Accredited official statistics

6 – Woodland Species

Updated 7 May 2024

Applies to England

Data last updated: November 2023

Latest data available:

• Breeding birds in woodland (National Statistics) - 2022
• Butterflies of the wider countryside in woodland - 2022
• Abundance of woodland plant species (Official Statistics in Development) - 2022

Introduction

This indicator shows changes in relative abundance of species in woodland, namely birds, butterflies and plants. Woodland provides habitat from canopy to ground level, important food resources for birds and butterflies, as well as nesting opportunities for birds and cover from predators.

Type of indicator

State indicator

Assessment of Change

• Breeding birds in woodland (National Statistics):
  • Long term (1970 to 2021): Deteriorating
  • Short term (2016 to 2021): Deteriorating
  • Latest year (2022): Decreased
• Butterflies of the wider countryside in woodland:
  • Long term (1990 to 2022): Deteriorating
  • Short term (2017 to 2022): Little or no overall change
  • Latest year (2022): Increased
• Abundance of woodland plant species (Official Statistics in Development)
  • not assessed

Breeding birds in woodland (National Statistics)

Trend description for Figure 6.1

In 2022, the breeding woodland bird indicator for England was 39% below its 1970 value (Figure 6.1). The greatest decline occurred between the early 1980s and the early 1990s. The index was stable, at around 80% of the 1970 numbers, between 1995 and 2012, since when it has decreased further by 15% in the short term.

Figure 6.1: Trends for the abundance of breeding birds in woodland in England, 1970 to 2022

Download the data for Figure 6.1 in ods format

Figure 6.2: Long-term and short-term changes in individual species trends for breeding birds in woodland in England, 1970 to 2021

Download the data for Figure 6.2 in ods format

Notes about Figures 6.1 and 6.2

• this indicator is taken from the Defra National Statistics publication Wild bird populations in England
• this indicator includes individual measures for 34 species of woodland birds
• Figure 6.1 shows the unsmoothed trend (dashed line) and the smoothed trend (solid line) together with its 95% confidence interval (shaded area)
• Figure 6.2 shows the percentage of species within the indicator that have increased, decreased, or shown little change, based on set thresholds of annual change
• Figure 6.2 is presented as a stacked bar chart and the legend is presented in the same order as the stacks in the bar chart

Source: British Trust for Ornithology, Defra, Joint Nature Conservation Committee and the Royal Society for the Protections of Birds

The overall trend masks different underlying trends for specialist species, those which are highly dependent on woodland habitats, and generalist species, which are found in a wide range of habitats, including woodland. In 2022, the woodland specialists index was 51% lower than in 1970, while the index for woodland generalists was only 10% lower (Figure 3.3). Over the short term, the woodland specialists have decreased by 17% and the woodland generalists have decreased by 12% (Figure 6.3).

The declines in woodland birds have several known and potential causes, such as a lack of woodland management (including the cessation of traditional practices such as coppicing) and increased deer browsing pressure, both of which result in a reduced diversity of woodland structure and reduced availability of suitable nesting and foraging habitats. Changes in farmland management, such as the removal of hedgerows, adversely impacted many of the species in the indicator which have substantial populations outside of woodland, for example, in farmland and gardens. In addition, several declining woodland birds are long-distance migrants, and a decline in the extent or quality of habitats used outside of England in the non-breeding season (in stop-over sites or African wintering areas) may be a significant factor affecting these species. Positive factors include the increasing area of woodland cover and milder winters potentially having a beneficial impact for some species.

Figure 6.3: Trends for the abundance of specialist and generalist woodland birds in England, 1970 to 2022

Download the data for Figure 6.3 in ods format

Notes about Figure 6.3

• this indicator includes individual measures for 34 species of farmland birds, of which 22 species are woodland specialists and 12 species are woodland generalists
• Figure 6.3 shows the unsmoothed trends (dashed lines) and the smoothed trends (solid lines)

Source: British Trust for Ornithology, Defra, Joint Nature Conservation Committee and the Royal Society for the Protection of Birds.

Butterflies of the wider countryside in woodland

Trend description for Figure 6.4

The abundance of butterflies in woodland has significantly declined since the start of 1990, decreasing by 47% (Figure 6.4). In the short term, since 2017, the abundance of butterflies has shown little or no change.

