Resolving conceptual confusion and quantifying cross-taxa patterns of 'density dependence' in population ecology.

Abstract

Density dependence represents a causal relationship between the size of a population and at least one of its measureable demographic rates. It encapsulates the demographic and evolutionary role of a range of social and trophic mechanisms (e.g., cannibalism, competition, cooperation, parasitism, predation), whose effects on crowding and extinction are themselves modified by the population’s abundance (density feedback). The concept is applied in conservation and management to assess critical matters such as harvest quotas, pest/invasion control and thresholds of extinction. I review the use of density dependence in ecology. I quantify a temporal increase in the number of ecological papers examining this concept across an augmenting number of study taxa, and little consensus around the meaning of density dependence and associated concepts in a questionnaire survey among 136 ecologists (Chapter 1). Next, I revise the vocabulary of density dependence in a historical context, finding more than 60 terms, many of which are polysemous, synonymous, or grounded in opinionated statements; I name five unequivocal qualifiers of density feedback (compensatory, delayed compensatory, overcompensatory, depensatory/Allee effect) linked to known population phenomena (stability, cycles, chaos, decline), and dissect the semantic differences between density dependence and population regulation (Chapter 2). Using empirical methods, I show that the strength of density feedback increases with the pace of species’ life histories (Chapter 3), yet is only negligibly correlated with coarse climatic gradients (Chapter 4). These results suggest that broad life-history information can assist management and conservation actions when detailed demographic data are unavailable; and that many demographic processes might operate at spatial scales specific to populations, not species. Subsequently, I provide the first empirical cross-taxa demonstration that density effects on single fertility/survival rates (components) have weak association with feedback at the population level - a phenomenon I call ‘ensemble’ density feedback (Chapter 5). The major implication is that population processes can buffer variation in demographic rates, and management/conservation can be misled when based only on component density feedbacks. In my corollary discussion (Chapter 6), I advocate for phenomenological models to characterise long-term population trends, argue that better integration of temporal and spatial demography could circumvent ongoing semantic conundrums, and highlight the need for a code of ecological nomenclature. Stronger emphasis on the comprehension, mathematical description and application of density feedback through ecological disciplines, from students to seasoned academics, is absolutely necessary for ecology to become one of the most influential branches of modern science, a tool of knowledge for improving societal and environmental well-being.