EQUIPROPORTIONAL TEMPERATURE-DURATION RESPONSES AND THERMAL INFLUENCES ON DISTRIBUTION AND SPECIES SWITCHING IN THE COPEPODS METADIAPTOMUS-MERIDIANUS AND TROPODIAPTOMUS-SPECTABILIS

The temperature-dependence of development was studied in two ecologica lly divergent freshwater calanoids, Metadiaptomus meridianus (Douwe) a nd Tropodiaptomus spectabilis (Kiefer). Egg durations were determined between 10 and 35 degrees C, and food satiated post-embryonic developm ent times between 12 and 32 degrees C. All responses were basically in verse monotonic functions of temperature, adequately described by Bele hradek's equation. M. meridianus generally developed faster than T. sp ectabilis. Its egg development was faster at all temperatures, and whi le its naupliar durations were shorter only up to +/- 15 degrees C, it s overall post-embryonic development was more rapid up to +/- 24 degre es C in females and +/- 28 degrees C in males. Relatively speaking, ho wever, T. spectabilis is clearly more warm-adapted than M. meridianus. The respective distributions (warm subtropical lowlands vs cooler upl ands) of these copepods in the southern African subcontinent, as well as reversible switches between these species observed in two Natal imp oundments are consistent with their contrasting thermal responses, alt hough additional considerations apply in respect of the species altern ations. T. spectabilis was replaced by M. meridianus in L. Midmar in s pring 1981 and 1989, and in L. Albert Falls in spring 1990. Reciprocal replacements occurred in Midmar in autumn 1984, and in Albert Falls i n late summer 1991. Both spring switches in Midmar coincided with cool spring temperatures, although the consequent Shifts in growth rate ad vantage predicted from the measured temperature-duration responses onl y partly explain the switches in this warm-temperate reservoir. Parasi tism of T. spectabilis by an ellobiopsid was observed during both swit ching events in Midmar, and perhaps augmented the change, although its effects on the host are indeterminate. Both species showed exactly pa rallel temporal changes in fecundity during the recent switches in bot h reservoirs, indicating closely similar trophic niches in the adults at least, and mitigating the possibility of trophic influences as dete rminants of the replacement. A dramatic but inexplicable increase (aro und 50% at 20 degrees C) in the development time of T. spectabilis was noted between 1988 and 1990, and perhaps contributed too. The protrac ted historical dominance of T. spectabilis in thermally suboptimal con ditions in Midmar is ascribed to a general competitive superiority pre sumed from its K-selected attributes, in contrast to the r-selection e vident in M. meridianus. This alternation between species with contras ting life styles is of fundamental ecological interest. Studies on Alb ert Falls, commenced in 1989, suggest an even greater competitive supe riority of T. spectabilis, in keeping with the warmer conditions in th is larger sister reservoir below Midmar. Overall, the species switches are intelligible largely as integrated manifestations of contrasting fecundity, temperature-dependent development, seasonality attributes a nd competitive ability, and parasite susceptibility of these copepods in habitats which tend to be marginal, especially for T. spectabilis i n Midmar. Equiproportional development is apparent in these taxa. The implications of this apparently general feature to the estimation of c opepod production is considered briefly with particular reference to w arm and tropical waters.