Effects of fire on landscape heterogeneity in Yellowstone National Park,Wyoming

Abstract. A map of burn severity resulting from the 1988 fires that occurred in Yellowstone National Park (YNP) was derived from Landsat Thematic Mapper (TM) imagery and used to assess the isolation of burned areas, the heterogeneity that resulted from fires burning under moderate and severe burning conditions, and the relationship between heterogeneity and fire size. The majority of severely burned areas were within close proximity (50 to 200 m) to unburned or lightly burned areas, suggesting that few burned sites are very far from potential sources of propagules for plant reestablishment. Fires that occurred under moderate burning conditions early during the 1988 fire season resulted in a lower proportion of crown fire than fires that occurred under severe burning conditions later in the season. Increased dominance and contagion of burn severity classes and a decrease in the edge: area ratio for later fires indicated a slightly more aggregated burn pattern compared to early fires. The proportion of burned area in different burn severity classes varied as a function of daily fire size. When daily area burned was relatively low, the proportion of burned area in each burn severity class varied widely. When daily burned area exceeded 1250 ha, the burned area contained about 50 % crown fire, 30 % severe surface burn, and 20 % light surface burn. Understanding the effect of fire on landscape heterogeneity is important because the kinds, amounts, and spatial distribution of burned and unburned areas may influence the reestablishment of plant species on burned sites.     

On the ecology of Colurellidae (Rotifera)

A material consisting of 45 colurellid rotifers from diverse waters in south and central Sweden was analyzed to reveal their relationships to their substrate. Periphytic substrates were generally preferred. Most species are obviously mobile and more or less euryecious, but a few show a predilection for bog environments. Some colurellids are euryhaline, but when inhabiting fresh water they prefer rivers and other environments with a high oxygen concentration, probably because of problems with osmoregulation.     

Nemertea inhabiting the Haploops (Amphipoda) community of the northern Øresund with special reference to the biology of Nipponnemertes pulcher (Hoplonemertea)

The densities of nemerteans and associated fauna on a soft-bottom sampling station (27–30 m deep) in the Øresund were determined from 47 cores (each 135 cm² in cross-section; 20 + cm deep) collected from September to December 1989; these data were compared with 14 cores taken from the same location in December 1982. Nine species of nemerteans were identified from cores and dredge samples: Palaeonemertea — Callinera-like sp.; Heteronemertea — Cerebratulus fuscus, C. marginatus, Lineus bilineatus, Micrura fasciolata, M. purpurea; Hoplonemertea — Amphiporus bioculatus, A. dissimulans, Nipponnemertes pulcher. Mean numbers of heteronemerteans were 32 and 10 m^?2 in 1982 and 1989, respectively, and hoplonemerteans were 90 and 71 m^?2 , respectively. Only one palaeonemertean was collected during both years. Mean densities of the dominant species, N. pulcher, were similar for the two years, 74 and 68 m^?2. The dominant groups of macrofauna (n m^?2) in 1989 were ostracods (1028), amphipods (618), polychaetes (514), and ophiuroids (449). Amphipods (>90% Haploops spp.) and polychaetes (at least 30 spp.) are the major potential prey for hoplonemerteans and heteronemerteans, respectively. Laboratory feeding experiments with N. pulcher revealed that it consumed amphipods (Haploops tenuis and H. tubicola) at a rate of 2.6 worm^?1 d^?1 during the first 12 hours, but after 36 hours and beyond the rate was maintained at approximately 0.2 worm^?1 d^?1. Beyond 12 hours this nemertean showed a tendancy to only partially evacuate its prey. It was demonstrated experimentally that N. pulcher has a supply of toxin capable of killing six amphipods in approximately one hour. Limited tests showed that N. pulcher fed on the cumacean Diastylis tumida, but not on the amphipod Maera loveni or the ostracod Philomedes globosus, and that Amphiporus dissimulans readily attacked Haploops spp., but not Maera or Philomedes. Although the results of laboratory experiments are tentative, they do suggest that suctorial hoplonemerteans can exert a potentially significant effect on benthic communities. Employing seven species of polychaetes as prey for Cerebratulus fuscus and Micrura fasciolata, only the latter responded positively to one of them, Glycera alba. The hermit crab Pagurus bernhardus violently rejected N. pulcher in all feeding trials.     

Winter feeding ecology of cod (Gadus morhua) in a fjord of southern Norway

We investigated the winter feeding ecology of cod ( Gadus morhua ) in the Risørfjord and Flødevigen areas on the Skagerrak coast, southern Norway. Diets from the ice-covered Risørfjord were compared with diets from the more coastal Flødevigen area. In the Risørfjord area the diet featured numerically both decapods (56.0%) and fish (27.8%), but fish dominated by mass (75.1%). The most numerous dietary items from the Flødevigen area were decapods (48.1%) and isopods (30.6%), although fish still made up the bulk of the diet's mass (52.6%). Diets at Flødevigen shifted from winter to spring, as polychaetes became important numerically (67.8%) and also contributed substantially by weight (53.6%). Since cod feed opportunistically, differences between areas and seasons probably reflected differences in prey diversity and abundance. Prey size variability increased with increasing predator length, but maximum prey size relative to predator length was constant at about 9%. Prey fish increased in both length and numbers with the length of cod, particularly in the Risørfjord area, where fish were more important in the diet than at Flødevigen. However, the majority of fish consumed in both areas were small gobiids. Based on growth patterns observed in otoliths, with opaque zones (indicating faster growth) formed during winter, the quality and quantity of the winter diet may determine annual growth rates of cod on the Skagerrak coast. Fish and decapods constituted important, high energy food sources, and there was little indication that cod were food limited during winter in this area.     

