- LAST MODIFIED: 21 February 2022
- DOI: 10.1093/obo/9780199941728-0141
- LAST MODIFIED: 21 February 2022
- DOI: 10.1093/obo/9780199941728-0141
Are there limits to the number of species on Earth? If resources on the planet are finite, it is reasonable to think that there must be a limit to the number of individuals (and species) it can sustain. Yet life on Earth is, and has been, extremely diverse and dynamic, shedding doubts on whether species diversity is, or has ever been, close to this limit. The concept of diversity-dependent diversification (DDD) is at the heart of this debate and postulates there is an ecological limit regulating and constraining species diversification. The term “diversity-dependence” was first used in the field of paleontology as an analogy to “density-dependence” in population ecology. The hypothesis is that diversification slows down as communities fill with species because of competition among species for limited resources. The resulting DDD patterns are a logistic increase in the number of taxa through geological time and/or a negative relationship between the standing diversity and the net diversification rate (via decrease in speciation rate and/or increase in extinction rate). In the past decades, many studies have tested the DDD predictions. These studies have often found support for a bounded diversification pattern, although they differ in temporal, taxonomic, and geographical scales, and use either paleontological (fossil diversity over time) or neontological (molecular phylogenies of extant species) data. Interspecific competition is the main mechanism used to explain observed DDD patterns; however, other processes can generate DDD-like patterns, and theoretical models have aided in generating more mechanistic and testable predictions. This article outlines empirical paleontological and neontological studies supporting (and opposing) the DDD hypotheses, as well as the models and mechanisms underlying the observed patterns.
The study of diversity-dependent diversification (DDD) involves the field of macroevolution. The dynamics of diversification are described as rates of species origination and extinction (see Rabosky and Slater 2014). The discussion on the existence of ecological limits to diversification started in the 1970s within the field of paleontology, as a debate about equilibrial versus directional (or random) diversification. This debate was greatly promoted by the theory of island biogeography and the first large compilations of taxonomic diversity in the fossil record. Signor 1990 provides a thorough review of this early debate, and Benton and Emerson 2007, although promoting the non-equilibrial point of view, provide an updated review. In the 2000s, with the increasing availability of time-calibrated molecular phylogenies of extant species, the DDD debate spread to the fields of evolutionary biology and ecology (see the section titled “Is There Density Dependent Diversification?” in Cavender-Bares, et al. 2018). Rabosky 2013 dedicates a review to the topic, which led to an official debate on the determinants of species richness, organized by Society president Trevor Price, at the American Society of Naturalists meeting in January 2015, followed by written dialogue outlining the arguments that were presented at the meeting. On one side of the debate, Rabosky and Hurlbert 2015 argues that species diversification is dominated by ecological limits. On the other side, Harmon and Harrison 2015 argues that species richness is dynamic, and communities are open and unsaturated.
Benton, M. J., and B. C. Emerson. 2007. How did life become so diverse? The dynamics of diversification according to the fossil record and molecular phylogenetics. Palaeontology 50:23–40.
A review of the evidence for equilibrium or exponential diversification patterns, including a discussion on the limitations of the fossil record, and on the idea that species diversity can spur diversification instead of constraining it.
Cavender-Bares, J., S. Kothari, and W. Pearse. 2018. Evolutionary ecology of communities. In Oxford Bibliographies in Evolutionary Biology. New York: Oxford Univ. Press.
This article gives a thorough overview of the field of community ecology and the importance of an evolutionary perspective on ecology. The section “Is There Density Dependent Diversification?” is particularly relevant for the topic of this overview.
Harmon, L. J., and S. Harrison. 2015. Species diversity is dynamic and unbounded at local and continental scales. American Naturalist 185:584–593.
A perspective paper, part of the American Society of Naturalist debate, that supports the view that ecological communities are open and highly dynamic, and that evidence for ecological limits to diversification is weak and inconsistent.
Rabosky, D. L. 2013. Diversity-dependence, ecological speciation, and the role of competition in macroevolution. Annual Review of Ecology, Evolution, and Systematics 44:481–502.
A review on DDD from the neontological (instead of paleontological) perspective, including phylogenetic approaches, a discussion about how interspecific competition generates DDD patterns, and a detailed explanation of the DDD models and predictions.
Rabosky, D. L., and A. H. Hurlbert. 2015. Species richness at continental scales is dominated by ecological limits. American Naturalist 185:572–583.
A perspective paper, part of the American Society of Naturalist debate, that supports that ecological limits regulate diversification of species through diversity-dependent feedback mechanisms.
Rabosky, D. L., and G. J. Slater. 2014. Macroevolutionary rates. In Oxford Bibliographies in Evolutionary Biology. New York: Oxford Univ. Press.
An overview of the concept of macroevolutionary rates, or the rate of an evolutionary process measured over geological timescales. The process measured is usually morphological (trait) evolution or taxonomic diversification (speciation and extinction).
Signor, P. W. 1990. The geologic history of diversity. Annual Review of Ecology and Systematics 21:509–539.
An overview of the empirical paleontological patterns in the marine and terrestrial realm found until then, the possible controls on global and local diversities, and the theoretical models of diversity dynamics through time.
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