14
M. E. SOLOMON
major disturbing factor (Voilte, 1946; Solomon, 1949; MacArthur,
1955; Pimentel, 1961a; Elton, 1958).
Insect populations tend on the whole to fluctuate much more violently
than those of birds and mammals, which have a relatively small reproductive potential, exercise parental care of the young, and are less
influenced by variations in weather. These vertebrates are more socially
organized than most invertebrates, and have developed patterns of behaviour which tend to regulate their abundance in accordance with the resources of the environment, a subject recently discussed a t length by
Wynne-Edwards (1962). Fish are to a great extent sheltered from
weather effects, and are long-lived enough for the impact of a heavy mortality or high survival of one year’s fry to be buffered by the older and
the succeeding generations.
2. Types of Fluctuation
Although, superficially, fluctuation is perhaps the most definite and
easily measured of population phenomena, there are so many different
causes of variation in abundance that comparisons between different
populations, or over different intervals of time, must be made with
caution. Abundance in a particular habitat may be influenced by an
annual breeding season, with annually varying reproductive success,
followed by a period of variable decline in numbers due to factors such
as weather conditions, food shortage, natural enemies and disease, until
the next breeding season. Clearly the time of year and the age distribution should be taken into account when these fluctuations are assessed.
In comparisons of the abundance from year to year these complications
are generally avoided by counting at a particular stage of the life cycle.
Other causes of fluctuations are migratory, dispersive or aggregative
movements, as in locusts, or, less strikingly, in the spruce budworm
(Morris, ed., 1963).
Another complication is that fluctuations between generations are
often caused by lagging density effects, notably in the parasite-host
oscillations : thus, while a parasite may be regulating its host, it may be
regulating it through an oscillatory course of marked systematic
increases and decreases in abundance, instead of steering it towards
a constant (or randomly varying) level of abundance. Other types of
lagging density-dependent factors can have a similar effect. The striking
fluctuations in Nicholson’s laboratory populations of blowflies, as
already mentioned, were caused by the delayed effects of intense competition for limited resources.
A field exam9le of this type is provided by the work of Wallace
(1957) on the lucerne flea, Xminthurus viridis (L.), in W. Australia.
.This collembolan is a pest of pastures containing clovers. It is very
M. E. SOLOMON
major disturbing factor (Voilte, 1946; Solomon, 1949; MacArthur,
1955; Pimentel, 1961a; Elton, 1958).
Insect populations tend on the whole to fluctuate much more violently
than those of birds and mammals, which have a relatively small reproductive potential, exercise parental care of the young, and are less
influenced by variations in weather. These vertebrates are more socially
organized than most invertebrates, and have developed patterns of behaviour which tend to regulate their abundance in accordance with the resources of the environment, a subject recently discussed a t length by
Wynne-Edwards (1962). Fish are to a great extent sheltered from
weather effects, and are long-lived enough for the impact of a heavy mortality or high survival of one year’s fry to be buffered by the older and
the succeeding generations.
2. Types of Fluctuation
Although, superficially, fluctuation is perhaps the most definite and
easily measured of population phenomena, there are so many different
causes of variation in abundance that comparisons between different
populations, or over different intervals of time, must be made with
caution. Abundance in a particular habitat may be influenced by an
annual breeding season, with annually varying reproductive success,
followed by a period of variable decline in numbers due to factors such
as weather conditions, food shortage, natural enemies and disease, until
the next breeding season. Clearly the time of year and the age distribution should be taken into account when these fluctuations are assessed.
In comparisons of the abundance from year to year these complications
are generally avoided by counting at a particular stage of the life cycle.
Other causes of fluctuations are migratory, dispersive or aggregative
movements, as in locusts, or, less strikingly, in the spruce budworm
(Morris, ed., 1963).
Another complication is that fluctuations between generations are
often caused by lagging density effects, notably in the parasite-host
oscillations : thus, while a parasite may be regulating its host, it may be
regulating it through an oscillatory course of marked systematic
increases and decreases in abundance, instead of steering it towards
a constant (or randomly varying) level of abundance. Other types of
lagging density-dependent factors can have a similar effect. The striking
fluctuations in Nicholson’s laboratory populations of blowflies, as
already mentioned, were caused by the delayed effects of intense competition for limited resources.
A field exam9le of this type is provided by the work of Wallace
(1957) on the lucerne flea, Xminthurus viridis (L.), in W. Australia.
.This collembolan is a pest of pastures containing clovers. It is very