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.