Published online by Cambridge University Press: 16 November 2009
INTRODUCTION
Accentuated seasons are a common phenomenon in large parts of the globe. This typically involves a pulse of plant growth during the favourable season, and an extended unfavourable season with no plant growth, which may be due to temperature variations in high latitudes or water limitations in arid environments. The consequences for large herbivores will be one season with abundant food resources and another season with very low food resources. This is especially pronounced in northern high latitudes where the favourable season can be as short as a month or less.
Polar areas receive less solar radiation than other parts of the globe on an annual basis, and this radiation is also to a large part lost to space due to reflection by clouds, snow and ice. This radiation imbalance gives low temperatures and low annual primary productivity. However, solar radiation levels vary greatly between seasons; in the summer the poles receive higher levels of radiation than any other place on Earth, whereas there may be a total lack of incoming solar radiation during winter. As plants experience 24 hours of daylight during summer, productivity on a daily basis may be very high in areas where the geology and topography are favourable for weathering and the transport of nutrients. Thus the pulsed plant growth may be of short duration but very strong.
The large annual variation in plant growth imposes constraints on herbivores as life history tactics must be adjusted to fit the seasonal pattern of the system.
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