Trends in species loss. These graphs present data on recorded animal extinctions since 1600. The majority of extinctions have occurred on islands, with birds and mammals particularly affected (although this may reflect to some degree our more limited knowledge of other groups).
Why are extinction rates highest for birds and mammals? Answer: Perhaps attributes of birds and mammals, such as higher energetic requirements due to being endothermic, make them more susceptible to extinction. Alternatively, these data may reflect insufficient information. Other groups are less well studied than are birds and mammals. We simply are unaware of many extinctions in these groups.
Human populations in hotspots. The rich biodiversity in many hotspots is under pressure from (a) dense and (b) rapidly growing human populations.
Why do population density and growth rates differ among hotspots? Answer: Many factors affect human population trends, including resource availability, governmental support for settlement in new areas or for protecting natural areas, and the extent to which governments attempt to manage population growth.
The economic value of maintaining habitats. (a) Mangroves in Thailand and (b) rainforests in Cameroon provide more economic benefits left standing than if they are destroyed and the land used for other purposes.
If shrimp farms established on cleared mangrove habitats make money, how can clearing mangroves not be an economic plus? Answer: The mangroves provide many economic services. For example, without them, fisheries become less productive and storm damage increases. However, because the people who benefit from these services do not own the mangroves, governmental action is needed to ensure that the value of what are economists call "common goods" is protected.
Extinction and the species-area relationship. The data present percent extinction rates as a function of habitat area for birds on a series of Finnish islands. Smaller islands experience far greater local extinction rates.
Why does extinction rate increase with decreasing island size? Answer: On smaller islands, populations tend to be smaller. As we discuss later in this chapter, small populations are vulnerable to many problems, which individually or in concert can heighten the risk of extinction.
World catch of whales in the twentieth century. Each species is hunted in turn until its numbers fall so low that hunting it becomes commercially unprofitable.
Why might whale populations fail to recover once hunting is stopped? Answer: As discussed in this chapter, populations that are small face many problems that can reinforce one another and eventually cause extinction.
Loss of genetic variability in small populations. The percentage of polymorphic genes in isolated populations of the tree Halocarpus bidwilli in the mountains of New Zealand is a sensitive function of population size.
Why do small populations lose genetic variation? Answer: As we discussed in chapter 21, allele frequencies change randomly in a process called genetic drift. The smaller the population size, the greater these random fluctuations will be. Thus, small populations are particularly prone to one allele being lost from a population due to these random changes.
1). Historically, island species have tended to become extinct faster than species living on a mainland. Which of the following reasons cannot be used to explain this phenomenon?
a). Island species have often evolved in the absence of predators and have no natural avoidance strategies.
b). Humans have introduced diseases and competitors to islands, which negatively impacts island populations.
c). Island populations are usually smaller than mainland populations.
d). Island populations are usually less fit than mainland populations.
2). An endemic species is
a). one found in many different geographic areas.
b). one found naturally in just one geographic area.
c). one found only on islands.
d). one that has been introduced to a new geographic area.
3). Conservation hotspots are best described as
a). areas with large numbers of endemic species that are disappearing rapidly.
b). areas where people are particularly active supporters of biological diversity.
c). islands that are experiencing high rates of extinction.
d). areas where native species are being replaced with introduced species.
4). Which of the following does not distinguish the current mass extinction event from past events?
a). This is the only extinction event to be triggered by a single species.
b). This is the only extinction event from which biodiversity may not recover.
c). This is the only extinction event in which the majority of losses are occurring in mainland areas.
d). This is the only extinction event in which the majority of losses are occurring on islands.
5). The ability of an intact ecosystem, such as a wetland, to buffer against flooding and filter pollutants from water is a(n) ________ value of biodiversity.
a). direct economic
b). indirect economic
6). Which of the following is currently considered the leading cause of extinction?
a). overexploitation of species
b). competition from introduced species
c). habitat loss
7). What percentage of Madagascar's original forests have been lost due to human activity?
8). Numbers of migratory songbirds are declining in North America. Which of the following factors is important in this decline?
b). human disruption of breeding behavior
c). habitat fragmentation in the United States
d). global climate change
9). Most large whale species have been driven to the brink of extinction. Which of the following is the most accepted explanation for this situation?
b). habitat loss
d). competition from introduced species
10). A keystone species is one that
a). has a higher likelihood of extinction than a nonkeystone species.
b). exerts a strong influence on an ecosystem.
c). causes other species to become extinct.
d). has a weak influence on an ecosystem.
Answer: b Test Your Visual Knowledge
1). List the possible factors that explain the trend in the graph.
Answer: ban on DDT spraying, captive breeding programs, public support for conservation, providing adequate nesting areas
Apply Your Knowledge
1). Do you know of any endangered or threatened species that live near you? What factors have led to their decline? What efforts are being made to preserve any endangered species in your area?
Answer: Answer will vary.
2). Exotic species can have detrimental impacts on native species. Are all exotics bad? Can you name any instances in which exotics have not had detrimental effects?
Answer: Exotic species that do not outcompete native species for access to resources usually do not have a detrimental effect on ecosystems. The ring-necked pheasant is an exotic species that was intentionally introduced to the United States. It has not been a problem species and there are many efforts to keep its populations in the United States healthy.
3). Recent advances in genetic technologies have allowed scientists to develop microbes that can metabolize toxins in the environment, such as pesticides and oil slicks. What are the dangers of releasing these genetically modified organisms into the wild?
Answer: Species introductions can often have negative consequences for native species. Releasing genetically modified microbes may cause a change in microbe communities that could, in turn, have negative consequences on organisms that interact with those microbes.