The effects of forest fragmentation on bird species in Madagascar : a case study from Ambohitantely Forest Reserve on the central high plateau.

Abstract

Considering the high rate of endemism in Madagascan organisms, which are mostly restricted to forest ecosystems, and the accelerating rate of deforestation affecting the island, it is critical to understand the effects of forest fragmentation on Malagasy biota to allow for better management of species within ecosystems. Ecological and human-induced changes have led to the disappearance of forest from vast areas of the island, including on the central High Plateau. The Ambohitantely Special Reserve, located on the central High Plateau at 1500 m asl, was selected as the study site for research on the effects of forest fragmentation on forestependent bird species in Madagascar. The Ambohitantely special Reserve covers 5600 ha of which 50% are natural forest, 35% are anthropogenic grassland savannah and 15% are exotic plantations. The forest, described as East Malagasy moist montane forest, is 2737 ha in area, of which 1487 ha are comprised of 513 forest fragments scattered around the largest block totalling 1250 ha. To investigate the effects of forest fragmentation on forest-dependent bird species, seven forest fragments were selected, ranging from 0.64 ha to 136 ha, in addition to the largest block, referred to as the control site. The bird species composition and relative abundance in different-sized fragments were assessed in reference to the control site, by using a combination of two standardized sampling methods: mistnetting and point-counts. A total of 1804 mistnet-days were accrued, 1026 in the control site and 778 in the seven forest fragments, leading to the capture of 491 birds of 26 species. A total of 160 point-counts was made at 39 different sample plots totalling 53 h 29 min of censusing, and a record of 30 species. A total of 72 bird species including 54 breeding forest affinities of the avifauna of Ambohitantely were defined with reference to 32 forest sites scattered across Madagascar. The species composition in all fragments are fully nested subsets of the control site and the species distribution in the fragments does not represent random subsets of the control site. The analysis of the bird communities in different size fragments indicates that the occurrence of bird species reflects a regular pattern of species extinction in relation to decreasing size of forest fragment. Species composition is discussed in reference to Ambohitantely's long history of isolation that may have led to extirpation of bird species from this site. The higher bird taxa decrease in number or totally disappear as a result of their ecological specialization. Equally, they are the most affected by edge effects (e. g. Atelornis pittoides). The main ecological trends in disappearance or extinction of species is a decrease in the number of small-bodied insectivorous understorey species and mid- and upper-stratum small-bodied insectivorous and nectarivorous species. Insectivorous species are particularly affected by forest fragmentation and three forest-dependent species found in Ambohitantely Forest are particularly sensitive to forest fragmentation: Newtonia amphichroa, Calicalicus madagascariensis, and Cyanolanius madagascarinus. The biological (flora species composition and forest structure) and the physical (temperature, moisture, and light) changes generated by the fragmentation of the forest have a greater impact on highly specialized foraging guilds and this explains the pattern of current bird species composition in Ambohitantely Forest. The influence of forest fragmentation on the altitudinal migration process is debated and conservation measures such as the establishment of forest corridors are proposed to improve the protection of biota found in the Ambohitantely Special Reserve.