Where are they? And if so, how many?
Is the core of an animal or plant population always found in the center of its range, as a classic ecological model predicts? In most cases, but not always. Luckily, exceptions from the model are easy to predict, according to the findings of an international research team involving ecologist Trevor Fristoe from the University of Konstanz.
How are the individuals of a species distributed within the species’ range, and where does the abundance peak? The answers to these questions are central to the development of evolutionary and ecological models and provides the basis for effective conservation measures and policy making. Because of its simplicity, a classic model for describing geographic patterns of a species' abundance – the "abundant-center model" – is widely used in ecology, but at the same time subject to scientific critique. In a recent article in Ecography, an international research team involving Konstanz-based ecologist Trevor Fristoe has now put the model to a thorough test.
Two simple assumptions
The classic model is based on two straightforward assumptions. The first is that environmental conditions determine a species’ abundance: Some environments are favourable for a species, so many individuals will live there, and some are less favourable. The second assumption is that environmental conditions should be similar between locations that are close to each other and become increasingly dissimilar as the distance between locations increases. “Considering these two assumptions together, species should be most abundant in the center of their range and become increasingly rare towards the edge,” Fristoe summarizes the model. Fristoe is lead author of the article and ecologist and evolutionary biologist from the Department of Biology at the University of Konstanz.
Although this model has been applied in ecology and biogeography for many decades, it has lately been subject to serious criticism. The reason: with better access to large amounts of data on the location of species and their populations in recent years, an increasing number of species have been identified that do not seem to fit the model. “Some recent studies have called into question the value of the model altogether, suggesting that abundant-center distributions are actually quite rare in nature. However, other authors have pointed out issues in the methods and data used in these analyses, so the question was still open,” Fristoe says.
Good, but with room for improvement
To re-examine the model, Fristoe and his colleagues tested it against a large, high-quality dataset on the distribution of more than 400 birds from the North American Breeding Bird Survey. “We wanted to know whether this classic idea really deserved to be discarded. So we did a careful evaluation – using an authoritative dataset on a really well-sampled group of animals,” Fristoe explains. They found that the classic model actually provides a very good starting point for making predictions on the shape of species distributions, as about two thirds of the bird species from their dataset followed the abundant-center pattern.
Nevertheless, about one third of the species did not match the predicted pattern, and the researchers wanted to know more about these exceptions and the circumstances that cause them. Their analyses showed that the classic model works very well in landscapes where the underlying assumptions are met, i.e. those where the environment changes gradually and uniformly in all directions. Deviations, on the other hand, predictably occur in complex environments, such as species ranges extending along coastlines or mountain ranges – with abrupt changes to the climate or habitat in one or more directions. For these cases, Fristoe and his colleagues propose an extended version of the classic model, which they call the “abundant-core”.
Softened assumptions, better results
The extended model still predicts that there is an area of highest abundance – the core – within a species’ range, with the abundance decreasing towards the edge until the species is no longer found. In contrast to the classic model, however, this core does not necessarily have to be located in the center of the range.
Testing the extended model against the bird dataset, Fristoe and his colleagues found that almost 90 percent of the bird species followed the proposed abundant-core pattern. For many of the cases in which the highest abundance of a species is not found in the center of its range, the extended model allows prediction of where in the range the abundance peak (the core) is located instead. “This is a clear improvement over the classic model, which is already good, and shows that our extension allows a more flexible application of the model’s basic idea, even to those landscapes where the original reaches its limits,” Fristoe concludes. He adds: “Large-scale ecological and evolutionary processes have an impact from local communities all the way up to global patterns of diversity. Our results will help us to better explore and understand them.”
Key facts:
- Original publication: Trevor S Fristoe, Bruno Vilela, James H Brown, and Carlos A Botero (2022) Abundant-core thinking clarifies exceptions to the abundant-center distribution pattern. Ecography; DOI: https://doi.org/10.1111/ecog.06365
- International research team extends a classic model for how the abundance of species is distributed within their ranges to allow its application to a broader spectrum of biogeographic conditions.
- Lead author Trevor Fristoe is ecologist and evolutionary biologist from the Department of Biology at the University of Konstanz.
- The study is based on data from the North American Breeding Bird Survey
- Open Science: The article in Ecography is open access; Links to the data from publicly available sources and the script used for analyses can be downloaded from DRYAD and zenodo, respectively.
