Species at the edge of their range: The significance of the thermal environment for the distribution of congeneric Craspedolepta species (Sternorrhyncha : Psylloidea) living on Chamerion angustifolium (Onagraceae)

There is much current discussion about the factors that control the distrib ution and abundance of animal species, particularly at the edges of their r ange. The significance of temperature for survival and development is compa red in two closely related psyllid species (Craspedolepta nebulosa and C. s ubpunctata) living on the same host plant (Chamerion angustifolium) (Onagra ceae) but displaying different distributions along latitudinal and altitudi nal gradients. The following measurements were made at critical periods dur ing the life cycle (a) winter supercooling points (SCPs), (b) tolerance of short (1 min) and long term (1-25) days exposure to sub-zero temperatures a bove the SCP, (c) tolerance of short term exposure to high spring/summer te mperatures and (d) comparative field development rates among species and si tes during the early critical part of the growing season. Successful comple tion of the life cycle is related to heat availability during the growing s eason. This appears to limit the distribution of the Craspedolepta species, rather than their survival response to thermal extremes. No significant di fferences were found between the two species in the supercooling point or i n their long and short term survival responses at low or high temperatures.