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TRENDS in Ecology and Evolution
Vol.21 No.9
481
Letters
Ecologists have already started rebuilding community ecology from functional traits Michael Kearney1 and Warren P. Porter2 1 2
Department of Zoology, The University of Melbourne, Parkville, VIC 3010, Australia Department of Zoology, University of Wisconsin, Madison, WI 53706, USA
McGill et al. [1] have recently called for a major change in how researchers approach community ecology, whereby the interaction between functional traits of organisms and environmental variation through space and time is considered against a background biotic ‘interaction milieu’. They suggest that this can be achieved through a mechanistic approach to the fundamental and realized niches, using general performance currencies such as energy and nutrients. Although we agree entirely, we would like to point out that the tools required to approach community ecology in this way are well developed and are already being applied under three different guises: dynamic energy budget models, the geometrical framework of organismal stoichiometry, and biophysical ecology. Dynamic energy budget models and the geometric framework address two of the four research themes outlined by McGill et al.: functional traits and performance currencies. Dynamic energy budget model theory [2–4] models the relationship between surface area, feeding behaviour and energy acquisition through partitioning the organism into structural body mass and reserves (i.e. stored energy density). Thus, it relates functional traits to a performance currency of energy. The ‘supply-side’ nature of dynamic energy budget models is especially attractive because it provides a link between organism traits, food availability, and energy for growth, maintenance and reproduction. This enables relatively easy integration of dynamic energy budget models with individual-based models of population dynamics; a ‘long-term’ goal discussed by McGill et al. [1] that has in fact recently been achieved with considerable success [5]. Whereas dynamic energy budget models often focus on energy as a performance currency, the geometrical framework focuses particularly on nutrient stoichiometries [6,7]. The basic process of the geometrical framework is to model the nutrient relationships of an organism as an n-dimensional hyperspace whose axes comprise carbohydrates, lipids proteins and other key nutrients (with obvious similarities to Hutchinson’s niche concept [8]). Nutrient intake, assimilation and output rates are modelled dynamically as an organism grows and reproduces, with the organism tracking a moving maximum-fitness target within the nutrient hyperspace. The dynamic energy budget model and geometrical framework theories therefore enable mechanistic analysis of how functional traits of individual Corresponding author: Kearney, M. (
[email protected]) Available online 10 July 2006 www.sciencedirect.com
organisms, including their surface area, feeding and digestive systems, and nutrient requirements, affect key ecosystem-level processes using the performance currencies of energy and mass acquisition. Recent developments in the field of biophysical ecology now provide the opportunity to link functional traits and performance currencies to McGill et al.’s third theme, environmental gradients, under their suggested framework of fundamental niches. Biophysical ecology [9] applies the principles of heat and mass transfer to calculate how different environmental conditions interact with functional traits, such as body size and behaviour, to affect the performance currencies of energy, nutrient and water exchange. It thus enables the calculation, from first principles, of the climatic component of the fundamental niche [10]. Integration of biophysical ecology with GIS data sets on climatic conditions, topography and vegetation [10–12] then enables visualisation of the distribution of the fundamental niches of species across environmental gradients through space and time [10]. This functional trait-based approach to modelling the distributions of species goes beyond correlative habitat models [13] to tackle McGill et al.’s second major research question ‘What traits and environmental variables are most important in determining the fundamental niche?’. The three approaches that we describe have developed independently and there is enormous potential for their integration to obtain a truly mechanistic view of the fundamental niches of organisms. An addition to this integration challenge will be to find ways of incorporating the fourth theme of McGill et al., the biotic ‘interaction milieu’, and its effect on constricting fundamental niches to realized niches. Although we applaud McGill et al.’s appeal for a functional-trait approach to community ecology, we hope to have shown that we are in fact further along the path to realizing their vision than they might have appreciated. References 1 McGill, B.J. et al. (2006) Rebuilding community ecology from functional traits. Trends Ecol. Evol. 21, 178–185 2 Kooijman, S.A.L.M. (2000) Dynamic Energy and Mass Budgets in Biological Systems, Cambridge University Press 3 van der Meer, J. (2006) Metabolic theories in ecology. Trends Ecol. Evol. 21, 136–140 4 Nisbet, R.M. et al. (2000) From molecules to ecosystems through dynamic energy budget models. J. Anim. Ecol. 69, 913–926 5 Klanjscek, T. et al. Integrating dynamic energy budgets into matrix population models. Ecol. Model. (in press)
