Expanding the mutualistic niche: parallel symbiont turnover along climatic gradients

Keystone mutualisms, such as corals, lichens, or mycorrhizae sustain
fundamental ecosystem functions. Range dynamics of these symbioses are,
however, inherently difficult to predict because host species may switch
between different symbiont partners in different environments, thereby
altering the range of the mutualism as a functional unit. Biogeographic
models of mutualisms thus have to consider both, the ecological amplitudes
of various symbiont partners, and the abiotic conditions that trigger
symbiont replacement. To address this challenge, we here investigate
‘symbiont turnover zones’ – defined as demarcated regions where symbiont
replacement most likely occurs due to overlapping abundances of symbiont
ecotypes. Mapping the distribution of algal symbionts from two species of
lichen-forming fungi along four independent altitudinal gradients, we
detected an abrupt and consistent β-diversity turnover suggesting parallel
niche partitioning. Modeling contrasting environmental response functions
obtained from latitudinal distributions of algal ecotypes consistently
predicted a confined altitudinal turnover zone. In all gradients this
symbiont turnover zone is characterized by ~12°C average annual
temperature, ~5°C mean temperature of the coldest quarter, marking the
transition from Mediterranean to cool temperate bioregions. Integrating
conditions of symbiont turnover into biogeographic models of mutualisms is
an important step towards a comprehensive understanding of biodiversity
dynamics under ongoing environmental change.