Friday, April 4, 2014

Cover cropping

Cover crops are typically defined as crops used to protect agricultural soils and to improve soil productivity. Historically, farmers have relied on green manure crops to add nutrients and organic matter to their soil. Typically, green manure crops are grown for a specific period of time, and then plowed under and incorporated into the soil while green or shortly after flowering. Cover crops have also been used to protect the soil from wind and water erosion, to interrupt disease cycles and suppress weeds; and sometimes as supplemental feed for livestock or to provide an additional food source for pollinators and other beneficial insects. 

This traditional form of plant diversification may also promote natural regulation of agricultural pests by supporting alternative prey that in turn enable the increase of generalist arthropod predator densities and diversities. The larger the densities of these predators, the higher the consumption of herbivore pests - provided that the pest remains the favorite prey. However, predator diet composition changes induced by cover cropping are poorly understood.

A group of French researchers used a metabarcoding approach to assess the diet of eight ground-dwelling predators commonly found in banana plantations in Martinique. They used a shortened fragment of COI from the gut contents of predators to identify their prey and to identify predators of the major pest of banana, Cosmopolites sordidus.  The researchers were particularly interested in differences in the composition of predator diets between a bare soil plot and a cover cropped plot of the banana plantation as the cover crop Brachiaria decumbens is increasingly used to control weeds and improve physical soil properties. They were able to demonstrate that the use of a cover crop in banana plantations altered the arthropod food web, with significant changes in the frequency of consumption of some of the prey. An increase in alternative prey in the diet of the predators induces a diet shift that seems to dampen the positive effects of cover crops on pest regulation. The predators actually increase consumption of non-pests without increasing consumption of pests. 

The study closes with a general assessment of the use of metabarcoding for research on trophic interactions:
In conclusion, it is essential to disentangle trophic interactions in order to achieve a better understanding of ecosystem resilience and persistence following disturbances, such as plant diversification. DNA metabarcoding allows direct inference of trophic interactions and enables the assessment of arthropod diet. Although the method has limitations, including the inability to discriminate between direct predation, secondary predation, and scavenging, it has the potential to be very useful for describing arthropod food webs. Here, we identified new and unexpected trophic interactions in the predator–prey system in banana plantations. The accurate determination of trophic networks will challenge current models of trophic interactions and will contribute to food web theory and ecosystem management. In addition to its application to individual food webs, DNA metabarcoding could be used to link different food webs, such as those that describe micro-organisms, plants, arthropods, and larger animals.

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