Indicator description
The indicator presents the change in genetic diversity in native
breeds of cattle and sheep in the UK between 2001 and 2007, as
measured by their effective population size. This is calculated for
each breed by assessing a number of factors, including the number
of breeding animals, the frequency of breeding events and the
degree to which breeding animals are related. The effective
population size represents the size at which the rate of loss of
genetic variation is equivalent to the rate of loss if that number
of individuals were mating randomly each generation. The greater
the effective population size, the lower the risk of in-breeding
and the greater the diversity retained over time.
The indicator shows the change in the average effective
population sizes for the breeds most at risk of loss of genetic
diversity. Those breeds most at risk are those with the lowest
effective population size (strictly the lower 20 per cent tail of
the distribution).
There are six sheep breeds identified as most at risk of loss of
genetic diversity in 2007. To respect the wishes of breed
societies, not all of the breeds are listed below. However, four
are already included in the Rare Breeds Survival Trust (RBST)
watch-list: Boreray (critical), Castlemilk Moorit (vulnerable),
Manx Loaghtan (at risk) and Soay (at risk). Of the four cattle
breeds at greatest risk, one, Chillingham, is classified by the
RBST as ‘critical’.
The assessment of change for the indicator was judged by
assessing whether the observed changes could not be explained by
chance alone:
For sheep breeds
- Of the six breeds in the lower 20 per cent tail in 2001,
five had increased their effective population size by 2007 (no more
than would be expected by chance alone). Of these five breeds, two
had moved above an effective population size of 50 by 2007. The
other breed at or below the 20 per cent line in 2001 had decreased
to a value of less than 50 in 2007.
For cattle breeds
- Of the four breeds in the lower 20 per cent tail in 2001, all
had maintained or increased their effective population size by
2007. Assuming breeds sampled for the indicator are
representative of cattle breeds generally, the odds of this
happening by chance are 1 in 16. For one of the breeds, the
change to the effective population size resulted in a move above
the threshold of 50 individuals.
The mean effective population size for sheep breeds has risen by
4.5 (12 per cent) and for cattle breeds by 8.3 individuals (32 per
cent). The short-term change for cattle breeds is assessed as
improving but because of the degree of uncertainty in comparing the
samples between years, the measure for sheep breeds is assessed as
showing little or no overall change, despite the small positive
increase. There has been no reported UK extinction of any
breed of sheep or cattle since 2001.
Figures 5 (ii) and 5 (iii) show the size class distribution for
effective populations of sheep and cattle breeds in the UK. The
United Nations Food and Agriculture Organisation (FAO) recommends a
minimum effective population size of at least 50 individuals to
reduce the rate of inbreeding to less than one per cent and
ensure the long-term survival of the breed. The figures show that
there has been comparatively little change in the number of breeds
with an effective population size of less than 50 individuals since
2001 (shown as bars with bold borders on the figures).
In summary, there was no evidence of overall change for the
sheep breeds based on the data presented, with most trends
encouraging but not definitive. However, there was evidence
of a significant increase for cattle breeds between 2001 and
2007.
Figure 5 (ii). Distribution of size classes for effective
populations of sheep breeds in the UK, 2001 to 2007
Figure 5 (iii). Distribution of size classes for effective
populations of cattle breeds in the UK, 2001 to 2007
Background
In a particular breed it is inevitable that some genetic
variation will be lost over time. Although it is not always
possible to measure the genetic variation in all traits of
interest, the average rate of loss in genetic variation can be
estimated, since this rate is related to the genetic size of the
breed. The genetic size of the breed can be described by
calculating the ‘effective population size’.
The indicator was developed by collating information for native
breeds of cattle and sheep listed in the UK Country Report (Defra,
2002). Effective population size is most readily estimated from
pedigree data. However, for breeds for which pedigree information
was not available, an estimate of effective population size can be
obtained from the numbers of parents and proportions of animals
selected for breeding.
