Macroinvertebrate assemblages in streams of the St. Paul National Park, Palawan, the Philippines


Hendrik Freitag


College of Science



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This dissertation focuses on insect and decapod assemblages of two rivers and their estuaries of the St. Paul Subterranean River National Park in Palawan and methodological aspects of emergence collection in tropical streams. Organismic and abiotic data were retrieved during a sampling period from September 2000 to August 2001 at 6 (insect emergence) or 7 sites along the longitudinal course of the Cabayugan / Underground River and 2 sites of the Panaguman River. An emergence trap designed for use in the tropics is presented. A special modification allowed a fast and easy replacement of the screen when damaged by flooding, as was required when traps are used downstream of the headwaters. Problems concerning the use of emergence traps in the humid tropics are discussed based on the experience at the longitudinal stream sections from spring to estuary. Sampling of the middle to lower stream sections and the estuaries caused higher temporal incompleteness of the data sets. The collections obtained by these traps are the first samples of aquatic insect emergence covering an entire longitudinal river course and the first emergence data from streams in the Philippines and Greater Sunda. Their composition and longitudinal patterns are presented mainly on order or family level. A decline of total emergence towards estuaries was observed, affecting all major orders. Diptera (86%), among them 67% Chironomidae, dominated all sites. High abundances in Ceratopogonidae, Odonata, and Coleoptera were found, compared to other emergence studies from tropical and temperate latitudes. Ephemeroptera displayed a highly variable contribution to the emergence in this and other studies. In the Panaguman River they appear to be limited by environmental variables such as pH. Trichoptera are likely to tolerate a wider range of environmental conditions and they are consequently able to fill further niches where Ephemeroptera are under-represented. Except for the scarce abundances of Plecoptera observed in this and other studies from the tropics, no substantial differences in the emergence composition at order level were found between temperate and tropical rivers of lower order, however, with a remarkable local variation. The components of the riparian, the non-aquatic and the non-emergent fauna contributing to the collections are discussed based on trap features. The samples of the Decapoda assemblages were obtained with colonization samplers at two distinct sub-habitats of each stream section and additionally with drift nets, which were used at all freshwater sites. Thirty five species of Decapoda belonging to 10 families were recorded including 9 Atyidae, 8 Palaemonidae and 7 Grapsidae. Among these there are six taxa not previously recorded in Palawan, three endemics and the new species Parathelphusa cabayugan Freitag & Yeo, 2004 which is described together with another new species of the genus, Parathelphusa manguao Freitag & Yeo, 2004 from the only natural freshwater lake of Palawan Island, Lake Manguao. The two new species are each easily distinguished from their closest congeners by characters of the male first pleopod, in conjunction with a unique combination of other morphological characters. A key to the Parathelphusa species of the Philippines is provided. The longitudinal zonation patterns of the decapod assemblages are illustrated by species congruity curves and Detrended Correspondence Analyses (DCA). The latter were correlated with the environmental variables water temperature, pH, electrical conductivity, dissolved oxygen, BOD, stream width, stream order, water depth, discharge, altitude, the site distance from the estuary and the source as well as various descriptive variables for the substrates and for the stream hydraulics. The topographic features altitude and distance from the estuary were found to be the major determinants for the decapod zonation in these streams. The DCA produced best results when pooled data of colonization and drift were used and when mass occurrences of unidentifiable juveniles were excluded from the ordination. The diversity was highest at the estuarine sites and lowest in the anthropogenically altered middle stream course. A complete species turn over from the headwater to the estuary was observed for the Cabayugan River when unidentifiable juveniles had been excluded. Regarding that many of the recorded species are amphidromous or potamodromous, the role of the river course as a passage appeared to be functioning. The 8 km long hypogean section of the Underground River was not seen to inhibit migration processes which these species must undergo during their ontogenesis. This section additionally accommodated trogloxene and troglobiont taxa. For those decapods depending on saltwater during their ontogenesis, dispersal appeared to be transitional within the Southeast-Asian archipelago. The high species richness particularly found for Trichoptera, aquatic Coleoptera and Decapoda underlines the character of this region as a biodiversity hotspot.





Decapoda (Crustacea)—Philippines—Palawan; Aquatic insects—Philippines—Palawan

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