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Part 3
Processes and Patterns of Biodiversity
What are the processes that regulate species diversity?
Speciation
Neutral processes that regulate species diversity
Processes in variable environments
Patterns of Species Biodiversity
Patterns in space
Patterns in time
 

What are the processes that regulate species diversity?

The patterns of species diversity in an area or at any one time are set by some combination of three factors: chance, history and necessity.

Chance: random processes of birth, death and migration. A lizard might arrive unpredictably on a remote island, for example, because the log it was on happened to float in the right direction.

History: correlation through time as a function of reproduction. In other words, if a species was abundant in the near past, chances are that it will be abundant today. Also, progeny tend to cluster near the parents, therefore, we tend to find organisms in "pockets" rather than evenly distributed in space.

Necessity: The laws of growth, competition and interaction. Different species flourish in different conditions. The number of species that can coexist will depend on how complex the environment is and on how strongly they compete with one another. And, of course, the number of species of herbivores, predators and parasites will depend on the number of plants, prey and hosts.

In order to understand the processes that drive biodiversity, it must be recognized that there is diversity in space and diversity over time. We can sample a large tract of forest to determine how plant species are distributed over the entire area of that large tract or we can sample a smaller patch of forest to determine how the species distribution changes over many years or even several seasons. Both scenarios are addressing patterns of species biodiversity.

 

In the long term, the total number of species belonging to any particular group will be governed by processes of speciation and extinction. Immigration may be a source of new species to a given area.

 

Speciation

What is a species?

A species can be defined as a group of individuals that can mate with one another but not with members of other groups. New species of organisms therefore arise when they become sexually incompatible with other groups. Some organisms are difficult to define as a species using this definition - those that are self-fertilizing or asexual, for example.

Speciation is, for all practical purposes, a historical factor; speciation of most of the organisms that now exist occurred long ago.

Recent changes in the composition of plant and animal communities occur on ecological rather than geological time scales as the result of processes such as immigration, competition and predation. The overall diversity of a particular region will depend on its capacity to support life - its size and productivity - and on the variety of habitats that it includes.

 

Speciation can occur gradually via geographic speciation (allopatric) or competitive speciation (sympatric) or abruptly through mechanisms such as polyploidy (sympatric). These three modes are explained below (Rosenzweig, 1995, Chapter 5 ).

 

Geographical Speciation

A barrier restricts gene flow between populations so that they evolve separately and eventually become different species. The barrier may break down and the isolates may again interact but they do not interbreed. (Rosenzweig, 1995).

The dynamics of speciation will depend on two processes: 1) the rate at which geographical isolates are formed and 2) the rate at which these isolates evolve into separate species.

1. Rates of isolate formation are influenced by spatial factors

a) geographical circumstance: archipelagos and mountain ranges are very effective isolating barriers. Once something does overcome the barrier to colonize these areas, speciation tends to occur relatively rapidly. Therefore, mountains and archipelagos will tend to increase rates of speciation there.

b) geographical range: size of the geographical range of an organism makes it more or less likely to include a barrier. A range can be too small, making it unlikely that a barrier will pass through it. A very large range may wholly encompass a barrier, so that individuals (or propagules such as pollen or spores) can pass around it. Therefore, intermediate-sized ranges are most likely to be divided by a barrier (knife-shaped).

2. Rates of speciation following isolate formation are influenced by two factors:

a) sexual divergence: the isolates may evolve different mating behaviour, for example, by flowering at different times of the year

b) ecological divergence: natural selection will cause different isolates to evolve differently because no two places are exactly alike.

Speciation is more likely to occur in large populations than in small populations because they contain more variation. This variation cannot be selected effectively, however, if the population is completely interbreeding. The most likely situation for divergence to occur, therefore, is when a a relatively small and unrepresentative group (a "propagule") is split off, or isolated, from a large ("parent") population.

