Changes in survivorship,behavior, and morphology in native soft‐shell clams induced by invasive green crab predators

Many studies on invasive species show reduced native densities, but few studies measure trait‐mediated effects as mechanisms for changes in native growth rates and population dynamics. Where native prey face invasive predators, mechanisms for phenotypic change include selective predation, or induced behavioral or morphological plasticity. Invasive green crabs, Carcinus maenas, have contributed to declines in native soft‐shell clams, Mya arenaria, in coastal New England, USA. We tested the hypothesis that clam ability to detect chemical cues from predators or damaged conspecifics would induce greater burrowing depth as a refuge from invasive crabs, and greater burrowing would require increased siphon growth. To determine how crab predation affected clam survivorship and phenotypic traits in the field, clams in exclosure, open, and crab enclosure plots were compared. Crab predation reduced clam density, and surviving clams were deeper and larger, with longer siphons. To determine whether the mechanism for these results was selective predation or induced plasticity, phenotypes were compared between clams exposed to chemical cues from crab predation and clams exposed to seawater in laboratory and field experiments. In response to crab predation cues, clams burrowed deeper, with longer siphons and greater siphon mass. Overall, crab predation removed clams with shorter siphons at shallow depths, and crab predation cues induced greater burrowing depths and longer siphons. Longer siphons and greater siphon mass of deeper clams suggests clams may allocate energy to siphon growth in response to crabs. By determining native behavior and morphological changes in response to an invasive predator, this study adds to our understanding of mechanisms for invasive impacts and illustrates the utility of measuring trait‐mediated effects to investigate predator–prey dynamics.     

Conservation status of wetlands in Zimbabwe: past and present

The conservation status of dambos, a particular type of wetland associated with the headwaters of rivers draining the central plateau area of Zimbabwe, is examined in terms of past and present utilisation and erosion problems. Pre- and post-1950 phases are recognized within both the commercial farming areas and the Communal Lands (or peasant farming areas). The pre-1950 phase on commercial farms was one of exploitation of dambos mainly for winter wheat production, such cultivation contributing towards locally serious erosion. Consequently legislation was passed to restrict disturbance of wetlands, especially for cultivation. The post-1950 phase on commercial farms was one of conservation, with minimal use of dambos for grazing and limited erosion. The pre-1950 phase within the Communal Lands, the former native reserves, was one involving progressive modification of the traditional farming system through, in particular, centralisation. Dambo cultivation, a well-established practice, continued throughout this place, but towards the end of this period there were localised signs of degradation associated, in part, with grazing. The post-1950 phase was one of increased pressure on land with a growing human and livestock population. Thus dambos were required to support more and more cattle throughout the year, whilst efforts to curtail dambo cultivation were not well-recieved by peasant farmers and may have compromised general conservation in the reserves.     

Failure of a mine waste dump in Zimbabwe: Causes and consequences

A combination of factors are considered important in causing the failure and subsequent development of a flowslide of a gold mine waste dump (or tailings dam) at Arcturus, near Harare, Zimbabwe. These factors comprise poor basal drainage, steep perimeter walls, saturation of the walls and basal sediments through continued spigoting of slurry during a period of heavy rainfall, and the effect of this saturation on the tailings. Properties of the tailings, eyewitness accounts, documentary evidence, and site characteristics are discussed. The failure and subsequent development of a fatal 300-m flowslide are reconstructed in a five-phase developmental model. The general applicability of the results is discussed.     

Changing patterns of population distribution in Zimbabwe

This paper critically evaluates the causes and consequences of changes in population distribution in Zimbabwe during the colonial period and since independence in 1980. Five main aspects of population geography are examined. Firstly, by way on background on contemporary patterns of settlement and land use, the history of tenure policies is outlined. Secondly, the distribution of the African population as revealed in the 1982 census is described and major changes between the census years of 1962, 1969 and 1982 are discussed. Thirdly, changing patterns of settlement and land use within the peasant farming areas (Communal Lands) are examined in the context of increasing population pressures. Fourthly, trends in the growth of the main urban centres and the process of urbanisation of the African population are described. Fifthly, post-independence development policies directed at effecting changes in the distribution of population are discussed with particular reference to the land resettlement programme. The present distribution of population is seen as the product of interacting political, economic and environmental factors, knowledge of which is essential in devising appropriate development strategies to achieve a more equable balance of population and resources.     

FRESH SNOW CHEMISTRY FROM HIGH MOUNTAIN REGIONS IN CENTRAL HIMALAYAS

Investigations of atmospheric composition in the Himalayas has been limited in both temporal and spatial scales, mainly due to difficult logistics. Ideal sites for monitoring atmospheric composition and its evolution should be free from local pollution and representative of the remote troposphere (HUEBERT et al., 1980). As the Himalayas are far removed from highly industrialized regions they provide suitable locations to monitor the chemistry of the remote troposphere and to study the evolu…     

格陵兰冰芯记录的大气环流变化及其与南极冰芯的比较(英)

The GISP2, central Greealand, glaciochemical series (sodium, potassium, ammonium,calcium, magnesium, sulfate, nitrate and chloride) provides a unique view of the chemistry of the atmosphere and the history of atmospheric circulation over much of the Northern Hemisphere. Interpretation of this record reveals the controls on both high and low frequency climate events of the last 110 000 years.Changes in insolation on the order of the major orbital cycles control the long-term behavior of atmospheric circulation patterns through changes in ice volume (sea level) and related positive feedbacks.Events such as the Heinrich events (massive discharges of icebergs first identified in the marine record)are found to operate on a 6 100 year cycle due largely to the lagged response of ice sheets to changes in insolation and consequent glacier dynamics Rapid climate change events (massive reorganizations of atmospheric circulation) are demonstrated to operate on 1 450 year cycle possibly in response to internal oscillations in the ocean-atmosphere system or due to changes in solar output. Changes in insolation and associated positive feedbacks related to ice sheets assist in explaining favorable time periods and controls on the amplitude of these massive rapid climate change events.Comparison of the GISP2 glaciochemical series with an ice record from Taylor Dome in Antarctica indicates considerable similarity suggesting that both polar regions experience marked changes in climate. While preliminary evidence points to similar phasing of several major climate events in the two polar regions exact phasing cannot as yet be determined, because dating of Antarctic ice core records is not as well-established as the dating for Greenland ice cores.     

Patterns of gullying in Zimbabwe

In Zimbabwe four contrasting gully locations and associated gully patterns are present, reflecting spatial variations in precipitation, bedrock, soil type and landuse. Deep dendritic gullies, associated with landslip activity and subsurface piping, characterise the wet Eastern Highlands and areas with sodic soils. Shallow discontinuous gully systems characterise bornhardt terrain in the Communal Lands. Dambo gullies tend to form a continuation of the fluvial system. The pattern and temporal variation of gullies was studied in three areas of Communal Land (Mhondoro-Ngesi, Mangewende, Mutema) with particular reference to the influence of average slope and landuse on gully development. In areas of sodic soils the gullies appear to be largely the product of natural processes and their development has only partly been influenced by landuse changes. In contrast the shallow gullies developed on granitic/gneissic terrain tend to occur on cultivated or recently cultivated land. The case studies indicate that a full assessment of gully development can only be achieved if a temporal perspective is also considered.