In a community, it is almost always true that some species are common while most are rare. This pattern is demonstrated by plotting species abundance distributions (see Magurran 2003 or Verbeek 2014 for excellent overviews) observed in many real life communities. Local abundance is not the only facet of rarity – species can also be defined as rare or common in terms of their overall geographic range and its habitat breadth. The late Deborah Rabinowitz, (a professor of Ecology and Systematics at Cornell), was the first to highlight and study the multidimensional nature of species rarity (Fig. 1).

Understanding the connections between different kinds of rarity and their respective drivers could help us better understand how species survive across multiple spatial scales. Perhaps more important is the potential to identify and predict which species will be imperiled by more than one kind of rarity.

Other studies have often observed a positive correlation between local abundance and geographic range size (see reviews here and here by Kevin Gaston, Tim Blackburn, and John Lawton; the latter, in particular, is a great book that contains critical examinations of many macroecological concepts and hypotheses that continue to be highly relevant today, 16 years since its publication).

Fig. 1. Three dimensions of rarity: range size (blue polygons), habitat breadth (different habitats denoted by different shades of grey), and local abundance (orange points). In this example, species B is rarer in all three dimensions than species A. A “triple extinction jeopardy” occurs when there is a general concordance in the three rarity dimensions across all species occurring in a given geographic region.

Fig. 1. Three dimensions of rarity: range size (blue polygons), habitat breadth (different habitats denoted by different shades of grey), and local abundance (orange points). In this example, species B is rarer in all three dimensions than species A. A “triple extinction jeopardy” occurs when there is a general concordance in the three rarity dimensions across all species occurring in a given geographic region.

Two hypotheses were relatively well-supported: the niche breadth hypothesis formulated by James H. Brown, and the niche position hypothesis developed by Richard Gregory and Kevin Gaston. In the first hypothesis, niche breadth drives both local abundance and geographic range size. In the second hypothesis, the authors argued that niche position (not breadth) underlie abundance and range, in which species that use the most common niches would achieve the greatest local abundance as well as range size. Investigations into these hypotheses have mostly focused on terrestrial or marine communities. Because freshwater stream systems are substantially different from both terrestrial and marine systems—for one thing, dispersal of a species greatly depends on its position in the stream network—we wanted to examine the mechanisms that govern rarity in freshwater fish.

To do this, we used a dataset of freshwater fish communities occurring in natural stream sites across the US. This dataset was previously used to show that environmental filtering (and secondarily, predation) but not competition drive community assembly in freshwater fishes. We limited our analysis to the stream sites that were not dominated by non-native species, and used phylogenetic regression and path analytic models to figure out how local abundance, niche (habitat) breadth, and range size of species occurring in these sites relate to one another. Through our analyses, we uncovered potential mechanisms by which species life history, habitat affinities, and biogeography drive variation in the three rarity dimensions.

So what did we find? Well, we found while habitat breadth drives both local abundance and geographic range size, the latter two variables were not positively correlated. As for why is that so, I’ll leave it as a mystery for now (a clue, which is perhaps already obvious to you, is that species life-history and ecological traits are really key to this finding). Overall, our results support the notion that macroecological dynamics in freshwater stream communities are indeed quite different from terrestrial or marine communities.

We are currently finalizing the manuscript for submission so watch this space for updates!

– Xingli Giam