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Karl Ramjohn
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Agro-ecosystems: Ecology & Management
« Thread started on: Aug 10th, 2008, 4:10pm » |
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AGRO-ECOSYSTEMS: ECOLOGICAL CHARACTER AND MANAGEMENT OF RICE, SUGAR CANE & CITRUS PRODUCTION
INTRODUCTION
The purpose of this article is to:
• Define agro-ecosystems in the context of man-made environmental management units;
• Describe the ecological character of some typical agro-ecosystems of the former Caroni (1975) Ltd (rice, sugar cane and citrus);
• Establish and describe how these agro-ecosystems differ from natural ecosystems.
From an ecosystem perspective, agricultural production systems may be regarded as restructured and simplified natural environments, as they rely on plant-based biomass productivity through photosynthesis, nutrient recycling via microbial activity, etc. However, their functioning requires subsidised ecological processes, and their habitat structure and complexity (as well as spatial arrangement of biota) are intentionally simplified and controlled, to focus on the productivity of a small number of targeted elements. Agricultural ecosystems are land-use units comprising soil, crop, tree, weed, livestock, pathogen and insect sub-systems that continuously interact to transform solar energy, water, nutrients, labour and other (e.g., agro-chemical) inputs into biomass yields as food, feed, fuel, fibre and pharmaceuticals. In this context, agro-ecosystem management requires a significant amount of human intervention, as compared to natural or semi-natural ecosystems.
AGRO-ECOSYSTEMS
Definitions
An agro-ecosystem is a restructured or simplified “natural” production system. The main objective is to produce one (or a limited number) of natural components, for example food crops. Agro-ecosystems do not support themselves, they must be managed to sustain their productivity. This has a number of ecological consequences.
Energy Relations and Cycling: Unidirectional Flow
Agro-ecosystems are characterized by a unidirectional flow in energy and materials, as opposed to the cyclic, more closed feature of natural systems. The unidirectional flow derives from the specific component extraction, which is the function of agro-ecosystems. The products are harvested and removed from the ecosystem, frequently for use at an external location.
Harvesting of crops removes large amounts of organic material from the system. This leads to a decrease in productivity, as plant material which should have been cycled back into the system is no longer available. The management response to this is inputs to the system to compensate for the removed material, i.e., to attempt to maintain the ecological processes which sustain the viability and functioning of the system. This is done by placing fertilizers on the land surface, consisting of either naturally-derived organic material or artificial (chemically-designed) fertilizers. This placement on the surface of the land can lead to the fertilizers affecting non-target areas by surface run-off. This can lead to increased amounts of nutrients entering the rivers and streams and eventually the sea, affecting water quality (nutrification) and disturbing the associated ecosystems.
Soil Quality: Deterioration
The unidirectional flow also has consequences on soil quality. Continuous cropping (same crop) depletes the soil of specific nutrient resources leading to soil deterioration. This can be addressed by a number of methods including fallowing (leaving the soil uncultivated for a period of time, re-establishing the soil nutrients by natural regeneration) or crop rotation (alternating the type of crop planted at a particular site). During the cropping cycle, soil is left exposed for long periods. Exposure of the sun can lead to a hardening of the upper layers of soil. Where there is significant evaporation, salinisation of the soil can take place. Exposure of soil also changes the surface albedo, increasing the reflectance of the surface. The increased reflection of the sun’s radiation can affect local climates. Erosional losses can take place as a result of rain wash effects.
Vegetation Successions: Control of Weeds
The management of agro-ecosystems requires the maintenance of a specific type of vegetation. This necessitates attempting to keep the system at a particular level of succession. The natural ecology attempts to re-establish itself, resulting in weeds (adventives) occurring. Control of weeds can be attempted by the use of herbicides (designed to kill particular types of plants) or by shading (use of trees to control the amount of sunlight reaching the soil level).
Diversity Reduction: Pests
Agricultural land use leads to simplified ecosystems. This reduction in diversity can encourage the colonization of agro-ecosystems by pest species, particularly phytophagous insects. A field of monoculture crop can be regarded as a “island” for pest dispersal. Large “islands” are more easily located by pests, increasing the probability of colonization. One method of dealing with this problem is to manage the spatial extent of the crop, i.e., a series of small plots with barriers, such as hedgerows.
