Trends In Population Density Research Articles

Land degradation and livestock production in Zimbabwe's communal areas

AbstractThis paper considers evidence of land degradation and its impact on livestock production systems in the communal areas of southern Zimbabwe. Various measures of rangeland degradation, using both primary and secondary production indicators, are examined. An apparent contradiction emerges between the high observed levels of soil erosion and the lack of impact on livestock production, as measured by population density trends, cattle production parameters, cumulative stocking levels and extraction rates. Farmers' observations of land degradation processes support the quantitative data. A possible resolution of the contradiction lies in an assessment of the spatial and temporal dimensions of degradation patterns; this points to the fact that current erosion is concentrated in areas that are not key grazing resources for cattle. The analysis suggests an agenda for degradation research that concentrates on evaluating the time span over which economic impacts of soil erosion are felt and assessing the spatial pattern of soil loss, with a special focus on key grazing resource patches.

Magpie Pica pica and Songbird Populations. Retrospective Investigation of Trends in Population Density and Breeding Success

Magpie Pica pica and Songbird Populations. Retrospective Investigation of Trends in Population Density and Breeding Success

DENSITY, PARASITISM, AND DISEASE INCIDENCE OF LARVAE OF THE BERTHA ARMYWORM, MAMESTRA CONFIGURATA WALKER (LEPIDOPTERA: NOCTUIDAE), IN MANITOBA, 1973–1986’

AbstractPopulations of larvae of the bertha armyworm, Mamestra configurata Wlk., in four physiographic regions of Manitoba showed similar trends over time: a decline from the outbreak of 1971–1972 to very low densities in 1975–1977, an increase to a peak during the years 1979–1981, and a subsequent decline. During the period of peak larval populations, brief (1 or 2 years) outbreaks [at least some fields with > 20 larvae per square metre) occurred at five locations in two regions, the Swan River Plain and the Valley River Plain, but not in the Western Uplands or the Manitoba Lowlands. In the first two regions, larval densities rose rapidly (from < 1.6 to > 13.8/m2) in 1 year. Although the general trend of population density was similar, there were differences in density among and within regions, and in the timing, severity, and duration of peak populations. Two parasitoids (Banchus flavescens Cress., Athrycia cinerea (Coq.)) and two pathogens (a nuclear polyhedrosis virus (NPV) and fungi of the Entomophthorales) occurred regularly in larval populations. Of these, B. flavescens had the highest constancy among collections and may help to keep bertha armyworm populations at low densities. NPV was rarely found among larvae from low-density populations but appeared in all populations that reached outbreak levels. No single biotic agent could be associated with the population declines because of multiple parasitism and the difficulty in partitioning mortality when only a single sample could be taken. The rapid increase of bertha armyworm larvae from very low to outbreak levels in 1 year will prevent predictions of outbreaks from being based on larval densities in the preceding year.

Interspecific competition between introduced house finch populations and two associated passerine species.

House Finches (Carpodacus mexicanus), natives of western North America, have expanded their range in the eastern United States since their 1940 release in New York City. Range and the relation of House Finch population growth to the population dynamics of House Sparrows (Passer domesticus) and Purple Finches (Carpodacus purpureus) were examined, using data from the Breeding Bird Survey and the Christmas Bird Count. The House Finch population grew exponentially throughout its eastern range. Significant negative relationships in population density, relative to spatial and temporal control populations, were found between House Finches and House Sparrows in summer and winter, and between House Finches and Purple Finches in summer. Purple Finch and House Sparrow populations outside of the House Finch range appeared to have no effect on each other throughout the study. Neither changes in 74 weather variables, nor changes in forest, field or developed habitat explained the observed trends in population density. The results indicate that House Finches compete with these two species, but winter migration complicates the picture.

Short‐term forecasting of peak population density of the grain aphid (Sitobion avenae) on wheat

SUMMARYThe trends in population density of Sitobion avenae in wheat fields were analysed for a total of 32 sites over the period 1975 to 1983 in eastern England and the Netherlands. The peak population density on each field was positively correlated with the population densities at the end of ear emergence, mid‐anthesis and the end of anthesis. It was also positively correlated with the observed rates of increase of the aphids on the crop immediately before these stages. Both parameters were incorporated in multiple regressions to forecast peak population density, and data from a further nine populations monitored in 1984 was used to validate the model.The accuracy of these forecasts, based on two counts on the crop, increased from ear emergence to the end of anthesis, the forecast at mid‐anthesis of peak density being much more accurate than any other published method.

