Gaining and Losing Biodiversity

Island biogeograpy | Measuring diversity | Gaining biodiversity | Losing biodiversity

Globally, diversity naturally has increased over time, though the great mass extinctions have decreased it for a while. The most famous of the mass extinctions is the one that claimed the dinosaurs, but we are currently in the midst of a human-created mass extinction. Local diversity, on the other hand, is constantly increasing and decreasing at very short time scales. There are many factors that affect diversity, and the major natural circumstances are given here. Human-generated impacts on diversity have almost always been negative, and are covered in the Conservation Issues section.

Island Biogeography

One of the first major theories of biodiversity, the theory of island biogeography, formulated by R. MacArthur and E.O. Wilson applies to the patterns of diversity found on islands. In it, islands start out empty of species, who arrive from a large area (referred to as the mainland, though it doesn't have to actually be a continent) and from neighbouring islands.

The chance that a species will land on the island depends mostly on the distance that the island is from the mainland; the greater the distance the less often a species will find its way to the island. In the figure to the right, island A is closer to the mainland than island B, and more species find their way to it. Species on smaller islands have smaller populations, making them more vulnerable to extinction. The number of species present on the island is a balance between the rate at which new species arrive and old species go extinct on the island.

This theory is neutral, meaning that all species are considered to be equal. In reality, some species are better at dispersing than others and are thus more likely to be found on islands. The exact species that are actually present has been found to be fairly random, though, and the theory does a good job of predicting the number of species to be found. What the theory calls islands doesn't have to be actual islands; lakes are effectively islands, as are isolated patches of habitat, and the theory has been extended to deal with peninsulas, bays, and other only partially isolated areas.

Measuring Diversity

To detect changes in biodiversity there has to be a way to measure it. Although at first glance biological diversity seems to be an obvious idea, quantifying it is much more difficult. Making an attempt to express it as a single number is futile, as a single number cannot hope to convey the different components. There are three common ways to measure diversity:

Numbers:

	Area 1

It is possible to measure how many species are found in an area, or how many alleles (defined above) a species has for a single locus, or how many functional groups (defined below) or taxonomic groups higher than species are present in an ecosystem. This is considered a reasonable if incomplete way of measuring diversity, and can be expressed as the number of species found per unit area, per unit mass, or per number of individuals identified. What controversy exists about this component is mainly about how to standardize measures that are taken at different scales.

Evenness:

	Area 2

If almost every individual in an area is from the same species, the diversity would not seem high, even if there are many species present. Evenness measures to what extent individuals are evenly distributed among species (if one is looking at the species level). The most common values that are used are species number and species evenness. How to represent even these two components as a single number has been controversial (see Magurran 1988 and Smith and Wilson 1996 for examples of indices and the controversy), and the number of different abundance indices is large, although certain ones (the Shannon and Simpson indices, for instance) are far more commonly used than others.

Difference:

	Area 3

A site with many species is considered to have high diversity, but what if those species are all very closely related? If another site had fewer species, but those species were more distantly related, would that second site have a lower or higher diversity? Measuring the evolutionary distance between the different units is important, as it is on a different level than something like species number, which doesn't measure how different the species are. Measurements of difference include disparity and character diversity.

Three sample areas are given to the above right, each of which is most diverse in a different way. The top area (area 1) has the greatest number of species, four in total. But half of the individuals in the sample are from the same species. The middle area (area 2) has fewer species, only three, but it has a greater evenness; there is an equal chance of getting an individual from each of the three species. The bottom area (area 3) has even fewer species, just two, but it has the greatest difference. While the other samples contain only insect species, this one contains both insects and a mammal, which is very distantly related to insects.

Gaining Biodiversity

Mutation
Mutations increase genetic diversity by altering the genetic material (almost always DNA) of organisms. Once mutations arise, they are passed on to the mutated organism's offspring, and in time may either disappear if the line dies out. Depending upon the specific mutation, the result can range from no effect whatsoever to the creation of an entirely new species. Although this gives rise to differences in organisms, it is an extremely slow process compared to the other ways in which local diversity increases. Ultimately, though, this is the only way in which diversity is truly created.

Speciation
The creation of a new species is known as speciation. Species are typically defined as being unable to successfully breed with other species (the so-called Biological Species Concept), although there are other ways of defining species. The origin of new species naturally has the largest immediate effect on species-level diversity; the immediate changes to genetic and ecosystem diversity are usually minimal, though the effects will grow in time. Speciation can occur through several different means, including geographical isolation, competition, and polyploidy. These are described below.

Geographical Isolation: Geographical isolation, such as new mountain chains or a lake whose level lowers enough that it splits into two separate lakes, can divide a population into two separate populations. The two isolated populations continue to evolve separately from one another. Eventually they can diverge to a great enough degree, either through adaptation to their differing environments or through random mutations, that they are no longer able to interbreed and are considered to be different species.