Large fluctuations in numbers between years are typical features of butterfly populations, principally in response to weather conditions. Overall, 2022 was an average year for butterflies in England, with 46% of all species contributing to the England indicators (23 out of 50) falling in abundance from the previous year. 2022 was the warmest year on record, although the weather fluctuated considerably each month with cooler and unsettled periods alternating with warmer, more settled spells. The rainfall was mostly below average with drought conditions in second half of the summer, especially in eastern parts of England.

Figure 6.4: Trends for the abundance of butterflies of the wider countryside in woodland in England, 1990 to 2022

Download the data for Figure 6.4 in ods format

Figure 6.5: Long-term and short-term changes in individual species trends for butterflies of the wider countryside on farmland in England, 1990 to 2022

Download the data for Figure 6.5 in ods format

Notes about Figures 6.4 and 6.5

• this indicator includes individual measures for 24 species of butterflies, the woodland index, however, only includes 23 trends. This is because an aggregate trend is used for small skipper and Essex skipper; these 2 species have been combined due to historical difficulties with distinguishing between them in the field
• Figure 6.5 shows the percentage of species trends within the indicator that have shown a statistically significant increase, a statistically significant decrease or no statistically significant change (little change)

Source: Butterfly Conservation, British Trust for Ornithology, Defra, Joint Nature Conservation Committee and the UK Centre for Ecology & Hydrology

The long-term decline of woodland butterflies is thought to be chiefly due to a lack of woodland management and loss of open spaces in woods. Species of the wider countryside showing the largest long-term decline in woodland include common blue; large skipper; marbled white; peacock; wall; small tortoiseshell; small copper; small or Essex skipper; and gatekeeper. No species of butterfly have undergone a significant short-term decline in woodland (since 2017). The ringlet displayed the same trends in woodland as on farmland, increasing significantly over the long term, but showing no significant short-term change.

Abundance of woodland plant species (Official Statistics in Development)

The Biodiversity Indicators project team would welcome feedback on the novel methods used in the development of this indicator.

Data collection for the National Plant Monitoring Scheme was severely affected by the COVID-19 pandemic. Due to this, the species abundance estimates for 2020 are likely to be biased and should be treated with caution.

This indicator measures, in small plots, change in the abundance of plant species considered indicative of good habitat condition in UK woodland, using modelled abundance data from the National Plant Monitoring Scheme (NPMS). Plant populations form the environment in which most other species exist, as well as providing numerous ecosystem services. Drivers of change are well-understood for many UK habitats.

Trend description for Figure 6.6

Between 2015 and 2022, average indicator plant abundance for the broadleaved woodland and hedges habitat type included within this UK indicator shows an increase of 2% (Figure 6.6).

Figure 6.6: Trends for the abundance of plant species in one broad UK woodland habitat, 2015 to 2022

Download the data for Figure 6.6 in csv format

Notes about Figure 6.6

• figure 6.6 shows the unsmoothed trends (dashed lines); the variation around the lines shown (the shaded areas) is the standard deviation of 1,000 simulated trend indices calculated according to the method of Soldaat, L.L., Pannekoek J., Verweij, R.J.T., Van Turnhout, C.A.M. and Van Strien, A.J. (2017). A Monte Carlo method to account for sampling error in multi-species indicators. Ecological Indicators, 81: 340–347 doi:10.1016/j.ecolind.2017.05.033.
• abundance is measured by the percentage area covered by a species within a plot.
• the figures in brackets indicate the number of species or species aggregates included in the composite index for that particular habitat type.
• data collection for the National Plant Monitoring Scheme was severely affected by the COVID-19 pandemic, and the species abundance estimates for 2020 are likely to be biased and thus should be treated with caution.

Source: Botanical Society of Britain and Ireland, Joint Nature Conservation Committee, National Plant Monitoring Scheme, Plantlife and the UK Centre for Ecology & Hydrology

The National Plant Monitoring Scheme (NPMS) was designed to monitor UK habitats of conservation importance. This is achieved through the establishment of small plots in areas of habitats targeted by the scheme. The abundances of plant species, measured as the percentage area covered by a species within a plot, are recorded each year. Surveyors record from different lists of indicator species depending on their level of experience and the habitat within which a plot is located. Both the placement of plots, and the selection of 1 kilometre national grid squares within which the plots are located, are subject to statistical methodologies designed to minimise bias (Pescott et al., 2019a).

