Native species richness of commercial plant vendors in the Midwestern United States

The Commercial Species Pool: An Overview

Because approximately 44% of all species native to the region can be purchased from vendors, the Midwest has a uniquely diverse commercial pool of available species when compared to the few studies similar to this one. For example, our figure was higher than species available in European grasslands, for which 39% of target species were available (Ladouceur and others 2018). Vidal and others (2020) found 561 native species in the native nurseries of southeastern Brazil, representing nearly 40% of the species recommended for use in restoration in the Atlantic Forests and Cerrado. Our percentage of commercially available species in the Midwest is comparable to that of White and others (2018). These authors found that nearly 74% of tallgrass prairie species native to the Midwest were commercially available, whereas 26% of vascular plant species are sold throughout the US. Given our broader species pool focus with respect to the Midwest, we note our proportion is intermediate to those found by White and others (2018).

These previous studies investigating native plant species in the commercial trade focused more narrowly on native plant nurseries. Our broader focus on the commercial rather than the restoration pool recognizes the diverse end uses of native plant materials. In our data set, however, the commercial pool and the “restoration species pool” from native nurseries would have been similar in composition because 95% of available species were found in native nurseries.

Vendor Differences and Their Conservation Value

Our vendor classifications highlighted clear differences in species richness among the different types, notably between native and conventional vendors. For example, tree farms, which are inherently limited in scope, were species poor, generally selling fewer than 20, and commonly fewer than 15, native species. Our “other” category was infrequently encountered and generally species poor, probably because this category encompassed specialty growers and niche product sales. General nurseries/garden centers are frequent vendors and vary considerably in diversity.

Perhaps the most important implication of our findings was the conservation value of native nurseries. Despite being the most infrequently sampled vendor type, native plant nurseries were disproportionately diverse, offering more than 40% of the species native to the Midwest and approximately 95% of those found in the commercial pool. Native nurseries had a sizable pool of unique species not found elsewhere in the commercial pool, which was mostly driven by vendor singletons (Figure 2). Together, these factors suggest that native nurseries are important regional repositories (ex situ banks) of species and genetic diversity (Haase and Davis 2017).

Despite the richness and unique composition of native nurseries, the biodiversity value of conventional vendor types should not be disregarded. First, 66% of species in the commercial pool can be purchased from conventional vendors (Figure 2), including the majority of rare species. Second, even if conventional vendors typically sell common species, these species may have high biodiversity value. For example, some of the most encountered species across the data set were native shade trees, which function as important food sources and habitat matrix species in human-dominated landscapes (Burghardt and others 2010; Narango and others 2017). Third, the sheer frequency of conventional vendors increases their conservation value. This frequency of conventional vendors can facilitate the spread of native plant diversity to urban and suburban landscapes. For example, Cavender-Bares and others (2020) found that the plant composition of Minnesotan household yards closely matched what was commercially available in conventional vendors. Finally, we acknowledge that our sampling methods could have undervalued the species richness of conventional plant vendors because we did not record nativars. Future research could expand our methodology to include nativars, as well as compare nativar versus unaltered richness among vendors (see Coombs and others 2020). Nativars can have similar or improved biodiversity value compared to their non-cultivar forms (for example, Baker and others 2020); nativars can also be suitable materials when used in gardening contexts (Kramer and others 2019). However, the question of the biodiversity value of nativars is likely dependent on the degree of change from the non-cultivar form and the responding variable in question (see White 2016; Baisden and others 2018). Nativars represent a “black box” of ecological effects and should be researched more, since existing research is scarce and emphasizes insect visitation, and because nativar biomass and coverage in human landscapes is clearly large given their commercial popularity.

The aforementioned differences in vendors’ species diversity, frequency in the landscape, end uses, and targeted end users could lead to complementarity of vendor function with respect to diffusing biodiversity on the landscape (Figure 7). In other words, the vendor types (namely, conventional versus native nurseries), because of their differential composition, locations, and target markets, could lead to spatially and ecologically distinct benefits and ecosystem services.

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Figure 7.

