Determinants of food resource assimilation by stream insects along a tropical elevation gradient.

Food resource availability varies along gradients of elevation where riparian vegetative cover exerts control on the relative availability of allochthonous and autochthonous resources in streams. Still, little is known about how elevation gradients can alter the availability and quality of resources and how stream food webs respond. We sampled habitat characteristics, stable isotope signatures (δ13C, δ15N, δ2Η) and the carbon, nitrogen and phosphorus composition of basal food resources and insects in 11 streams of similar size along an elevation gradient from 1260 to 4045 m on the northeastern slope of the Ecuadorian Andean-Amazon region. Algal-based (autochthonous) food resources primarily supported insects occurring at higher elevations, but at low elevations there was a shift to greater allochthony, corresponding with lower light availability and reduced epilithon resource abundance. Additionally, percent phosphorus (%P) of both autochthonous and allochthonous food resources and of body tissue for some abundant insect taxa (stonefly Anacroneuria and mayfly Andesiops) declined with increasing elevation, despite the greater autochthony at high elevation. Allochthonous food resources were always a lower quality food resource, as indicated by higher C:N, N:P, and lower %P, across elevation in comparison to autochthonous resources, but autochthonous resources had higher %P than allochthonous resources across all elevations and comprised a greater portion of high-elevation insect resource assimilation. Aquatic insects may be able to compensate for the lower quality of both resource types at high elevations through altered body stoichiometry, even though higher quality autochthonous-based foods are in high abundance at high elevations.