Habitat loss and degradation are the most important threats to diversity
The next time you fly in an airplane over Earth's surface, look down and ask yourself, “What species lived here before these farms and cities were here? Where do the species native to this place live now, and how do they move about?” From 30,000 feet above the landscape, you will find yourself face to face with the source of the diversity crisis: the scale of the human impact on the planet.
Earth has been modified greater than 70% of its land surface (Winkler et al. 2021), and all marine ecosystems have been affected by humans (Halpern et al. 2008). One species, Homo sapiens, is now appropriating about 25% of Earth's primary production (Haberl et al. 2014).The influence of human activities on natural habitat is the most important factor contributing to global declines in diversity (Sax and Gaines 2003). There are areas of extreme human influence, such as agricultural regions and certain coastal waters, and areas of little human influence, such as deserts and some polar seas. Overall, however, most of the lands and waters of Earth are at least moderately affected by humans (see Figure 3.5B). Addressing the loss, fragmentation, and degradation of habitat caused by human activities is central to conservation work. Habitat loss refers to the outright conversion of habitat to another use, such as urban development or agriculture, while habitat fragmentation refers to the breaking up of once continuous habitat into a series of habitat patches amid a human-dominated landscape. Habitat degradation refers to changes that reduce the quality of the habitat for many, but not all, species. Concepts 24.2 and 24.3 will address habitat fragmentation and its effects in detail; in this and the following sections, we'll cover habitat loss and habitat degradation.
On a continental scale, the extent of loss of some habitats is staggering (see Figure 24.12).
Similar losses can be observed on more local scales, as in the forests of western Ecuador (see Figure 23.4). Another example is provided by the Atlantic forest of Brazil (Ranta et al. 1998). This moist tropical forest has many endemic species, perhaps because it has been isolated from the Amazon rainforest for millions of years. Of South America's 904 mammal species, 73 are endemic to this forest, and 25 of those endemics are threatened with extinction. The forest's location also coincides with that of 70% of Brazil's human population. As a result, more than 92% of this habitat has been cleared to make room for agriculture and urban development, and what remains has been highly fragmented, pushing many species to endangerment.How has the loss of Atlantic forest habitat affected diversity? Brooks and colleagues (1999) asked why there have been no reports of extinctions among birds of this region. They offered three possible explanations, which may apply to patterns of biological decline in other regions as well. First, the birds may be adjusting to living in forest fragments. Second, the most vulnerable species might have gone extinct before they were known to biologists. Their third explanation, which they see as the most plausible, is that the time lag between deforestation and extinction has not yet played out. By 2021 five to seven species of birds were reported to have gone extinct, with nine more listed as critically endangered (Develey and Phalan 2021). Unless drastic measures are taken, additional species are doomed to extinction. Moreover, the loss of bird species will have negative effects on other species. As bird populations in the Brazilian Atlantic forest have dwindled to low numbers, reductions in seed size and seedling survival have been observed in plant populations that depend on
these birds for seed dispersal (Galetti et al. 2013).
Habitat degradation is extremely widespread, and it has diverse causes, including invasive species, overexploitation, and pollution. We'll turn now to one of those causes, invasive species.