Cross-ecosystem bottlenecks alter reciprocal subsidies within meta-ecosystems.

Reciprocal subsidies link ecosystems into meta-ecosystems, but energy transfer to organisms that do not cross boundaries may create sinks, reducing reciprocal subsidy transfer. We investigated how the type of subsidy and top predator presence influenced reciprocal flows of energy, by manipulating the addition of terrestrial leaf and terrestrial insect subsidies to experimental freshwater pond mesocosms with and without predatory fish. Over 18 months, fortnightly addition of subsidies (terrestrial beetle larvae) to top-predators was crossed with monthly addition of subsidies (willow leaves) to primary consumers in mesocosms with and without top predators (upland bullies) in a 2 × 2 × 2 factorial design in four replicate blocks. Terrestrial insect subsidies increased reciprocal flows, measured as the emergence of aquatic insects out of mesocosms, but leaf subsidies dampened those effects. However, the presence of fish and snails, consumers with no terrestrial life stage, usurped and retained the energy within in the aquatic ecosystem, creating a cross-ecosystem bottleneck to energy flow. Thus, changes in species composition of donor or recipient food webs within a meta-ecosystems can alter reciprocal subsidies through cross-ecosystem bottlenecks.