Project #82334 - Find My Ecosystem Assignment

Ecosystem-Pelagic Fish


When you finish this activity you should be able to:

  • Explain the meanings of the terms population density, species richness, species evenness and biodiversity.
  • Calculate population density, species richness & the Simpson's Diversity Index for an ecosystem
  • Apply the Simpson's Diversity Index to two or more ecosystems to assess their relative diversity.


What you will turn in:


TO COMPLETE THIS EXPLORATION ACTIVITY START HERE! Start with the introduction and read your way through the information and perform the activity as indicated (for example answer a question, complete a procedure, enter data).  All the information you need is outlined in detail here:


Biodiversity is a "hot topic" in conservation biology & ecology now days. In the wild, habitats are shrinking as the human population increases, and many species are becoming extinct - some without humans even discovering them.  Species diversity is important because it is a major component of biodiversity and tends to increase the sustainability of ecosystems. Though we may be able to qualitatively assess an ecosystem, the best science seeks to "quantify" or "quantitatively" assess an ecosystem. That is, we want to use numbers instead of general descriptions in order to more accurately compare the two. So how do we do that? How do we "quantify" diversity. It turns out there are a number of different methods that can be used. In this activity we will use two simple methods - the species richness and a "Simpson's Diversity Index".

It may seem that a clear picture of the diversity in an ecosystem can be quantified by using a simple survey of the number of different species. This is what we call the species richness. However, species richness alone is not the best measure of biodiversity. It is not until we also consider the population density and distribution of those species, or the species evenness, that the biodiversity of a community is more accurately portrayed. 

A population is defined as a collective group of organisms of the same species that occupy a particular area.  The population is the fundamental unit of study in marine ecology. In undertaking a study of a population, ecologists often want to determine the population density – the number of individuals (or biomass) with in a defined area (or volume).  Population estimates are valuable for estimating biodiversity. Many different methods have been devised to measure population density.  The most straightforward method for determining population density is to do a total count of the population – ALL individuals are counted.  This type of census is possible when populations are small and the organisms are large and conspicuous (like manatees or some species of seabirds). Later in the semester we will examine other ways to estimate population when a total count is not possible.

In order for us to best describe the biodiversity then we need to consider a combination of species richness and species evenness, which requires knowing the population density. One equation that does just that is called the Simpson's Diversity Index.

  • Species richness is the total number of species present in the community.
  • Species evenness is the relative distribution of individuals among the species present in a community.
  • Population density is the number of individuals of a particular species within a given area (or volume).
  • Simpson's Diversity Index is a simple mathematical measure that characterizes diversity in a community (the equation produces a number between 0-1).

Using these measures we can now quantify the diversity of an ecosystem. The greater the species richness, the higher the diversity.  The higher the Simpson's Diversity Index is (or the closer to 1 it is), the higher the diversity of the ecosystem.

The question you should be asking now is WHY is biodiversity important? How does this relate to the "health" of an ecosystem? The answer is that a community with high biodiversity is more likely to survive "adverse conditions".

Before we begin assessing our ecosystems, take a minute to write a 1-2 sentence answer to this question:

Question 1. Biologists see high biodiversity as an indicator of a healthy ecosystem and constantly fight to conserve the species in these areas!

  • What types of “adverse” conditions might affect an entire biological community (all the species that exist there) - name at least one natural and human induced?
  • Choose one of the scenarios you listed and describe how that would impact a "low" diversity community compared to a similar ecosystem with "high" biodiversity? Imagine revisiting these ecosystems some time later, which has recovered and why?


Quantitative Assessment of Ecosystems

Alright, as a new biologist you are ready to start collecting data and determining just how diverse your ecosystem is! To do this we need to:

  1. measure the population density of each species in the ecosystem
  2. determine the species richness
  3. calculate the Simpson Diversity Index

I have assigned each of you a different ecosystem to assess. To find your ecosystem assignment click here to open a Google Doc with the class list -> Find My Ecosystem Assignment

Question 2. What ecosystem were you assigned? Please include this on your Word document.


Your next task is to begin assessing your ecosystem. Follow my steps below to work your way through the calculations:

Step 1. Open the file that has the image of your assigned ecosystem (Find My Ecosystem Assignment). 

Determine what species are present in your “community” to the best of your ability. Identify them by common name and record them in your homework document. Below is a table that you may want to copy or recreate in your word document.

Note: For this exercise it is NOT that important to be able to properly ID each species (even I would have a tough time doing that!), but rather to identify when you have different species. If you don't know what the name of the organism is you may give it a generic, yet somewhat descriptive name, for example "yellow fish species 1" and "blue fish species 2". 

Question 3. Is your first impression of your "community" that it has a high or low biodiversity? Why?


Step 2. Count the total number of individuals for each species and record them in your table. Do this to the best of your ability -- for those species that there are a LOT of, just do your best to count them so you have a close estimate.

Step 3. Once you have all the populations counted and recorded, the next step is to determine your species richness and your Simpson's Diversity Index. The table I have set up is intended to help walk you through the steps and make the math much easier! If you would like to use excel please feel free. However, the document you submit to me should have all the data completed in a table as formatted below.  (Or download the EA1 Assignment Template Preview the documentView in a new windowand fill your answers in)

Here is a link to an example where I have included explanations of all the calculations you will need to complete --> Example Simpson's Diversity Index (Links to an external site.)


(common name or scientific name)

Number of Individuals (n)






















































The symbol means calculate the “sum” of


Species Richness


∑ =

N = ∑n =_________

∑ n(n-1)=



The formula for the Simpson’s Diversity Index "D" is:

 Simpson's Diversity Index.jpg


n = the number of individuals of each species

N = the total number of individuals for all species (add up your n's)

Where D = Simpsons Diversity Index (a number between 1-0)

This may look kind of scary, but in reality it is pretty easy! If we approach the equation step-by-step we can see that we only need to know how to add, subtract, multiply & divide - I promise!  Also, use my Example Simpson's Diversity Index (Links to an external site.) to see a detailed explanation of the steps.  Be sure to have a calculator handy!


Question 4. Calculate the Simpson’s Diversity Index for your community (show your work by completing the table) and record your value for “D” (*HINT: You already calculated the entire numerator in your table):

 D = ____________ 

-----> Did you remember the last step to subtract from 1? This is important or your interpretation of D will be incorrect.  

D will equal 0 for a community with a single species (i.e. no biodiversity) and will approach 1 as diversity is maximized.  So, the closer to 1, the higher the biodiversity!


Question 5. What is your species richness?


You are almost done with this part of the activity...

Once you have finished your calculations complete the following form where you will input your:

  1. Ecosystem assignment
  2. Species Richness (R)
  3. Simpsons Diversity Index (D)

If the form below is not visible, please click here to enter your data: Link to Data Entry Form (Links to

Subject Science
Due By (Pacific Time) 09/16/2015 11:30 pm
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