Cascading effects from bears to plants via a protection-service mutualism

Background/Question/Methods

Top predators can influence primary production and other basal resources in food webs by generating trophic cascades. Trophic cascades are characterized by strong food chains; in terrestrial systems these typically occur when predators suppress herbivores and indirectly benefit plants. Previous studies have shown that these cascades can be altered by protection-service mutualisms, such as mutualisms between ants and plant-sucking insects, or by the presence of natural enemies of predators. Yet, few studies have measured plant performance when both protection-service mutualisms and a fourth trophic level are present. We used field experiments and structural equation modeling (SEM) to investigate how plant performance (reproduction and growth) depends on interaction chains in a web composed of plants, herbivorous insects, mutualist ants, other arthropod predators, and black bears as top predators of ants. We manipulated plant-sucking treehoppers and their ant protectors, expecting that plant performance would respond negatively to their presence. Simultaneously, we tracked black bear damage to ant nests and investigated whether cascading effects of ants would weaken when nests were disturbed. Lastly, we manipulated ants and other arthropod predators to elucidate how the cascading effects depend on these competing predators.

Results/Conclusions

We found that treehoppers and their ant protectors were detrimental for plant reproduction, but not for plant growth. Ant-protection also benefited other herbivores, adding to the negative effects of ants on plants. Plants near bear-disturbed ant nests had significantly greater reproduction than those near undisturbed nests; SEM analyses indicate that this can be explained by the suppression of the ant’s interactions with herbivores when nests were disturbed. We also found that the strength of the cascade from bears to plants depends on abundances of the competing arthropod predators. Treehoppers were only suppressed when there were few ants and many other arthropod predators, which is when plants responded with greatest reproduction and growth. These results demonstrate how a protection-service mutualism can mediate cascading effects in food webs and highlight the need to integrate mutualisms into trophic cascade theory. In addition, our results indicate that black bears can influence plants via cascading effects, suggesting that bears should be managed with their potential to influence primary producers in mind.