1 How Butterflies Keep Their Cool: Physical and Ecological Traits Influence Thermoregulatory Ability
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1 How butterflies keep their cool: physical and ecological traits influence thermoregulatory ability 2 and population trends. 3 Andrew J. Bladon1*, Matthew Lewis1, Eleanor K. Bladon1, Sam J. Buckton1,2,3, Stuart Corbett4, Steven 4 R. Ewing5, Matthew P. Hayes1, Gwen E. Hitchcock2, Richard Knock2, Colin Lucas6, Adam McVeigh7, Rosa 5 Menéndez7, Jonah M. Walker1, Tom M. Fayle8, Edgar C. Turner1 6 1Department of Zoology, University of Cambridge, Downing Street, Cambridge, CB2 3EJ, UK. 7 [email protected]; [email protected]; [email protected]; [email protected]; 8 [email protected]; [email protected]; [email protected] 9 2The Wildlife Trust for Bedfordshire, Cambridgeshire & Northamptonshire, The Manor House, Broad 10 Street, Cambourne, Cambridge, CB23 6DH, UK. [email protected]; 11 [email protected] 12 3Yorkshire Wildlife Trust, 1 St George’s Place, York, YO24 1GN, UK. 13 4RSPB Winterbourne Downs, RSPB Manor Farm, Newton Tony, Salisbury, Wiltshire, SP4 0HA, UK. 14 [email protected] 15 5RSPB Centre for Conservation Science, RSPB Scotland, 2 Lochside View, Edinburgh Park, Edinburgh, 16 EH12 9DH, UK. [email protected] 17 649 Mill Road, Beccles, Suffolk, NR34 9UT, UK. [email protected] 18 7Lancaster Environment Centre, Lancaster University, Lancaster, LA1 4YQ, UK. [email protected]; 19 [email protected] 20 8Biology Centre of the Czech Academy of Sciences, Institute of Entomology, Branisovska 31, 37005, 21 Ceske Budejovice, Czech Republic. [email protected] 22 * Author for correspondence: [email protected] 23 1 24 Abstract 25 1. Understanding which factors influence the ability of individuals to respond to changing 26 temperatures is fundamental to species conservation under climate change. 27 2. We investigated how a community of butterflies responded to fine-scale changes in air 28 temperature, and whether species-specific responses were predicted by ecological or 29 morphological traits. 30 3. Using data collected across a UK reserve network, we investigated the ability of 29 butterfly 31 species to buffer thoracic temperature against changes in air temperature. First, we tested 32 whether differences were attributable to taxonomic family, morphology or habitat 33 association. We then investigated the relative importance of two buffering mechanisms: 34 behavioural thermoregulation versus fine-scale microclimate selection. Finally, we tested 35 whether species’ responses to changing temperatures predicted their population trends from 36 a UK-wide dataset. 37 4. We found significant interspecific variation in buffering ability, which varied between families 38 and increased with wing length. We also found interspecific differences in the relative 39 importance of the two buffering mechanisms, with species relying on microclimate selection 40 suffering larger population declines over the last 40 years than those that could alter their 41 temperature behaviourally. 42 5. Our results highlight the importance of understanding how different species respond to fine- 43 scale temperature variation, and the value of taking microclimate into account in conservation 44 management to ensure favourable conditions are maintained for temperature-sensitive 45 species. 46 47 Keywords: Behavioural thermoregulation, butterflies, climate change, generalist, microclimate, 48 population trends, specialist, temperature. 2 49 50 Introduction 51 Climate change affects the distribution, abundance and phenology of species. These changes can 52 comprise range shifts, with increases in abundance in the cooler parts of species’ ranges and declines 53 in abundance in warmer parts (Fox et al., 2015; Lehikoinen et al., 2013; Parmesan et al., 1999), and 54 altered activity patterns, with species emerging or becoming active earlier in the year or in warmer 55 conditions (Sparks & Yates, 1997; Thackeray et al., 2010). Research on the impacts of climate change 56 is now well-advanced, with many studies predicting accelerating effects on the natural world as 57 warming progresses (Dennis & Shreeve, 1991; Devictor, Julliard, Couvet, & Jiguet, 2008). 