Concept for range limits with Pinus albicaulis

Background/Question/Methods Species range limits are not determined by climate alone. Darwin noted the differences in abiotic and biotic controls on northern and southern range limits and reasoned that their relative effects changed geographically. Such differences were first articulated in terms of “niche” for ongoing climate change a quarter century ago, and the importance of biotic interactions, beyond competition, in controlling range limits has been recognized in the decades since, but a satisfactory conceptual framework and complementary analyses are still lacking. Soberón (2007) represented the niche concept for a given species as a Venn diagram. Its geographical occurrence was determined from the intersection of three circles—depicting where the species would have positive population growth within the abiotic environment, the limits of organism dispersal, and where the species can coexistent with or exclude competitors. We rethink this conceptual framework with specific attention to the regeneration niche. We develop the conceptual framework for the system centered on the whitebark pine – Clark’s nutcracker (Pinus albicaulis – Nucifraga columbiana) mutualism; the system includes seed predation by red squirrels, symbiosis with ectomycorrhizae, infestation by mountain pine beetle, and infection by white pine blister rust.

Results/Conclusions First, the biotic interaction area must be wholly within the abiotic and dispersal areas: the species must be present before biotic interaction can occur. Second, the interaction includes facilitation. Third, biotic interactions can expand the area of the abiotic conditions and the geographic area reached by dispersal. The abiotic space can be expanded through facilitation, which ameliorates microclimate, or by symbiosis through mycorrhizae. The expansion of the area of dispersal includes zoochory, especially “directed” dispersal. We show that at range limits biotic interactions can changes a species’ niche: 1) as the abiotic constraints tighten at the range edge, the biotic interactions countering this effect become relatively more important even as they become increasingly constrained; and 2) zoochory can become unreliable and may constrain range more than abiotic conditions, but directed dispersal can expand the niche. At range limits regeneration sites are near the edge of the abiotic niche, but where it is expanded by facilitation; sites are near the limits of dispersal, but directed dispersal expands the area. The geographic areas of the abiotic, biotic, and dispersal dimensions of niche are all reduced, but the relative area increases where biotic interactions expand the abiotic and dispersal dimensions.