PMID: 16676535;Abstract:
Generalized, facultative mutualisms are often characterized by great variation in the benefits provided by different partner species. This variation may be due to differences among species in the quality and quantity of their interactions, as well as their phenology. Many plant species produce extrafloral nectar, a carbohydrate-rich resource, to attract ant species that can act as "bodyguards" against a plant's natural enemies. Here, we explore differences in the quality and quantity of protective service that ants can provide a plant by contrasting the four most common ant visitors to Ferocactus wislizeni, an extrafloral nectary-bearing cactus in southern Arizona. The four species differ in abundance when tending plants, and in the frequency at which they visit plants. By adding surrogate herbivores (Manduca sexta caterpillars) to plants, we demonstrate that all four species recruit to and attack potential herbivores. However, their per capita effectiveness in deterring herbivores (measured as the inverse of the number of workers needed to remove half of the experimentally added caterpillars) differs. Using these among-species differences in quality (per capita effectiveness) and quantity (number of workers that visit a plant and frequency of visitation), we accurately predicted the variation in fruit production among plants with different histories of ant tending. We found that plant benefits (herbivore removal and maturation of buds and fruits) typically saturated at high levels of ant protection, although plants could be "well defended" via different combinations of interaction frequency, numbers of ant workers per interaction, and per capita effects. Our study documents variation among prospective mutualists, distinguishes the components of this variation, and integrates these components into a predictive measure of protection benefit to the plant. The method we used to average saturating benefits over time could prove useful for quantifying overall service in other mutualisms. © 2006 by the Ecological Society of America.
Abstract:
Pollinator attraction, pollen limitation, resource limitation, pollen donation and selective fruit abortion have all been proposed as processes explaining why hermaphroditic plants commonly produce many more flowers than mature fruit. We conducted a series of experiments in Arizona to investigate low fruit-to-flower ratios in senita cacti, which rely exclusively on pollinating seed-consumers. Selective abortion of fruit based on seed predators is of particular interest in this case because plants relying on pollinating seed-consumers are predicted to have such a mechanism to minimize seed loss. Pollinator attraction and pollen dispersal increased with flower number, but fruit set did not, refuting the hypothesis that excess flowers increase fruit set by attracting more pollinators. Fruit set of natural- and hand-pollinated flowers were not different, supporting the resource, rather than pollen, limitation hypothesis. Senita did abort fruit, but not selectively based on pollen quantity, pollen donors, or seed predators. Collectively, these results are consistent with sex allocation theory in that resource allocation to excess flower production can increase pollen dispersal and the male fitness function of flowers, but consequently results in reduced resources available for fruit set. Inconsistent with sex allocation theory, however, fruit production and the female fitness function of flowers may actually increase with flower production. This is because excess flower production lowers pollinator-to-flower ratios and results in fruit abortion, both of which limit the abundance and hence oviposition rates, of pre-dispersal seed predators.
Abstract:
Mutualisms are almost ubiquitously exploited by species that gain the benefits that mutualists offer to each other, but that offer nothing in return. This paper investigates the possible dynamical outcomes of a mechanistically formulated model system, involving two obligate mutualists and one exploiter. The model is based conceptually on a mutualism between a plant species and its pollinating seed parasite, in the presence of an obligate, nonpollinating seed parasite. Of particular interest are the conditions under which the exploiter species can invade and coexist with a mutualism that, by itself, possesses an equilibrium stabilized by other density-dependent regulating factors. Two types of models are used in the analyses: a deterministic, nonspatial model described by a set of discrete time equations, and an individual-based simulation incorporating stochastic interactions and spatial structure. Comparing the results of these two models uncovers the temporal dynamics of both well-mixed local systems and spatially distributed populations. Using these two situations, we examine how the predictions of the nonspatial model are affected by habitat structure. In the nonspatial case, and without the exploiters, there are typically two stable equilibria: one having zero densities for both mutualist species, and the other having nonzero densities. This bi-stability is a characteristic feature of obligate mutualisms. When the exploiter species is included, the system always retains the stability of the zero-density equilibrium, but the dynamics of the upper equilibrium can be more complicated, including limit cycle and extinction dynamics. Simulation results demonstrate that spatial structure is a highly stabilizing influence on the three-species system as long as the exploiter's dispersal distance is large relative to the seed dispersal distance of the plant species. Given this condition, the model predicts spatial distributions that are marked by a patchy distribution of plants, with mutualists and exploiters situated about these patches.
