Conserving cowpea using GIS tools

Diversity of cowpea seeds. Photo by IITA.
Diversity of cowpea seeds. Photo by IITA.

The germplasm collections in genebanks are an invaluable resource for the future. The conservation of this biodiversity is tied to agricultural production and represents a safety net for the food security of future generations.

In addition to the conservation, multiplication, regeneration, and characterization of these collections, another central function of a genebank is the expansion of germplasm collections to cover as much agrobiodiversity as possible.

IITA works on cowpea improvement and holds the largest cowpea germplasm collection in the world (15,115 accessions); 10,814 (71.5%) of these were collected in Africa or acquired from African national programs.

High protein food legume
The cowpea is a very important, widely adapted, and versatile grain legume of high nutritional value. It is mainly produced and consumed in Africa where it provides a major low-cost dietary protein for millions of smallholder farmers and consumers who cannot afford high protein foods, such as fish and meat. Food legumes, particularly cowpea, have high protein contents. Cowpea contains 24% protein, 62% soluble carbohydrates, and small amounts of other nutrients.

It is a very low-input crop, traditionally grown in intercropping systems. Cowpea contributes to soil fertility through nitrogen fixation and is also cultivated to prevent soil erosion.

The worldwide area cultivated with cowpea in 2008 was estimated to be 11.8 million ha with an annual production of 5.4 million t of dried grains (FAOSTAT 2010).

Production in Africa represents about 91% of the global production. West Africa, with 10.7 million ha, accounts for most of Africa’s production, with Nigeria and Niger being the leading cowpea growing countries (FAOSTAT 2008). The area planted with cowpea is substantial in Senegal, Mauritania, Mali, Burkina Faso, Côte d’Ivoire, Ghana, Bénin, Togo, Chad, Cameroon, Central African Republic, Congo, Uganda, Tanzania, Sudan, Ethiopia, Kenya, Angola, Somalia, Zambia, Mozambique, Zimbabwe, Botswana, Namibia, South Africa, and Madagascar (NRC 2006).

Ex situ conservation in IITA genebank: long-term storage, −20 °C. Photo by IITA.
Ex situ conservation in IITA genebank: long-term storage, −20 °C. Photo by IITA.

Cowpea diversity
At IITA, cowpea is maintained in two storage conditions, medium (5°C) and long-term (-20°C) at an optimal water content of 7−8% fresh weight basis. The viability of most accessions stored at –20°C for 25 years remains as high as 90%.

To avoid losses of genetic diversity and to guide future sampling, researcher Anne Rysavy of the University of Hohenheim (now with the University of Tuebingen), GIS Specialist Kai Sonder (now with CIMMYT), and the head of IITA’s Genetic Resources Center, Dominique Dumet, assessed the geographic coverage of the current collection to get an overview of the crop’s conservation status. The study identified areas in Africa where the probability of finding more and diverse Vigna unguiculata accessions is highest and where further collection should be done.

GIS tools
Gap analysis is an evaluation technique applied to provide wide geographic information on the status of different species and their habitats using satellite data and different computer tools and by digital map overlays in a geographic information system (GIS). Gaps refer to geographical areas that are underrepresented in the collection and where cowpea is expected to occur based on agrometeorological and other factors.

GIS can be a powerful tool for analyzing spatial distribution of a species. Combined with biophysical information from germplasm collections, it can help in conducting surveys and prioritizing future sampling areas. Areas that have not yet been sampled can be targeted for collecting missions so that the material can be conserved ex situ or using in situ conservation strategies.

Specifically, the study analyzed, corrected, and georeferenced the available passport data for cowpea. It also applied different GIS tools to identify gaps in previous collection areas, and predicted areas where new diversity is likely to be collected and/or areas where diversity erosion risk is highest, e.g., from climate change, civil war, deforestation, etc.

This study used spatial analysis tools and software applications, such as FloraMapTM, HomologueTM, ArcGISTM, and DIVA-GIS, including the predictive models EcoCrop, BIOCLIM, and DOMAIN to perform the gap analysis on the existing cowpea germplasm collection at IITA and identify potential areas for future conservation activities.

First the country coverage of georeferenced cowpea accessions was determined. Then, ecogeographical site descriptors (temperature, precipitation, length of growing period, and altitude) were extracted to determine areas with environmental conditions favorable to cowpea. Based on this, regions with similar environmental conditions were identified using GIS techniques.

Distribution of the 10,814 cowpea accessions
Distribution of the 10,814 cowpea accessions

Gaps in cowpea collection
Study results provide an overview of the actual distribution, agroclimatic preferences, and potential distribution of cowpea.

The geographical scope of the study focused on sub-Saharan Africa. Results indicated that cowpea can be found approximately between 15°N and 20°S, and over a large range of climates—temperature as well as precipitation. However, it occurs most likely in subtropical to tropical conditions characterized by warm temperatures (annual average >20°C) and relatively high annual precipitation (>250 mm).

The distribution of the total number of cowpea accessions held in the IITA genebank is very diverse with a certain concentration in West Africa (see map). Nigeria and Niger account for nearly 50% of all accessions. The origins of the remaining 50% are unequally distributed across the continent. Several countries such as Burundi, Equatorial-Guinea, Eritrea, Guinea-Bissau, Namibia, and Rwanda are not represented.

Depending on the country, the total number of accessions collected within Africa ranged between one (Algeria and Angola) and 3,813 (Nigeria). Nigeria ranked first with 35.3% and Niger second with 11.6% (1,249 accessions).

Cameroon, Botswana, and Zambia accounted for 15% of the total number of accessions, each contributing 5%. Tanzania, Malawi, Bénin, Egypt, Ghana, Mali, Burkina Faso, and Senegal accounted for 24.4%.

All the methodologies used identified areas where, according to agroecological similarities, the probability is high of finding more cowpea accessions and no collections have been carried out yet, or very few accessions have been collected. They proved to be useful approaches to conserving the genetic diversity of crop species.

Based on the predictive models, the following countries were identified as the priority for the acquisition of new germplasm: Angola, Burundi, Guinea-Bissau, Eritrea, Equatorial-Guinea, Namibia and Rwanda, especially since no collections have yet been made in these countries. In addition, further sampling is recommended in countries with small numbers of georeferenced accessions, such as Botswana, Congo, DRC, Gambia, Lesotho, Liberia, Madagascar, Sierra Leone, Sudan, Swaziland, and Uganda.

Germplasm acquisition will be done through the duplication of existing national collections at IITA with the support of the Global Crop Diversity Trust (GCDT), and specific collecting missions to capture missing diversity. The GCDT has commissioned IITA to lead the development of a global conservation strategy for the genetic resources of cowpea and its wild relatives with an emphasis on Africa.

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