208
H . KLOMP
fluctuate irregularly. This raises the question of the causation of density
fluctuations.
2. Although fluctuations do occur, with some exceptions the density
of such a population changes within relatively narrow limits. It is often
stated that this is due to regulation, i.e. high densities cause mortality
and emigration to exceed reproduction and immigration, and low
densities induce the reverse effect. These density-governed reactions
form the central problem in many population studies. Nevertheless,
especially in field populations our knowledge of the relationships which
stabilize numbers is still very scanty.
3. The density of any one species is often stabilized at different levels
when in different types of suitable habitats. This must be due to an
environmental influence on the relation between regulating factors and
density. The study of this problem is a comparative one, and it will be
the last stage to be solved, and then only after a thorough analysis has
been made of the co-action of factors affecting reproduction, mortality
and migration.
The present paper is devoted solely to the first two problems; the
third being left out of consideration because the investigations reported
here apply only to one area. Our aim was to analyse the variability of
numbers of an insect, and the relations between density and factors
affecting mortality and reproduction. Therefore, we measured the density of all stages of the insect, its reproduction, and the influence of as
many mortality factors as possible.
A. T H E A R E A O F INVESTIGATION
The study has been carried out in a plantation of Scots pine (Pinua
sylvestris), in the north-western part of the National Park “De Hoge
Veluwe”, near Otterlo, province of Gelderland, the Netherlands. The
area is situated on flat preglacial sands and the trees were 45 years old
and 10 m high in 1950, when the investigations were begun. The size of
the plantation is 20 ha (approximately 50 acres), but the observations
were all performed in the central part of it, hereafter referred to as the
study area, the size of which is 6.8 hectares. This area is divided in six
blocks by narrow paths (Plate 1 and Fig. 5 ) .
The sands in the study area are covered with a litter layer of 3-5 cm
thickness, which is nearly completely overgrown with moss, with some
patches of grass and blueberries. It is a pure stand of Scots pine; other
trees and shrubs are lacking. The growth of the trees is slow and their
density normal with a fairly good closure between the crowns. A
characteristic view of it is given in Plate 1.
H . KLOMP
fluctuate irregularly. This raises the question of the causation of density
fluctuations.
2. Although fluctuations do occur, with some exceptions the density
of such a population changes within relatively narrow limits. It is often
stated that this is due to regulation, i.e. high densities cause mortality
and emigration to exceed reproduction and immigration, and low
densities induce the reverse effect. These density-governed reactions
form the central problem in many population studies. Nevertheless,
especially in field populations our knowledge of the relationships which
stabilize numbers is still very scanty.
3. The density of any one species is often stabilized at different levels
when in different types of suitable habitats. This must be due to an
environmental influence on the relation between regulating factors and
density. The study of this problem is a comparative one, and it will be
the last stage to be solved, and then only after a thorough analysis has
been made of the co-action of factors affecting reproduction, mortality
and migration.
The present paper is devoted solely to the first two problems; the
third being left out of consideration because the investigations reported
here apply only to one area. Our aim was to analyse the variability of
numbers of an insect, and the relations between density and factors
affecting mortality and reproduction. Therefore, we measured the density of all stages of the insect, its reproduction, and the influence of as
many mortality factors as possible.
A. T H E A R E A O F INVESTIGATION
The study has been carried out in a plantation of Scots pine (Pinua
sylvestris), in the north-western part of the National Park “De Hoge
Veluwe”, near Otterlo, province of Gelderland, the Netherlands. The
area is situated on flat preglacial sands and the trees were 45 years old
and 10 m high in 1950, when the investigations were begun. The size of
the plantation is 20 ha (approximately 50 acres), but the observations
were all performed in the central part of it, hereafter referred to as the
study area, the size of which is 6.8 hectares. This area is divided in six
blocks by narrow paths (Plate 1 and Fig. 5 ) .
The sands in the study area are covered with a litter layer of 3-5 cm
thickness, which is nearly completely overgrown with moss, with some
patches of grass and blueberries. It is a pure stand of Scots pine; other
trees and shrubs are lacking. The growth of the trees is slow and their
density normal with a fairly good closure between the crowns. A
characteristic view of it is given in Plate 1.