A single square meter of upper Midwest prairie may contain 10 to 15 species [1]. A square foot of land in one of England’s chalk grasslands may contain more than 30 species, many of them no taller than the top of your boot [2]. This is equivalent to a plot of ground approximately the size of a record album cover, with each species covering an average of less than 5 square inches.

How does this kind of diversity persist? As Jonathan Silvertown asks in his book Demons in Eden [2], why doesn’t a single species, a super-fit super-competitor, take over the entire prairie? One answer to this question has long been that coexisting species—species that grow together within a plant community—must differ in how they utilize resources, and physiological trade-offs keep any single plant from dominating in every community. There have been excellent studies demonstrating examples of such trade-offs. In one study conducted in Amazonian forests [3], habitat specialization to soil types in tree species was found to involve an interaction between herbivore defense and growth rate: trees that can grow most rapidly are the best competitors on richer soils, but they were devoured by insects on poorer soils. The trees that fare best on the poorer soils are better defended against insects, but at the cost of their ability to grow rapidly.

However, numerous studies of plant distributions within communities fail to find significant differences among species in how they utilize available resources. If ecological differences between species explain diversity within communities, why can’t we find strong differences? A recent article by James Clark of Duke University [4] demonstrates that even when differences are not evident among species, the differences among individuals within species may allow those species to coexist.

Clark and colleagues studied 33 tree species in 11 forests of the southeastern United States over the course of six to 18 years (depending on the site), studying variation in growth responses for more than 22,000 trees. This is a fantastic dataset, as it is both broad in species and within-species sampling and relatively long in years. Clark studied how each individual tree responded to environmental fluctuations from year to year. He found, first, that different coexisting species tend to respond in the same way to environmental variations. This would suggest that trees do not partition their environment in a way that would promote coexistence. But then he compared the growth rate of every pair of individual trees growing within 20 meters of each other and the reproductive effort (measured in seeds per year) for every pair of individual trees growing within 60 meters of each other. He found something very striking: while among-species differences in response to environmental variation are not significant, the correlation in growth rate and reproductive effort is much higher between individuals of a single species than between individuals of different species. Two individuals of different species will be likelier to live within 20 meters of each other than two individuals of the same species. In Clark’s own example, “On average, both species benefit in wet years, but it is not the same individuals that are benefitting and not to the same degree.” It is variation within species, as much as variation among species, that leads to diversity of plant communities.

In a previous article , I discussed how plant biodiversity entails diversity both within and among species. Clark’s article is an excellent example of how diversity within species leads to diversity among species.

NOTES:
1.            Leach, M.K. and T.J. Givnish, Gradients in the composition, structure, and diversity of remnant oak savannas in southern Wisconsin. Ecological Monographs, 1999. 69(3): p. 353-374.

2.            Silvertown, J., Demons in Eden: The paradox of plant diversity. 2005, Chicago and London: The University of Chicago Press.

3.            Fine, P.V.A., I. Mesones, and P.D. Coley, Herbivores Promote Habitat Specialization by Trees in Amazonian Forests. Science, 2004. 305(5684): p. 663-665.

4.            Clark, J.S., Individuals and the Variation Needed for High Species Diversity in Forest Trees. Science, 2010. 327(5969): p. 1129-1132.

Based on his best-selling book, Bringing Nature Home, Dr. Tallamy explains the link between human well-being, native plant species and native insects and wildlife. He will recommend plants that can support the most wildlife and sustain biodiversity.

Dr. Tallamy is the chair of the Department of Entomology and Wildlife Ecology at the University of Delaware. Register now and walk away an inspired gardener!

Andrew L. Hipp, PhD
Plant Systematist and Herbarium Curator
The Morton Arboretum

In the fourth chapter of The Origin of Species, Darwin presents a phylogenetic tree depicting the relationships among a hypothetical set of individuals. Natural selection, Darwin explains, causes the branches on this tree to diverge from each other in ecology, behavior, and form. The tree thus portrays how a single population can give rise to a multitude of varieties.

After describing how natural selection creates varieties from a single population, Darwin writes:

If we suppose the amount of change . . . in our diagram to be excessively small, these three forms may still be only well-marked varieties; but we have only to suppose the steps in the process of modification to be more numerous or greater in amount, to convert these three forms into doubtful or at least into well-defined species. Thus the diagram illustrates the steps by which the small differences distinguishing varieties are increase into the larger differences distinguishing species.

Natural selection, Darwin tells us, causes populations to become adapted to their environment, and, consequently, to differentiate from one another, and it is this same process that eventually leads to the formation of new species, new genera, new families, and even phyla and kingdoms. Stated another way, the wide range of biological and ecological diversity we see today is a product of natural selection and lots of time. Darwin’s insight was that the mechanism by which populations diversify within species is the same mechanism by which species, genera, or families diversify. Microevolution—evolution within species—and macroevolution—evolution at the species level and higher—work hand-in-hand.

I have been thinking recently about how macroevolutionary and microevolutionary approaches provide complementary views on biological diversification. Let me give you an example from our work in the evolution of sedges (Carex: Cyperaceae), a fascinating group of plants that represent more than 5% of the diversity of the Great Lakes region.

