Environmental variation shifts the relationship between trees and scatterhoarders along the continuum from mutualism to antagonism

The Role of Indirect Effects in Coevolution along the Mutualism-Antagonism Continuum

AbstractThe web of interactions in a community drives the coevolution of species. Yet it is unclear how the outcome of species interactions influences the coevolutionary dynamics of communities. This is a pressing matter, as changes to the outcome of interactions may become more common with human-induced global change. Here, we combine network and evolutionary theory to explore coevolutionary outcomes in communities harboring mutualistic and antagonistic interactions. We show that as the ratio of mutualistic to antagonistic interactions decreases, selection imposed by direct partners outweighs that imposed by indirect partners. This weakening of indirect effects results in communities composed of species with dissimilar traits and fast rates of adaptation. These changes are more pronounced when specialist consumers are the first species to engage in antagonistic interactions. Hence, a shift in the outcome of species interactions may reverberate across communities and alter the direction and speed of coevolution.

Ungulate presence and predation risks reduce acorn predation by mice in dehesas

Foraging decisions by rodents are key for the long-term maintenance of oak populations in which avian seed dispersers are absent or inefficient. Decisions are determined by the environmental setting in which acorn-rodent encounters occur. In particular, seed value, competition and predation risks have been found to modify rodent foraging decisions in forest and human-modified habitats. Nonetheless, there is little information about their joint effects on rodent behavior, and hence, local acorn dispersal (or predation). In this work, we manipulate and model the mouse-oak interaction in a Spanish dehesa, an anthropogenic savanna system in which nearby areas can show contrasting levels of ungulate densities and antipredatory cover. First, we conducted a large-scale cafeteria field experiment, where we modified ungulate presence and predation risk, and followed mouse foraging decisions under contrasting levels of moonlight and acorn availability. Then, we estimated the net effects of competition and risk by means of a transition probability model that simulated mouse foraging decisions. Our results show that mice are able to adapt their foraging decisions to the environmental context, affecting initial fates of handled acorns. Under high predation risks mice foraged opportunistically carrying away large and small seeds, whereas under safe conditions large acorns tended to be predated in situ. In addition, in the presence of ungulates lack of antipredatory cover around trees reduced mice activity outside tree canopies, and hence, large acorns had a higher probability of survival. Overall, our results point out that inter-specific interactions preventing efficient foraging by scatter-hoarders can reduce acorn predation. This suggests that the maintenance of the full set of seed consumers as well as top predators in dehesas may be key for promoting local dispersal.

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.

Sowing forests: a synthesis of seed dispersal and predation by agoutis and their influence on plant communities

Granivorous rodents have been traditionally regarded as antagonistic seed predators. Agoutis (Dasyprocta spp.), however, have also been recognized as mutualistic dispersers of plants because of their role as scatter-hoarders of seeds, especially for large-seeded species. A closer look shows that such definitions are too simplistic for these Neotropical animals because agoutis can influence plant communities not only through seed dispersal of large seeds but also through predation of small seeds and seedlings, evidencing their dual role. Herein, we summarize the literature on plant-agouti interactions, decompose agouti seed dispersal into its quantitative and qualitative components, and discuss how environmental factors and plant traits determine whether these interactions result in mutualisms or antagonisms. We also look at the role of agoutis in a community context, assessing their effectiveness as substitutes for extinct megafaunal frugivores and comparing their ecological functions to those of other extant dispersers of large seeds. We also discuss how our conclusions can be extended to the single other genus in the Dasyproctidae family (Myoprocta). Finally, we examine agoutis' contribution to carbon stocks and summarize current conservation threats and efforts. We recorded 164 interactions between agoutis and plants, which were widespread across the plant phylogeny, confirming that agoutis are generalist frugivores. Seed mass was a main factor determining seed hoarding probability of plant species and agoutis were found to disperse larger seeds than other large-bodied frugivores. Agoutis positively contributed to carbon storage by preying upon seeds of plants with lower carbon biomass and by dispersing species with higher biomass. This synthesis of plant-agouti interactions shows that ecological services provided by agoutis to plant populations and communities go beyond seed dispersal and predation, and we identify still unanswered questions. We hope to emphasise the importance of agoutis in Neotropical forests.

Exclusion of interspecific competition reduces scatter-hoarding of Siberian chipmunk Tamias sibiricus: A field study

Although food availability and the abundance of seed predators have been postulated to affect seed dispersal, it is not clear how seed-eating animals modify their scatter-hoarding strategies in response to different levels of interspecific competition. We placed paired germinated and ungerminated acorns of Quercus mongolica on 30-cm high platforms to exclude potential interspecific competition of the predominant larder hoarders Apodemus peninsulae and Myodes rufocanus, to investigate seed dispersal by a predominant scatter-hoarder, Tamias sibiricus, in the field in north-eastern China. Our results showed that T. sibiricus ate more acorns in situ in the absence of interspecific competition. In the presence of interspecific competition of A. peninsulae and C. rufocanus, however, more acorns were scatter-hoarded by T. sibiricus. Regardless of interspecific competition, germination of acorns showed no significant effects on seed dispersal patterns, inconsistent with the "seed perishability hypothesis" that animals avoid hoarding seeds with high perishability. Exclusion of interspecific competition, though relatively increasing the per capita seed abundance, appears to reduce seed dispersal, scatter-hoarding and seedling establishment. Therefore, we propose that moderate interspecific competition rather than competition exclusion may benefit seed scatter-hoarding and seedling establishment.

