STUDIES ON THE CEREAL ECOSYSTEM
173
area (b) than elsewhere (see Fig. 18(vii)). In this area 50% of the surviving young sampled in October had a t least one pair of Syngamus in
the trachea in the two years of poor chick survival, 1968 and 1971;
the occurrence was 25% in 1969 and only 12% in 1970 when the chick
survival approached that expected from the abundance of arthopods.
Other diseases were less important, but were also found mainly in this
area. Area (b) is characterized by a very high density of pheasants and
contains several pens from which hand-reared birds are released each
summer. The invertebrate hosts of Synganzus, especially Lumbricidae,
were collected near these pens and found to contain high numbers of
infective larvae. Laboratory experiments have shown that partridge
chicks are very susceptible to infection with these larvae and that the
disease is often fatal. The anomaly at area (b) was therefore attributed
to disease. In contrast, disease was not a problem in area (a).
There was circumstantial evidence that predation was likely to be
unusually high among chicks in area (a) and that this was aggravated
by the cessation of predator trapping and by a temporary disappearance
of rabbits in the winter of 1969-1970.
c. P R E D A T I O N
Prior to 1970 area A (which includes area (a)) did not appear to be
anomalous. For the previous nine years the productivity of grey
partridges as measured by the August count was significantly similar
to that at area B (which includes area (b)) (see Fig. 18(i)). Where z =
the mean young:old ratio on area A and y = the mean young:old
ratio on area B then
x f 0.058 = 1*01y+ 0.06 (r2 = 0.96, P < 0.001)
So that in 1970, when the mean young:old ratio in area B was 3-25,
a mean ratio of 3-34 could be expected in area A, with 95% confidence
limits at 3-22 and 3.46. The observed ratio was 0.66 in A, due to poor
chick survival in area (a). The difference was even greater in 1971
when 32 pairs of grey partridge in area (a) produced only 16 young
to the age of eight weeks. In 1972, 31 pairs produced only three young
in this area.
Directly comparable figures are available for the numbers of predators killed per ha in areas A and B for the period 1960-1967. These
showed that the number of predators removed from each area was
similar and fluctuated in parallel. All gamekeeper-trapping in area A
ceased towards the end of 1969. The number of predators which would
normally have been removed from area A in the year ending August
1970 can be estimated from the number of predators removed from
173
area (b) than elsewhere (see Fig. 18(vii)). In this area 50% of the surviving young sampled in October had a t least one pair of Syngamus in
the trachea in the two years of poor chick survival, 1968 and 1971;
the occurrence was 25% in 1969 and only 12% in 1970 when the chick
survival approached that expected from the abundance of arthopods.
Other diseases were less important, but were also found mainly in this
area. Area (b) is characterized by a very high density of pheasants and
contains several pens from which hand-reared birds are released each
summer. The invertebrate hosts of Synganzus, especially Lumbricidae,
were collected near these pens and found to contain high numbers of
infective larvae. Laboratory experiments have shown that partridge
chicks are very susceptible to infection with these larvae and that the
disease is often fatal. The anomaly at area (b) was therefore attributed
to disease. In contrast, disease was not a problem in area (a).
There was circumstantial evidence that predation was likely to be
unusually high among chicks in area (a) and that this was aggravated
by the cessation of predator trapping and by a temporary disappearance
of rabbits in the winter of 1969-1970.
c. P R E D A T I O N
Prior to 1970 area A (which includes area (a)) did not appear to be
anomalous. For the previous nine years the productivity of grey
partridges as measured by the August count was significantly similar
to that at area B (which includes area (b)) (see Fig. 18(i)). Where z =
the mean young:old ratio on area A and y = the mean young:old
ratio on area B then
x f 0.058 = 1*01y+ 0.06 (r2 = 0.96, P < 0.001)
So that in 1970, when the mean young:old ratio in area B was 3-25,
a mean ratio of 3-34 could be expected in area A, with 95% confidence
limits at 3-22 and 3.46. The observed ratio was 0.66 in A, due to poor
chick survival in area (a). The difference was even greater in 1971
when 32 pairs of grey partridge in area (a) produced only 16 young
to the age of eight weeks. In 1972, 31 pairs produced only three young
in this area.
Directly comparable figures are available for the numbers of predators killed per ha in areas A and B for the period 1960-1967. These
showed that the number of predators removed from each area was
similar and fluctuated in parallel. All gamekeeper-trapping in area A
ceased towards the end of 1969. The number of predators which would
normally have been removed from area A in the year ending August
1970 can be estimated from the number of predators removed from