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:hen k > 0 suggest that the cue or the signal associated with S is costly. Second, the major difference between S and D is that
does not allow N to escape from herbivory in mixed pairs. That is why the fitness of N, when playing against S, reduces to
whereas Ws will be
W N= W0- 2m
Ws = W0 - C(I + k)-l/2 [l+(1-r)e]m
where r is the population frequency of S. The population frequency of N is q = 1 - r. Also in this case we have three
possibilities (Fig. 3a):
(1) N will be an ESS when C/m > (3 - e)/2(l+k).
(2) S will be an ESS when C/m < 3/2(1+k).
(3) There will be a unstable equilibrium 0 < _ < 1 when both N and S are ESSs.
In other words, no stable coexistence is possible since S will be an ESS above the equilibrium frequency ?, and N will be an
ESS below the equilibrium (Fig. 3b).
In summary, this subgame has two interesting aspects. The first is that nondefensive plants do not benefit from S
because herbivores are assumed to be able to make a distinction between N and S types. The second is that the defensive
plants will benefit from each other if feeding aversion is maintained by a cue that the neighbors share. As a consequence, the
fitness of defensive plants will increase as the frequency of S approaches 1, while the fitness of nondefensive plants remains
constant (Fig. 3b). This situation corresponds to "synergistic selection" discussed by Guilford and Cuthill (1991) in the
evolution of aposematism, and by Tuomi and Augner (1993) in relation to plant defenses.
Costs of Defenses
When allowing D and S to play against each other, a plant adopting D will obtain the same payoffs as when playing
against N (Fig. 2). A plant adopting S will obtain the same payoffs when the neighbor is D or S. When r is the population
frequency of S, we get the following fitnesses
W D=W0 - C - 1/2 m(1 +er)
W s =W 0- C(1 + k)- 1/2 m
where W Ddeclines as a function of er, while Ws is independent of both e and r. There are four qualitatively different
situations:
(1) S will be a pure ESS either ifk < 0, or ifk = 0 and e > 0.
(2) S and D are equally fit if k = 0 and e = 0.
(3) D will be a pure ESS if k > 0 and C/m > e/2k.
(4) There is an unstable equilibrium 0 < } < 1 if k > 0 and C/m < e/2k.
Consequently, the costs of defenses play a prominent role here. If S is expensive relative to D, lethal defense is most
likely to be selected for. Fig. 4a indicates the last two situations obtained for k > 0. The unstable equilibrium
? = 2Ck/em
represents the point where the fitness curves intersect each other (i.e., Ws = WD,Fig. 4b). When Fmoves closer to zero,
the situation becomes more favorable to S. This will happen when k becomes smaller, or when e and m become larger.
The reverse changes will accordingly favor D. We interpret this so that lethal defenses may be more economical against
rare herbivores (m small) and sedentary herbivores (e small), while defenses inducing conditional feeding aversion
should be more effective against common herbivores that are mobile and move from a plant to another (both m and e
high).
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