A NEW SPECIES OF AKALAT (SHEPPARDIA) NARROWLY ENDEMIC IN THE EASTERN ARC OF TANZANIA

Auk, The, Jan 2004 by Beresford, Pamela, Fjeldsa, Jon, Kiure, Jacob

ABSTRACT.-The group of small forest robins, or akalats, that dwell in lowland and montane forests in Africa have complicated parapatric or partly overlapping distributions, the details of which are still being discovered. Here, we use external morphology and mitochondrial and nuclear DNA sequence data to determine the reciprocal monophyly of several populations, including one form that is related to Sheppardia lowei and S. montana. Those data corroborate the recognition of a new species, Sheppardia aurantiithorax, and show relatively high levels of sequence divergence among populations of the different species. The discovery of this new species, narrowly endemic in the Eastern Arc montane forests, emphasizes the complex biodiversity of the region and underscores the need for prompt and effective conservation measures. Received 22 September 2002, accepted 6 January 2004.

RESUMEN. - Los zorzales pequenos ("akalats") que se encuentran en los bosques montanos y de tierras bajas de Africa tienen complejas distribuciones que son parapatricas o se superponen parcialmente, y cuyos detalles estan aun siendo descubiertos. En este estudio utilizamos la morfologia externa y secuencias de ADN mitocondrial y nuclear para determinar la monofilia reciproca de varias poblaciones, incluyendo una forma que esta relacionada con Sheppardia lowei y S. montana. Estos datos corroboran el reconocimiento de una nueva especie, Sheppardia aurantiithorax, y muestran niveles relativamente altos de divergencia en secuencias entre poblaciones de las diferentes especies. El descubrimiento de esta nueva especie, que es estrechamente endemica a los bosques montanos del arco oriental, enfatiza la compleja biodiversidad de la region y recalca la necesidad de implementar medidas efectivas de conservation con prontitud.

THE TANZANIA-MALAWI (Tanganyika-Nyasa) montane avifauna, which Moreau (1966) described as an "area of great complexity," continues to reveal new information on distributions and patterns of local endemism in many groups of vertebrates (e.g. Burgess et al. 1998) and plants (e.g. Lovett and Wasser 1993). Those patterns are thought to be attributable to geologic uplift and isolation of mountain blocks during the Tertiary, as well as to shifting global climate regimes (Wasser and Lovett 1993). The Tanzania-Malawi system includes the crystalline Eastern Arc mountains and volcanic mountains to the south-west. The Eastern Arc mountains are block-faulted nonvolcanic mountains of Karoo-age origin (-250 mya), with the final uplift mainly in the upper Miocene. The mountains harbor a high proportion of species presumed to be relatively old and relict; the persistence of such taxa is currently attributed to the beneficial effect of the Indian Ocean on the east-facing mountains (Fjeldsa and Lovett 1997). However, the mountains also have several endemic species that are part of widespread and species-rich genera; their presumably more recent speciation may be related to Pleistocene climatic instability or to tectonic changes. Some faults are thought to have been affected by Pleistocene activity in the Gregory Rift (Griffiths 1993, Partridge et al. 1995).

Species richness of birds is relatively high (Brooks et al. 2001), and distributions of endemic species are complicated, with different patterns of overlap among isolated montane forest blocks (Stuart et al. 1993). Knowledge of bird distributions has improved considerably since the analysis by Stuart et al. (1993), and the general pattern that seems to emerge indicates a strong nestedness of species distributions (Cordeiro 1998), with peak concentrations in the East Usambara and eastern Udzungwa mountains and many range disjunctions, which suggests that biogeographic patterns are strongly influenced by local species loss. However, sharp replacements suggesting vicariance within the Eastern Arc are also known, and new such forms are being discovered as various isolated forests and mountains are surveyed. Dinesen et al. (1994) described a new, highly distinctive galliform; more recent discoveries include a new population of that galliform (J. Fjeldsa et al. unpubl. data), the new akalat described here, and additional forms yet to be described and named (J. Fjeldsa unpubl. data). Few phylogenetic analyses have been published by which these patterns of richness and endemism can be interpreted in a biogeographic context.

The akalats are one of several groups of birds that have differentiated in eastern Africa. These small forest robins have traditionally (e.g. White 1962) been placed in the family Turdidae, subfamily Turdinae. Sibley and Monroe (1990) proposed that the group was related to the flycatchers and placed them in their subfamily Muscicapinae, in the tribe Saxicolini. However, Dowsett and Forbes-Watson (1993) kept them in the family Turdidae, in the subfamily Saxicolinae. all treatments broadly agree on the affinity of the genera Saxicola, Erithacus, Sheppardia, Cossypha, Alethe, Luscinia, Erythropygia, Oenanthe, and several smaller genera, but relationships among and within genera are poorly understood at present. Pasquet et al. (1999) found that some members of Sibley and Monroe's (1990) "Saxicolini" were not monophyletic with respect to "Muscicapini," results recently corroborated with different muscicapine taxa by Beresford (2003). Although the composition of major groups within the Muscicapidae remains to be clarified, the genus Sheppardia was partly recovered by Roy et al. (2000) with mitochondrial markers and by Beresford (2003) with mitochondrial and nuclear markers. However, neither study obtained good support as currently measured in phylogenetic analyses (bootstrap and decay index values).