Why some species become at risk

Through a long history of evolution, each species has become adapted to fit into a particular ecological niche. When things change beyond a certain limit, some species no longer prosper, and their numbers decline until they becomes at risk.

There are many, often complex and interrelated, reasons why certain species decline and become at risk. Various outside forces (factors in a species' environment) obviously influence how well a species is able to survive. In addition, more subtle internal forces, such as the specific biological requirements of a species and its ability to adapt to change, determine whether and how well a species can cope with the outside changes in its environment.

Below are some of the most prevalent environmental factors contributing to species decline.

Habitat loss and degradation

The single most prevailing factor responsible for the endangerment of species today is habitat loss and degradation. In fact, about 60 percent of species that the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) identified as being at risk are affected by habitat problems. If a species cannot find suitable conditions in which to live, it simply will not survive.

As the human population grows, development increases and spreads over the landscape to satisfy human wants and needs. The construction of houses, buildings, and roads; logging of vast tracts of forest for paper and building materials; mineral extraction; and conversion of wild habitats into agriculturally productive fields all mean that habitat for wild species shrinks. And when habitat shrinks, species are squeezed out.

A habitat does not have to be totally destroyed to make it unsuitable for some species. The mere presence of people and associated disturbance can cause some species to abandon certain habitat or prevent them from breeding successfully.

And human presence affects species in many other ways as well. Vehicles on roads are particularly dangerous to some snakes that like to bask on the warm pavement and to some birds that tend to feed near roadways. The lights from vehicles and from street lights and buildings have been shown to seriously affect some moth populations. Control of water flow in rivers, usually for the generation of electricity, changes conditions downstream, often rendering these water bodies unsuitable for certain species, or restricts their ability to travel to parts of the system they need for feeding or to reproduce. The building of dams or tilling of soil near rivers and streams causes siltation and increases water turbidity, factors responsible in the decline of some fish and mollusc populations.

Genetic and reproductive isolation

When habitats are destroyed or become degraded, a species' range becomes fragmented. As parcels of suitable habitat become smaller and barriers between these islands of suitable habitat become greater, remnant populations become increasingly isolated. This means that individuals become restricted to fewer and fewer reproductive partners, and so genetic exchange and mixing can no longer occur. The more limited a population's genetic variability, the less able is the population to deal with change, disease or other factors and the less likely it is to survive over the long term.

The Kentucky Coffee-tree illustrates the problem of genetic isolation resulting from habitat fragmentation. Most of the remaining trees produce single-sex clones but are located so far apart that pollen from one can no longer reach others. Thus, although the trees flower, they do not produce seeds. Without help, most of the Canadian population of the Kentucky Coffee-tree will exist only as long as the current individuals can continue to reproduce vegetatively.

Suppression of natural events

It is not only the activities that directly damage habitats that are important. Sometimes it is the natural occurrences that people prevent from occurring that are just as detrimental. Suppression of forest fires, for example, has allowed woody species to invade open areas, shading out species that need more open habitats. Control of water flow in rivers has put an end to periodic spring flooding, thereby preventing the replenishment of adjacent ponds. This has altered the moisture regime and therefore the habitat as well, making it no longer suitable for some species.

Environmental contamination

Today, many species are exposed to environmental contaminants, probably the second most important factor (after habitat loss and degradation) that causes species endangerment. DDT, Fenitrothion (once sprayed on forests to control spruce budworm) and carbofuran (used to control grasshoppers) are several chemicals that do a great deal of damage to wildlife. In addition, many chemicals are discharged by industry into the air or water, or leach off forested or agricultural lands into water bodies and can be detrimental to a variety of organisms. Acid rain has done its share of damage.

Chemicals discharged into the environment can have indirect effects. Recent studies indicate that depletion of the ozone layer, caused by chemicals that are released into the atmosphere may be having serious detrimental effects on wildlife. For example, it is believed to be responsible for reducing the survival of amphibian eggs before they hatch.

Over harvesting and excessive trade

In the early twentieth century, over harvesting of wildlife (both plants and animals) for food or pelts was the most important threat to wildlife. It drove species like the passenger pigeon and the great auk to extinction, and reduced the numbers of eskimo curlews to such low numbers that they were never able to recover. Still today, species such as the right whale, sea otter, aurora trout and american ginseng are either over harvested or are still suffering the aftereffects of previous over harvesting.

Several species are also threatened by excessive trade. These species are harvested as a food resource and for their pelts. They are also taken from the wild and traded for their medicinal properties, as hunting trophies, as ornamental plants or as pets. Some of the coveted Canadian species are the peregrine falcon, the goldenseal, orchids, sturgeons and bears. This trade is controlled by the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) in Canada and worldwide.

Climate change

Climate change, whether naturally occurring cycles or the slow climate change caused by human activities, affect species in an insidious way that is difficult to evaluate or quantify.

Sudden climatic events, like extreme storms, take their toll as well. One example is the crusting over of the tundra in a large part of the Canadian Arctic during several winters, which prevented peary caribou from being able to dig down to reach their food. To date, the animals have not recovered from the massive starvation and die-offs that occurred.


Outbreaks of disease can decimate populations. Under normal circumstances, effects are limited, as they occur in rather restricted areas. But when all individuals of a species occur in a small remnant range, disease can have devastating effects on the entire population, and hence on the species' survival.

Invasive species

The introduction of invasive plants and animals by humans has also taken its toll on native wildlife. It is quickly becoming an increasingly important factor causing species decline. House Sparrows and Starlings are at least partially responsible for declines in Bluebird populations. Zebra Mussels are responsible for the rapid decline of a number of freshwater clams and mussels. Scotch Broom is invading native habitats on Vancouver Island and choking out native herbaceous vegetation, including some rare species. And the introduced White Mulberry is hybridizing with the native red mulberry and threatening the loss of the native species through genetic swamping.

There are many factors affecting species. In many cases, it is impossible to identify a single reason for a species' decline. Often it is the insidious cumulative effects of several factors that result in the endangerment of species.

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