•To preserve Earth’s biodiversity, we need to both maintain the processes that
generate new species and provide conditions that will keep extinction rates at
a typical level
•There are four reasons why scientists cannot accurately predict the number of
oDo not know how many species live on Earth.
oDo not know where species live (i.e. animal ranges are poorly known).
oDifficult to determine when a species actually becomes extinct.
oDo not know what will happen in the future.
•Regardless, there are some methods for estimating probable rates of extinction
resulting from human actions (e.g. habitat destruction).
•Species area relationship—mathematical relationship between the size of
an area and the number of species that area contains.
•Findings suggest that a 90 percent loss of habitat will result in the loss of half
of the species that live in and depend on that habitat.
•Current rate of loss of tropical evergreen forests is about 2% of remaining
forest each year; if this continues, at least 1 million species there could
become extinct during this century.
•To estimate the risk that a population will become extinct, conservation
biologists develop statistical models that incorporate information about pop.
size, its genetic variation, and the morphology, physiology and behaviour of its
•Species in imminent danger of extinction in all or a significant part of their
range are labelled endangered species.
•Threatened species are likely to become endangered in the near future.
•Rarity is not ALWAYS a cause for concern b/c some species that live in highly
specialized habitats (e.g. panda) have probably always been rare and are well
adapted to those conditions.
•Species whose populations suddenly shrink at rapid rates (i.e. “newly rare”)
are usually at high risks.
•Species with special habitats or dietary requirements are more likely to
become extinct that species with more generalized requirements.
•Populations with only a few individuals confined to a small range can easily be
eliminated by local disturbances.
57.3 What Factors Threaten Species Survival?
Species are endangered by habitat loss, degradation, and fragmentation
•Habitat loss is the most important cause of species endangerment.
•A habitat loss also affects the remaining habitats that are not destroyed.
•As habitats are progressively lost, the remaining habitat patches become
smaller and more isolated and becomes increasingly fragmented.
•Small habitat patches are qualitatively different from larger patches of the
same habitat in ways that affect the survival of species.
•Small patches cannot maintain populations of species that require large areas;
can support only small populations of many of the species that can survive in
•The fraction of a patch that is influenced by factors originating outside it
increases rapidly as patch size decreases So, species from surrounding