MÓDULO FORMATIVO 2

I believe there are two primary mechanisms that could explain the relationships I have observed between speckled wood’s habitat associations and climate. Firstly,

microclimatic differences between woodland and open habitats could have a negative effect on the survival and performance of individuals outside of woodland and the magnitude of this effect could vary with macroclimatic conditions. Alternatively, climate may affect the density of speckled wood populations which could have an indirect effect on habitat associations through density-dependent dispersal into alternative non-woodland habitats under climatically favourable conditions. These mechanisms are not necessarily mutually exclusive and may act in concert, or different factors may be more important in different parts of the butterfly’s range.

3.5.1.1 The role of microclimate over winter

Speckled wood is more restricted to woodland in places with colder winters. Woodland provides a buffered microclimate, with narrower diurnal fluctuations in temperature and less extreme minimum temperatures compared with open habitats (Morecroft et al. 1998, Suggitt et al. 2011). All organisms have minimum temperatures beyond which

they cannot survive (Bale 2002) and above this cold exposure can result in chill injuries which negatively affect fitness (Hutchinson and Bale 1994). Increased restriction of speckled wood to woodland in regions with cold winters could arise because conditions outside of woodland lead to high mortality rates and reduced fitness of survivors and hence smaller more vulnerable populations, whereas the buffering effect of the woodland means individuals can survive within these habitats. In regions with milder winters, however, microclimate conditions outside of woodland may have less of an effect on the survival and fecundity of individuals in open habitats resulting in larger populations outside of woodland.

The hypothesis that winter cold might be a limiting factor for speckled wood is supported by evidence from Sweden where winters are colder than in Britain and the butterfly is thought to be entirely restricted to woodland (Gibbs et al. 2011c). In the south of Sweden, speckled wood can overwinter in the larval or pupal life stages, as in Britain, but larvae enter a true diapause (Wiklund and Friberg 2011) whereas in Britain larvae can resume feeding when temperatures reach their development threshold (Blakeley 1996). Furthermore, in central Sweden where winter temperatures are even lower, it is thought that speckled wood are only able to over winter as pupae which are thought to be more cold tolerant than larvae (Nylin et al. 1995).

3.5.1.2 The role of microclimate over summer

The increased restriction of speckled wood to woodland in areas with warm and dry summers could also be due to physiological constraints on the butterfly’s occurrence outside of woodland in these climatic conditions. Low rainfall leads to desiccation of host plants which cause mortality through starvation or negatively affect the fitness of insect herbivores in less extreme conditions (Scriber 1977). The speckled wood butterfly is thought to be particularly sensitive to host plant desiccation. Female adults

select host plants for egg-laying that are in particularly humid locations (Wiklund and Persson 1983) which suggests they avoid locations where eggs or host plants will desiccate. Evidence that populations decline in years following dry summers supports this idea (Roy et al. 2001, Morecroft et al. 2002). Laboratory studies show that

individuals reared on drought-stressed plants take longer to develop and achieve lower adult body mass (Talloen et al. 2004), which is supported by field evidence that shows individuals are smaller (and thus are likely to have lower fecundity) in drier years (Gibbs et al. 2011a). Levels of humidity and soil moisture are higher in woodland than open habitats (Chen et al. 1993) thus host plant desiccation is likely to be greater in open habitats. In regions where temperatures are hot and rainfall is low, individuals may be unable to tolerate host plant desiccation in open habitats but in regions where rainfall is high conditions may be suitable outside of woodland. Furthermore, physiological differences between individuals originating from woodland and open habitats means those from open habitats are more susceptible to egg desiccation (Gibbs et al. 2010c, Gibbs et al. 2010b, Gibbs et al. 2010a) and suffer more from host plant desiccation (Gibbs et al. 2011b). Host plant desiccation may, therefore, drive a stronger association of speckled wood with woodland in regions where summers are relatively hot and dry such as the south east of England and Catalunya (Suggitt et al. 2012). Previous studies have shown that speckled wood becomes more restricted to woodland in drought years (Schweiger et al. 2006), supporting the role of host plant desiccation in affecting patterns of habitat use.

3.5.1.3 Microclimate and temporal changes in habitat associations

Association of the butterfly with woodland has weakened most over time at the

butterfly’s leading-edge range margin, and particularly in the east of England (Fig. 3.4) where both winter temperatures and summer rainfall have increased. Thus changes in

climatic conditions in these regions could have reduced lethal or sublethal effects of cold or host plant desiccation in “open” habitats, allowing speckled wood to occupy these habitats. Relaxation of these physiological constraints over time could have, therefore, led to an increase in numbers of the butterfly outside woodland habitats. In contrast, the butterfly’s association with woodland does not appear to have changed much in the south-west of England over time, where its association with woodland was historically comparatively weak and has remained weak. This is a region of England in which winters are mild and summers are warm and wet and so conditions may have been suitable for the butterfly to occur outside of woodland prior to recent climate change.

3.5.1.4 The role of density

A second, not necessarily mutually exclusive hypothesis is that variation in speckled wood’s association with woodland is an indirect result of the effect of climate on population density. It would be expected that speckled wood densities are lower in regions where climatic conditions are less favourable. Woodland is the preferred habitat of the speckled wood butterfly (Merckx et al. 2003) and at low densities they may remain in woodland, but when densities are high, density-dependent pressures could result in emigration of individuals from woodland into less favoured habitat types (Baguette and Schtickzelle 2006, Nowicki et al. 2009). Speckled wood males are known to be territorial and defend sunspots in which they wait for passing females to mate with (Davies 1978) and so high densities could force males to move to subordinate habitats in search of territories. The same climatic variables as discussed above (winter cold and summer rainfall) could drive variation in population density. The butterfly’s habitat associations are also related to summer temperature which could also be an important driver of density. For example, thermal availability for development is greater when

summers are warmer, leading to completion of more generations per year (Shreeve 1986b). Warm summer conditions also increase thermal availability for egg-laying perhaps resulting in greater realised fecundity. Both of these factors could result in higher population growth rates and lead to density-dependent dispersal into other non- woodland habitats.

The butterfly’s association with woodland has weakened most over time in areas where the butterfly has expanded its distribution in the east of England (Figs. 2.4 and 3.4). As climate has changed over time and conditions become more suitable, the butterfly has expanded its distribution (Hill et al. 1999a). Its rate of range expansion is higher in areas with greater woodland cover (Hill et al. 2001) and so appears to colonise woodland first. Following colonisation numbers build up in woodland and then stabilise (Pollard et al. 1996). This could be the point at which the butterfly begins to experience density-dependent pressures and moves out into alternative habitats. Weaker patterns of change over time in other parts of the butterfly’s range, such as the south-west of England could be because this area has long-established populations of speckled wood which had already undergone density-dependent spillover into alternative habitats prior to the beginning of the analysis in the 1970s.