Keystone Species: A Framework for Selecting Native Plants

Over the past several years, the news has been abuzz with information regarding the importance of planting native plants to support pollinators. In North America, native plants are generally defined as plants that grew in an area before the colonization by European settlers. Since they have grown in a region for hundreds of years, they have evolved with the pollinators and other wildlife of the area, and close relationships have developed between them. For instance, about ⅓ of our native bees are referred to as specialists, meaning that they require pollen from specific species of plants to reproduce. The absence of these plants would be catastrophic for these bees. With this in mind, the focus has been on incorporating native plants into our gardens, but not all species provide the same level of support for our pollinators, local ecosystem, and food web.

What are Keystone Species?

While supporting our pollinators is important because, through pollination, they are helping to create new plants, we also need to consider other insects that play a key role in our local food webs. Much of the local food web is supported by caterpillars and, like native specialist bees, many caterpillars also feed on only specific species of plants, referred to as host plants. As they feed on these plants, they take up the energy within them. In turn, some of these caterpillars will become food for other insects, birds, or bats, thus extending the importance of these plants within the food web.

Research into the native flora of North America has revealed that only about 5% of a region's native plants are host plants to 70 to 75% of the region's butterfly and moth (Lepidoptera) species, as well as providing pollen to many of the native specialist bees. These plants have been designated as keystone species.

Keystone plants are defined as "species that have the maximum amount of habitat benefit to wildlife" per the National Wildlife Federation. The U.S. Environmental Protection Agency (EPA) delineates North America into ecoregions — geographic areas characterized by similar environmental conditions that shape the composition of plant and animal communities and the ecological interactions among them. There are three levels of ecoregion maps. Level I starts out broadly, with 15 ecoregions across North America. Pennsylvania contains both ecoregions 5 and 8. The Level II map breaks down those 15 ecoregions further into 50 ecoregions. This takes specific plant communities into consideration. Level III hones down even further, describing the smaller ecological areas within the Level II regions. There are 182 ecoregions at this level. Keystone plant designations are made at the ecoregion level, reflecting the fact that ecological relationships vary across the continent.

Identifying Keystone Plants by Region

Regional keystone plant lists are now readily accessible to home gardeners. Two primary resources are maintained for this purpose:

National Wildlife Federation 

Homegrown National Park

Both websites break down North America into ecoregions. The ecoregion for a specific area can be obtained by inputting a ZIP code. The National Wildlife Federation provides a listing of the top keystone plants for the Level I ecoregions. Homegrown National Park provides a more comprehensive list at Level II. Both websites provide the common name and the scientific (botanical) name for each plant.

To ensure you purchase the specific plant you want, it is important that you use the scientific name. Not all "sunflower" or "yarrow" plants are the same. The scientific name consists of a genus and species name. Plants within the same genus will have similar fruits, flowers, roots, stems, buds, and leaves. The species name generally gives more specific information about the plant. This could be information regarding bloom color, habit, or leaf shape. Both names are necessary for correct identification.

Ecoregions of Pennsylvania

Keystone species are defined as those that are most critical in maintaining healthy ecosystems. According to the EPA ecoregion maps, Pennsylvania falls into two Level I ecoregions and five Level II distinct ecoregions:

• Ecoregion 5 - Northern Forests
  • 3 - Atlantic Highlands
• Ecoregion 8 - Eastern Temperate Forests
  • 1 - Mixed Wood Plains
  • 3 - Southeastern USA Plains
  • 4 - Ozark/Ouachita-Appalachian Forests
  • 5 - Mississippi Alluvial and Southeast USA Coastal Plains

Ecoregion 5, Northern Forests, includes parts of New England, the Canadian Maritimes, New York state, and northern areas of Pennsylvania, Michigan, Wisconsin, and Minnesota. Some of the notable keystone species that support a variety of caterpillars and specialist bees include white oak (Quercus alba), black cherry (Prunus serotina), pussy willow (Salix discolor), and pale leaf sunflower (Helianthus strumosus).

Ecoregion 8, Eastern Temperate Forests, covers most of eastern North America, from Maine to Florida, west to Illinois and eastern Texas, plus Nova Scotia and parts of Quebec and Ontario. White oak and black cherry are also predominant in this ecoregion. Other notable species include river birch (Betula nigra), sugar maple (Acer saccharum), black willow (S. nigra), stiff leaf goldenrod (Solidago rigida), and blue wood aster (Symphyotrichum cordifolium).

For more information about keystone species in Pennsylvania, Penn State Extension has the following articles on keystone trees, shrubs, and perennials.