Answers to Review Questions
1. Abiotic and biotic agents of change include those listed in Table 17.1. Intense disturbances such as hurricanes, tsunamis, fires, and volcanic eruptions can cause major damage but are relatively infrequent.
Other agents of change, such as sea level rise, competition, or parasitism, may not cause major damage initially but may be frequent or constant and have dramatic effects over time. Still others, such as predation, may be relatively frequent but not very intensive, thus forming patches of available resources.2. Primary succession involves the colonization of habitats devoid of life. Species colonizing these habitats must deal with stressful conditions and transform their habitats to create soils, nutrients, and food. Secondary succession involves the reestablishment of a community in which most, but not all, of the organisms have been destroyed. Under these conditions, colonizing species benefit from the biological legacy of the preexisting species, but they are likely to face more competition for resources than the species involved in primary succession.
3. A hypothetical community might be a newly cleared vacant lot in an unnamed city. The facilitation model would be supported if the first species to arrive were stress-tolerant and had the ability to modify their habitat in positive ways. In this case, those early species would facilitate the growth of later species, which would be better competitors but less stress tolerant. Over time, these later species would dominate as they outcompeted the facilitating species. The tolerance model assumes that the earliest species modify the environment, but in ways that neither help nor hinder later species. Later species are merely those that live longer and tolerate stressful conditions longer than early species. Finally, the inhibition model would be supported if the early species created conditions that benefited themselves but inhibited later species. Only through the removal of those inhibitory early species—for example, via disturbance or stress—would later species be able to displace them.
4. It is hard to know whether a community is stable because stability depends on the spatial and temporal scale at which the community is observed. All communities fluctuate and change over time, but how long must we wait for a community to return to some original state before we assume it is stable? There is no single answer. Although Sutherland did observe the formation of alternative communities on his tiles when predators were manipulated, did he follow the communities long enough, and at a large enough spatial scale, to show stability? Again, it depends on how you define “stability,” leaving us with an unresolved question.