The design of the NPMS included the definition of a set of 11 broad habitat types, within which 28 finer habitat types are nested. These fine-scale habitats are linked to existing classifications such as the British National Vegetation Classification. Surveyors can choose, based on their knowledge of a habitat, whether to record a plot at the broader or finer level. The current indicator summarises species’ percentage cover (abundance) data at the broad habitat level. This is done using a model that is able to account for both the range of percentage covers that a species may exhibit in a habitat when present, and the fact that species may often be absent from any given plot (Pescott et al., 2019b). Such data are often described as ‘zero-inflated’. This model is applied across years for each species and habitat combination, and the indicators presented here for each broad habitat are the result of combining the resulting species and habitat time trends across the relevant set of NPMS habitat indicator species.

The broad UK woodland habitat measure presented in this indicator is a subset of those for which the largest numbers of NPMS plots currently exist. See the technical background document for more detail.

Further information

Relevance

Bird and butterfly populations are considered to provide a good indication of the broad state of the environment because they occupy a wide range of habitats and there are long-term data on changes in populations which help in the interpretation of shorter-term fluctuations. Butterflies also respond rapidly to changes in environmental conditions and habitat management, are representative of many other insects, in that they utilise areas with abundant plant food resources and play a complementary role to birds as an indicator, because they use the landscape at a finer spatial scale.

Plants are a large part of the fundamental fabric of which habitats are made and directly indicate changes to environmental conditions and habitat management. Plants provide essential habitats and food for wildlife, and essential ecosystem services for humans, such as reduced erosion, nutrient cycling, oxygen production, and climate regulation.

These indicators show progress towards commitments to improve the status of our wildlife and habitats. They are relevant to outcomes 1 and 3 in Biodiversity 2020: A strategy for England’s wildlife and ecosystem services (see Annex A of the publication). The indicators are also relevant to international goals and targets (see Annex B of the aforementioned publication).

The UK and England Biodiversity Indicators are currently being assessed alongside the Environment Improvement Plan Targets, and the new Kunming-Montreal Global Biodiversity Framework Targets, when this work has been completed the references to Biodiversity 2020 and the Aichi Global Biodiversity Framework Targets will be updated.

Background

Breeding birds in woodland

The woodland bird measure has been supplied by the British Trust for Ornithology (BTO), the Royal Society for the Protection of Birds (RSPB), and JNCC and is compiled using data from the Common Bird Census (CBC) and the Breeding Bird Survey (BBS). Within the woodland bird measure there are 34 species (Table 6.1). Each species is given equal weighting and the index is the geometric mean of the individual species indices.

Bird count data from more than 3,000 sites surveyed annually by volunteers are analysed using log linear models to calculate population trends for each species. The longer-term changes in the indicator are assessed using the version of the indicator generated from the smoothed species trends, with bootstrapping used to generate confidence limits. Further details about species and methods can be found on the BTO website (See the Web links for further information section of this document).

Table 6.1: Species included in the woodland bird indicator

Common.name	Species.name	Category
blackbird	Turdus merula	Generalist
blue tit	Cyanistes caeruleus	Generalist
bullfinch	Pyrrhula pyrrhula	Generalist
chaffinch	Fringilla coelebs	Generalist
dunnock	Prunella modularis	Generalist
great tit	Parus major	Generalist
lesser whitethroat	Sylvia curruca	Generalist
long-tailed tit	Aegithalos caudatus	Generalist
robin	Erithacus rubecula	Generalist
song thrush	Turdus philomelos	Generalist
tawny owl	Strix aluco	Generalist
wren	Troglodytes troglodytes	Generalist
blackcap	Sylvia atricapilla	Specialist
chiffchaff	Phylloscopus collybita	Specialist
coal tit	Periparus ater	Specialist
garden warbler	Sylvia borin	Specialist
goldcrest	Regulus regulus	Specialist
great spotted woodpecker	Dendrocopos major	Specialist
green woodpecker	Picus viridis	Specialist
jay	Garrulus glandarius	Specialist
lesser redpoll	Carduelis cabaret	Specialist
lesser spotted woodpecker	Dendrocopos minor	Specialist
marsh tit	Poecile palustris	Specialist
nightingale	Luscinia megarhynchos	Specialist
nuthatch	Sitta europaea	Specialist
redstart	Phoenicurus phoenicurus	Specialist
siskin	Carduelis spinus	Specialist
sparrowhawk	Accipiter nisus	Specialist
spotted flycatcher	Muscicapa striata	Specialist
treecreeper	Certhia familiaris	Specialist
tree pipit	Anthus trivialis	Specialist
willow tit	Poecile montanus	Specialist
willow warbler	Phylloscopus trochilus	Specialist
wood warbler	Phylloscopus sibilatrix	Specialist

