Coexistence of similar zooplankton species by differential adaptation to reproduction and escape in an environment with fluctuating food and enemy densities : III. Laboratory experiments.

(1) Daphnia hyalina (H) and D. cucullata (C) have coexisted for at least 50 years in the prealpine Klostersee, Federal Republic of Germany. On the basis of field data it was hypothesized that coexistence is facilitated by a compensatory mechanism: H would be a better reproducer but worse escaper than C. This would produce an advantage for H whenever reproduction is higher than mortality due to predation. C would dominate when mortality outweighs reproduction. To test this hypothesis, a number of experiments on growth, reproduction, mortality by fish predation, and competition were performed. (2) At 15° C, equivalent developmental stages were larger in H than in C. H grew faster, its growth slowed down later, and it lived longer (Fig. 2). Brood intervals were slightly shorter and both the number of eggs per brood and the total number of broods were much greater in H than in C (Fig. 3). On the other hand, generation time (time from birth to the release of the first clutch of eggs) was longer in H than in C (Figs. 2 and 4). (3) Intrinsic rates of natural increase were calculated from the growth data of the individuals by the method of Edmondson (1968). If both species are allowed to grow and reproduce to their maximal sizes, H is about 20% better than C. The major contribution to population growth comes from the first three to four broods. By artificially modifying the data on (a) the maximal age of the mothers, (b) the maximal size of the mothers, (c) the brood size, and (d) the generation time, it can be shown that the maximal size of the mothers has the greatest impact on population growth, and accounts for the greatest differences between the growth rates of both species. Varying the brood size has the smallest effect (Figs. 5 and 6). (4) Predation experiments with minnows (Phoxinus laevis) at 20° C and 500 Lux gave the following results: (a) Starting with unexperienced fish, predation increased during the first weeks of experimentation. After about 10 experiments the fish showed no further trend of improvement (Fig. 7).-(b) The fish preyed over the whole range of prey sizes (500-1500 μ body length) but large prey types were preferred (Fig. 8). Since H is larger than C, more H were eaten than C. When both species were simultaneously exposed to predation, the relation between prey size (body length L) and predation rate m was curvilinear and best described by the equation m∼L2.5-3.5 (Fig. 9).-(c) All size classes of H were eaten faster than equallysized C (Fig. 8). Thus H had a double predation disadvantage. (5) To test the compensatory functions of differential growth and selective predation, competition experiments without decimation, with unselective decimation (by the experimenter), and with selective predation (by fish) were performed at 20° C. (a) Without predation (food as limiting resource) and with unselective decimation (near-exponential growth, food not limiting), H outcompeted C very fast (Fig. 10, curves 1 and 2). This was mainly due to a marked depression of the growth rate of C (Table 3).-(b) Selective predation was a powerful antagonist of differential reproduction (Fig. 11). Selective predation slowed down the displacement of C and there was a tendency of stabilization at 80% H (Fig. 10, curves 3-5).-(c) With selective predation, the growth rates of both species were depressed, probably because predation, the growth rates of both species were depressed, probably because larger and egg-bearing individuals were preferentially eliminated (Table 3).-(d) The slower displacement of C in the face of predation had two causes, first, a greater mortality of H by selective predation, and second, a stronger decrease of the growth rate of H, probably due to the selective elimination of the largest reproducing individuals (Table 3).-(e) There was a marked and significant decrease of log r H/r C (growth advantage of H) during the course of the experiments. This could account for the observed trend toward stabilization. (6) The relevance of the experiments for the interpretation of field data and evolutionary aspects of coexistence are discussed.