Introduction
Species geographical range is a fundamental unit in macroecology and is a main predictor of extinction risk across organisms (Brown et al. 1996, Purvis et al. 2000, Chichorro et al. 2019). As the distribution of a species provides important information on their ecology and evolution, understanding what drives the vast variability in species range size has for long been of interest to paleontologist, biogeographers, macroecologists and evolutionary biologists (Brown et al. 1996, Gaston 2003, Gaston 2009, Gaston & Fuller 2009). Several mechanisms might underlie species geographical ranges, such as environmental and physical constraints, differences in niche requirements, population abundance, latitudinal gradients, differences in body size or trophic level, colonization-extinction dynamics, species age and dispersal ability (Gaston 2003). Although all these mechanisms can simultaneously interact to produce the empirical range sizes, dispersal has received the most interest, likely because it is one of the most prominent processes affecting range size (Hanski et al. 1993, Lester et al. 2007, Sheth et al. 2020). However, despite major efforts to link dispersal ability to range size, the theoretical expectation of a positive dispersal-range size relationship has received mixed empirical support both among and within taxa (Lester et al. 2007, Luiz et al. 2013, Alzate et al. 2019a, Sheth et al. 2020).
Here we define dispersal as any movement of individuals or propagules potentially leading to gene flow across space (Ronce 2007, Clobert et al. 2012). Both theoretical and experimental work have shown that dispersal can positively affect the mechanisms that allow attaining large geographical ranges (Holt 2003, Holt et al. 2005, Sexton et al. 2009, Alzate et al. 2019b). For instance, dispersal promotes range expansion by facilitating the colonization of new habitats and promoting local adaptation (Holt & Gomulkiewicz 1997, Alzate et al. 2019b). Dispersal also prevents range contraction by decreasing extinction risk and allowing populations to persist in suboptimal habitats (Hufbauer et al. 2015, Alzate et al. 2019b), as it provides demographic (Brown & Kondric‐Brown 1977) and genetic (Holt & Gomulkiewicz 1997) rescue. Furthermore, simulation models (spatial explicit neutral models) predict a positive effect of dispersal on species range size (Rangel-Diniz-Filho 2005, Alzate et al. 2019c). An outstanding question is therefore why positive effects of dispersal on range size are found in some cases, but not in others.
Discrepancies between expected and observed dispersal-range size relationships might emerge for different reasons. Firstly, studies differ in their dispersal ‘definitions’ and therefore also differ in which phenotypic traits are considered to be associated to dispersal ability. Which dispersal-related traits to choose is not a trivial question, particularly because dispersal and dispersal distance are emergent properties (complex traits) resulting from the interactive effects of various dispersal-related traits (e.g., morphology, physiology, behavioural and life history traits), which are highly dependent on the type of organism studied (Ronce 2007, Bonte et al. 2012, Matthysen 2012, Ronce & Clobert 2012, Travis et al. 2012, Sheth 2020, Green et al. 2021). Moreover, dispersal can occur at different life stages (e.g., seeds, eggs, juveniles, adults), it is composed by three phases including departure (i.e., decision to leave the old habitat), transfer (i.e., displacement from the old to a new habitat) and settlement (i.e., arrival and settlement in the new habitat), and can be shaped by external factors and evolve (Ronce 2007, Matthysen 2012). Although dispersal kernels are in general good proxies for dispersal abilities across species, there are still difficulties to measure the tail of the kernel, which is of major importance when scaling up to distribution dynamics (Clobert et al. 2012). Therefore, different measures of dispersal may capture different components of the dispersal process, and may thus affect the resulting dispersal-range size relationship.
