Functional diversity, species-specific responses, and grazing pressure: the effect of Juncus effusus in grazed subtropical wetlands

Background/Question/Methods Unpalatable plant species often act as biotic refuges by protecting neighboring plants from herbivores. This positive interaction can increase functional diversity in grazed ecosystems by protecting species sensitive to grazing that may have otherwise been lost.  The frequency of positive interactions is expected to increase as consumer pressure increases while competitive interactions are expected to increase when consumer pressure is low.  However, because neighboring species may vary in their tolerance to competitive effects from the unpalatable species, variation in facilitative responses can be expected among species that are sensitive to grazing.
We investigated community effects and plant interactions of a dominant unpalatable plant, Juncus effusus L. in grazed wetlands in south-central Florida.    Using experimental grazing exclosures and transplants of one native and one exotic species each of grasses and forbs with or without Juncus, we tested several hypotheses:  1) Juncus effusus would increase functional diversity within grazed wetland plant communities; 2) plant interactions with Juncus would range from competition in ungrazed areas to facilitation in grazed areas and would differ among transplanted species; and 3) these relationships would be mediated by grazing and would  increase as grazing intensity increased (0.15-1.7 cows/ha in our study system).

Results/Conclusions

The abundance of species sensitive to grazing was significantly higher (p=0.004) within Juncus tussocks compared to plots without Juncus in grazed areas, which  supported our hypothesis that Juncus would preserve plant functional diversity in grazed wetlands.  Grazing had a negative effect on all four transplanted species. Facilitation occurred in grazed plots as indicated by increased survival for three of four species and increased biomass for the two grass species when Juncus was present.  The native forb did not obtain grazing refuge from Juncus. When grazing was removed, Juncus had negative effects on survival and biomass for all species except the non-native forb.  Analysis of variation of relative interaction intensity (RII) showed that facilitative effects of Juncus varied in magnitude among species; the native grass and non-native forb showed the highest facilitative response, the non-native grass showed a moderate response and the native forb showed none. This suggests that a range of responses to plant interactions along stress gradients is possible depending on the traits of beneficiary species. The magnitude of facilitation (RII) was highest at intermediate grazing intensity (0.15-0.5 cows/ha) but decreased at higher intensities, supporting the idea that in intensely grazed systems, the positive effects of unpalatable plants are reduced.