Identifying the paths leading to variation in geographical range size in western North American monkeyflowers

Abstract

Aim Closely related species can vary tremendously in size of geographical range, yet the causes of such variation are poorly understood. Prominent hypotheses about range size emphasize effects of niche properties and habitat connectivity via the amount and occupancy of suitable habitat, respectively. Previous studies have examined single hypotheses in isolation; however, we assessed the relative importance of these effects along with their potential interactions, using monkeyflower species (genus Mimulus) as a study system. Location Western North America. Methods We used primary occurrence data and climatic layers to estimate climatic niche breadth and position (relative to average regional climate), connectivity of climatically suitable habitat, and geographical range size of 72 monkeyflower species. Using path analysis, we then assessed the relative importance of climatic niche properties and connectivity of climatically suitable habitat in explaining variation in the amount and occupancy of climatically suitable habitat, respectively, and in turn, variation in geographical range size. Results We documented strong support for the hypothesized effects of climatic niche breadth, but not niche position and connectivity of climatically suitable habitat. Amount of climatically suitable habitat explained more variation in range size than occupancy of climatically suitable habitat, with amount and occupancy of suitable habitat together explaining c. 83% of the variation in range size. Main conclusions To our knowledge, this is the first study to show that climatic niche breadth, via its effects on the amount of climatically suitable habitat, is a strong predictor of geographical range size, thereby improving our understanding of the mechanisms driving species rarity.

Determinants of geographic range size in plants

“…Though experiments can only quantify niche breadth for a relatively small number of species and niche axes, they are a potentially powerful complement to verify tests of the niche breadth hypothesis based on correlative methods for a large number of species. For example, a study of western North American monkeyflowers found strong support for the climatic niche breadth hypothesis when using herbarium records to quantify both niche breadth and range size for 72 species (Sheth et al , ), and weaker support (possibly due to small sample sizes) when quantifying niche breadth experimentally with thermal performance curves for 10 of those species (Sheth & Angert, ).…”

“…Colors in the map correspond to the mean range size of species recorded in each grid cell. (b) Frequency distribution of range size for species of western North American monkeyflowers (redrawn from Sheth et al , ). As observed in many species assemblages, most species have small geographic ranges, and only a small number of species have large geographic ranges, but range size can vary dramatically even among closely related species.…”

“…Range filling is often estimated as the proportion of suitable habitat (shaded blue grid cells) in which a species is known to occur (blue dots). For (b–d), see Sheth et al () for more information.…”

Realized climate niche breadth varies with population trend and distribution in North American birds

“…Understanding how fitness and reproductive success vary across climate niche space could be an important avenue for future investigation, as it has been with density-dependent habitat selection (Morris, 1989;Sillett et al, 2004). While others have defined ecological niche as the set of conditions under which the intrinsic rate of increase is non-negative (Sheth et al, 2014), we make no statements here about fitness or growth rate and instead use the range of conditions occupied to define realized niche, as is consistent with ecological niche modelling (Pearson & Dawson, 2003;Sober on & Peterson, 2005) and niche theory (Pulliam, 2000). While others have defined ecological niche as the set of conditions under which the intrinsic rate of increase is non-negative (Sheth et al, 2014), we make no statements here about fitness or growth rate and instead use the range of conditions occupied to define realized niche, as is consistent with ecological niche modelling (Pearson & Dawson, 2003;Sober on & Peterson, 2005) and niche theory (Pulliam, 2000).…”

“…This expands long-standing ecological niche theory that occupied niche breadth increases with population growth (Vandermeer, 1972). Niche breadth has been used as a static, explanatory variable of population trends (Green et al, 2008;Jiguet et al, 2013) or geographical distribution (Sheth et al, 2014), or as a latent variable in the distribution-abundance relationship (Brown, 1984;Heino et al, 2008;Borregaard & Rahbek, 2010). Niche breadth has been used as a static, explanatory variable of population trends (Green et al, 2008;Jiguet et al, 2013) or geographical distribution (Sheth et al, 2014), or as a latent variable in the distribution-abundance relationship (Brown, 1984;Heino et al, 2008;Borregaard & Rahbek, 2010).…”