Determining the likely abiotic thresholds for survival, development, or reproduction in the laboratory, or perhaps with limited caging experiments, is only the first step in explaining their relevance to population dynamics and ultimately to the abundance and distribution of a species (or suite of species) (Kingsolver 1989; Holt et al. 1997). Several critical questions remain, including which stage, gender, or age group is most likely to experience the critical population bottleneck (van der Have 2002), how physiological characteristics and fitness are actually related (Feder 1987; Kingsolver 1996), and how frequently environmental extremes might be encountered. In some cases, the immature stages might be most critical because development generally proceeds within a smaller range of environmental variables (e.g. temperature) than the adult organisms can survive (van der Have 2002). In consequence, both abundance and distribution might be determined by growth conditions faced by the immatures (Bryant et al. 1997). Alternatively, the time available for adult flight (and oviposition), and therefore realized fecundity of adults might explain population fluctuations (Kingsolver 1989). In Drosophila pseudoobscura survival of cold winter conditions by males is relatively insignificant because it is females and the sperm they carry from previous matings that are most important for subsequent population recovery (Collett and Jarman 2001). Surprisingly, investigations of ontogentic variation in physiological traits and its implications for survival, and consequently its role in determining insect abundance and distribution, remain relatively scarce.
That year-to-year variation in environmental conditions has a large effect on insect populations has long been appreciated (Andrewartha and Birch 1954), and is widely supported by a range of studies (e.g. Roy et al. 2001). However, the importance of extreme events, and the way in which their likely recurrence and impact on populations should be investigated, have enjoyed less attention, particularly in the context of microclimates (Parmesan et al. 2000; Sinclair 2001a). For example, populations of Euphydryas editha (Lepidoptera, Nymphalidae) were driven to extinction as a consequence of three extreme weather events (and human landscape alteration). In 1989 minimal snow led to early April (rather than June) emergence of adults and their subsequent starvation owing to an absence of nectar. A year later emergence was once again early for the same reason, and a 'normal' snowstorm in May resulted in high mortalities. In 1992, unusually low temperatures killed most of the host plants, leaving caterpillars with no source of food (Thomas et al. 1996; Parmesan et al. 2000). Similarly, unseasonably warm temperatures in the Arctic lead to surface ice-formation and considerable mortality of several populations of soil-dwelling species (Coulson et al. 2000).
Many of the studies examining extreme environmental events have emphasized not only their extreme values, but also their duration, the rates at which they are approached, and their likely return times (Gaines and Denny 1993; Sinclair 2001a). These parameters might be more important than simple mean, variance, and absolute extremes of the climate of an area in determining the likely persistence of a population. Fortunately, there is a variety of techniques available for analysing extreme values (Ferguson and Messier 1996; Denny and Gaines 2000; Sinclair 2001b), and the importance of doing so at the microclimate level is being increasingly recognized (Sinclair 2001a; Williams et al. 2002).
For example, along the east coast of Australia, highest daily maximum temperature in the hottest month of the year does not vary with latitude. By contrast, mean daily maximum temperature declines with latitude, suggesting that the annual
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