Elevational Gradients of Diversity and Species Composition of Hemerodromiinae (Diptera: Empididae) at Doi Inthanon, Thailand: Has Historical Partitioning between Seasonally Dry Lowland and Aseasonal Moist Mountain Forests Contributed to the Biodiversity o

Abstract

Community structure, species richness and diversity of Hemerodromiinae (Diptera: Empididae) were investigated along an altitudinal gradient between 700 m and 2,500 m above mean sea level on the Doi Inthanon mountain in northern Thailand (18º 35’ N, 98º 29’ E). The diversity and richness were maximal at the upper-mid to sub-summit elevations corresponding to speciose communities associated with continually moist montane evergreen forests. A relatively depauperate species assemblage was associated with seasonally water-stressed biotopes at lower elevations. Relaxation of seasonal aridity at higher elevations was considered an important determinant of species richness and abundance for Hemerodromiinae, especially Chelipodini which require moist soils for development. Elevational patterns of strictly aquatic Hemerodromiini were reasoned to arise from geometrical constraints limiting the number of suitable streams at altitudinal extremes.- there are more water courses at intermediate altitudes where abundance and diversity are greatest. Historical factors underlying current diversity and abundance patterns are discussed and hypothesised to involve three elements. (1) Historically concurrent orogenesis of mountain ranges, development of a seasonally arid monsoon climate in Southeast Asia and dry periods associated with glacial maxima forced altitudinal migrations into and subsequent radiation in aseasonal moist forests that developed on nascent mountains. (2) Immigration from the northwest and south along ‘corridors’ of montane moist forest, and (3) dispersal of lowland forms inhabiting seasonally dry biotopes in response to climatic drying.