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Dr Brian Coppins
Ms Sally Eaton
Dr Christopher Ellis
Ms Louise Olley
Dr Rebecca Yahr

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Introduction

Climate change is a significant threat to biodiversity, especially because it accompanies additional pressures such as habitat loss and fragmentation. There is increased risk because effective migration or gene flow (adaptation) in response to climate change may be severely limited in fragmented landscapes.

RBGE has played a key role in examining the exposure to climate change of Britain's native species [1], especially lichens.

We developed the first predictive bioclimatic models for lichen diversity (Figure 1), and have been involved in testing some of the fundamental assumptions which underpin this widely-used approach.

Development of Bioclimatic Models

We developed predictive bioclimatic models for 26 lichen species with contrasting spatial distributions (Figure 1). This was a starting point for understanding the exposure of species with different distributions to climate change scenarios [2] as well as highlighting key areas of uncertainty [2, 3].

To expand beyond climate change, we investigated the relative importance of climate, compared to pollution and landscape-scale habitat in controlling species richness and composition [4, 5]. This demonstrated that the measured importance of contrasting drivers will change quite dramatically depending on the regional context and the methods by which multiple correlated variables are investigated [5, 6].

Testing Climatic Equilibrium

The statistical modelling procedures that are widely used in bioclimatic modelling tend to be correlative; species distributions are compared to climatic variables. However, species often have spatially aggregated distributions for a variety of reasons, e.g. owing to dispersal limitation. Combining aggregated distributions with spatially correlated climate variables can lead to statistically 'good' though biologically spurious models. Bioclimatic models thus inherently assume that species distributions (responses) directly reflect sensitivity to macroclimatic variables (explanatory variables), an assumption known as 'climatic equilibrium'. This may be a particular problem for epiphytes, which are nested within the microhabitats of forest canopies (Figure 2).

To confirm or refute the assumption of climatic equilibrium for lichen epiphytes, we tested independent evidence for macroclimatic sensitivity in two ways:

(i) by showing that for the model epiphyte, Lobaria pulmonaria (Figure 3), its North American range broadly matches sampled experimental growth rates that are significantly explained by the macroclimate [7], and

(ii) by providing robust tests of the extent to which lichen distributions might be controlled by macroclimate, e.g. (i) comparing the climatic sensitivity of species which occur on different intercontinental landmasses (Britain and North America), and the extent to which these independently derived ranges are climatically equivalent [8], and (ii) by using our studies in Historical Biogeography to demonstrate a time-series shift in lichen communities that is consistent with the end of the Little Ice Age and recent climate warming [9].

Figure 1. Bioclimatic projections showing the loss of suitable bioclimatic space for a suite of montane lichens occurring in Britain, using UKCIP02 scenarios [cf. 2].
 
Figure 2. The interior of an ancient oakwood with a canopy dwarfed by physical exposure on the west coast of Scotland. Such habitats retain microhabitat complexity that may be decoupled from macroclimatic sensitivity at the scale employed by bioclimatic models, e.g. 5km grids.
 
Figure 3. Lobaria pulmonaria on a mature oak at Cawdor Wood, north-east Scotland.

References:

[1] Ellis, C.J. (2011) Predicting the biodiversity response to climate change: challenges and advances. Systematics and Biodiversity, 9: 307-317.

[2] Ellis, C.J., Coppins, B.J., Dawson, T.P. & Seaward, M.R.D. (2007) Response of British lichens to climate change scenarios: trends and uncertainties in the projected impact for contrasting biogeographic groups. Biological Conservation, 140: 217-235.

[3] Ellis, C.J., Coppins, B.J. & Dawson, T.P. (2007) Predicted response of the lichen epiphyte Lecanora populicola to climate change scenarios in a clean-air region of northern Britain. Biological Conservation, 135: 396-404.

[4] Ellis, C.J. & Coppins, B.J. (2009) Quantifying the role of multiple landscape-scale drivers controlling epiphyte composition and richness in a conservation priority habitat (juniper scrub). Biological Conservation, 142: 1291-1301.

[5] Ellis, C.J. & Coppins, B.J. (2010) Integrating multiple landscape-scale drivers in the lichen epiphyte response: climatic setting, pollution regime and woodland spatial-temporal structure. Diversity and Distributions, 16: 43-52. See Corrigendum: Diversity & Distributions, 16: 312.

[6] Ellis, C.J. & Coppins, B.J. (2010) Partitioning the role of climate, pollution and old-growth woodland in the composition and richness of lichen epiphytes in Scotland. The Lichenologist, 45: 601-614.

[7] Eaton, S. & Ellis, C.J. (2012) Local experimental growth rates respond to macroclimate for the lichen epiphyte Lobaria pulmonaria. Plant Ecology and Diversity, 5: 365-372.

[8] Braidwood, D. & Ellis, C.J. (2012) Bioclimatic equilibrium for lichen distributions on disjunct continental landmasses. Botany, 90: 1316-1325.

[9] Ellis, C.J., Yahr, R., Belinchón, R. & Coppins, B.J. (2014) Archaeobotanical evidence for climate as a driver of ecological community change across the anthropocene boundary. Global Change Biology, 20: 2211-2220.