Arthropods can provide ecosystem services or dis-services to agricultural systems; herbivores can cause substantial crop losses, whereas their natural enemies can provide pest regulation services that otherwise would be difficult to achieve. Two major factors that affect arthropod communities within plantations are farming practices and surrounding landscape. Previous studies showed that natural enemies are less abundant in plantations with simplified landscapes and intense management practices, while pests does not always respond to these factors. Given the role of arthropods in plantations, here we seek to understand the cascading effects that landscape matrix and intensive management practices may have on them and ultimately on plant damage and fruit production.
A piecewise structural equation model was used to assess the direct and indirect effects of management and landscape upon herbivores and their natural enemies, and the cascading effects of these on papaya crop damage and fruit production in 11 papaya plantations around the region of the Chamela-Cuixmala Biosphere Reserve, a tropical dry forest in Jalisco, Mexico. Mexico is considered to be the center of origin and diversity of Carica papaya and is nowadays one of the main exporters of this fruit, together with Brazil and Belize.
Results/Conclusions
We found a management intensity gradient in papaya plantations. There were no differences in arthropod community composition between surveyed plantations. The model suggested that, in the papaya crop system, intensive management practices relate directly with less abundance of natural enemies (β = -0.56, p=0.0017 ) and with less fruit production (β = -0.50, p=0.0187 ), and has cascading effects over plant herbivores (β =0.37, p=0.0000) and leaf damage (β = 0.05, p=0.0000), but there was no significant relation between leaf damage and fruit production. Model showed that the abundance of natural enemies contributes directly to higher fruit production (β = 0.10, p=0.0000). Percentage of surrounding forest relates directly with higher abundance of herbivores located at ground level (β =0.78, p=0.0000) but had no cascading effects on plant damage nor production.
Our results indicate that intensive management can have direct adverse effects on natural enemies of pests and on crop production, and that plantations with high abundance of natural enemies had higher fruit production. The structural equation model proved useful for synthesizing field observations into a single scenario that can reveal the weights of the effects of exogenous variables in complex systems, like agroecosystems, and highlight relative important factors to take into account for decision making.