Individual crop varieties may correspond to populations that share agroecological characteristics, or to certain criteria perceived by farmers that may not correlate to genetic criteria. A variety may be adapted to an ecological niche, such as soil type, soil moisture, exposure, cropping season, or intercropping system. The ecological or climate factors may affect the population structure by interacting with farmer efforts that cause mixing or inflow of genes. Mixing occurs between ecological regions, and re-segregation to local type occurs through human pressure (a preferred phenotype) and through environmental pressure (through selection of the best performing plants). In general, de facto conservation takes place under challenged conditions that may cause larger populations to segregate into local populations according to ecological niches. Understanding and measuring this may be difficult and costly, but are important in the design of a conservation program.
Any in situ conservation program will need to define its targets for conservation, and the population structure of each crop will inform a management strategy. A program for a self-pollinating crop like wheat may seek to preserve a set of distinct farmer varieties, or may seek to preserve fields that contain mixed populations that compete under local conditions. An out-crossing crop like maize may call for the maintenance of a crop population at a village or regional level spanning individual farmers (Louette and Smale, 2000). Another approach may be to work with a single or select group of farmers, who can later serve as a local seed source for their community.
Analysis of crop populations and management strategies may be influenced by the ecological model used. A model based on niche theory would seek to understand the diversity of varieties (and competition for resources) within a farmer's field or a village. A meta-population model may lay more stress on the flows between farms and between villages. Selecting the unit of analysis is also an important component in the design of in situ conservation projects, where the costs of expanding the scope of the project to a larger region have to be balanced against the inclusion of a more comprehensive level of crop diversity. The unit of analysis for an in situ conservation program could range from an individual cooperating farmer, a series of farmers within a village, or a series of villages within a region. In each case it will be important to document the flows of genes and seeds into, and within, each system.
The institutions that influence seed flows are basic elements of the farmer seed system. Characterization of these is needed both to understand the stability of a crop variety and the breadth of genetic sources used by farmers in an evolutionary process. Seed sources both within the village (or relevant unit of analysis) and outside the village should be identified. Other institutions may be regional markets, commodity traders, seed or input suppliers, or labor migration by local farmers. Furthermore, seed sources may be influenced by cultural factors such as common linguistic groups or kinship networks.
The flows of seeds or genetic materials can be documented at two principal levels. First are flows from the larger population (regional) to a smaller population (farmers seed lot) and the second are flows between subpopulations. Furthermore, the flows may be uni-directional, e.g., only from outside the farm into the population, or multidirectional, between many farmers simultaneously. Approximation of the rates of flow, and the human mediated transactions can be recorded through asking the sources of new seed lots, the customary sources for seed renewal, or potential sources in case seed is depleted. Finally, in out-crossing species, the level of cross-pollination in farms with small plot sizes is significant and could be important both within a given farmer's field and across fields in a village (Louette and Smale, 2000).
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