Land use change in the Atlantic Forest affects carbon and nitrogen sources of streams as revealed by the isotopic composition of terrestrial invertebrates

Mudanças no uso do solo na Mata Atlântica afetam fontes de carbono e nitrogênio em riachos como é revelado através da composição isotópica dos invertebrados terrestres

Terrestrial invertebrates link terrestrial systems to aquatic ones, making vegetal material produced in the watershed available to aquatic food webs. In this study, using carbon and nitrogen stable isotopes, we evaluated the importance of introduced C₄ grasses as a source of carbon in aquatic food webs of headwater streams of the coastal Atlantic Forest located on the north coast of the State of São Paulo, in the southeastern region of Brazil. Terrestrial invertebrates were collected in two streams: one where the main land cover was pristine montane Atlantic Forest (forest stream) and another where the main land cover was introduced C₄ forage grasses for livestock (pasture stream). The average δ¹³C of terrestrial invertebrates collected in the forest stream (−26.3±2.1‰) was significantly (p<0.01) smaller than the average δ¹³C of terrestrial invertebrates collected in the pasture stream (−15.7±4.7‰), denoting a larger contribution of C₄ grasses to terrestrial invertebrates of the pasture stream. The average δ¹⁵N of terrestrial invertebrates of the forest stream (4.1±2.4‰) was significantly (p<0.01) lower than the average δ¹⁵N of terrestrial invertebrates of the pasture stream (9.5±2.7‰). The relative contribution of C₃ and C₄ plants to terrestrial invertebrates was estimated using SIAR. In the forest stream, the C₃ contribution was on average 0.75 (0.72 minimum to 0.79 maximum), and the C₄ contribution was on average 0.25 (0.21 minimum to 0.28 maximum). In the pasture stream, the C₃ contribution decreased to 0.20 (0.14 minimum to 0.26 maximum), and the C₄ contribution increased to 0.80 (0.74 minimum to 0.86 maximum). These results have several implications for the ecosystem functioning as well as for recent changes in environmental policies of Brazil. The lower nutritional value of C₄ grasses may not only decrease invertebrate performance, but also alter the stoichiometry of several components of the aquatic food webs with potential consequence for the whole ecosystem functioning. On the public policy side, recent changes in the Brazilian Forest Act, a series of laws that regulate land cover at the property level, reduced the width of the forested riparian area with potentially dangerous consequences for aquatic ecosystems.

Atlantic Forest; land use; streams; stable isotopes; terrestrial invertebrate