Sammendrag
Prey preference of top predators and energy flow across habitat boundaries are
of fundamental importance for structure and function of aquatic and terrestrial
ecosystems, as they may have strong effects on production, species diversity,
and food-web stability. In lakes, littoral and pelagic food-web compartments
are typically coupled and controlled by generalist fish top predators. However,
the extent and determinants of such coupling remains a topical area of ecological
research and is largely unknown in oligotrophic high-latitude lakes. We analyzed
food-web structure and resource use by a generalist top predator, the
Arctic charr Salvelinus alpinus (L.), in 17 oligotrophic subarctic lakes covering a
marked gradient in size (0.5–1084 km2) and fish species richness (2–13 species).
We expected top predators to shift from littoral to pelagic energy sources with
increasing lake size, as the availability of pelagic prey resources and the competition
for littoral prey are both likely to be higher in large lakes with multispecies
fish communities. We also expected top predators to occupy a higher
trophic position in lakes with greater fish species richness due to potential substitution
of intermediate consumers (prey fish) and increased piscivory by top
predators. Based on stable carbon and nitrogen isotope analyses, the mean reliance
of Arctic charr on littoral energy sources showed a significant negative
relationship with lake surface area, whereas the mean trophic position of Arctic
charr, reflecting the lake food-chain length, increased with fish species richness.
These results were supported by stomach contents data demonstrating a shift of
Arctic charr from an invertebrate-dominated diet to piscivory on pelagic fish.
Our study highlights that, because they determine the main energy source (littoral
vs. pelagic) and the trophic position of generalist top predators, ecosystem
size and fish diversity are particularly important factors influencing function
and structure of food webs in high-latitude lakes.
Benthic, energy mobilization, food-chain
length, habitat coupling, lake morphometry,
predation, resource competition, stable
isotope analysis, trophic niche
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