Taxonomic Prevalence:
Across the tree of life, from ocean depths to mountain peaks, various
animals change their elevational distribution and/or depth between
seasons (Figure 1; Hsiung et al., 2018; Milligan et al., 2020). Studies
on altitudinal migration have largely focused on insects (Kimura, 2021),
mammals (primarily ungulates and bats;
McGuire & Boyle,
2013), and especially birds
(Barçante et al.,
2017), whereas studies on other terrestrial organisms (e.g. reptiles,
amphibians, and non-insect invertebrates) are comparatively scarce
(Hsiung et al., 2018). Altitudinal migration is widespread and can even
bridge aquatic and terrestrial biomes in some species (e.g., salmonOncorhynchus spp. ). Although salmon and other diadromous taxa are
not typically included in discussions of altitudinal migration, or are
mentioned only in passing (e.g., Hsiung et al., 2018), they do undertake
altitudinal migration as some species move from sea-level up to
~2000 m in elevation in the mountains of western North
America over the course of their breeding cycle (Crossin et al., 2004).
Correspondingly, they experience many of the same challenges and drivers
as “traditional” (i.e., terrestrial) altitudinal migrants, such as
changes in air pressure, habitat type, and predation risks. Anadromous
and catadromous fish also connect the study of altitudinal migration to
aquatic species that may seasonally move across bathymetric gradients
(Milligan et al., 2020). Many of these bathymetric movements are also
seasonal (not to be confused with diel vertical migration) and involve a
biologically relevant shift in distribution, and thereby would be
included in our broad definition of altitudinal migration. An expanded
conceptual framework for altitudinal migration that includes all species
that undergo seasonal vertical movement will facilitate communication
among researchers of different taxonomic groups.
Ignoring the nuances that we outline here risks misclassifying the
altitudinal migratory status of the taxa at hand. We believe that most
reviews or databases concerning altitudinal migration consistently
underestimate the number of species that are altitudinal migrants
(Barçante et al.,
2017). This underestimation is in part due to a lack of data on many
species, but is also related to how altitudinal migration is defined. We
suspect this is especially true when studies exclude populations that
are altitudinal migrants as well as latitudinal migrants, which would
constitute a type II error or a ‘false negative’. However, the reverse
pattern also occurs: sometimes species’ irruptive movements (i. e. Pine
Siskin, Spinus pinus,Boyle, 2017) or
opportunistic movements to avoid inclement weather (O’Neill & Parker,
1978) are sometimes classified as altitudinal migration, or a type I
error. Without a consistently applied definition, studies of altitudinal
migration—especially macroevolutionary studies that incorporate
hundreds or even thousands of taxa—may inaccurately classify migratory
behavior, which could lead to erroneous inferences and biases. Regional
biases also exist: some geographic regions have received more attention
than others (Boyle, 2017), both in terms of the number of publications
and the rigor of scientific study
(Schunck et al.,
2023). A consistent definition also allows comparisons among regions
that may have heretofore been using different definitions of what
constitutes an altitudinal migrant.
Novel discussion and comparisons may reveal new insights into the
evolutionary drivers, ecological interactions, conservation implications
of seasonal shifts in vertical distributions, and contextualize
altitudinal migration alongside other types of animal movement. By using
a common definition for altitudinal migration, new comparisons and
questions can be made under a more unified conceptual framework. For
example, are certain taxonomic groups more inclined to be altitudinal
migrants, or are these groups just better studied (i. e. birds)? Are
certain ecological or morphological traits associated with evolutionary
gains or losses in altitudinal migration? Across mammals and birds,
migrants are on average more similar to each other for numerous
ecological traits than they are to their more closely related resident
counterparts, suggesting evolutionary constraints on migratory
phenotypes
(Soriano-Redondo et
al., 2020), but does this pattern hold for ectotherms? Does this vary
regionally (i.e frugivore/nectavore are under-represented in the
Palearctic; Pageau et al., 2020)? Do terrestrial and aquatic altitudinal
migrants share similar comparative evolutionary associations, such as
body size differences between residents and migrants, or differences in
tempo of life history traits? With a rapidly changing climate our
understanding of the traits or lack thereof of altitudinal migrants may
allow for a better understanding of species response. How do differences
in mobility and physiology impact the capacity for altitudinal migration
among different animal groups? Animal movement across ecosystems affects
food webs, nutrient recycling, and resource availability, such as when
diadromous fish bring nutrients from the ocean to the terrestrial
realms. But what is the broader role of altitudinal migration in such
ecosystem services and how does this vary among different groups of
altitudinal migrants? While habitat loss and broad-scale global change
have disrupted many ecosystems
(Brodie et al., 2021;
Wootton et al., 2023), how have changes in the migratory routes and
abundances of altitudinal migrants affected ecosystem services? Using a
united definition improves our comparative framework by recognizing
potentially confounding variables when contrasting taxa and biomes.