Novack-Gottshall, P.M. 2007. Using a Theoretical Ecospace to Quantify the Ecological Diversity of Paleozoic and Modern Marine Biotas
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Novack-Gottshall, P.M. 2007. Using a theoretical ecospace to quantify the ecological diversity of Paleozoic and modern marine biotas. Paleobiology 33:274-295 (+ online appendices). Please note the following corrections: p. 283. Replace p=1.005 with p=0.1005 p. 284, Fig. 3 caption. Replace p=1.005 with p=0.1005 p. 289: Add at end of Acknowledgements: This is Paleobiology Database Publication 62. Paleobiology, 33(2), 2007, pp. 273–294 Using a theoretical ecospace to quantify the ecological diversity of Paleozoic and modern marine biotas Philip M. Novack-Gottshall Abstract.—The process of evolution hinders our ability to make large-scale ecological compari- sons—such as those encompassing marine biotas spanning the Phanerozoic—because the com- pared entities are taxonomically and morphologically dissimilar. One solution is to focus instead on life habits, which are repeatedly discovered by taxa because of convergence. Such an approach is applied to a comparison of the ecological diversity of Paleozoic (Cambrian–Devonian) and mod- ern marine biotas from deep-subtidal, soft-substrate habitats. Ecological diversity (richness and disparity) is operationalized by using a standardized ecospace framework that can be applied equally to extant and extinct organisms and is logically independent of taxonomy. Because indi- vidual states in the framework are chosen a priori and not customized for particular taxa, the framework fulfills the requirements of a universal theoretical ecospace. Unique ecological life hab- its can be recognized as each discrete, n-dimensional combination of character states in the frame- work. Although the basic unit of analysis remains the organism, the framework can be applied to other entities—species, clades, or multispecies assemblages—for the study of comparative paleo- ecology and ecology. Because the framework is quantifiable, it is amenable to analytical techniques used for morphological disparity. Using these methods, I demonstrate that the composite Paleozoic biota is approximately as rich in life habits as the sampled modern biota, but that the life habits in the modern biota are significantly more disparate than those in the Paleozoic; these results are robust to taphonomic standardization. Despite broadly similar distributions of life habits revealed by multivariate ordination, the modern biota is composed of life habits that are significantly en- riched, among others, in mobility, infaunality, carnivory, and exploitation of other organisms (or structures) for occupation of microhabitats. Philip M. Novack-Gottshall. Department of Biology, Duke University, Box 90338, Durham, North Caro- lina 27708-0338 Present address: Department of Geosciences, University of West Georgia, Carrollton, Georgia 30118-3100. E-mail: [email protected] Accepted: 28 January 2007 Ecological communities, however, do exist, but titatively by comparative paleoecologists. I what are linked in them by biotic factors are not will argue below that the answers to these and the faunistic units, the species, but the ecological similar questions are impeded by a method- units, the life forms. ological limitation in our ability to compare —G. Thorson (1957: p. 470) communities (or other ecological entities) when they are separated by vast expanses of Though the technical difficulties are very great, time and space and when they share few or no they could probably be solved by anyone who re- evolutionary homologies. Their solution hing- ally wanted to compare the furry growth of di- es on the ability to compare quantitatively all atoms on a stone in a stream with the larger- kinds of entities directly on the basis of their scale patches of woodland that have about the ecological capabilities. same sort of uniformity when viewed from an Taxonomy has remained a typical yardstick airplane. for such comparisons. It has formed the dom- —G. E. Hutchinson (1965: p. 77) inant basis for comparing the structure of Pa- leozoic and Recent communities (Bretsky Is the modern marine biota composed of the 1968; Ziegler et al. 1968; Walker and Laporte same life habits as ancient ones? Which biotas 1970; Levinton and Bambach 1975; West 1976; are ecologically more diverse, in terms of both Miller 1988; Radenbaugh and McKinney 1998). the number of life habits and the disparity Although all of these studies considered var- (similarity) of these life habits? These are basic ious ecological characters (e.g., trophic guilds, questions that ought to be answerable quan- abundance), their primary impetus was the ᭧ 2007 The Paleontological Society. All rights reserved. 