Pollination mutualism over the edge: growth form and fig traits respond differently during niche expansion to arid environments

Abstract

Background: It is unclear how climate shifts may interfere with the diversification of tightly associated species interactions. Obligate pollination mutualisms are ideal for investigating the underlying processes of niche evolution by environmental change. Objectives: Niche evolution that corresponds with phenotype-by-environment interactions that include climate shifts is expected to affect the stability of partner associations of mutualisms. Here, we compare the evolution of fig syconium size and growth form strategies for coping with soil water stress during a range expansion by Ficus into arid environments. Methods: Niche models of 31 Ficus taxa were used to define contemporary habitat requirements of each species, which are then incorporated into Bayesian mixed models to examine the response of each trait to the soil water variables, other environmental variables and phylogenetic dependencies. Results: The results showed that phylogenetic constraints best explained syconium size variation, while environmental lability best characterised growth form variation. Two species adapted to arid environments that exhibited convergence in these traits had significantly different habitat requirements compared to forest and savanna species. Conclusion: The niche expansions by Ficus ilicina and Ficus cordata involved distinct historical processes. The findings suggest selection for traits that varied at different temporal scales limit the rate of niche expansions, and potentially destabilise mutualist partnerships and promote host shifts.