Composite indicators can mask a lot of variation among the species within them. The bar chart provided alongside the headline chart (Figure 6.2), shows the percentage of species within the indicator that have increased, decreased or shown little change. Whether an individual bird species is defined as increasing or decreasing has been decided by its rate of annual change over the time period (long or short) of interest.

If the rate of annual change would lead to a population decrease of 50% (halving), or a population increase of 100% (doubling) or more over 25 years, the species is said to have shown a ‘strong decline’ or a ‘strong increase’ respectively. Rates of change less than these but above an increase of 33% or below a decrease of 25% are labelled ‘weak’. Asymmetric thresholds are used for declines and increases to represent an equivalent symmetrical proportional change in an index. These thresholds for decline are based on the rates used in the Birds of Conservation Concern status assessment for birds in the UK.

Note that for most species, particularly over the longer period, the change is statistically significant.

Butterflies of the wider countryside in woodland

The woodland butterfly indicator is a composite (multi-species) index compiled by Butterfly Conservation (BC) and the UK Centre for Ecology & Hydrology (UKCEH) from data collated through the UK Butterfly Monitoring Scheme (UKBMS) including the Wider Countryside Butterfly Survey (WCBS). It uses butterfly count data collected at UKBMS butterfly transect sites in woodland together with additional data from randomly selected 1 km squares of the WCBS primarily comprised of woodland (totalling 2,534 sample locations across England), see the UKBMS sites details map for further information.

The indicator includes 24 species of butterflies associated with woodland; however, the woodland measure only includes trends for 23 species because an aggregate trend is used for small skipper (Thymelicus lineola) and Essex skipper (Thymelicus sylvestris). These 2 species have been combined due to historical difficulties with distinguishing between them in the field (Table 6.2).

Table 6.2: Species included in the England woodland butterfly indicator

Common.name	Species.name
brimstone	Gonepteryx rhamni
brown argus	Aricia agestis
comma	Polygonia c-album
common blue	Polyommatus icarus
gatekeeper	Pyronia tithonus
green-veined white	Pieris napi
holly blue	Celastrina argiolus
large skipper	Ochlodes sylvanus
large white	Pieris brassicae
marbled white	Melanargia galathea
meadow brown	Maniola jurtina
orange-tip	Anthocharis cardamines
peacock	Aglais io
purple hairstreak	Neozephyrus quercus
ringlet	Aphantopus hyperantus
small copper	Lycaena phlaeas
small heath	Coenonympha pamphilus
small tortoiseshell	Aglais urticae
small white	Pieris rapae
small/Essex skipper	Thymelicus sylvestris/lineola
speckled wood	Pararge aegeria
wall	Lasiommata megera
white-letter hairstreak	Satyrium w-album

The year-to-year fluctuations in butterfly numbers are often linked to natural environmental variation, especially weather conditions. Therefore, in order to identify underlying patterns in population trends, the assessment of change is based on smoothed indices. The smoothed trend in the composite indicator is assessed by structural time-series analysis. A statistical test is used to compare the difference in the smoothed index in the latest year versus other years in the series. Within the measures, each species is given equal weight, and the annual figure is the geometric mean of the component species indices for that year.

Populations of individual species within the measure may be increasing or decreasing irrespective of the overall trends. The bar chart provided alongside the headline trend chart (Figure 6.5), shows the percentage of species within the indicator that have shown a statistically significant increase, a statistically significant decrease or shown no statistically significant change (little change). A table summarising the estimated long-term and short-term changes for each species together with an assessment of the individual species trends can be found in the statistical dataset trends in populations of selected butterfly species, 1990 to 2022.

