Beetles, bunnies, and fungi: grassland plants face the same growth-defense tradeoffs across many consumer types

Background/Question/Methods

Plant investment in defense against enemies may come at a cost to plant growth rates (growth-defense trade-off, GDTO) or a cost to plant competitive ability (competition-defense trade-off, CDTO). Evidence of these trade-offs in plant communities in part comes from grassland experiments manipulating herbivore or more rarely pathogen abundances. However, no studies have explicitly tested the generality of the GDTO or CDTO hypotheses in the context of a broad suite of consumers ranging from soil fungi to large mammalian herbivores. Past work suggests that the cost of defense against pathogens or herbivores may be similar, and therefore trade-offs may be uniform in plant species regardless of consumer type. To test the generality of the GDTO and CDTO hypotheses across a broad range of consumer types, we used a grassland experiment that increases nutrient supply and removes multiple consumer groups (soil fungi, foliar fungi, arthropods, large mammals) via pesticides or fencing. By observing aggregate plant species responses to treatments and using standardized major axis regression, we tested for positive (in support of GDTO) or negative (in support of CDTO) associations between consumer removal and the nutrient addition treatment and also investigated whether associations varied by the type of consumer removal.

Results/Conclusions

Plant species response to mammal, arthropod, foliar fungi, and soil fungi removal were all positively associated with response to nutrient addition (slope=1.01, p< 0.0001, R2=0.47) supporting the GDTO hypothesis. In other words, plant species that increased with removal of nutrient limitation (poor nutrient competitors) also increased with removal of consumers – regardless of the type of consumer. The slope value of this positive association did not significantly differ among the various consumer removal treatments (likelihood ratio test, p=0.95) suggesting that different consumer groups do not confer different allocation constraints on the GDTO. These findings extend previous work supporting the idea that the GDTO is more prevalent than CDTO in grassland plant species. While plants differ in their ability to compete for limiting resources and defend against enemies, the surprising lack of variation in trade-offs across fungi, arthropods, and large mammals here suggests that well-defended species, that are also better nutrient competitors, tend to be protected from a wide array of consumer types. More broadly, this work provides new insights into the potential mechanisms maintaining grassland biodiversity and the life-history constraints that all plant species may fall under as they encounter pressures from competitors, pathogens, and herbivores.