Indicator assessment
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% tail of
the distribution).
The mean effective population size for sheep
breeds has risen by 4.5 (12%) and for cattle breeds by 8.3
individuals (32%). 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.
Description of trends
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, five 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 4
cattle breeds at greatest risk, one, Chillingham, is classified by
the RBST as ‘critical’.
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 1% 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 addition to there being little or no
overall change in the mean effective population size for sheep
breeds and an increase in the mean effective population size for
cattle breeds at greatest risk, there has been no reported UK
extinction of any breed of sheep or cattle since 2001.
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 2008 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% of sheep and 58% 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% 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).
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 6 breeds in the lower 20% tail in
2001, 5 had increased their effective population size by 2007 (no
more than would be expected by chance alone). Of these 5 breeds,
two had moved above an effective population size of 50 by 2007. The
other breed at or below the 20% line in 2001 had decreased to a
value of less than 50 in 2007.
For cattle breeds
- Of the 4 breeds in the lower 20% 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.
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.
Further development
This indicator has been developed rapidly over
the last year 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 5 – 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 the largest
ex situ conservation project 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 aim to collect 10% of the
World flora (about 30,000 species) by 2010 and 25% (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 – 2008
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.
Reference
Scotland R, W and Wortley A, H. 2003. How many species of
seed plants are there? Taxon. 52, 101-104