THE evolution of Darwin's finches on the Galapagos Islands is a dramatic examDarwin's finches.gifple of geographic speciation. It is believed that the 13 species of Darwin's finches that are found on the Islands descended from an ancestral pair of South American finches that landed there accidentally over 100 000 years ago. This pair found an area free of predators and probably adapted to the various unfilled niches. For instance, one finch population evolved a longer bill and the facility for using sticks to prod insect holes in cactus, and over time evolved into the Woodpecker finch. Other finches evolved thicker bills for eating the large seeds of the prickly pear cactus, and became the Large ground finch; smaller thick bills were ideal for eating small seeds; while other bills and habits adapted themselves to insect predation. Gradually, a number of such variations led to the radiation of 13 different species from 1. However, this classification of "species" is tenuous, as some of the species are thought to be able to interbreed.

 

Competitive speciation

Competitive speciation occurs when one portion of a population exploits a new ecological niche or opportunity (food, life history attribute, habitat, etc.) that was previously unexploited and becomes sufficiently different as to be considered a new species. This is the most controversial mode of speciation (Rosenzweig, 1995).

The place where a population lives may contain two (or more) different kinds of resource, for example, two species of food plant. Some individuals may use one plant more effectively, and some the other plant. These specialists are likely to be more successful than individuals who are not as effective in using either plant. Specialists who mate among themselves will be exceptionally successful, because their offspring are likely to inherit their specialization. The evolution of appropriate mating preferences may then lead to the appearance of two separate groups, who in time become so strongly isolated that they become different species.

The problem with this explanation is that random mating among the different phenotypes and genetic recombination break up any adaptive combinations of genes faster than they can be selected. For sympatric speciation to work, therefore, some strong force has to hinder recombination. In other words, something would have to prevent an individual suited to environment A from mating with an individual suited to some environment in between A and B, even though they are the same species and in the same location. (Rosenzweig, 1995)

STRONG evidence for competitive effects was shown in a 1969 paper by Guy Bush who studied the fruit flies of the genus Rhagoletis. Prior to European arrival in North Americafruitfly.gif, R. pomonella fed exclusively on hawthorn and R. indifferens on native pin cherry. Each of these species has now formed a new host race, the former adapted to domesticated apple and the latter fed on cherries. Both these fruits were introduced by Europeans and both flower at slightly different times from their native counterparts; therefore the races that became adapted to the domesticated fruits had a different reproductive cycle from those that fed on native fruit. This temporal difference has resulted in races of the same species in the same location that are reproductively isolated - one of the main criteria defining different species. (Bush, 1969)

Polyploidy

Atriplex.gifMost familiar organisms have two sets of chromosomes, inheriting one set from each parent. Such organisms are called diploid. Polyploid individuals have more than two sets. They arise through cytological irregularities during cell division or through the fusion of abnormal gametes. Once formed, they are often sexually isolated from their parent population. For example, a tetraploid individual (having four sets of chromosomes) forms diploid gametes. When these fuse with the haploid gametes produced by normal individuals, they give rise to triploid progeny (3 sets of chromosomes), which are sterile. This is why polyploidy can result in instant, or abrupt, speciation.

 

Which type of speciation is most prevalent?

Polyploid series of related species are easy to identify and can be common, especially among plants. More generally, most speciation probably requires isolation, followed by divergence: good examples of sympatric speciation are hard to find. (Rosenzweig, 1995)

 

Neutral processes that regulate species diversity

Neutral processes are those that occur independently of any differences among species, as though the species were genetically identical.They will, therefore, affect diversity regardless of the ecological characteristics of a region. For example, there is a continual rain of seeds and spores onto the soil, and which species happen to land in a site suitable for growth is largely a matter of chance.

Immigration

Immigration provides a continual source of new diversity for a region. How important it is depends on the balance between the number of propagules that come from outside and the number produced by resident individuals. If the area is large (a few square kilometers), most young individuals will be recruited from the resident population, but in small areas (a few square meters), reproduction by residents may be overwhelmed by immigration. Thus the importance of immigration increases as the size of the area decreases.

