A fundamental goal of conservation is to prevent species from becoming extinct, be it regional or global. But how do we define a species’ risk of extinction?
A species can be described as Critically Endangered if the probability of extinction in 10 years or three generations is considered to be more than 50%; endangered if there is more than a 20% chance of extinction in 20 years or five generations; vulnerable if there is a greater than 10% chance of extinction in 100 years; near-threatened if a species is close to qualifying for a threat category or judged likely to qualify in the near future; of least concern, if a species does not meet any of these threat categories.
Based on the above criteria, for example, 12% of bird species, 20% of mammals, and 32% of amphibians are threatened with extinction (being critically endangered, endangered, or vulnerable.
Species that are at high risk of extinction are almost always rare, but not all rare species are at risk. We need to ask what precisely we mean by rare. A species may be rare in the sense that its geographic range is small, or in the sense that its habitat range is narrow, or because local populations, even where they do occur, are small.
Species that are rare on all three counts, such as the giant panda (Ailuropoda melanoleuca), are intrinsically vulnerable to extinction. However, species need only be rare in one sense in order to become endangered. For example, the peregrine falcon (Falco peregrinus) is broadly distributed across habitats and geographic regions, yet, because it exists always at low densities, local populations in the USA have become extinct and have had to be re-established with individuals bred in captivity.
However, rare species are not necessarily endangered because of their rarity. In fact, it appears that many species, probably most, are inherently rare. All things being equal, driving a rare species to extinction will be easier simply because a local effect may be enough to push it over the edge.
Some of the threats to biodiversity are overexploitation, habitat disruption, invasive species and climate change.
Overexploitation
The essence of overexploitation is that populations are harvested at a rate that is unsustainable, given their natural rates of mortality and capacities for reproduction.
Sharks provide an interesting example. Among the most feared of species (although attacks are much rarer than held in the popular imagination), large numbers are taken for sport, many others to make shark fin soup, while a large proportion of the estimated annual 200 million shark kills are accidental by-catches of commercial fishing. Evidence is mounting that many species of shark have been declining in abundance, a trend that should come as no surprise given their late ages of maturity, slow reproductive cycles and low fecundities.
Sharks are among the most important predators in the marine environment, and their enforced rarity may have widespread repercussions in ocean communities.
A feature of animals that are collected for ornamentation, whether for their body parts or as exotic pets, is that their value to collectors goes up as they become rare. Thus, instead of the normal safeguard of a density-dependent reduction in consumption rate at low density, the very opposite occurs.
Harvesting of pangolins for their scales and meat, and as curiosities, has led to a drastic decline in population size for this fascinating creature.
The phenomenon is not restricted to animals. New Zealand’s endemic mistletoe (Trilepidia adamsii), for example, parasitic on a few forest understorey shrubs and small trees, was undoubtedly overcollected to provide herbarium specimens. Always a rare species, its extinction (recorded from 1867 to 1954 but not seen since) was due to overcollecting combined with forest clearance and perhaps an adverse effect on fruit dispersal because of reductions in bird populations.
Habitat disruption
Habitats may be largely affected by human influence in three major ways. First, a small area of the habitat available to a particular species may simply be destroyed, for industrial, commercial and urban development or for the food production and other natural resources such as timber. Second, habitat can be degraded through pollutants to the level that situations turn out to be untenable for specific species. Third, habitat may be disturbed by human activities.
Forest clearance has been, and is still, the major reason for habitat destruction. Much of the native temperate forest in the developed world was destroyed long ago, while current rates of deforestation in the tropics are 1% or more per annum. As a consequence, more than half of the wildlife habitat has been destroyed in most of the world’s tropical countries.
The process of habitat destruction often results in the habitat available to a particular species being more fragmented than was historically the case. This can have several repercussions for the populations concerned. Degradation by pollution can take many forms, from the application of pesticides that harm non-target organisms, to acid rain with its adverse effects on organisms as diverse as forest trees, amphibians in ponds and fish in lakes, to global climate change that may turn out to have the most pervasive influence of all.
Aquatic environments are particularly vulnerable to pollution. Water, inorganic chemicals and organic matter enter from drainage basins, with which streams, rivers, lakes, and continental shelves are intimately connected. Land use changes, waste disposal and water impoundment and abstraction can profoundly affect their patterns of waterflow and the quality of their water.
