Evol Ecol Res 17: 757-770 (2016)     Full PDF if your library subscribes.

Replacement and displacement driven by innovations, competition, and extinctions

Jörgen Ripa and Jacob Johansson

Theoretical Population ecology and Evolution Group (ThePEG), Department of Biology, Lund University, Lund, Sweden

Correspondence: J. Ripa, Theoretical Population ecology and Evolution Group (ThePEG), Department of Biology, Lund University, Sölvegatan 37, SE-223 62 Lund, Sweden. email: jorgen.ripa@biol.lu.se


Background: A key innovation may allow one group of species to radiate and replace another, but the role of competition and background extinctions for the replacement process is vigorously debated and unclear.

Questions: Is a minimum competitive advantage necessary to initiate replacement? Are background extinctions of the incumbent species necessary? What speciation and extinction dynamics characterize the replacement process and how do they differ from radiation into an empty niche space?

Model features: We used a classic eco-evolutionary model describing asexual species competing for resources distributed along a single niche axis. Successive evolutionary branching generated an adaptively radiating community. Evolutionary innovation was introduced as an increased carrying capacity. Externally driven extinctions were also added.

Mathematical methods: The model was studied using individual-based simulations. We monitored the radiation of the new group into the niche space of the original group and recorded the time to near-complete (90%) and complete (100%) replacement, along with rates of extinction and speciation of new and old groups.

Results and conclusions: We demonstrate that replacement may occur by the radiation of a competitively superior group. Increasing the competitive advantage shortened replacement times, but there was no particular threshold advantage. Origination rates in new groups were comparable to extinction rates of the original groups, keeping the total number of species approximately constant – tentatively a result of niche widths and overall available niche space in the underlying model being unaffected by these innovations. Extra background extinctions speeded up the process, consistent with observed patterns in the fossil record that replacement is sped up during mass extinctions, but an increased extinction rate also introduced uncertainty and a higher risk of global extinction of the superior group.

 The replacement process was mainly driven by evolutionary competitive exclusion. The old, inferior type had an elevated rate of extinction matched by an increased speciation rate of the superior type. Incumbency also played a role, but we found little support for a strict incumbent replacement scenario, where replacement is driven entirely by background extinctions of the old group and opportunistic speciations of the new group.

 Compared with adaptive radiation into an empty niche space, replacement was slow and relict species from the original group could linger in marginal and previously unexploited niche space that provided a niche refuge and prevented complete replacement. Similar patterns are also found in the fossil record.

Keywords: adaptive radiation, competition, extinction, incumbent replacement, key innovation, macroevolution, relict species.

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