Some organisms are dispersed much more broadly than others. The very small spores of ferns, for example, may be carried by wind for hundreds of kilometers from their parental site. The seeds of plants such as dandelions and poplars are much larger but have special devices to facilitate wind transport. Marine creatures such as corals and starfish have larvae which are carried for great distances on ocean currents. Immigration will be much more important in such creatures than it will be in oak trees or land snails, for example, which produce larger propagules with no special devices to ensure long-distance dispersal.

The effectiveness of immigration in providing new recruits to an area is seen dramatically after a natural disaster destroys all life in an area. After the devastation of a volcanic eruption, for example, plants and animals quickly return: first the groups with effective long-distance dispersal, and later, those who disperse more slowly but are better competitors once they arrive. (Rosenzweig, 1995)

Extinction

Extinction of a species or a population will occur for one of two reasons: as a result of accidents (environmental fluctuations) or because of population interactions.

a) accidents: events that trigger extinctions for no predictable reason - volcanos, rising sea level, an ice storm, any environmental circumstance that wipes out an ecological niche.

b) population interactions that are not neutral processes: predation and competition can result in negative growth rate and ultimately, extinction. However, on their own, predation and competition rarely cause extinctions directly; they cause population densities to become very low and then a random accident may drive the vulnerable population to extinction.

The probability that enviornmental or population fluctuations will cause an extinction depend on how abundant the organism is and how large its range is.

a) abundance: if the chance that any given individual will die in a given period of time is p, then the chance that all individuals in a population of size N will die within that same period of time is pN. If the population is large, the probability that this will happen is very small: for example, if p = 0.5 in a given year, then the probability that all individuals in a population of N = 1000 will die at the same time is so small that it is unlikely to occur in a billion years. If, on the other hand, N = 10, the population is likely to become extinct within a thousand years, a relatively short period, and certainly much shorter than the time necessary to produce a new species. Small populations are thus at high risk of chance extinction.

b) range: disturbances that kill all the individuals in a given area happen all the time. Smaller and more localized disturbances are more frequent than large and widespread disturbances - treefalls are more frequent than forest fires, landslips more common than earthquakes. A species that is restricted to a few small sites is therefore at higher risk of being extinguished by an environmental fluctuation than one that occurs at many sites over a large area. (Rosenzweig, 1995)

Theory of Island Biogeography

The equilibrium theory of island biogeography states that the number of species on a given island is regulated by the balance between immigration of new species and extinction of species.

The total number of species found on an island depends on the size of the island and the distance from the source of immigrants or propagules (mainland or other islands). The smaller the island, the higher the probability that the population can be wiped out by random fluctuations in size, environmental conditions and mortality. An island that is near to the source population will receive more immigrants than one that is far away because more individuals will be able to cross the barrier (water, or other hostile habitat). Therefore, we expect to find more species on larger islands that are closer to the mainland or to other islands and fewer species on small distant islands.

By extension, the same principle applies to any stretch of land, whether it is separated by land or not. And, of course, one can readily view lakes as islands of water surrounded by land. The theory, therefore, is broadly applicable to all sorts of situations.

Island biogeography theory is a neutral theory, because species are assumed to have the same rates of extinction and immigration. It leads to a characteristic species diversity on islands of given size and isolation as the result of a dynamic equilibrium between the processes of extinction and re-colonization. We can therefore use the theory to predict patterns of species diversity.

(MacArthur and Wilson, 1963; Huston, 1996, Ch.4; Rosenzweig, 1995, p.220-263)

 

Area

Species-Area Curves: In general, diversity vs area.gifthe bigger the area sampled, the more species found. This relationship between species and area can be plotted to generate a species/area curve. Such a plot can give us useful information such as the total number of species in a region (which is the number at which the curve levels off with increasing area indicating that no more species are found), and the rate of species increase with area between different regions (calculated from the slope of the curve).

Such species area curves have been described for a wide variety of organisms including vascular plants, birds, mammals, fishes, and terrestrial and aquatic invertebrates. For a list of citations on this topic, see Huston, 1996, p.37.

