Relationship between fundamental and realized niches in terms of frost and drought resistance.

The objective of the present study was to relate the fundamental niches of 16 grassland species, defined in terms of their frost and drought tolerance, to their realized macroclimatic niches derived from the geographical distributions of the species. Eight species pairs each consisting of two congeneric species with different distributions and exposed to different levels of frost and drought stress were selected. The ranges of the two species in each genus differed in the degree to which their geographical ranges extended into cold or dry regions. Frost resistance was analysed by measuring electrolyte leakage and expressed as temperature at which 50% leakage occurred (LT50). The genera investigated responded differently to frost stress. In two genera, the species with ranges extending into colder regions showed a lower LT50 than species confined to warmer regions. In two other genera, the opposite pattern was recorded and the species in the remaining genera did not differ in their frost resistance. Responses to drought stress were quantified by recording their responses to different moisture treatments in a glasshouse experiment. No differences in the responses to drought stress were recorded for species with geographical ranges that extended differently into dry regions. In general, there was only a poor match between fundamental and realized niches for only a few genera and only for resistance to frost and not drought. Thus, it is not possible to predict the geographical distribution ranges of two congeneric species based on their performance in experiments, and vice versa. Therefore, it is likely that the assumptions made in modelling species distributions are incorrect and as a consequence the forecasts of future distributions of species based on these models unreliable.