| Dams have modified rivers worldwide, with regions such as the Sierran foothills of California being heavily impacted by changed in river flow, sediment transport, chemical and physical properties of rivers downstream of artificial reservoirs. Another important and undesirable consequence are changes in temperature such that conditions downstream of a dam may not be suitable to the riverine organisms that evolved with a given catchment. One iconic river-breeding organism is the foothill yellow-legged frog, Rana boylii. At present, R. boylii occupies less than half its historic range, due to the existence of large dams in many river systems. Dam-associated threats include loss of habitat when rivers are converted to lakes, and mortality when extreme variation in stream flow causes stranding and scouring of early life stages. |  |
Low flow conditions downstream of dams promote invasion by non-native species, including predators, competitors and sub-optimal food resources for the frogs. However, some populations persist even in spite of unfavorable water flow and temperature conditions. In collaboration with Dr. Sarah Kupferberg, we aimed at understanding the role of plasticity in thermal preference and performance in allowing river fauna to persist under temperature challenging conditions. We monitored water temperature in six river catchments inhabited by Rana boylii, pairing one regulated (i.e., dammed) and unregulated river in each catchment. We also compared thermal preference and performance of tadpoles from these catchments, in field and common-garden rearing experiments.
We found substantial geographic variation in frog population distribution and abundance based on river size combined with water temperature, but overall frogs in regulated rivers bred in colder waters than they did in free-flowing rivers. Sierran rivers were colder during spring (breeding and egg deposition) but became warmer during the summer ( when tadpoles develop) than coastal rivers. Tadpoles from all rivers had a positive linear growth response to temperature, but individuals from inland rivers displayed higher growth rates. Consistent with a counter-gradient model of selection in which the response to temperature change is in the opposite direction of the change, individuals from cooler rivers selected warmer temperatures. When reared under common conditions (photo right), however, tadpoles showed similar temperature (see thermal gradient below) preferences regardless of source river.
Plasticity in thermoregulatory behavior and growth performance may explain how small populations of Rana boylii are able to survive in river where cold water is released from reservoirs downstream of large dams. To further promote the persistence of these small populations, regulated rivers could be managed with a goal to increase the availability of edgewater, shallow habitats where tadpoles can thermoregulate and complete development by the end of summer.
Catenazzi A, Kupferberg SJ. 2017. Variation in thermal niche of a declining river-breeding frog: From counter-gradient responses to population distribution patterns. Freshwater Biol. ##:1–11. https://doi.org/10.1111/fwb.12942
Catenazzi A, Kupferberg SJ. 2017. Variation in thermal niche of a declining river-breeding frog: From counter-gradient responses to population distribution patterns. Freshwater Biol. ##:1–11. https://doi.org/10.1111/fwb.12942
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