Showing papers on "Undergrowth published in 1982"

Population Variability and Extinction in the Avifauna of a Tropical Land Bridge Island

Abstract: Newly created oceanic islands slowly accumulate species. In contrast, habitat islands created by rising water level or by human reduction of the area of a habitat (for example, by deforestation) support a full complement of local species at their formation. With time these islands lose species, a phenomenon called relaxation (Diamond 1972, Wilcox 1980). Several explanations may account for relaxation in island relicts. (1) As island size decreases, diversity of habitats present decreases. Species dependent entirely or in part on lost habitats disappear ("habitat diversity hypothesis;" Able and Connor 1979, Wilcox 1980). (2) Islands passively intercept dispersers in proportion to their area. As island size declines, recolonization probabilities decline. Thus, species lost from a small land bridge island are less likely to recolonize ("disperser intercept hypothesis;" Able and Connor 1979, Martin 1980, Wilcox 1980). (3) Smaller islands have more species with low population sizes, a condition which predisposes such species for extinction from random causes ("rarity hypothesis"; Willis 1974, Terborgh and Winter 1980). Corollaries of the "rarity hypothesis" relate bird size and ecological specialization to extinction probability (Willis 1974). (4) On theoretical grounds, species unable to maintain stable populations in the face of environmental vagaries (species with high "population variability") may be prone to local extinction (Leigh 1975, 1981). Terborgh and Winter (1980) suggested from qualitative considerations that species dependent on resources likely to be variable and/or patchy in their distribution (especially frugivores and nectarivores) are prone to extinction. Tracking of those resources in space results in locally variable population sizes for such species. To my knowledge, no quantitative field data are available to test the "population variability hypothesis." To what extent do these hypotheses account for loss of species on Barro Colorado Island (BCI), Panama, a 14.8-km2 land bridge island formed when the Panama Canal was created by damming the Chagras River early in the 20th century? I have used two approaches to explore that question. First, I have examined in detail the avifauna of BCI and its presumed source fauna in the adjacent mainland forest of the Pipeline Road region of Parque Nacional Soberania. Soberania is a 22 000-ha tract of lowland forest on the mainland adjacent to BCI. At its nearest point, BCI is ;200 m from a peninsular extension of that forest, although less-disturbed forest is not so close. As many as 5060 species of forest birds are missing from BCI (Karr 1982), well above earlier conclusions that 15-18 forest species are extinct on BCI (Terborgh 1974, Willis 1974, Wilson and Willis 1975, Willis and Eisenmann 1979). The approach used in earlier estimates of avian extinctions on BCI led to underestimation of extinction rates. Further, the high extinction rate seems to be due to the restricted habitat mosaic of BCI relative to that found on a similar-sized mainland area. Undergrowth and ground species and species associated with foothill forest are especially prone to extinction (Karr 1982). Second, I have conducted a long-term study of undergrowth species in mainland forest near BCI, with the objective of identifying population attributes of species prone to extinction. I report those findings here. Birds of forest undergrowth have been sampled periodically since 1968 with mist nets in an area of 2 km2 along the Pipeline Road (centered at Limbo Hunt Club [LHC]) and 9 km east of BCI. A total of 4722 captures of 114 species of birds is available in samples collected from August 1968 through March 1982. The methodology for this study was operation of 12-14 mist nets in the undergrowth of forest for periods of 3-6 d (see Karr [1979, 1981] for details of methodology). All data were collected by me except for 10 monthly samples (826 captures) from 1977 to 1978 collected by Schemske and Brokaw (1981). Net shyness sometimes occurs in areas that are regularly sampled with mist nets. However, sampling in recent years has been limited to two 3-6 d periods each year, and.there is no sign of net shyness with this low sampling frequency. Indeed, 5070W of captures are of birds banded during earlier sampling periods. In this paper I concentrate on two of the four hypotheses described above: rarity and population variability. I do not consider the disperser intercept hypothesis since it is incompatible with the empirical observation that the undergrowth forest birds discussed in detail here are very reluctant to cross water gaps. The habitat diversity hypothesis will be discussed only briefly here as it relates to population variability since it is the primary focus of another paper (Karr 1982). I assume that the relative abundances of species today in the vicinity of Limbo Hunt Club are similar to those of the bird community of BCI at the time BCI became an island.

Early Succession Following Clearcutting of Aspen Communities in Northern Utah.

