Chapter 22
Summary
This chapter is concerned with giving you the basics about biodiversity.
Measurement of Biodiversity
The first section deals with defining and measuring it. Biodiversity is measured in three basic ways:
a) Species richness - the number of species in an area
b) Heterogeneity - the releative abundance of different species.
Most communities which have small numbers of species tend to contain relatively few common species and many rare species; that is, most species a rare.
If we were to graph this we would get a "hollow curve". This pattern occurs when there are a small number of species in a community and one environmental factor is dominant. The model predicts that "the greatest number of species would have minimal abundance", however as it is typical in ecology every rule has numerous exceptions.
There are many examples in which the pattern of abundance in a community doesn't fit the "hollow curve" but the "bell shaped curve" otherwise known as the "normal distribution" or "log-normal" curve. Here the majority of species are of moderate abundance, and there are both a minority of very rare and very common species.
Biodiversity Gradients
There are biodiversity trends. The one investigated throughout the chapter is the trend that biodiversity increases as one moves towards the equator. This is referred to as the "...one of the great patterns found in community ecology." (p456).
This apears to be true for many plant and animal species such as forest trees in Malaysia compared to Michigan, or ants in Brazil compared to Alaska. But there are many exceptions to this trend. For example, in Australia there are more marsupial species to be found in arid regions than in tropical ones.
In North American mammals one investigator has identified five diversity gradients including the north-south trend.
Possible Factors Responsible for Diversity Gradients
1) History
More time for evolutionary radiation
Tropics are free of severe changes such as glaciation and species can radiate without disturbance creating greater diversity than in cold areas. In other words, in the absence of disturbance we can say that tropical areas are more mature than temperate or polar regions.
Ecological and Evolutionary Time
This can explain diversity changes. Ecological time involves the time needed for a species to colonize a vacant niche.
Evolutionary time involves the time needed for an organism to evolve to fill a vacant niche. Also, in general, evidence suggests that the number of species increases over evolutionary time. Rates of extinction tends to be lower in the tropics and higher in the colder areas.
2) Spatial Heterogeneity
The more complex and heterogeneous the environment the more diverse the flora and fauna tends to be, and complexity increases towards the tropics.
Topographical relief affects diversity. This is the case for North American mammals. This is because:
a) it produces a variety of habitats
b) it creates isolated populations which generates speciations
This pattern does not work for birds or trees nor does it work for oceanic commnities either.
3) Competition
Interspecific competition drives natural selection in the tropics but it is the severity of the environment that drives it at the poles.
In the tropics numerous different species live alongside each other and to survive they must avoid competing with one another. They do so my becoming very specialized.
Niche breadth/length and overlap
The more species there are in a community the narrower their niches are. This is because of Gause's hypothesis which states that "...two similar species rarely occupy the same niche, but will displace one another..."
Where there is overlap, that is where niches of organisms cross over, competition is avoided by sharing resources. The example of species of Anolis lizards in the Caribean is given. Here they eat the same types of insects but will select prey of a certain size so as to avoid competition with other lizards.
Competition is a certain factor in determining biodiversity in animals but less so in plants.
4) Predation
Predation increases biodiversity
The example of a rock pool is given. Here Pisaster, a starfish, was removed from a pool. It was noticed that fierce competition for resources resulted and led to some species disappearing.
Predator and prey species are more abundant in the tropics, and prey populations are kept low. This means that if there are less prey species there is less competition them. Less competition may allow for a greater variety of prey species and consequently a greater variety of predators - well so the argument goes.
5) Climate and Climatic Variability
Climatic stability leads to greater variety. This is due to a lack of calamity and other disturbance and this gives species greater evolutionary time to radiate. See the section on disturbance for an alternative view.
Species-energy-richness model
Species diversity is limited by available energy (measured in terms of evapotranspiration which is a product of temperature and solar radiation). This is consistent for trees, British birds and N.American vertebrates.
6) Productivity
One would expect that highly productive areas would have high biodiversity. I presume this refers to production of biomass although it's not stated in the text. Nevertheless, evidence does not support this idea, in fact high productivity leads to reduced biodiversity.
7) Disturbance
The absence of disturbance leads to competitive exclusion. Disturbance leads to extinction if it occurs frequently enough.
Intermediate disturbance hypothesis
Communities if left alone should tend towards equilibrium in which species are selected by interspecific competition, and generally competition may reduce diversity. But equilibrium is prevented by various factors such as predation, catastrophies etc. Increasing disturbance of this kind tends to stimulate biodiversity.
Ecological succession
Distubance of a forest will lead to regrowth in which colonizing varieties will estabish and change the diversity until the forest has recovered.
However there are many exceptions to these models in which disturbance reduces biodiversity. (Note you need to balance this section with 'evolutionary time' above which suggests a lack of disturbance increases biodversity)
Conclusion
No single factor can account for biodiversity, but a combination of several. It has to be noted that ecology is often case specific; that is, what is true for one instance cannot necessarily be extrapolate to others
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