Biotic Versus Abiotic Control of Primary Production Identified in a Common Garden Experiment

Abstract

Understanding drivers of ecosystem primary production is a foundational question in ecology that grows in importance with anthropogenic stresses (e.g., climate change). Traditionally, ecosystem production is considered to be abiotically controlled at large spatial scales (e.g., precipitation, temperature, etc.), which underlies forecasting climate change impacts. Using a "common garden" experiment over 10 years at two sites with the same plant and grasshopper species, we show that primary production is strongly influenced by biotic factors (herbivory and plant adaptations to it) at finer spatial scales by creating positive feedbacks, which reverse relative productivity of ecosystems expected from abiotic conditions alone. Our results without herbivory indicate that one site has 26% less annual net primary production (ANPP) than the other site. With herbivory, the sites reverse in ANPP, so the site with lower ANPP without herbivory now is 15% greater than the site with higher ANPP without herbivory, as they respectively increase by 6% and decline by 33%. This reversal is due to changing nitrogen availability (N), as N becomes 16% greater at the higher ANPP site with herbivory, respectively a 3% increase and 41% decline in N. The ANPP and N changes are observed, even though the sites are a few kilometers apart and have the same grasshopper and plant species.

Figure 1

“Common garden” results. (a) Comparison of ANPP (±SE) measured without grasshoppers (2005–2006) for pots with Site A plants and soil, Site B plants and soil, and control plots at each site. ANOVA results are presented. (b) The relationship between ANPP versus proportion of grass blades eaten by grasshoppers in the prior year (2007–2013) at each site for control plots and pots with that sites’ plants. Results are presented separately as the slopes differ (P < 0.0002, see ANCOVA in Table S2.4).

Figure 2

(a) The inter-annual change in ANPP (ANPP effect = current year ANPP/prior year ANPP) and the inter-annual change in growing season N (N effect = current year N/prior year N) in experimental pots, and regression statistics. (b) The inter-annual change in growing season N (N effect = current year N/prior year N) and the percentage of litter remaining (100% X litter in Sept/prior year ANPP) in experimental pots, and regression statistics. (c) Relative abundance of fast (P. pratensis) to slow (P. smithii) decomposing plants at each site with each plant source after grasshoppers were present in experimental pots. The number of plants sampled are also provided.

Figure 3

The ANPP response to grasshopper herbivory (ANPP with grasshoppers ÷ ANPP without grasshoppers) for plants from Site A at Site A and plants from Site B at Site B, and quadratic regression statistics.