chapter 18Review
In this chapter, we examined the state of the world’s biodiversity. We learned that the genetic diversity of many wild and domesticated populations has declined substantially over the past century. In addition, the species diversity of most major taxonomic groups has also declined, with large proportions of birds, mammals, and amphibians being threatened or near-
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Module 59 The Sixth Mass Extinction
Explain the global decline in the genetic diversity of wild species.
Declines in the abundance of individuals in a population can lead to reductions in genetic diversity that cause inbreeding depression. Inbreeding depression can cause offspring to inherit two copies of a harmful mutation and experience reduced survival and reproduction.
Discuss the global decline in the genetic diversity of domesticated species.
Humans have bred a wide variety of domesticated plants and animals, but in recent decades farmers have focused on the most productive varieties and many of the other varieties have disappeared over time. Such reductions in genetic diversity limit the options available to respond to new diseases or changing environmental conditions.
Identify the patterns of global decline in species diversity.
Of the estimated 10 million species on Earth, only about 50,000 have been assessed to determine whether their populations are increasing, stable, or declining. In examining those groups with the most complete data, scientists have found that every group has a substantial percentage of species that are threatened or near-
Explain the values of ecosystems and the global declines in ecosystem function.
Ecosystems can have intrinsic values, which are independent of any benefit to humans, or they can have instrumental values, which provide a benefit to humans and can be assigned a monetary value. Instrumental values include provisions, regulating services, support systems, resilience, and cultural services. Recent assessments of ecosystem function have found that more than half of those assessed are either declining or used at a rate that cannot be sustained.
Module 60 Causes of declining biodiversity
Discuss how habitat loss can lead to declines in species diversity.
The loss of habitat means that fewer individuals can be sustained in the habitat that remains. Smaller populations can then suffer from inbreeding depression. A reduction in habitat can also prevent the normal migration of species to important seasonal habitats and cause increased interactions with other species that have negative effects.
Explain how the movement of exotic species affects biodiversity.
Exotic species are those that are moved to new parts of the world where they are not native. Some of these species spread rapidly in their new locations and cause the demise of native species either as competitors, predators, herbivores, or pathogens.
Describe how overharvesting causes declines in populations and species.
Overharvesting plants and animals at rates that exceed the production of new individuals can cause population declines and even extinctions. Many of these extinctions have occurred due to unregulated harvesting in the past. However, in most parts of the world, governments have imposed harvest regulations to ensure that harvests occur in a sustainable manner.
Understand how pollution reduces populations and biodiversity.
Some pollutants can have direct lethal effects on species. Many other pollutants, however, can have sublethal effects that prevent or inhibit reproduction or alter ecosystems in ways that indirectly harm species.
Identify how climate change affects species diversity.
Climate change has the potential to alter the distribution of environmental conditions around the world. When conditions change and species are unable to move to more hospitable conditions, scientists predict that these species will either decline in abundance or go extinct.
Module 61 The Conservation of Biodiversity
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Identify legislation that focuses on protecting single species.
The primary pieces of legislation in the United States to protect species are the Marine Mammal Protection Act and the Endangered Species Act. Internationally, nations created the Convention on Biological Diversity in order to conserve biodiversity, to use biodiversity sustainably, and to share equitably the benefits that emerge from the commercial use of biodiversity.
Discuss conservation efforts that focus on protecting entire ecosystems.
There has been a continual increase in the amount of aquatic and terrestrial habitats that have been protected around the world. When preserving such habitats, scientists consider the size, shape, and connectedness of these habitats as well as the presence of edge habitats. They have also incorporated the need to balance human use and habitat protection by designing biosphere reserves.