Impacts of Agricultural Land Use on Stream Ecosystems of the Coffee-Growing Region of Columbia

Abstract

The pressures of a growing population and a fluctuating economy have caused extensive land use transformations in the Andean region of Colombia, as more than 63% of the natural land cover has been replaced by cattle pastures and crop fields. To date, the specific consequences of this development for stream ecosystems remain unclear. In this study, land use, habitat, and macroinvertebrate community characteristics were measured in 30 first order streams of the coffee-growing region of Colombia. This information was analyzed using structural equation modeling to evaluate effects of agriculture on macroinvertebrate community acting indirectly through effects on riparian condition and instream habitat characteristics. Landform and land use were measured in catchments and riparian zones of the 30 streams using digital elevation models and QuickBird satellite imagery. Stream habitat was evaluated in a 100-meter reach using water physicochemical characteristics, discharge, channel morphology, substrate, and type of flow. Macroinvertebrate tolerance was measured using a version of the Biological Monitoring Working Party adapted for the region (BMWP-Univalle). The results supported the hypothesis that agricultural land use has strong negative impacts on stream ecosystems, as reflected in the aquatic macroinvertebrate community. These negative impacts occurred indirectly, appearing to act through a reduction of riparian forest width and availability of coarse substrates, and an increase in the percent of slow-flowing habitats and the ammonia nitrogen concentration in the water. The extent of riparian forest had a positive indirect influence on the macroinvertebrate community by reducing the percent of slow-flowing habitats in the reach. Furthermore, the percent of slow-flowing habitats in stream reaches proved to be an important indicator of habitat deterioration in the studied systems. These results indicate that local farming practices such as elimination of the riparian forest, excess application of fertilizers, and cattle grazing in riparian zones are responsible for most of the impacts of agriculture on stream habitat and communities. Land management practices such as establishment of riparian forest buffer strips, control of grasses in the riparian zone, and fences to prevent the access of cattle to the stream channel are recommended to mitigate the negative effect of agriculture on these systems. The results also highlight the importance of local studies of iii land use, given that the effects of agriculture are strongly affected by local farming practices and environmental conditions.