Chapter 6 Review

chapter 6Review

Key Terms

Learning Objectives Revisited

• Explain how nature exists at several levels of complexity.

  Nature exists at several levels of complexity: individuals, populations, communities, ecosystems, and the biosphere.

• Discuss the characteristics of populations.

  Populations can have distinct population sizes, densities, distributions, sex ratios, and age structures.

• Contrast the effects of density-dependent and density-independent factors on population growth.

  Density-dependent factors influence an individual’s probability of survival and reproduction in a manner that is related to the size of the population. Density-independent factors have the same effect on an individual’s probability of survival and reproduction in populations of any size.

• Explain the exponential growth model of populations, which produces a J-shaped curve.

  The exponential growth model describes rapid growth under ideal conditions when resources are not limited. The J-shaped curve occurs because the population initially grows slowly when few individuals are present to reproduce but then grows rapidly as the number of reproducing individuals increases.

• Describe how the logistic growth model incorporates a carrying capacity and produces an S-shaped curve.

  The logistic growth model incorporates density-dependent factors that allow rapid initial growth but then cause population growth to slow down as populations approach their carrying capacity. When we allow lag times between when resources change in abundance and when populations produce new offspring, we can observe population overshoots and die-offs.

• Compare the reproductive strategies and survivorship curves of different species.

  Organisms have a range of reproductive patterns. At the extremes are r-selected species, which experience rapid population growth rates, and K-selected species, which experience high survivorship and slow population growth rates. Patterns of survivorship over the life span can be graphically represented as type I, II, and III survivorship curves.

• Explain the dynamics that occur in metapopulations.

  Metapopulations are groups of spatially distinct populations that are connected by occasional movements of individuals. These movements reduce the probability of any of the populations going extinct.

• Identify species interactions that cause negative effects on one or both species.

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  Competition is an interaction between two species that share a limiting resource. Over time, competition for a resource can cause natural selection to favor those individuals that have reduced overlap in resource use and this can lead to spatial, temporal, or morphological resource partitioning. Predation is an interaction in which animals partially or entirely consume another animal. Predators can affect the abundance of prey populations and cause the evolution of antipredator defenses in prey populations. Parasitism is an interaction in which one organism lives on or in another organism. Those parasites that can cause disease in their hosts are known as pathogens. Herbivory is an interaction in which animals consume producers. In some cases, herbivores can have dramatic effects on plant and algal communities by removing the most palatable species.

• Discuss species interactions that cause neutral or positive effects on both species.

  Mutualisms are interactions that benefit two interacting species by increasing the chances of survival or reproduction for both. One of the most common mutualisms is the interaction between flowering plants and their pollinators. A second well-known mutualism is between acacia trees and the ants that defend the trees in exchange for food and a place to live. Commensalisms are interactions in which one species benefits but the other is neither harmed nor helped. Examples include birds perching on trees and marine fish using coral reefs for protection from predators.

• Explain the role of keystone species.

  Keystone species play a role in the community that is far more important than its relative abundance might suggest. Common examples include predators that alter the outcome of competition in intertidal communities and beavers that create large ponds by constructing dams on streams.

• Explain the process of primary succession.

  Primary succession occurs on surfaces that are initially devoid of soil, such as bare rock that is exposed after the retreat of glaciers or the cooled lava from a volcanic eruption. Over time, plants and animals arrive at the site and modify the environment, making it more favorable for other species to arrive and persist.

• Explain the process of secondary succession.

  Secondary succession occurs in areas that have been disturbed but have not lost their soil. A common example is the bare soil left behind when farmers stop planting crops in a field. Over time, plants and animals colonize the site, alter the environmental conditions, and favor the persistence of other species.

• Explain the process of aquatic succession.

  Lakes and ponds experience sedimentation over long periods of time and this slowly fills in the basin. Over thousands of years, the lakes and ponds can be slowly converted into terrestrial habitats.

• Describe the factors that determine the species richness of a community.

  The species richness of a community is typically higher at latitudes that are closer to the equator. Richness is also higher in older sites where evolution has been producing new species for longer periods of time. Finally, more species exist in larger habitats and habitats that are closer to sources of new species, as is the case for oceanic islands that are located close to continents.