Reciprocal subsidies between rivers and terrestrial habitats
are common where terrestrial leaf litter provides energy to aquatic
invertebrates while emerging aquatic insects provide energy to terrestrial
predators (e.g., birds, lizards, spiders). We examined how aquatic insect
emergence changed seasonally with litter from two foundation riparian trees,
whose litter often dominates riparian streams of the southwestern United
States: Fremont (Populus fremontii) and narrowleaf (Populus angustifolia)
cottonwood. P. fremontii litter is fast-decomposing and lower in defensive
phytochemicals (i.e., condensed tannins, lignin) relative to P. angustifolia.
We experimentally manipulated leaf litter from these two species by placing
them in leaf enclosures with emergence traps attached in order to determine how
leaf type influenced insect emergence. Contrary to our initial predictions, we
found that packs with slow-decomposing leaves tended to support more emergent
insects relative to packs with fast-decomposing leaves. Three findings emerged.
Firstly, abundance (number of emerging insects m−2 day−1) was 25 % higher on
narrowleaf compared to Fremont leaves for the spring but did not differ in the
fall, demonstrating that leaf quality from two dominant trees of the same genus
yielded different emergence patterns and that these patterns changed
seasonally. Secondly, functional feeding groups of emerging insects differed
between treatments and seasons. Specifically, in the spring collector-gatherer
abundance and biomass were higher on narrowleaf leaves, whereas
collector-filterer abundance and biomass were higher on Fremont leaves.
Shredder abundance and biomass were higher on narrowleaf leaves in the fall.
Thirdly, diversity (Shannon’s H′) was higher on Fremont leaves in the spring,
but no differences were found in the fall, showing that fast-decomposing leaves
can support a more diverse, complex emergent insect assemblage during certain
times of the year. Collectively, these results challenge the notion that leaf
quality is a simple function of decomposition, suggesting instead that aquatic
insects benefit differentially from different leaf types, such that some use
slow-decomposing litter for habitat and its temporal longevity and others
utilize fast-decomposing litter with more immediate nutrient release.