New paper out: Parasitoid diversity reduces the variability in pest control services

My latest paper from the organic farms study in the UK has just come out. You can download from here or contact me for a copy.

Macfadyen, S., Craze, P., Polaszek, A., van Achterberg, K. & Memmott, J. (2011) Parasitoid diversity reduces the variability in pest control services across time on farms. Proceedings of the Royal Society B, 278: 3387-3394.


Recent declines in biodiversity have increased interest in the link between biodiversity and the provision and sustainability of ecosystem services across space and time. We mapped the complex network of interactions between herbivores and parasitoids to examine the relationship between parasitoid species richness, functional group diversity and the provision of natural pest control services. Quantitative food webs were constructed for 10 organic and 10 conventional farms. Parasitoid species richness varied from 26 to 58 species and we found a significant positive relationship between parasitoid species richness and temporal stability in parasitism rates. Higher species richness was associated with lower variation in parasitism rate. A functional group analysis showed significantly greater parasitoid species complementarity on organic farms, with on average more species in each functional group. We simulated parasitoid removal to predict whether organic farms experienced greater robustness of parasitism in the face of local extinctions. This analysis showed no consistent differences between the organic and conventional farm pairs in terms of loss of pest control service. Finally, it was found that the different habitats that make up each farm do not contribute equally to parasitoid species diversity, and that hedgerows produced more parasitoid species, significantly more so on organic farms.

Interesting paper – network analysis….a guide for community ecologists

I found this great ‘invited view’ paper in Basic and Applied Ecology that explains the problems and pitfalls of using a range of network metrics to describe network topology. Very useful for any ecologist involved with, or wishing to embark on, network analysis.

The full reference is:

Nico Blüthgen (2010) Why network analysis is often disconnected from community ecology: A critique and an ecologist’s guide. Basic and Applied Ecology 11:185-195.

Link to Journal website

Interesting paper

Googling Food Webs: Can an Eigenvector Measure Species’ Importance for Coextinctions?

Predicting the consequences of species’ extinction is a crucial problem in ecology. Species are not isolated, but connected to each others in tangled networks of relationships known as food webs. In this work we want to determine which species are critical as they support many other species. The fact that species are not independent, however, makes the problem difficult to solve. Moreover, the number of possible “importance’” rankings for species is too high to allow a solution by enumeration. Here we take a “reverse engineering” approach: we study how we can make biodiversity collapse in the most efficient way in order to investigate which species cause the most damage if removed. We show that adapting the algorithm Google uses for ranking web pages always solves this seemingly intractable problem, finding the most efficient route to collapse. The algorithm works in this sense better than all the others previously proposed and lays the foundation for a complete analysis of extinction risk in ecosystems.

Stefano Allesina and Mercedes Pascual
PLoS Computational Biology Vol 5, Issue 9, e1000494

The authors have developed an algorithm based on the one Google uses to rank web-pages to order species in a network in terms of their importance for coextinctions. Their algorithm outperformed other measures of robustness to species loss. When examining 12 published food webs their results suggest that the position of a species in the food web is an important determinate of impact on extinction cascades.

Interesting paper

Compartments in a marine food web associated with phylogeny, body mass, and habitat structure
Ecology Letters (2009) 12(8), 779-788
Enrico L. Rezende, Eva M. Albert, Miguel A. Fortuna, Jordi Bascompte

doi: 10.1111/j.1461-0248.2009.01327.x

Rezende et al. examined network structure in a marine food web (containing 3313 interactions between 249 species/trophic groups!) and unequivocally showed the presence of compartments (or subunits) in this network. These are link-dense regions of the network where species interact more closely with other species within the module than between modules. Modules may may be important for the propagation of disturbance impacts throughout a network.  More importantly Rezende et al. identified some potential mechanisms behind this interesting network structure (body size, phylogeny and spatial structure).  Shark species played an important role in this network.

Presentation of research at INTECOL next week

The 10th International Congress of Ecology is on in Brisbane next week (16-21st August)

I am due to give a presentation in the Ecosystem Services symposium (Tuesday 18th 3:15pm Mezzanine 2). The presentation abstract is here:

The provision of the ecosystem service of pest control on farms now and in the future

Sarina Macfadyen1, Rachel Gibson2, Paul Craze3, William O.C. Symondson4, Jane Memmott2

1CSIRO Entomology
2School of Biological Sciences, University of Bristol
3Biology and Environmental Science, University of Sussex
4Cardiff School of Biosciences, Cardiff University

A precautionary approach to biodiversity management is often justified on the basis that the maintenance of a diversity of species is useful for the provision of a particular ecosystem service now and as a form biological ‘insurance’ against disturbances in the future. Natural pest control is one ecosystem service that is thought to be threatened by agricultural intensification. Here we examine the complex network of interactions between insect herbivores and their parasitoids to understand the relationship between parasitoid species richness, functional group diversity and the provision of natural pest control services across time. We utilise 20 farms that display a gradient of parasitoid species richness as a result of farming system. We hypotheses firstly, that there will be a strong correlation between parasitoid species richness and variability in parasitism rate at the whole-farm level. Secondly, those farms with greater parasitoid species richness within functional groups will experience better pest control services in the future. Finally, we use species interactions to identify the key parasitoid species important for providing pest control services in this context.