Zum Adaptivwert der zyklomorphen Dornenbildung von Brachionus calyciflorus Pallas (Rotatoria)

Interactive effects of algal level and predator density (Asplanchna sieboldi) on the life-history strategy and morphology of Brachionus calyciflorus

Predation plays an important role in mediating the coexistence and the community structure of rotifera. In response to predation stresses, prey rotifers develop morphological defenses and change their life-history strategy for the reallocation of energy investment. Yet, how these changes respond to different total energy ingestion remains largely unknown. In the present study, we investigated the life-table demographic, population growth and morphological parameters of Brachionus calyciflorus in response to various densities of Asplanchna sieboldi at two food levels (1.0  ×  10⁶ and 2.0  ×  10⁶ cells per ml of Scenedesmus obliquus). To resist predators, B. calyciflorus developed long postero-lateral spines. The life-table demographic results showed that predation only significantly shortened the generation time of B. calyciflorus at the algal level of 2.0  ×  10⁶ cells per ml, but not affect other demographic parameters. The population growth tests revealed that predator density, algal level and their interactions all significantly influenced the population growth rate of prey rotifers. At 1.0  ×  10⁶ cells per ml of S. obliquus, treatments with two, four, and eight A. sieboldi individuals per 50 ml significantly reduced the population growth rate of B. calyciflorus. In contrast, the population growth rate was not affected in response to two A. sieboldi individuals per 50 ml at 2.0  ×  10⁶ cells per ml of S. obliquus. These results suggested that algal level probably affected the inhibitory effects of predation on the population growth of prey rotifers, and the underlying mechanisms should be further investigated.

The effects of refuges on predator-prey interactions: a reconsideration

Prey refuges are widely believed to prevent prey extinction and damp predator-prey oscillations. A review of the empirical evidence suggests that refuges are indeed capable of playing the former role. But the conditions under which they do so are not understood, nor is there any solid evidence for an effect on population fluctuations. The intuitive view that refuges act to stabilize equilibria and damp predator-prey oscillations is based in several theoretical studies of extremely simple models. Using a more realistic model, I show that several kinds of refuges can exert a locally destabilizing effect and create stable, large-amplitude oscillations which would damp out if no refuge was present. This finding contrasts sharply with the usual view. I argue that current evidence is tol weak, and the range of theoretically possible effects is too broad, to justify any simple characterization of refuge effects in nature. Manipulative empirical studies are an important first step toward correcting this situation, and I discuss some important factors to consider in their design.