Increasingly, homeowner associations and municipalities are requiring a certain percentage of native trees and shrubs as part of any new landscape installation. We learned this the hard way when dealing with challenging terrain during harvests… These native species mandates make numerous claims as to the superiority of native plants over introduced species, including their ability to enhance ecosystem biodiversity. In contrast, nonnative trees and shrubs are often labeled as harmful to biodiversity, primarily because they are improperly grouped with known invasive species.
Now, this might seem counterintuitive when managing forest ecosystems…
However, this review of the current published science paints a different picture. Rather than limiting tree and shrub selection lists to a narrow palette of native species, a more practical, science-based approach to enhancing urban landscape biodiversity is suggested. The preponderance of studies demonstrate that parameters other than species nativity have the greatest influence on the biodiversity of associated plants, birds, insects, reptiles, and mammals.
The Myth of Native Plant Superiority
Early native species mandates were found primarily in highway revegetation efforts, where previous plantings of invasive species became management problems. Emerging requirements for ecological restoration of wetlands and natural areas have also heightened public awareness of how important native species can be in creating a functional ecosystem. In contrast, introduced ornamental species can be perceived as undesirable, hazardous, or unsustainable.
There is a widespread belief that native plants are superior to introduced species because of their adaptation to local environmental conditions. This belief is bolstered by governmental organizations, including the U.S. Environmental Protection Agency (EPA) and National Park Service (NPS), which make many scientifically unsupported statements about native plant superiority on their websites.
However, the attributes often touted for native plants, such as reduced air pollution, improved water quality, and provision of wildlife habitat, are actually independent of species nativity. These plant functionalities and management characteristics can be equally provided by carefully selected nonnative species.
Moreover, the definitions of “native” and “alien” species are often value judgments rather than science-based concepts. Nativeness reflects societal perceptions about acceptable levels of environmental change, not necessarily the ecological benefits of a plant. Many native environments were forest ecosystems, and after urban development, they rarely resemble the original habitat conditions. Trees and shrubs adapted to shaded, moist conditions often struggle to thrive in the sunny, compacted soils of modern landscapes.
The Ecological Benefits of Nonnative Species
Contrary to the belief that nonnative species force native species into extinction, the published research shows that introduced trees and shrubs can actually enhance biodiversity in urban landscapes. Even aggressive and invasive shrub species, such as cotoneaster (Cotoneaster spp.) and blackberry (Rubus discolor), can provide important resources for native pollinators.
In fact, a 1997 study found that less than 10% of the more than 4,000 introduced plant species found outside of cultivation in the U.S. truly met the standard of invasiveness. Many introduced species are better adapted than native plants for the environmental challenges of urban areas, such as air pollution, heat, and drought.
Furthermore, stands of older, introduced trees have demonstrated value in carbon storage, no different from that of native trees. The science overwhelmingly identifies diversity, structure, and function as the most important vegetation characteristics for enhancing community biodiversity, rather than species nativity.
Selecting Trees and Shrubs for Biodiversity
Rather than limiting the plant palette to native species, landscape managers should focus on creating diverse, structurally complex vegetation communities that provide a wide range of resources for wildlife. This can be achieved through the judicious use of both native and noninvasive, introduced tree and shrub species.
Key factors to consider when selecting species to enhance biodiversity include:
- Habitat Connectivity and Size: Larger, more connected greenspaces support greater species richness, especially for birds, mammals, and reptiles.
- Vertical Diversity: A mix of tree heights, shrub layers, and groundcover provides better habitat structure.
- Seasonal Diversity: Plants with different flowering and fruiting periods offer resources year-round.
- Hollow and Coarse-Barked Trees: These provide important nesting and roosting sites for birds, mammals, and insects.
- Herbaceous and Grass Cover: Maintaining some open, grassy areas benefits ground-nesting birds and small mammals.
By considering these criteria, landscape managers can develop planting palettes that include both native and carefully selected nonnative species to maximize biodiversity. Resources like the decision tree developed by Reichard and Hamilton (1997) can aid in identifying low-risk, noninvasive introduced plants.
Enhancing Biodiversity in Specific Landscapes
Certain urban landscape types, such as golf courses and public/residential gardens, present unique opportunities for biodiversity enhancement. These environments naturally contain a high percentage of edge habitats, which research has shown to be important refuges for rare and endangered species.
Golf course managers can leave out-of-bounds areas unmanaged and increase vegetation surrounding water bodies, creating valuable habitat. Similarly, public and private gardens and landscapes, with their diverse collections of trees and shrubs, can be significant contributors to regional biodiversity. Homeowners and landscape managers should be engaged with science-based recommendations for attracting and retaining desirable wildlife through simple actions like reducing lawn area, increasing shrub and tree cover, and providing food and water sources.
Conclusion
The published research overwhelmingly demonstrates that nonnative plants, when selected and managed appropriately, can play a valuable role in enhancing biodiversity in urban landscapes. By focusing on functional diversity, habitat structure, and resource availability rather than species nativity, landscape professionals can create thriving, biodiverse communities that support a wide range of plant and animal species.
Through science-informed decision-making, urban planners and landscape managers can move beyond the myth of native plant superiority and develop innovative, ecologically-functional greenspaces that benefit both people and nature. It’s time to shift the focus from plant origin to plant function in order to harness the full biodiversity potential of our landscaped environments.
Example: Mixed-Species Reforestation Project 2023
