Species abundances e1 e2 e3 e4 e5 3 500 300 200 094

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Species Abundances E1 E2 E3 E4 E5 3 500, 300, 200 0.94 0.93 0.90 0.94 0.91 4 500, 299, 200, 1 0.75 0.71 0.61 0.94 0.90
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E4 and E5 remain relatively constant with sampling variations and hence tend to be independent of sample size. Computed as ratios and S , the number of species, is both the numerator and denominator. Cancels impact of number of species in the sample. In reality, E4 and E5 are the ratios of the number of very abundant species ( N2 ) to the number of abundant species ( N1 ). Relative evenness can also be compared by examining the steepness of rarefaction curves. Higher evenness is equated with a steeper rarefaction curve. Habitat 20 has highest evenness, habitat 36 has the lowest.
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Uses in ecology Changes in diversity ( H ’) and equitability ( J ) in a control plot and fertilized plot 1850-1950 Decline in diversity and equitability with time in the fertilized plot. Possibly due to higher growth rates of taller species allowing them to dominate and exclude other smaller species.
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Not all Species are Equal! In a functional role, not all species in a community appear to be equal . 1. Few common species with a high population density or relative abundance. Called dominants . Dominance is the converse of diversity. Simpson's is often used as a measure of dominance. A value of 1 would represent total dominance (only one species present). Dominance may not be numerical abundance but may be biomass, functional importance, or size of the individuals. 2. Many rare species with a low population density or relative abundance. Called rarities .
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3. Keystone species – may be less abundant but play a crucial role in the function of the community. Keystone species have a unique and significant role through their activities and their overall effect on the community may not be related or proportional to their abundance. If removed, major changes in community structure occur along with a significant loss of diversity. Role of keystone species may be to create or modify habitats or to influence the interactions between other species.
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Examples of keystone species are 1. Herbivores e.g. African elephant in savanna communities 2. Predators e.g. sea otters ( Enhydra lutris ) eat sea-urchins in kelp sea-weed beds. Kelp beds provide habitats for many other species. If sea otters decline, sea-urchin populations increase resulting in overgrazing of kelp beds and loss of habitat for many species. 3. Coral Oculine arbuscula – more than 300 invertebrate species live among its branches.
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Keystone species are like pieces in a Jenga game In a game of Jenga, players successively take away parts and place them on top until the structure becomes unstable and crashes. Each part can thus be a keystone. When parts are replaced at other positions, the stability of the Jenga structure can be maintained. Keystone species may be like pieces in a Jenga game.
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