Lichens that contribute to the formation of soil facilitate future plant growth. The lichens and mosses that are pioneer species give way to grasses and shrubs. The next community is the appearance of conifers. Finally, a community of broadleaf hardwoods appears in the site. Secondary succession takes place after an original biological community is damaged and the area undergoes regeneration.
Secondary is much more rapid than primary succession due to the presence of soil. An example of secondary succession would be recovery of an area after a fire or forest regeneration in an abandoned field. Grasses and weeds are the pioneers in secondary succession. Communities continue to change during succession until the final community the ecosystem can support is established. The final community that resists change is known as a climax community. If fire maintains the ecosystem then they are called fire climax communities. These communities regenerate quickly and are able to maintain their stability due to moderate, fairly frequent disturbances.
Chapter 4 Questions
Use the following choices for questions 1–4.
a. commensalism
b. mutualism
c. competition
d. predation
e. parasitism
1. aphids make a secretion called honeydew that ants feed upon, and the ants protect the aphids from predators
2. an epiphytic orchid living on an oak tree