Intraguild Predation and Competition Mediate Stage-Structured Interactions between Wood Frog (Rana sylvatica) and Upland Chorus Frog (Pseudacris feriarum) Larvae

Although anuran larvae with generalist feeding morphology have traditionally been treated as lower-level consumers, emerging studies indicate that the larvae of many species are intraguild predators of other anurans. We conducted two experiments to examine priority effects of the Wood Frog (Rana sylvatica) on the Upland Chorus Frog (Pseudacris feriarum). Wood frogs hatch before upland chorus frogs and could potentially function both as intraguild predators and competitors of the latter. In a laboratory experiment, R. sylvatica did not prey on P. feriarum hatchlings that were greater than 1-day old. In a mesocosm experiment in which food level and the density of R. sylvatica hatchlings were manipulated, R. sylvatica engaged in stage-structured intraguild predation on egg masses of P. feriarum. The number of P. feriarum embryos surviving to hatching was inversely related to the initial density of R. sylvatica, but was independent of food level. Growth rates of P. feriarum hatchlings were inversely related to R. sylvatica density, suggesting strong interspecific competition. Collectively, these results illustrate that the density of an intraguild predator may mediate the strength of both stage-specific intraguild predation and interspecific competition. Our findings suggest that food webs in many seasonal pond communities are more complex than previously thought and need to be revised to incorporate intraguild predation by anurans.

Figure 1

Relationship between the mean mass of Rana sylvatica larvae during the egg predation stage (31 March) and the initial number of R. sylvatica hatchlings per pool for low (r² = 0.93) and high (r² = 0.94) food treatments.

Dashed trendline = high food treatment; solid trendline = low food treatment.

Figure 2

Relationships for low and high food treatments between (A) the mean number of Rana sylvatica larvae feeding on Pseudacris feriarum egg masses and the initial number of R. sylvatica established in pools, (B) the survival of P. feriarum to hatching and the mean number of R. sylvatica feeding on egg masses, and (C) the survival of P. feriarum to hatching and the initial number of Rana sylvatica established in pools.

Dashed trendlines = high food treatment: respective r² for (A), (B), and (C) = 0.77, 0.54, and 0.63; solid trendlines = low food treatment: respective r² for (A), (B), and (C) = 0.64, 0.34, and 0.73.

Figure 3

Relationship between the average mass of Rana sylvatica (A) and Pseudacris feriarum (B) on 28 April and the initial density of R. sylvatica hatchlings for low and high food treatments.

Dashed trendlines = high food treatment: respective r² for (A) and (B) = 0.94 and 0.91; solid trendlines = low food treatment: respective r² for (A) and (B) = 0.97 and 0.93.

Figure 4

Relationship between the percent survival of Rana sylvatica (A) and Pseudacris feriarum (B) to 28–29 April and the initial density of R. sylvatica hatchlings for low and high food treatments.

Dashed trendlines = high food treatment: respective r² for (A) and (B) = 0.09 and 0.03; solid trendlines = low food treatment; respective r² for (A) and (B) = 0.05 and 0.76.

Contributor Notes

Department of Biology, The University of North Carolina at Asheville, One University Heights, Asheville, North Carolina 28804. E-mail: (JWP) petranka@unca.edu. Send reprint requests to JWP.

Received: 20 Mar 2006

Accepted: 09 Aug 2006