How do pollinators respond to the shape of agricultural landscapes?

The global debate rumbles on about pollinator decline. In the UK, the recent European Commission directive banning neonicotinoid pesticides has at least partly been a catalyst for some very public debate on why decline is happening, and what could be done about it (with the BBC rushing out a nicely-balanced edition of their science programme Horizon, exploring a few of the factors that may be driving the disappearance of pollinators).

This posting ties in with my talk at INTECOL 2013 in London (if you’re there, it’s in the Ecosystem Services session in Capital Suite 13 on Wednesday 21st at 2.15pm).

Aside from disease and poisoning, one factor that is frequently pointed to is the huge changes that have been made to the landscape in recent years.  The intensification of agriculture has meant that the ‘wild’ bits of the landscape have been taken away through changes in field management, and the steady creep of urbanisation.  These wild bits, even if they’re simply hedgerows and the other untidy bits at the edges of fields, are hugely important for providing nesting sites, refuge and food for wild pollinators and the other beasties that contribute to making agricultural systems work.  If we take these messy little spaces away, not only do we remove the resources that these beneficial species use, but we also make it much more difficult for those existing beneficial species already present to gain access to the parts of our managed agricultural species that are not close to these refuge areas.

Working with Heather Whitney (University of Bristol), I’ve done some work looking at how the shape of the agricultural environment affects the ability of pollinators to access it. In a paper published in Ecological Modelling, we considered a simple case where the environment was considered to be a square grid of hedgerows, with pollinators nesting in the hedgerows. The pollinators were considered to only fly a set distance from their nest (realistic, since many solitary bees fly a maximum  of about a kilometre from their nest), and the model demonstrated that if this distance was small, and kind of environmental manipulation that increased the size of fields beyond a certain point may have a detrimental effect upon the amount of wild space available to a pollinator.

However, the model was extremely simplistic.  Although I believe very strongly in keeping exploratory models as simple as possible, it felt like there was too many rigid assumptions made when we assumed that the landscape was a square grid.  In order to make the landscapes more realistic, we took two approaches: firstly, simulating random landscapes filled with hedgerows, and secondly, using landscape data from the UK, where there is a large amount of variation in wild refuge space within the landscape, as you can see from the four sample landscapes given below.

These landscape-informed models, published in PLoS One, demonstrated again that pollinators that only fly short distances from their nest (less than about 125 metres, which is relevant for some solitary bees such as Andrena hattorfiana) are affected heavily by landscape manipulations, but are unlikely to benefit from having wild land added to the environment unless it is targetted specifically for them (the equivalent of trying to help an isolated island community by building a new hospital for them on the mainland.  For species travelling more than 125 metres, adding wild space into the (British) landscape is a good thing, regardless of the exact distance the species travels.

So, we should maybe consider how far specific pollinators are able to travel when we are considering their conservation.  Lots of work is being conducted by  research groups across the world to quantify and observe the lengths of these commuting distances, and many research teams are finding that pollinators are thriving in response to many unexpected resources such as urban gardens. We still have a lot of work to do to explore how different species choose to move through the environment, and how this can be manipulated to benefit them and us.

Further reading

Rands SA & Whitney HM (2010). Effects of pollinator density-dependent preferences on field margin pollination in the midst of agricultural monocultures: a modelling approach. Ecological Modelling 221: 1310-1316 | abstract | pdf (postprint version)

Rands SA & Whitney HM (2011). Field margins, foraging distances and their impacts on nesting pollinator success. PLoS One 6: e25971 | full text | pdf