In Florida and elsewhere, cities are a mixture of exotic and native species. These novel or “recombinant ecosystems” represent unique plant and animal communities that have been purposely or accidentally created by humans. The question posed for this blog is whether “native purity” is a viable option for urban green infrastructure. I would suggest that returning urban habitats to a semblance of past, native assemblages of species is unrealistic in most cities.
Instead, we should think about restoring urban habitats in terms of “reconciliation ecology” where the goal is not to return to pristine, indigenous habitats but to implement strategies that simply increase the diversity of native species in urban areas. Below, I touch on why we should increase native diversity in cities and why we should be careful about the types of exotics used in cities.
Natural Heritage and the Extinction of Experience: With more and more people living in cities, the chance to experience and appreciate indigenous flora and fauna is in yards, neighborhoods, and city parks. With the same generalist (exotic) species dominating cities around the world, this homogenized environment is what urban residents experience and they lose touch with their unique natural heritage. This exposure to a more degraded habitat and isolation from native plants and animals reduces peoples’ connection to nature and willingness to invest time and money for biodiversity conservation. Implementing strategies that increase the diversity of native plants and animals would expose citizens to local species, creating a sense of place. This strong identity with native plant and animal species ultimately raises awareness and support for biodiversity conservation.
Exotics and risk of environmental impacts: Exotic plants in urban landscapes can require a large amount of care, which increases natural resource consumption and impacts on the environment. For example, turfgrass and some ornamental plants (e.g., roses) installed on nutrient poor soils in Florida means a homeowner has to irrigate, use herbicides and pesticides, and fertilize extensively to keep the grass and plants healthy. Excess fertilizers (e.g., phosphate and nitrate that is not taken up by yard plants) end up in local wetlands and waterbodies when nutrients run off the landscape after a storm event. With pesticides, these chemicals are not specific to the pest insect and kill many native pollinators such as bees, beetles, wasps, and butterflies.
Furthermore, the maintenance of turfgrass and ornamentals can actually cause a net increase in CO2 and other greenhouse gases. This is due to the use of fertilizers, irrigation, and mowing of a manicured landscape, which takes fossil fuels, thus releasing CO2 into the atmosphere. Greenhouse gas emissions from yard management practices are greater than the amount of carbon stored by lawns and manicured ornamentals.
Exotics and risk of decreasing native plant and animal diversity: Selecting and installing primarily exotic plant species would ultimately decrease native plant diversity because of the simple fact that native plants are absent from urban areas. Also, native animal diversity, in general, is correlated to native vegetation diversity. For example, native urban bird diversity increases with native vegetation; more native plants serve as host plants for native butterfly larvae; and native bee diversity increases with the occurrence of native plants. However, some exotics can provide food and shelter for native species. For example, butterflies obtain nectar from pentas (e.g., exotic cultivars of Pentas lanceolata) in Florida and elsewhere around the world.
Another risk associated with using exotic plants is that one may introduce invasive species into the environment. While many exotics are not invasive, some introduced plants are not designated as invasive until it is too late, having already spread from urban to natural areas. Here is a place to determine the current status of invasive plants in Florida – https://assessment.ifas.ufl.edu/. When exotic plants are used, known invasive plants should be eliminated and where information is lacking about the invasive status of a plant, perhaps the precautionary principle should be applied. This states that if the risk is serious enough, the absence of full scientific certainty should not be used to postpone actions to prevent environmental degradation.
In summary, I propose that a goal for urban green infrastructure is not native purity, but a sensible mix of native and exotic plants. In cities, this means increasing the use and conservation of native plant and animal species; using exotics that are low maintenance and provide an ecosystem service such as sequestering and storing carbon; using exotics that provide shelter and food for native animals; and not planting any exotics that have the potential of becoming invasive.
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