Blog by Derek Gruar, Senior Research Assistant, RSPB Centre for Conservation Science
Over the years we have been able to share some great success story from RSPB Hope Farm. While we have widely publicised the increases in bird numbers on the farm, what is less well known is the success we have had in increasing the numbers of our butterflies.
One of my jobs as the researcher based at the farm is to monitor the butterfly populations; to do this I walk three set routes (transects) through the farm, these transects are just over 5.5 km in total length and are surveyed on a weekly basis from 1st April until late September (a 26 week survey season), over the course of the summer almost 150km is walked surveying butterflies, the equivalent distance of walking from Cambridge to London and back.
Surveying butterflies is a very pleasant job and does have some distinct advantages over the bird survey work that I also do on Hope Farm, firstly surveys need to be on relatively warm and sunny days and start around 11am (a far cry from the 4am alarm call needed to get out for summer bird surveys!).
Photo: Marbled white butterfly, by Ben Andrew (rspb-images.com)
Contributing to UK Butterfly Monitoring Scheme
During the surveys I count all butterflies within a 5 meter band around me as I walk the survey routes. The long survey season allows us to record all species likely to be found on the farm including butterflies that emerge early in the season (such as orange tip) and species that are multi-brooded (peacock, common blue) that occur at two different time periods during the summer. The data is submitted to Butterfly Conservation and contributes to the UK Butterfly Monitoring Scheme.
We also produce a Hope Farm Butterfly Index to give a measure of how our butterfly population has changed over time to when we first took over the management of the farm using the first full survey season in 2001 as our baseline year. We use the same species as the DEFRA wider countryside species index used to measure national trends.
In that first season over 2300 butterflies of 19 species were recorded over the 3 transects for the 26 week survey season. This increased to its highest value in 2015 with almost 4500 butterflies of 24 species were recorded over the season, new species recorded included brown argus and marbled white. We have now recorded 26 species on the farm, with purple hairstreak recorded for the first time in July 2017.
In terms of the index at its high point in 2015 the Hope Farm butterfly index stood at 2.66. This means that there has been an average increase of 166% in our butterfly population since 2001. This compares to a stable or slightly declining national trend over the same period.
Photo: Butterflies trends at RSPB Hope Farm
So how is 2017 faring? After a cool start to the year the hot dry summer weather saw good numbers of ringlet, gatekeeper and meadow brown on the farm. I am 17 weeks through the 2017 season and over 3700 butterflies have been recorded so far, if there is a warm end to the year then this is the chance a new record number of butterflies may be set.
More flowering plants on the farm for our butterflies
This poses the question why has the butterfly population increased? The simple answer is that we have increased the amount of flowering plants on the farm. When the first surveys were done in 2001 there was little in the way of environmental features on the farm. Crops were sown close to the hedgerows and these hedges would have been cut on an annual basis. Since then most of our fields have 6m wide margins with a mixture of grasses and those planted with pollen and nectar providing flowers such as birds foot trefoil, knapweed, vetches and mallow. Our hedgerows are now cut in a rotation every 3 years which also provides more flowers for insects to feed on.
In 2007, the Hope Farm Butterfly index sees a large increase this coincides with the farm entering into our Entry Level Environmental Stewardship (ES) scheme, where we receive governmental funding for providing nature friendly habitats on the farm. This enabled us to increase the number of grass and flower rich margins on the farm and the butterfly numbers appear to have responded. Using GIS software we plan to look at the effect of this habitat creation and the subsequent butterfly numbers.
In the new Countryside Stewardship scheme that has replaced ES it is possible for landowners to plant flower rich habitats that will be attractive to butterflies and other pollinating insects: a number of options pay over £500/ha with an ask of 1% of the farm area in the scheme allocated to flower-rich options. With the correct promotion and take up of these options then our countryside can become richer with butterflies.