174 Chalker-Scott: Nonnative, Noninvasive Woody Species Can Enhance Urban Landscape Biodiversity if they are properly planted and established (NPS) • save money (EPA) Despite the lack of evidence to support these assertions (Kendle and Rose 2000; Davis et al. 2011), native plant superiority continues to be pro- moted and entrenched into policy as community planting mandates (Ingram 1999; Smith 2013) and sometimes to justify nonnative tree removal (Engar- dio 2013). Mandates include “native only” policies, such as those adopted by the Village of Riverside, Illinois, U.S. (2010), as well as policies requiring a certain percentage of natives in new tree plantings (New York City Council 2010). In this latter policy, the Council states that “native plants provide habi- tats for local birds, insects, and other animals that are indigenous to our region”—even though this urban environment surely bears little resemblance to the ecosystem that existed prior to development. Thus, the increased interest in native plant and animal species is entwined with a heightened aware- ness of their sometimes tenuous existence in urban areas. Long-term studies have documented the loss of native species in urbanized landscapes. Though this decrease in biodiversity is largely driven by land-use changes, such as the transformation from rural to urban land use and the historic drainage of wetlands (Knapp et al. 2010), there is a perception that biodiversity loss is due to nonnative species forcing native species into extinction. In preamble to its biodiversity law for public landscapes, the New York City Council (2010) states that “nonna- tive species oſten out-compete native plant species leaving native species and the animals that depend on them vulnerable to depletion or even extinction.” This perception is augmented by a related corol- lary—that only native plants can fill ecological roles in landscapes (Lukas 2011). For instance, Tallamy and Shropshire (2009) claim that insects prefer native to nonnative species. The authors correctly point out that productive landscapes are critical to supporting terrestrial food webs and that plant genera with no local species harbor fewer lepidopteran species than plant genera with local species representation. How- ever, the authors label entire plant genera as “native” or “alien” based solely on whether the genus con- tains species native to the northeastern U.S. In doing so, nearly 200 introduced woody species are folded ©2015 International Society of Arboriculture into “native” genera, some of which (e.g., Berberis, Lonicera, Prunus, Rosa, and Rubus) contain aggres- sively invasive, nonnative species, which are spread by members of that food web. Extrapolating the results of this study to determine the effect of invasive species on biodiversity is not possible, as those spe- cies were combined with natives at the genus level. METHODOLOGY The purpose of this review is to answer the ques- tion: Do native and nonnative woody species differ in how they affect species diversity? To address this question, the author considered information drawn from a global survey of relevant, peer-reviewed, scientific articles found across several databases (AGRICOLA, BIOSIS, CABI, and ISI Web of Sci- ence) from 1990 to 2014. Both native and nonna- tive (or the synonymic terms alien, introduced, and invasive) were required search terms. While there is no scientific consensus on the exact defi- nitions of these terms (Sagoff 2005), they served to identify relevant papers. Ecological restoration research was excluded from consideration, as na- tive species are required for installation. Likewise, the author did not include publications on geo- graphically isolated and/or exceptionally sensitive areas, such as Hawai’i, U.S., where even noninvasive introduced species can have significant disruptive effects on native ecosystems (Lukas 2011). Thus, in- terpreting the findings of this paper in the context of such sensitive places should be done with caution. Native planting mandates can require removal of nonnative trees and shrubs, which are oſten well-established components of landscapes and whose removal can be disruptive to that land- scape’s function. Therefore, a second set of search terms, which define the author’s species of interest, were included—trees and shrubs. Both terms were included as the line between large shrubs and small trees is blurry. The search was limited to research focused on woody plants, not ground covers, lawns, or flowering annuals and herbaceous perennials. The third set of search terms focused on urban landscapes. The author defined urban landscapes to include settings such as public parks, golf courses, remnant natural areas, botanical gardens, private residences, and traffic roundabouts. Scientific arti- cles that met the “urban landscape” definition also needed to include measures of biodiversity apart
July 2015
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