When Sandra Anderson and her colleagues studied fruit set of Rhabdothamnus solandri on the mainland and on the island, why was it important to measure fruit set when flowers were hand pollinated?

When Sandra Anderson and her colleagues studied the fruit set of Rhabdothamnus solandri, also known as the New Zealand Gloxinia, they were interested in understanding the reproductive success of this species both on the mainland and on an island. Measuring fruit set when flowers were hand pollinated is important for several reasons:

1. **Controlled Pollination**: Hand pollination allows researchers to ensure that pollination has occurred. In natural conditions, it can be uncertain whether flowers have been visited by pollinators or if the pollination was effective. By hand pollinating, researchers can control for this variable and be certain that any subsequent fruit set is due to the pollination event they facilitated.

2. **Pollinator Limitation**: By comparing the fruit set from hand-pollinated flowers to those left to natural pollination, researchers can determine if there is a pollinator limitation. If hand-pollinated flowers set more fruit, it suggests that the natural pollinator population is not sufficient to maximize fruit set, indicating that the plant may be pollen-limited.

3. **Reproductive Potential**: Hand pollination can reveal the maximum reproductive potential of the flowers by ensuring that they receive an adequate amount of pollen. This helps in understanding the best-case scenario for fruit set in the absence of pollinator constraints.

4. **Isolation Effects**: On islands, plant species often face different ecological conditions, including pollinator communities that may be less diverse or abundant than on the mainland. Hand pollination can help determine if differences in fruit set between mainland and island populations are due to differences in pollinator availability or effectiveness.

5. **Genetic Factors**: Hand pollination can also help rule out other factors that might affect fruit set, such as genetic differences between mainland and island populations that could influence the plant's ability to set fruit after pollination.

6. **Environmental Factors**: By controlling the pollination process, researchers can focus on other environmental factors that might influence fruit set, such as nutrient availability, habitat quality, or climatic conditions, without the variable of pollinator activity.

In summary, hand pollination is a critical method in plant reproductive studies as it allows researchers to isolate the effects of pollination from other variables, providing a clearer understanding of the factors influencing fruit set and the reproductive ecology of the species under study.