Climate change studies need to develop models for species risk that are mechanistic and predictive, with conservation strategies explored through the use of scenarios. This study focused on a diverse group for climate change analysis – lichen epiphytes – to develop a heuristic model for quantifying risk that has two key components. First, it draws on the classic ecological concept – ‘das Gesetz der relativen Standortskonstanz’ – which explains how the suitable niche space of a species (at a microhabitat-scale) may occur under different local circumstances in contrasting macroclimatic zones. To quantify this pattern, conservation priority cyanolichen and tripartite epiphytes were sampled across a steep climatic gradient, to characterise their different microhabitat preferences in optimal and sub-optimal macroclimates. Second, the model used climatically controlled growth rates as a functional response to climate variability, leading to an increase in generation time for sub-optimal climates. Together, the macroclimate-microhabitat and growth rate data parameterise a mechanistic population model that was used to explore the effect of environmental change scenarios, including: 1. Climate change leading to longer generation times, and 2. A reduction in habitat quality, e.g. through a tree disease scenario such as ash dieback. The advantage of this population approach was its down-scaling to better understand a species’ local vulnerability. Accordingly, the study suggested how management at landscape or habitat-scales can be used to offset the negative effects of climate change. Because extinction rates for the epiphyte populations are low, and established individuals are relatively long-lived, there is a time-lag during which conservation can increase the resilience of threatened populations. However, multiple threats, such as climate change and tree disease combined, severely shorten this window of opportunity.

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