A food web represents all the food interactions between living organisms in an ecosystem. Among these interactions there is for instance predation, parasitism, decomposition of organic matter and plant consumption. In an agrosystem, cultivated plants and other plant species constitute a basic trophic level: that of producers. At a higher trophic level are the herbivores. Some are considered pests because of their negative impact on crop yields. These pests’ natural enemies are one or more higher trophic levels.
In an agrosystem, nutrient cycling on which crop plants depend is influenced by many trophic interactions. One can also hope to control pests by relying on their natural enemies: crop auxiliaries. The idea behind such a biological control (or biocontrol) is based on the notion of trophic cascade. In other words, the increased pressure of the higher trophic levels on the trophic level of pests impacts the base level, crops.
The interactions that take place within a network can be complex, resulting in counterintuitive and unwanted results in terms of biocontrol. For instance, an increase in abundance and/or diversity of auxiliaries may be accompanied by a decline in crop yields. In this case, predation, or even cannibalism, between natural enemies leads to less predation pressure exerted on pests. Ultimately, the crop suffers more damage. Conversely, by having pests in particular places, during particular periods or development stages, a complementarity can emerge between crop auxiliaries, increasing crop yield.
Agroecology promotes production systems that develop natural processes such as biocontrol or the decomposition of organic matter. In this context, a clear understanding of the structure and dynamics of food webs can be useful in effectively managing an agrosystem.
References to explore
Deguine J.P., Ferron P., Russell D. 2008. Vers une gestion agroécologique des communautés. In : Protection des cultures : De l’agrochimie à l’agroécologie. Editions Quae, pp. 123-130. ISBN : 978-2-7592-0167-9
Sauvion N., Calatayud P.A., Thiery D. Et Marion-Poll F. (eds). 2013. Interactions insectes-plantes. Editions Quae, 784p. ISBN 978-2-7592-2018-2
Suty L. 2010. La lutte biologique : Vers de nouveaux équilibres écologiques. Editions Quae, 136p. ISBN : 978-2-7592-0612-4