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Coexisting congeners: demography, competition, and interactions with cardenolides for two milkweed-feeding aphids

Summary: Coexisting congeners: demography, competition, and interactions
with cardenolides for two milkweed-feeding aphids
Kailen A. Mooney, Patricia Jones and Anurag A. Agrawal
K. A. Mooney (mooneyk@tritrophic.org), Dept Ecology and Evolutionary Biology, Univ. of California at Irvine, 321 Steinhaus Hall, Irvine, CA
92697-2525, USA. KAM, P. Jones and A. A. Agrawal, Dept Ecology and Evolutionary Biology, Corson Hall, Cornell Univ., Ithaca, NY
14853, USA.
Explaining the coexistence of closely related species sharing a single resource has been a long-standing challenge in
ecology. Here we report on studies comparing the aphids Aphis nerii and A. asclepiadis that feed sympatrically on the
milkweed Asclepias syriaca in northeastern North America. We sought to identify tradeoffs among species' attributes that
might promote coexistence, but in most instances A. nerii was superior to A. asclepiadis. Aphis nerii was 84% more
fecund, fed upon 880% more phloem sap, and was affected 70% less by intraspecific competition as compared to
A. asclepiadis. In interspecific competition, A. nerii reduced A. asclepiadis abundance by 77%, whereas A. asclepiadis did
not affect A. nerii. In dispersal trials, 10% of winged A. nerii but only 1% of A. asclepiadis successfully moved from non-
host plants to A. syriaca. We also investigated whether there were differences in aphid interactions with milkweed
cardenolides. Jasmonic acid increased milkweed cardenolides by 33%, a realistic amount similar to that induced by
several leaf-chewing herbivores. Nevertheless, jasmonate-induced cardenolides failed to affect aphid performance and
aphid feeding had no effect on milkweed cardenolide concentration. Yet cardenolides were important for aphid resistance
to predators; A. nerii sequestered 25% more cardenolides and was preyed upon 50% less than A. asclepiadis. Interactions
with cardenolides thus again favored A. nerii over A. asclepiadis. Given that A. nerii and A. asclepiadis are decidedly not
equivalent in their demography, competitive ability, defense and dispersal, our results strongly refute the notion that


Source: Agrawal, Anurag - Department of Ecology and Evolutionary Biology & Entomollogy, Cornell University
Mooney, Kailen A. - Department of Ecology and Evolutionary Biology, University of California, Irvine


Collections: Environmental Sciences and Ecology