58 59 Changing temperatures can have a particularly marked effect on butterflies, with species at the edge 60 of their distribution showing the most dramatic shifts (Dennis & Shreeve, 1991; Hill, Thomas, & 61 Huntley, 1999; Menéndez et al., 2006; Parmesan et al., 1999). In the UK, where the butterfly fauna is 62 dominated by species at their poleward range limit, climate change is generally predicted to drive 63 range expansions and increases in abundance of butterflies (Thomas & Lewington, 2016; Warren et 64 al., 2001; C. J. Wheatley, unpublished data), although the negative impacts of habitat fragmentation 65 and degradation could counteract these effects (Oliver, Thomas, Hill, Brereton, & Roy, 2012; Warren 66 et al., 2001). In contrast, for a handful of cold-adapted northern or montane species, climate change 67 will likely result in declines (Franco et al., 2006). Given these effects, many habitats are predicted to 68 experience turnover of species and altered species richness as time goes on (González-Megías, 69 Menéndez, Roy, Brereton, & Thomas, 2008; Menéndez et al., 2007). Such changes may be linked to 70 the direct effects of temperature on individuals (Bladon et al., 2019; Calosi, Bilton, & Spicer, 2008), 71 temperature-mediated impacts on water balance (Smit & McKechnie, 2015) and oxygen availability 72 (Pörtner & Knust, 2007), or through the impacts of changing temperatures on species interactions 73 (Diamond et al., 2017). For example, temperature can directly affect the speed of insect life cycles 74 (Rebaudo & Rabhi, 2018; Wilson & Maclean, 2011) and reproductive rates, affecting population 3 75 growth rates (Mills et al., 2017). Higher temperatures can also change butterfly activity patterns and 76 facilitate more frequent, longer or more effective territorial and mate-locating behaviours, potentially 77 increasing breeding success (Advani, Parmesan, & Singer, 2019; Hayes, Hitchcock, Knock, Lucas, & 78 Turner, 2019; Ide, 2010; Rutowski, Demlong, & Leffingwell, 1994). 79 80 In general, species can respond to changing temperatures in three main ways: by directly buffering 81 their temperature through physiological or metabolic means (which we term “physiological 82 thermoregulation”), by changing their behaviour in situ and therefore increasing warming or cooling 83 (“behavioural thermoregulation”), or by shifting their distribution to more favourable microclimatic 84 conditions (“microclimate selection”). Behavioural thermoregulation is widespread in ectotherms 85 (Abram, Boivin, Moiroux, & Brodeur, 2017) and taxa such as butterflies display a number of 86 behavioural mechanisms to control their temperatures. In cooler conditions, these include angling 87 their body and wings so that the surface is perpendicular to the sun and absorbs more energy (Kemp 88 & Krockenberger, 2002; Pivnick & McNeil, 1986; Wasserthal, 1975) or even acts to concentrate 89 sunlight (Shanks, Senthilarasu, ffrench-Constant, & Mallick, 2015), thereby increasing body 90 temperature. In contrast, in hotter conditions, butterflies can adopt postures that reduce the surface 91 area exposed to the sun or that reflect more sunlight (Dreisig, 1995; Rutowski et al., 1994), thus 92 reducing body temperature. Microclimate selection is also common, with individuals selecting sunny 93 locations to warm up or shady locations to cool down (Hayes et al., 2019; Ide, 2010; Kleckova & Klecka, 94 2016; Kleckova, Konvicka, & Klecka, 2014). Over slightly larger spatial scales, individuals can also select 95 a microhabitat with the preferred temperature (Dreisig, 1995; Rutowski et al., 1994), enabling 96 individuals to maintain their body temperature under shifting climatic conditions (Kleckova, Konvicka, 97 & Klecka, 2014). This can result in individuals within a species preferring more northerly slopes at the 98 southern end of their range, but more southerly slopes at the northern end of their range (Oliver, Hill, 99 Thomas, Brereton, & Roy, 2009; Suggitt et al., 2012). Understanding the ability of species to buffer 4 100 against changing temperatures, and the means by which they do this, is therefore important for 101 predicting the future impact of climate change on whole communities. 102 103 Models exploring the long-term effects of climate change on species distributions and population 104 trends are well-advanced, although these are typically based upon weather station data collected 105 from standardised environments (Bramer et al., 2018) and therefore do not take into account the fine- 106 scale impacts of local habitat structure and topography on temperature (“microclimatic 107 temperature”). Microclimatic temperature can differ dramatically from the climatic mean, and varies 108 with topography, vegetation cover and altitude (Suggitt et al., 2011). Microclimatic temperature is 109 also variable over short time-frames, with extremes often exceeding meteorological means in exposed 110 areas, but being much more stable in sheltered areas (Maclean, Suggitt, Wilson, Duffy, & Bennie, 111 2017). It is also microclimatic temperatures that individual organisms experience on a day-to-day basis 112 and which can affect the distribution and abundance of species at the local scale. Therefore, 113 information on how individuals respond to fine-scale microclimatic temperature variation over short 114 time-periods (Bladon et al., 2019; Ide, 2002; Kelly, Godley, & Furness, 2004) may need to