Extent of endangerment
Threatened species of animals fall into the categories of critically endangered, endangered, and vulnerable, as defined below. IUCN (1996) classified more than 50 percent of species in every one of the vertebrate classes into one of the threatened categories.
The situation with plants is similarly alarming. IUCN (1997) classified 12.5 percent of vascular plants as threatened, with a much higher proportion of gymnosperms (32 percent) than angiosperms (9 percent) being threatened. Estimates for invertebrates and microbes are not available as the number of extant species in these groups is not known.
Recorded extinctions
ince 1600 for different groups of animal and plants on islands and mainlands are given in
Table 1. The proportions of species in different groups that have gone extinct are small, being only 1-2 percent in mammals and birds. However, the pattern of extinctions is a matter for concern as the rate of extinction has generally increased with time (Figure 1) and many species are threatened. Further, many extinctions must have occurred without being recorded; habitat loss must have resulted in many extinctions of undescribed species of invertebrates and plants.
The majority of recorded extinctions, and a substantial proportion of currently threatened species, are on islands. For example, 81 percent of all recorded bird extinctions are insular, yet only about 20 percent of bird species have existed on islands.
Projected extinction rates
Several projections of extinction levels into the future are given in
Table 2. While these estimates are crude and vary widely, there is a consensus that extinction rates are destined to accelerate markedly, typically by
1,000-fold
or more above
'normal'
background extinction rates.
Average
lifespans
of species provide an alternative way of viewing rates of extinction. The average
lifespan
of an animal species in the fossil record, from origin to extinction, is around 1-10 million years, with the higher number being more typical. For birds and mammals, rates of documented extinction over the past century correspond to species'
lifespans
of around 10,000 years. Three different methods outlined in Lawton and May (eds.),
Extinction Rates
(1995) suggest an average
lifespan
for bird and mammal species of around 200-400 years if current trends continue i.e. current extinction rates are 5,000-25,000 times those in the fossil record.
What is an endangered species?
The IUCN
(1996) has defined criteria to classify species into
critically endangered, endangered, vulnerable and lower risk. These are based on population biology principles developed largely by Mace &
Lande
('Assessing extinction threats',
Conserv. Biol. 5, 1991, pp. 148-157). They defined a
threatened species as one with a high risk of extinction within a short time frame. For example, a critically endangered species has a risk of extinction of 50 percent within 10 years or three generations, whichever is longer (Table 3).
IUCN (1996) set out simple rules to define these categories in terms of the rate of decline in population size, restriction in habitat area, the current population size and/or the probability of extinction. A critically endangered species exhibits any one of the characteristics described under
A-E
in
Table 4, i.e. it has either an 80 percent or greater decline in population size over the last 10 years (or three generations), or an extent of occupancy of less than 100 square kilometres, or a population size of less than 250 mature adults, or a probability of extinction of 50 percent or more over 10 years (or three generations), or some combination of these. For example, there are only about 65
Javan
rhinoceroses surviving in Southeast Asia and the numbers are continuing to decline, so this species falls into the category of critically endangered.
There are similar, but less threatening characteristics required to categorize species as endangered, or vulnerable. Species falling outside these categories are designated as lower risk. IUCN has also defined categories of extinct, extinct in the wild, conservation dependent, near threatened and data deficient (IUCN 1996).
While there are many other systems used throughout the world to categorize
endangerment,
the IUCN categorization system is used as the basis of listing species in the IUCN Red Books of endangered animals (IUCN 1996).
Importance of listing
It is of great importance to define
endangerment,
as it is the basis for legal protection for species. For example, most countries have Endangered Species Acts that provide legal protection for threatened species and usually require the formulation of recovery plans. In addition, trade in threatened species is banned by countries that have signed the Convention on International Trade in Endangered Species (CITES). This provides important protection for threatened parrots, reptiles, cats, fish, whales, etc.
What causes extinctions?
Human-associated factors
The primary factors contributing to extinction are directly or indirectly related to human impacts. Since the human population is growing rapidly (Figure 2), the impacts of these factors are continually increasing. The human population reached 6 billion on 12 October 1999, the last billion increase (20 percent) having occurred in only 12-14 years. The human population will continue to increase. By 2050, the population is projected to rise to 8.9 billion, with a range of projections between 7.3 and 10.7 billion. However, in 'How many billions to go?', V. Smil has shown that the rate of increase has declined from a peak of just over 2 percent per year to below 1.5 percent in the early 1990s (Nature 401, 1999, p. 429).
The total human population is projected to climax at 10-11 billion around 2070 and then begin to decline. Even the lower projection of a peak population size of 7.7 billion in 2040 represents a 28 percent increase above the current population. Consequently, human impacts on wild animals and plants will continue to worsen in the foreseeable future.
Stochastic factors
Human-related factors can reduce species to population sizes where
they are susceptible to stochastic effects. These are naturally occurring
fluctuations experienced by small populations. These may have
environmental,
catastrophic, demographic, or genetic (inbreeding
depression,
and loss of genetic diversity) origins. Even if the original cause
of
population decline is removed, problems associated with small population size
will still persist.