Abstract: Changes in aspen reproduction and undergrowth production and composition were recorded over a 3-year period following clearcutting. Aspen suckers increased from 2,300 per hectare prior to cutting to a maximum of 44,000 per hectare the second post-cut year, and dropped to approximately 25,000 per hectare by the third year. Undergrowth production on the cut units increased from 1,013 kg/ha prior to cutting to 3,000 kg/ha after three growing seasons; production on the uncut control areas increased from 1,199 kg/ha to 1,539 kg/ha during this period. The significant increase in undergrowth is attributed to the reduction in competition from the removal of the aspen overstory. Clearcutting appeared to increase the proportion of shrubs in the undergrowth and decrease the proportion of forbs. A similarity index comparing the cut and uncut areas suggested that the greatest change in species composition occurred the first year after cutting, with a gradual return towards the precut conditions. The extensive aspen (Populus tremuloides) forests found at intermediate elevations in the intermountain Rocky Mountain West are highly valued multiple-use lands. They are noted for production of livestock forage, wildlife habitat, and scenic beauty, and they are a potentially valuable source of wood products. Through the process of natural plant succession many of these forests are becoming dominated by conifers, frequently within a single aspen generation. Such conversion concerns resource managers because it is usually accompanied by substantial reductions in forage and wildlife habitat. As a consequence, considerable interest has developed in periodically setting back the process of succession in order to maintain communities that are dominated by aspen. Prescribed burning and clearcutting are the 2 main management alternatives for halting succession to conifers and regenerating the aspen forest. Both of these methods involve drastic disturbance of the plant community. Intelligent use of either requires that we understand not only the effect of such disturbance upon the trees, but upon other components of the community as well. Burning is a viable alternative for rejuvenating an aspen forest where fuel conditions are amenable to fire and where the trees have little or no commercial value. Recently we published descriptions of early succession following prescribed burning of aspen communities (Bartos and Mueggler 1981). Clearcutting may be desirable either where burning is not feasible or where the value of the wood is sufficient to finance the cutting operation. Although the ability of western aspen to reproduce prolifically following clearcutting is well documented (Jones 1975, Schier 1976, Schier and Smith 1979), very little information is available regarding the changes in undergrowth likely to occur when the tree overstory is removed by clearcutting. This report describes aspen reproduction and changes in undergrowth composition and production over a 3-year period following Authors are range scientist and plant ecologist with the Intermountain Forest and Range Experiment Station, USDA Forest Service, located at the Forestry Sciences Laboratory, 860 N. 12 E. Logan, Utah, 84321. The authors wish to thank Dr. William A. Laycock for his help in the initial design of the study and in sampling in 1973. clearcutting aspen communities on the Wasatch National Forest in northern Utah.

The starred robin in natal, part 1: behaviour, territory and habitat

Abstract: Summary Oatley, T. B. 1982. The Starred Robin in Natal, Part 1: Behaviour, territory and habitat. Ostrich 53:135-146. Starred Robins Pogonocichla stellata were studied in forests in the Natal midlands. Aspects of behaviour, including vocalizations and the use of plumage patterns in displays are described. Unlike many other forest birds the Starred Robin has a quiet song and is seldom aggressive toward conspecifics; males nevertheless effectively defend territories in the breeding season. Such areas invariably contain at least one thicket of dense undergrowth, and range in size from 0,50 to 0,75 ha. The extent of the territory and the effective range of the song are evidently related.

Influence des sols et arbres noyes sur quelques caracteristiques chimiques de l’eau du reservoir lg2

Abstract: In order to evaluate the modifications of some chemical properties of the water of La Grande 2 reservoir after flooding, experiments were done on spruce and pine undergrowth soil, on peat bog and on spruce and pine tree. The influence of continuously aerated water was first tested; then, after oxygen consumption, the same sample was used to evaluate the changes occurring under anoxic conditions. The results indicate that at the beginning of flooding in oxygen saturated water or at the beginning of anaerobic conditions, a rapid increase in conductivity, colour, tanins, potassium and calcium occurs. These variations diminish rapidly after a few days. Generally, the release of biogenic substances is higher during continuous aeration than in anaerobic conditions. The contribution of trees to these changes is much more important than their respective undergrowth soils.

Resting site preferences of the tsetse Glossina morsitans submorsitans Newstead (Diptera: Glossinidae) in Mali.

Abstract: Resting site preferences of the tsetse Glossina morsitans submorsitans Newstead were studied in a savanna woodland and gallery forest in the Northern Guinea Savanna of Mali. Five site-types [boles, fallen logs, undergrowth (‘bushes’), branches and tree canopies] were observed. Flies alighting to rest on these site-types were trapped on tanglefoot. Observations were made in the warm rainy, warm dry and hot dry seasons. Boles were dominant as the preferred resting sites in the savanna woodland in all seasons; fallen logs were the next preferred site-type, while few flies were collected from the undergrowth, branches and tree canopies. Approximately 90% of flies on boles were collected below 1.0 m and none were found above 2.5 m. During the hot dry season in the gallery forest, the undergrowth was the most preferred site-type; unshaded undergrowth accounted for approximately 98% of all flies on the undergrowth. Fallen logs and boles were the next preferred site-types; branches and tree canopies recorded very few flies. Probable reasons for the results obtained and the importance of these results in tsetse control operations are discussed.