As you have seen in Key Concept 56.1, biodiversity is central to the interconnectedness of various levels of biological organization, from genes to species to communities. Thus the loss of diversity at one scale affects the loss of diversity at other scales (Figure 58.1). For example, when human-caused activities reduce the number of individuals in a population, they reduce what is called the effective population size, or the number of individuals that can contribute offspring to the next generation (Focus: Key Figure 58.2), resulting in further population size decline, or what has been termed an “extinction vortex.” How does the effective population size continue to decline? First, small populations are more likely to experience *genetic drift and inbreeding, which can lead to reduced genetic diversity and inbreeding depression. Second, when population sizes are small, there is also a greater chance of demographic stochasticity, or fluctuations in population size as the result of random differences among individuals in reproduction and survival. For example, a small population might have, by chance, fewer females than males, resulting in fewer offspring than expected if the sex ratio were reversed. Finally, external mortality events such as extreme weather or habitat destruction can have devastating effects on small populations by increasing their chance of extinction even if only a small number of individuals are killed.

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Biodiversity loss is also connected at higher metapopulation, species, and ecosystem levels (see Figure 58.1). For example, you know from Key Concept 54.4 that the larger the metapopulation, the greater the chance that dispersal among source populations can rescue sink populations from extinction. As more local populations become extinct, and as more metapopulations are compromised, there is a greater risk that species will become extinct as well. Unfortunately, if some of those species play critical roles in their ecosystems—if they are, for example, keystone or foundation species—the possibility exists that other species will be threatened with extinction as well. As species diversity declines in ecosystems, biodiversity loss manifests itself at larger regional, continental, and even global scales (see Figure 58.1).

There are many examples of how a series of unfortunate events at different levels of biodiversity has led to the extinction of species (or subspecies, in the case below). Consider the heath hen (Tympanuchus cupido cupido), a large bird in the grouse family (Figure 58.3A). Heath hens were common from Maine to Virginia prior to the 1800s but were hunted extensively for food. By 1830 only one population, on the island of Martha’s Vineyard, Massachusetts, was left. By 1908 only 50 birds remained, and a reserve was established to recover the population, which eventually grew to several thousand birds. Unfortunately, in 1916 a fire and a series of environmental factors that included a hard winter, an influx of predators, and a poultry disease caused the population size to decline dramatically. By 1928 demographic stochasticity and inbreeding depression set in, leaving only 13 individuals—2 females and 11 males. Four years later, the heath hen was extinct.

Question

Of the 59,033 species categorized by the IUCN, 1 percent are extinct and 20 percent are threatened. Birds have suffered the most extinctions (~150 species), but amphibians are most at risk of extinction (~1,900 species).