There are currently between 1.5 to 1.8 million named species in the world, about half of which are insects. The largest group of insects is the beetles (the order Coleoptera), with 300 000 species. In contrast, there are only 4 500 species of mammals.
These numbers are just for the known and named species. No one knows how many species are still to be discovered. Estimates for the total number of species on the planet range from three million to 100 million, though most generally accepted estimates are between five and 20 million. This range of estimates emphasizes how little we presently know about biodiversity. Some groups are better known and more completely identified than others; new mammal species are rarely found, but an seemingly endless stream of new insect species are found wherever one looks. It would be almost impossible to identify all the species of bacteria (most of which would be new) in a kilogram of earth from an average temperate forest.
Despite this large number of species currently inhabiting the planet, almost all species that have ever existed are now extinct. No one knows how many species have existed in the 3.8 billion years that life has been on this planet, but the current number is a small proportion of that.
An estimated 140 000 species live in Canada, only half of which have been identified. A list of some of the major groups is given in Mosquin et al. 1995, and some of the groups are given here. Most of the larger organisms (mammals, birds, trees) have been almost completely identified, and it's the smaller creatures that account for most of the unidentified species; over one fifth of all species in Canada are insects. Estimates of how many species of the more obscure groups, such as the nematodes, are little more than guesses.
Exotic species are constantly being introduced to new locations around the world, and Canada is certainly no exception. In many cases, the introduced species die out, leaving no permanent impact. Other times, though, they can grow out of control and directly interfere with native ecosystems, perhaps even dominating them and driving native species to extinction. A few of the better-known examples of successful exotic species in Canada include Dutch elm disease, purple loosestrife, zebra mussels, sea lampreys and starlings. The number of exotic plant species is staggering; it is estimated that 800 of the 5 000 plant species in Canada are exotic introductions and successful enough to maintain populations in the wild!
Introductions of exotic species can be accidental or intentional. Accidental introductions of aquatic species often occur when species arrive with ships, either attached to them or carried in the bilgewater, or new canals are built, allowing species to make their way around barriers that previously stopped their spread. These are both aquatic types of accidental introductions; accidental land introductions are rarer in Canada, usually arriving in imported goods.
Intentional introductions are usually for food or decorative purposes. Plants and sport fish are most commonly introduced in Canada. Most tree species found on city streets and a surprising number of agricultural species that seem native, such as apples and potatoes, are actually introduced. Game fish have been introduced over a large part of the country, and are still introduced annually into many lakes to maintain levels for sport fishing.
The Great Lakes have been the site of many of Canada's introductions of exotic species. The cargo of ships along with the ships themselves often carry new species, and the docks and canals created to allow the ships access to the Great Lakes have also allowed new species to bypass the falls, rapids and stretches of land that once prevented their spread.
Flowers, butterflies, birds, and other pleasing organisms are typically used to symbolize biodiversity. Large mammals such as the lynx and panda are especially popular, and the term "charismatic megafauna" has been used to describe them. Other, less attractive species, such as slugs, fungi or worms are rarely mentioned despite their importance in the functioning of ecological systems. The vast majority of species belong to these less charming groups; there are more Canadian species of earthworms (Oligochaeta) than mammals, and there are more Canadian species of clams and mussels than birds.
The more popular, large, or economically-important a group is, the more is known about it, including its distribution. Canada is such a large country, and so sparsely settled for the most part, that intensive surveys are virtually impossible to perform for the entire country (see the Patterns of Biodiversity section for more information on biological surveys).
The ideal distribution maps would be a regular grid with the abundance of every species in every grid space clearly marked. Unfortunately, none of the maps that are in this section are of this type, and there are extremely few of these sorts of distribution maps anywhere in the world. When they do exist, they are usually for small areas; the resources needed for proper large-scale surveys are large.
When the distribution maps in this section have large solid areas (known as area maps), the species isn't found everywhere in the indicated area. The area instead represents where the species is likely to be found when the conditions are right, which is most of the indicated range. The actual shape of the areas is still determined as much as possible by actual records. More well-known species tend to have these sorts of maps.
Dot maps, formed by scattered dots, are usually sparse due to insufficient records. Each dot represents a location where the species was found, but areas that don't have dots have often simply not been searched yet. These maps show where a species definitely is, but where else it may be or where it is not found is uncertain. Although the species are found elsewhere, their pattern of distribution isn't known well enough to guess where, and so only actual records are shown. Less popular or hard to identify species have these sorts of maps unless an intensive effort has been made to obtain a more complete distribution.