Answers to Review Questions
1. Complicating factors discussed in the text include (1) limited knowledge about the dispersal capabilities of the organism under study, (2) the fact that populations may have a patchy structure, and (3) the fact that individuals may be hard to define.
The first two factors— limited information about dispersal and patchy populations—can make it difficult to determine the area within which individuals interact, and hence what constitutes a population. The third factor—difficulty in defining individuals—applies to the many organisms that reproduce asexually to form clones. In such organisms, it can be hard to determine what an individual is, thus making it difficult to estimate abundance.2. The simplest reason that no species is found everywhere is that much of Earth does not provide suitable habitat. There can, in turn, be many reasons why portions of Earth are not suitable for a particular species. For example, the abiotic or biotic conditions of an environment may limit the growth, survival, or reproduction of the species, as may disturbance or the interaction between abiotic and biotic conditions. Furthermore, a species may be absent from environments where we would expect it to thrive because of dispersal limitation or historical factors (including evolutionary history and continental drift).
3. A species distribution model is a tool that predicts the environmental conditions occupied by a species based on the conditions present in its current distribution. Species distribution models can be used to predict the future distribution of an introduced species by collecting as much information as possible about environments where the species currently is found. Those data are then used to construct a species distribution model, which in turn is used to identify currently unoccupied locations that are likely to provide suitable habitat for the species. For such predictions to accurately reflect the future spread of the organism, information also must be gathered about its dispersal capabilities.
4. For a conservative estimate, assume there are 20 otters per square kilometer, each of which eats 20% of its body weight in food each day. Since urchins, on average, weigh 0.55 kg each, a kilogram of urchins consists of roughly 1/0.55 = 1.82 urchins. Thus, the number of urchins per square kilometer that an otter population would be expected to eat each year is as follows: (20 otters/km[10]) ? (0.2 ? 23 kg/otter/day) ? (365 days/year) ? (1.82 urchins/kg) = 61,116 urchins/km2/year.