Abstract
AbstractWe assessed the effects of herbivory on competition, water stress, and potentially biological nitrogen fixation on three species of endemic shrubs using variations in the stable isotope ratios (13C/12C and 15N/14N) in leaves of chaparral shrubs in a long‐term field experiment. While variations in isotopic ratios of plants are often attributed to abiotic stresses, impacts of biotic interactions are rarely evaluated. Our site was a low‐nutrient, chaparral community on the central California coast. In this system, deer browsing on Ceanothus rigidus, which have symbiotic N‐fixing bacteria (Frankia), was intense and suppressed growth, while the two non‐fixing shrubs (Arctostaphylos pumila and Ericameria ericoides) were not browsed heavily. For Ceanothus, excluding deer increased both plant size and the δ15N value to ~0‰; δ13C values also increased as the plants increased in mass. In Arctostaphylos and Ericameria, stable isotope values did not change, while plant sizes remained the same or even declined when deer were excluded. We interpret the change in Ceanothus δ15N values as due to increased N fixation after evaluating possible alternative explanations. The increase in Ceanothus δ13C values may be due to increased water stress with substantial shrub growth. More broadly, herbivore suppression of N fixation may impact ecosystem processes such as productivity and N cycling, as well as an ecosystem's ability to respond to increased CO2.
Highlights
Herbivory can have cascading impacts on the structure and functioning of ecological communities as well as ecosystem processes that include nutrient dynamics and biogeochemistry (Schmitz 2008)
The symbiosis between nitrogen (N)fixing bacteria and plants may be responsive to herbivory, since the dynamics of this mutualism are modulated by plant allocation of compounds to support the symbiotic bacteria (Denison 2000, Bronstein 2001, Ness et al 2009), which might be reduced if plants are stressed (Thomas and Berry 1989)
There were no initial differences among treatments within species (P > 0.11 for all except in Ericameria control leaves, which had slightly higher d15N values than ÀD+R leaves [P ~ 0.06])
Summary
Herbivory can have cascading impacts on the structure and functioning of ecological communities as well as ecosystem processes that include nutrient dynamics and biogeochemistry (Schmitz 2008). Herbivores affect plants directly by reducing biomass, carbon fixation, and survival, or by increasing fertilization through liquid and solid waste They have indirect impacts through interactions with other species that include plants, symbiotic microbes, and other herbivores, and through changes they create in habitat structure and soil nutrient dynamics (Schmitz 2008, Tao and Hunter 2011, Strickland et al 2013). Excluding deer from an oak savanna increased the abundance of only one of three N-fixing legumes, Lathyrus venosus, enhancing its productivity and increasing [N] in surrounding soil (Ritchie and Tilman 1995, Knops et al 2000) Such studies demonstrate that herbivory affects plant growth and/or abundance, but they do not show direct effects on Nfixation rates per se because biological N-fixation (BNF) rates were not measured. Impacts of biotic interactions such as herbivory on BNF rates are largely unexplored in natural terrestrial systems
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