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.