Plant introductions have been conducted since the earliest period of Western colonization and Native American populations introduced edible and useful plants from other regions along their trade routes. However, these introductions were made into agricultural systems, or were introduced as garden ornamentals.
While some introductions have reproduced aggressively and can be considered invasive, many others have adapted to local conditions and have naturalized.
Ecosystems are not static, but evolving and as mentioned earlier, ecosystems lose and gain species through evolutionary time. The issue for biodiversity and sustainability of ecosystems arises from the degree to which introductions disrupt functioning ecosystems. To again quote E. O. Wilson: “Eliminate one species, and another increases in number to take its place. Eliminate a great many species, and the local ecosystem starts to decay visibly.”
Naturalized species perform valuable functions as ornamentals, provide habitat, shelter, and food for some bird, animal, and insect species. They have, however, decreased the overall diversity of the ecosystems they have colonized by displacing other species. Although they provide some ecological services, they will not function to the same degree as the species they displaced in intact ecosystems that have evolved over evolutionary time.
In addition, if they have displaced specialist species that, for instance could only be pollinated by a particular bee species, then that loss will have cascaded through the ecosystem, with the potential loss of many other plant and animal species.
In highly disturbed sites, even within remnant ecosystems, introduced plants may prove better adapted to soil and hydrological conditions and this very well may merit their use, even though this is contrary to the goal of increasing the use of native plants in the city. Intelligent and informed planting design recognizes a number of complex characteristics that can’t be confined to a narrow discussion of native vs. non-native origins.
Opportunities to increase biodiversity of New York City‟s existing ecosystems through planting practices will be carefully managed by New York City‟s land management professionals and landscape architects, and indeed we are now instructed to take concrete steps to do so. We can best meet this challenge by preserving the best of the remaining open space ecosystems that are as yet unprotected and through sound management and restoration of our surviving ecosystems.