Vegetation Structure Parameters Research Articles

Guild Structure of a Riparian Avifauna Relative to Seasonal Cattle Grazing

The avifauna within the willow (Salix spp.) community on the Arapaho National Wildlife Refuge (NWR) was dominated (96% of all observations each year) by 11 species of passerine birds during the summers of 1980-81. Using 28 vegetation variables measured or calculated for randomly selected points and points where birds were sighted, we assigned the species to 3 distinct response guilds relative to historical patterns of seasonal grazing. A eurytopic response guild (habitat generalists) included yellow warblers (Dendroica petechia) (YEWA), savannah sparrows (Passerculus sandwichensis) (SASP), and song sparrows (Melospiza melodia) (SOSP). A stenotopic response guild (habitat specialists) included willow flycatchers (Empidonax traillii) (WIFL). Lincoln's sparrows (Melospiza lincolnii) (LISP), and white-crowned sparrows (Zonotrichia leucophrys) (WCSP). The intermediate, mesotopic response guild included American robins (Turdus migratorius) (AMRO), red-winged blackbirds (Agelaius phoeniceus) (RWBL), and brown-headed cowbirds (Molothrus ater) (BHCO). Population densities of the eurytopic response guild differed little between healthy (historically winter-grazed) and decadent (historically summer-grazed) willow communities within a year. Densities of species in the mesotopic response guild differed more dramatically, and stenotopic responseguild species were absent or accidental in decadent willows. Information on habitat use patterns of the individual species between years supported the definition of response guilds; vegetation structure was most variable in habitats of eurytopic species and least variable in habitats of stenotopic species. Comparisons between used and available vegetation features indicated that species in the stenotopic response guild used locations that differed from random on the basis of bush spacing. We hypothesize that the response-guild structure primarily reflects the impact of cattle upon the horizontal patterning of the vegetative community. J. WILDL. MANAGE. 52(2):280-290 Grazing by domestic cattle is the most common historical use of native rangelands in the western United States (Busby 1979). Ecologists have described the impacts of grazing upon breeding bird communities by comparing avian populations on adjacent sites that have been subjected to different levels of grazing (Kantrud 1981, Mosconi and Hutto 1982), on the same site before and after exclusion of grazing (Duff 1979), or both (Bock et al. 1984). Those studies generally have described patterns of species presence or abundance relative to grazing impacts upon vegetation. Birds generally respond to changes in vegetation structure as a consequence of grazing rather than to the presence of cattle per se (Bock and Webb 1984). However, avian responses to vegetative changes are neither easily defined nor consistent among sites (Wiens and Dyer 1975). Ryder (1980) reviewed studies of grazing impacts upon avian communities and categorized species as either increasers or decreasers (in abundance) with the introduction of cattle into their habitats. Studies of grazing impacts, however, generally have not elaborated on the nature of avian behavior or population responses, especially relative to all bird species within a community. Previous studies in northern Colorado have shown that seasonal grazing practices over a 75100-year period may alter the horizontal and vertical structure of a shrub-willow community as potential habitats for birds (Knopf and Cannon 1982). Our objectives were to define avian guilds within the shrub-willow community which respond similarly to structural vegetation parameters, statistically test the validity of the response-guild structure with information on species densities and habitat use, and identify vegetation parameters that cattle alter that appear important to sensitive bird species. We thank E. C. Patten for his interest and local support. Field assistance was provided by B. Bergstrom, K. Bergstrom, J. F. Ellis, E. A.

Modes of colonization of an heterogenous alluvial area on the edge of the garonne river by Phalaris arundinacea L

AbstractThe modes of colonization of a heterogenous alluvial area by Phalaris arundinacea L., a pioneer species, were studied on the edge of the Garonne river downstream of Toulouse (S.W. France). The heterogeneity of the location is essentially represented (1) by the type of soil: sands and gravels on the banks of the Garonne and silts in the hollows at a certain distance from the river; (2) by the period of submersion: zones flooded only during short periods and zones submerged during six months of the year; and (3) by the fertility of the substratum.The response of the Phalaris arundinacea population were measured using vegetation structure parameters and descriptions of the development cycle. Water plays the most important part in the identification of space colonization modes. Two types of individual plants appear. Those living in submerged environments for a very short time are characterized by both a high biomass and sexual reproduction energy allowance, and by unimodal growth. Those living in environments flooded over long periods are characterized by both a low biomass and sexual reproduction energy allowance, and by a bimodal growth in two successive generations: a submersion generation and a low‐water generation. Specific competition reduces the differences between the two types of individual plants from the point of view of population cover and of the structure and density of the individual plants. Phalaris arundinacea L. is shown to have a remarkable capacity for colonizing sites with distinctive ecological characteristics.