Exploring population level herbivore effects on an endangered terrestrial orchid

Background/Question/Methods

The effects of herbivory on plant populations are not well known for many species despite a large body of literature related to plant-herbivore interactions.  This is likely related to difficulty in translating herbivore damage of individual plants to herbivore effects on population dynamics.  In order to investigate population level effects of herbivores on an endangered terrestrial orchid, Spiranthes parksii. We experimentally manipulated herbivore pressure and used the data to parameterize a demographic population model for S. parksii.  In May 2009, we established thirty-two 1.5x1.5m plots and fenced half of them with 2m fences made of 10cm mesh to exclude large native herbivores.  We collected orchid abundance, demographic, and herbivore damage data every ten days following establishment of exclosures.  We used these data to develop periodic stage-structured population models for each plot.  The periodic model multiplies the projection matrices for each sampling period to create an annual projection model which accounts for seasonal differences in demographic parameters.  The annual projection matrix was used to predict an asymptotic annual growth rate (λ) for each plot, which we compared using a t-test to determine differences between λ for protected and unprotected plots.

Results/Conclusions

In unprotected plots, fewer flowering stalks produced inflorescences because of greater removal by herbivores (p=0.0080).  This herbivore damage to reproductive structures represents a direct loss of fitness.  Large herbivores did not have an effect on rosette abundance (p=0.1288) and rosettes in protected and unprotected plots experienced similar levels of herbivory (p=0.0713).  However, average time aboveground was lower both overall (p=0.0141) and for the rosette stage (p=0.0435) in unprotected plots because of large herbivore removal of flowering stalks and entire rosettes.  Surprisingly, these herbivore effects on S. parksii population parameters do not translate into population level differences.  Despite direct loss of fitness through consumption of flowering stalks prior to inflorescence development, population level effects of large herbivores are not manifested in reduced population growth rates, λ is the same for protected and unprotected plots (p=0.1883).  This is likely due to a substantial effect of small herbivores (eg, invertebrates) on parameters related the rosette stage of the S. parksii lifecycle.  Loss of resources allocated to photosynthesis, which contribute to future reproduction through effects on growth parameters, can indirectly affect population fitness.  This disconnect necessitates the use of both experimentation and demographic modeling to fully understand implications of herbivory for plant population dynamics.