Mutual cheating strengthens a tropical seed dispersal mutualism

Costs of floral larceny: A meta-analytical evaluation of nectar robbing and nectar theft on animal-pollinated plants

Mutualistic interactions are biological markets in which different species exchange commodities to mutual benefit. Mutualisms are, however, susceptible to exploitation, with some individuals taking without reciprocating. While it is generally assumed that exploiters will inflict fitness costs, evidence for such costs is mixed and difficult to generalize due to their context-dependent nature. Animal-pollinated flowers are commonly exploited by larcenists, non-pollinating animals that consume floral rewards often without transferring pollen. The impacts of larcenists on plant reproduction vary widely, suggesting they inflict differing costs on plants, but which types of floral larceny are most and least costly, and why, has received little attention. We employed a meta-analytical approach to explore the effects of flower larceny on nectar traits, pollinator visitation, and plant reproduction. We focused on the effects of two contrasting forms of larceny: primary nectar robbing-nectar consumption through holes constructed in the corolla rather than entering flowers legitimately-and nectar theft-nectar consumption by entering flowers but with no pollen transfer. We found that both robbing and theft had negative impacts on nectar quantity and quality, but that only theft negatively affected pollinator visitation rates. Similarly, robbers had no impact on either female or male reproductive success, whereas thieves consistently reduced both male and female reproductive success. These effects were not associated with plant mating systems nor with the identities of robbers and effective pollinators, challenging previous generalizations. This study provides a comprehensive evaluation of larcenists' costs to animal-pollinated plants, revealing that nectar theft is more detrimental to plant reproduction than nectar robbing. These results enhance our understanding of the intricate dynamics of mutualism exploitation in ecological and evolutionary contexts.

Granivore abundance shapes mutualism quality in plant-scatterhoarder interactions

Conditional mutualisms involve costs and benefits that vary with environmental factors, but mechanisms driving these dynamics remain poorly understood. Scatterhoarder-plant interactions are a prime example of this phenomenon, as scatterhoarders can either increase or reduce plant recruitment depending on the balance between seed dispersal and predation. We explored factors that drive the magnitude of net benefits for plants in this interaction using a mathematical model, with parameter values based on European beech (Fagus sylvatica) and yellow-necked mice (Apodemus flavicollis). We measured benefits as the percentage of germinating seeds, and examined how varying rodent survival (reflecting, e.g. changes in predation pressure), the rate of seed loss to other granivores, the abundance of alternative food resources, and changes in masting patterns affect the quality of mutualism. We found that increasing granivore abundance can degrade the quality of plant-scatterhoarder mutualism due to increased cache pilferage. Scatterhoarders are predicted to respond by increasing immediate consumption of gathered seeds, leading to higher costs and reduced benefits for plants. Thus, biotic changes that are detrimental to rodent populations can be beneficial for tree recruitment due to adaptive behavior of rodents. When scatterhoarder populations decline too drastically (< 5 individuals ha-1 ); however, tree recruitment may also suffer.

Plant hormones mediate the interaction between oak acorn germination and rodent hoarding behaviour

The interaction between animals and plants for seed dispersal and predation has received much attention; however, the underlying physiological mechanisms driving the responses of both seeds and animals remain unclear. We conducted a series of behaviour and physiology experiments to examine the role of plant hormones in regulating seed germination and rodent hoarding behaviour in the Quercus variabilis and Leopoldamys edwardsi systems. We found that acorns that were partially consumed by rodents had increased gibberellin (GA) levels and shortened germination time. Rodents preferred scatter-hoarded abscisic acid (ABA)-treated and intact acorns but consumed germinated and GA-treated acorns; such treatment differences disappeared for inactivated acorns by boiling water. Moreover, we found that seven potential compounds may be linked to seed germination and rodent hoarding behaviour. Our results indicate that acorns of oak showed rapid germination when facing predation risk, while rodents could identify the germination status of seeds for hoarding; GA and ABA may play an important role in regulating seed germination of oak and hoarding behaviour of rodents.

Behavioral adaptation of sympatric rodents to early germination of oak acorns: radicle pruning and embryo excision

The seed germination schedule is a key factor affecting the food-hoarding behavior of animals and the seedling regeneration of plants. However, little is known about the behavioral adaptation of rodents to the rapid germination of acorns. In this study, we provided Quercus variabilis acorns to several rodent species to investigate how food-hoarding animals respond to seed germination. We found that only Apodemus peninsulae adopted embryo excision behavior to counteract seed germination, which is the first report of embryo excision in nonsquirrel rodents. We speculated that this species may be at an early stage of the evolutionary response to seed perishability in rodents, given the low rate of embryo excision in this species. On the contrary, all rodent species preferred to prune the radicles of germinating acorns before caching, suggesting that radicle pruning is a stable and more general foraging behavior strategy for food-hoarding rodents. Furthermore, scatter-hoarding rodents preferred to scatter-hoard and prune more germinating acorns, whereas they consumed more nongerminating acorns. Acorns with embryos excised rather than radicles pruned were much less likely to germinate than intact acorns, suggesting a behavioral adaptation strategy by rodents to the rapid germination of recalcitrant seeds. This study provides insight into the impact of early seed germination on plant-animal interactions.

Mutual cheating strengthens a tropical seed dispersal mutualism

While cheating can cause the degradation or collapse of mutualisms, mutualisms may theoretically stabilize or strengthen if the cheating is mutual. Here, we present an asymmetric two-player game model to explore the evolutionary dynamics of mutual cheating in a mutualistic interaction. We found that the interaction evolved towards mutual cheating if cheating can help both partners obtain higher benefits or if counter-cheating yields more benefits to victims than simply tolerating exploitation by partners. Then, we present empirical evidence for such mutual cheating strengthening a seed dispersal mutualism in which rodents disperse seeds by scatter hoarding, rodents sabotage seed germination by pruning radicles, and seeds escape rodents by resprouting. By tracking >8000 Pittosporopsis kerrii seeds throughout the dispersal process in a tropical forest in southwest China, we found that rodents provided better dispersal to seeds that they pruned, i.e., pruned seeds were dispersed farther and were more likely to establish seedlings than unpruned seeds. Compared to unpruned seeds, pruned seeds retained more of their nutrients, i.e., dry mass of pruned seeds was greater than that of unpruned seeds, and were stored for longer by rodents. These findings indicate that mutual cheating benefited both partners. Payoffs estimated from the field experiments indicated that mutual cheating was indeed favored in rodents and plants P. kerrii, and that neither partner was enslaved by the other under mutual cheating. Rather, the mutualism remained stable because the partners were able to exploit each other, and each partner attempted to gain the maximum benefits from the interaction. Our findings indicate that mutual cheating between two mutualists can enhance and stabilize mutualisms.