Grazing herbivores reduce herbaceous biomass and fire activity across African savannas.

Fire and herbivory interact to alter ecosystems and carbon cycling. In savannas, herbivores can reduce fire activity by removing grass biomass, but the size of these effects and what regulates them remain uncertain. To examine grazing effects on fuels and fire regimes across African savannas, we combined data from herbivore exclosure experiments with remotely sensed data on fire activity and herbivore density. We show that, broadly across African savannas, grazing herbivores substantially reduce both herbaceous biomass and fire activity. The size of these effects was strongly associated with grazing herbivore densities, and surprisingly, was mostly consistent across different environments. A one-zebra increase in herbivore biomass density (~100 kg/km2 of metabolic biomass) resulted in a ~53 kg/ha reduction in standing herbaceous biomass and a ~0.43 percentage point reduction in burned area. Our results indicate that fire models can be improved by incorporating grazing effects on grass biomass.

Host use by 2 sibling species of bogus yucca moths in relation to plant hardness and saponin content.

Plant defenses allow plants to deter or kill their insect herbivores and are considered to be a major driver of host use for herbivorous insects in both ecological and evolutionary time. Many closely related species of insect herbivores differ in their ability to respond to plant defenses and in some cases are specialized to specific plant species. Here we tested whether both mechanical and chemical plant defenses are a major factor in determining the host range of 2 sibling species of Prodoxid bogus yucca moths, Prodoxus decipiens (Riley) and Prodoxus quinquepunctellus (Chambers) that feed within the inflorescence stalk of Yucca species. These 2 moth species have separate suites of host plant species, yet narrowly overlap geographically and share 1 Yucca species, Y. glauca. We surveyed the lignin and cellulose content, the force required to the puncture the stalk tissue, and saponin concentration across 5 Yucca species used as hosts. Lignin, cellulose concentrations, and stalk hardness differed among Yucca species but did not correlate with host use patterns by the moths. Saponin concentrations in the stalk tissue were relatively low for yuccas (<1%) and did not differ among species. The results suggest that these moth species should be able to use each other's hosts for egg deposition. Additional factors such as larval development or competition among larvae for feeding space may serve to keep moth species from expanding onto plants used by its sibling species.

Integrating defense and leaf economic spectrum traits in a tropical savanna plant.

Introduction: Allocation to plant defense traits likely depends on resource supply, herbivory, and other plant functional traits such as the leaf economic spectrum (LES) traits. Yet, attempts to integrate defense and resource acquisitive traits remain elusive. Methods: We assessed intraspecific covariation between different defense and LES traits in a widely distributed tropical savanna herb, Solanum incanum, a unique model species for studying allocations to physical, chemical, and structural defenses to mammalian herbivory. Results: We found that in a multivariate trait space, the structural defenses - lignin and cellulose - were positively related to the resource conservative traits - low SLA and low leaf N. Phenolic content, a chemical defense, was positively associated with resource acquisitive traits - high SLA and high leaf N - while also being associated with an independent third component axis. Both principal components 1 and 3 were not associated with resource supply and herbivory intensity. In contrast, spine density - a physical defense - was orthogonal to the LES axis and positively associated with soil P and herbivory intensity. Discussion: These results suggest a hypothesized "pyramid" of trade-offs in allocation to defense along the LES and herbivory intensity axes. Therefore, future attempts to integrate defense traits with the broader plant functional trait framework, such as the LES, needs a multifaceted approach that accounts for unique influences of resource acquisitive traits and herbivory risk.