7775

• UCREFRP Catalog Number: 7775
• Author: Carlson, C. A. and R. T. Muth.
• Title: Inland Fisheries Management in North America, Chapter 15
• USFW Year: 1993.
• USFW - Doc Type: Endangered Species Management.
• Copyright Material: YES

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CHAPTER 15

and compressed extinction as that which we now face. Predictions of the
percentage of today's species that will be iost by the end of this century range
from 15 to 50 (Shen 1987).
Many see a need for human intervention to slow the accelerated rate of
extinction, but attempts to reduce the extinction rate are unlikely to succeed
unless causes of extinction are understood. Humphrey (1985) listed three biolog-
ical circumstances that lead to extinction: natural rarity, lack of habitat, and
inbreeding depression (loss of genetic diversity). Persistent rarity often arises
from a position high on a grazing food chain or from large body size. An organism
that has evolved to maximize its fitness in a particular limited habitat is a
candidate for extinction if that habitat is threatened. Liability to extinction is
decreased if a species occurs in a large number of semiautonomous populations
over a wide geographical range. Without sufficient numbers of individuals, genetic
diversity in biological populations may be decreased to harmful levels. The
concept of minimum viable population-a level of abundance above which
persistence, without loss of fitness, for a given length of time is assured-has
recently come into vogue (Soule 1987).
Environmental change resulting in extinction may be physical or biological in
nature. Habitat disruption, a type of physical change, is the foremost cause of
plant and animal extinctions, and it is usually difficult or impossible to rectify.
Wolf (1987) stressed the significant effects offragmenting once-continuous habitat
into island-like refuges; animals typically die out in a manner referred to as faunal
collapse. Habitat destruction in tropical forests, where up to half the species on
earth reside (Myers 1986), is widely recognized as the greatest threat to wild
plants and animals. Extinction from biological causes may come about from
improved abilities of predators (including people) or competitors, which evolve
over time, or invasion by nonnative species. Humans have contributed to
extinction through commercial hunting, predator and pest control, and collection
of organisms for medical research, zoos, and sources of houseplants. Impacts of
alien organisms introduced by humankind may combine with effects of human
exploitation to devastate native species. Ehrlich and Ehrlich (1981) provided
excellent reviews of how species endangerment by humans occurs directly (e.g.,
by overexploitation and predator control) or indirectly (e.g., in their words,
through paving over, plowing under, spewing, cutting down, transporting, and
recreation).

15.1.2 Reasons to Minimize Extinction

Ehrlich and Ehrlich (1981) grouped reasons to care about extinction into (1)
direct economic benefits to humans; (2) indirect (ecological) benefits to humans:
and (3) compassion, aesthetics, fascination, and ethics. Direct benefits include
allowing us to meet future agricultural, pharmaceutical, and industrial needs.
Potentially useful species may become extinct before humans discover and exploit
them as sources of food, fiber, shelter, recreation, medicines, lubricating oils and
waxes, energy, biological pest controls, or research subjects.
The ecological benefits of maintaining diverse natural systems are less obvious,
more difficult to convey to nonbiologists, and sometimes the subject of contro-
versy among ecologists. Myers (1979a) emphasized the role of biological diversity
in contributing to stability of ecosystems. Ehrenfeld (1976) described the diver-