Interactions in communities. Tropical rainforest diversity. Simpson's index of diversity. Practice: Community ecology. Next lesson. Current timeTotal duration Google Classroom Facebook Twitter. Video transcript - [Instructor] So in this table here we have two different communities, community one and community two, and each of them contain three different species.
And we see the populations of those three different species. And we also see that the total number of individuals in each community is the same.
They both have a total of 1, individuals. Now, my question to you, just intuitively based on the data in this table, which community would you say is more diverse and why, community one or community two? All right, now let's think about this together. So as we already talked about, they have the same number of individuals, and you might be thinking that the number of species could be related to the diversity, and you'd be right.
The number of species does contribute to the diversity, but we're dealing with a situation where both communities have the same number of species. They each have three species. But when we look at the data, it's clear that community two is mostly species A and you have very small groups of species B and species C, while community one is more evenly spread.
So just intuitively it feels like community one is maybe more diverse, but this was just on my intuition or our intuition, and the numbers are pretty clear here. It's evenly distributed amongst the species here, and here it's very heavily weighted on species A, but it might not always be this clear.
Sample 2 is therefore considered to be less diverse than sample 1. A community dominated by one or two species is considered to be less diverse than one in which several different species have a similar abundance.
As species richness and evenness increase, so diversity increases. Simpson's Diversity Index is a measure of diversity which takes into account both richness and evenness. Simpson's Diversity Indices. The term 'Simpson's Diversity Index' can actually refer to any one of 3 closely related indices. Simpson's Index D measures the probability that two individuals randomly selected from a sample will belong to the same species or some category other than species.
There are two versions of the formula for calculating D. Either is acceptable, but be consistent. With this index, 0 represents infinite diversity and 1, no diversity. That is, the bigger the value of D, the lower the diversity. This is neither intuitive nor logical, so to get over this problem, D is often subtracted from 1 to give:.
Simpson's Index of Diversity 1 - D. The value of this index also ranges between 0 and 1, but now, the greater the value, the greater the sample diversity. This makes more sense. In this case, the index represents the probability that two individuals randomly selected from a sample will belong to different species. Another way of overcoming the problem of the counter-intuitive nature of Simpson's Index is to take the reciprocal of the Index:. The value of this index starts with 1 as the lowest possible figure.
This figure would represent a community containing only one species. The higher the value, the greater the diversity. The maximum value is the number of species or other category being used in the sample. For example if there are five species in the sample, then the maximum value is 5. For example, the diversity of the ground flora in a woodland, might be tested by sampling random quadrats.
Otherwise, it assumes the response variable contains frequencies. In either case, if negative values are encountered an error is reported. This syntax is used when the response variable is a group-id variable.
The group frequencies will be computed automatically. It may be referred to by a different name or have a slightly different formulation in various fields. Note: Dataplot statistics can be used in a number of commands. References: Edward H. Simpson , "Measurement of diversity," Nature , Following example from page 23 of:.
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