12. Individual-Based Models and Assessment of Population Variability
193
interval, it moved to another cell, possibly distant from the original cell, although
it incurred higher travel costs from flying longer distances.
The growth of the nestlings was calculated on the basis of the energetic value of
the food they received from their parents. Food requirements for growth and
starvation thresholds were taken into account. For example, if a nestling did not
attain a predetermined threshold of accumulated food before the rainy season
started or if it received less than a specified threshold of food over 5 consecutive
days, it died and was removed from the simulation. If the parents could not meet
their own energetic needs, they abandoned their nest, causing their nestlings to
perish.
Extensive simulations were performed in an effort to examine the causes of
decline of the wading birds (Fleming et al. 1994; Wolff 1994). Instead of summarizing these results, we look here at the effects of a single mechanism on colony
success. In particular, we demonstrate how the type of competition for food
among nestlings can have an effect on numbers of successful fledglings at the
population level. The relationship between success at the individual level and
spatial heterogeneity depends on whether there is a scramble or contest competition for resources.
Altricial birds, such as storks, show total dependency on their parents for food.
Egg laying (and hatching) is asynchronous, and it has been suggested (Fujioka
1985; Mock et al. 1987), especially in resource limited situations, that older
nestlings have a competitive advantage over their younger siblings. We considered two different scenarios. In the first situation, the resource was unlimited,
and as Figure 12.1 shows, all nestlings fledged successfully. In this scenario, all
150 nestlings fledged successfully, so the probability-of-success histogram (Fig.
12.2) was trivial. Both contest and scramble competition were simulated for the
nestlings, but the results were indistinguishable.
In the second scenario, we decreased resources available initially by 30% in an
effort to simulate the effects of competition. However, in contrast to nestlings,
resource competition by adults is purely exploitative (i.e., there are no social
dominance structures among them). The effects are shown in Figures 12.3 and
12.5. In the cases in which there was a 30% reduction of resources, assuming
scramble competition, the chronology of hatching did not play any role. In the
probability-of-success histogram (Fig. 12.4) the three hatching-order categories
ended up with almost the same rates (9, 8, and 10%).
When the older nestling had a competitive advantage over its siblings, the
probabilities to fledge successfully were very different (70%, 30%, 8%) (Fig.
12.6). In the case of contest competition, the parents in all but two nests (that were
deserted) raised only one young. These results are intuitively clear. A modification of the rules for feeding among nestlings radically altered which of the nestlings fledged successfully.
We can also ask how the availability of resources affects the reproductive
success of the colony under either competition scenario. The results show threshold effects for both cases (Figs. 12.7 and 12.8). For scramble competition and a
reduction to about 80% of the initial resources, the probabilities to fledge
193
interval, it moved to another cell, possibly distant from the original cell, although
it incurred higher travel costs from flying longer distances.
The growth of the nestlings was calculated on the basis of the energetic value of
the food they received from their parents. Food requirements for growth and
starvation thresholds were taken into account. For example, if a nestling did not
attain a predetermined threshold of accumulated food before the rainy season
started or if it received less than a specified threshold of food over 5 consecutive
days, it died and was removed from the simulation. If the parents could not meet
their own energetic needs, they abandoned their nest, causing their nestlings to
perish.
Extensive simulations were performed in an effort to examine the causes of
decline of the wading birds (Fleming et al. 1994; Wolff 1994). Instead of summarizing these results, we look here at the effects of a single mechanism on colony
success. In particular, we demonstrate how the type of competition for food
among nestlings can have an effect on numbers of successful fledglings at the
population level. The relationship between success at the individual level and
spatial heterogeneity depends on whether there is a scramble or contest competition for resources.
Altricial birds, such as storks, show total dependency on their parents for food.
Egg laying (and hatching) is asynchronous, and it has been suggested (Fujioka
1985; Mock et al. 1987), especially in resource limited situations, that older
nestlings have a competitive advantage over their younger siblings. We considered two different scenarios. In the first situation, the resource was unlimited,
and as Figure 12.1 shows, all nestlings fledged successfully. In this scenario, all
150 nestlings fledged successfully, so the probability-of-success histogram (Fig.
12.2) was trivial. Both contest and scramble competition were simulated for the
nestlings, but the results were indistinguishable.
In the second scenario, we decreased resources available initially by 30% in an
effort to simulate the effects of competition. However, in contrast to nestlings,
resource competition by adults is purely exploitative (i.e., there are no social
dominance structures among them). The effects are shown in Figures 12.3 and
12.5. In the cases in which there was a 30% reduction of resources, assuming
scramble competition, the chronology of hatching did not play any role. In the
probability-of-success histogram (Fig. 12.4) the three hatching-order categories
ended up with almost the same rates (9, 8, and 10%).
When the older nestling had a competitive advantage over its siblings, the
probabilities to fledge successfully were very different (70%, 30%, 8%) (Fig.
12.6). In the case of contest competition, the parents in all but two nests (that were
deserted) raised only one young. These results are intuitively clear. A modification of the rules for feeding among nestlings radically altered which of the nestlings fledged successfully.
We can also ask how the availability of resources affects the reproductive
success of the colony under either competition scenario. The results show threshold effects for both cases (Figs. 12.7 and 12.8). For scramble competition and a
reduction to about 80% of the initial resources, the probabilities to fledge