Large-scale patterns of insect generation time (partly a function of developmental rate and partly of diapause) can also provide insight into the likely determinants of broad-scale variation in diversity. Of the growing number of macroecological patterns that are exciting renewed interest among ecologists and conservation biologists, the latitudinal gradient in diversity is one of the most well known, but perplexing. As has long been acknowledged, overall taxonomic diversity increases from high to low latitudes (Gaston 2000). The perplexing nature of this general pattern has to do both with its exceptions, and with the plethora of mechanisms proposed to explain it (Rohde 1992).
Among these mechanisms, three have come to the fore in recent debate. These are the extent to which species richness is dictated by available energy, the extent to which the large area of the tropics might account for its high species richness, and the idea that higher richness in the tropics is a consequence of rapid evolutionary rates in this region (Rohde 1992,1998; Waide et al. 1999; Chown and Gaston 2000; Gaston 2000). From the perspective of generation time it is this last mechanism that is of most interest.
Rapid evolutionary rates in the tropics are thought to be a consequence of short generation times, increased mutation rates at higher temperatures, and an acceleration of selection resulting from the former processes and the general increase of physiological processes at higher temperatures (Rohde 1992; Allen et al. 2002). This naturally begs the question of the extent to which generation time and number of generations vary over large geographic gradients, and the nature of any systematic patterns in this variation. There has been considerable work at the intraspecific level examining geographic variation in generation time and the number of generations that insects can complete per season (Mousseau and Roff 1989; Masaki 1996). However, the nature of interspecific variation in development time, and how this interacts with growing season length to produce geographic variation in number of generations per annum is less clear.
The large-scale studies that have been undertaken indicate that the lower development threshold (LDT—the temperature below which growth and development does not take place) shows the expected decline with increasing latitude, and that there is a negative relationship between LDT and the sum of effective temperatures (SET) (the sum of day degrees above the LDT required for an insect stage to complete development) (Honek 1996). These empirical relationships mean that SET should increase with latitude, and that at lower temperatures polar species should have more rapid development rates than their tropical counterparts and vice versa (Honek and Kocourek 1990). The latter implies equivalent development rates across species from different latitudes, and this is indeed what is found (Fig. 7.2). For total development (egg to adult) the relationship is complicated by variation between species in size, higher taxonomic group membership, and dietary specialization (Honek 1996, 1999), but considerable variation in
Was this article helpful?