Determinants of geographic range size in plants

Abstract

Summary Geographic range size has long fascinated ecologists and evolutionary biologists, yet our understanding of the factors that cause variation in range size among species and across space remains limited. Not only does geographic range size inform decisions about the conservation and management of rare and nonindigenous species due to its relationship with extinction risk, rarity, and invasiveness, but it also provides insights into fundamental processes such as dispersal and adaptation. There are several features unique to plants (e.g. polyploidy, mating system, sessile habit) that may lead to distinct mechanisms explaining variation in range size. Here, we highlight key studies testing intrinsic and extrinsic hypotheses about geographic range size under contrasting scenarios where species' ranges are static or change over time. We then present results from a meta‐analysis of the relative importance of commonly hypothesized determinants of range size in plants. We show that our ability to infer the relative importance of these determinants is limited, particularly for dispersal ability, mating system, ploidy, and environmental heterogeneity. We highlight avenues for future research that merge approaches from macroecology and evolutionary ecology to better understand how adaptation and dispersal interact to facilitate niche evolution and range expansion.

Beyond range size: drivers of species’ geographic range structure in European plants

“…Species’ niche breadth was a strong biogeographic determinant of species’ ranges, with similar effects on the predicted and observed range metrics. The widely recognized niche effects on range and area size (Moore et al 2018, Sheth et al 2020) were associated with further niche effects on species’ range structure (observed and predicted geographic range filling and patch size distribution, and predicted range fractality). Niche breadth can therefore be a useful tool in global predictions of species’ range structure and internal range rearrangements, especially in a climate change context (Broennimann et al 2006, Connor et al 2018).…”

“…glaciations) or human land use limiting dispersal (Miller and McGill 2017) were more dramatic in selected groups of phylogenetically related species, while species with closer observed to predicted matches benefited from more stable climates. Another hypothesis posits that phylogenetically older species may have more fragmented ranges than their younger relatives (Sheth et al 2020), due to e.g., increasing complexity of biotic interactions with species’ age. While these hypotheses need further testing, our results underscore that we lack a good understanding of the phylogenetic structuring of species’ ranges (Gaston et al 2003, Sheth et al 2020).…”

“…Another hypothesis posits that phylogenetically older species may have more fragmented ranges than their younger relatives (Sheth et al 2020), due to e.g., increasing complexity of biotic interactions with species’ age. While these hypotheses need further testing, our results underscore that we lack a good understanding of the phylogenetic structuring of species’ ranges (Gaston et al 2003, Sheth et al 2020).…”

“…For the range size metrics, inferring realized from predicted range and area sizes was the most problematic for species with intermediate observed values of these metrics. This may arise due to both methodological and ecological constraints: for small ranges, the convergence between observations and predictions may be a modeling artefact due to models being more easily overfit; for large ranges, the convergence might be due to geographical constraints from landmass availability and continental barriers, which can drive species’ global range size especially in highly dispersive species (Broennimann et al 2006, Sheth et al 2020). Alternatively, both narrow climate endemics of Europe (Ohlemüller et al 2008) and species with large niches might be less prone to modeling biases from niche truncation.…”

“…Understanding what drives observed large-scale distribution patterns is key to predicting species’ future geographic ranges and vulnerability to global change (Broennimann et al 2006). Analyses of species’ geographic ranges have mostly focused on determinants of range size (Morueta-Holme et al 2013, Sheth et al 2020). However, examining occupancy patterns throughout a species’ range may enable a much better understanding of dispersal and extinction dynamics in response to global environmental changes (Crooks et al 2011, Cianfrani et al 2018).…”