Final remarks
In less than a century, macroecology has shed light on our understanding of species distribution patterns and about the processes and mechanisms governing them, at least from a theoretical point of view. Nowadays, we know that dispersal is a central process driving macroecological and macroevolutionary patterns in complex ways (e.g., Onstein et al. 2017, Alzate et al. 2019c, Sheard et al. 2020), but we still have not reached a consensus, based on empirical evidence, whether dispersal will positively affect geographical range sizes, or whether other variables, such as physiological tolerance (Pie et al. 2021), are more relevant for range size expansion, or even interact with aspects of the dispersal process (as shown here for endo- vs. ectotherms). Here, we show that differences between studies are largely responsible for different dispersal-range size relationship outcomes, which leads us to the following conclusions.
First of all, we need a better understanding of the dispersal process and envision it as a three-stage process (departure, transfer and settlement), with multiple traits (morphological, behavioral, physiological or life-history) acting differently in each of these stages (Clobert el al. 2012, Laube et al. 2013). It is important to be aware that many dispersal-related traits, which lead to net displacement, may be selected for functions other than dispersalper se (Burgess et al. 2005). In benthic marine organisms, dispersal-related traits (e.g., pelagic larvae, spawning mode) are often a by-product of traits selected for feeding, as part of the egg size-number trade-off, predation avoidance or retention of propagules (Burgess et al. 2015). In plants dispersal related traits, like seed size and plant height, can evolve as a result of the seed size-seed number trade-off and as to avoid competition for sunlight (Burgess et al. 2015). In animals, for which one type of locomotion is used for several ecological functions, dispersal can result from movement for foraging, exploration, mate and shelter seeking (Burgess et al. 2015). Regardless of whether traits are selected for dispersal or are an eco-evolutionary by-product, using a more complete picture of dispersal will allows us to capture its complexity in a more realistic manner and to better explain species geographical ranges. We should aim to use multiple dispersal-related traits, dispersal syndromes or co-variations of multiple dispersal-related traits (a multivariate dispersal phenotype) instead of using individual traits as dispersal proxies (Ronce & Clobert 2012).
Second, we propose ‘evolution’ and past dynamics as the fourth stage of the dispersal-range size paradigm. There is a lack of integration between macroecology and macroevolution (McGill et al. 2019); paleontological studies have pointed out the intricate relationship between dispersal, range size, speciation, extinction and species ages (Jablonski 1986), but very few studies have considered time for dispersal and range expansion (e.g., species age) or speciation/extinction dynamics when examining determinants of range size. Possibly, this is because suitably large and complete phylogenies, and reliable molecular clock models to estimate diversification rates, have been lacking until recently. In addition, past changes in paleoclimates, landscape connectivity, orogeny, and barriers, all have major impacts on dispersal and range size (Hagen et al. 2021), and should ideally be considered to fully capture this fourth temporal dimension to the dispersal-range size relationship. Finally, understanding the distribution of ranges, niche widths (e.g., right or left skewed), level of specialization, and distribution of niche properties (i.e., ecological opportunity for range expansion) within a studied system/clade, may allow the dissection of ecological and evolutionary processes influencing the dispersal-range size relationship.