Density Anomalies and Regulatory Mechanisms in Land Bird Populations on the Florida Peninsula

Pineland bird populations censused at 13 sites in Florida and on nearby Bahama Islands showed declining density trends southward along the peninsula and a steep rise in the Bahamas. These distributional trends in population density were apparently not paralleled by comparable trends in food supply. In one foraging guild, the pine foliage gleaners, the ratio of foraging biomass to food supply was approximately four times as great in the Bahamas as in southern Florida. Concluding that the Florida-Bahama differences in population density in at least the pine foliage insectivore guild cannot be attributed to differences in food levels, I hypothesize that the low breeding densities in southern Florida are related to the peripheral position of the area in the geographic ranges of most of the species, and that densities are suppressed in these peripheral zones by the persistent swamping of incipient adaptive mutations there by gene flow from the range centers to the north where populations are adapted to different environmental conditions. This hypothesis is supported by evidence in the literature that undifferentiated species (no subspeciation recognized by taxonomists and hence, presumably, no major barriers to gene flow) have been less successful in maintaining numbers in south Florida than differentiated species. The relatively low densities in northern Florida vis-a-vis the Bahamas I attribute to reduced mean adaptedness in the region, stemming from gene flow between habitat patches in the mosaic of habitat types that characterizes the northern Florida vegetation. The overall density gradients in the winter resident community, roughly paralleling those of the permanent resident (breeding) community, again reflect the position of geographic ranges and range centers in the area. I attribute declining densities toward the range boundaries in these species to the effect of natural selection in persistently focusing annual migration orientation on core areas of optimal habitat and to the low probability that mutant individuals, potentially adapted to conditions at some peripheral area, will ever reach that area in the face of counterselection in migration orientation. Finally, I conclude that the definitive guild approach can provide access to consumer-resource ratios in natural communities and that comparative studies using this approach may reveal that consumer densities are often maintained at levels well below the resource-determined carrying capacity of the habitat.

Estimates of Some Attributes of a Population of Nuttall's Cottontails

Density, natality, and mortality of a population of Nuttall's cottontails (Sylvilagus nuttallii) in shrub-juniper scabland of central Oregon were monitored from August 1972 to March 1974. Monthly censuses indicated population density ranged from 6.6 to 254.4/100 ha. Densities declined 89 and 85 percent from August to December in 1972 and 1973, respectively; within these periods, greatest losses occurred at times of unusually low minimum daily temperatures. In 1972, 4 litters were produced (April-July) of which 10, 22, 68, and 80 percent, respectively, survived to 30 August. In 1973, only 3 litters were produced (April-June) of which 25, 21, and 11 percent survived to 30 August. The decline in production and survival of juvenile cottontails in summer 1973 was believed to be related to reduced precipitation during the reproductive period; precipitation was 64 percent less in 1973 than in 1972. Differential survival of litter groups indicated that drought had the greatest impact on younger cohorts, perhaps through reduction in succulent forage for weanlings. J. WILDL. MANAGE. 42(1):159-168 This is a report on our attempts to estimate density, natality, and mortality throughout most of 2 annual cycles for a population of Nuttall's cottontails, and to show relationships between changes in these attributes and differences in ambient temperatures and amounts of precipitation. In studies of lagomorph population dynamics, semiannual estimates of density commonly are made prior to the hunting season in autumn and prior to or during the breeding season in spring (Lechleitner 1958, Lord 1959, Meslow and Keith 1968, Petrusewicz 1970, Gross et al. 1974). Occasionally, single annual estimates made in midsummer (Korschgen and Stuart 1972, Fitch 1947, Janson 1946), or in autumn (Edwards and Eberhardt 1967, Andersen 1957), or monthly indices to roadside activity (Lord 1963, Kline 1965) are used to establish trends in population density. Mortality (and its antithesis, survival) usually is calculated on an annual basis (Lord 1959, Meslow and Keith 1968, Eberhardt et al. 1963), although Lord (1961) attempted to calculate monthly mortality from lens weights. Reproduction in lagomorphs, particularly after Wight and Conaway (1962) reported synchronous breeding in eastern cottontails (S. floridanus) and Casteel (1966) verified immediate postpartum breeding in the same species, frequently is reported on the basis of mean number of young in each of the synchronous litters, and the number of litters produced during each breeding season (Meslow and Keith 1968, Powers and Verts 1971, Trethewey and Verts 1971, Hill 1972, Gross et al. 1974). However, to our knowledge, simultaneous estimates of density, natality, and mortality (survival) at short, regular intervals throughout the annual cycles for a population of wild lagomorphs have not been attempted. This study was supported, in part, by grants to the senior author from the Theodore Roosevelt Memorial Fund of the American Museum of Natural History and from the Grants-in-Aid Committee of the American Society of Mammalogists. H. Holechek generously supplied space and utilities for field headquarters. Numerous students in the Department of Fisheries and Wildlife, Oregon State University, aided in conduct1 This is Technical Paper No. 4057, Oregon Agricultural Experiment Station. 2 Present address: Woodward-Clyde, 4791 Business Park Blvd., Suite 1, Anchorage, Alaska 99503. J. Wildl. Manage. 42(1):1978 159 This content downloaded from 157.55.39.178 on Fri, 05 Aug 2016 05:15:24 UTC All use subject to http://about.jstor.org/terms 160 POPULATION STATISTICS OF NUTTALL'S COTTONTAILS McKay and Verts ing censuses. H. F. Horton, E. C. Meslow, and P. A. Vohs, Jr., provided critiques of the manuscript.

