Consequences and causes of geographic variation in the body size of a keystone predator, Notophthalmus viridescens

Two subspecies of the predatory aquatic salamander Notophthalmus, N. viridescens viridescens and N. v. dorsalis, differ in adult body size and geographic distribution. We tested whether experimental populations of the two predator subspecies differed in their effects on prey populations of B. americanus, and whether observed differences in predator body size were genetic and/or environmentally induced. We compared the effects of predation by both Notophthalmus subspecies on larval Bufo americanus by experimentally manipulating the densities (0, 2, or 4 newts/m³) and subspecies of Notophthalmus (N. v. viridescens or N. v. dorsalis) added to artificial ponds. Both Notophthalmus subspecies significantly reduced B. americanus survival, but differed significantly in this effect. Fewer Bufo survived with the larger subspecies, N. v. viridescens, than with the smaller Notophthalmus subspecies, N. v. dorsalis. The Notophthalmus subspecies differed in their patterns of adult and larval growth. Adults of the smaller subspecies, N. v. dorsalis, had a significantly higher growth rate than the larger subspecies, N. v. viridescens, under common environmental conditions, suggesting that differences in predator size were partly genetic, rather than entirely environmentally induced. Larval N. v. dorsalis metamorphosed significantly later in the season than larvae of N. v. viridescens, suggesting that larval N. v. dorsalis had a lower growth rate than larval N. v. viridescens. Differences in adult and larval growth, together with differences in the minimum adult size observed in natural populations, suggest that differences in the rate or duration of pre-adult growth may contribute substantially to observed differences in size.