Abundance and Composition
In addition to diversity increasing and decreasing, it can also change by alterations in the relative numbers of individuals in species or by the particular species that are present. Understanding how the specific species and numbers present got there and interact is the focus of this section. In addition to two theoretical techniques that are used to work out how diversity takes shape, some of the known ways in which abundance and composition are affected are covered.
One question that comes up when dealing with biodiversity is why there are so many species in the first place. Why doesn't a single species outcompete and eliminate the rest? The answer is that no species can be perfect at everything; it must instead make trade-offs between different abilities, and the species that we see around us are the results of these different trade-offs. Characteristics that are traded off include the ability to compete vs. the ability to disperse offspring; being able to thrive in average conditions vs. being able to take advantage of sudden pulses of resources; and being able to compete for different resources in a varying landscape. So many species exist because they all have different niches.
If the environment varies in some way, then species that are specialized to those variations should be found there, allowing more species to exist in an area as the variation increases. Variation provides the new niches for species. For variations in space, such as bare rock or marshy areas, specialized species will be found in those areas. Three-dimensional structures, such as trees or kelp beds, also provide more variation and let more species coexist. If the variation is in time, such as seasons, diversity will be different at different times. For example, spring ephemerals (plants that grow in the short period in spring before trees produce leaves and reduce the light) will only be found in early spring, and only if they can obtain enough light in the early spring.
A niche is the "role" of a species in a community, and can be defined as the conditions in which the species can survive or the way of life that it follows. An example of a simple niche description could be "large grazing herbivore." Based on the diversity-production patterns that have been observed, niche differentiation is the rule, meaning that species tend to find niches in which they can avoid competition rather than engaging in direct competition with other species for resources. When two species share the same niche, one will eliminate the other by outcompeting it.
Keystone species are species that are more important to an ecosystem than one would expect based on their abundance. This importance comes from their niches and interactions affecting the system as a whole, rather than only affecting the species that they directly interact with. Removing or adding keystone species to a community can result in enormous changes to the rest of the community through the effects they have on other species. The resulting cascade of interactions can have drastic effects on the ecosystem.
One of the better-known keystone species is the sea otter, Enhydra lutris. They are found in the waters off the west coast, where one of their main prey species are sea urchins. Sea urchins, in turn, eat algae such as kelp. By keeping the population of sea urchins low, the otters indirectly let kelp flourish. An increase in kelp coincides with a decrease in barnacles, mussels, and chitons. Fish species that can use the kelp for cover increase, and other species also take advantage of the structured environment. Rock greenlings, harbour seals and bald eagles are more common in areas with sea otters. When sea otters were removed from some areas, the sea urchins and other herbivores quickly managed to severely reduce the kelp, allowing barnacles and mussels to flourish at the cost of other species.
An example of a keystone species found throughout Canada is the beaver. Beavers modify large amounts of land through the flooding caused by their dams. While the dams are being actively used by the beavers ponds and lakes are formed, allowing many aquatic species to thrive. Once the pond fills with sediment, succession (see Gaining and Losing Diversity in this section) begins. If beavers are removed from an area, many species that live in the ponds caused by beavers would drop in numbers or go locally extinct.
Disturbances and catastrophes change which species are found in an ecosystem and their relative abundance. By disturbing the system, the catastrophe mostly effects the current stage of succession and effectively sets the disturbed section into an earlier successional stage. This reduces the uniformity of succession and allows plants and animals who would not be present in the final stage of succession to persist in the system.
When species from earlier stages are present, diversity increases. They also allow succession to occur at a faster rate, as the species that are needed for a given stage are relatively nearby in other recently disturbed areas.
Lastly, random chance can play a very important role in determining composition and abundance in an ecosystem. The order in which species show up can determine which one makes the dominant tree species in the forest, for example (see Assembly Rules, above). An insect species that is specialized on a particular host plant will usually go extinct if the host species does, no matter how well-adapted and otherwise successful it is. If the insect had lived on another plant species, it would not have gone extinct.
Poor conditions can make the difference for a species that is not very abundant, whether it is an invader or a struggling species that has been in the area for a long time. A particularly wet year is good for mushrooms, while a dry year is bad for them. What kind of first year it experiences in a new territory can make the difference between an invading species of mushroom flourishing or failing.