Model Answer:

A species’ population size is effectively determined by how many individuals enter into and exit a given region. Thus, population size depends on birth rates and immigration rates (individuals “entering” into a region), in addition to death rates and emigration rates (individuals “exiting” a region).

Model Answer:

The exponential growth curve represents a population with a constant per capita growth rate. At this stage, the environment can still support a growing number of individuals of one species. Neither the scarcity of resources nor overcrowding has limited the population size. Thus, the curve keeps extending upward on the right side of the graph. The straight line on the left side of the graph (representing a slight incline) has to do with a population having relatively few individuals to start. A handful of individuals can only produce so many offspring at once. However, if these individuals continue to reproduce, and so do their progeny (and so on), population size can rapidly increase.

The logistic growth curve also contains a period of exponential growth; however, the curve levels off in a horizontal line on the right side of the graph. This population size (where the graph levels offs) represents the carrying capacity of a given environment. At this point, the environment does not have enough resources to support any more members of a species. As a result, more individuals in a population die and the birth and death rates equilibrate, leading to a (relatively) constant population size over time.

Model Answer:

Density-dependent factors that affect a species’ population size include inter- and intraspecific competition for resources and space. For example, as resources are exhausted and competition intensifies, a population’s birth rate decreases but the death rate increases. However, competition isn’t the only density-dependent factor that influences population size. As a population grows and its density increases, diseases can also spread (increasing the death rate).

Density-independent factors that affect population size include natural disasters (e.g., floods, droughts, hurricanes) and/or unexpected changes in temperature. Such occurrences can severely decrease the size of a population, regardless of how many individuals inhabit a given area.

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Model Answer:

K- and r-strategists differ fundamentally in the number of offspring they produce, and in the amount of resources and energy they dedicate to their progeny. r-strategists produce several offspring, but do not invest much energy in the long-term raising or protection of their young. This is a particularly effective strategy for a species that inhabits an area where resources are limited, or for a species that has multiple predators. Thus, r-strategists tend to inhabit “unpredictable” environments, assuring that some of their progeny will survive due to the sheer number they produce.

Conversely, K-strategists devote a large degree of energy and resources into raising their young, with the trade-off of only producing a few offspring at a time. This strategy is employed by species living in “predictable” environments, where resources are more readily available and predators are limited.

Model Answer:

A physiological trade-off is when an organism contributes more resources and/or energy to one particular function over another (i.e., in plants, producing more seeds while sacrificing defensive structures such as thorns).

Model Answer:

A "habitat island" is a region that can support a given species, but is surrounded by environments that cannot support the same species. Thus, much like islands in the ocean, habitat islands are isolated areas. For example, a lake would be considered a habitat island for trout, as these fish could not survive in the forest surrounding the lake. Other examples of habitat islands include actual islands in the ocean (i.e., ants on an island can't live in the surrounding ocean) or a valley surrounded by mountains (i.e., grasses able to inhabit the valley would not be able to survive at higher mountain elevations).

Model Answer:

The species diversity of a habitat island depends on the colonization and extinction rates of organisms inhabiting (or attempting to inhabit) this area. Initial colonization rates depend both on the size of a habitat island (more area means the potential to support a greater number of species) and its proximity to other habitat islands (from which organisms can migrate). In turn, extinction rates depend on the types of species colonizing a habitat island and their interactions with one another.

Many of the factors that influence species diversity on a habitat island can also be taken into consideration during conservation efforts for an endangered species. Imagine that a conservationist wants to introduce members of an endangered species into a new environment. She must evaluate what other organisms have colonized this environment, and the extent of human activity in this area. She must also determine if the size of this potential habitat is large enough to support the introduction of a new species. If few resources are available and/or the species will be heavily hunted (either by humans or other predators), this will likely result in the deaths of any endangered animals introduced into this environment. Furthermore, the conservationist has to assure that she introduces enough animals into the environment so that they will be able to find one another and mate (i.e., individuals must be in close proximity to one another). If steps are taken to assure that the above conditions are met, there is a good chance that members of this endangered species can take root and survive in this new habitat.