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)
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