Population Cycles and Activity Patterns of the Dusky Salamander, Desmognathus fuscus fuscus

Population densities of the dusky salamander were determined in a Georgia Piedmont stream using a mark-recapture technique for density estimation. Highest densities were obtained from late spring to early summer when the salamanders are more active under optimal and stable environmental conditions. Lowest densities were recorded in winter, indicating the inactivity and torpidity of D. fuscus. Nocturnal estimates were greater than corresponding diurnal estimates throughout the study. Seasonal trends in population densities are chiefly attrilbuted to temperature, reproduction condition of adult females and relative abundance of the three age-size classes. Differences between diurmal and nocturnal density estimates are mainly attributed to the interaction of light, relative humidity and the differential mobility of the population components. Mark-recapture and population estimation techniques are discussed and evaluated. INTRODUCTION Desmo gnathus fuscus fuscus, a semiaquatic plethodontid, is a predator common in woodland streams of the Piedmont Region of the eastern United States. Its anatomy and life history (Wilder, 1913, 1917; Noble and Evans, 1932; Dunn, 1917, 1926), home range and activity (Barbour et al., 1969), dispersion patterns (Stewart and Bellis, 1970), homing ability (Barthalmus and Bellis, 1969, 1972), and position in the evolution of the desmognathines (Tilley, 1968) are well-documented. Most recent attention has been given to metabolic studies (Fitzpatrick et al., 1972) and the influence of urbanization on populations of this salamander (Orser and Shure, 1972). Mark-recapture as a technique for estimating certain population parameters has been applied to a variety of animal species. Despite its applicability, mark-recapture has seldom been used to estimate salamander densities (Spight, 1967; Burton and Likens, 1971; Orser and Shure, 1972). The Hayne (1949) method of density estimation is based on the increase in percentage of recaptured individuals as the number of marked individuals increases over time. Population density is estimated by the line of best fit to a plot of the percentage recaptured vs. the cumulative number previously marked. The Hayne method assumes a uniform population size with no recruitment or loss of animals during the population estimate. It further assumes equal probability of capture of marked and unmarked individuals. A previous study (Orser and Shure, 1972) indicated possible limitations in the application of mark-recapture and Hayne's method for density estimations of D. fuscus. This study also suggested that a comparison of diurnal and nocturnal population density estimations 1 Present address: Stone and Webster Engineering Corp., Division of Environmental Engineering, 225 Franklin St., Boston, Mass. 02107.

The population genetics of the potato cyst‐nematode, Heterodera rostochiensis: mathematical models to simulate the effects of growing eelworm ‐resistant potatoes bred from Solatium tuberosum ssp. andigena

SUMMARYTo follow population changes when potato varieties with resistance to Heterodera rostochiensis derived from Solanum tuberosum ssp. andigena were grown on infested land, a computer programme was written including three mathematical relationships: (1) a law relating multiplication to pre‐cropping density; (2) two mathematical models of inheritance of ability to overcome resistance; (3) a law relating the proportion of larvae able to become female to pre‐cropping population density. The programme also included four parameters: (1) the maximum possible reproductive rate; (2) the fraction of the population (eggs) not participating in reproduction when potatoes are grown and carried over to the following year unchanged; (3) the fraction carried over annually when other crops are grown; (4) the frequency of larvae able to become female in the population initially. Population density was measured relative to the equilibrium density and was therefore independent of the units in which density is usually measured.After supplying a range of parameters for all the above to include those likely to be encountered in practice, the changes expected (a) in the frequency of larvae able to become female in the roots of resistant varieties and (b) in population density were computed for resistant varieties grown continuously or alternately with susceptible varieties in crop rotations of different lengths. Because well established field populations are relatively dense, observed reproductive rates are small and rarely approach the maximum possible. Reproductive rate is therefore a relatively unimportant determinant of genetic change. The fraction of the population carried over to the following year is more important because it affects the length of a crop rotation necessary to make loss of potato yield acceptable, determines what the multiplication rate will be and influences the speed of genetic change by providing a reservoir of initial type males which backcross with any genetically different females that may develop on the roots of resistant plants.No experiment seems to have been done specifically to determine the parameters needed to calculate population changes. Some values can be obtained from the literature but mostly they must be guessed.When the law relating the proportion of larvae able to become female to pre‐cropping population density was included in the computations, it had little effect initially but later, after several generations, it delayed genetic change.Two field experiments, one by Huijsman (1961) another by Williams (1958) and Cole & Howard (1962 a), provide some of the variables needed to compute trends in population density. Best fitting variables were computed for the data in these experiments by the method of maximum likelihood. The computed parameters for one experiment were not very realistic but those for the other were in line with what would be expected in practice and tended to favour the hypothesis that larvae able to become female in the roots of resistant plants are double recessives (aa).The computations lead us to suggest that the best policy for potato growers who have fields suitable for resistant varieties is to alternate resistant with susceptible varieties in a crop rotation containing potatoes every 3 or 4 years.