10.2. Response variables and calculations
Net income is the difference between costs from income. We are interested in the change in net income () given a particular increase in number of hives ():
where is the price that the farmer obtain for each metric tonne of crop, is the increase in metric tonnes of crop because of the addition of hives (), is the cost of producing each tonne (i.e. variable costs such as harvest and transportation costs), and is the cost of renting each hive. As mentioned before, at least two treatments are needed to estimate yields without honey bees and subtract them from yields with honey bees (). In case information for several treatments is available, i.e. several densities of colonies are evaluated, a functional form of yield () with increased number of hives () can be estimated. This function should be used to obtain values for any number of hives within the measured range (Fig. 19).
The data to perform this valuation should be measured at the field scale (e.g. crop yield) or obtained through questionnaires to the farmers and beekeepers (Olschewski et al., 2006). Fruit or seed yield (tonnes per ha) should be measured at ripeness or harvest. The crop price () and production costs (including the costs of renting hives) should be obtained from questionnaires.
If honey bees promote yield quality (e.g. bigger and well-formed fruits) in addition to yield quantity (), changes in crop prices (P) may occur with or without hives. In this case, the price obtained with the desired number of hives should be used in the above formula to estimate ∆NI (note that not only but also all the harvest, Y, can show enhanced quality and price). On the other hand, if losses of honey-bee colonies are replaced with other sources of pollination (e.g. hand pollination), this change in production costs should be also accounted in the above formula. Finally, we must account for some management inputs, such as planting hedge rows with flowering species to improve honey-bee colony health, that incur high initial costs but low costs in subsequent years. Therefore, the temporal scale of analyses, as well as equations used to estimate the net income, will depend on the management practices to be evaluated.
By now it should be evident that the quality of the data gathered has a strong influence on the resulting values for the contribution of honey bees. Several of the ideas discussed here (e.g. replication, scale) can also be applied to other objectives such as the evaluation of the impacts of adding hives on the pollination of surrounding wild vegetation or the diversity and abundance of wild pollinators.