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Part 3
| Processes and Patterns of Biodiversity |
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Which type of speciation
is most prevalent?
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 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. 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. 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. |
Species-Area Curves: In general, |
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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. 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. |
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. 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) |
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Patterns of Species Biodiversity
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Part 2: Importance of Biodiversity |
Part 4: Surveys of Biodiversity |
ple
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. 
,
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.
(
Most
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.
the
bigger the area sampled, the more species found. 
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. 
Disturbance
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 
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. 
Habitat
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). 
