Can redistribution of breeding colonies on a landscape mitigate changing predation danger

摘要

The reproductive success of colonially breeding species depends in part upon a trade-off between the benefit of adilution effect against nestling predation within larger colonies and colony conspicuousness. However, there maybe no net survivorship benefit of dilution if smaller colonies are sufficiently inconspicuous. This raises thequestion about how the size distribution of breeding colonies on a landscape might change as the predationdanger for nestlings changes. In southwest British Columbia, Canada, bald eagle Haliaeetus leucocephaluspopulations have increased exponentially at -5% per year in recent decades and prey upon nestlings of colonialbreeding great blue herons Ardea herodias faninni. Motivated by field data on reproductive success in relation tocolony size, modeling is used to ask under which circumstances trading off a dilution benefit against colonyconspicuousness can improve population reproductive success. That is, which colonial nesting distribution,dispersed and cryptic versus clumped and conspicuous, best mitigates predation danger on nestlings. Whenpredators are territorial, the modeling predicts a dispersed nesting strategy as attack rate increases, but not aspredator numbers increase. When predators are non-territorial, the modeling predicts a dispersed nestingstrategy as predator numbers and/or attack rates increase. When predators are both territorial and non-territorial,colonial nesting within a predator's territory improves reproductive success when attack rates are low. Thissuggests nesting in association with territorial predators may offer decreased levels of predation when comparedwith nesting amongst non-territorial predators. Thus a change in the colony size distribution of coloniallybreeding species might be anticipated on a landscape experiencing a change in predation danger.