The indicator was compiled in 2005 by collating information from
breed societies of native breeds. This information included
pedigree files for breeds with electronic recording or numbers of
breeding animals used each year (and numbers of years of active
breeding for males and females) for breeds without electronic
recording. In total, responses obtained from breed societies
representing 53 per cent of sheep and 58 per cent of cattle breeds
native to the UK.
The indicator was calculated in the following way:
i. For each, the effective population size
for each breed was estimated from the data supplied by breed
societies;
ii. The distribution of size classes for
effective population across the species was plotted (see
description of trends section); and
iii. The average effective population size
for the lower 20% tail of the distribution was calculated.
In step (iii), 20 per cent was chosen because, given the number
of breeds with available information, it gives high weight to the
breeds most at risk, without being too sensitive to events
surrounding a single breed. The indicator decreases when breeds
become extinct or when management within breeds deteriorates (i.e.
the risk of in-breeding increases). It will be sensitive to those
breeds most at risk, and insensitive to events in breeds where the
effective population size remains high.
Further details of the methods used to calculate effective
population size are given in an accompanying technical report (see
web links below).
Further development
This indicator has been developed rapidly and is likely to be
refined in future. The current indicator has included about half of
the UK native breeds of sheep and cattle. The indicator would
benefit from additional data from breed societies that have not yet
been able to respond to requests for information. There may also be
further methodological or presentational refinements. Central
collation of data is an objective of the FAnGR National Action
Plan, which will simplify future updating of the indicator,
although this is likely to be on a five to 10 year
interval.
Supplementary data on conservation of plant genetic
resources
In common with indicators at global and European level, the
genetic resource indicator is based on diversity in native farm
animals. Additional data on the conservation of plant genetic
resources are available but to-date it has not been possible to
develop and agree an indicator of plant genetic resources. This
section provides supplementary information of the conservation of
plant resources in the Millennium Seed Bank project managed by the
Royal Botanic Gardens, Kew.
The Millennium Seed Bank (MSB) is one of the largest ex
situ conservation projects in the World. It aims to collect
seeds, herbarium specimens and data from species worldwide,
including the entire UK seed-bearing
flora, and to conserve these collections to international
standards. These will include the rarest, most threatened and most
useful species.
Seed banks provide an insurance policy against the extinction of
plants in the wild. They complement in situ conservation
methods, which conserve plants and animals directly in the wild.
The Millennium Seed Bank already holds seeds from species thought
to be extinct in the wild. In addition, seed banks provide a
controlled source of plant material for research, education and
public awareness.
Figure 5 (iv) shows the cumulative number of species of world
seed-bearing flora collected and conserved in the MSB. The Royal
Botanic Gardens at Kew had collected 10 per cent of the World flora
(about 30,000 species) by 2010, and aim to collect 25 per cent
(75,000 species) by 2025. The total number of seed-bearing species
is estimated at about 300,000 species, based on research by
Scotland and Wortley (2003).
Figure 5 (iv). Number of species from around the World
conserved in the Millennium Seed Bank, 2000 to 2010
Figure 5 (v) shows the number of distinct geographic areas
(hectads) from which collections of threatened seed-bearing plants
have been made across Great Britain (GB). A hectad is an area of 10
km x 10 km. Threatened seed-bearing plants are those listed as
critically endangered, endangered or vulnerable in the GB Vascular
Plant Red List. Seeds are collected according to a written protocol
that ensures a representative sample is taken from each site (see
web link to field manual below).
The greater the number of hectads, the wider the sampling area
and the more likely it is that the total sample contains greater
genetic diversity. The Royal Botanic Gardens, Kew have set a target
of at least five hectads per threatened species (about 1,100 ‘plant
populations’ in total).
Figure 5 (v). Number of sampling ‘hectads’ from which UK
threatened plants have been collected
References
Scotland R.W. & Wortley A.H. (2003) How many species of seed
plants are there? Taxon. 52, 101-104.