A web of flies (and bees)

If you were looking for me in the summer of 2017, you’d stand a good chance of finding me on the slopes of an obscure northern hill, knee-deep in ticks with my head buried in a field guide to hoverflies. Which to be fair sounds like the kind of thing I’d be doing anyway, but this time I had a purpose: I was conducted fieldwork for my undergraduate dissertation, (hopefully) the first of many such endeavours. For those who don’t know, I’m an ecology student at Lancaster University (UK), and a 10,000-word dissertation is a key element of my course. Having finally finished this, I thought I’d give you a window into my experience.

I’m something of a butterfly nerd, so I decided that insect pollinators would form the major focus of my dissertation; I’m also a plant nerd, to that played out well for me. As almost everyone knows by now, given recent events in the world of neonicotinoid pesticides [see this National Geographic article], insect pollinators are extremely important both in agriculture and uncultivated landscapes. In their search for nectar and pollen, bees, butterflies, hoverflies and their like transfer pollen between plants, allowing them to reproduce; without the critical service, our wild plant communities would be severely impoverished, and our agricultural system crippled. Many scientific studies [e.g. Hayter & Cresswell, 2006; Albrecht et al., 2012] have demonstrated a strong link between how many insect pollinators (individuals and species) exist in an ecosystem, and how successfully flowering plants reproduce.

However, I’m a real sucker for making my life harder than it really needs to be, so I thought, why stop at species diversity? This dangerous line of though led me into the shadowy world of functional diversity. For those who want a more in-depth discussion of this, you can refer to my previous blog post here, but basically this involves collating organisms into groups based on the functions they perform in their ecosystems – farmland birds being a good examples, which can be grouped into small seed-eaters like buntings and finches, insectivores like warblers, large generalist herbivores like gamebirds and pigeons, and carnivorous hawks, owls and falcons. The concept of ‘functional diversity’ is still relatively new on the ecology scene compared to more conventional measures of diversity, so I really had my work cut out for me. I wanted to see if there were links between the diversity of plants and pollinators on Warton Crag, a limestone hill just a few miles north of Lancaster, which is swathed in lowland calcareous grasslands and ash woods, and supports important populations of rare butterflies like the pearl-bordered fritillary and northern brown argus.

First of all, I had to figure out what was there on the Crag. I used twelve transects, each 15m long and running through tall bracken stands, rough bramble scrub, or short grassland, depending on how recently it had been managed. Scrubby areas on the Crag quickly become overrun with bracken, which shades out almost everything else but thistles; so the bracken is routinely cut back to ground level, on a multiple-year rotation to provide a mixture of habitats. I counted the number of flowers 30cm to either side of the transect’s midline and identified them to species level where possible (and genus level where not – I’m looking at you, eyebrights). Then I sat back for twenty minutes and watched for pollinators visiting the transect (in most cases there were remarkably few, but occasionally I was really spoiled for choice).

The really hard part began once I had my data. Not having the resources to measure the tongue of every bumblebee I encountered, I had to jury-rig a functional classification for my plants and insects based on flower morphology and method of feeding and pollen carriage, which I think worked rather well, all things considered. So I could compare data from different transects, I calculated indices of species and functional richness (simply the number of species or functional groups present on a transect), Pielou’s evenness index for species evenness (see Pielou, 1966), and a functional evenness index derived from Bulla’s O index of species evenness (see Mouillot et al., 2005; Bulla, 1994).

Regardless of how recently a transect was managed, community composition in terms of plants and insects was strongly skewed in favour of a few, very common species, with most species making up only a tiny percentage of the community.

Abundance ranks
Figure 1 | Whittaker plots for a. plants and b. pollinators, on managed and unmanaged transects, relative to the most abundant species (rank 1).

Scrub management was found to have no significant impact on any measure of diversity, for both plants and pollinators – which is good, as it suggests that the habitat management being conducted to benefit fritillaries can continue without damaging the Crag’s wider biodiversity. Additionally, scrub clearing only slightly simplified the food webs I produced from observing interactions between plants and pollinators.

Plant-pollinator web.png
Figure 2 | Plant-pollinator species interaction web constructed from data collected on managed bracken transects. Black bars represent pollinators (top) and plants (bottom) – unfortunately I’ve no room here to explain the abbreviations.  Their width represents the relative abundance of the species. The grey bars represent pollinator visits to flowers, with wider bars indicating a stronger, more common interaction.
Pearl-bordered fritillary
Figure 3 | Female pearl-bordered fritillary Boloria euphrosyne resting. © Iain Leach, UK Butterflies.

As expected, I found significant correlations between plant and pollinator diversity, across all transects and in all indices. However, the linear correlations between species and functional diversity indicates that there is little functional redundancy on the Crag (i.e. different species performing the same, or very similar, ecological functions). This leaves Warton Crag’s plant and pollinator communities susceptible to heavy losses from extinction: if one species vanishes, nothing can replace the services it provided, impacting the other species that relied upon it. But I hate to end on such a gloomy subject, so I’ll mention that the scrub clearance does seem to be benefiting the pearl-bordered fritillary, which has had a pretty good run over the last couple of years.

Writing up this dissertation was surprisingly fun, probably because I had such a good connection to the subject matter. However, there are a few things I wish I’d known before I started the whole process:

  1. Make a little bit of headway every day: even just ten words is still ten more than zero, and you’ll thank yourself later.
  2. If you’re going to choose a relatively new topic, be prepared to wade through some frustratingly impenetrable papers on the subject.
  3. Remember to enjoy your time in the field. It’s rare enough to escape the library, and Warton Crag is part of a designated Area of Outstanding Natural Beauty for a reason.
  4. A driver’s license can be extremely useful when you’re trying to reach far-flung field sites, invest in one if you can.
  5. Wide-brimmed hats are great, buy three to go with any outfit.

And on that note, I shall take my leave. I’m hoping to get back into the swing of blogging now that most of my undergraduate degree is over and the content of my dissertation has been dis-embargoed. So until next time,


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