6 Jul 2015
The Composition and Ecological Function of Birds in the Agricultural Landscape of Nyandarua, Central Kenya

BRANTA — Paul Kariuki Ndang’ang’a

The Composition and Ecological Function of Birds in the Agricultural Landscape of Nyandarua, Central Kenya

Institution: Jomo Kenyatta University of Agriculture and Technology, Kenya
Supervisors: John B Njoroge, Kamau Ngamau, Wariara Kariuki
Details: PhD 2013 (completed)
Subject Keywords: agriculture, ecosystem service, pest control, common birds


There has been limited studies in Africa on how the composition of agricultural landscapes affect the capacity of the land mosaic to support natural flora and fauna and maintain ecological processes. As a contribution to this knowledge in the highland agricultural landscape of Nyandarua in Kenya, five related objectives were addressed. Firstly, the effects of vegetation composition and structure on bird species diversity and species richness of various foraging guilds were examined. Four rounds of bird and vegetation surveys were undertaken. Bird species diversity and richness of respective foraging guilds, except insectivores, increased with increasing heterogeneity of vegetation cover types. Two gradients of increasing vegetation structural heterogeinity were most important in influencing bird community composition and had positive effects on overall species diversity and presence of most of the species assessed: increasing closed habitat due to woody plant density, and increasing fallow and cultivation versus decreasing grassland/pasture cover. Secondly, the effects of crop diversity on avian species richness and abundance were examined. Crop diversity had significant positive effects on species richness. Cereal cover had negative effects on species richness, overall bird abundance and abundance of granivores. Thirdly, bird foraging behaviour and its implications for provision of ecosystem services and crop damage was investigated. Most birds foraged from the ground, often consuming weed seeds, particularly during the dry season. The relatively high rate of invertebrate intake by two aerial foraging species and high number of insectivorous bird species recorded in the area suggested that invertebrate predation could be high and potentially contribute to pest regulation. Fouthly, bird exclusion experiments on Kale plants were undertaken to quantify the extent to which foraging birds contributed to reduction in the number and subsequent impact of invertebrate pests. On average per week during the dry season, exclusion of birds from kale plants led to aphid load increase of about three times greater than when birds could access the kale plants, and 130% in proportion leaves whose underside were heavily infested by aphids. This suggested that birds could make a significant contribution to pest control especially in the dry season. Lastly, the extent to which birds play a role in controlling weeds within cultivated fields was investigated. Bird exclusion experiments were used to test the prediction that post-dispersal weed seed predation would be higher, and thus weed seed germination lower, in bird-accessible compared to bird-inaccessible plots. Germinating weed seedlings were more in bird-inaccessible treatments across all but one replicate site, with estimated overall average loss of approximately 20% seedlings in bird-accessible treatments and paired sample tests showed significantly higher numbers of germinating seedlings in bird-inaccessible treatments. This suggested that birds could play a role in subsequent weed control. Overall birds were found to prefer structurally heterogenous vegetation in farmlands and were confirmed to provide significant ecosystem services in agricultural landscapes. The ecological knowledge contributed by this study could guide in planning for the extent and type of crop and non-crop cover in the landscape, and justifying integration of conservation of birds and other wild fauna in the agricultural landscape.


Ndang’ang’a, P.K., Njoroge, J.B.M., Ngamau, K., Kariuki, W., Atkinson, P.W., Vickery, J. 2013. Avian foraging behaviour in relation to provision of ecosystem services in a highland East African agroecosystem. Bird Study , 60: 156-168.
Ndang’ang’a, P.K., Njoroge, J.B., Ngamau, K., Kariuki, W., Atkinson, P.W., Vickery, J. 2013. Effects of crop diversity on bird species richness and abundance in a highland East African agricultural landscape. Ostrich , 84: 33-39.
Ndang’ang’a, P.K., Njoroge, J.B.M., Vickery, J. 2013. Quantifying the contribution of birds to the control of arthropod pests on kale, Brassica oleracea acephala, a key crop in East African highland farmland. International Journal of Pest Management , 59: 211-216.
Ndang’ang’a, P.K., Njoroge, J.B.M., Githiru, M. 2013. Vegetation composition and structure influences bird species community assemblages in the highland agricultural landscape of Nyandarua, Kenya. Ostrich , 84(3): 171-179.
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