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Limitations of a Generalist: a Field Comparison of Foraging Snakes

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[The assumption that a "jack-of-all-trades" is master of none was examined by comparing a snake that forages under water and in air (a terrestrial-aquatic generalist) with a congener that forages only under water (an aquatic specialist). Thamnophis eques is a generalist species because across its geographic range it feeds at the air/water interface, underwater and on land, taking amphibious, aquatic and terrestrial prey; T. melanogaster is an aquatic specialist, everywhere feeding underwater on aquatic prey. Both snakes were studied at an isolated desert pond where the main prey was the Rio Grande leopard frog (Rana berlandieri). Thamnophis eques ate frogs and late-stage tadpoles, and small numbers of other anurans, mammals and lizards, whereas T. melanogaster ate leopard frog tadpoles of all developmental stages and a few frogs. Different frog life stages were captured by the two snakes even when foraging simultaneously in the pond. Thamnophis eques foraged mainly over the water surface and shoreline and also strayed from the pond, whereas T. melanogaster foraged mainly underwater and only in the pond. When the frog population crashed in the second year, T. melanogaster continued to prey on tadpoles and reproduced normally, but the numbers, foraging activity and reproduction of T. eques declined drastically. Apparently, the terrestrial-aquatic generalist was unable to forage profitably on tadpoles, probably because it is ineffective underwater (compared with a congener specialised in that medium). Despite its broad diet and foraging repertoire, T. eques may be a truly effective predator only on frogs at the air/water interface; possibly aquatic prey are taken only when especially vulnerable, and terrestrial prey can only be added to the diet opportunistically. In contrast, T. melanogaster appears limited to strictly aquatic foraging, never opportunistically adding terrestrial prey. Different use of tadpole developmental stages by the two snakes indicates that the reported special vulnerability of late stages holds for generalist but not aquatically specialised snake predators. Failure of T. melanogaster to forage when tadpoles were at peak abundance could be due to an inability to forage profitably when the mean temperature is below 19°C, a limitation that frogs may exploit by ovipositing early., The assumption that a "jack-of-all-trades" is master of none was examined by comparing a snake that forages under water and in air (a terrestrial-aquatic generalist) with a congener that forages only under water (an aquatic specialist). Thamnophis eques is a generalist species because across its geographic range it feeds at the air/water interface, underwater and on land, taking amphibious, aquatic and terrestrial prey; T. melanogaster is an aquatic specialist, everywhere feeding underwater on aquatic prey. Both snakes were studied at an isolated desert pond where the main prey was the Rio Grande leopard frog (Rana berlandieri). Thamnophis eques ate frogs and late-stage tadpoles, and small numbers of other anurans, mammals and lizards, whereas T. melanogaster ate leopard frog tadpoles of all developmental stages and a few frogs. Different frog life stages were captured by the two snakes even when foraging simultaneously in the pond. Thamnophis eques foraged mainly over the water surface and shoreline and also strayed from the pond, whereas T. melanogaster foraged mainly underwater and only in the pond. When the frog population crashed in the second year, T. melanogaster continued to prey on tadpoles and reproduced normally, but the numbers, foraging activity and reproduction of T. eques declined drastically. Apparently, the terrestrial-aquatic generalist was unable to forage profitably on tadpoles, probably because it is ineffective underwater (compared with a congener specialised in that medium). Despite its broad diet and foraging repertoire, T. eques may be a truly effective predator only on frogs at the air/water interface; possibly aquatic prey are taken only when especially vulnerable, and terrestrial prey can only be added to the diet opportunistically. In contrast, T. melanogaster appears limited to strictly aquatic foraging, never opportunistically adding terrestrial prey. Different use of tadpole developmental stages by the two snakes indicates that the reported special vulnerability of late stages holds for generalist but not aquatically specialised snake predators. Failure of T. melanogaster to forage when tadpoles were at peak abundance could be due to an inability to forage profitably when the mean temperature is below 19°C, a limitation that frogs may exploit by ovipositing early.]

Affiliations: 1: Centro de Ecologia, Universidad Nacional Autónoma de México, A.P. 70-275, Delegación Coyoacán, 04510 DF, México

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