However, the most widespread method of managing pests is through the use of pesticides. These chemicals are usually sprayed directly onto fields, and this increases the potential for non-target species being affected, including the natural predators of the pests. The pesticides can also enter fluvial systems and affect ecosystems. Despite dilution of the pesticides in rivers, there are implications for trophic relationships. Large numbers of small organisms support smaller numbers of larger organisms. A species at the base of the foodweb can be exposed to a non-lethal dose; however, the level of contamination increases for each trophic level. At higher levels of the food chain, the initial small doses can be magnified by a sufficient factor to be lethal or cause physiological damage.
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Karl Ramjohn
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Re: Agro-ecosystems: Ecology & Management
« Reply #1 on: Aug 10th, 2008, 5:31pm » |
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METHODOLOGY
A field visit was paid to representative agro-ecosystems managed by Caroni (1975) Ltd in western Trinidad, on November 04, 1998. This included rice and sugar cultivation areas just south of Caroni Village, and a citrus plantation east of Longdenville, Chaguanas. This served the purpose of:
• An introduction to the important crops and some of the preparation, cultivation and land management methods.
• Identification of the ecological character of managed agro-ecosystems and some of the associated problems, especially pest management and pollution.
ECOLOGICAL CHARACTER
The “ecological character” of an ecosystem refers to the structure and inter-relationships between the biological, chemical and physical components. Therefore, in defining the ecological character of an agro-ecosystem, the overall complex of structure and interactions must include crops, weeds, pests, land, humans, climate, physical preparations, and chemical additives. All of these must be taken into account in defining the overall agro-ecosystem management unit and formulating plans for their sustainable management.
Rice
Rice paddies can be regarded as man-made wetland systems, characterized mainly by herbaceous vegetation of a specific variety. The inundation is ensured by man-made physical barriers (banks) and the source of water is via irrigation canals or by rain-fed accumulation. Considerable site preparation is required. This includes ploughing and mud-rolling. The underlying soils at the Caroni Rice Scheme are generally the “Bejucal Clay”. However, for rice cultivation, water supply is a much more significant limiting factor than soil type. The rice plants do not naturally colonise the site, they are generally scattered on the site via aircraft. The inundated nature of the rice paddies generally assists in weed control. Two weeks after planting, herbicides are applied to eliminate or reduce selected weeds. Nitrogenous fertilizer is also applied. Pesticides are frequently mixed into herbicide, to target such pest species as caterpillars, and insect hoppers. A significant pest in the later stages of the life-cycle is the paddy bug, which feeds on rice grains. Sampling is conducted to test for threshold quality, and as required, further pesticide application is done. For the Caroni Rice Scheme there have been several attempts to use biological controls, to cut down on pesticide use.
Monoculture encourages the dispersal and colonization by certain pest species; however, diversity is generally not high. On the edges of access roads and bunds throughout the rice paddies, grasses tend to grow. These include guinea grass, cat-tail and “jungle rice”. This leads to potential “edge effects”, i.e., an increase in the diversity of insect species. Even though specific pest species may be lower in number, the increased diversity can complicate pest management. Rice plants are herbaceous and not structurally complex, which limits the number and diversity of insect species which can be supported.
Rice paddies support large numbers of birds, indicating the presence of invertebrates, such as snails. The diversity and number of sensitive invertebrates are good indicators of the overall ecological health of the system.
Sugar Cane
Sugar cane fields exhibit landscape similarity to savannas. These fields are characterized by one dominant grass type, with occasional clumps of trees. The sugar cane is planted on banks, to prevent inundation of the fields during the wet season. Land preparation takes place every six to eight years. Harvesting of the crop is generally preceded by the systematic burning of the sugar cane fields.
