Terrestrial ecologists ignore aquatic literature: Asymmetry in citation breadth in ecological publications and implications for generality and progress in ecology

Abstract

The search for generality in ecology should include assessing the influence of studies done in one system on those done in other systems. Assuming generality is reflected in citation patterns, we analyzed frequencies of terrestrial, marine, and freshwater citations in papers categorized as terrestrial, marine and freshwater in high-impact “general” ecological journals. Citation frequencies were strikingly asymmetric. Aquatic researchers cited terrestrial papers ~ 10 times more often than the reverse, implying uneven cross-fertilization of information between aquatic and terrestrial ecologists. Comparisons between citation frequencies in the early 1980s and the early 2000s for two of the seven journals yielded similar results. Summing across all journals, 60% of all research papers (n = 5824) published in these journals in 2002–2006 were terrestrial vs. 9% freshwater and 8% marine. Since total numbers of terrestrial and aquatic ecologists are more similar than these proportions suggest, the representation of publications by habitat in “general” ecological journals appears disproportional and unrepresentative of the ecological science community at large. Such asymmetries are a concern because (1) aquatic and terrestrial systems can be tightly integrated, (2) pressure for across-system understanding to meet the challenge of climate change is increasing, (3) citation asymmetry implies barriers to among-system flow of understanding, thus (4) impeding scientific and societal progress. Changing this imbalance likely depends on a bottom-up approach originating from the ecological community, through pressure on societies, journals, editors and reviewers.

Introduction

A prime motivation in research is to ask if one's results are applicable to systems beyond the one of focus. Given the immense diversity of ecological systems, answering this question is key to advancing our understanding of community and ecosystem functioning. In community and ecosystem ecology, however, the search for generality often seems hindered by a relative lack of awareness of progress in other systems. For example, several authors have commented on the gap in communication between scientists working in aquatic and terrestrial habitats (e.g., Steele, 1991, Chase, 2000, Stergiou and Browman, 2005a, Richardson and Poloczanska, 2008). In some respects, this terrestrial vs. aquatic “controversy” is based on real and important contrasts. Terrestrial and aquatic environments are inherently different, as reflected in several aspects of the ecology of the communities and organisms within them (Carr et al., 2003). However, most fundamental ecological processes are shared among systems. Further, contrary to the opinions of some (e.g., Richardson and Poloczanska, 2008), much of our conceptual framework applies across all systems with ideas flowing in both directions between aquatic and terrestrial arenas (e.g., Halley, 2005, Paine, 2005, Raffaelli et al., 2005).

But what is generality, exactly? As implied by the above-cited controversy, is it the extent to which results in one habitat apply to systems in other habitats? That is, do outcomes obtained in a study conducted in a terrestrial environment contribute to inductively reasoned inferences about the dynamics of systems occurring in the other major habitat types, marine or freshwater? We define this as “habitat” generality. Or is generality more theoretical, in which model results apply to a broad range of populations, communities or ecosystems? “Theoretical” generality was one of a triumvirate of characteristics (generality, precision, and realism) identified decades ago by Levins (1966) as a tradeoff that constrained the effects of model builders, with the asserted limitation that particular types of models (analytical, simulations) could satisfy two but not all three of these traits. Generality could also be “geographic,” such that results obtained in a particular type of ecosystem in one geographic region apply to the same type of ecosystems located in different geographic regions. Another possible form of generality is conceptual, in which ecological concepts such as top-down, bottom-up, trophic cascade, positive interactions and intermediate disturbance provide a deductive framework for investigating patterns and processes that may operate across broad suites of individual systems (e.g., Chase, 2000).

Here we focus on generality as reflected in citation patterns of terrestrial, marine and freshwater papers in publications categorized into these three groups. Our interest in examining habitat generality was prompted by personal experience and comments of colleagues (see e.g., Underwood, 2005) that reviewers often criticized submissions of marine ecological papers to general journals as not being of sufficiently broad interest; they were “too marine.” The usual guidance was to demonstrate that results were of interest to a non-marine audience, usually understood as terrestrial.

Such comments made us wonder if non-marine authors received similar criticisms. In lieu of directly surveying authors, a difficult and relatively subjective method, we approached this question through a literature survey. Assuming that the extent of comparison of one's results would be reflected in the frequency of citations of papers from other systems, and using papers sampled from general ecological journals, we estimated the proportions of marine, freshwater, and terrestrial citations in marine, freshwater and terrestrial publications in community and ecosystem ecology. To assess the publication scenario against which these citation frequencies occur, we also summarized the frequencies of all terrestrial, marine, freshwater papers in each journal. Of course, one would expect that, correcting for citations that may largely be independent of habitat (methodological, conceptual/theoretical, reviews/books, and theses, collectively termed “other”), most citations in a terrestrial (or marine or freshwater) paper, for example, will be terrestrial (or marine or freshwater). But if all authors are being urged to emphasize generality or breadth independently of habitat, then the proportion of citations of papers from habitats other than the subject of the paper should be similar regardless of whether the paper was terrestrial, marine or freshwater.