Sunday, October 13, 2013

Biodiversity and The RAIL Project


Remind Me what RAIL Is?

The RAIL Project is an aquatic ecology project I do with my students each fall at Dakota high school.  The school has a small, sluggish stream on the northeast corner of campus that acts as a drain for several square miles of the Clinton River watershed.  RAIL stands for Riparian Area Integrated Learning.  The "riparian area" of a stream is the strip of shrubs, trees, and grasses along the edges of a river or lake.  Riparian zones are important because they filter runoff and groundwater of sediment and nutrients, and they also slow the runoff so it has time to drop in temperature before entering the water.  These are all good things because too many nutrients will cause algae blooms that lead a stream to become "eutrophic," or full of slime; too much sediment can plug up a stream and stress out native species.  Low temperatures are good because dissolved oxygen exists at a higher concentration in low temperature water, so native macroinvertebrates- not to mention fish- are more successful.

The point of this project is to allow students to determine, by collecting their own data from the stream, whether the ecosystem of the Dakota Drain is healthy or not.  Numerous factors play in here to make this determination, but one of the most important is our calculation of the biodiversity of the river area.


Biodiversity 101

"Biodiversity" refers to how varied the biological community (mix of species) is, in a certain space at a certain time.  Any community, from a stream or forest area, to the suite of bacteria living under your armpit, can have its biodiversity calculated.  Most measures of biodiversity are a function of both "species richness," or how many species there are, and "evenness," or how evenly distributed they are.  More species and more evenness mean higher biodiversity.

The figures below illustrate what is meant by "richness" and "evenness."  Each different shape represents a species in a biological community.  Note that evenness doesn't really refer to the physical distribution of species within the sample area.  It simply refers to the relative fractions made up by each species.  For instance, although the purple triangles in the second box are "evenly" spread out in the physical space, they dominate the community with their numbers.  Thus that community is neither rich nor even.




There are lots of specific ways to calculate biodiversity, but the one my students use is called the "Shannon-Weiner Diversity Index."  The equation is seen below.


In English, the equation says this: "The Shannon-Weiner diversity index of a sample is the opposite of the sum of the natural logs of the percent of each species multiplied by the percent of each species, as represented by the sample."

That's a very compressed way of saying that the index is a representative number that integrates both the evenness of the sample (the percent of each species) and the number of species (which directly influences the percent of each species).  Never mind the steps of the calculation; it has seven discrete steps and is hard to explain without showing a big clunky data table.  An extremely high Shannon-Weiner diversity index is around 3.5, and a low index is anything less than around 2.0.  Generally an index has no unit.

But if you are interested in experimenting with this particular index, you can play around with this Diversity Index Calculator.  (Name and fill in the numbers for a few species, and it will spit out dozens of different biodiversity indices for you.  The one my students are using is the fifth one on the left under the heading "alpha diversity."  It's the second "Shannon" index.)


How do Students Learn to Use the SW Index?

Now students are not exactly familiar with indices or natural logs, and at first they don't really have a clear understanding of what is meant by "evenness."  What kind of work would bring home the concept of biological diversity?

In a word: Candy.

While I was student teaching at Allendale middle school, my cooperating teacher, Kieth Piccard, had his sixth-graders practice finding the SW diversity index with M&Ms, which they of course loved.

Like sixth graders, freshmen love doing work with candy.  I took Kieth's lesson a step further and gave the students four different "biological communities" to study: M&Ms (6 colors), Skittles (5 colors), Reese's Pieces (3 colors), and York Pieces (2 colors).  Each color represents a different species in a biological community.

Each student chose a single Dixie cup with a sample of one of these four communities.  First they simply graphed the community and looked to see if it appeared to be even.

Below are a few typical graphs that students produced for each community.  Some of them chose to do pie charts instead, which is fine because it more clearly illustrates the fractional nature of populations within communities.























It's pretty easy to see whether the communities are even or not.  Reese's Pieces and York Pieces tend to be highly lopsided toward orange and white, respectively.  M&Ms and Skittles are more even, but M&Ms seem to have much more orange and green than the other colors, while Skittles Riddles show a more egalitarian distribution of colors.  (Skittles Riddles, by the way, are puzzling to eat, because the red tastes like green apple, while the blue tastes like watermelon, and so forth.  Students love them.)

Next students had to count the total of the sample, find the percentage that each color represented, and then use the Shannon-Weiner diversity index calculation above to find the biodiversity of the sample.  I only let students eat their candy after they had correctly calculated the index, which presented a pretty powerful motivator.

Students then had to contrast the SW diversity values of the different communities by collecting the values from other students, then graphing them in a single integrated graph like the one to the left.  These are some pretty typical diversity values for each community.



Some students ended up producing the slightly different graph at right.  Notice that here the M&Ms have a higher diversity value compared to the Skittles, and the York Pieces have a slightly higher value than Reese's.

This led students to have discussions about why some of them got different values.  Some students who had M&Ms pointed out that their communities were not very even, which was why some of them found that their diversity value was lower than Skittles, which are more even.  The same was true of Reese's and York.  A few York samples were split evenly down the middle, while Reese's was lopsided.

These comparisons helped students to see that both the number of species (species richness) and the distribution of each (evenness) both determined how diverse their sample was.


The Rub

So here's the rub.  Students will be held responsible for finding the Shannon-Weiner diversity index of the Dakota Drain, based on the vertebrates and macroinvertebrates they find out there.  This will allow them to make a data-informed assessment of the health of our ecosystem.  They already have a sense that our community is uneven, because they are finding dozens of viviparid snails and non-biting midges, with relatively small values from other species.  As for vertebrates, some students have even pointed out that we should not include the 59 Canada geese, 3 red-tailed hawks, or 4 turkey vultures we've spotted, because they are only passing through (migrating) and thus don't really represent our community.  After one more day of data collection, students will begin to compile their data and put together formal reports on the health of our ecosystem.

My students are experiencing what it is to be real ecologists.  The challenges faced by field ecologists are manifold, but the students are consistently showing that they relish the challenges and want to say something meaningful about the Dakota Drain.  Saying something meaningful about reality is ultimately what science is, and it is what every curious person wants to do, whether they know it or not.