Biotic and abiotic factors alter the structure and function of plant communities. Herbivore communities have been declining while nitrogen deposition has increased due to anthropogenic causes changing plant composition across space and time. Understanding how changes in soil nutrients will mediate herbivores guild impacts on plant communities structure (richness, evenness, diversity and composition) and function (productivity) is critical. We asked the following questions: (1) What are the main effects of selective herbivores (small rodents) on grassland diversity, composition, and productivity over time?, (2) How do generalist herbivores (insects) influence plant community diversity and productivity in the presence vs. absence of rodents?, (3) How do soil nutrients influence plant community diversity and productivity in the presence vs. absence of rodents?, (4) How do soil nutrients and insects interact to affect plant community diversity and productivity in the presence vs. absence of rodents?
To test our questions, we manipulated soil nitrogen and insect herbivory in established rodent access and rodent reduced areas. We tracked plant community structure and function over two years as well as microclimate (light availability, soil moisture, soil temperature, and soil N).
Results/Conclusions
Herbivores, not soil nutrients, altered the structure and function causing our grassland system to be driven by top-down effects of herbivores that control diversity and productivity. Rodents increased species richness in May 2014 (p=0.047) and August 2015 (p=0.004) by 12% and 14%, respectively. N addition increased species richness in the rodent access plots by 13% but decreased richness in the rodent exclosure plots by 17% (August 2014, p=0.052). Rodent herbivores lowered plant evenness by 16% in May and 13% (p-values= 0.030, 0.015 respectively) in August of 2014, while insect herbivory impacted species dominance in August 2014 and May 2015 (p-values= 0.007, 0.034 respectively). This effect was dependent on the presence of rodents. In August 2014, plots with some insect herbivory had 33% lower evenness than plots with full or no insect herbivory, but this effect was gone with the absence of rodents. On the other hand, In May 2015 insect herbivory lowered evenness by 16%, in rodent absent plots having no impacts in rodent present plots. Diversity was 13% lower in rodent access than exclosure plots in August of 2014 (p= 0.052) alongside with changes in species composition. Taken together, rodents mediate biotic and abiotic effects on grassland structure and function.