Differences in adult survival drive divergent demographic responses to warming on the Tibetan Plateau

A central question in biodiversity conservation is whether species will
maintain viable populations under climate warming. Assessing species
viability under climate warming requires demographic studies integrating
vital rate responses to long-term warming throughout species’ life cycle.
However, studies of this nature are rare. Our Integral Projection Models
(IPMs) parameterised with demographic data show differing responses of two
functionally similar co-occurring species, Elymus nutans Griseb. and
Helictotrichon tibeticum (Roshev.) Holub, to 10 years of in situ active
warming by 2℃. Our IPMs estimated that the life expectancy is higher in H.
tibeticum (6.7 years) than that in E. nutans (4.5 years) under ambient
conditions, and the difference is larger under warmed conditions. We found
that while warming decreased individual-level growth in both species, H.
tibeticum, which has a longer life expectancy, compensated with increased
survival, and thereby increased projected population-level growth under
warming. Contrastingly, E. nutans, which has a shorter life expectancy, is
projected to have decreased population-level performance. Furthermore, our
elasticity analyses show that survival is the most important vital rate
for population viability in both species under both ambient and warmed
conditions. Moreover, our retrospective Life Table Response Experiment
(LTRE) analysis reveals that the contrasting fates of the two species
under warming mainly arise from the different responses of adult survival,
which is significantly promoted in H. tibeticum but slightly reduced in E.
nutans. Individual shrinkage occurred 1.6-fold more frequently under
warming than ambient conditions for both species, and made considerable
negative contributions to their population growth rates in warmed plots.
However, such negative effects are offset in H. tibeticum (but not E.
nutans) by the positive contribution to population growth rate of the
associated increased survival.  Our results illustrate that the
responses to climate warming may vary considerably between similar
co-occurring species, and species with a demographically compensatory
strategy may avoid population collapse. Furthermore, our study
demonstrates the potential of using life-history traits to predict
species’ viability facing warming, so as to inform biodiversity
conservation under climate change.