Interspecific synchrony of seed rain shapes rodent-mediated indirect seed-seed interactions of sympatric tree species in a subtropical forest

Animal‐mediated indirect interactions play a significant role in maintaining the biodiversity of plant communities. Less known is whether interspecific synchrony of seed rain can alter the indirect interactions of sympatric tree species. We assessed the seed dispersal success by tracking the fates of 21 600 tagged seeds from six paired sympatric tree species in both monospecific and mixed plots across 4 successive years in a subtropical forest. We found that apparent mutualism was associated with the interspecific synchrony of seed rain both seasonally and yearly, whereas apparent competition or apparent predation was associated with interspecific asynchrony of seed rain either seasonally or yearly. We did not find consistent associations of indirect interactions with seed traits. Our study suggests that the interspecific synchrony of seed rain plays a key role in the formation of animal‐mediated indirect interactions, which, in turn, may alter the seasonal or yearly seed rain schedules of sympatric tree species.

Prescribed fire and partial overstory removal alter an acorn-rodent conditional mutualism

In eastern North America, oak (Quercus) regeneration failure has spurred management using silvicultural approaches better aligned with the autecology of oaks. In particular, shelterwood harvests can create favorable intermediate light conditions for oak establishment and prescribed fire is predicted (by the oak-fire hypothesis) to favor oak regeneration. These approaches substantially modify forest structure and may affect crucial trophic interactions including the conditional mutualism between oaks and granivorous rodents that scatterhoard acorns, which shifts along a continuum from antagonistic to mutualistic depending on external factors. We investigated how overwinter survival and dispersal of northern red oak (Quercus rubra) acorns were influenced by location within or outside of group shelterwood harvests (small canopy gaps created throughout an intact forest stand) with and without prescribed fire. We conducted two concurrent experiments to test (1) dispersal and survival of acorns presented on the forest floor and (2) acorn pilferage rates from caches that mimic squirrel scatterhoards in shelterwood gap/group interiors, edges, and the uncut forest matrix in burned and unburned forest stands. In both experiments, acorn survival was generally higher in burned than unburned stands. Acorn survival from forest floor presentations was higher in the unharvested forest matrix than harvest gap interiors; however, there was no effect of proximity to harvest gaps on survival of cached acorns. Survival of cached acorns was associated with understory vegetative cover (-), coarse woody debris cover (-), and distance to nearest tree (+), but uncorrelated with canopy cover above the cache. Our results suggest that reduced understory cover following prescribed fire may increase perceived habitat riskiness for granivores resulting in higher acorn survival up to 2 yr post-fire. These findings unify the oak-fire and oak-granivore conditional mutualism hypotheses, and suggest that the environmental conditions following prescribed fire and group shelterwood harvests may shift the oak-granivore conditional mutualism in a direction beneficial for oak regeneration.

Endozoochory by granivorous rodents in seed dispersal of green fruits

Although the role of frugivores in seed dispersal has attracted scientific attention, it remains unclear whether granivorous rodents can act as frugivores to interact mutualistically with fruit-producing plants, especially those bearing green fruits inconspicuous to avian frugivores. In this study, we tracked fruit removal of the tara vine (Actinidia arguta (Siebold & Zucc.) Planch. Ex Miq.) and variegated kiwi vine (Actinidia kolomikta (Rupr. & Maxim.) Maxim.) in a temperate forest and presented fruits to the granivorous rodents Siberian chipmunk (Tamias sibiricus (Laxmann, 1769)), Korean field mouse (Apodemus peninsulae (Thomas, 1907)), and gray red-backed vole (Clethrionomys rufocanus (Sundevall, 1846) = Myodes rufocanus (Sundevall, 1846)) in the laboratory to answer this question. Seeds were collected from rodent feces to see the effects of gut passage on seed germination to determine the role of granivorous rodents in endozoochory of A. arguta and A. kolomikta. We presented clear evidence of endozoochory by granivorous rodents in seed dispersal of the two Actinidia species. Rodents appeared to play an alternative role in dispersing plants bearing green fruits. Moreover, we observed increased germination rates after gut ingestion by the granivorous rodents. Our study evidenced endozoochory of granivorous rodents and provided new insight into the mutualist interactions between rodents and plant species bearing fleshy fruits containing tiny seeds. We suggest future studies pay more attention to endozoochory of rodents and establish their mutualistic relationship with fruit-bearing plants in temperate forests.