ASN Vice-Presidential Symposium

"Beyond a pair-wise perspective on mutualisms"

Organized by Maureen Stanton (University of California Davis)

Friday July 12, 2002 8:30 am - noon

8:30 - 8:40 am Maureen L. Stanton (University of California Davis)

"A contra dance, not a tango: Beyond a pair-wise perspective on mutualisms"

In ecology courses and textbooks, mutualisms are typically portrayed as cozy interactions between pairs of species in which each partner receives unconditional benefits. Similarly, most models of mutualistic associations consider the ecological or evolutionary dynamics of a single pair of interacting species. In striking contrast to this deterministic, pair-wise view, detailed field analyses of such interactions reveal that the patterns and fitness consequences of species associations often vary in time and space. Multiple potential partners may co-exist within any given geographical range, and may compete for access to the interaction. Single individuals may change partners through their lifetime, and the fitness consequences of association with any given species can hinge upon the availability of alternative partners, as well as on the broader ecological context in which the interaction occurs. The goals of this symposium are to explore the consequences of such diffuse and conditional mutualistic interactions, as well as to articulate how this alternative view of mutualisms can shape future research efforts.

8:40 - 9:15 Nancy Knowlton (Scripps Institution of Oceanography)

"Mutualisms on coral reefs"

Coral reefs exist because of an obligatory symbiosis between corals and dinoflagellates (zooxanthellae). Some corals contain just a single type of symbiont, while others simultaneously host multiple types. In the latter case, the algae typically differ in their stress tolerance and their ability to colonize available hosts. Understanding the ecology of these relationships is important, because one of the greatest long-term threats to coral reefs is destabilization of this symbiosis ("coral bleaching") due to global warming. Some have argued that corals will be able to respond to changing conditions by switching to more stress tolerant symbionts, but essentially nothing is known about benefits provided to corals by different types of zooxanthellae.

9:15 - 9:50 Jason D. Hoeksema (University of California Davis) and Miroslav Kummel (University of Michigan)

"Understanding interspecific mutualism in diverse plant/soil-microbe systems: the importance of the degree and mechanism of competition among symbionts"

In potentially mutualistic interactions between plants and soil microbes, such as mycorrhizal fungi and nodule-inhabiting, nitrogen-fixing bacteria, plants generally encounter and associate with a diverse community of symbionts. The degree of partner-choice experienced by plants in these situations will determine the degree to which cheating or less mutualistic microbial symbiont genotypes can be avoided. In a biological market model framework, the overall price of trade will be more favorable for the plant to the degree that it can choose to trade with partners offering the most favorable trades. To the extent that direct interference (as opposed to plant-mediated) competition is prevalent among symbionts, plants are not expected to be able to choose the most favorable partners. This situation should select for cheaters or weak mutualists that outcompete other symbionts. We know little about the nature of direct and indirect interactions among potential symbionts in plant/soil-microbe systems. Experiments examining the strength of competition among ectomycorrhizal fungi for host-plant root tips suggest that, at least for fungi colonizing from spores, competition may be less important than other processes. However, these experiments do not distinguish the mechanism of interaction among symbionts, i.e. whether plants choose the most beneficial symbionts. Experiments examining ectomycorrhizal fungus associations by host plants along an environmental gradient support the hypothesis that plants can choose symbionts that offer the best price of trade in different environments. Much more work is needed to understand the nature of interactions among soil microbe symbionts of plants. This work should focus especially on the mechanisms of such interactions. A number of features of plant/soil-microbe interactions (such as the patchy nature of soil and the ephemeral nature of root systems) may minimize the degree of interference competition among potential symbionts, making dispersal and stochastic processes more important than equilibrium competition and allowing partner-choice by plants.

9:50 - 10:20 Mid-morning break

10:20 - 10:55 Richard Gomulkiewicz (Washington State University), John N. Thompson (University of California Santa Cruz), and Scott Nuismer (University of Texas)

"Coevolution of mutualisms embedded within and among communities: some simple models"

Interspecific interactions, including mutualisms, often occur within - and may be strongly contingent upon - the community context in which they occur. This talk will use simple models to explore the influences of a broader community setting on the coevolution of a pair of species that can (and do) interact mutualistically at particular localities or times. Two classes of models will be discussed, both inspired by empirical studies of the moth Greya politella and herb Lithophragma parviflorum. The first class considers a pair of species with a locally mutualistic interaction that occurs within an network of communities connected by gene flow. Interspecific interactions at other localities within this "selection mosaic" may be antagonistic, commensal, or absent altogether. The second class of models describes coevolution in a closed community of species for which pair-wise interactions vary from mutualism to antagonism through time. We will consider situations in which the time course of interaction is determined by a third species (such as a co-pollinator) that can potentially co-evolve with the other two. Analyses from both sets of models show that the dynamics of coevolution can only be predicted by considering the community context in which a local or transient mutualistic interaction occurs.

10:55 - 11:30 Judith L. Bronstein (University of Arizona, Tucson), William G. Wilson (Duke University) and William F. Morris (Duke University)

"The ecological dynamics of mutualist-exploiter communities"

Mutualisms do not occur in isolation within ecological communities, although most considerations of their ecological and evolutionary persistence do not take this fact into account. In particular, mutualisms are commonly associated with species that obtain benefits from one or both partners, but that fail to provide benefits in return. Concern has been focused on how such species can be prevented from exploiting mutualisms, under the assumption that their presence has the potential to drive mutualisms to extinction. Starting from the observation that exploiter species have in fact been associated with many well-studied mutualisms throughout most of their evolutionary histories, we examine instead the forces that might permit ecological coexistence of mutualists and exploiters. We consider three-species networks of obligate interactions, in which interactions between the two mutualists have a plus/plus effect; interactions between one mutualist and the exploiter have a plus/minus effect (i.e., the mutualist provides a benefit, but is exploited in return); and interactions between the other mutualist and exploiter have a minus/minus effect (i.e., these species compete for access to benefits provided by the third partner). Using spatially explicit, analytic and simulation models based on yucca/yucca moth interactions, we demonstrate the existence of wide ranges of parameter space in which three-species coexistence is in fact quite feasible. We then address three questions. First, does the likelihood of coexistence change depending on whether the exploiter or mutualist is competitively superior? We find three-species coexistence in both cases, although the spatial patterns generated differ somewhat depending upon who is the better competitor. Second, do exploiters (species that attack a product of the mutualism they exploit) have different effects than specialized predators (species that attack one of the mutualists themselves)? We show that in our models, mutualism is even more resilient to the presence of these predators than it is to exploiters; in fact, predators are generally unable to persist for long. Finally, what are the dynamics of four-species systems that include both types of antagonists? We show that, predators will usually be excluded by exploiters, although sometimes (depending on their relative dispersal abilities) they are able to coexist with or extinguish exploiters; these dynamics take place with surprisingly little effect on the mutualism that they co-exploit. Overall, our results point to a remarkable and perhaps unexpected ecological resilience of mutualism. We discuss the implications of our work for understanding the ecological and evolutionary dynamics of pairwise mutualisms embedded within broader communities. Finally, we identify explicit, testable predictions that these models make about spatial patterns of coexistence within mutualist/exploiter communities in nature.

11:30 - noon Panel discussion: Future directions for research