ESS Topic 2.3 - Measuring Biotic Components of the Ecosystem

2.3.1 Construct simple keys and use published keys for the identification of organisms.

Read pp.309-312 in the IBO ESS Course Companion.

In class, develop a dichotomous key for all the organisms listed in Figure 16.4: "A picture key of pond animals". Your key should start with the largest groups of organisms and split them into successively smaller groups until each organism is alone in its own group. No characteristic should be used twice to subdivide different groups.

Here’s another selection of critters to classify with your own dichotomous key:

Complete the "To-do" activity accompanying Figure 16.5: "A selection of arthropods" on p.312

ESS Topic 2.3 - Measuring Biotic Components of the Ecosystem

2.3.2 Describe and evaluate methods for estimating abundance of organ-isms.

Because it's usually impractical to sample every single organism in an ecosystem, we use statis-tical sampling to estimate the abundance of species within the ecosystem.

A transect is a line across an ecosystem along which quadrats may be placed for sampling.

Quadrats are square frames of a specific size, in which we count the number of each species of organism.

• The size of the quadrat depends on the size of organism anticipated (i.e. small quadrats for small organisms and large quadrats for large organisms).

• The number of quadrats used for sampling depends on the number of species found. If we plot the number of species found against the number of quadrats used, once the number of species is stable, it's no longer necessary to add more quadrats - we can as-sume we've found all the species in the sample area. Refer to Fig. 16.6 on p.315 of the IBO ESS Course Companion.

• Placing quadrats: Quadrats can be placed randomly, continuously, or systematically.◦ Random quadrats - use random number tables (easily generated online) to

determine placement of quadrats. Click here for an example of how to do this.

◦ Continual quadrats - place quadrats adjacently along a transect line and ev-ery species along the line. This is quite accurate but can be extremely time-con-suming.

◦ Systematic quadrats - place quadrats along a transect at regular, pre-deter-mined intervals.

Estimating abundance depends on the type of organism being sampled.

• Plants are stationary, so we can use the percentage cover to estimate their abun-dance. Simply put, estimate the % of each quadrat covered by each species' leaf area. The total % in a quadrat do NOT have to total 100%!

• We can also count the density of individuals - how many per square meter.• Frequency of individuals is the % of the number of quadrats where a particular species

is found (i.e. Acacia senegalensis was present in 47 of 92 quadrats, for a frequency of 51%).

Here's a Youtube video podcast showing how an AP Environmental Science class in North Carolina uses some of these field sampling techniques.

Capture, mark, release, recapture: Animals within an ecosystem are caught and marked so that they may be tracked in the future, then released back into their habitat. At a subse-quent time, another capture event is held, and previously-marked animals are noted in the field notes. We must be careful to make sure that the way the animals are marked doesn't make

ESS Topic 2.3 - Measuring Biotic Components of the Ecosystem

them more prone to be killed and eaten by predators or harm their chances of survival in any other way (i.e. making it more difficult to hunt or find food for themselves).

The Lincoln Index is an important tool for estimating population size via capture-release pro-grams. This method is frequently used but is not perfect. See some of the assumptions on p.317 of the IBO ESS Course Companion for further explanations. In class, compelte the Lincoln Index calculations on p.317. The formula for calculating the Lincoln index is explained be-low:

N = (n1 x n2)/m2 where:

• N = total population• n1 = number of animals first marked and released• n2 = number of animals captured in the second sample• m2 = number of marked animals in the second population

2.3.3 Describe and evaluate methods for estimating the biomass of trophic levels in a community.

Biomass, as the name implies, is a measurement of the mass of living material at a trophic level or within an ecosystem. Because water is NOT living, we do not include it in our calcula-tions.

Since all organisms are made of roughly the same organic molecules in similar proportions, a measure of their dry weight is a rough measure of the energy they contain. Therefore, mate-rial brought into the lab must be dried completely before measuring its mass. Normally, this is accomplished by placing the material in a warm - not hot - drying oven and allowing it to dry completely over a day or two before weighing it.

• Use the Lincoln index (above) to estimate the total population of a secies of organisms.• Once the mass of an average organism within the population is known, that mass can be

multiplied by the estimated population to determine the total biomass of the popula-tion of that species.

• This process is repeated until all species at a trophic level have been accounted for.• (Source:

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/F/FoodChains.html)

2.3.4 Define the term diversity.

ESS Topic 2.3 - Measuring Biotic Components of the Ecosystem

Diversity (commonly 'biodiversity' when discussing ecological principles) is a measure of the relative abundance of different living organisms within an ecosystem. Diversity is not simply the number of different species found in an ecosystem - consideration must be given to the rel-ative abundance of all species within the system.

Two ecosystems may have the same number of species (called 'species richness'), but vary considerably in their diversity. If one or two species make up most of the total number of living organisms within the system, that system is considered less diverse than one in which the num-bers of every species are relatively similar (called 'species evenness').

Look at the example on p.318 (recreated below). Both ecosystem 1 and ecosystem 2 have the same species richness - they each contain 3 different species. They also have similar numbers of organisms (70 and 72, respectively). However, the vast majority of organisms in ecosystem 2 are from a single species - species 'A'. Because the distribution of species in ecosystem 2 is skewed heavily to species A, ecosystem 2 is considered less diverse than ecosystem 1. How does Ecosystem # 3 compare to #1 and #2 in terms of species richness and species evenness?

Diversity of different ecosystems

Species # of Individuals

Ecosystem #1

A 25

B 24

C 21

Ecosystem #2

A 65

B 3

C 4

Ecosystem #3

A 28

B 32

C 12

Because ESS examines the science of ecosystems, we must be able to quantify or measure the diversity of ecosystems for the sake of comparison. How do we do that? Read below...

ESS Topic 2.3 - Measuring Biotic Components of the Ecosystem

2.3.5 Apply Simpson’s diversity index and outline its significance.

Simpson's diversity index is a mathematical model used to compare the relative biodiversity of different ecosystems. A full explanation can be found on p.318 of the IBO ESS Course Companion. In short, the higher the numerical value of Simpson's diversity index, the greater the biodiversity of a system.

D = [N*(N-1)]/[Σ(n*(n-1)]

Where...

• D = diversity index

• N = total number of organisms of all species found

• n = number of individuals of a particular species

Complete the "To do" activities # 1-3 on p.318 in class. In addition, respond to the following prompts in your notebook:

1. Calculate the diversity index 'D' for a sample from ecosystem #3, the North American tem-perate deciduous forest. Sample data are listed in Table 1, below.

2. What would happen to 'D' if all the oaks were harvested, leading to a 40% decline in the squirrel population, a decline of 25% in the woodpecker population, and a 30% decline in the warbler population in ecosystem #3?

Table 1: Species distribution in the eastern deciduous forest of North America, October 2009.

Species # of individuals per species

Red oak (Quercus rubrum) 55

Silver maple (Acer saccharinum) 60

Eastern white pine (Pinus strobus) 32

Eastern white-tail deer (Odocoileus virgini-anus)

88

Eastern grey squirrel (Sciurus carolinensis) 110

ESS Topic 2.3 - Measuring Biotic Components of the Ecosystem

Species # of individuals per species

Red bellied woodpecker (Melanerpes caroli-nus)

50

Yellow-rumped warbler (Dendroica coro-nata)

42

This site has a good step-by-step explanation of Simpson's Diversity Index.