Why did the pollinator cross the road?

The concept of the ‘edge effect’ has inspired long and varied discussion in the ecological literature. In essence, an edge effect is a change in animal or plant communities seen at a boundary between two types of habitat.

These changes are most obvious in plant communities, for example where a swamp segues into a savannah. So, historically, research into edge effects and ecotones (the zone surrounding the edge where two plant communities meet, and energy fluxes and dynamics change) was mostly focused on plants.

It wasn’t until the mid-1900s that people started considering how edges affected animals. Vegetation ecologists had already discovered that the zone surrounding habitat edges usually had more plant species than either of the two patches that met at the edge.

Then in 1930 Aldo Leopold noticed that game animals, like deer, were often found more frequently at forest edges than in the interior. These animals loitered at edges, where they could feed on all the extra plants and see danger coming more easily. And so the misconception arose that edges = more animals.

But this isn’t always true. Not all animals love fence-sitting as much as humans. Some species do thrive at edges, where they benefit from the extra resources, light, and energy created where the two habitats meet. Other species avoid edges, seeking safety or more stable microclimates in patch interiors. And some species might not really care about the edge itself, but their decision to step across it could be influenced by what’s on the other side.

Edge effects are two-sided. So if we want a robust estimate of how animals are influenced by an edge, it’s important to find out what’s happening on both sides of the edge…as we have shown in a new paper documenting wild pollinators across a woodland/plantation edge, just published online in the journal Agricultural and Forest Entomology.

So why would a pollinator, or any other insect, cross an edge between two different habitats?

It depends on the context. What type of insect is it? What are the characteristics of the edge? What are the different habitats (or patches) on either side of the edge in question?

For example, a ground-dwelling beetle is going to feel more threatened by crossing a two-lane road than a bumble bee would be, and a butterfly is less likely to fly beyond the edge of a meadow full of flowers, into a dense, dark forest.

And why should we care whether an insect will cross an edge or not?

Because human-dominated environments are full of edges. Highways, fences, walls, footpaths, roads, canals, railway lines, hedgerows, crop fields, open-cut mines…we humans just love straight lines, both physical and metaphorical.

These human-created edges have very different structure and dynamics to natural edges. Human-made edges usually have a sharp gradient and little or no transition zone between the two habitats on either side. They are also regularly influenced by management activities that constantly affect the dynamics at the edge…like pedestrian or vehicle traffic, mowing regimes, crop harvesting etc.

On the other hand, most insect species play a vital role in keeping ecosystems functioning and evolving, by pollinating flowers, controlling pest and disease outbreaks, improving water infiltration etc.

So if you care whether an endangered butterfly will survive after a highway is built through its habitat, or whether pollinators are crossing the fence into your crop fields to pollinate your crops, then it’s important to understand how those species respond to the edge in question.

For our study, the edges in question were the field boundaries between two monoculture almond plantations and adjacent patches of native mallee woodland. Almond trees need pollinators to set fruit, so it’s important to know whether any pollinator insects in the woodland do cross the fence into the plantation.

The edge between almond plantations and mallee woodland (Photo: James Abell)

We found that pollinators living in the mallee were likely to cross the edge into the plantation, but weren’t too keen to travel too far away from the diverse mallee resources. This pattern also changed over time (before, during and after the almond flowering event), indicating that edge effects are more complex than just identifying what happens across an edge at one moment in time.

So the barrier to pollinator dispersal into plantations was not necessarily the ‘edge’ between the two habitats, but more likely the lack of resources available on the plantation side. In a crop field, these resources change over time, particularly in perennial tree crops like almond, where flowers are only present once a year, for a few weeks…after crop flowering finishes, the plantation is a floral desert. So pollinators will be attracted by the abundance of flowers for the flowering period, but what do they do for the rest of the year?

Our study shows how important it is to consider what happens to pollinators in crop fields before and after crop flowering, and also how complex habitat edges are. We need to create new edges through wilderness to build towns, gardens, and crop fields…but we can reduce the impact of these edges on wildlife by making sure that the grass is just as green for them on the other side.

© Manu Saunders 2014

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