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Explain the reasons for great diversity of higher plants
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Species Diversity Introduction
Species Richness (s) is a relative term that refers to the number of species in a community, and is
directly associated with measuring the diversity of species in a given area. A related term,
evenness (E), is another dimension of diversity that defines the number of individuals from each
species in the same area. Together, these terms have been used to describe species diversity
patterns on Earth.
There are several hypotheses that have been proposed to explain species diversity patterns. Many
of these hypotheses are based upon the idea that species are more diverse near the equator than
near the poles. In other words, there is a recognized latitudinal gradient of species diversity on
Earth. The hypotheses that support this latitudinal gradient can be divided into two groups:
abiotic and biotic. The biotic hypotheses are those that explain species diversity patterns with
relation to living organisms. The abiotic hypotheses, on the other hand, explain species diversity
patterns with relation to non-living chemical and physical environmental factors.
Abiotic Species Diversity Hypotheses
Four commonly recognized abiotic hypotheses include:(1) The Time/Stability Hypothesis, (2)
The Area Hypothesis, (3) The Productivity Hypothesis, and (4) The Metabolic Hypothesis.
The Time/Stability Hypothesis suggests that diversity is directly related to the length of time
that an area of land has been around. Specifically, land that has been around for a long time,
undisturbed, tends to have more diversity, and is therefore more stable when compared to land
that has been around for a short period of time. For example, the land underneath a glacier that
has just receded will be less diverse than land that never had a glacier.
The Area Hypothesis explains species diversity relative to land area. According to this
hypothesis, the larger an area is, the more organisms it can support, which results in larger
populations. Larger populations are characterized by fewer extinctions and more prevalent
mutation events and are, consequently, more diverse.
The Productivity Hypothesis says that the more energy there is in a system, the more biomass
there will be in that system. More biomass supports greater species diversity.
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The Metabolic Hypothesis states that temperature has an impact on the metabolic rates of
organisms. This expands on the idea that every population has a net energy flow that acts upon it.
The main idea behind this hypothesis is that larger animals have relatively slower metabolic rates
when compared to smaller animals . For example, a mouse must eat half of its body weight to
keep from starving, while a human only needs to eat two percent of its body mass to live.
This phenomenon is explained by several ideas. One idea is that larger animals have
proportionately less surface area and lose heat more slowly. So, pound for pound, they need less
food. The other idea is that larger animals and plants must transport nutrients further and, as a
result, use them up more slowly than their smaller counterparts.
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