Competition: If a new resource, such as a new food source, becomes available to a population, some part of the population may become specialized in obtaining that resource. Being specialized in obtaining either the new resource or the original resource may be better than trying to obtain both. If so, then the specialists would be better off mating with the other specialists on the same resource, as mating with someone who uses the other resources will result in offspring that aren't specialized for either resource and at a disadvantage. In time, there is a chance that the population will split into two species, each specialized on one of the two resources. This can happen, but it is probably a fairly rare event.

Polyploidy: Speciation through polyploidy happens far more often in plants than in animals, as animals are much more sensitive to large changes in their genetic structure. Most species are diploid, meaning they have two ("di" meaning two) copies of each chromosome (large packages of DNA), one from each of their parents. An individual in a normally diploid species may have more copies of these chromosomes, being polyploid ("poly" meaning many), through errors at the cellular level. The additional copies of the chromosomes render them unable to produce functional offspring with normal members of their species. Plants often fertilize themselves to at least some extent, so polyploid species can arise from a single individual. This method of speciation is almost instantaneous, happening in a single generation, and is more common in plants than animals.

Immigration
Immigration increases diversity as new individuals and perhaps even new species enter an area, increasing its diversity. The rate at which immigration happens depends on the size of the area in question, how many species are there already, and how close the area in question is to the source of immigration. Even if a species is unable to survive in an area, a constant flow of immigrants to the area can keep the species present indefinitely. Island biogeography is the classic theory on the topic of how these factors affect immigration and more, and is explained above.

Most species that immigrate to a new ecosystem have only minor effects on their new system, though some drastically change it. Zebra mussels, native to the Caspian Sea and Ural river, were first recognized in the Great Lakes in 1988. It is most likely that they were brought over in ballast water. Since then they have spread throughout the Great Lakes and beyond, killing native mussel populations and fouling all manner of pipes and intakes.

Succession
Succession is the process through which an area gains species as successive communities of organisms replace one another until an endpoint is reached. This endpoint, or climax community, is commonly a forest in southern Canada. Succession may begin on bare rock, an abandoned field, the burned remnants of a forest, or any stage before the endpoint. A hypothetical bare field isn't bare for long before annual plants appear. They are replaced within a few years by perennial plants and shrubs, who in turn are replaced by pine trees. Eventually, hardwood trees invade and replace the pines, forming the hardwood climax community.

Different regions have varying climax communities; the tundra of the north is extremely different from the grasslands of the prairies or the west coast rainforests, though they are all the local endpoints of succession. One usually refers to the different stages of succession in terms of the plants rather than the animals because the plants precede the animals and provide the structure and environment that the animals live in. One exception to this is aquatic communities, where sponges, corals, bivalves and other animals are responsible for much of the three-dimensional structure of the community. The climax community is typically the most diverse stage of succession, and each stage of succession is more diverse than the one preceding it. This pattern depends on the group being looked at; plant diversity actually decreases at the final stage, while animal diversity increases to the end. Species that were common in the early stages of succession will not be common in the later stages, but may still be found if small disturbances in the area effectively set the disturbed area back to an earlier successional stage (see the page on Abundance and Composition for more details).

Losing Diversity

Extinction
Extinction is more an outcome than a process. Once a species goes extinct, all the diversity that it represented is lost forever. The vast majority of species that have ever existed are now extinct through natural processes, whether by mass extinction or by the more common individual extinction. Genes also go extinct if they fail to get passed on to the next generation, though it's not necessary for the entire species to go extinct as well. Ecosystems may be destroyed by severe disturbances, but they don't really go extinct unless the species that make them up are lost.

Species can also go locally extinct; in this case, they are said to be extirpated. Although the local loss of diversity is the same, the species still exists elsewhere and may be able to return in the future through immigration. Much the same thing can happen to genetic diversity, as particular alleles are lost in a population.

Competition
If one species outcompetes others to a dramatic extent, the result may be extirpation or perhaps even extinction of the other species and a reduction of diversity. Diversity, in the sense of evenness, will also be lowered if other species have their populations greatly reduced by a competitor or predator, even if the species aren't extirpated. As species that have been eliminated simply aren't around, it's rare to see this process happening unless a species has recently invaded or conditions have recently changed.

Disturbances
Disturbances can maintain diversity (see Abundance and Composition, below), but extremes can reduce diversity. Constant large-scale disturbance can eliminate many populations and keeps an area at the early levels of succession, which have lower diversity (see above). An area with no disturbances at all would end up completely at the final stage of succession. This would prevent the presence of the species that would normally be found at intermediate stages of succession, living in the disturbed areas.

Bottlenecks
Genetic bottlenecks happen when many individuals in a population die. In the example to the right, the population initially has many different types of shapes and colours, representing genetic diversity (A). The few individuals that are left after most die (B) have a small amount of the genetic diversity that originally existed, as much of the genetic diversity was lost with the rest of the population. Although the population's numbers quickly recover (C), the genetic diversity is much slower to respond, which can cause problems if conditions change in the future, as the reserves of diversity that would be useful won't be there.