As there are delays in data submission, data for previous years are also updated retrospectively. This means that the species indices for individual years may vary from previous publications.

Further details of the methods used can be found on the UKBMS website and in the Technical background document for this indicator.

Abundance of woodland plant species

The creation of the NPMS allowed for the creation of annual trends in the abundance of plants in habitats of conservation importance. Following 5 years of development, the scheme was launched by a partnership consisting of the Botanical Society of Britain and Ireland (BSBI), JNCC, Plantlife, and UKCEH in 2015. This indicator uses a subset of the species selected by the NPMS as indicative of good condition in those habitat types considered to be of most importance for the conservation of UK biodiversity, see the technical background document for a full list of species included. These species are monitored in small sample plots (between 25 and 100 square metres in area) according to a methodology that was designed to minimise biases in data collection. Results for the UK broadleaved woodland and hedges habitats are presented here in the woodland plant species richness indicator.

Since 2018, UKCEH, with input from all partners, have been developing a method of using NPMS data to indicate annual changes in habitat condition. The method is based on a hierarchical model, formulated in a Bayesian framework, that integrates information on a species’ abundance and occupancy. The occupancy estimates also take advantage of the fact that most plots are surveyed twice a year, allowing adjustments for false negatives (that is, species that are overlooked during surveys). Simulation tests and applications to real data indicate that the method is robust and produces ecologically sensible metrics.

The 1 kilometre squares of the NPMS were selected according to a weighted-random algorithm designed to introduce a known bias towards semi-natural habitats. However, within this design, a sampling bias exists in that, in common with other UK structured monitoring schemes based on volunteer participation, squares located within lowland areas are more likely to be sampled. Further work will focus on additional adjustment for bias (Pescott et al., 2019b).

Until 2013, this indicator was based on analysis of the change in plant species richness in the wider countryside. Data were taken from the UK Countryside Survey. This survey provides a random sample of vegetation plots located in arable and horticultural fields, agricultural grasslands, woodlands and associated boundary habitats in Great Britain. Key messages from the previous indicator are presented here; although now archived, the indicator can be viewed in full on the archived version of the JNCC website.

Web links for further information

• Kunming-Montreal Global Biodiversity Framework 2030 targets
• Environmental Improvement Plan
• Bat Conservation Trust: The National Bat Monitoring Programme
• Botanical Society of Britain & Ireland: Home page
• British Trust for Ornithology: Breeding Bird Survey - Methodology and survey design
• British Trust for Ornithology: Potential volunteering for surveys
• British Trust for Ornithology: Publications - Bird Trends
• British Trust for Ornithology, Royal Society for the Protection of Birds and Defra: Technical background document - birds
• Butterfly Conservation: The state of Britain’s butterflies
• Butterfly Conservation and UK Centre for Ecology & Hydrology: Technical background document - butterflies
• Countryside Survey Home page
• Defra: Butterflies in England: species of the wider countryside on farmland and in woodland
• Defra: Wild bird populations in England
• Defra and UK Centre for Ecology & Hydrology: Technical background document - plants
• Joint Nature Conservation Committee: Home page
• National Plant Monitoring Scheme: Home page
• Plantlife: Home page
• UK Butterfly Monitoring Scheme: Butterflies as indicators
• UK Centre for Ecology & Hydrology: Home page

References

• Pescott, O.L., Walker, K.J., Harris, F., New, H., Cheffings, C.M., Newton, N., Jitlal, M., Redhead, J., Smart, S.M. and Roy, D.B. (2019a). The design, launch and assessment of a new volunteer-based plant monitoring scheme for the United Kingdom. PLoS ONE, 14(4): e0215891.
• Pescott, O.L, Powney, G.P. and Walker, K.J. (2019b). Developing a Bayesian species occupancy/abundance indicator for the UK National Plant Monitoring Scheme. Wallingford, NERC/Centre for Ecology & Hydrology and BSBI, 29pp. DOI:10.13140/RG.2.2.23795.48161
• Soldaat, L.L., Pannekoek J., Verweij, R.J.T., Van Turnhout, C.A.M. and Van Strien, A.J. (2017). A Monte Carlo method to account for sampling error in multi-species indicators. Ecological Indicators, 81: 340–347 DOI:10.1016/j.ecolind.2017.05.033