EVOLUTION OF HOST-PARASITE DIVERSITY

Hosts and parasites often have extensive genetic diversity for resistance and virulence (host range). Qualitative diversity occurs when the success of attack is an all-or-nothing response that varies according to the genotypes of the host and parasite. Quantitative diversity occurs when the success of attack is a graded response that depends on additive genetic variation in the host and parasite. Community diversity occurs when parasites vary in the success with which they can attack different host species, leading to a mixture of specialists and generalists. I developed a series of models that classify components of host-parasite interactions according to whether they cause stabilizing or disruptive selection for resistance and virulence. Stabilizing selection reduces diversity by favoring a single optimal phenotype. Disruptive selection creates diversity by favoring a mixture of widely separated phenotypes. The evolution of maximal resistance and virulence are opposed by one of three forces: metabolic costs, frequency dependence, or negative genetic correlations among beneficial traits. The models predict that qualitatively inherited resistance and virulence traits typically cause greater diversity than quantitatively inherited traits. However, each natural system is composed of many stabilizing factors that reduce diversity and disruptive factors that promote diversity. I advocate a style of modeling in which families of related assumptions are compared by their equilibrium properties, and general conclusions from equilibrium properties are tested by complete dynamical analysis. The comparison among models highlights the need for empirical studies that compare levels of diversity among related host-parasite systems.     

On choice of substrate and habitat in bdelzoid rotifers

Information on the distributions of 77 bdelloid rotifers from diverse waters in south and central Sweden was analyzed to reveal their relationships to substrate and habitat. Only two species were reported from plankton, both in low frequency. An artificial substrate, white cotton, was utilized only by few bdelloids, mainly by species with eyes. Only one species occurred at higher frequency on the saprobic substrate Sphaerotilus. Most bdelloids prefer environments rich in oxygen, but some species may be found in soft-bottom sediments. Several species dwell preferentially in diverse habitats of bogs. Some bdelloids have an exceptionally broad ecological range, but even for these is it possible to distinguish a pattern of preference and avoidance.     

Diatom ecology in the phyllosphere of the common duckweed (Lemna minor L.)

Epiphyton associated with thick, floating mats of the common duckweed (Lemna minor L.) was studied at four sites in western Canada between 1985 and 1988. Maximum epiphyton abundance generally occurred in spring as biomass of the duckweed mat was increasing. Epiphytic biomass was low during summer and increased at some sites in autumn with mat decomposition. The community was composed mostly of diatoms and, during summer, photosynthetic bacteria. Species richness of the diatom flora was low, suggesting that duckweed mats are environments to which few species are adapted. Photosynthesis - irradiance curves indicated that duckweed epiphyton was not adapted to low light levels that occurred in the mat (< 1 % of ambient), suggesting they may survive via other means of nutrition. The mat phyllosphere was also characterized by wide spatial and temporal variation in temperature, and sharp vertical profiles of dissolved oxygen and nutrients.     

Quaternary and recent Lamprothamnium groves (Charophyta) from Argentina

Only recently have extant Lamprothamnium species been reported from the American continent. L. succinctum (A. Br. in Asch.) R. D. W. was found in Lago Titicaca, Bolivia and L. haesseliae Dont. en Laguna Luro and Laguna La Salada, Argentina.Fossil gyrogonites of L. succinctum and L. haesseliae, however, are here reported to be widely distributed in Quaternary sediments of Argentina, associated with other charophytes, ostracods and foraminiferans. Localities include Laguna del Siasgo, Laguna Salada Grande, Laguna Mar Chiquita, Laguna La Amarga and Salina del Bebedero. Lamprothamnium is a genus that prefers shallow, alkaline, hyposaline to mesosaline environments. Analysis of the characteristics of Laguna La Salada contributes information on the factors that influence the distribution of the genus.The morphology characteristics of the gyrogonites and oospores of L. haesseliae and the gyrogonites of L. succinctum are described. The use of Lamprothamnium as a biomarker and its application in the reconstruction of Quaternary saline environments are also discussed.     

The dynamics and retention of phosphorus in lentic and lotic patches of two river-lake systems

Two river-lake systems in a mosaic, lowland postglacial landscape (Masurian and Suwalskie Lakelands, northeastern Poland) of different length and flow were studied. The concentration of total and dissolved phosphorus (in water and in seston) and the retention of these two forms of P were analysed at several hydrological occasions in lakes of various morphometry and trophy, and in several stream fragments. The concentration and retention is equally highly variable in lake (lentic patches) and stream fragments (lotic patches) of the system. The polluted stream fragments at all occassions as well as the lakes in summer, are mainly exporting phoshorus. The patches which act as sources occur alternatively with patches which are P-sinks. This makes the whole system more or less balanced with respect to P movement in the landscape.