Insect community ecology

Terrestrial arthropods represent a significant component of functional biodiversity within agricultural ecosystems. They contribute to the overall sustainability of these systems through their involvement in a range of ecosystem processes, i.e. nutrient cycling and pest biocontrol.

Ecological theory states that multi-species plant communities should support a greater diversity of invertebrates relative to a monoculture crop; this is due to the wider range of potential niches they provide (Cardinale et al. 2007). It was hypothesised that invertebrate groups will be more abundant in the multi-species swards compared to the monocultures due to the presence of more plant species and functional groups.

Eight pasture mixtures were selected using a constrained simplex-centroid design with three functional groups; grasses, legumes and herbs. There was a constraint imposed on the design so that there was at least 40% grass in each mix, therefore no more than 60% legume or herb (see diagram). This was repeated at three levels of species richness, with 1, 2 or 3 species per functional group (24 mixtures). The design was replicated at four different nitrogen (N) rates (0, 45, 90, 135 kg N/hectare/year).

Grazing was simulated by cutting to a height of 4cm eight times during the growing season. Invertebrates were sampled using a Vortis suction sampler (Burkard Manufacturing Ltd.) two weeks after the previous harvest, i.e. at the midpoint of growth, three times a year. Samples consisted of 10 randomly positioned suction samples each of 10 sec. duration.

Results from this experiment will allow practical recommendations for methods to increase insect diversity & abundance using multi-species sward mixtures. This work will also allow us to explore the theory that insect diversity & abundance will increase along a gradient of plant species richness. We can also explore the effect of plant species versus function group redundancy on insect populations.

Cardinale, B.J. et al. (2007) Impacts of plant diversity on biomass production increase through time because of species complementarity. Proceedings of the National Academy of Sciences of the United States of America, 104, 18123-18128.
Cornell, J.A. (2002) Experiments with Mixtures: Designs, Models, and the Analysis of Mixture Data, 3rd edition. Wiley, Chichester.