Habitat disturbance is not such a pervasive influence as destruction or degradation but certain species are particularly sensitive. For example, diving and snorkeling on coral reefs, even in marine protected areas, can cause damage through direct physical contact with hands, body, equipment and fins. Often the disturbance is minor, but this can amount to cumulative damage and reduction in the populations of vulnerable branching corals.
In one analysis of 214 divers in a marine park on Australia’s Great Barrier Reef, 15% of divers damaged or broke corals, mostly by fin flicks. Impacts were much more likely to be caused by male than female divers, whilst specialist underwater photographers caused more damage on average (1.6 breaks per 10 minutes) than divers without cameras (0.3 breaks per 10 minutes).
Nature recreation, ecotourism and even ecological research are not without risk of disturbance and the decline of the populations concerned.
Invasive species
Invasions of alien species into new geographic areas sometimes occur naturally and without human intervention. Human action, however, turned this trickle into a flood. or legitimately achieve an anticipated public benefit by controlling a pest, producing new agricultural products, or providing novel recreational opportunities..
It is important to remember that not all introduced species become invasive. Many introduced species are assimilated into communities without much apparent effect. However, some are responsible for dramatic changes in native species and natural communities.
For example, the accidental introduction of the brown tree snake Boiga irregularis on Guam, a Pacific island, has reduced 10 species of endemic forest birds to extinction through nest predation. The snake’s gradual expansion from its beachhead population in the center of the island coincided in time with the loss of bird species to the north and south.
Similarly, the introduction of the predatory Nile perch (Lates nilotica) as a food source for humans in the highly biodiverse Lake Victoria in East Africa has driven most of its 350 endemic fish species to the brink of extinction. Preservative biologists are particularly concerned about the effects of the introduced species, where always domestic agencies are present, which are largely endemic (that is nowhere else in the world).
Early evidence suggests that another fungal pathogen, Geomyces introduced from Europe is responsible for white-nose syndrome, which infects cave hibernating bats in eastern North America and has spread from a point of origin in western New York State. The disease has decimated bat populations and threatens the extinction of species already listed as endangered: the Indiana bat, Myotis sodalis, and potentially the Virginia big-eared bat, Corynorhinus townsendii virginianus. How the fungus was introduced is unknown, but one logical presumption would be that recreational cavers unintentionally brought the fungus on clothes or equipment from Europe.
Indeed, one of the major reasons for the world’s great biodiversity is the occurrence of centers of endemism so that similar habitats in different parts of the world are occupied by different groups of species that happen to have evolved there. If every species naturally had access to everywhere on the globe, we might expect a relatively small number of successful species to become dominant in each biome. The extent to which this homogenization can happen naturally is restricted by the limited powers of dispersal of most species in the face of the physical barriers that exist to dispersal.
By virtue of the transport opportunities offered by humans, these barriers have been breached by an ever-increasing number of exotic species. The effects of the introductions have been to make an enormously diverse range of local church compositions much more homogeneous. However, it would be a mistake to conclude that the introduction of species into a region would inevitably lead to a decline in biodiversity there.
For example, there are various species of plants, invertebrates and vertebrates determined in continental Europe however absent from the British Isles (many due to the fact they have got thus far didn’t recolonize after the remaining glaciation). Their advent could be probable to reinforce British biodiversity.The large unfavourable impact mentioned above arises wherein competitive species offer a unique venture to endemic biotas illequipped to cope with them.
Climate Change
Climate change, and in particular the current ongoing anthropogenic warming trend, is recognized as a major extinction threat, especially when combined with other threats such as habitat loss. Anthropogenic warming of the planet has been observed and is due to past and ongoing emissions of greenhouse gases, mainly carbon dioxide and methane, into the atmosphere caused by fossil fuel burning and deforestation.
Scientists predict that climate change will affect regional climates, including precipitation and snowfall patterns, making habitats less hospitable to the species that live there. The warming trend will shift cooler climate zones toward the north and south poles, forcing species to move (when possible) with their adapted climate norms.
The rate of warming appears to be accelerating in the Arctic, which is thought to pose a serious threat to polar bear populations, which need sea ice to hunt seals during the winter months. Seals are an important source of protein for polar bears. Since observations began in the mid-20th century, there has been a trend towards decreasing sea ice cover.The reduction rate observed in recent years is much larger than previously predicted by climate models.
