198
J. HESLOP-HARRISON
a capacity to tolerate fluctuating climatic change of longer periods, such
as the “sun-spot” cycle recorded in the annuals rings of the redwoods,
and the irregular variations in winter temperatures and precipitation
that have been a feature of temperate latitudes in post-glacial times
(Manley, 1954).
Either because the periods favourable to growth are too short, or
because the amplitude of variability of environmental factors is beyond
tolerance, some habitats will not permit the survival of perennials.
However, adaptation to ephemeral habitats or to severe cyclical or
fluctuating changes can be attained by curtailment of the life cycle. The
annual habit represents a satisfactory solution to the problem of exploiting temporary habitats, or of surviving in a climate where there is
extreme but regular seasonal variation in vital habitat factors such as
temperature and rainfall. By fitting in with the major cycle of change
the annual avoids the requirement for adaptations giving tolerance to
the adverse season, which can be safely weathered in the seed ; it is left
then to face the longer and shorter period fluctuations. The methods
available to achieve this are of three general kinds. The “annual” cycle
can be distorted to accommodate irregular longer cycles ; this response is
seen in species of arid regions with sporadic rainfall, where the amount
of precipitation is itself a determinant of germination time. Or the
developmental plasticity of the individuals may be such that the phenotypes produced each year are adapted to match the conditions of that
year. Or, finally, the average phenotypes may possess an amplitude of
tolerance great enough to accommodate year-to-year fluctuations in
climate during the growing season.
The ephemeral habit is an adequate solution to survival in habitats
subject to severe and irregular disturbance. It commonly combines the
ability to suppress growth altogether during unfavourable periods, a
property which we may term avoidance, with adaptive developmental
plasticity and a high amplitude of individual tolerance.
These different kinds of physiological adaptation to change necessarily bear some relationship to community structure and stability. The
long-lived perennial habit is that characteristic of the stable climax
community such as forest or permanent grassland. In the case of forest,
the moderating and controlling effects of the dominants themselves on
soil and microclimate constitute a damping factor stabilizing the environment still further for tree seedlings and the herbaceous members of
the community. So far as the tree species are concerned, this is, of
course, an aspect of the “independence of environment” designated by
Huxley (1 942) as one of the marks of evolutionary progress. The annual
habit must necessarily be associated with a less uniformly favourable
environment and accordingly with communities of a less stable kind,
J. HESLOP-HARRISON
a capacity to tolerate fluctuating climatic change of longer periods, such
as the “sun-spot” cycle recorded in the annuals rings of the redwoods,
and the irregular variations in winter temperatures and precipitation
that have been a feature of temperate latitudes in post-glacial times
(Manley, 1954).
Either because the periods favourable to growth are too short, or
because the amplitude of variability of environmental factors is beyond
tolerance, some habitats will not permit the survival of perennials.
However, adaptation to ephemeral habitats or to severe cyclical or
fluctuating changes can be attained by curtailment of the life cycle. The
annual habit represents a satisfactory solution to the problem of exploiting temporary habitats, or of surviving in a climate where there is
extreme but regular seasonal variation in vital habitat factors such as
temperature and rainfall. By fitting in with the major cycle of change
the annual avoids the requirement for adaptations giving tolerance to
the adverse season, which can be safely weathered in the seed ; it is left
then to face the longer and shorter period fluctuations. The methods
available to achieve this are of three general kinds. The “annual” cycle
can be distorted to accommodate irregular longer cycles ; this response is
seen in species of arid regions with sporadic rainfall, where the amount
of precipitation is itself a determinant of germination time. Or the
developmental plasticity of the individuals may be such that the phenotypes produced each year are adapted to match the conditions of that
year. Or, finally, the average phenotypes may possess an amplitude of
tolerance great enough to accommodate year-to-year fluctuations in
climate during the growing season.
The ephemeral habit is an adequate solution to survival in habitats
subject to severe and irregular disturbance. It commonly combines the
ability to suppress growth altogether during unfavourable periods, a
property which we may term avoidance, with adaptive developmental
plasticity and a high amplitude of individual tolerance.
These different kinds of physiological adaptation to change necessarily bear some relationship to community structure and stability. The
long-lived perennial habit is that characteristic of the stable climax
community such as forest or permanent grassland. In the case of forest,
the moderating and controlling effects of the dominants themselves on
soil and microclimate constitute a damping factor stabilizing the environment still further for tree seedlings and the herbaceous members of
the community. So far as the tree species are concerned, this is, of
course, an aspect of the “independence of environment” designated by
Huxley (1 942) as one of the marks of evolutionary progress. The annual
habit must necessarily be associated with a less uniformly favourable
environment and accordingly with communities of a less stable kind,