Mike Benard
, Phenotypic plasticity, adaptation and population dynamics of Pacific Treefrogs (Pseudacris regilla).

I intensively studied the ecology and evolution of Pacific Treefrogs (Pseudacris regilla) on the Quail Ridge Reserve from 2001-2006. In this research, I focused on three major questions:

1) Do pacific treefrogs exhibit adaptive phenotypic plasticity in response to different degrees or types of predation risk?

Pacific Treefrogs lay their eggs in bodies of water that vary in predation risk, from ponds that contain no predators, to ponds that contain predatory insects, to ponds that contain predatory fish. The tadpoles develop different shapes in these different environments. In the presence of predatory insects, tadpoles develop deep tails and bodies, while in the presence of predatory fish, tadpoles develop shallow tails and bodies. When no predators are present, tadpoles have intermediate tail and body shapes.

To test if these environmentally-induced differences in tadpole shape are adaptive, I conducted a series of mescocosm and laboratory experiments. I found that this phenotypic plasticity is adaptive: when tadpoles with one predator-specific phenotype are exposed to a different predator, they suffer increased mortality. For example, tadpoles that develop the fish-specific phenotype (i.e. shallow tails) are more easily killed by predaceous diving beetles than tadpoles with the beetle-induced phenotype (i.e. deep tails). This trade-off in survival explains why phenotypic plasticity in tadpole shape has evolved in Pacific Treefrogs.

2) Do populations exhibit adaptation to geographic variation in the predators that feed on tadpoles (i.e., predaceous diving beetles or bluegill sunfish)?

I also investigated whether there were genetic differences in predator-induced defenses among Pacific treefrog populations. Eggs of Pacific treefrogs from different populations were reared in the lab, and exposed to four combinations of predator chemical cues (1 – No Predator, 2 – Beetle only, 3 – Fish Only, 4 – Combined Beetle and Fish). The pond type that a tadpole originated from (i.e. Fish Pond or Beetle Pond) affected the body shape that it developed. This finding demonstrates that tadpole phenotypic plasticity itself remains genetically variable, and can evolve in response to natural selection.

3) How does environmentally-induced variation in size at metamorphosis affect Pacific Treefrog survival?

The environmental conditions experienced by tadpoles affect their size at metamorphosis. To determine if variation in size at metamorphosis affects frog fitness, I conducted a mark-recapture study at the four ponds on the quail ridge reserve from 2002 through 2006. Over the course of the study I batch-marked over 13,500 metamorphosing frogs, and individually marked over 3,500 adult frogs. Within a population, size at metamorphosis was clearly under selection; larger metamorphs had higher survival to adulthood and bred on average one year earlier than smaller metamorphs. Additionally, variation in size at metamorphosis affected changes in the total adult population size. Thus, carry-over effects of the phenotypic plasticity of tadpoles can affect not only individual fitness, but also population dynamics.

Current Quail Ridge Research:

I continue to study the aquatic communities at the Quail Ridge Reserve. I return to the reserve each year to quantitatively sample the densities of Pacific Treefrog tadpoles, California Newt larvae, and aquatic insects. This data will be combined with data on weather and adult amphibian populations gathered through the use of the remote sensing network. Ultimately, this data set will allow us to investigate the factors that control the distribution and abundance of two geographically widespread amphibians, Pacific Treefrogs and California Newts.

Importance of the Quail Ridge Reserve to my research:

My research was only possible because of the many advantages of working on a UC Natural Reserve. The UC NRS staff has provided invaluable logistical support throughout my research. For instance, reserve staff helped me move mesocosms and thousands of liters of pond water around the reserve for experiments. Quail Ridge laboratory space allowed me to measure and safely mark anesthetized tadpoles, metamorphs and adult frogs on-site. I was able to spend 3-7 nights a week at Quail Ridge from January through July each year because of the living quarters and kitchen facilities. All-terrain vehicles on the reserve allowed me to access my field sites throughout the year, even on very rainy nights. The fact that access to Quail Ridge is limited to researchers and other approved visitors protects equipment such as mesocosms and dataloggers from interference or vandalism.

Publications derived from my research at Quail Ridge:

Benard, M.F. 2007. Predators and mates: conflicting selection on the size of male Pacific Tree Frogs (Pseudacris regilla). Journal of Herpetology. 41:317-320.

Benard, M. F. 2006 Survival trade-offs between two predator-induced phenotypes in pacific treefrogs (Pseudacris regilla). Ecology 87: 340-346

Benard, M. F. and Fordyce, J. A. 2003 Are induced defenses costly? Consequences of predator-induced defenses in western toads (Bufo boreas). Ecology. 84(1): 68-78

Mike Benard's Research Website: http://www.eeb.lsa.umich.edu/eeb/people/mfbenard/index.html
Pacific Treefrog website: http://www.mister-toad.com/PacificTreeFrog.html

Photo Credits: Title, Research, Beetle eating tadpole, tadpole shape, mesocosm, and adult P. regilla in pond (Mike Benard)

This page last updated: August 15, 2007  


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John Muir Institute of the Environment
109 The Barn, University of California, Davis, CA 95616
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