Herbivores are the chief drivers of their individual prey plant's performance and profoundly affect the composition and function of populations, communities and ecosystems.  Herbivore effect studies often focus exclusively on a consumer and its prey plant, and may neglect the potential for direct and indirect effects on co-occurring unpalatable or non-preferred plants.  Lack of information on unpalatable species responses to herbivores is likely due to the idea that unpalatable plants are expected to benefit from the presence of herbivores, who release unpalatable plants from competition with their palatable neighbors . However, generalist herbivores, especially dense populations of ungulate browsers and grazers, have the potential to exert a significant toll on unpalatable, non-prey plant population viability via indirect and direct mechanisms.  Here we test for the presence of costs and benefits to an unpalatable non-preferred species, Arisaema triphyllum,  growing in sites used by deer.  We used Trillium grandiflorum as our indicator of deer impact level.  We determined deer browse intensity on Trillium, and individual plant demographic traits (biomass, sex and stage and seed set for females) for Arisaema. Further, we measured three abiotic variables to quantify potential mechanisms of deer's indirect effects on plant growth.   

Results/Conclusions

For the unpalatable understory species, Arisaema triphyllum, we found surprising and insidious changes in its population structure and demography even though it is not browsed.  Deer browse level on a co-occurring palatable species significantly correlates with reduced growth and plant size and seed rain, lower proportion flowering, and male-biased sex ratios in Arisaema populations. These demographic costs are similar in sign and magnitude to those of browsed species. Further, more compacted, drier soils correlate with increased deer browse level, suggesting a potential indirect mechanism for the observed decline. If Arisaema's responses are typical of unpalatable species, implications for understory biodiversity are clear. Many unpalatable species in forests besieged by deer are likely to be in decline along with their palatable neighbors. Our study implicates deer overabundance in the cascade of forest species decline and the urgency of this conservation issue in North America.