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Friday, June 6, 2008

article : Plant Life Histories: Ecology, Phylogeny and Evolution. - book reviews

Ecology, Sept, 1998 by Gregory P. Cheplick

Silvertown, Jonathan W., Miguel Franco, and John L. Harper, editors. 1997. Cambridge University Press, New York. xviii + 313 p. $29.95, ISBN: 0-521-57495-1.

Plant ecologists often attempt to make broad, sweeping generalizations regarding the relationships between life history features and various components of the abiotic environment (e.g., correlations between seed mass and habitat dryness). The objective is to unravel the selection pressures thought to have led to the evolution of the life history trait of interest. In such analyses that ignore phylogeny, life history measurements for different species are assumed to represent statistically independent data points and therein lies a problem, as the papers in this volume make abundantly clear.

This collection of papers revolves around the common theme of using phylogenetically independent contrasts (PICs) to "re-examine in phylogenetic perspective the perceived patterns of relationship between different plant life history traits, and between those traits and the presumed selective pressures that shape them." For their phylogenies, a fair number of the papers rely on Chase et al. (1993; Phylogenetics of seed plants: an analysis of nucleotide sequences from the plastid gene rbcL. Annals of the Missouri Botanical Garden 80:528580). In actuality, not all authors take the phylogenetic approach and some seem to be unaware of the general theme. Nevertheless, there is much material worth recommending, including the several papers that do not utilize a phylogenetic approach.

The fifteen contributions are organized into five sections. The three chapters in the first section on "Phylogenetic perspectives" adhere closely to the general theme. It is remarkable how well several generally accepted life history correlations, originally established without reference to phylogeny, hold up when phylogeny is controlled. For example, J. Silvertown and M. Dodd show that reproductive allocation is indeed higher in annuals than in perennials, and higher in early-successional perennials than in late-successional perennials, even when phylogenetically controlled tests are used. In their comparison of native and alien floras of the British Isles, M. J. Crawley et al. stress how aliens and natives are not drawn from species pools with similar taxonomic compositions, making a phylogenetically-controlled approach essential to the espousal of any generalizations.

The second and third sections deal with "Reproductive traits" and "Seeds," respectively. The papers cover mating systems, including selfing (S. C. H. Barrett et al. and D. J. Schoen et al.), genetic diversity (J. L. Hamrick and M. J. W. Godt), seed size in relation to dormancy (M. Rees) and dispersal (M. Westoby et al.), and a mathematically intense theory of packaging and provisioning in plant reproduction (D. L. Venable). Most of the contributions are of high quality and provide an excellent introduction to the recent literature as well as some new insights. Barrett et al. show how selfing has repeatedly evolved as a reproductive assurance mechanism in lineages of annual Polemoniaceae, while Schoen et al. use theoretical and experimental approaches to determine the relative importance of automatic selection and reproductive assurance in the evolution of self-pollination. The relation of seed mass to life history is explored in two chapters. Using comparative methods, Rees shows how tall, long-lived plants with animal-dispersed seeds tend to have heavier seeds than species with extensive lateral spread or species with long-lived seed banks. In an interesting twist, Westoby et al. question the assumptions underlying the phylogenetic approach, but use PICs to show an unexplained inverse correlation between seed mass and specific leaf area.

The fourth section, on "Recruitment and growth," includes an interesting comparative analysis of clonal plant associations with habitat features that reveals that clonality is more abundant under "wet, cold and unproductive circumstances" (J. M. van Groenendael et al.), a comparative study of life history variation (M. Franco and J. Silvertown), and a rather misplaced review of life history evolutionary theory by an animal ecologist (R. M. Sibly) that considers neither recruitment, growth, or phylogeny.

Two chapters in the final section ("Interactions") use phylogeny to address host-plant choices in insect-plant interactions (D. J. Futuyma and C. Mitter) and evolutionary trends in root-microbe symbioses (A. H. Fitter and B. Moyersoen). Although the final chapter does not specifically utilize a phylogenetic approach, it clarifies current definitions of "competitive ability" and explores how competitive hierarchies depend on environment (D. E. Goldberg).

In short, this affordable volume serves as an excellent introduction to the usefulness of PICs and related phylogenetic tools in advancing our present understanding of plant life history evolution. The phylogenetic perspective may indeed, as the editors maintain in the Preface, provide the means to address the important question of how general the observed trait correlations ofttimes reported by ecologists actually are. The book is relatively free of typographical errors, contains clear graphics, and appears to have been well-edited. Graduate students in both ecology and systematics who are interested in the grand questions of how and why adaptive traits have evolved in plants can find ideas for research in this volume. Many of the authors should be commended for helping to unite two groups in plant biology that all-too-often carry out their respective research programs in relative isolation: systematists and population ecologists. Clearly, researchers in these disciplines can find commonalities in the phylogenetic approach advocated by the editors to questions regarding adaptation and evolution.

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