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EN 100 Lab – Measuring Biodiversity

GOAL: To learn how to calculate the Simpson’s Diversity Index (D), an important index for measuring diversity.

BACKGROUND:

Remember from class that community diversity is an important metric for understanding the stability and resilience of an ecological community. The more species present, the more the food web is resilient to disturbances and/or can recover from them quickly. Ecologists use the terms species richness and species evenness to describe the diversity in an area.

Richness refers simply to the number of species per sample.

Example: We found 5 distinct tree species along a 1 mile road. The 1 mile stretch of road is the sample, and the richness is 5.

The more species present in a sample, the richer that particular sample is. A drawback of this measurement is it gives as much weight to those species which have very few individuals as to those which have many individuals. For example, 1 dandelion has as much influence on the richness of an area as 1000 violets do (each is only counted once towards richness), but the violets are clearly more ecologically “weighty” (e.g. more influential to the rest of the ecological community). It’s possible to have a community with really high richness (200 species), but all but one of them are super rare, and thus only one species is influencing most of the community. That is a very different community than one that has 200 species all of which are relatively common. Thus, to be able to account for both the number of species and the commonness of those species, ecologists not only measure the number of species they find, but also calculate the relative abundance, a measure of the number of individuals of a species found divided by the total number of individuals of all species in the area.

Relative abundance of species A =

This number can never be truly known but only estimated because the total number of species is hard to know in any research area (there are often lots of very rare species that take extensive sampling to find). As a result, relative abundance usually increases non-linearly (like an s-curve) with sampling (the more you sample, the more you find; but it takes longer to find the very, very rare species).

Evenness is a measure of the distribution of these relative abundances across all species in a sample.

Example: a road with 10 dandelions, 10 roses, and 10 bluebells has very high evenness (relative abundance for each is 10/30 or 0.33). A road with 1 dandelion, 2 roses, and 27 bluebells has very low evenness because bluebells are very abundant (have high relative abundance, 0.9) compared to the other species (0.03 and 0.06). Notice that both samples have the same number of total flower species (or richness; 3 in each).

Most ecologists would like to account for both richness and evenness in their sampling. Simpson’s Diversity Index is a measure of diversity which takes into account both:

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Where:

D = diversity (Simpson’s)

n = the number of individuals in a species

N = number of individuals across all species

This is a generalized formula for all possible sampling efforts. The Greek letter ∑ means “the sum of”. As a scientist, you would plug in your specific numbers. For instance, using the numbers in the example above (1 dandelion, 2 roses, and 27 bluebells = 30 total individuals), you would get:

D = 1 – [a(a-1) + b(b-1) + c(c-1)]

[N(N-1)] where a = dandelions, b = roses, and c = bluebells

D = 1 – [1(1-1) + 2(2-1) + 27(27-1)]

[30(30-1)]

D = 1 – (0 + 2 + 702)

870

D = 1 – 0.81

D = 0.19

Your answer will always be between 0 and 1, with 1 meaning higher diversity.

You are going to get to practice collecting diversity data and using this formula to calculate Simpson’s Diversity Index within a natural area of your choice.

PRACTICE PROBLEM

First, calculate the D (Simpson’s diversity) for this sample using the dataset below.

Species

# of individuals

(n-1)

A

15

B

25

C

7

D

50

E

2

How many total individuals across all species are there? N = ____________

Now, fill out the diversity formula and calculate D:

Diversity = 1 –

Diversity = 1 –

D = _________

Note: you should have gotten 0.66 as your answer. If you didn’t, go back and see what you missed.

SAMPLING ON YOUR OWN

Now you get to collect your own real data and calculate the Simpson’s Diversity Index.

1) Locate a nearby natural area, such as a forest or stand of trees where you can conduct field sampling. A local park or your own neighborhood will do. There just needs to be a lot of trees and you need to be able to safely (and legally) walk around (no trespassing!).

2) Take a 5 photos of your area, including a selfie.

3) Start at a point of your choosing; pick a direction that will lead you past lots of trees.

4) Walk for 200 to 500 steps in that one direction (depending on the size of your area and how spread out the trees are), noting every tree that you can see and identify without moving from your line. Try to get at least 30 trees.

5) As you move along your line (in science this is called a “transect”), fill out the following chart with your data. If you know the tree species’ name, list it in the chart. If not, make up a unique identifier for each species based on their different leaf shapes (e.g. star-shaped leaf, needle leaf, prickly edge leaf) or another characteristic – just make sure you don’t mix up your species!

Species

# of individuals

n(n-1)

6) Now, calculate Simpson’s Diversity Index for your transect sample. Show your work. Remember it has to be between 0 and 1.

7) Once you have your index, answer the following questions:

a. What was your species richness?

b. Which of your species had the highest relative abundance? The lowest?

c. If you were trying to calculate the diversity of an entire forest, do you think it would be better to have fewer or more sampling transects? Why?

d. Do you think an area that has been logged/thinned has higher or lower diversity than an area that has been left alone for a long time? Why or why not?

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