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We conducted field surveys and experiments to evaluate the hypothesis that
predation is an important driving factor determining the degree of
coexistence between red and green morphs of the pea aphid Acyrthosiphon
pisum. Theory suggests that the different colour morphs are differentially
susceptible to natural enemies and selection by predation which in turn
leads to variable relative abundances of red and green morphs among host
plants across landscapes. Our field surveys on pea and alfalfa revealed,
however, that the colour morphs tended to coexist closely in a ratio of
one red to three green aphids across fields with different host plant
monocultures. Experimentation involving manipulation of the relative
abundances of the two colour morphs on host plants pea and alfalfa with
and without predator presence revealed that red morphs had higher or same
fitness (per capita reproduction) than green morphs on both pea and
alfalfa only when in the proportion of one red/three green proportion.
Moreover, experimentation evaluating predator efficiency revealed that red
morphs are safest from predation when in a 1 : 3 ratio with green morphs.
These results suggest that in addition to predation selection effects, red
morphs may behaviourally choose to associate with green morphs in a narrow
1 : 3 ratio to maximize their fitness. This evidence, along with existing
published data on red and green morph anti-predator behaviour indicates
that a 1 : 3 red and green morph coexistence ratio is driven by a balance
between predation pressure and behavioural assorting by red morphs across
landscapes. In this way predators may have ecological-evolutionary
consequences for traits that affect the colour morphs' proportion and
tolerances to selective pressure.
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