0094-8373/07/3302-0007/$1.00 274 PHILIP M. NOVACK-GOTTSHALL presence of taxonomically similar entities. The It is important here to understand what I underlying assumption when using taxonomy mean by the term ecological character. We can in this way is that the ecological characters of start with the understanding that each organ- taxonomic groups are conserved during evo- ism exhibits unique phenotypic features (sen- lution, such that taxonomy acts as shorthand su Bock and von Wahlert 1965) that affect en- for ecology. Although this may be generally vironmental interactions. Collectively, these true at low taxonomic levels, and occasionally phenotypic features endow each organism high ones (Webb et al. 2002), there are many with ecological capabilities or characters (fac- exceptions. For example, Fauchald and Jumars ulties sensu Bock and von Wahlert 1965). For (1979) noted stark population-level differenc- now, I will focus on those autecological char- es within individual species of polychaetes, acters related to feeding, use of space, mobil- and Stanley (1968, 1972) and Miller (1990) not- ity, dispersal, reproduction, and body size; ed widespread life habit convergence among taken together, these describe an organism’s bivalve orders. As a general rule, Peterson et basic life habit. Ecological diversity, regarded al. (1999) demonstrated that conservatism is as the overall variety of life habits within some less likely above the familial level. Thus, al- group, can be most easily assessed by rich- though taxonomic comparisons may be suit- ness, the number of unique life habits in this able for documenting the ecological organi- group. It can also be assessed by ecological zation of taxonomically similar communities, disparity, a measure of how different each life such a basis is not useful when comparing tax- habit is from others in this group (modified onomically disparate communities. In short, from Foote 1993a). I propose below a common taxonomy is an indirect, and potentially mis- framework for such characters and formal def- leading, proxy for getting at ecological ques- initions for ecological richness and disparity. tions. Focusing on such ecological characters di- Morphology has been another vehicle for rectly has two benefits. First, it avoids the ecological comparisons (Van Valkenburgh problems of homology associated with mor- 1985, 1988, 1991, 1994; Foote 1996b; Wain- phological comparisons. Because distinct phe- wright and Reilly 1994; Van Valkenburgh and notypes can perform identical functions in nu- Molnar 2002; Lockwood 2004). The general merous ways (Bock and von Wahlert 1965; Al- premise of ecomorphology is that morpholo- faro et al. 2004, 2005; Wainwright et al. 2005; gy can be used as a proxy for the ecological Marks and Lechowicz 2006), there exists in na- characters of organisms. Such correspondence ture an innate tendency for ecological conver- has been well supported (e.g., Winemiller gence when emergent capabilities are benefi- 1991; Wainwright 1994). However, there seems cial. In a sense, such higher-order capabilities little potential in using these methods for are ‘‘screened off’’ (sensu Brandon 1984) from large-scale comparisons spanning phyla and their underlying morphological and function- long time scales because of the lack of appro- al causes. These characters are accordingly priate homologous characters. The most am- more suitable for large-scale ecological com- bitious comparisons include Paleozoic and Re- parisons. This may diminish, although not cent arthropods (Briggs et al. 1992; Wills et al. eliminate, the role of phylogenetic effects (Fel- 1994; Stockmeyer Lofgren et al. 2003) and an- senstein 1985b; Harvey and Pagel 1991). Sec- imal skeletons (Thomas and Reif 1993; Thom- ond, compared with analyses using the prox- as et al. 2000). There are few homologous (and ies of taxonomy or morphology alone, such a even functionally comparable) morphological focus better aligns results with the theoretical characters shared throughout benthic com- understanding of ecological diversifications munities composed of green algae, foraminif- (Grant 1999; Schluter 2000; Coyne and Orr era, corals, trilobites, bryozoans, brachiopods, 2004). and bivalves. It is essential to focus such com- Such benefits motivated the development of parisons on ecological characters directly, in- the guild concept. Originally focused on com- stead of on their underlying morphology or parisons among taxa sharing diet and forag- their consequences for taxonomy. ing habits (Root 1967), it was later modified to QUANTIFYING ECOLOGICAL DIVERSITY 275 include other categories—microhabitat, loco- thetic quantitative framework, while also al- motion, ecomorphology, timing of reproduc- lowing such analyses, is preferable for several tion and daily activities, among others (Schoe- reasons. First, it facilitates more robust docu- ner 1974; Bambach 1983, 1985; Simberloff and mentation of overall changes in ecospace