The butterfly effect: measuring the impact of agroforestry on biodiversity

As part of our Broadlears agroforestry project, we’ve been trying to monitor the impact of the trees on the field. This is an area where our amazing team of Conservation volunteers have once again come to the rescue – with a particular focus on the impact on birds and butterflies. Here, volunteer Steve brings us a mid-season update on how the butterflies have been doing this year.

Normally I would wait until October and the end of the butterfly survey season before undertaking a review of the year and an analysis of findings. However, here are a few notes and some initial findings that relate to the past ten weeks of 2018 (approx late June to late August), with a comparison with the same period in 2017.

Common Blue. Photo by Steve Turner

Common Blue. Photo by Steve Turner

Introduction and background to Broadlears Field Agroforestry butterfly survey

The brief was for a volunteer to carry out a butterfly survey in connection with the Agroforestry project at Broadlears Field at Dartington, located to the south west of Dartington Hall’s gardens and south of Upper Drive. This is a multi-cropping and multi-tenanted agricultural scheme that involves the cultivation of trees and crops alongside each other.

The site

The Agroforestry project covers a field extending over 48 acres. It is dissected by a service track and on each side, at an angle to the track, the field is planted out with rows of saplings of elderflower, a variety of apple and Sichuan pepper trees. During 2017 and 2018, the wide bands between each of the tree rows have been sown with red clover, which has been subject to periodic harvesting.

Aims of survey

The butterfly survey was instigated in order to provide base data to help measure the levels of biodiversity on the site and to provide indicators of environmental health in the locality. In turn, this data will provide some evidence of the ongoing conservation impact of the agroforestry project.


The survey is being carried out on a weekly basis each year between the start of April and the end of September, in accordance with Butterfly Conservation best practice. In 2017, it didn’t start until mid-summer, the time when the volunteer was first appointed, taking place over 15 consecutive weeks between June and September 2017.

The survey site covers the entire Broadlears Field and this site is divided into 10 survey routes called transects (eight in 2017), each of which are walked as part of the survey. For each transect, the number and species of butterfly identified are recorded on a survey sheet. Wind speed and direction, temperature and sunshine cover are also noted. Transects were identified around the perimeter field margins, along the internal service track margins and along the rows of selected tree species. Depending on the level of butterfly activity recorded and photographs taken, the survey typically takes between one and a half and two hours.

Survey findings

The total number of butterflies recorded during the past ten weeks (survey period week 12 to week 21 – approx. w/c 18 June to w/c 18 August) was around 660 in 2017 and 750 in 2018, an increase of 90 this year. The highest weekly totals were recorded in the first week of July in both years and were of a similar magnitude (160 in 2017 and 151 in 2018). In both years the peak volumes lasted over a period of three consecutive weeks – however, this peak started two weeks earlier in 2018.

The number of species of butterfly identified during the ten week period were very similar; 17 in 2017 and 15 in 2018. The most voluminous species for 2017 was the Meadow Brown and Ringlet. This remains the case in 2018 for the Meadow Brown but is no longer the case for the Ringlet (see note* below). Interestingly, however, there have been noticeable changes in the actual numbers recorded for each species.

Speckled Wood. Photo by Steve Turner

Speckled Wood. Photo by Steve Turner

Significant gains in numbers have been recorded in 2018 for certain species of butterflies:

Large White: 2017 – 38 / 2018- 139
Small White: 2017 – 16 / 2018 – 49
Common Blue: 2017 – 8 / 2018 – 22
Meadow Brown: 2017 – 327 / 2018 – 435

In contrast, these gains have been offset by some notable losses in 2018:

Small Skipper: 2017 – 29 / 2018 – 1
Ringlet*: 2017 – 148 / 2018 – 26
Marbled White: 2017 – 5 / 2018 – 0

There have also been fewer Small Tortoiseshell, Peacock and Painted Lady sightings so far in 2018.

Some conclusions

The reasons for the increase in the numbers of some species recorded and the decline in others are as yet unclear. There is some speculation that the unusually cold spring may have impacted on the hibernation periods of caterpillars and in turn the butterfly chrysalis stage but as yet I’m not aware of any clearly evidenced reasons for these changes. More information may be available by the end of the season in October when a more detailed analysis may be possible. I may get some more information from Butterfly Conservation in due course.

These totals continue to be distributed across transects in a very uneven way – the largest numbers (2018 analysis pending – over 90% during the 2017 June-Sept period) were recorded along the field margins and the track dissecting the field. Very few (2018 analysis pending – less than 7% during the 2017 June-Sept season) were found along the rows of recently planted trees. Butterflies recorded in these locations continue to appear to be ‘in transit’ rather than to be feeding specifically along and within the tree belts.

It was noticeable that in 2017 butterflies were not particularly attracted to the flowering clover in the broad agricultural strips. This appears to be less the case in 2018, when numerous butterflies of varied species (but particularly the Meadow Brown) were seen to be feeding on the clover crop. These flowers continue to remain hugely attractive to a vast number and type of bees and a variety of moths (including the Silver Y moth) once again shelter within the clover foliage.

The clover crop continues to be particularly invasive, both within the individual tree belts and the wildflower habitats of the field margins. This may have implications for food sources for butterflies in subsequent years. The suggestion still applies that it may be mutually beneficial, both in terms of the future productivity of the fruit trees and the improvement of the butterfly feeding habitat, to consider under-planting a mix of wildflowers to attract pollinators. This would probably have resource and maintenance implications for the various agro-forestry partners.

–  Steve, Dartington Volunteer

Find out more about volunteering at Dartington ⇒

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