Document: JOH-3-99-141

Trophic links between rivers and riparian lizards: Indirect effects of aquatic insect subsidies on terrestrial resources.

SABO, J.L.* and M.E.POWER

University of California, Berkeley, CA 94720 USA ¹

Abstract:
Resource flow between rivers and surrounding watershed habitats can have direct and indirect effects on recipient food webs. We document strong links between riverine insects, riparian lizards, Sceloporus occidentalis, and terrestrial invertebrates. We used 2 m high "subsidy shields" to experimentally reduce aquatic insect flux at the river-land interface in two separate experiments. First, we used lizard enclosures with either reduced or ambient subsidy levels to evaluate the effects of riverine subsidies on lizard growth rates. Second, we employed experimental plots open to lizard movement with or without subsidy shields to measure the numerical response of lizards to riverine subsidies. We also measured the abundance of ground dwelling terrestrial invertebrates in both experiments to assess the direction of indirect effects on in situ prey. Lizards grew up to seven times faster in ambient vs. reduced subsidy enclosures (P < 0.05). Moreover, lizards emigrated at significantly higher rates from reduced subsidy plots open to lizard movement, leading to two-fold decreases in density in shielded plots (P < 0.05). Average density of lizards (0.034 I*m²) in ambient open plots was nearly identical to that used in the enclosure experiment. Fixed densities of lizards in enclosures had stronger effects on terrestrial invertebrates in reduced vs. ambient subsidy enclosures (P < 0.05). This suggests that if lizard densities are held constant, subsidies may release terrestrial resources from predation pressure by lizards and that seasonal declines in subsidy availability may result in pronounced negative effects as a result of lizard diet shifts to in situ resources. In contrast, we observed no significant difference in terrestrial invertebrate prey abundance between ambient and reduced subsidy plots open to lizard movement. We hypothesize that numerical tracking of aquatic resources by lizards may compensate for strong negative effects brought on as a result of diet shifts by lizards to terrestrial resources. These experiments demonstrate that riverine subsidies can enhance the performance and local density of riparian consumers, and that subsidies can exert either positive or negative indirect effects on terrestrial prey depending on the temporal and spatial scales of resource availability.

Keywords: rivers,riverine insects,riparian lizards,terrestrial invertebrates

This abstract is being presented at: 11:30 AM in session:
Symposium # 3: Linking Communities Across Ecosystem Boundaries: A Symposium in Memory of Gary A. Polis.