Ecological Morphology: Integrative Organismal Biology

Ecology, Dec, 1995 by Stephen F. Norton

Two themes run through this volume: 1) the search for adaptive mechanisms leading to the ecomorphological patterns that we observe and 2) the need to incorporate an explicit phylogenetic dimension in ecomorphological studies. As several authors point out, the search for mechanisms requires a broader comparative focus than provided by morphology and ecology alone. Greater insights can result from an integration of ecomorphological studies with the other disciplines that comprise organismal biology, including physiological ecology (Bradley; Garland and Losos), behavioral ecology (Emerson et al.; Garland and Losos), functional morphology (Wainwright), developmental biology (Travis; Reilly), biomechanics (Emerson et al.; Denny; Norberg), and paleontology (Van Valkenburgh).

Demonstrating a causal basis of correlations between morphological and ecological characteristics requires the establishment of plausible mechanisms of operation. Wainwright outlines the application of functional morphology to this problem via a comparison of the functional potential of the different forms. Alternatively, Emerson et al., Denny, and Norberg compare the observed forms to theoretical optima derived from biomechanical models. Both approaches can provide an important service in ecomorphological studies by separating meaningful correlations from spurious correlations or autocorrelations and by directing the selection of relevant ecological and morphological characters.

The chapters by Denny on the interaction of hydrodynamic forces and the morphology of rocky intertidal invertebrates and algae and by Norberg on the role of aerodynamics in bat design provide a provocative contrast of the "success" achieved when applying biomechanical models to analyzing ecomorphological patterns. The congruence between wing design, flying behavior, and ecology of bats presented by Norberg is in striking contrast to the more chaotic patterns found in the intertidal in which widely divergent forms can present similar hydrodynamic profiles. The different outcomes may reflect 1) the radically-different taxonomic scales of these studies and 2) the ability of bats to control the aerodynamic conditions under which they operate (and therefore be fine-tuned through natural selection) versus the uncontrollable and variable nature of the hydrodynamic forces faced by sessile/sedentary intertidal organisms.

The development of a plausible mechanism is not enough to show that an ecomorphological correlation is adaptive. For example, Travis points out that for phenotypic plasticity to be adaptive the optimum morphotype for a species must be different under different ecological conditions. To demonstrate that a feature is adaptive Arnold (Arnold, S. J. 1983. Morphology, performance and fitness. American Zoologist 23: 347-361) proposed a two-step process in which the performance consequences of morphological variation are assessed first and then the fitness consequences of performance variation are assessed. Garland and Losos present an important modification of Arnold's proposal in which they highlight the potential role of behavior to act as a filter between performance studies and fitness. They illustrate their discussion of the implications and limitations of their expanded framework by examining the locomotory biology of snakes and lizards.

There is clear agreement among the contributors that ecomorphological studies should include a phylogenetic component. Losos and Miles point out the difficulties (both statistical and interpretive) that result when phylogenetic patterns are ignored. They describe several approaches that incorporate phylogenetic information in an ecomorphological analysis. Their chapter also highlights new questions that can be addressed by the addition of a phylogenetic perspective (e.g., examining the rate and direction of evolutionary change, examining the degree of correlation of changes in morphological and ecological characteristics, separation of in situ character displacement from differential colonization). Bradley clearly demonstrates the utility of phylogenetic information in an ecomorphological study as he traces the evolution of strategies used by mosquito clades to expand the range of larval habitats from freshwater into hypersaline environments. His analysis demonstrates that among euryhaline mosquitoes an osmoconformer strategy has evolved at least twice and an osmoregulatory strategy, complete with novel salt-secreting organs in the hind-gut, has evolved at least three times. If it was not clear before, this volume unquestionably indicates that incorporation of phylogeny has emerged as a new standard in ecomorphology.

The volume presents two potential drawbacks for the readers of Ecology. First, the volume is far more evolutionary and functional than it is ecological. By far the most ecological treatment of the current state of ecomorphology is presented by Ricklefs and Miles. Other chapters with direct ecological content include Emerson et al. who develop a model for the scaling of predator-prey relationships derived from optimal foraging theory, and Wainwright who discusses how functional differences in foraging mechanism and ability shape interactions among centrarchid fishes. Second, with the exception of the chapters by Denny and by Bradley, the examples and perspectives emphasize vertebrate biology. The narrow taxonomic coverage (e.g., the near absence of references to studies of higher plants) may discourage researchers whose interests extend beyond vertebrates. However, regardless of the taxa you study, if you are interested in questions that ask how organisms have adapted to their environment you will benefit by a close consideration of the ideas and approaches presented in this volume.