Monday, January 30, 2017

Teaching biodiversity

The biodiversity crisis is one of the most pressing issues facing mankind.  But what is biodiversity? How does it come about?  Why are there so many kinds of organisms?  And what are the consequences of lost biodiversity?  Sounds like something we could devote a whole course to in our undergraduate curriculum.  But when we do, it's usually only to a handful of upper level ecology students.  What about our general student population, all those pre-meds we educate, or the public at large?  We would like our general citizenship to be well-informed on the dimensions and importance of biodiversity, but at the very least, our own biology majors should be not only informed, but also advocates.  How are we all doing on that?

The one, and sometimes only, opportunity we have to reach all of our undergraduates is in freshman biology.  A second opportunity comes if we can get students to take an organismal course like introductory botany, invertebrate zoology, mycology, or entomology.  The following suggestions apply equally well to all such courses, but I will focus on Freshman Biology, which in my university is taught in two separate one-semester courses: Cellular Processes and Biodiversity.  We use one of the standard large college texts for introductory biology.

The second course, Biodiversity, sounds good, but it has an ambitious amount of material to cover.  It is divided roughly into three sections:  Evolution, Diversity, and Ecology.  The section on Diversity, which might get five weeks out of the semester, is usually a rapid march through the kingdoms and phyla.  Plants usually only get 1-2 weeks of discussion in lecture,  fungi even less, more often than not by an instructor who has not had much training in botany or mycology.  The sections on animal and plant biology in the textbook are largely ignored in our courses, but they contain much additional information that can and should be brought in to help elucidate biodiversity issues.

The net result is that after this course in Biodiversity students have only a superficial introduction to diverse kinds of life, and have no real understanding of why there are so many different kinds of organisms.     The coherent biodiversity message will only  emerge if an experienced and skillful instructor is motivated to do so, and willing to synthesize material from different parts of the book.  The more we can integrate our discussions of evolutionary and ecological principles with questions about biodiversity, rather than treating them as three separate topics, the better. This is a tall order for the limited time available in a freshman course, so you have to choose your battles carefully.

Here are some ideas, with links to my blog essays that discuss these topics in detail:

1.  Think in terms of  adaptation.  This is the key concept  that links evolution, ecology, and biodiversity together. Adaptation is what results from the evolutionary process, it defines how organisms interact with their environment, and it is what differentiates the distinctive lifestyle of one organism from another.  Organism A is different from organism B because, since their common ancestry, they have had different adaptive histories, and have diverged into different lifestyles.  They have come to live in different environments  or to survive in the same environment in different ways.
The flowering structures of skunk cabbage are
adapted for getting a jump on the spring
flowering season by generating sufficient heat
to melt their way through the remaining snow.
Photo by Sakaori, from Wikimedia Commons

2.  Every feature has a function.  Everything we see in an organism, from the shape of a leaf to the color of tail feathers in birds, has some adaptive function, or did sometime during the adaptive history of the organism.  There is a story for every characteristic feature of a species.  Pollination biology presents many attention-grabbing examples.

3. "What good is half an eye?" Make use of some of the great questions posed by anti-evolutionists (though answered wrongly by them).  By focusing  on a single topic, like vision, one can trace the origins of light detection in bacteria, through the simple eyespots of protists, the simple eyes of flatworms, and then the diverse kinds of eyes found in cephalopods (squids, octopi), insects, and vertebrates.  There never was half an eye, always light detecting systems that became more complex and varied over time.  We can even bring in plants with their light-detecting systems  involved in phototropism (bending toward light), and photoperiodism (determining when plants bloom.)

 4. "If humans evolved from apes, why are there still apes?"  Another great question that illustrates the diversifying nature of evolution.  From the common ancestor of chimpanzees and humans, the chimp lineage continued to hone their adaptations for life in the forest, while human ancestors adapted for life in the open savanna, with skeletal changes that allowed them to stand upright and walk comfortably on two legs.  Students really take interest in human evolution, and so it is worthwhile to spend time on it.

Animals are eating machines, with mouth and eyes at the
front end, and locomotory organs along the side. Their
food resources are in compact packages - other organisms -
that they digest internally.
2. "What is the difference between plants and animals?"  I always began my botany classes with this question.  Almost everyone starts with the observation that plants are photosynthetic and animals eat stuff. True as it is, it is only  the beginning. The next thing that might come up is that animals move and plants don't.  Then as an instructor, you ask "why?"   Asking why is not to be anthropomorphic.  It's really asking "what are the adaptive advantages for plants not to move?"

Plants are anchored in one spot and create
branched systems above and below ground for
gathering diffuse resources. Meristems continually
create new roots, twigs and leaves.  And materials
are transported internally by manipulating water

One can then direct the discussion to the stationary, indefinitely branching body of a plant, adapted to create  an expansive antenna system for gathering light and other diffuse resources, in contrast to the discrete bodies of animals with mouth and sensory organs at the front end, and locomotory organs along the sides.  One must then discuss the system of meristems that enable varied plant architectures built of repetitive leaf-bearing units, and the hydrostatic nature of plant cells and plant processes that substitute for muscular activity in animals.

3. Why are there no moss trees? Everyone knows, at least by the time they get to college, how animals make babies.  The varied equipment and various strategies for getting sperm and egg together are a wonderful theme for exploring animal diversity, but how do plants do it? Plant (or fungus) reproduction, however, is always a challenge.    If you're stuck in one spot, and a potential mate is 50 meters away, how do get your sperm to her?  The astute student will immediately shout "pollen grains."  But how many know that there are actually sperm cells produced within pollen grains, and that pollen grains, and the structures that house the eggs are actually tiny. haploid individuals?

Eggs and sperm cells in ferns are produced on a
separate, independent, short-lived plant
(gametophyte)  that develops from a spore
released from the main plant (sporophyte).
Meiosis occurs during the production of the
spores, rather than in the production of gametes.
From Haupt, A. W. 1953. Plant Morphology.

That brings up the dreaded life cycle.  Students hate them and instructors who don't know their significance tend to skip over them. Memorizing a life cycle does not explain why life cycles exist.  What is the adaptive value of having separate tiny haploid plants to bear the sperm and eggs?  The place to begin is with ferns.  There is adaptive value in breeding with distant, genetically different individuals.  That would be impossible if the fern plant produced sperm and egg cells directly, as sperm cells on dry land can't get very far.  So instead, it produces spores, which can easily disperse over great distances.  Often enough, spores from genetically different ferns land together, These spores then germinate to produce tiny gamete-producing plants that can breed with one another.  The fertilized eggs then develop into a new fern plant.

The leafy, long-lived phase of a
moss life cycle is the egg and
sperm producing gametophyte.
The simple sporophytes consist
only of a single sporangium and
its stalk, which develops from
the fertilized egg, and which
remains attached to the gametophyte
plant for its short existance.
The answer to the question about moss trees is that a moss is actually a gamete-producing plant, and must remain small so it can mingle with genetically different plants for successful reproduction.  Mosses lack independent spore-producing plants, having instead small diploid spore-producing structures that emerge directly from the fertilized eggs, but remain attached to the parent plant. Thus there is no part of the moss life cycle that can get really large.

 Pursuing these sorts of discussions is of more value than memorizing the characteristics of all the phyla of invertebrates, or the differences between club mosses and horsetails.  Horsetails can be brought in, however, as an early example of the kind of multiple elongating (intercalary) meristems used by bamboos for their rapid growth in height.(convergent evolution).  Ultimately, we can try to understand why there are so many kinds of plants, and how to avoid the extinction of all those species.

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