Niche conservatism above the species level

Abstract

Traits that enable species to persist in ecological environments are often maintained over time, a phenomenon known as niche conservatism. Here we argue that ecological niches function at levels above species, notably at the level of genus for mammals, and that niche conservatism is also evident above the species level. Using the proxy of geographic range size, we explore changes in the realized niche of North American mammalian genera and families across the major climatic transition represented by the last glacial-interglacial transition. We calculate the mean and variance of range size for extant mammalian genera and families, rank them by range size, and estimate the change in range size and rank during the late Pleistocene and late Holocene. We demonstrate that range size at the genus and family levels was surprisingly constant over this period despite range shifts and extinctions of species within the clades. We suggest that underlying controls on niche conservatism may be different at these higher taxonomic levels than at the species level. Niche conservatism at higher levels seems primarily controlled by intrinsic life history traits, whereas niche conservatism at the species level may reflect underlying environmental controls. These results highlight the critical importance of conserving the biodiversity of mammals at the genus level and of maintaining an adequate species pool within genera.

Fig. 1.

Change in log₁₀ area from late Pleistocene (LP) to late Holocene (LH) for North American genera. Genera listed left to right from highest LP rank to lowest LP rank. Bars shows difference in geographic range area between the two periods: blue indicates that genus range decreased in LH, red indicates genus range increased in LH. Note that most genera increased in range size in the LH.

Fig. 2.

Examples of the LP and LH geographic range sizes for North American mammalian genera. Included are differences in late-Pleistocene to LH range sizes for (A) monotypic genus Antilocapra americana (0 ranks changed), (B) monotypic genus Cryptotis parva (reduction in two orders of rank), (C) the smallest range Aplodontia rufa (0 ranks changed), (D) the largest range expansion Sigmodon spp. (increase in 23 ranks), and (E) the largest range contraction Thomomys spp. (decrease in 13 ranks).

Fig. 3.

Correlation between North American LP and LH generic range sizes (R² = 0.6246).

Fig. 4.

Differences in the number of species within a genus due to late-Pleistocene extinction did not significantly influence changes in geographic range rank between time intervals. Example shown is genus Canis, which ranked second in range size in LP and seventh in LH. (A) LP Canis species, (B) LH Canis species. Note that Canis dirus became extinct at the end of the Pleistocene.