Agro-Ecosystems最新文献

Population dynamics is rarely considered in current pest control programs. Instead of the usual aim of killing a certain percentage of the target species, we demonstrate here the importance of considering dynamic aspects of populations when attempting management (e.g., pest control) of vertebrate populations. Conceptually we treat heterogeneous landscapes with pest-suitable patches intermingled with non-suitable areas. These pest-suitable patches are also regarded as separate objects for local pest management. We discuss density-independent and density-dependent natality and mortality rates for separate patches. In the landscape context we treat immigration into, and extinction in local patches. In this analysis we also consider the importance of transition habitats for local and regional population development.

We summarize our conclusions in a list of features which ought to be examined before a species' pest status can be evaluated and management programs are planned. Important information for such a purpose is the form and magnitude of natural mortality rates as functions of density, magnitude of equilibrium densities, the occurrence of presaturation dispersal or not, and the costs of reducing natality and of increasing mortality. Due to the immature state of population ecology, we emphasize that our suggestions have to be regarded as hypotheses to be tested in actual management situations.

The rain forest in West Africa is dominated by ferralitic soils, which are old, highly weathered and low in nutrient reserves and cation exchange capacity. Their fertility is mainly dependent on a delicately balanced cycle of organic matter.

Agri-silviculture as practised in this area is held to be a progressive variant of shifting cultivation in that it encourages multiple use of the land and immediate reforestation of the sites after crop harvest while providing the forest owner/cultivator with extra income from sales of poles and timbers.

Little attention has been given to the soil, which is inherently poor and further impoverished by export of portions of the meagre nutrient reserve. Drawing on various sources, this paper examines the impact of clearing and burning, cultivation and cropping and the monospecific tree fallows, on the productive potentials of these soils. It is concluded that much has still to be learnt of the restoration of soil fertility under short rotation crops, the loss of nutrients during cropping and the behaviour and response of ferralitic soils to various intensities of cropping and management methods. Some possible topics for research are suggested.

The opportunity presented by the introduction of a coal-based granular formulation of the carbamate nematocide, aldicarb for use on sugar beet and potatoes in the United Kingdom has been used to evaluate a number of new methods for the assessment of the environmental hazard presented by pesticides. In an intensive field trial with sugar beet, well established biological procedures for recording changes in animal populations and behaviour were combined with a planned programme involving the sampling of animals, plants and soil. Emphasis was placed on sampling and observing mammals in addition to birds which have been more extensively studied in the past. The samples were submitted to a variety of chemical and biochemical analyses designed to reveal the extent to which the pesticide was distributed temporally and spatially and the effects it produced on individuals. The results indicate that although the pesticide is not persistent, it is mobile especially in wet soil, systemic in plants and can be found widely in the local vertebrate fauna for up to 90 days after application. Specific hazards were identified as arising from the ingestion of uncovered granules and from worms containing residues coming to the surface following application in wet conditions.

Subsequently a surveillance exercise was mounted on the commercial use of the pesticide at eight sites where sugar beet and potatoes were grown. The hazards identified in the field trial were confirmed and their practical importance assessed, resulting in modifications to the application method in order to eliminate them.

The value of the various methods employed in the trials and the significance of the results are discussed. It is concluded that the approach used will find wider and more general use in assessing the environmental hazard posed by new pesticides.

The distribution of total nonstructural carbohydrate (TNC) concentration and accumulation was analysed in various components of three selected grasses, a perennial C₄ (Dichanthium annulatum) and two annuals, Echinochloa colonum (C₄) and Polypogon monspeliensis (C₃). The first two species were grown during dry summer and first and third during the winter season. Three moisture regimes were chosen, viz. full field capacity, half field capacity and unirrigated condition. The plants were clipped three times; each time 40 and 80% of the current live green was removed. In all the four instances, the TNC levels increased with greater soil moisture availability. Dichanthium grown in winter showed a drastic reduction in both concentration and accumulation due to soil moisture deficit while irrigated plants reflected higher TNC compared to Polypogon. Dichanthium grown in the summer had a higher value of TNC than Echinochloa in all the components. Clipping reduced the TNC levels in all the instances except in the spike. The extent of recovery from clipping seemed to depend upon the phenophase; plants in vegetative growth recovered earlier in summer Dichanthium compared to Echinochloa and Polypogon in which spike development delayed the build-up of TNC in the leaves.

The rural inhabitants (campesinos) of the lowland tropical region of southeastern Mexico have managed their traditional agro-ecosystems for centuries with a focus on sustaining yields on a long term basis rather than maximizing them in the short term. Recently introduced agricultural technology in the region has been rapidly displacing and even eliminating local practices in favor of large-scale commercial farming and cattle raising, yet without achieving the production levels originally proposed. This is accompanied by a loss of diversity in local cropping systems, leading to an ever-increasing dependence on imported food products, poorer nutrition, and degradation of natural resources.

Modular production units are proposed in order to help achieve once again the diversity and stability of productivity originally characteristic of the traditional agro-ecosystems. The primary focus of the units center around the application of ecological principles with the incorporation of empirical knowledge of varieties and practices still extant in the region. The basic structure of the units is described.

The ecological processes seen to be functioning in local agro-ecosystems are also applied in the modular units. This includes high species diversity in both time and space, high rates of biomass accumulation, closed nutrient cycling, and biological control mechanisms for weeds, pests, and disease. The perspectives for such systems of production are discussed.