Plant vendor types are complementary in providing native plants for different contexts. Circles represent vendors, and circle size represents the species diversity of that vendor. The pool of native and conventional vendors each have a variety of materials. Conventional vendors tend to emphasize potted plants, whereas materials are more evenly spread in native plant nurseries as seed mixtures, individual species’ seed, potted plants, and plugs. Arrows represent the relative volume of sales to the types of end uses; native nurseries are likely to provision the bulk of materials destined for use in semi-natural and lightly managed landscapes, whereas conventional vendors emphasize human-dominated landscapes. Native and conventional vendors can also potentially exchange plant materials. Figure created using BioRender.

Species-Level Patterns: Commercial Dominance and Rarity

In natural communities, a well-recognized ecological pattern (sometimes referred to as a “law” of ecology; McGill and others 2007) is that rare species are common, and common species are few. Our investigation of vendors extends this finding to the commercial trade as well. Despite the high total diversity of species, we found that the vast majority of native species are either unavailable or not widely available, matching others’ findings about native species rarity in the commercial trade (Ladouceur and others 2018; White and others 2018; Vidal and others 2020).

Multiple and non-mutually exclusive explanations account for why so many species are rare in the commercial pool. Obviously, some species should not be expected to belong to the commercial pool in any capacity because of their unfavorable attributes in human or natural landscapes, such as weedy or nuisance species such as Amaranthus tuberculatus (Moq.) Sauer (roughfruit amaranth; Amaranthaceae) or Toxicodendron radicans (L.) Kuntze (eastern poison ivy; Anacardiaceae). However, what might explain the absent or rare status for the many species that are not problematic? Some species may simply be bland or unattractive in appearance, respond poorly to cultivation, or be obscure to growers and (or) customers compared to more popular options. For example, Liatris cylindracea Michx. (Ontario blazing star; Asteraceae) highlights these aforementioned characteristics when compared to its showier and commercially popular congeners, L. pycnostachya Michx. (prairie blazing star) and L. spicata (L.) Willd. (dense blazing star). Furthermore, many ecologically specialized species (such as native orchids or bog species) are unlikely to find widespread commercial use given their intolerance of cultivated conditions, even if they are aesthetically pleasing to consumers. In ecological restoration, some species will be favored over others because of their reliable supply, lower prices, and robust growth compared to underutilized species (Dunne and Dunne 2003; Broadhurst and others 2016). Consequently, we suggest that commercial rarity results from a combination of social, economic, and ecological causes.

In contrast, we found a select group of “dominant” species, which were found across a gamut of vendors (Table 3). These species are likely the most dominant because they are found in conventional vendors, and likewise, they clearly benefit from a demand from general consumers. For example, this demand could be attributable to these species’ natural aesthetics, ease of cultivation, and in the case of many trees, functional value (for example, Quercus rubra L. [Fagaceae] as a generic shade tree). If we had recorded nativars, we speculate that some occasional species within our data set (such as Acer rubrum L. [red maple; Aceraceae] and Thuja occidentalis L. [arborvitae; Cupressaceae]) would appear common along with the species listed in Table 3. Furthermore, there were rare species within our data set (for example, Hydrangea arborescens L. [wild hydrangea; Hydrangeaceae] and Phlox paniculata L. [fall phlox; Polemoniaceae]) that were far more common as nativars and (or) unspecified hybrids. These examples highlight additional possible approaches to recording species in commercial pools and reinforce the potential value of research on nativars, discussed above (see also Coombs and others 2020). Nonetheless, even if we had included nativars in the study, it is clear that relatively few species from the total pool achieve commonness in the commercial trade.

Materials

Generally, our findings on material types reinforced those about vendor differences and species rarity. Our study highlighted “material rarity” in the commercial trade. Most species could be expected to be found as about one material per vendor, and many species within the commercial pool are found as only a single type of material.

Our examination of native plant materials also highlighted the differences among vendor types. Native nurseries sold a more diverse array of materials for each species compared to conventional vendors, while general nurseries/garden centers and tree farms primarily sold potted plants (Figure 6B; Table 4). Perhaps this finding was because native plant nurseries target customers interested in native specimen plants and gardening (who might purchase potted plants, plugs, and bareroots), as well as practitioners conducting ecological restoration (who might purchase seed and seed mixes). We suggest that recording sold materials is useful information for characterizing the commercial species pool.