Behind the scenes of “Understanding the relationship between dispersal and range size” - an sDiv Postdoc paper as result of a two year synthesis project

by Adriana Alzate (former sDiv postdoc, now at Wageningen University)

This journey began two years ago in 2020, when we were stuck at home in the middle of a global pandemic. Some of us were left with too much time to think (or procrastinate) on the possible 1000 research questions for our next decade. I was at that time writing a proposal to apply for the sDiv individual postdoc grant and waking up at 5am every morning to get some work done before our daughter woke up and found herself in the current tedious world.

My sDiv proposal idea was about synthesizing dispersal and its multiple effects on macroecological and macroevolutionary patterns. For instance, the geographical range of a species, a fundamental unit in macroecology and the main predictor of extinction risk, is expected to be positively associated with dispersal. High dispersal ability enables range expansion by promoting colonization of new habitats, providing genetic and demographic rescue, promoting local adaptation, and reducing extinction risk. Therefore, differences in dispersal ability should lead to differences in range sizes between organisms. During my PhD, using correlational studies and spatially explicit neutral models, I showed that dispersal positively affects range sizes of reef fishes. However, the theoretical expectation of a positive relationship between range size and dispersal ability has received mixed empirical support both within and among taxa. I questioned then why if dispersal is such an obvious process affecting range expansion and range size, it was such a tricky thing to prove.

As you might know, reviews and meta-analyses became very popular during pandemic times. They require no fieldwork and can be done from your quarantine quarters. If you have a computer, an internet connection and access to bibliographic databases and articles, you are good to go. Quite timely, with the successful acceptance of my sDiv postdoc proposal, there was the announcement of the E4 award open competition of Ecography for review manuscripts contributions. I thought (naively) that maybe we can easily and quickly write a review to solve once and for all the enigma whether dispersal has an effect on species range sizes. Renske Onstein, my main supervisor and collaborator at iDiv (now @Naturalis in Leiden, Netherlands), who almost always supports my crazy ideas, was also up for the challenge. We proposed to investigate why and under which scenarios studies find positive, none or negative dispersal-range size relationships. For this, we have systematically collated more than 100 scientific articles that studied this question across distinct taxa (invertebrates, plants, mammals, fish, birds, insects, amphibians and reptiles).

Our proposal was accepted, but soon we realized that this commitment was more difficult and would take more time than originally anticipated. We then decided to explore other possibilities and asked iDiv’s synthesis professor Jonathan Chase whether this could be something for Ecology Letters (he is synthesis editor for the journal). In a very professional manner, he said that it was possible, but that the whole process would have to go through the then editor-in-chief, Tim Coulson, first. Luckily, he also thought that our idea had merit. We thought that we could get away with a vote-counting type of review, but that was far from what happened and after a quick immediate rejection from Coulson, he sent us straight to do a proper meta-analysis. And that is how it started our journey on meta-analytical research: a new world. I went asking around to people who had experience on this. In the iDiv consortium I had and still have fortunately several meta-analysis experienced colleagues, and asked Roel van Klink, Becks Spake and Stephan Kambach. I followed some lectures from Julia Koricheva and did lots of self-reading (like Koricheva’s book and the metafor’ tutorials from W. Viechtbauer).

I remember that my sDiv colleagues were skeptical about making sense of such a mess. To start with, all studies measured range size and dispersal differently. After almost two years and a lot of work our study has now been published in Ecology Letters and even the cover picture of the issue was featuring our study (Fig. 1). In summary, the majority of studies reported neutral dispersal-range size relationships, followed by a large proportion of studies reporting positive relationships and very few reporting negative relationships. These mixed outcomes were common to all type of organisms studied. However, when doing a meta-analysis (when you actually focus on effect sizes and not on p-values that are very much dependent on statistical power and sample sizes), we found that dispersal actually has a positive effect on range size. Furthermore, one key finding was that the relationship between dispersal and range size strongly depends on which traits you decide to use as proxy of dispersal. For instance, Hand-wing index has a stronger positive effect on bird range sizes than body size or relative muscle mass was a very poor proxy for dispersal in insects. Having big muscle might make you look strong, but that does not help you when running a marathon or when expanding your range, as muscles are costly. Another interesting finding was that the effect of dispersal on range size was stronger when studying groups of species that are not too similar (e.g., genus level) or not too different (e.g., phylum, division, kingdom level). This was particularly happening for plants and it struck me that scientists studying animals usually study birds or insects or fish or mammals, a taxonomic group at the Class level, whereas scientist studying plants… well they study plants. We really should become more knowledgeable about plants.

Now you might think I solved all my questions. Far from reality and other questions emerged from this research to keep me busy at least for the next quinquennium.


Alzate, Adriana, Onstein, Renske E. (2022): Understanding the relationship between dispersal and range size. Ecology Letters, DOI: https://doi.org/10.1111/ele.14089

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