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This study evaluates the hypothesis that biological grazing refuges have an important role in plant-grazer interactions of grasslands with a long history of grazing. We assessed the hypothesis that clumps of the spiny cactus Opuntia polyacantha provide biological refuges from cattle grazing, affecting cover and seedhead production of associated vascular plants in the shortgrass steppe of the North America. The study was based on sampling inside and outside Opuntia clumps in eight long-term moderately grazed pastures established 60 yr ago and their respective ungrazed controls. Opuntia clumps provided a refuge for seedhead production of the dominant grass (Bouteloua gracilis) and for cover and seedhead production...
Annual differences in reproductive effort by female long-billed curlews (Numenius americanus) were clearly associated with changes in environmental conditions. During pre-laying in 1977 and 1978, long-billed curlews foraged almost exclusively on their breeding territories where vegetation height was generally short. Females did little flying prior to laying, and heavier females with relatively greater wing-disc loading laid larger egg and clutch volumes earlier in the season than did lighter females with lesser wing-disc loading. Tall, dense residual vegetation covered most upland breeding territories during pre-laying in 1979; and until grazing livestock broke down or removed this standing-dead vegetation, long-billed...
Categories: Publication;
Types: Citation,
Journal Citation;
Tags: Blackwell Publishing on behalf of Nordic Society Oikos,
Oikos
Experiments were conducted in two Rocky Mountain streams (Gunnison County, Colorado) to determine the context within which predatory stonefly larvae locally depress their prey populations. In feeding trials where no migration of prey (Baetis bicaudatus, Ephemeroptera) was allowed from flow-through boxes containing natural substrates, Megarcys signata and Kogotus modestus (Perlodidae) significantly depressed prey densities over 14 and 38 h, respectively. However, increasing predator density had no apparent effect on per capita predation rate. In 1984 enclosure experiments allowing prey but not predator migration, Kogotus significantly reduced total prey density and that of two relatively sedentary prey species (Cinygmula...
Categories: Publication;
Types: Citation,
Journal Citation;
Tags: Blackwell Publishing on behalf of Nordic Society Oikos,
Oikos
Theory predicts that habitat fragmentation and varying corridor length and width will affect animal populations in adjoining habitat patches due to varying migration rates. Previous work on the moss/microarthropod microcosm showed that connecting moss patches with moss corridors maintained species richness and individual species abundance. By contrast, in this study there was little evidence for differences in species richness between landscapes of varying connectivity and corridor length and width. The ? diversity, the cumulative species richness of entire connected systems, followed the same pattern. Similar non-significant results were obtained for species abundance. Contrary to a previous study, I found no evidence...
Mosses are important ecosystem engineers in mires. Their existence may be threatened directly or indirectly by anthropogenic drying, which further leads to shading and changed competition conditions via increased arboreal plant cover. Yet, some species are able to acclimate to the changing habitat, while some give way to new colonizers. In the shaded conditions, acclimation or adaptation to low light levels is likely to be a winning strategy to survive. We studied the light responses of photosynthesis and photosynthetic pigment concentrations in mosses from an open mire and its shaded, i.e. drained and forested counterpart. Against our expectations, the Sphagnum species found only in the open habitat had lower photosynthetic...
Endophytic fungi, especially asexual, systemic endophytes in grasses, are generally viewed as plant mutualists, mainly through the action of mycotoxins, such as alkaloids in infected grasses, which protect the host plant from herbivores. Most of the evidence for the defensive mutualism concept is derived from studies of agronomic grass cultivars, which may be atypical of many endophyte-host interactions. I argue that endophytes in native plants, even asexual, seed-borne ones, rarely act as defensive mutualists. In contrast to domesticated grasses where infection frequencies of highly toxic plants often approach 100%, natural grass populations are usually mosaics of uninfected and infected plants. The latter, however,...
