dynamics of dissolved nutrient patches. We have yet to identify the frequency of occurrence and magnitude spectra of such patches in specific microbial food webs. They undoubtedly represent interesting ecological niches for bacteria, and they will also contribute much to our understanding of the flow of nutrients and energy in aquatic ecosystems if they prove to be major pathways.References and notes 1. J. C. Goldman, Bull. Mar. Sci. 35, 462 (1984). 2. E. M. Purcell, Am. J. Phys. 45, 3 (1977). 3. G. A. Jackson, Limnol. Oceanogr. 32, 1253(1987. 4. W. Bell and R. Mitchell, Biol. Bull. 143, 265 (1972 ѨC Ѩt where C is concentration, K D is a dissociation constant (100 M), and ␣ is a sensitivity constant (1000 s). Simulations were performed by allowing cells to move against a concentration field (3). Cells were moved a distance determined by their swimming velocity and heading angle from a physical location whose attractant concentration was C 1 to another location of concentration C 2 , within each simulated time step dt. The change in mean run duration could thus be calculated asNegative changes were ignored. Run durations are Poisson distributed (12). The probability of tumbling after each time step is dt/( ϩ ⌬). The desired Poisson process was implemented with a random generator to decide whether or not to tumble after each time step. Tumbles were simulated as reversals, and a Brownian rotation of 1 rad s Ϫ1 was introduced.Swimming velocities and mean run durations were acquired from tracks of live cells. 23. The steady-state concentration field C(r) was calculated as C͑r͒ ϭ E4D ͱr where oxygen exudation rate E ϭ 0.25 fmol s Ϫ1 was estimated from light-saturated photosynthesis of a cell 4 m in diameter. The diffusion coefficient D ϭ 10 Ϫ5 cm 2 s Ϫ1 . The inverse square-root dependency on the distance r from the source was introduced instead of inverse proportionality (3) to more closely approximate its shape in the flat chamber. 24. The concentration field C(r,t) of a spreading patch was calculated aswhere M is the amount of matter released (1 pmol The regular cyclic fluctuations in vertebrate numbers have intrigued scientists for more than 70 years, and yet the cause of such cycles has not been clearly demonstrated. Red grouse populations in Britain exhibit cyclic fluctuations in abundance, with periodic crashes. The hypothesis that these fluctuations are caused by the impact of a nematode parasite on host fecundity was tested by experimentally reducing parasite burdens in grouse. Treatment of the grouse population prevented population crashes, demonstrating that parasites were the cause of the cyclic fluctuations.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. British Ecological Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of Animal Ecology. Summary1. An extensive post-mortem survey of grouse revealed that birds killed by predators in spring and summer had significantly greater burdens of the caecal nematode Trichostrongylus tenuis than grouse shot during the autumn. Furthermore, grouse that appeared to have died through the effects of parasites carried greater worm burdens than grouse killed by predators. 2. The proportion of grouse with high levels of parasite infection increased with the intensity of predator control as measured indirectly through keeper density. These two empirical observations suggest that predators selectively prey on heavily infected grouse. The interactions between parasites and predators were examined experimentally by reducing the worm burdens of female grouse with an oral anthelmintic. Nests of treated and untreated females were subsequently located either by research workers flushing the incubating female or by dogs trained to locate birds by scent.The dogs found significantly fewer of the treated than control birds, suggesting that female grouse with large parasite burdens emit more scent and are more vulnerable to mammalian predation. 4. A modified mathematical model of the grouse-nematode system is described which incorporates the effects of both random and selective predation of heavily parasitized grouse. An analysis of the model illustrates the importance of interactions between grouse, parasites and predators in determining the relative densities of each. In particular, when predators selectively remove heavily parasitized individuals, then low levels of predation can lead to increases in the size of the host (or prey) population.