The InVEST Crop Pollination model focuses on wild pollinators providing an ecosystem service. The model estimates insect pollinator nest sites, floral resources, and flight ranges to derive an index of pollinator abundance on each cell on a landscape. If desired, the model can creates an index of the value of these pollinators to agricultural production, and attributes this value back to source cells.
Quoted from: https://storage.googleapis.com/releases.naturalcapitalproject.org/invest-userguide/latest/croppollination.html
The InVEST pollination model focuses on wild bees as a key animal pollinator. It uses estimates of the availability of nest sites and floral resources within bee flight ranges to derive an index of the abundance of bees nesting on each cell on a landscape (i.e., pollinator supply). It then uses floral resources, and bee foraging activity and flight range information to estimate an index of the abundance of bees visiting each cell. If desired, the model then calculates a simple index of the contribution of these bees to agricultural production, based on bee abundance and crop dependence on pollination. The results can be used to understand changes in crop pollination and crop yield with changes in land use and agricultural management practices. Required inputs include a land use and land cover map, land cover attributes, guilds or species of pollinators present, and their flight ranges. To estimate wild pollinator contributions to crop production requires information on farms of interest, the crops grown there, and the abundance of managed pollinators. The model’s limitations include not accounting for pollinator persistence over time or the effects of land parcel size.
The Model
A wide range of animals can be important pollinators (e.g., birds, bats, moths and flies), but bees are the most important group for most crops (Free 1993). As a result, the InVEST Pollination model focuses on the resource needs and flight behaviors of wild bees. Many people think of honeybees, managed in artificial hives, when they think of pollinators, but wild bees also contribute to crop pollination. In fact, for several important crops (e.g., blueberries), native species are more efficient and effective pollinators than honeybees (Cane 1997). These native bees, in addition to feral honeybees living in the wild, can benefit crops without active management of captive hives. This is the pollination service associated with habitat conservation.
This model is adapted from Lonsdorf et al. (2009). For bees to persist on a landscape, they need two things: suitable places to nest, and sufficient food (provided by flowers) near their nesting sites. If provided these resources, pollinators are available to fly to nearby crops and pollinate them as they collect nectar and pollen. The model translates land cover into an index of suitability (0-1) for bees to create a pollinator source map. Higher scores indicate sources of greater relative bee abundance. To calculate the index, the model assumes that bees require two types of limiting resources to persist on a landscape - nesting substrates and floral resources. Given an input of land cover that describes the landscape, various suitability values of each land cover type are assigned based on their ability to provide these resources.
The Pollination model then uses the nest supply index to estimate the pollinators visiting crop fields. It assumes the supply from nearby parcels contributes more than those farther away. We use insights from Olsson et al. (2015) and weight bee visitation by the floral quality of the parcel to reflect the central place foraging behavior that bees are known to follow. In other words, the visitation rates to two parcels equally distant from a nest are proportional to the floral qualities of the parcels.
Additionally, this model incorporates the potential use of managed bees into a yield index. With information on the location of crops and their dependence on pollinators, the model uses a simple yield function to project how wild pollinator abundance in agricultural areas and the use of managed bees contributes to an index of crop yields. The inclusion of managed bees reflects guidance from recent work by the USDA-Funded Integrated Crop Pollination Project (icpbees.org) to integrate the combined benefits of managed and wild bees.
These steps are described formally below.
How it Works
The model is an index-based model, and requires the following biophysical data:
a land use / land cover (LULC) map;
a biophysical table paired with the LULC raster to map LULC types to nesting suitability and floral resources across seasons;
a pollinator guild table with properties about active seasons, nesting preferences, mean flight distances, and relative abundances for each species or group of wild pollinators;
a farm shapefile indicating the geospatial location of farms, crop type, dependence on pollinators, abundance of managed pollinators, as well as on-farm nesting sites and floral resources.
Because bees are proficient fliers, they integrate over several elements of a landscape, moving between nesting habitats and foraging habitats (Ricketts et al. 2006). The distances they typically fly affect both their persistence and the level of service they deliver to farms. The model therefore requires a typical foraging distance for each pollinator species. These data can be supplied from quantitative field estimates (e.g., Roubik and Aluja 1983), proxies such as body size (Greenleaf et al. 2007), or from expert opinion.
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