Secondly, true biological differences between study organisms might determine how dispersal correlates with range size. For instance, it is possible that the dispersal-range size relationship differs between active (e.g., reptiles, birds, mammals) and passive dispersers (e.g., plants, diatoms, marine molluscs), because dispersal (the transfer phase) for passive dispersers is outside the control of the individual, as it depends on external forces (e.g., currents, wind, gravity, other organisms) with a high stochastic component (Matthysen 2012). Moreover, the nature of the dispersal medium of marine and terrestrial realms can lead to differences in dispersal ability between terrestrial and marine organism. In marine systems, passive rather than active dispersal is favoured, which may lead to fewer dispersal-related adaptations in marine than in terrestrial systems (Burgess et al. 2005). The higher connectivity between marine habitats may also erase the link between dispersal ability and range sizes (Mora et al. 2012). In terrestrial systems, dispersal may be more difficult without specialised adaptations (such as wings to fly, fleshy fruits to attract animal dispersers, etc.) (Burgess et al. 2015). In addition, human activities can uncouple the dispersal-range size relationship by decreasing species distributions (Webb & Gaston 2000). This might be more common on terrestrial than in marine systems, where large scale human impact has a longer history. Similarly, dispersal may be less limiting in endotherms than ectotherms, because endotherms possess broader thermal niches and higher thermal tolerances due to high metabolic rates compared to ectotherms, facilitating settlement. Even though endo- and ectotherms might not differ in the dispersal capacities required during the transfer phase of dispersal, endothermy might have an advantage during the settlement phase. Thus, physiological tolerance rather than dispersal ability may in some cases be the limiting factor when it comes to range size (Pie et al. 2021).
Thirdly, evolutionary history may affect the current dispersal-range size relationship by determining time and potential for (past) range expansion, or the evolution of dispersal-related traits that facilitated past long-distance dispersal (Onstein et al. 2019). For example, range size is likely to vary with species age because species need time to expand their range (Willis 1922, Webb & Gaston 2000, Gaston 2003), and the dispersal-range size relationship may thus be obscured when studying species of different ages. Furthermore, past climate changes (e.g., since the last glacial maximum), population connectivity, and availability of suitable settlement environments, may have affected the rate of range size expansion (e.g., ‘Reid’s Paradox’), and may explain distinct dispersal-range size relationships across biogeographical realms and climate zones that differ in their glaciation history, for example (Svenning & Skov 2004, Svenning et al. 2008).
Lastly, besides biological and evolutionary reasons, intrinsic differences between studies, such as study design, methodology, data, or analytical approach might explain absence of a relationship between dispersal and range size. Studies examining how dispersal affects range size differ in taxonomic scope (e.g., ‘genus’, ‘family’, ‘phylum’), taxonomic unit of analysis (e.g., at ‘genus’ or ‘species’ level), the measure of species range size (e.g., ‘extent of occurrence’ or ‘area of occupancy’), range completeness (e.g., ‘partial’ or ‘complete’ measures of range size), and the number of considered species and dispersal-related traits to capture variation in the dispersal-range size relationship. For example, range size measures can under- or overestimate true range sizes by including or excluding discontinuities in the spatial distribution of taxa. Similarly, high taxonomic units of analysis (e.g., ‘genus’ or ‘family’ instead of ‘species’) ignores within-clade variation in dispersal. The benefit of using comprehensive data, that is, bigger areas that include complete ranges, over partial data (i.e., smaller areas that do not include complete ranges) has been shown for understanding the body size - range size relationship in animals, with more consistently positive relationships when using comprehensive data (Gaston & Blackburn 1996).
Despite all possible caveats and warnings about several of these problems (Blackburn & Gaston 1998, Alzate et al. 2019a, Johnson et al. 2021), a comprehensive methodological framework to study dispersal and range size is missing. Here we performed a systematic review to investigate the causes of variation in the dispersal-range size relationship by collating 478 dispersal-range size relationships from 86 independent studies. Firstly, we investigated and synthesized the spread of evidence for the dispersal-range size relationship between regions, realms and clades. Secondly, we quantified how differences between studies regarding dispersal and range size characteristics related to transfer, settlement and evolution, and potential methodological differences (range size definitions, spatial corrections and taxonomy) can affect the overall dispersal-range size relationship (Table 1). Finally, we discuss these results in the context of the complexity of the dispersal process – from departure, to transfer, to settlement.
Table 1. Moderator candidates of the dispersal-range size relationship as synthesized in this study. Variables were classified into six groups depending on their hypothesized effect on the dispersal-range size relationship: departure/transfer variables that directly affect movement/transfer of species, settlement variables or corrections that influence the potential and realized niche space, time variables or corrections related to evolutionary history and past dynamics that may influence range size, and three methodological type variables that may bias the inference of the dispersal-range size relationship. Description of each variable and the prediction of why and how it potentially influences the dispersal-range size relationship is provided.