Any species participates in what biologists call a "specific ecology," a relationship to its environment described by measurement of both environmental factors affecting the species and the species's response to these factors. Environmental factors include such variables as amount of light and abundance of food; species' responses typically include changes in the rates of birth, death, immigration, and emigration.
Since environmental stresses limit population growth, variations in environment correlate directly with variation in local species' population density. The relationships between particular environmental stresses and specific modes of response are difficult to define and predict; however, the relationship between the various response modes and changes in population density is relatively simple. The rate of change in population density is equal to the immigration rate plus the birth rate, minus the death rate, minus the emigration rate. Introduction of iodine into an area populated by sea urchins, for example, might affect the birth rate and immigration rate negatively, and the death rate and emigration rate positively, shifting the rate of change of population density toward the negative.
1. The purpose of this passage is toA. question the existence of specific ecologies
B. reveal the effects of the introduction of iodine into a population of sea urchins
C. discuss the specific factors that limit sea urchin population growth
D. predict the relationships between various environmental factors and specific modes of response
E. define "specific ecology" and illustrate how local population density changes when certain factors are altered
2. The author discusses the introduction of iodine into a population of sea urchins in order toA. question the usefulness of statistical approaches to ecology
B. illustrate how population density is affected by species' response modes
C. explain the concept of "specific ecology"
D. demonstrate how chemical pollutants affect a marine environment
E. show that population density remains constant over time despite environmental changes
3. It can be inferred that ecologists probably have the LEAST precise understanding of which of the following?A. The relationship between variation in environment and variation in local population density
B. How species' response modes affect population density
C. The effect of introducing a toxin into an area on the immigration rate in the area
D. What types of behavior comprise species' responses to environmental stresses
E. The relationship between a given environmental stress and a given mode of response
4. It can be inferred that a given species's "specific ecology" would include which of the following factors?I. The availability of food favored by the species
II. The number of predators present in the species's local environment
III. Increase in the birth rate due to favorable environmental conditions
A. I only
B. III only
C. I and II only
D. II and III only
E. I, II, and III