Trophic cascades in agricultural landscapes: indirect effects of landscape composition on crop yield

Abstract

The strength and prevalence of trophic cascades, defined as positive, indirect effects of natural enemies (predatory and parasitic arthropods) on plants, is highly variable in agroecosystems. This variation may in part be due to the spatial or landscape context in which hese trophic cascades occur. In 2011 and 2012, we conducted a natural enemy exclusion experiment in soybean fields along a gradient of landscape composition across southern Wisconsin and Michigan, USA. We used structural equation modeling to ask (1) whether natural enemies influence biocontrol of soybean aphids (SBA) and soybean yield and (2) whether landscape effects on natural enemies influence the strength of the trophic cascades. We found that natural enemies (NE) suppressed aphid populations in both years of our study, and, in 2011, the yield of soybean plants exposed to natural enemies was 37% higher than the yield of plants with aphid populations protected from natural enemies. The strength of the :rophic cascade was also influenced by landscape context. We found that landscapes with a higher proportion of soybean and higher diversity habitats resulted in more NE, fewer aphids, and, in some cases, a trend toward greater soybean yield. These results indicate that landscape context is important for understanding spatial variability in biocontrol and yield, but other factors, such as environmental variability and compensatory growth, might overwhelm the beneficial effects of biocontrol on crop yield.

Biomass Production a Stronger Driver of Cellulosic Ethanol Yield than Biomass Quality

“…However, it should be possible to ensure a consistent supply of biomass by developing a diversity of feedstock sources including switchgrass monocultures and corn stover (~6 Mg ha -1 ), mixed grass stands (~6 Mg ha -1 ), and miscanthus (~16 Mg ha -1 ) into a landscape mosaic surrounding a biorefinery. In addition, the resulting land use and land cover mosaic would likely provide significantly higher levels of other ecosystem services such as higher crop yields in pollinator-dependent crops (Liere et al, 2015), better wildlife habitat (Robertson et al, 2011), and climate stabilization among others (Landis et al, 2017;Ventura et al, 2012). Moreover, where the feasibility and availability of feedstock supply align, these fuelsheds could be expanded with a distributed network of preprocessing and aggregation depots where pretreatment and densification occur .…”

“…Environ. [In preparation]) by improving soil health (Robertson et al, 2008;Duran et al, 2016) and increasing pest and pathogen suppression (Werling et al, 2014;Liere et al, 2015;Landis et al, 2017). However, yield stability is likely to come at the expense of attaining the highest possible biomass yields in years with favorable growing conditions (Webster et al, 2010;Duran et al, 2016).…”

Intersection between biodiversity conservation, agroecology, and ecosystem services

“…For example, in soybean fields in the US Midwest, landscape heterogeneity (called landscape diversity in their study) enhanced abundance of ladybird beetles and removal rates of their aphid prey (Gardiner et al 2009a). Conversely, in these same landscapes, biological control services of soybean aphids decreased in less heterogeneous landscapes Liere et al 2015), but there were no significant effects of changes in the proportion of natural and semi-natural habitats to biocontrol services (Liere et al 2015).…”

“…In some cases, the effect of landscape diversity on natural enemies is stronger than the percent of natural habitat cover (Liere et al 2015). This is likely because natural enemies utilize resources from multiple habitat patches and rely on heterogeneous landscapes that provide 'partial resources' (Westrich 1996) or 'landscape complementation' (Dunning, Danielson, and Pulliam, 1992) to fulfill their resource needs.…”

“…The effect of landscape drivers on yield is difficult to detect because yield depends on a variety of factors including soil and crop type, timing of pest infestation, and weather conditions. The effects of landscape context on yield can been detected, however, when local factors are experimentally controlled (Liere et al 2015). Carefully planned experiments and population models are needed to understand how landscape and local factors interact to affect not only the organisms mediating ecosystem services but also the population dynamics of pest populations and, ultimately, if these effects significantly impact yield.…”