This study aim was to incestigate the impact of land clearing on changes of macroinvertebrate communityin the Thamyai stream, Nam Nao National Park, Phetchaboon province, Thailand. The study was conducted between April and May 1996. Fifty sampling ststions were established along the stream course, about 1500 m length. These stations included the stream sites located comparatively in (i) healthy with well protected forest, (ii) modified land for cropplantation with sparse vegetation cover, and lastly (iii) intensive agricultural lands with minimal forest cover. Three replicated were randomly sampled from each station using the Surber sampler. Nine water quality variables and percent forest cover at each sampling site were examined and analyzed. Variations of macroinvertebrate community as well as environmental attributes were analyzed by ANOVA. Correlation between macroinvertebrate abundance and environmental factors was also tested. Multivariated methods;classification and ordination analyses in the PATN package, were used to identify association pattern between sites and environmental attributes.
The study results showed that percent forest cover was significantly correlated with certain immediate environmental variables of the streams. These were ambient and water temperatures, total dissolved solids (TDS), eletrical conductivity (EC), and the composition of streambed substrates and underlaying detritus. The sites located in densely forested areas had low ambient and water temperatures, high levels of TDS and EC, and also with high propotional composition of pebble and gravel substrates. In contrast, the site located in the forest-clearing zone, the ambient and water temperatures were relatively high, but TDS and EC comparatively decreased.Their substratum type was different from those sites in the forestland, which it was mostly composed of sand, silt, leaf-packed and twigs. Macroinvertebrate community structure significantly varied along the stream length. The site located in the dense forest had markedly higher taxon richness and individual density than sites in the cleared land.
Among macroinvertebrate taxa assemblage, Trichoptera, Diptera, Coleoptera and Ephemeroptera, their taxon richness and density showed a significant correlation with altered percent forestcover and those environmental sttributes. In other words, taxa community were more diverse in the forestland than in the modified zone. Mesogastropoda mollusk, in contrast, did not reveal significantassociation with any environmental factors
The variation of percent cover along the stream length, remarkably, had an influential effect on sensitive macroinvertebrate assemblages (Ephemeroptera, Plecoptera, and Trichoptera-EPT). Generally, along the stream course, the EPT were abundant in forestland stretches, while they colonized minimally in streambeds in the agricultural areas.The higher EPT taxon richness and density were well associated with the greater percent forest cover. The varied forest canopy subsequently altered immediateenvironmental attributes sites, including substrate types, temperature, TDS and EC. All these interrelated milieus were prominently attributable to EPT taxa assemblage. Another macroinvertebrate group, especially Chironomid larvae, its density was not significantly different between sites. in other words, the midge larvae were relatively tolerant to environmental changes.
In summary, macroinvertebrate community colonized along the stream length was varied according to the altered forest cover. The different forest canopy subsequently affected the immediate environmental attributes and habitat at a site, which further influencing the extent to which the macroinvertebrate fauna colonization.