2. Plant diversity

Summary

Figure 2 (i). Change in plant species richness in the wider countryside, 1990 to 2007

 

 

 

 

 

Assessment of change in plant diversity in the wider countryside

	Long term	Since 1998	Latest year
Arable and horticultural land	1990-2007		N/A
Woodland and grassland	1990-2007		N/A
Boundary habitats	1990-2007		N/A

 

 

 

• Within arable fields there was an increase in plant species richness (number of species per survey plot) both in the longer term (since 1990) and shorter term (since 1998).
• In woodland and grassland, plant diversity has declined in both the longer and shorter term. For neutral grassland, broadleaved woodland and improved grassland, plant species richness fell over the longer term by 19, 7 and 5 per cent respectively and by 10, 2 and 3 per cent since 1998.
• In boundaries, plant species richness of the ground flora has also declined in both the long and shorter term. For streamsides, field boundaries and hedgerows plant species richness fell over the longer term by 13, 6 and 5 per cent respectively and by 7, 3 and 5 per cent since 1998.

Indicator description

This indicator presents the change in plant species richness in survey plots across Great Britain between 1990 and 2007 for a range of widely occurring habitats. The results from seven habitat types are presented, grouped into three measures for the assessment.   Species richness is presented as an index in the indicator so that the percentage change between the years in plots of different size can be compared. Table 2.1 shows actual species richness in each habitat along with information on the type and size of the survey plots.

Within arable and horticultural land there has been increase in plant species richness both in the longer term (1990–2007) and shorter term (1998–2007).   Plant diversity increased to 12 per cent above the 1990 baseline by 1998 and 45 per cent above the baseline by 2007. The long-term increase between 1998 and 2007 was larger than the short-term increase between 1990 to 1998.

In woodlands and grasslands, plant diversity has declined in both the long and shorter term. Within broadleaved, mixed and yew woodlands, plant species richness fell between 1990 and 1998 and between 1998 and 2007, although the decrease was only significant between 1990 and 2007 (i.e. the observed change may be due to random fluctuations rather than a genuine change). Within improved grasslands there was a similar decrease between 1990 and 1998 and between 1998 and 2007, although again it was only significant between 1990 and 2007.

In the smaller, targeted neutral grassland plots, species richness fell significantly between 1990, 1998 and 2007.

In boundary habitats (hedgerow ground flora, field boundaries and streamsides), plant diversity also fell significantly between 1990, 1998 and 2007.

Relevance

Today’s countryside has been shaped and maintained largely by agriculture. Wild plants in agricultural fields and field boundaries provide important food sources and cover for farmland birds and other species which have also declined over the same period. For this reason, hedgerows and arable field margins are Priority Habitats in the UK Biodiversity Action Plan.

Neutral grassland is a semi-natural habitat that was once common in lowland Britain but has declined dramatically in extent since the 1950s. Table 2 shows that neutral grassland has one of the higher values for plant diversity despite having the smallest plot size. It is because of this diversity, the scarcity of the habitat and its associated fauna that the best examples are identified as Lowland Meadow Priority Habitats in the UK Biodiversity Action Plan.

The woodland index records plants on the woodland floor – the ground flora. Woodland ground flora such as bluebells (Hyacinthoides non-scripta), wood anemones (Anemone nemorosa) and wild garlic (Allium ursinum) are aspects of woodland biodiversity that are highly appreciated by the public.

The vegetation on the banks of small rivers, streams and ditches provides an important habitat for small mammals, birds and invertebrates throughout the countryside. Such streamsides also provide a relatively undisturbed refuge for plants which were formerly much more extensive in unimproved meadows and wetlands.

Background

The Countryside Survey includes a random sample of vegetation plots located in arable and horticultural fields, agricultural grasslands, woodlands and along boundary habitats in Great Britain. Although there is a version of Countryside Survey for Northern Ireland, there are no equivalent data available for species richness.

The indicator compares species richness per plot for the same plots surveyed in 1990, 1998 and 2007. For each broad habitat type, the data are converted to an index (on a scale of 0-100) to compensate for the difference in plot size between habitats.

As a result of agricultural intensification (use of herbicides, artificial fertilisers and cropping and land management practices) over many years arable fields and improved grassland already had low plant diversity in 1990. There is some evidence that arable set-aside schemes in England contributed to a slight increase in diversity by 2007.

Supplementary data from the Botanical Society of the British Isles

Countryside survey provides data from across Great Britain on changes in the extent and condition of different habitats. However, because it is a sample-based survey of the whole country, it tends to record only widespread plant species. Further data on a wider range of species are available from species records collected by the Botanical Society of the British Isles (BSBI). The BSBI compared the distribution of species found in Great Britain in two systematic surveys completed in 1987-8 and 2003-4. These surveys recorded plant species present in a grid of 2km x 2km plots (tetrads).

The change in extent between the two dates can be calculated for 860 species, many of which are typically associated with semi-natural habitats. To combine these data and provide a measure of change for groups of species associated with different habitats, the change values for the individual species are weighted to account for their relative abundance. The weighted statistics are then combined to give a ‘change statistic’ for each habitat. The change statistic is a measure of the relative change in the geographical extent of a plant species. It does not represent the number of tetrads gained or lost between the years, because of the weighting applied before combining individual change values (it more closely represents the degree of loss in tetrads at the edge of the range of the species). A positive figure indicates an increase in extent, a negative figure indicates a reduction. The higher the figure, the greater the change in extent at the edge of the range of component plants. Further details on the method are given in Braithwaite et al. (2006).

Table 2.2 shows the mean change statistic for plants associated with a range of artificial and semi-natural habitats. This shows similar results to the main indicator, with an increase in plants associated with agricultural land and a decrease in species of woodland and grassland. The figures also suggest this decline extends to upland habitats. The method does not lend itself to an assessment of change in boundary and improved grassland habitats.

The average change statistic for individual species show much larger changes than the habitat groupings, for instance, within neutral grassland species, wild carrot (Daucus carota) has a change statistic of -20 and meadow cranesbill (Geranium pratense) has a change statistic of +22.

Table 2 (ii). Change in plant species associated with selected habitats from BSBI surveys conducted in 1987-88 and 2003-04

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