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Artist's impression of Jurassic organisms on the sea floor

Long-standing theories in biology assume that competition between individual organisms limits how much a species can use a resource and therefore how geographically widespread that species can become. Geographic distributions, in turn, affect community structure and extinction rates, so the small-scale competition between animals or plants could regulate much larger-scale patterns of biogeography and biodiversity. This concept, known as ‘ecological release’ forms the cornerstone of many fundamental evolutionary theories. However, much of the modern evidence for this assumption is based on studies of vertebrates in terrestrial systems, and the crucial link between competition and geographic ranges has seldom been tested at a species-level or in marine ecosystems.

Artist's impression of Jurassic organisms on the sea floorThe mid Jurassic is an example of a mass extinction event in the oceans. Before the extinction (T1), species may have been limited in geographic range because competitors excluded each other. After the extinction (T2), it has long been assumed that the survivors spread out into the space of their fallen rivals, so-called ‘ecological release.’ On geologic scales, theory predicts the number of species and amount of competition to change in the opposite way as do species’ geographic range size and chance of surviving extinction. The current study found no support for this idea, however. Artistic reconstruction by Andrew Orkney.

A new study by researchers from the University of Oxford Department of Earth Sciences, Friedrich-Alexander University in Erlangen, and the Berlin Natural History Museum suggest that competition may not be as important on large scales as prevailing theory would suggest. The team examined bivalve and brachiopod species’ distributions throughout the Phanerozoic (the last 485 million years). These two animal groups are filter-feeding marine invertebrates of similar body size, and their fossils are abundant and well-studied.

Oysters, scallops, and brachiopods competing for space on the seafloor© Martin Aberhan

The study focused on filter-feeding animals in marine ecosystems. Animals like oysters, scallops, and brachiopods must compete for space on the seafloor. Illustration by Martin Aberhan.

The new study published today in the journal Current Biology reports that, contrary to expectations, geographic range size varied independently with species count. If the distribution of different species was limited by competition, species would instead have been less widespread on average, when there were more potential competitors in the same region. Similarly, after mass extinction events like the end-Cretaceous extinction 65 million years ago (when non-avian dinosaurs perished), survivors had been predicted to expand into the space left by fallen competitors.

‘It’s like when a lot of people share a workspace. As soon as one person moves out, everyone else spreads their stuff out onto the empty desk,’ explained DPhil student Gwen Antell, the lead author. Surprisingly, however, it appears that species which survived biotic crises did not tend to expand in distribution. In fact, animals’ ranges changed seemingly independently of whether the number of potential competitors increased or decreased.

The results suggest that over Earth’s history, factors other than competition have driven dominant evolutionary and macroecological patterns in marine animals. For instance, habitat availability could play a large role in how far a species can spread.

Maps showing survival and extinction of Jurassic species in different scenarios© Andrew Orkney

Species’ geographic distributions are expected to expand after a mass extinction (Earth at bottom left), such as the biotic crisis around 175 million years ago. However, new evidence suggests that species remained equally widespread (Earth at bottom right). Current Biology.

Associate Professor Erin Saupe, senior author on the study, noted, ‘we often assume that species expand their geographic ranges after major extinction events in the fossil record, but our work shows this is not the predominant pattern, at least in the marine record of bivalves and brachiopods. It is really important to test these assumptions.’

The research was funded by the Clarendon Foundation, Leverhulme Trust, and German Research Foundation (DFG). The article was released online at Current Biology on 12 December:

Antell, G.S., Kiessling, W., Aberhan, M., and Saupe, E.E. Marine biodiversity and geographic distributions are independent on large scales. Current Biology (2019).

Story courtesy of the Department of Earth Sciences

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