1. This study assessed the ecological effects of substituting pasture
for forest through changes to benthic macroinvertebrate assemblages. 2.
Rapid river assessment protocol (RRAP) scores were determined for eight
river stretches, four stretches from each one of the two matrix
category, where benthic macroinvertebrates were also sampled. 3. All
forest fragment stations were sorted into the natural category, but one,
which was categorized as altered; all pasture stations were sorted into
the impacted category; there were statistically significant differences
in terms of RRAP scores and macroinvertebrate assemblage structures
between the two matrix types; 18 taxonomic units contributed the most to
this difference; Concerning their water-quality tolerance, seven were
considered intolerant, eight tolerant, and one resistant to pollution
while two could not be placed into any pollution resistance category;
39.44% of all specimens collected was herbivores, 8.06% was
detritivores, 7.92% was predators, and 44.57% of them could not be
categorized into a feeding category; forest fragment river stretches had
greater taxon richness and relative abundances of pollution-intolerant
taxa and predators than pasture river stretches; a positive relationship
was found between RRAP score and intolerant taxa abundance. 4. These
results may be a consequence of changes to habitat properties related to
the matrix substitution, such as greater fine sediment suspension,
greater nutrient concentration, and lower availability of food
resources; the stress imposed by these changes mainly selects the more
resistant taxonomic groups over the more sensitive ones, such as
Ephemeroptera, Plecoptera, and Trichoptera; this indicates a reduction
in water quality of river stretches without riparian forests.
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