A new macroecological pattern: The latitudinal gradient in species range shape

Abstract

Aim We analyse the latitudinal variation of range shape, testing whether ranges at higher latitudes or of larger size tend to be elongated in an east–west direction, as expected from climatic niche constraints. Additionally, we assess whether poleward range limits are less stochastic than equatorward limits. Location Global; all terrestrial areas except Antarctica. Time period Contemporaneous. Major taxa studied Birds and mammals. Methods We quantified the shape of the distribution ranges of 10,057 species of birds and 5,411 species of mammals, and assessed the relationship of range shape with latitude and range size using generalized additive models. The observed pattern was compared with that predicted by a null model, in which species ranges were constrained only by the shape of the landmasses. Furthermore, we divided each range in two polygons and measured their separate perimeters, to compare the limits of the poleward and equatorward portions of each range. Results In terrestrial mammals and birds, only species living at higher latitudes in the Northern Hemisphere and with large range sizes have ranges that consistently tend to be elongated in an east–west direction. The difference between observed and null range shapes increases from the Southern Hemisphere and the tropics towards the north. The difference between equatorward and poleward semi‐perimeters was relevant only in the Southern Hemisphere, and it is probably explained by the shape of the continents. Main conclusions The existence of a latitudinal gradient in range shape is consistent with a different role of deterministic and stochastic processes in the tropics versus high latitudes. Specifically, the results are in line with a scenario in which tropical niche conservatism and relatively evolutionarily labile lower thermal tolerance increase the relevance of climate in determining range limits towards the poles, with stochastic factors (e.g., dispersal limitation) more relevant in shaping low‐latitude range limits.

Determinants of geographic range size in plants

“…relative longitudinal and latitudinal extent) can help disentangle the importance of barriers compared to environmental gradients (Brown & Maurer, ). For instance, a study of gradients in range shape for mammals and birds found that the geometric constraints caused by the continental shape of South America and Africa indeed limit longitudinal range extents stronger than climate gradients (Castro‐Insua et al , ). To our knowledge, there are no similar studies of global gradients in plant range shapes.…”

Widespread underfilling of the potential ranges of North American trees

“…Range shape, the ratio between the latitudinal and longitudinal extent of a range, can be used to further detect dispersal limitations in underfilled ranges (Baselga et al., 2012). Under neutral ecological dynamics (Hubbell, 2001), a species should expand its range equally in all directions, resulting in circular ranges with shape ratios equal to 1 (Cain, 1944; Castro‐Insua et al., 2018). Such ranges are limited by a species' intrinsic dispersal ability, influenced by its functional traits (Cain, 1944; Castro‐Insua et al., 2018; Rapoport, 1982).…”

“…Under neutral ecological dynamics (Hubbell, 2001), a species should expand its range equally in all directions, resulting in circular ranges with shape ratios equal to 1 (Cain, 1944; Castro‐Insua et al., 2018). Such ranges are limited by a species' intrinsic dispersal ability, influenced by its functional traits (Cain, 1944; Castro‐Insua et al., 2018; Rapoport, 1982). Species with greater dispersal distances, more abundant propagules, shorter generations, or generalist ecological preferences should have advantages during establishment (Angert et al., 2011); this lack of intrinsic limitations allows such species to attain larger ranges (Baselga et al., 2012).…”

“…(2012) for more background and Castro‐Insua et al. (2018) for details on how kilometres are calculated for latitudinal and longitudinal extents. (b) Example simulated range (grey) and realized range (green) for Honey Locust ( Gleditsia triacanthos ), along with calculations for range shape.…”

Local climatic changes affect biodiversity responses to land use: A review

“…Researchers investigating the effects of climate and its interaction with land‐use change on biodiversity often use the term “ climatic niche ” (e.g. Castro‐Insua, Gomez‐Rodriguez, Svenning, & Baselga, ; Dallas, Decker, & Hastings, ; Sapes, Serra‐Diaz, & Lloret, ; Sunday, Bates, & Dulvy, ), which aims to reflect the climatic elements of a species' multidimensional niche sensu Hutchinson (Hutchinson, ; Sapes et al, ). Within a species' fundamental niche (Hutchinson, ), the climatic elements encompass the species' climatic tolerances, which are often estimated through physiological tests (Frishkoff et al, ; Peterson et al, ).…”