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Using Pollinator Networks To Recover Whole Ecosystems
By: Denise Knapp, Ph.D., Director of Conservation and Research
About 87% of the world’s flowering plants need insects and other animals to pollinate them in order to reproduce. That is why the growing evidence for pollinator decline in many places of the globe, including California and the rest of the American West, is of real concern. Now, what if the plant needing pollination is rare? Could be double trouble. At Santa Barbara Botanic Garden, one of the many ways that we work to conserve and recover our region’s rare plants is by investigating and understanding their pollinator networks. Networks are also a useful tool to assess environmental health. It’s “the interactions between organisms that breathe life into ecosystems,” explains one of my favorite scientific papers, “Conservation and restoration of plant-animal mutualisms on oceanic islands” by Christopher N. Kaiser-Bunbury, et al.
Pollinator networks reveal not only the animals visiting our rare flowery friends but also the animals using the rare plants’ neighboring plants. This helps us to understand the connections between all of these different organisms. For the rare plant, it reveals 1) whether it has enough different kinds of visitors to be resilient to change, 2) what the priority pollinators are that can be supported to boost its pollination services, and 3) which more common plants attract those priority pollinators. Networks also point to pollinator species that visit a lot of different plants in the area and thus may be important for supporting pollination services in general. In fact, these generalist species have been called the linchpins of networks, since they help keep the system robust to environmental changes — that’s especially important these days.
The Garden’s Invertebrate Ecology Team or “bug team” as they are affectionately called internally, together with collaborator Jenny Hazlehurst, Ph.D., from California State University, East Bay, recently investigated the pollinator networks of five different rare plants on San Clemente Island, located west of San Diego and owned by the U.S. Navy. Those rare plants are the angelic white San Clemente Island larkspur (Delphinium variegatum ssp. kinkiense), the delicate San Clemente Island woodland star (Lithophragma maximum), the creamy San Clemente Island bush mallow (Malacothamnus clementinus), the marvelously magenta southern island mallow (Malva assurgentiflora ssp. glabra), and the little lilac-colored island rockcress (Sibara filifolia). This work took us to different habitat types and populations scattered around the island, including within the actively bombed Shore Bombardment Area, otherwise known as SHOBA. (We of course had to wait for a day when they weren’t bombing.) Over six visits in spring 2019 and at each of seven sites, Dr. Hazlehurst and Stephanie Calloway, our conservation technician, (with help from me and Casey Richart, Ph.D., the Garden’s first invertebrate ecologist) surveyed 10 plots, each 6.5-by-13 feet (2-by-4 meters) containing these rare plants for 30 minutes each.
The plant that was the big winner for flower visitors was the southern island mallow (figure 1a), which attracted 18 different unique species — from bees and wasps to flies and beetles (figure 1b). This is great news for a plant that is only found in a few locations on Earth. Two bees that visited this mallow more than most (as noted by the thicker segments connecting the pollinator to the plant in figure 1b) were the bindweed turret bee (Diadasia bituberculata) and the metallic green sweat bee (Agapostemon texanus). You can see how well the turret bee fits into the flower (figure 2), undoubtedly getting covered in pollen to ferry to the next plant. Both bee species also visit the much more common island false bindweed (Calystegia macrostegia) (figure 3). Since this is an easy plant to grow, the Navy planted more of it to support the bindweed turret bee on San Clemente Island.
On the other end of the spectrum was the sweet little island rockcress (figure 4), which did not receive any flower visitors, despite hours of searching. Luckily, the rockcress can self-pollinate — although any plant benefits from mixing genes from time to time (inbreeding isn’t good for plants any more than it is for humans). Since our survey, another group who maps and monitors rockcress populations regularly on San Clemente Island observed three small insects visiting it: large-tailed aphideaters (Eupeodes volucris), which is a type of flower fly, soft-winged flower beetles (Family Melyridae), and a type of predatory thrips (tiny, slender insects with fringed wings) in the Genus Aelothrips. The hairy little flower beetles appear to be a good fit for the rockcress, and typically this family is fairly abundant; hopefully they are doing the job for the endangered island rockcress.
