Biodiversity and pest control: organic vs conventional farms

Good webs or ecological networks as they are
more generally known as, to do with the links between species. So traditionally ecologists have looked at
species lists, patterns of abundance and diversity but over the last decade there has been increasing
interest in how these species are linked together because this has a huge effect on things like
the provision of ecosystem services such as pollination, pest control, our capacity to
mend these communities if they are damaged and degraded, their response to stress and
so on. The structure of the study we used was a big ambitious project and what we did
first of all was to choose ten pairs of organic and conventional farms in the South West of
the UK. So we had 20 farms. To actually choose 20 farms you’ve got to look at 80 so there
is a lot of leg work involved in actually getting the choice right. You have got a list of criteria – the farms
need to be matched in size, type and so on – otherwise you are comparing chalk and cheese.
Having got our farms in place, and these go from Stroud down to Shepton Mallett basically,
we then constructed ecological networks for each of the farms so this is a tears worth
of work. We are sampling the farms once a month, all 20 farms and we have got a huge
team of people out there gathering up, counting plants, collecting caterpillars, leaf miners
and ruling the parasitoid. We use that information to construct these networks which shows who’s
eating who on the farms. We can then analyse the structure for those networks. So that’s
the descriptive stage. The next stage is the manipulative stage because
to me a food web project – you either have a perturbation where there is some environmental
woe affecting it in some way or you deliberately manipulate it and predict what will happen.
So our manipulation in this case was to introduce a novel pest to the farms and obviously we
can’t use a real live pest or else the farmers won’t like that at all for all the obvious
reasons! What we did, we had a surrogate pest, so we introduced a plant that does grow on
farmlands at all. It’s a plant called paracantha which is a
very urban shrub and we planted a small plot of 50 plants at each farm, we then introduced
species of leaf miner that is specific to that plant so it won’t grow on any crops,
it won’t move into the native habitat at all. We then used that as a bioassay for the health
and the parasitoid communities on each of the farms. With the following years after
we put the plant in we sampled the leaf miners and saw how many species of parasitoid we
got on the organic and conventional farms and also the percent morality – how well it
was controlled. Specifically, what was the biotic resistance to a novel pest on the two
types of farms. We got two levels of information concerning the results. The first one is that
there is more biodiversity on the organic farms than the conventional farms, significantly
more species are found at all three trophic levels on the network so more plants, more
herbivores and more parasitoid. That didn’t translate into an increase in ecosystem service
test of pest control though on the organic farms which was really interesting. The results
might be different if we were to repeat the work in a more intensively farmed area but
here in the South West of England which is a hotspot for organic farming we got the same
level of biotic resistance, as we call it, to a novel pest on the conventional farms
and that means the conventional farms really do have these services in tact and perhaps
learning to manage that network of interaction better we might be able to reduce pesticide
use in the long term. The organic farms can certainly be reservoirs
of biodiversity in the landscape. We don’t know that much about the movement of parasitoids
at the farm scale so it is a tricky one to answer and you could actually say that organic
farms providing reservoirs of biocontrol agents, so they move onto conventional land, that’s
one approach. You could also say they are actually gaining from the pest control or
neighbouring farms. We don’t know which way the benefits are going at the moment. But
having organic farms around is certainly good for biodiversity in the landscape.

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