Trophic cascades are permutations across at least two links in a food chain, and are most frequently invoked when there are shifts in the primary trophic level (plants) caused by changes at least two trophic levels above plants. There is currently a serious lack of data in many terrestrial systems, including deserts, on trophic cascades. Here, we experimentally manipulated lizards in a desert dune community in White Sands National Monument and measured the effect on lower trophic levels. Six experimental arrays, each consisting of 3 10 by 10 meter treatments, were built in June 2017. The three treatments were (1) aluminum flashing fence with full lizard removal (exclosures), (2) a partial fence treatment where lizards are not removed and can come and go, to control for potential fence effects and (3) control plots with no fences and no lizard removal. We collected data on lower trophic levels and the efficacy of lizard removal at the beginning of the study and periodically over the next two years. Response variables include web building spider densities (initiated in June 2018), arthropod biomass and community composition (pitfall and sticky trapping), plant cover, plant community composition, plant growth and reproductive output, and herbivory (leaf damage).
Results/Conclusions
Exclosures were effective in reducing lizard densities. We found an average of 0 lizards in exclosures, 1.3 lizards in partial plots and 2.4 lizards in control plots in surveys (one way ANOVA; F[2, 15] = 3.538, p = 0.055). Pitfall trap data indicates no difference in abundance or biomass between treatments for Collembola, Coleoptera, Formicidae or Araneae June 2017, August 2017, November 2017 or March 2018. Work is continuing for additional dates, taxa, and for sticky traps. In our web building spider surveys, we found an average plot density of 6.5 spiders in exclosure plots, 2.1 spiders in partial plots, and 1.83 spiders in control plots (one way ANOVA; F[2, 15] = 9.361, p = 0.0023). We did not find a significant difference in spider densities in August of 2018 (one way ANOVA; F[2, 15] = 0.83, p = 0.454). Quantitative results for plant cover, plant community composition, plant growth, plant reproduction and herbivory are forthcoming with additional lab work, but qualitatively no major changes in plants have been observed following lizard exclusion. Early conclusions are that that lizard removal can have effects on lower trophic levels in a desert community, but this effect is limited and dependent on season.