In sedges, chromosome rearrangements evolve rapidly within and among species. Many species possess numerous chromosome arrangements, but each species still appears to be genetically, ecologically, and morphologically coherent, often across a wide geographic range. This is striking: with so much chromosomal variation, we might expect sedges to have splintered into even more species than they have. Sedges are quite diverse, with more than 2,000 species worldwide, and chromosome evolution seems likely to have played an important role in their diversity. In our lab, we use molecular methods to estimate evolutionary relationship between species, and statistical methods to estimate how chromosome rearrangements evolve between species. These are primarily macroevolutionary approaches. We have found using these approaches that chromosome breakages and fusions evolve rapidly to different equilibrium points in different sedge lineages, such that different branches on the sedge tree of life are marked by different dynamics of chromosome evolution. We know that one such transition in chromosome evolutionary dynamics occurred between 2-8 million years ago, somewhere near the ancestor of a lineage of more than 35 sedges that characterize wetlands, grasslands, and woodlands of the Great Plains and eastern North America. These shifts in evolutionary dynamics have the effect of driving chromosome rearrangements apart in different lineages, and they consequently limit the opportunities for hybridization between lineages. We find, using a macroevolutionary approach, a plausible mechanism by which changes in the dynamics of chromosome evolution may lead to increased species diversity.

This story from a macroevolutionary perspective raises the question of whether chromosome rearrangements play a role in genetic diversification within species. To get at this, we are working with collaborators Paul Rothrock and Richard Whitkus  to investigate how chromosome rearrangements affect gene flow among populations within two sedge species, one from eastern North America and one from the West Coast. In both species, we find a striking pattern: the difference in chromosome number between individuals or populations correlates significantly with their genetic distance. Furthermore, it is not the existence of a single rearrangement that limits gene flow, but the interaction between rearrangements. What this suggests is that as populations accumulate different chromosome rearrangements over time, they lose the ability to exchange genetic material. Chromosome evolution consequently has the potential to work together with ecological diversification to create new species. Moreover, we find that there is almost no genetic structure within the eastern North American species, in contrast to the deep genetic breaks found between closely related species. These findings tell us that chromosome rearrangements may play a role in allowing populations to diverge from one another by limiting hybridization between those populations, but that the chromosome rearrangements we find within species are insufficient on their own to cause speciation. They are most likely working hand-in-hand with ecological divergence or the evolution of reproductive incompatibilities to form the diversity we see today.

We know many details now about the generation of biodiversity that Darwin didn’t know, and the relative importance of natural selection and neutral genetic drift in diversification has been argued at length throughout the latter half of the 20th century. But the importance of evolution by natural selection as a unifying mechanism of biological diversification cannot be overstated. When we talk about plant biodiversity, we are really talking about diversity both within and among species.

For further reading:
 Darwin’s phylogenetic tree, from chapter four of The Origin of Species.
The Origin of Species, 6th (1872) edition.
Hipp et al. 2009. The most current review of what we know about the pattern and process of chromosome evolution in sedges.

What were Darwin’s key ideas and how are modern scientists using them? Who was Darwin? Try this video clip that features Dr. Andrew Hipp, Plant Systematist and Herbarium Curator at The Morton Arboretum speaking about the implications of Darwin’s work on his won research. And we have more. Click here for questions, answers, videos and a whole new learning experience!

Andrew L. Hipp, PhD
Plant Systematist and Herbarium Curator
The Morton Arboretum

Plant diversity is the focus of my work, and I have been asked from time to time why I or anyone else should be interested in it. Why bother studying the biological limits of and evolutionary relationships among plant species? Why worry about how species traits evolve and how those traits—flowering time, for example, or a particular chromosome arrangement—influence plant diversity? My answer is that these questions are the heart of understanding how species come into existence, how they persist over time, and how we can hope to preserve them.

There are many practical outcomes of this research. Biodiversity research tells us what species there are and how they can be identified. It identifies risks to their continuing existence. It tells us how important population genetic structure is to the restoration of plant communities. It tells us which are the dominant species of our forests and woodlands, how they adapt to changes in environment across their range, how they cohere genetically and ecologically, and how they assemble together to form communities. This is the baseline data needed to conserve biodiversity.

“But why,” you may ask yourself, “should I care about biodiversity in the first place?” There are many reasons. Communities that are diverse in species are more productive and more stable than communities that are simpler. This means that the plant communities with more species in them are more resistant to disease, to natural disaster, and more able to support more animals and, consequently, more people. Communities that are genetically diverse likewise support a greater diversity and greater abundance of herbivores and have greater flexibility to evolve in response to the changing climate. These communities do the practical work of protecting our water supply and atmosphere, feeding the animals we depend on, and moderating our climate. Finally, people are happier and their lives are richer when they come to know a greater diversity of life, just as knowing a greater diversity of people makes our lives richer. This strikes me as reason enough to study and protect biodiversity.

In a 2007 interview with Bill Moyers, the eminent naturalist E.O. Wilson observed that the human-caused extinction event we are currently living through is similar in magnitude to the meteor-induced extinction 65 million years ago that drove the dinosaurs to extinction. The result by the end of the century may be a loss of half of the species currently existing. Moyers asked, “How would that change life on earth?” Wilson responded, “We would just live in an impoverished environment. It would be a lot tougher. We wouldn’t have as many pollinators. We wouldn’t have as many future crops and genes to feed ourselves. We wouldn’t have the same kind of security given to us free in terms of water management… It should be a horror to people.”

To answer the question posed at the top of this posting, the study of biodiversity, ecology and evolution are central to understanding what makes the world livable, and how we can keep that world livable and at the same time enrich our own lives. In this series of articles I’ll focus on recent or ongoing research in diversity, ecology and evolution that may not be immediately available outside of the technical literature, but that provides us important tools and/or insight into making our natural world richer, healthier and more beautiful.

I’d like to hear from you. Please post your comments right here!