One major pest of sugar cane is the froghopper. This is generally controlled by a combination of biological and chemical (pesticides) mechanisms. Sugar cane plants are not structurally complex; however, their relatively large size encourages pest colonization. The vast acreage of sugar cane monoculture supports a low diversity of species and can be regarded as a “desert” for wildlife. Sugar cane in Trinidad is generally cultivated on soils formerly covered by seasonal deciduous forest, in an area with an average annual rainfall of between 1500 – 2000 mm. Sugar cane cultivation tends to leave the soils relatively exposed.
Citrus
Citrus plantations are dominated by trees, of one general type; they can be regarded more from the perspective of woodland rather than a forest, as they lack the fundamental processes and diversity supported by true forests. The trees in plantations are generally grafted; the root stock is usually “sour orange”, while the scion is usually “white grapefruit” or “Valencia orange”. The trees are planted on banks with the adjacent rows separated by drainage ditches.
Citrus trees are structurally more complex than sugar cane or rice plants, and consequently they support a larger number and higher diversity of insect pests and other parasites, including fungi, mites, scale insects and ant colonies. Fungi tend to grow on the excreta of scale insects on the leaves. This compromises the photosynthesis and overall efficiency of the citrus production systems. Spiders and ants tend to feed on some of the pests; however, ants protect the scale insects, to safeguard their supply of the sugar-rich excreta of the insects. Yellow spots of a fungal nature were noted on the leaves of some of the grapefruit trees.
The soils on which the plantations are located in Trinidad are usually those formerly supporting cacao monoculture. The soils tend to be exposed; however, little undergrowth was observed, apart from mosses, ferns and small mushrooms. This is a result of management practices such as the use of herbicides. The lack of ground cover also controls the spread of vines.
Agro-ecosystem Indicators
A number of ecological indicators can be used to assess the overall health and sustainable productivity of agro-ecosystems. The monitoring of these changes in ecological character is a useful tool in agro-ecosystem management. Some criteria of condition include:
• Crop productivity
• Soil productivity
• Irrigation water quantity and quality
• Density of beneficial insects
• Density of pests
• Foliar symptoms
• Indicators of genetic diversity of crops
• Indicators of contamination of natural resources
• Indicators of landscape quality
Based on the site visits to the agro-ecosystems managed by Caroni (1975) Ltd, the citrus plantations showed the most obvious signs of unsustainability. No detailed studies of indicators were conducted at the sites; however, the high incidence of pests and leaf spots, gave some indications of the productivity problems associated with citrus production.
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Karl Ramjohn
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Re: Agro-ecosystems: Ecology & Management
« Reply #2 on: Aug 10th, 2008, 5:32pm » |
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CONCLUSIONS
• The purpose of agricultural systems is to provide food for humans and/or domesticated species. The result is an effective “one-way” ecosystem.
• Biological, chemical and physical processes and interactions are managed to provide useful components. The functions cannot sustain themselves, and as such, agro-ecosystems require intensive management: heavy subsidies of energy, materials and human labour.
• For effective management of agro-ecosystems, the overall “ecological character” of any particular system must be understood, to achieve the goals of management (food production). This ecological character must take all components into account: physical (climate and soils), biological (crops, pests, weeds, beneficial insects), humans, inputs (labour, energy, chemicals) and interactions.
• The overall sustainability of an agro-ecosystem (i.e., its ability to continue achieving the goal of useful component production) can be assessed and monitored by using ecological indicators. These should be developed as a critical part of the management plan for any agro-ecosystem.
REFERENCES
Based on:
Ramjohn, Karl. Agro-ecosystems: Ecological Character and Management of Rice, Sugar Cane and Citrus Production. Tropical Environment Research & Management Center, Trinidad & Tobago. November 1998.
Related Resources:
> http://hydroterrestrial.pulseblog.net/The-first-blog-b1/Spatial-Ecological-Assessment-of-Land-use-Land-cover-Caparo-River-Valley-Republic-of-Trinidad-Tobago-b1-p2.htm
> http://tropicalenvirophoto.phpbbnet.com/viewtopic.php?f=2&t=2
> http://sustainablelanduse.wordpress.com
> http://www.its2hot.in
> http://www.sustainabilityforum.com/forum/
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