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6 Raubenheimer, D. and Simpson, S.J. (2004) Organismal stoichiometry: quantifying non-independence among food components. Ecology 85, 1203–1216 7 Simpson, S.J. et al. (2004) Optimal foraging when regulating intake of multiple nutrients. Anim. Behav. 68, 1299–1311 8 Hutchinson, G.E. (1957) Concluding remarks. Cold Spring Harb. Symp. Quant. Biol. 22, 415–427 9 Gates, D.M. (1980) Biophysical Ecology, Springer-Verlag 10 Kearney, M. and Porter, W.P. (2004) Mapping the fundamental niche: physiology, climate and the distribution of nocturnal lizards across Australia. Ecology 85, 3119–3131
11 Porter, W.P. et al. (2000) Calculating climate effects on birds and mammals: impacts on biodiversity, conservation, population parameters, and global community structure. Am. Zool. 40, 597–630 12 Porter, W.P. et al. (2002) Physiology on a landscape scale: plant-animal interactions. Integr. Comp. Biol. 42, 431–453 13 Kearney, M. Habitat, environment and niche: what are we modelling? Oikos (in press)
0169-5347/$ – see front matter ß 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.tree.2006.06.019
Letters Response
Response to Kearney and Porter: Both functional and community ecologists need to do more for each other Brian J. McGill1, Brian J. Enquist2, Evan Weiher3 and Mark Westoby4 1
Department Department 3 Department 4 Department 2
of of of of
Biology, McGill University, Montreal, QC, Canada, H3A 1B1 Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ 85721, USA Biology, University of Wisconsin – Eau Claire, Eau Claire, WI 54702, USA Biological Sciences, Macquarie University, NSW 2109, Australia
Kearney and Porter [1] have correctly identified one of our themes from our recent article in TREE [2]: community ecologists should pay more attention to, and make greater use of, functional (physiological) ecology. Kearney and Porter [1] cite several good examples of the kind of exciting, recently published studies in functional ecology that community ecologists would benefit from incorporating into their thinking. Other recent important efforts in energetics [3,4], stoichiometry [5] and biophysics [6,7] take different approaches. In our paper [2], we made three other points that go beyond Kearney and Porter’s point of identifying work in functional ecology that is of use to community ecologists. First, we emphasized three types of quantitative measures (traits, performance currencies and environment) and the need to explore all possible combinations of these factors. Thus, in addition to the combinations found in the work highlighted by Kearney and Porter, we emphasize, for example, exploring how traits vary along environmental gradients and which morphological traits link to which physiological traits. Second, we also emphasized exploring not only the fundamental, but also the realized niche processes (i.e. the interaction milieu or species interactions). Most of Kearney and Porter’s examples [1] do not address these realized niche processes: for example, we encourage asking which traits affect species interactions and how do species interactions change with the environment? Finally, we emphasized a shift in approach away from ANOVA-based ecology towards looking at the mathematical relationships between quantitative measures of traits and environment. None of these three agendas necessarily requires DOI of original article: 10.1016/j.tree.2006.06.019. Corresponding author: McGill, B.J. (
[email protected]) Available online 10 July 2006 www.sciencedirect.com
the types of physiological ecology put forth by Kearney and Porter [1]. These research agendas are currently still in a necessary pattern-finding phase and tend to not explicitly include any mechanism. The search for mechanism in these areas in the future might lead to Kearney and Porter’s physiological ecology, but it could equally well lead to evolutionary ecology or behavior. We suggest that functional ecology has not yet solved all of the needs of community ecologists. We addressed our paper to community ecologists and the changes that they need to make. The examples that we and Kearny and Porter provide not to the contrary, an equally long and necessary paper could have been written calling for select branches of functional ecology to pay more attention to community ecology [8] and thereby achieve more relevance to applications in conservation. We do not attempt this here, but briefly suggest that to achieve maximal usefulness and relevance to the larger field of ecology areas of functional ecology with aspirations to informing community ecology need to come out of the laboratory and into the field by: Becoming more comparative between species. Community ecologists study 5–200 species at a time. Physiological data need to have the same span. Placing more emphasis on measures of fitness that have an influence on the fate of the species over multiple generations, instead of focusing on factors such as instantaneous energetic requirements, leaf carbon assimilation rates, or single clutch size. Building more links between physiological traits and more easily measured, ecologically relevant traits. The links between metabolism and body size [2] and between carbon assimilation rates and leaf life span [9] are good examples that need to be expanded.