 

Productivity

Naturally, you do not always find the same number of species in areas of the same size because some areas support far more individuals than others. There are more species in a hectare of tropical forest, for example, than there are in a hectare of tundra. The capacity of an area to support growth is called its productivity.

Productivity, defined as gross primary productivity, is the solar energy that is captured and converted to carbon compounds in an ecosystem.

In general, more productive areas support more species.diversity vs productivity.gif But the pattern is usually more complicated than this. In many systems, the relationship between primary productivity and diversity has been shown to be unimodal, or hump-shaped: diversity is highest at intermediate levels of productivity. Such a pattern has occurred at regional scales in many biomes and for many groups of animals and plants, including desert rodents, tropical mammals, marine benthic communities and megafauna, freshwater plankton, montane ferns and bryophytes.

Many ecologists believe that productivity has a great influence on diversity, however, the mechanism by which it affects diversity is still poorly understood, although Rosenzweig (1995) summarizes the most common hypotheses to explain the mechanism in Chapter 12 of his book.

(Rosenzweig, 1995, p.40.; Huston, 1996, p.29)

 

Processes in variable environments

Neutral theories that assume species to be ecologically identical provide a simple explanation of basic patterns such as species-area relationships. However, they are by no means completely satisfying because we know that there are often obvious and important differences among species. This offers an alternative explanation for patterns of diversity. For example, larger areas may support more species because they provide a greater variety of habitats. We can therefore investigate how species diversity is affected by how conditions of growth vary in space and time.

Competition Within Functional Types

For species that are functionally analogous, competition for resources plays an extremely important role in shaping their diversity. One of the earliest axioms of ecology, the Principle of Competitive Exclusion, states that if two species are competing for the same limiting resource, they can't coexist and one will outcompete the other and drive it to extinction, given sufficient time.

Explanations for areas with high diversity of functionally analogous species fall into two categories. One maintains that if the area is sufficiently patchy or heterogeneous, then competitive exclusion may eliminate a species in a given patch that would still exist in another patch and thus high species diversity would be maintained over the entire area in a dynamic equilibrium of local extinctions and re-immigrations. The other maintains that competitive exclusion does occur, but the rate at which a system proceeds towards competitive equilibrium is extremely slow and affected by many factors, including environmental fluctuations, predation and herbivory.

(Gause, 1935; Huston, 1996, p.80-86)

 

Spatial Heterogeneity

Since competition plays such an important role in structuring diversity for functionally analogous species, why hasn't there evolved one superorganism that competitively excludes all other species? The reason is because most landscapes and larger areas are spatially and temporally heterogeneous. Some species are better adapted to one habitat, some to another. Moreover, being well-adapted to one way of life may mean that you are necessarily less well-adapted to another. An annual plant producing broadly dispersed seeds that readily colonizes new patches of bare soil, for example, will eventually be displaced by larger, longer-lived perennials. A landscape that is a mosaic of different kinds of patch will thereby provide suitable conditions for the growth of a variety of different species.

One kind of environmental heterogeneity is created by organisms themselves. Such "structural species" create a substrate and resource that can then be utilized by other species ("interstitial species") thereby increasing the number of co-occurring functional types in an area as well as the number of functionally analogous species, at the landscape level. For example, bird diversity (interstitial species) is positively correlated to structural complexity or species diversity of trees (structural species). In aquatic environments, diversity associated with structural species such as corals or sponges is strongly associated with diversity of fish and invertebrates (interstitial species).

