Ecological restoration is the process of assisting the recovery of an ecosystem that has been degraded, damaged, or destroyed. To that end, there may be multiple goals when designing a restoration project, including stabilizing streambanks, providing pools and riffles in streams for aquatic species, providing habitat for grassland birds, or simply increasing the vegetative diversity on a site. An often-overlooked goal that goes hand in hand with plant diversity may be to increase the abundance and diversity of native bees and/or other pollinators. Although an increased diversity of plants on a site may very well increase the diversity of bees present, there may also be individual plant species that provide an outsize role in pollinator relationships.
When designing a seed mix for restoration projects, you must consider many variables. First and foremost, you can look to an undisturbed reference site to see what species are growing there. If that species isn’t available, you can refer to the NRCS ecological site descriptions. When considering soils, we look at whether they are sandy, rocky, or dense clays. What about aspect? Is the site a gentle east-facing slope, a moister north-facing drainage, or a harsh, hot south-facing site. Even after all those considerations, there is the question of if you can even purchase the species required.
Which Plant Species to Select
Digging even deeper, the plant ecology team has pondered what species of plants may provide services for the widest range of pollinators. If we are targeting specific species for seed collection, which of those could provide the most bang for our buck, so to speak, when it comes to helping conserve our native bees?
To that end, Boulder County Parks & Open Space plant ecology staff worked with renowned bee experts, Dr. Adrian Carper at the University of Colorado, and Olivia Messinger-Carril, a native bee researcher and author of three books on native bees, to look at some of our more common wildflowers and shrubs and use a diagram to graphically represent individual plant pollinator relationships.
Using the Global Biodiversity Information Facility database, Dr. Carper provided a list of more than 1,700 observations of bees and their interactions with plants documented in Boulder County. We pared the list down to 45 plant species that were either common in the county or of particular interest in restoration work, many of which we target for seed collections. Using that information, Messinger-Carril created an interactive graphic that illustrates which plants host which bees and, conversely, which bees are generalists, visiting a wide variety of plants, and which are specialists, faithful to just one or two species.
Results
The results are fascinating (see the interactive webpage of the data). For instance, scrolling through the data, one can see the value of willows in hosting many visits from a select group of bees. Most people would not associate willows as a key pollinator plant. Certainly, they are not known for showy flowers, but they do flower early in the spring, with very small flowers (clustered together in a catkin) and, as such, provide an important nectar and pollen source at a time when bees are just emerging after the winter months and most other plants are still dormant.
The chart below reveals how willows host seven different genera of bees, with outsize visits by Adrena and Lasioglossum species. Adrena bees, or mining bees, are solitary ground nesters. Lasioglossum are sweat bees, the largest genera of bees, with 1,800 species worldwide, many of which also nest in the ground.
Hairy yellow aster (Heterotheca villosa) is a common native plant, one which we collect, grow, and seed on projects. Not only is it useful for its ability to establish in disturbed areas with poor soil, but the following graphic also reveals it has an outsize impact on the diversity of bees it hosts — visited by no less than 16 different genera of bees, the highest percentage of all the flowers referenced.
Surprisingly, we also find that some species, (not shown here), like Yarrow and Gayfeather, two other species we target for collection, have very little visitation from bees, though likely may be visited by other pollinators such as butterflies. Bumblebees were the most commonly observed genera of bees, and their generalist nature is apparent, visiting 35 of the 45 species, including some invasive plants, such as diffuse knapweed (Centaurea diffusa) and musk thistle (Carduus nutans).
Although this work is a rough snapshot of bee-plant relationships, it is not a completely accurate picture. Are bumblebees really that numerous, or are they just most often observed by people, because of their large size? Are plants like whiskbroom parsley only utilized by one single bee species, or is it because observations of small, less common visits to, say, sunflowers or lupines?
Regardless, the information provides us with a rough idea on which plants we may want to prioritize to include in our seed mixes for restored sites as we try to provide a functioning ecosystem for not just larger wildlife, but also bees and their important pollinator services. It also highlights the complex interrelationships of plants and their pollinators, as well as the beauty taking place among us throughout the year.