PLANT-SOIL FEEDBACKS
Returning native species to habitats degraded by biological invasions is a critical conservation goal. A leading hypothesis poses that exotic domination of invaded landscapes is reinforced by plant-soil feedbacks. Such feedbacks occur when invaders modify nutrient cycling, shift myccorhizal and pathogenic fungal communities, or release allelopathic compounds in ways that feed back to benefit themselves over competitors. But how important are plant-soil feedbacks relative to other community level processes, such as competition, or dispersal? In addition, if feedbacks are important for invasive species persistence, how long are they maintained? The projects below address these questions.
THE RELATIVE IMPORTANCE OF PLANT-SOIL FEEDBACKS: Santa Cruz Island CA
My dissertation work explored the relative roles of plant-soil feedbacks, competition and seed dispersal limitation in ecosystem recovery on Santa Cruz Island, CA. Santa Cruz Island was grazed by feral sheep for over a century, turning hillsides once dominated by native shrubs into exotic annual grasslands. Since sheep removal in the 1980’s, some native shrubs have started to move into these exotic dominated landscapes, and I was interested in whether shifts in ecosystem processes due to invasion affect re-establishment dynamics. To this end, I quantified colonization and competition constraints to the re-establishment of dominant native shrubs Eriogonum arborescens and Artemisia californica. I also measured the relative effects of exotic grasses and native shrubs on soil N and P cycling in the field and experimental monocultures. Finally, I used an intact core method to repeat plant-soil feedback experiments in the field over three years that varied widely in rainfall. Results showed that exotic grasses could increase or decrease nitrogen cycling rates relative to different species of native shrub. I demonstrated that counter to common expectations, exotic plant effects on nutrient cycling created plant-soil feedbacks that facilitated rather than inhibited native species recovery. In addition, the effects of competition and climate were much stronger than those of feedbacks, and thus played a more critical role in determining which native species recover, and the years in which they do so. Therefore, understanding the relative strength of different ecological factors is important for determining community succession.
publications (click on the journal name to access a pdf):
Yelenik, S.G. and Levine, J.M. 2011. The role of plant-soil feedbacks in driving native recovery. Ecology, 92:
66-74.
Yelenik, S.G. and Levine, J.M. 2010. Processes limiting native shrub recovery in exotic grasslands after non-
native herbivore removal. Restoration Ecology, 18: 418- 425.
Yelenik, S.G. and Levine, J.M. 2010. Re-establishment of native shrubs into California exotic annual
grasslands: are ecosystem services returned? Ecological Applications, 20: 716-727.
Levine, J.M., Pachepsky, E., Kendall, B., Yelenik, S.G., and Hille Ris Lambers, J. 2006. Plant-soil feedbacks
and invasive spread. Ecology Letters, 9: 1005-1015.
publications (click on the journal name to access a pdf):
Yelenik, S.G. and Levine, J.M. 2011. The role of plant-soil feedbacks in driving native recovery. Ecology, 92:
66-74.
Yelenik, S.G. and Levine, J.M. 2010. Processes limiting native shrub recovery in exotic grasslands after non-
native herbivore removal. Restoration Ecology, 18: 418- 425.
Yelenik, S.G. and Levine, J.M. 2010. Re-establishment of native shrubs into California exotic annual
grasslands: are ecosystem services returned? Ecological Applications, 20: 716-727.
Levine, J.M., Pachepsky, E., Kendall, B., Yelenik, S.G., and Hille Ris Lambers, J. 2006. Plant-soil feedbacks
and invasive spread. Ecology Letters, 9: 1005-1015.
SELF-REINFORCING IMPACTS OF PLANT INVASIONS CHANGE OVER TIME: Hawaiʻi Volcanoes National Park, HI.
While feedbacks are often invoked as evidence for the existence of alternate stable states, species effects on ecosystem function are likely to change in direction and strength over time. For example, invaders that initially increase the level of a particular resource to their own benefit (positive feedback) may lead to different resources being limiting over time, facilitating co-existence of other species (negative feedback). Thus, if viewed in the short term feedbacks will presumably lead to invader persistence, but when studied over longer timescales feedbacks may breakdown, benefiting the establishment of other species. My NSF-funded work with Carla D’Antonio (UCSB) explored the long-term effects of grass invaders, and the consequences of shifting impacts for plant communities in Hawaiʻi Volcanoes National Park (HAVO). Earlier work suggested that grass invasions of Hawaiian submontane ecosystems lead to persistent invaded states in part due to plant effects on soils. Invasive grasses were previously demonstrated to cause high soil N-turnover rates, which, coupled with grasses being N-limited, contributed to their persistence. However, our data suggests that positive feedbacks between grasses and N cycling are breaking down. Soil N cycling rates, which were elevated early in exotic grass invasion, have returned to values no different than native dominated woodlands, and N content in plants has decreased over time. Isotopic data and ecosystem N budgets suggest that the breakdown of feedbacks was due to invaded grassland sites being “leaky” for N, potentially through processes such as leaching and denitrifcation. While it is possible that native species will re-establish, depauperate seedbanks and slow growth rates may preclude this possibility, allowing for a new suite of invasive species to enter. Indeed, a large outplanting experiment showed that the species most likely to gain benefit from shifting exotic grass impacts was another invader, Morella faya, an N-fixing shrub. Thus, although ecosystem impacts and feedbacks shift over time, this may not benefit native species recovery.
publications (click on the journal name to access a pdf):
Yelenik, S.G. and D’Antonio, C.M. 2013. Self-reinforcing impacts of plant invasions change over time.
Nature, 502: 517-520..
publications (click on the journal name to access a pdf):
Yelenik, S.G. and D’Antonio, C.M. 2013. Self-reinforcing impacts of plant invasions change over time.
Nature, 502: 517-520..