Question

Birds living on oceanic islands tend to have a smaller clutch size than the same species (or close relatives) breeding on the mainland (Klomp \(1970,\) p. 85 ). Explain this on the basis of Lack's hypothesis.

Short answer

Island birds have smaller clutches to maximize offspring survival under resource limitations, according to Lack's hypothesis.

Step-by-step solution

Understanding Lack's Hypothesis

Lack's hypothesis, proposed by David Lack, suggests that the clutch size of birds is optimized through natural selection to maximize the number of surviving offspring. According to Lack, a larger clutch size might increase the number of chicks, but beyond a certain point, it could lead to less food and care per chick, thereby reducing their survival chances.

Examining Oceanic vs. Mainland Environments

Birds on oceanic islands usually encounter limited resources, such as food availability, and fewer predators compared to those on the mainland. Resource limitation means that attempting to raise a large number of chicks could overwhelm the parent birds' capacity to provide sufficient food, reducing chick survival rates.

Applying Lack's Hypothesis to Island Birds

According to Lack's hypothesis, because island birds face these limited resources, producing smaller clutches allows them to invest more resources and care in each chick. This maximizes the survival per chick, which is favored by natural selection. In contrast, mainland birds might produce larger clutches due to more abundant resources and different predation pressures.

Key concepts

Clutch Size Adaptation

Birds have unique traits that support their survival and reproduction in various environments. One such trait is clutch size. Clutch size refers to the number of eggs a bird lays at one time.

David Lack's hypothesis plays a crucial role in understanding clutch size adaptation. According to his theory, natural selection optimizes clutch size to ensure the greatest number of offspring survive and reach maturity. While laying more eggs may seem beneficial, there is a trade-off. Larger clutches require more resources, making it harder for parent birds to provide enough food and care for each chick. Consequently, beyond an optimal point, an increase in clutch size may lead to lower survival rates for the offspring.

In simpler terms, birds adapt their clutch size based on resource availability. If resources are sparse, as they often are on isolated islands, smaller clutches allow parents to focus their resources effectively on fewer chicks. On the other hand, in environments with abundant resources, larger clutches might be viable since parents can adequately care for more chicks.

Island Biogeography

Island biogeography is a fascinating field that investigates how distribution and diversity of species are influenced by island environments. Islands often have unique ecological challenges and benefits.

• Limited Resources: Islands usually have fewer resources like food and shelter, which affects bird populations.
• Isolation: Due to isolation, islands can have fewer predators, influencing bird behavior and traits, including clutch size.
• Specialized Adaptations: Island species often develop specialized adaptations to thrive under these distinct conditions.

Indeed, these environmental factors push birds to adapt their breeding strategies, leading often to a smaller clutch size. The limited resources mean it becomes necessary for birds to invest intensely in a small number of offspring to maximize their chances of reaching adulthood. Moreover, without the presence of numerous predators, island bird populations can stabilize more around these adapted reproductive strategies.

Natural Selection in Birds

Natural selection is a vital process in evolution, shaping how species develop traits best suited for their environments. In birds, traits like clutch size are a result of natural selection.

Birds on islands provide a clear example of natural selection at work. Due to the unique ecological pressures and limited resources on islands, natural selection favors birds that can successfully raise their offspring under these conditions. This results in smaller clutch sizes, where parental investment ensures that each chick has the best chance of survival.

• Survival of Offspring: Parents aim to maximize the number of chicks that they can successfully raise to maturity.
• Resource Allocation: There is a balance between the number of offspring and the resources parents can provide, influencing clutch size decisions.
• Adaptation: Over time, natural selection on these habitats leads to a fine-tuning of these traits, ensuring that birds maintain a balance between reproductive output and chick survival.

Overall, these adaptive strategies driven by natural selection highlight how bird species continue to evolve, ensuring their survival in diverse ecological niches.