Biodiversity loss is the result of four major, and potentially interacting, factors. First, habitat loss and degradation (including pollution) have been primary culprits contributing to the extinction of tens of thousands of species. Second, overharvesting for food, clothing, ornamentation, pets, and medicines has eased for some species, but others are still under threat. Third, deliberate or inadvertent introduction of non-
learning outcomes
You should be able to:
Describe, with examples, how human activities have resulted in habitat loss, fragmentation, and degradation.
Give examples of situations where birds, mammals, and exotic animals have been overharvested, and describe and analyze the ecological implications of these examples.
Describe and analyze methods by which species can invade or be introduced into new regions, and infer some possible results of these introductions.
Correlate changes in climate with changes in species or ecosystems, giving evidence for the correlation.
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How does habitat fragmentation contribute to the extinction of species?
The more fragmented a habitat is, the smaller and more isolated the fragments are and the greater the edge effects are. Fragmentation reduces the population sizes of species and isolates them from one another, reducing their ability to disperse among the greater metapopulation. Once population sizes are small and isolated, they become susceptible to extinction vortex processes (see Figure 58.2).
Refer to Figure 56.9. Describe how the loss of wolves has likely resulted in a decrease in hardwood trees in Yellowstone National Park (see Figure 58.9).
Gray wolves have been shown to create a trophic cascade that benefits hardwood trees such as aspens. Research by William Ripple and colleagues in Yellowstone National Park found that gray wolves, by feeding on elk, released aspen seedlings from herbivory, thus creating mature aspen forests. After wolves were hunted to extinction in Yellowstone in 1926, the number of aspens declined precipitously because of intensive elk grazing, as is shown in Figure 56.9.
Give three reasons why invasive species can have negative effects on biodiversity and ecosystems.
Three reasons why invasive species can have negative effects on biodiversity and ecosystems are (1) they can threaten native species with extinction through predation, competition, and disease transmission; (2) they can change the functions and services of ecosystems by affecting fire cycles, water availability, and sedimentation; and (3) they can change the genetic diversity of native species through hybridization events.
Suppose you have ecological data for a species of butterfly over the last 50 years. The data show that this species’ range has moved north by 100 km and that its emergence occurs 4 days earlier than 50 years ago. Do these data suggest the butterfly species is responding to climate warming? Explain.
Yes, these data strongly suggest that climate warming is causing the butterfly species to shift its distribution to cooler, more optimal temperature conditions farther north. Likewise, emergence time is occurring earlier, when temperature conditions are optimal for that stage of the species’ life history.
A major objective of understanding how and why biodiversity loss is occurring is to determine how conservation and management strategies might help preserve species. In the next section we will consider some positive steps that are being taken to recover and protect biodiversity.