A practical question confronting conservation biologists, politicians, and the voting public is the design and management of nature preserves - national parks, wilderness areas, wildlife refuges - any chunk of habitat that has been set aside with at least one of the intentions being the preservation of biological diversity. Biological diversity, or "biodiversity" can be measured at various levels of biological organization, the two most common being: a) species diversity, that is the number of different kinds of species and their relative abundances, and b) genetic diversity (the genetic variety present within a given species, as well as among different kinds of species). Both kinds of variety are critical for the maintenance of a healthy, viable population of organisms and for the long-term health of an entire ecosystem (why?).
How big should a nature preserve be? Most of us would reply intuitively that "bigger is better" - but can we provide sound ecological reasons why bigger is better? Challenge your students to think of some of these reasons. All else being equal, a big preserve can hold more individuals than a small one, on average. Much less obvious is the prediction that a large preserve will also contain a greater variety of organisms, on average, than a small one. Why might this be so?
- Locate a schoolyard habitat such as a patchy lawn or a weedy area. Divide the class into groups and distribute the hoola hoops and embroidery hoops. If the 80cm and 16 cm varieties are chosen, then you will have big preserves 5 times the size of small ones and that's convenient for later analysis.
- Each group throws the hoola hoop at random (throwers CLOSE EYES) and censuses all living things inside by recording the type of organism and how many of each. If you have a particularly diverse area, restrict the census to just plants or just animals.
- Each group does the same thing with the embroidery hoop.
- It is not necessary to correctly identify all species but it is very important that different groups choose consistent name for the same creatures so that they are recorded under the same name on everybody's data sheet.
- Tally the class results.
- Compare the total number of living things in big vs. little, the number of different kinds of things in big vs. little, and the number in each population. Discuss what a "viable" population might be for various organisms, (e.g., if you have just one caterpillar in a reserve is that likely to sustain the population over a length of time or are more caterpillars (in more preserves) needed?
- For advanced students: Plot the cumulative number of species types found on the y axis, and the number of plots sampled on the x axis. This is called a species-area curve. It shows the importance of sample size, among other things. If you get a graph that goes up, then levels off, this means that at the corresponding number of samples, you are no longer picking up new species even though you take more samples. This way, you know how many samples it takes to get a very good notion of how many different critters/plants are in the entire area. How many hoola hoop samples does it take to reach this point? How many embroidery hoop throws? What would you predict? This relationships reinforces the concept that bigger will hold more variety, on average.
- Still more advanced: What if the conservationists and other citizens are faced with the choice below:
- These two preserve "designs" have equivalent area, but one has the area broken into 5 small chunks and the other has all of the area contained in one large chunk. Which preserve system would contain the greatest variety of species? Based on our mathematical intuition alone, we might conclude that the sum of the species types found in preserve system 1 should equal the number of species sampled in preserve system 2. However, in the real world, the answer is: it depends. And what it depends on most, apparently is a more complex understanding of the ecological ties in the habitats involved. For example, if the big preserve, by virtue of being big, contains a greater variety of landscape features (rivers, low areas, high areas, wet areas, dry areas), then more species may be found there. On the other hand, depending upon how spread apart the little preserves are, they may encompass more different types of terrain.
- Still, you are left with the problem of smaller populations that can be sustained in each of the smaller chunks. Get the picture? The answer may be complex and depends a lot on the particular situation. Try it on your schoolyard and see what happens. If your school had $500 to buy wildlife preserves, would you spend that money in one big chunk and carefully select it (where would you put it on your schoolyard?) or would you buy 5 small pieces where would you put those?) Here is a good reason for having the hoola hoop measure 5 times the diameter of the embroidery hoop!
The same exercise can be done sampling bird species in a neighborhood. This exercise uses the same 5:1 ratio for area sampled as the hoola/embroidery hoop technique. A single "yard" can count for one small preserve and 5 yards constitute a big preserve. Have groups of students count and identify all of the bird species present in several large and small preserves. (When I did this I enforced a rule of all census-takers remaining on the front sidewalk - no walking into backyards, although they were allowed to count birds that could be seen anywhere in the preserve). Also, this is more scientific if you make a rule for when to sample large and small - don't leave it up to the census takers. For example, Yard 1 = first small, skip 2 houses, next 5 yards = large preserve , skip 2 houses, then second small preserve, and so on.