THERMAL TIME AND ECOLOGICAL STRATEGIES - A UNIFYING HYPOTHESIS

Rates of embryogenesis and of development and growth in several nemato des are linearly related to temperature over a considerable range. On this basis, published data on the thermal time requirements are compar ed for a tropical and a temperate species of plant parasitic nematode Meloidogyne javanica and M. hapla respectively, the two being closely related and morphologically and biologically similar. M. hapla has a l ower base temperature (T-b) and a higher thermal constant (S) than M. javanica with the relative values being almost inversed. Consequently, above their respective T-b values the slope of the relationship betwe en rate of development and temperature was greater for the tropical sp ecies than that for the temperate species. A mathematical exploration of the relationship between T-b and S was made assuming that, over a n arrow range, Tb X S was a constant. With this assumption, for any give n average environmental temperature (T-e) the optimum base temperature for minimum developmental duration was T-e/2, and the temperature at which the duration of development was equal for the otherwise identica l species was shown to be the sum of their base temperatures. The prac tical effect of the differences in T-b and S was to give M. hapla, the temperate species, a shorter life cycle and hence a competitive advan tage at temperatures below 21 Angstrom C and M. javanica, the tropical species, the advantage above that temperature. It is argued that a ne gative correlation between T-b and S is likely to be widespread, and p rovides a mechanism for regulating the distributions of related, compe ting organisms. Support for the hypothesis that the value of S tends t o decrease as T-b increases is derived from data on the embryogenesis of an animal parasitic nematode Haemonchus contortus and from seed ger mination studies. Contrary results and exceptions are also briefly dis cussed. The observed interaction between T-b and S may be fundamental to many poikilothermic organisms and plants and provides an explanatio n for tropical species generally having higher T-b values than tempera te ones. The ecological implications of different values of T-b and of S, including their relationship with organisms which are ''r'' or ''K '' strategists are briefly discussed.