STUDIES ON TEE CEREAL ECOSYSTEM
113
became available and as cereal growing became relatively more economic, the forage legumes were no longer essential. Their use in undersown
leys was limited to areas in which extensive arable grazing could
continue, and by 1968-1969 the annual acreage was down 80% on the
1949-1960 total (e.g. see Table 11). Meanwhile, the use of fertilizer
nitrogen increased three-fold (calculated from Tomlinson, 197 1). These
changes must have had a considerable effect on the insect fauna, and
the decline in lucerne and sainfoin may well explain the virtual absence
of some of the species of Phytonomus (Co1eoptera:Curculionidae) in our
surveys in Hampshire, Angus, Yorkshire and Sussex. We have found
P. austriacus Schrank. ( = punctatus F.), but P. posticus Gyll., which
was once widespread on English farms, was only present in one area
of Suffolk where lucerne was regularly grown. It did not occur on
lucerne in Sussex where lucerne has not been continually present.
Judging from the numbers present in partridge food samples in the
1930’s their absence today must have greatly increased the foraging
activity of game chicks. The larvae are unusual among the weevils in
that they feed exposed on the leaves and, like sawfly larvae (Hymenoptera-Symphyta-Tenthredinidae), are therefore a favoured food of the
grey partridge chick (Potts, 1970a).
c. F I E L D SIZE-HEDQE REMOVAL A N D GEOGRAPHICAL
CONCENTRATION O F CROPS
On many farms there is now no need to enclose, protect or shelter
livestock, and progressively more mechanized methods of cultivation
have been introduced. Thus, a new set of factors now limits field size.
Fields, as apart from crop units, have become larger (see Plate 2)
and the process is for the most part irreversible. Indeed, the lack of
fences or hedges imposes restrictions on the flexibility of the farm
economy.
In practice the size-frequency distribution of fields has changed from
being normally distributed around 5 ha (though in some parts of
eastern England the figure was a good deal higher) to a very positively
skewed size-frequency distribution. Examples of this are given in Fig. 2.
The upper limit to field size will usually be determined by the upper
limit to crop size, but many aspects of this have not been studied.
However, any crop barrier 200 m wide will delay the spread of cereal
leaf disease (A.D.A.S. Arundel Cereal Survey data) and it seems likely
that smaller crop sizes slow down the rate of spread of these diseases;
this is currently being investigated. As with so many changes in
agriculture, hedge removal has been localized; in parts of West Sussex,
for example, there has not yet been much change. Moore et al. (1967)
113
became available and as cereal growing became relatively more economic, the forage legumes were no longer essential. Their use in undersown
leys was limited to areas in which extensive arable grazing could
continue, and by 1968-1969 the annual acreage was down 80% on the
1949-1960 total (e.g. see Table 11). Meanwhile, the use of fertilizer
nitrogen increased three-fold (calculated from Tomlinson, 197 1). These
changes must have had a considerable effect on the insect fauna, and
the decline in lucerne and sainfoin may well explain the virtual absence
of some of the species of Phytonomus (Co1eoptera:Curculionidae) in our
surveys in Hampshire, Angus, Yorkshire and Sussex. We have found
P. austriacus Schrank. ( = punctatus F.), but P. posticus Gyll., which
was once widespread on English farms, was only present in one area
of Suffolk where lucerne was regularly grown. It did not occur on
lucerne in Sussex where lucerne has not been continually present.
Judging from the numbers present in partridge food samples in the
1930’s their absence today must have greatly increased the foraging
activity of game chicks. The larvae are unusual among the weevils in
that they feed exposed on the leaves and, like sawfly larvae (Hymenoptera-Symphyta-Tenthredinidae), are therefore a favoured food of the
grey partridge chick (Potts, 1970a).
c. F I E L D SIZE-HEDQE REMOVAL A N D GEOGRAPHICAL
CONCENTRATION O F CROPS
On many farms there is now no need to enclose, protect or shelter
livestock, and progressively more mechanized methods of cultivation
have been introduced. Thus, a new set of factors now limits field size.
Fields, as apart from crop units, have become larger (see Plate 2)
and the process is for the most part irreversible. Indeed, the lack of
fences or hedges imposes restrictions on the flexibility of the farm
economy.
In practice the size-frequency distribution of fields has changed from
being normally distributed around 5 ha (though in some parts of
eastern England the figure was a good deal higher) to a very positively
skewed size-frequency distribution. Examples of this are given in Fig. 2.
The upper limit to field size will usually be determined by the upper
limit to crop size, but many aspects of this have not been studied.
However, any crop barrier 200 m wide will delay the spread of cereal
leaf disease (A.D.A.S. Arundel Cereal Survey data) and it seems likely
that smaller crop sizes slow down the rate of spread of these diseases;
this is currently being investigated. As with so many changes in
agriculture, hedge removal has been localized; in parts of West Sussex,
for example, there has not yet been much change. Moore et al. (1967)