Regulation of Freshwater Community Structure at Multiple Intensities of Dragonfly Predation

We examined the role played by predaceous dragonfly nymphs, Celithemis fasciata (Odonata: Libellulidae), in the regulation of the community structure of a benthic macroinvertebrate assemblage in Par Pond, an 1100-ha reservoir in South Carolina. Effects of predation intensity on species richness, evenness, and density were evaluated by adding zero, two, four, and eight large dragonfly nymphs (antepenultimate and penultimate instars) to previously sieved (0.85-mm mesh) bottom sediment containing benthic macroinvertebrates. Predator and prey assemblages were then placed in individualfieldmicrocosms that consisted of polyethylene trays surrounded by underwater screens (mesh <2 mm diagonally) and suspended 15 cm belowfloatingplatforms. Twelve replicates of each treatment level were run during each of three 6-wk experimental periods: April-May 1980, August-October 1980, and January-February 1981. In addition, colonization of microcosms by invertebrates was quantified, and samples from natural, unenclosed benthic fauna were collected seasonally along a transect for comparison with experimental assemblages. We tested whether predators enhanced, depressed, both increased and decreased, or had no effect on the complexity of the community structure. The dual effect of predation on community structure is predicted by Connell's "intermediate disturbance hypothesis." Results showed that dragonfly nymphs can significantly influence the structure of the benthic community. However, the results did not show that invertebrate predation is the sole or even the primary regulator of community structure. Species richness was significantly greater at intermediate treatment levels (thus supporting Connell's general hypothesis), but the increase was not great (a range of -10%). The mechanisms by which species richness is maximized at intermediate intensities of predation are not entirely evident, but are probably a combination of prey refuges and nonselective predation with patch switching. In contrast, species evenness, as measured by equitability and by Simpson's index, was greatest at the highest predation level (which does not support the intermediate disturbance hypothesis). Dragonflies appeared to exert a greater influence on prey density than on community diversity

Putting New Life Into Pastures

Many of the old permanent bluegrass pastures in Iowa can almost be made “ to feed two cattle where one fed before.” We found this out from a study of the problem at Mt. Pleasant during the 6 years, 1933 to 1938. In the past 3 years some 250 Iowa farmers in 45 counties have cooperated in demonstrating the possibilities of increasing the returns from permanent pastures by introducing clover into old grass sods and from applying lime and phosphate fertilizer in addition to reseeding

Photonic realization of topologically protected bound states in domain-wall waveguide arrays

We present an analytical theory of topologically protected photonic states for the two-dimensional Maxwell equations for a class of continuous periodic dielectric structures, modulated by a domain wall. We further numerically confirm the applicability of this theory for three-dimensional structures.Comment: 6 pages, 5 figures. To appear in the Phys. Rev.