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Inter-annual variation in amphibian larval interspecies interactions.
Ford J
,
Green DM
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The outcomes of species interactions can vary by life stage, year, and surrounding environmental conditions. Amphibian species are expected to compete most strongly during their tadpole stage when they exist in the highest densities. Changes in arrival timing, surrounding aquatic communities, and yearly conditions could all affect the outcome of larval competition. In Long Point, Ontario, the Fowler's toad (Anaxyrus fowleri) is at the northern edge of its range and overlaps with the more common American toad (Anaxyrus americanus). Both species breed in ponds that encounter high inter-annual variation. To determine whether these species compete strongly, and if this effect was replicated across multiple years, we raised both species as tadpoles together and, apart, in mesocosms in 2018 and 2021. We measured survivorship to, weight at, and time to metamorphosis for both species in both years. We determined that the presence of American toad tadpoles consistently had a detrimental effect on Fowler's toad tadpoles, even though this effect presented itself differently across years. Our study suggests that competitive exclusion by American toads could be occurring at the edge of the Fowler's toad's range. This study further demonstrates the importance of studying communities across multiple years to understand the full scope of species interactions.
FIGURE 1.
(a) Average survivorship at metamorphosis of American toad tadpoles and Fowler's toad tadpoles in single-species and mixed-species mesocosms in 2018 and 2021. (b) Average weight to metamorphosis of American toad tadpoles and Fowler's toad tadpoles in single-species and mixed-species mesocosms in 2018 and 2021.
FIGURE 2.
Weight of toadlets emerging from each experimental group of mesocosms in 2021, starting from the first day of toadlet emergence (June 16th). Each point represents an individual toadlet. (a) Single-species American toad tadpole mesocosms with a density of 50 tadpoles. (b) Single-species American toad tadpole mesocosms with a density of 100 tadpoles. (c) Single-species Fowler's toad tadpole mesocosms with a density of 50 tadpoles. (d) Single-species Fowler's toad tadpole mesocosms with a density of 100 tadpoles. (e) Mixed-species American toad and Fowler's toad tadpole mesocosms with a density of 100 tadpoles (50 of each species). Illustrations by Jessica Ford.
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