Snapshot isolation and isolation history challenge the analogy between mountains and islands used to understand endemism

Abstract

Aim Mountains and islands are both well known for their high endemism. To explain this similarity, parallels have been drawn between the insularity of “true islands” (land surrounded by water) and the isolation of habitats within mountains (so‐called “mountain islands”). However, parallels rarely go much beyond the observation that mountaintops are isolated from one another, as are true islands. Here, we challenge the analogy between mountains and true islands by re‐evaluating the literature, focusing on isolation (the prime mechanism underlying species endemism by restricting gene flow) from a dynamic perspective over space and time. Framework We base our conceptualization of “isolation” on the arguments that no biological system is completely isolated; instead, isolation has multiple spatial and temporal dimensions relating to biological and environmental processes. We distinguish four key dimensions of isolation: (a) environmental difference from surroundings; (b) geographical distance to equivalent environment [points (a) and (b) are combined as “snapshot isolation”]; (c) continuity of isolation in space and time; and (d) total time over which isolation has been present [points (c) and (d) are combined as “isolation history”]. We evaluate the importance of each dimension in different types of mountains and true islands, demonstrating that substantial differences exist in the nature of isolation between and within each type. In particular, different types differ in their initial isolation and in the dynamic trajectories they follow, with distinct phases of varying isolation that interact with species traits over time to form present‐day patterns of endemism. Conclusions Our spatio‐temporal definition of isolation suggests that the analogy between true islands and mountain islands masks important variation of isolation over long time‐scales. Our understanding of endemism in isolated systems can be greatly enriched if the dynamic spatio‐temporal dimensions of isolation enter models as explanatory variables and if these models account for the trajectories of the history of a system.

Two ways to be endemic. Alps and Apennines are different functional refugia during climatic cycles

“…Virtually all European taxa showed differentiation among these areas and signal of (repeated) post‐glacial poleward expansion (forest plants, Petit et al, 2003; butterflies, Dapporto et al, 2019; Schmitt, 2007; mammals, Seddon et al, 2001; springtails, Fiera et al, 2017). The high incidence of ME endemics dated to the onset of the Pleistocene and limited to Peninsula‐Sicily fits with the definition of neoendemics, described as recently diverged species that failed to disperse out of their ancestral area (Flantua et al, 2020).…”

“…Their incidence in the Alps could have contributed to the absence of a significant higher genetic differentiation of Alpine compared to Peninsula‐Sicily endemics. In general, the endemics of the Alps fit with the definition of paleoendemics, described as relict species whose ranges became spatially restricted (Flantua et al, 2020).…”

Climate Vulnerability Assessment of the Espeletia Complex on Páramo Sky Islands in the Northern Andes

“…Last but not least, extending the modeling approach implemented in this work to other key pressures (e.g., fire, Wu and Porinchu, 2019;Rivadeneira et al, 2020;Zomer and Ramsay, 2020) and plant groups (Luteyn, 1999) in the Páramo, as well as to other sky islands around the mountains of the world (Hoorn et al, 2018;Pausas et al, 2018;Nürk et al, 2019;Testolin et al, 2020), and more broadly to other island-like systems (Papadopoulou and Knowles, 2015;Lamichhaney et al, 2017;Cámara-Leret et al, 2020;Flantua et al, 2020), will help understanding climate change effects on unrelated taxa experiencing similar evolutionary processes (Condamine et al, 2018;Cortés et al, 2020;Nürk et al, 2020). Such systems offer natural experiments to assess the role of colonization and adaptation (Ding et al, 2020;McGee et al, 2020;Tito et al, 2020) in the past and ongoing responses to climate change, which undeniably will complement ecological predictive modeling.…”

Changes in plant diversity in a water-limited and isolated high-mountain range (Sierra Nevada, Spain)

“…This applies to most of the Mediterranean mountain ranges (Jiménez-Alfaro et al 2014). Owing to the orographic isolation of their alpine refugia, which were even ice-free during Pleistocene glacial periods (Flantua et al 2020;Gomez-Ortiz et al 2013), Mediterranean mountains host an exceptionally high number of cold-adapted endemic plants (Fernández Calzado et al 2012;Kazakis et al 2007;Noroozi et al 2018;Stanisci et al 2005), and, thus, they are exposed to a high risk of climate change-induced biodiversity loss on the species level. Although observational studies in high-mountain vegetation in the context of climate change are still underrepresented in semi-arid areas (Giménez-Benavides et al 2018), upward shifts of species including drought-tolerant shrub species were found in Mediterranean mountains by re-visiting historical survey sites (Evangelista et al 2016).…”