We were really hoping to find Greya politella moths on the San Clemente Island woodland star, since we know that they have evolved together and have a specialized relationship. This relationship benefits both the moths, which will only lay eggs in woodland star flowers, and the flowers, which get pollinated (and only lose a few seeds in the process). We weren’t allowed to go at night into the bombed canyons full of cactus (go figure), but that is when many moths are active — so we purchased the most advanced remotely triggered camera system we could find. Despite steep slopes and technical difficulties, Calloway got images of two different woodland star visitors: the southern-Channel-Islands-endemic urbane digger bee (Anthophora urbana clementina) and the widespread generalist alfalfa looper (Autographa californica) moth, which you can see in figure 5. Alas, there were no Greya moths, which have never been recorded on the island.
That alfalfa looper moth also came into play with another of our rare plants, the San Clemente Island larkspur. In this case, not only did the adult moth visit the rare flower to drink nectar but its larval stage (known as a caterpillar for moths and butterflies) also visited the larkspur to eat the plant itself (figure 6a, 6b). Dr. Richart raised a caterpillar to adulthood in our lab to confirm its identity. Is the alfalfa looper having a net positive or negative effect? We’d need to study more to find out. Photos seem to indicate, though, that the moth is able to drink nectar with its long proboscis (mouthparts) without getting any pollen on it. Chances are, it’s not one of the larkspur’s better helpers.
The endemic urbane digger bee, which has such a lovely and unusual rusty color (figure 7), may be important as a pollinator of several rare island plants. Not only was it documented visiting the woodland star but also the larkspur, bush mallow, and southern island mallow. This would be a good pollinator for the Navy to keep an eye on, both for its endemism and its likely importance as a pollinator. They should not only make sure that it has enough nectar sources but also that it has enough bare soil to nest in. The digger bee, like about 70% of our other native bees, is a ground nester.
Bees range from generalists that will use many types of plants, like our urbane digger bee and the European honeybee (Apis mellifera), to specialists that will only use a small group of plants, like mallows, cacti (Family Cactaceae), or sunflowers (Genus Helianthus). But it’s been shown that on islands, generalists are typical. It makes sense, since it’s hard enough to disperse to a remote island, let alone find your favorite plant. In one way, this is good news for the islands’ resilience to ecological change. But in another, the loss of a few super-generalists could spell doom for the plants that depend on them. This is a good reason to keep an eye on the urbane digger bee.
Another important generalist flower visitor on the island may surprise you: it’s a fly. Or rather, several kinds of flies, known as flower flies in the Family Syrphidae. One species, Copestylum avidum, was found at every site, while another, Copestylum marginatum, was found at all but one site. Flower flies visit four out of five of our rare plants: the larkspur, bush mallow (figure 8), island rockcress, and southern island mallow. Furthermore, two different kinds of flies called bee flies (Genus Bombylius) visited both the larkspur and the bush mallow. Flies (Order Diptera) are the second most important order among flower-visiting insects worldwide. In some circumstances, they may equal or rival bees as effective pollinators. Furthermore, they can be much more abundant, and many visits may add up to the same amount of service. More research is needed to determine whether the flower flies and bee flies on the island are good pollinators compared to the local bees, but it seems that this is another important group for conservation.
Of course, not all floral visitors are effective pollinators, and even among pollinators, different degrees of pollination services are provided. Photographs can be a great way to get a first sense of how well an insect can do the job, as we’ve shown here. We take as many photographs of these interactions as we can, which helps to generate hypotheses about which visitors are the most important pollinators. These hypotheses help us to design and pursue further studies where we investigate the amounts, and types, of pollen an insect carries to determine how effective they are as pollinators. In 2024, we’ll be following up with both the San Clemente Island woodland star and the island rockcress to survey and collect flower visitors. We’ll also then investigate pollen loads using a combination of microscopy and DNA barcoding. There is always more to explore and understand and so, the conservation journey continues.
