Plant species richness and diversity in urban and peri-urban gardens of Niamey, Niger

Urban and peri-urban agriculture (UPA) significantly contributes to food and nutritional security of urban dwellers in many African countries. Economic and demographic pressures often lead to transformation of subsistence-oriented traditional homegardens into commercial production units. Such transformation is claimed to result in decreasing plant diversity, particularly of local species. A study was therefore undertaken in 51 gardens of Niamey, Niger, to assess the factors determining plant diversity and the suitability of UPA for in situ conservation of plant genetic resources. In each garden, the number and abundance of all human-used plant species were determined, and species density, Shannon index and Shannon evenness were calculated. In the 51 surveyed gardens, a total of 116 plant species were cultivated, most of them for the production of fruits or vegetables. Annual vegetables dominated, particularly exotic species grown for sale. In the cold season, an average of 14 species were cultivated per garden, the Shannon index was 0.96 and evenness was 0.39. Commercial gardens had a species richness similar to that of subsistence gardens, but a lower evenness (P < 0.005), caused by the dominance of a few vegetable species. Gardens of immigrants had a lower Shannon index than those of members of the local Djerma ethnic group. Stepwise multiple regression analysis showed significant influence of various variables on plant species richness and diversity parameters: garden size (richness and Shannon index), ethnicity of the gardener (richness and evenness), gender of the gardener and cash-oriented production (evenness), household size (richness) and garden possession status (Shannon index). Cluster analysis revealed the existence of five garden types. The highest species richness and diversity, particularly of perennial and local species, was found in large, peri-urban, commercial gardens managed by relatively wealthy, elderly gardeners with large families and a regular non-agricultural income.