Acorn Ecology
85
6.4 Abiotic and Biotic Interactions
The results obtained in Prades support the hypothesis that acorn production
is determined by both water and nutrient availability. Interannual variability
in rainfall may partially explain the high interannual variability in acorn
production. Acorn fate, however, is the result of more complex interactions,
which include the amount of acorn production, consumption by animals,
and dispersal. Consumer densities are in turn influenced at some spatial and
temporal scales by acorn production, as well as by other food sources.
In low crop years, even low density populations of rodents may consume
nearly all the available acorns. In high crop years, the survival probability of
acorns increases strongly (Crawley and Long 1995) and some acorns are expected to escape predation even when rodent population densities are high
(satiation hypothesis; Silvertown 1980). Rodents can be considered as generalist consumers (Obrtel and Holisova 1983; Hansson 1985); their populations
do not seem to be regulated by the acorn crop but by the availability of other
food resources during the months in which acorns are not available. In forests with higher diversity of food sources for rodents throughout the year
(Hoffmeyer and Hansson 1974), or in areas with more heterogeneous landscapes providing a higher diversity of food supply (Telleria et al. 1991; Montgomery and Dowie 1993), rodent populations are expected to increase enormously in years of high production of acorns. Then, these populations can
potentially deplete other resources which eventually will regulate their
populations. Therefore, through rodent predation, acorn survival is less likely
to occur in heterogeneous landscapes with higher rodent densities, especially
in low crop years.
On the other hand, the weevil Curculio glandium is the main specialist
consumer of acorns, potentially affecting a very high proportion of seeds.
Weevil populations are expected to be regulated by the acorn production of
previous years (Dempster and Pollard 1981), and in this case the number of
produced acorns appears more relevant than their total biomass. This species
shows winter diapause, and a wide interannual variability in acorn crop is
expected to favour strategies of longer diapause (Crawley and Long 1995) or
wider dispersion. On the other hand, in the long term, alternate low acorn
crops may favour the maintenance of relatively low density populations of
weevils, increasing the escape from infection in high crop years (Crawley and
Long 1995).
Viability of acorns by escaping predation in pure or mixed holm oak forests depends to some degree on both number and size of the seed production
in previous years through its interaction with the features of the life history
and the population regulation of potential consumers. Nevertheless, the spatial distribution of acorn crops and other food sources and their effects on
animal movements strongly contribute to avoiding a rigid correspondence
between holm oak seed production and consumer activity. In addition to
85
6.4 Abiotic and Biotic Interactions
The results obtained in Prades support the hypothesis that acorn production
is determined by both water and nutrient availability. Interannual variability
in rainfall may partially explain the high interannual variability in acorn
production. Acorn fate, however, is the result of more complex interactions,
which include the amount of acorn production, consumption by animals,
and dispersal. Consumer densities are in turn influenced at some spatial and
temporal scales by acorn production, as well as by other food sources.
In low crop years, even low density populations of rodents may consume
nearly all the available acorns. In high crop years, the survival probability of
acorns increases strongly (Crawley and Long 1995) and some acorns are expected to escape predation even when rodent population densities are high
(satiation hypothesis; Silvertown 1980). Rodents can be considered as generalist consumers (Obrtel and Holisova 1983; Hansson 1985); their populations
do not seem to be regulated by the acorn crop but by the availability of other
food resources during the months in which acorns are not available. In forests with higher diversity of food sources for rodents throughout the year
(Hoffmeyer and Hansson 1974), or in areas with more heterogeneous landscapes providing a higher diversity of food supply (Telleria et al. 1991; Montgomery and Dowie 1993), rodent populations are expected to increase enormously in years of high production of acorns. Then, these populations can
potentially deplete other resources which eventually will regulate their
populations. Therefore, through rodent predation, acorn survival is less likely
to occur in heterogeneous landscapes with higher rodent densities, especially
in low crop years.
On the other hand, the weevil Curculio glandium is the main specialist
consumer of acorns, potentially affecting a very high proportion of seeds.
Weevil populations are expected to be regulated by the acorn production of
previous years (Dempster and Pollard 1981), and in this case the number of
produced acorns appears more relevant than their total biomass. This species
shows winter diapause, and a wide interannual variability in acorn crop is
expected to favour strategies of longer diapause (Crawley and Long 1995) or
wider dispersion. On the other hand, in the long term, alternate low acorn
crops may favour the maintenance of relatively low density populations of
weevils, increasing the escape from infection in high crop years (Crawley and
Long 1995).
Viability of acorns by escaping predation in pure or mixed holm oak forests depends to some degree on both number and size of the seed production
in previous years through its interaction with the features of the life history
and the population regulation of potential consumers. Nevertheless, the spatial distribution of acorn crops and other food sources and their effects on
animal movements strongly contribute to avoiding a rigid correspondence
between holm oak seed production and consumer activity. In addition to