1.3.5.4 Changes in morphology and reproduction

A change in fecundity is one of the mechanisms altering species distributions (see Section 1.3.5.2). Temperature can affect butterfly egg-laying rate and microhabitat selection; recent warming has been shown to increase egg-laying and thus population size for one species (Davies et al., 2006). The egg sizes of many bird species are changing with increasing regional temperatures, but the direction of change varies by species and location. For example, in Europe, the egg size of pied flycatchers increased with regional warming (Jarvinen, 1994, 1996). In southern Poland, the size of red-backed shrikes’ eggs has decreased, probably due to decreasing female body size, which is also associated with increasing temperatures (Tryjanowski et al., 2004). The eggs of European barn swallows are getting larger with increasing temperatures and their breeding season is occurring earlier. Additionally, in the eggs, concentrations of certain maternally supplied nutrients, such as those affecting hatchability, viability and parasite defence, have also increased with warming (Saino et al., 2004). Studies from eastern Poland, Asia, Europe and Japan have found that various birds and mammals exhibit trends toward larger body size, probably due to increasing food availability, with regionally increasing temperatures (Nowakowski, 2002; Yom-Tov, 2003; Kanuscak et al., 2004; Yom-Tov and Yom-Tov, 2004). Reproductive success in polar bears has declined, resulting in a drop in body condition, which in turn is due to melting Arctic Sea ice. Without ice, polar bears cannot hunt seals, their favourite prey (Derocher et al., 2004).

These types of changes are also found in insects and plants. The evolutionary lengthening and strengthening of the wings of some European Orthoptera and butterflies has facilitated their northward range expansion but has decreased reproductive output (Hill et al., 1999a; Thomas et al., 2001a; Hughes et al., 2003a; Simmons and Thomas, 2004). The timing and duration of the pollen season, as well as the amount of pollen produced (Beggs, 2004), have been found to be affected by regional warming (see Section 1.3.7.4).