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OCR for page 83
In the Light of Evolution: Volume II—Biodiversity and Extinction
Part II
CONTEMPORARY PATTERNS AND PROCESSES IN PLANTS AND MICROBES
Charismatic animals are most often the focus of conservation efforts, but much of the biological world is potentially at extinction risk from current human activities. Chapters in this section address some potential concerns about biodiversity and extinction in plants and microbes.
The anthropogenic introduction of alien species is perhaps second only to habitat loss as a cause of recent and ongoing species extinctions. The problem is especially acute on oceanic islands, where countless native animals have gone extinct following the arrival of humans and their hitchhiking associates. In Chapter 5, Dov Sax and Steven Gaines examine historical records from islands around the world to ask whether native plant species likewise often have gone extinct when exotic plants were introduced and became naturalized. The answer seems to be a clear no, at least yet. One possibility is that native plant species on islands are accumulating an extinction debt that will be paid in future species losses; alternatively, the number of native plus exotic plants on islands may reach a stable equilibrium or saturation point that is much higher than the endemics alone had been able to achieve. The authors examine the evidence pertaining to these competing hypotheses, and explore the ramifications for future plant biodiversity on islands depending on which scenario proves to be more nearly correct.
The task of tallying extant species and estimating extinction risks can be daunting even for relatively well-studied biotas. Such scientific exercises can also be highly informative, as Stephen Hubbell and colleagues illustrate in Chapter 6 by applying neutral biodiversity theory (Hubbell,
OCR for page 84
In the Light of Evolution: Volume II—Biodiversity and Extinction
2001) to estimate the number, abundance, range size, and extinction risk (under alternative scenarios of future habitat loss) for medium- and large-sized trees in the Amazon Basin. Their quantitative analysis suggests that more than 11,000 tree species inhabit this extraordinarily biodiverse region. The good news for biodiversity conservation is that more than 3,000 of these species have large population sizes and therefore are likely to persist well into the future (barring catastrophic climatic or other environmental changes). The bad news is that for the large class of rare Amazonian trees (more than 5,000 species likely to consist of fewer than 10,000 individuals each), estimated near-term extinction rates are 37% and 50%, respectively, under optimistic and non-optimistic projections concerning ongoing deforestation practices by humans.
With regard to tallying numbers of taxa and characterizing local, regional, or global patterns of biodiversity, microbes offer even stiffer challenges than many plant and animal taxa. In Chapter 7, Jessica Bryant and colleagues associated with Jessica Green tackle such problems on a mesogeographic scale by applying DNA sequence data (from the 16S ribosomal gene) and other information to questions about microbial biodiversity along an elevational habitat gradient in the Colorado Rocky Mountains. Bacterial taxon richness along their climatic-zone transect decreases monotonically from lower to higher altitudes, and detectable phylogenetic structure (nonrandom spatial clustering of related taxa) occurs at all elevations. In comparable analyses of plants along the same gradient, the authors uncovered qualitatively different outcomes with regard to both taxon richness and species assemblage. These findings indicate that whatever ecological and evolutionary forces shape microbial communities, the biodiversity patterns will not always mirror those in macrobiota.
An important follow-up issue for microbial (or other) taxa is whether the composition of natural communities predictably influences the responses of those communities to environmental alteration. Traditionally, microbial communities often have been treated as “black boxes” in functional ecological models, a situation that Steve Allison and Jennifer Martiny would like to see rectified. In Chapter 8, these authors review experiments and observations from the scientific literature to address questions about the composition of a microbial community following exposure to environmental perturbations. Is the microbial community resistant to the disturbance (tend not to change in taxonomic composition)? Is it resilient (change in makeup but then return quickly to the predisturbance condition)? If an altered composition is sustained, is the new community functionally redundant to the original? Based on the authors’ literature review, the answers to these questions usually seem to be “no,” “no,” and “no.” Allison and Martiny emphasize that all such conclusions remain provisional pending further research of this nature, and they suggest several promising empirical and conceptual approaches.
