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c_ >l.s S S S
m
N < DN DN < D
c DN DN D
k =0 k = 0.4- k = 1
Figure 5.--Basic game dynamics when neighbor effects are excluded (e = 0). N, D, and S correspond to populations where
q = 1, p = 1 and r = 1 respectively.
The third situation corresponds to "evolutionarily stable" associational refuges as both plant types coexists (Sabelis and de
Jong 1988, Tuomi and Augner 1993). The coexistence is possible because both types have a selective advantage when rare,
but a disadvantage when common (Fig. 2b). In our case, stable polymorphism requires that D has a higher fitness value for p
close to 0. However, when p increases toward 1, WN increases more steeply than W D. The slope of W Nas a function p is em,
while the slope of W Dis (1/2)era. The reason for this difference is that h' = 1/2. Consequently, the associational protection
has a greater value for N that suffers h = 1, while D suffers only a half of that value. If both N and D would win equally from
the neighbors' defenses (i.e., h' = h), associational protection would not maintain stable polymorphism in the present case.
The major lesson of this subgame is that the associational effects counteract the evolution of lethal defenses. We
have proposed that this implies a dilemma for plant defenses: the defenses should protect the plant without benefiting
nondefensive neighbors too much (Tuomi et al. submitted). If the defensive plants kill all herbivores, the nondefensive type
can easily invade. Consequently, one could expect that "less lethal" defenses could do a better job in the long run. However,
when adopting sublethal defenses the plant itself will suffer more damage by herbivores. There are at least two solutions for
the dilemma. First, if a plant is allowed to adopt a lethality-level (d) between 0 and 1, associational refuge effects select for
sublethal defenses with d < 1 (Tuomi et al. submitted). Second, another solution of the dilemma could be a defense strategy
(S) that is potentially "lethal" to the herbivores, but that is associated with chemical or visual traits which herbivores can use
as cues in order to avoid the potentially "lethal" defenses. Below, we analyze this second possibility. Since we have set d = 0
for S, we thus assume that feeding aversion is induced before the amount of ingested food reaches any toxic level.
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