Delicate pollinator-plant relationships: Forged through coevolution and critical considerations for risk assessment
Pollinator-plant relationships represent some of the most striking examples of mutualism and coevolution in all of nature. We’ve discussed in our pollinator protection article how native bees can be generalists, often thriving on a diverse availability of flowering plants. This diversity is a benefit to both bee and plant alike. From a bee’s perspective, diverse food sources can supply a well-rounded diet and from a plant’s perspective, diversity in pollinators helps to assure successful reproduction. However, these symbiotic interactions can be driven by specific pollinator preferences, sometimes to the point of becoming a completely obligate relationship.
Plants are pollinated by different types of animals, the most common of which include bees, flies, butterflies, beetles, moths, bats, and birds. Each of these animal pollinators displays specific preferences, such as color, scent, flower shape, presence or absence of nectar guides, and characteristics of nectar and pollen. For example, without a sense of smell, birds are attracted to bright red or orange flowers. In return, plants relying on avian pollination have evolved to be typically odorless, bright in color, with funnel or cup-shaped flowers and sometimes a strong base for perching. Plants that have evolved for pollination by bats or moths emit scents at night to account for nocturnal activity patterns: musty scents for bats and strong sweet scents for moths. (Next time you’re admiring a beautiful flower, challenge yourself to consider what traits may have evolved in response to pollinator preferences! Information from the U.S. Forest Service provides some great details to get you started.)
Evolutionary pressure can also yield extreme examples of pollinator-plant relationships, some of which represent complete species-to-species dependence. For example, the death camas (Anticlea elegans) is a beautiful flowering plant that is deathly toxic to most pollinators and animals. Yet even this plant has developed a single pollinator relationship with the solitary andrenid bee (Andrena astragali). Curiously, the adult andrenid bee is also unable to consume the pollen or nectar of the death camas, but its kids love it! Once hatched, the bee larva consumes the pollen ball. It is not completely understood if the larvae are immune to the plant toxins or if the toxic chemicals degrade prior to consumption.
The intricate relationships between pollinators and plants has become a focal point for toxicological consideration in Endangered Species Assessments for US EPA. During these assessments, we conduct a three-step consultation process to evaluate the potential risk to Federally-listed threatened or endangered species. While direct effects of chemicals are characterized, we also examine indirect effects during Step 1 of an Endangered Species Assessment (ESA). Indirect effects refers to impacts on other species for prey, habitat, or symbiotic interactions upon which a listed species relies and is the last step in the decision tree for Step 1 of the ESA. For example, a chemical may not be directly toxic to an endangered pollinator, but an indirect effect could occur if the chemical affects the plant(s) that the pollinator relies upon for a food source. It is the last step in the decision tree for Step 1 of the ESA. Up until this point, an individual species has not been determined as a “no effect” species. In the case of “indirect effects”, a species may have an obligate or a generalist relationship to another species that is directly affected by the chemical being assessed. An obligate relationship infers that one or both of the listed species depends on the other for its survival. A generalist relationship means that a species is able to survive on a variety of other species and is not solely dependent on one for survival.
An example is the endangered species, Dicerandra immaculata (Lakela’s mint), a small fragrant shrub. This species occurs in six isolated sites in the southern Indian River and northern St. Lucie counties in Florida. Lakela’s mint has a generalist relationship, relying solely on pollinating insects for survival. Another example is the Karner blue butterfly (Lycaeides Melissa samuelis), an endangered species located mainly in Wisconsin but also found in Indiana, Michigan, Minnesota, New Hampshire, New York, and Ohio. This species has an obligate relationship to the wild lupine (Lupis perennis) because the butterfly’s caterpillars only feeds on wild lupine leaves, making this species dependent on the wild lupine for its survival.
Understanding the precise species interactions is critical for an in-depth look at potential exposure and risk of a chemical to a listed species. The beautifully intricate pollinator-plant relationships, which are the result of millions of years of coevolution, serve as a prime example of these interactions!