Abundance and Composition

Why do we have so many species? | Variable Environments | Niches | Keystone Species | Catastrophes | Chance

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.

Why do we have so many species?

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.

Variable Environments

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.

Niches

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.

Niche Packing
One approach to understanding the number of species and their relative abundance is called "niche packing". Any ecosystem has a limited amount of resources, and it is assumed that there are rules about how the resources can be used. The rules deciding how the resources are allocated to species and the species fit into their niches (i.e. how the niches are packed) determines how many species can exist in the system and how abundant each species is. Each species is added to the system one by one, with each species following the same rules. Rules include whether new species invade already occupied niches or only unused niches, or whether the size of the niche makes a difference to its chance of being invaded.

In the illustration to the right, each bar represents the total resources available to the community. Each colour represents a different species, with the amount of colour reflecting the number of individuals of the species. The two bars have been filled with the same number of species, but by using different rules, and the species reside in niches of various sizes. One species, represented by the red, dominates the bottom system, while in the top one it is only slightly more abundant than the species represented by yellow and orange.

Niche packing is studied in two ways. The first is by examining how species are packed in nature and trying to come up with the rules that most closely match reality. The second is by deciding how the niches are packed by various theories and them comparing the results to reality. Both approaches have the problem that the rules that generate the patterns may create the same patterns found in nature without being the actual rules followed by species.

Assembly Rules
Assembly rules look at why certain types of species are found together in a community by beginning with a theoretical community with no species and adding species one by one according to certain rules. This approach differs from niche packing by focusing on the niches that have already been filled rather than only the sizes of the niches that species occupy. Diffuse competition, the competition faced by a species by several other, usually closely related, species is very important in this approach, as every new species is treated as an invader and has to be able to fit into an already crowded community.

Which type of species is added next depends upon what type of species are already present and which rules are being followed. One common rule in these models says that the niches of new species added to the community should be as different as possible from those of the species already present. Similar areas will not necessarily have the same species, as the order that they appear in will affect which other species may successfully invade. By comparing the results from the models to the patterns seen in nature, insights into how communities form can be gained.

 

Keystone Species

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.

Catastrophes

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.

Chance

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.