Geographical abundance distributions of coastal invertebrates: using one‐dimensional ranges to test biogeographic hypotheses

Abstract

Aim It is often assumed that species generally reach their highest densities in the centre of their range and decline in abundance towards the range edges. A number of mechanisms have been proposed that could theoretically support this pattern, and several ecological theories have been developed based on the assumption that this pattern occurs in nature. However, few studies have quantified geographical patterns of species abundance throughout species ranges. This is largely because of the logistical challenges of sampling throughout the large spatial areas of most species ranges. We use intertidal invertebrates, which have relatively well defined linear ranges, to test the hypothesis that species are most abundant in the centres of their ranges. Location Our sampling programme covered all or most of the ranges of twelve intertidal invertebrate species along the Pacific coast of North America, from Cabo San Lucas (Baja California, Mexico) to Shelikof Island (AK, USA). Method We sampled invertebrate density at forty‐two field sites using quadrat and transect methods. We used a shape fitting procedure to find idealized range shapes that best fit the sampled distributions of abundance. The idealized range shapes represented both a distribution where abundance was highest at the range centre and distributions where abundance was highest at one or both of the range edges. Results Overall, this suite of species did not show the expected pattern of high abundance near the range centre. Six of the species showed patterns indicative of high densities near one of their range edges, whereas only two showed patterns with high densities near their range centres. Furthermore, nine of the twelve species had sites near the range edges in which density ranked in the top 20% of all sites. Main conclusions The hypothesis that species are most abundant in the centre of their ranges cannot be generalized to this diverse suite of intertidal organisms. The diversity of distribution shapes that we found suggest that evolutionary and ecological theories that assume high abundance at range centres should be re‐examined with consideration of alternative abundance distributions. We suggest that sampled geographical distributions of abundance can be combined with demographic and physical factor data taken at the same scale to test hypotheses related to the causes of range boundaries and the responses of species ranges to climatic change.

Patterns of demography for rocky-shore, intertidal invertebrates approaching their geographical range limits: tests of the abundant-centre hypothesis in south-eastern Australia

“…Although all three study species have planktonic larvae that spend up to a few weeks in the water column (Wisely and Blick 1964;Underwood 1974;Caffey 1985), recent genetic investigations into the connectivity of populations of rocky, intertidal marine species across the Ninety Mile Beach biogeographical barrier have revealed that degree of population connectivity across this barrier is not related to the mode of larval dispersal, but rather, to flexibility in habitat utilisation (Ayre et al 2009). The present study supports the mounting evidence (Sagarin and Gaines 2002b;Gilman 2005;Sagarin et al 2006) that the abundant-centre hypothesis is too simplistic a model to predict the abundance distributions of planktonically developing benthic marine invertebrates. Our results indicate that abundances of rocky-shore, intertidal invertebrate species are highly variable and perhaps unpredictable across the range, and that their recruitment can be extremely patchy and episodic.…”

“…Moreover, the predictability of the abundances of each species varied with scale at which the abundance was assessed. Sagarin and Gaines (2002b) and Gilman (2005) found similarly large variations in the large-and small-scale patterns of abundance along the geographical ranges of 13 rocky-shore, intertidal marine invertebrates on the western coast of the USA.…”

“…Tests of this prediction at appropriately large spatial scales are, however, limited to a few studies in the northern hemisphere. On the western coast of the USA, only 2 of 13 intertidal marine invertebrates had abundant-centre distributions, whereas others either showed no obvious patterns or had a skewed distribution in abundance from north to south (Sagarin and Gaines 2002b;Gilman 2005). These studies concluded that the underlying assumptions of the abundant-centre hypothesis may conflict with specific characteristics of the life histories and patterns of environmental variation across the ranges of marine species (Gilman 2005), and that future studies should combine sampling geographical distributions of abundance with demographic and physical data (Sagarin and Gaines 2002b).…”

Physiological tolerances account for range limits and abundance structure in an invasive slug

“…ignoring other spatial variation), density does not peak in the centre, but rather is the highest close to the coastal margin, declining inland up to approximately 200 m. For an island of this overall, elliptical shape and with a considerable change in elevation, a central peak in abundance might be expected to take a torus-like form, but this was clearly not the case for D. panormitanum. Complex abundance structures are now considered the norm rather than the exception, with little evidence for an 'abundant centre distribution' either across entire ranges or partial components thereof (Brewer & Gaston 2002;Sagarin & Gaines 2002b;Gaston 2003;McGeoch & Price 2004;Sagarin et al 2006).…”

Differences in feeding adaptations in intertidal and subtidal suspension-feeding gastropods: studies on Crepidula fornicata and Crepipatella peruviana

“…Because species are usually distributed over large areas latitudinally, species range limits are most often studied on a broad geographic scale (e.g., Sagarin and Gaines 2002;Sorte and Hofmann 2004;Kuo and Sanford 2009). Such studies have been important for identifying potential biotic and abiotic limiting factors for a host of different species and their potential adaptations at range edges (e.g., Fawcett 1984;Zacherl et al 2003;Lima et al 2007; reviewed by Bridle and Vines 2007;Sexton et al 2009).…”