Studying germination in the native and non-native range of a species can provide unique insights into processes of range expansion and adaptation; however, traits related to germination have rarely been compared between native and non-native populations. In a series of common garden experiments, we explored whether differences in the seasonality of precipitation, specifically, summer drought vs summer rain, and the amount and variation of annual and seasonal precipitation affect the germination responses of populations of an annual ruderal plant, Centaurea solstitialis, from its native range and from two non-native regions with different climates. We found that seeds from all native populations, irrespective of...
Bromus tectorum is a winter annual grass that has become extensively naturalized in western North America. Its seeds are usually at least conditionally dormant at dispersal and lose dormancy through dry afterripening. Germination response to temperature for recently harvested seeds and rate of change in germination response during afterripening were examined for collections from 21 western North American populations representing a wide array of habitats. Analysis of variance showed highly significant among-population differences in germination response variables. Principal components analysis of 20 germination variables revealed groups of populations that could be characterized by distinct response syndromes. Degree...
Contemporary evolution may explain the success of some exotic plant invasions. However, the evolutionary response of recipient native plant populations to exotic invasion has received relatively little attention. Because plant populations are genetically variable, contemporary evolution may also occur in native populations following entry of invasive species. Previously, we documented molecular differences in native populations; here we extend these studies to evaluate growth of native species in a common garden experiment. We seek to determine if three populations of two native grass species (Hesperostipa comata and Sporobolus airoides) demonstrate evidence of contemporary evolution in response to invasion by Acroptilon...
We examined the potential effects of spatial heterogeneity and its development on the distribution, abundance, and functioning of nitrogen fixing and non-fixing components of a model ecosystem. CAECO, a spatially explicit model individual based approach, simulated the interactions between nitrogen fluxes and plant species community dynamics. Self-organized spatial patterns of nitrogen concentrations and plant occupancy were observed as the system approached an apparently meta-stable state. Nitrogen limitation was tested using chronic and gradient nitrogen amendments to the landscape. The dynamic arrangement of ecosystem components was sufficient to maintain indefinite nitrogen limitation at a local scale. However,...
Anthropogenic influences are simultaneously perturbing multiple biochemical cycles, as well as climate, in the biosphere (Falkowski et al. 2000). To address such effects, an explicitly multi-variate approach is needed. Furthermore, to understand the full gamut of ramifications of such changes for the functioning of ecosystems and for biodiversity in the long- and short-term, a perspective that connects fluxes of chemical elements to underlying physiological and genomic mechanisms and to evolutionary processes is needed. One such perspective is offered by biological stoichiometry, the study of the balance of energy and multiple chemical elements in living systems (Elser and Hamilton 2007), a perspective that has...
Although the link between the nitrogen (N): phosphorus (P) stoichiometry of biota and availability has received considerable attention in aquatic systems, there has been relatively little effort to compare the elemental composition of biota and supply in terrestrial habitats. In this study, I explored the effects of a prominent topo-edaphic gradient, from dry hilltop to wet slope-base, and native ungulates on N and P of soils, plants, and rates of in situ net mineralization in grasslands of Yellowstone National Park. Nitrogen and P measurements were made May?September, 2000, in paired, grazed and 38?42 year fenced, ungrazed grassland at five topographically variable sites. Similar to findings from other grassland...
Categories: Publication;
Types: Citation,
Journal Citation;
Tags: Oikos,
decomposition,
microbe,
mineralization: nitrogen,
rhizosphere,
Nitrogen-fixers can contribute significant amounts of nitrogen (N) and impact ecosystem functioning in diverse aquatic and terrestrial ecosystems. What determines N-fixer abundance still remains poorly understood. Here we experimentally investigate major environmental controls on the abundance of N-fixers: nitrogen to phosphorus (N:P) ratio and light. We grew a N-fixer, cyanobacterium Anabaena flos-aquae, in a multispecies community of freshwater phytoplankton in replicated factorial design treatments with two N:P ratios and two light levels. We show that low N:P ratios promote the dominance of the N-fixer in the community, but only under high light. Under low light, N:P ratio did not have a significant effect on...
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