Changes in the Structure of Avian Community along a Moisture Gradient in an Urbanized Tropical Riparian Forest

In larger river valleys of southern Africa, riparian forests are among the main vegetation types. The forests are regarded as ones of the most diverse and distinct terrestrial habitats on earth. They also play a crucial role in wildlife conservation as corridors and refugia for wildlife. It is hypothesized that the closer to the river bank, the higher the species diversity, while population densities may increase with the increase of the distance from the river bank. In 2014, quantitative studies (by means of the mapping method) on avian community breeding in such forest on Zambezi River in Katima Mulilo were undertaken to measure this gradient. Four plots were designed along an increasing distance from the river bank in this forest. In terms of the number of species, there was a significant gradual decrease with the increasing distance from the bank (from 91 to 54 species). The number of breeding pairs per 100 ha increased with the moisture gradient (from 222 to 677). Shannon's (H') and Simpson's (D) indices were higher in the two plots closer to the bank river (H'= 3.44-3.85; D = 0.97-0.99), than in the two remaining plots (H' = 3.13-3.15; D = 0.92-0.93). Also the Pielou's Evenness Index was higher in two plots closer to the bank (J' = 0.84-0.85 vs. 0.77-0.78). The proportion of granivores in the avian community declined along the moisture gradient, while that of insectivores and frugivores – increased. These gradual changes can be linked to a parallel gradual changes of the amount of fruits and invertebrates.