(Huston,1996, p.86-90; Palmer, 1994)

Disturbance

The theory of competitive exclusion is conditional: it assumes that communities reach equilibrium (or climax). However, competitive exclusion rarely occurs because equilibrium is interrupted by disturbances. In fact, field research has revealed that high numbers of species coexisted in natural communities where niches overlapped and diversity could not be explained by spatio-temporal heterogeneity nor by niche specialization. One scientist suggested that species coexisted in high diversity systems because competitive equilibrium was prevented by environmental factors, and that coexistence was a non-equilibrium state. (Hutchinson, 1961)

disturbances.jpgDisturbance regimes, therefore, play an important role in the maintenance of regional species diversity, mainly by preventing, or slowing down competitive exclusion by one dominant species by causing mortality or by slowing down growth rates. (Huston, 1996; Hutchinson, 1961)

Disturbances can be abiotic or biotic. Dynamics of a rocky intertidal shore illustrate how both biotic and abiotic processes may act to maintain the diversity of organisms. In rocky intertidal communities, starfish prey on the competitively dominant mussels and prevent them from excluding all other functionally analogous organisms. Wave action also has a similar effect in controlling the most competitive species, through desiccation or battering by debris. (Huston, 1996)

The intermediate disturbance hypothesis: If an area is frequently disturbed, it makes sense that there will not be many species living in it because they don't have an opportunity to recover. However if an area is rarely disturbed (that is, it has time to reach equilibrium), then species diversity is frequently low because competitive exclusion has occurred and resulted in local extinctions. It has been shown repeatedly that the highest levels of species diversity were maintained at some intermediate level of disturbance. A community that is not in equilibrium will tend to have more species coexisting, even if they overlap in niches, because there has not been enough time for competitive exclusion to occur.

 

Patterns of Species Biodiversity

In discussing the processes that regulate species diversity, we have already presented some spatial patterns of diversity, for instance, how species diversity increases with area (species-area curves), how it peaks in areas with intermediate productivity or intermediate rates of disturbance. Below are additional examples of patterns of species biodiversty in space and time.

Patterns in space

divresity vs latitude.gif Latitudinal Gradients: Scientists have observed that species diversity declines with increasing distance from the Equator, either north or south. There is no doubt that the tropics are very rich in species diversity. And this pattern is ancient; it has existed for thousands, if not millions, of years, across many taxa, from trees to fossil foraminifera.

 

 

 

diversity vs habitat diversity.gifHabitat Variety: The more variable the habitat, the greater the species diversity within it. This pattern was offered as one of the reasons why there are more species in a bigger area (more area covers a greater variety of habitat).

 

 

 

 

Patterns in time

Seasonal patterns

Diversity of species can vary during different seasons of the year. Good examples of organisms whose diversity varies seasonally are birds and insects. Insects have very different life history stages so if an area is sampled for diversity, the time of year at which sampling occurs may dramatically affect the diversity estimates. For example, some terrestrial beetles have larval stages that are underground and they only emerge to the surface as adults. The same problem can occur when estimating stream diversity since many adult forms of stream insects will emerge from the water which can affect diversity estimates depending on the season sampling occurs. Birds are another problematic taxon because many are migratory and the bird diversity of an area may be affected by the absence of seasonal breeders and the presence of migrants passing through. These seasonal patterns are most noticable in temperate areas but are also documented in the tropics. Seasonal patterns can occur in both terrestrial and aquatic habitats (Rosenzweig, 1995).

Successional patterns

After a disturbance (such as fire or agriculture), plant and animals species begin to reoccupy the habitat, grow, and get replaced or out-competed by other species. This pattern of gradual temporal shift in the species composition of a community os called succession. It results from a variety of processes including migration, dispersal, growth, competition and environmental change. For plants, diversity increases with succession until woody species (trees and brushes) establish, whereby diversity then decreases. For animals, diversity generally increases with succession (this has been observed for birds and insects).

Evolutionary patterns

Evolutionary patterns: Over 600 million years, the number of different types of organisms has been increasing. Some clear patterns of increasing diversity have been established over evolutionary time.

225 mya (million years ago): the number of different phyla stopped increasing

65 mya: the number of new classes stopped increasing. But since then, many new orders, families, genera and species have evolved. Patterns of increasing diversity over evolutionary time scales have been plotted for plants and marine invertebrates using fossil evidence. However, there have been many studies showing that diversity of organisms has not followed an increasing pattern over evolutionary time scales but has only fluctuated.

 

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