Fruit rot, antifungal defense, and palatability of fleshy fruits for frugivorous birds

We tested several predictions concerning fungal rot and antifungal defense of vertebrate-dispersed fruit. (1) Fungal fruit to should be generally deterrent to frugivores. But because fungi may vary in their negative effects, antifungal defenses should be allocated in relationship to net effects of particular species on dispersal. (2) Defenses of fruits under selection for high palatability should be directed primarily toward microbes (@'microbespecific@' defenses). Lower quality fruits, for which high palatability is not crucial to dispersal, may be defended by more general defenses. (3) Preferred fruits should be defended to a lower degree than less preferred fruits, as a result of consistently high dispersal rates (Removal Rate model). We tested our hypotheses using real fruits and artificial-pulp media of six temperate ericaceous plants in feeding trials with captive birds and in fungal growth trials. Plants had been assigned to two temperature fruiting classes (Summer Small-seeded, SS, Fall-Low-quality, FL), and differed in ripening phenology, persistence times, and fungal resistance in the field. We compared feeding rates on fruits rotted by members of a suite of fruit rot fungi. We also determined feeding rates on artifical-pulp media that either contained ripe fruit extracts or did not, and we determined fungal growth rates on identical media. We found that: (1) fungal species depressed feeding rates in general accordance with their reported vertebrate toxicity. Relative to nontoxic fungi, toxic fungi were inhibited more strongly by secondary fruit pulp extracts. (2) Artificial-pulp media of SS fruits were preferred in feeding trials, due both to basic nutritional composition and nondeterrent (microbe-specific) secondary defense. (3) In accordance with the Removal-Rate model, artificial-pulp media of the preferred SS fruits showed significant antifungal activity, but at a much lower level than that of FL fruits. Our results suggest that a primary function of secondary chemicals in ripe fruits may be to defend against destructive fungi that compete with seed dispersers for the pulp. Our results generally support the hypothesis that selective pressures may result in ripe fruit defenses that are directed primarily against organisms having strong negative effects upon dispersal. As such, factors influencing risk of pulp consumption by nondispersers may be useful predictors of fruit defense patterns.