ENVIRONMENTAL IMPACT PAPER
The Ecological Impacts Of The Invasion Of Pyrus Calleryana In Kentucky
Erin Brennan
Western Kentucky University
BIOL 543 Environmental Science Concepts
28 April 2021
Introduction
Callery pear (Pyrus calleryana) is an ornamental pear tree commonly used in horticulture that is native to China, Korea, and Taiwan (Miller et al. 2015). Callery pear is one of the most widely used landscaping trees in the United States, with varieties including the popular cultivars Bradford Pear, Redspire, and Cleveland Select (Culley et al. 2011). The Bradford cultivar was first introduced to the United States in 1908 in an effort to produce breeding resistance to the fire blight disease afflicting fruiting pears (Miller et al. 2015). While unsuccessful, Callery pear became extremely popular in horticulture and landscaping with over 20 ornamental cultivars having been developed since the 1950s (Miller et al. 2015). The trees are fast-growing, have small fruit, and have copious amounts of bright white blooms in the spring. Callery pear varieties are commonly found lining roadways and in landscaping, mainly in urban and suburban areas across the state of Kentucky, but these trees have also become a mainstay along the country roads that lead to the many horse farms throughout the Bluegrass.
Figure 1. Callery pear cultivation
The Problem
After its rise to popularity and subsequent years of extensive horticultural use, the Callery pear began to appear in disturbed areas and along roadways across the United States. Callery pear trees then expanded into fields and grasslands, out-competing native, rare, and endangered plants and transforming these ecosystems. Callery pears have even begun to invade woodlands and forests, taking over the understory as they grow quickly and block sunlight to native, slower-growing plants. Callery pear is a threat to native plants and ecosystems as well as forest resources and ecosystem services that humans rely upon.
Invasive plant species
This rapid spread from cultivation as a sterile ornamental plant to a wild hybrid has earned its status as invasive plant species (Culley & Hardiman 2007). Invasive plants may also be known as nonnatives, exotics, aliens, nonindigenous harmful species, or simply, weeds (Westbrooks 1998). Invasive plants are “plants that have been introduced into an environment in which they did not evolve and thus usually have no natural enemies to limit their reproduction and spread,” (Westbrooks 1998). Once established, nonnative plants can cause serious ecological and economic damage (Westbrooks 1998). Despite its invasive nature, many varieties of Callery pear are still widely sold and planted across the United States (Miller et al. 2015).
Figure 2. Map of the suggested horticultural range for Callery pear cultivars in the United States (shown in gray). States marked with an “X” are those in which invasive populations of wild Callery pear in natural areas have been collected or observed.
Callery pear has escaped cultivation in 25 states and has been recently reported as a new invasive species in California, Michigan, Missouri, New Jersey, and West Virginia (Vincent 2005). There is evidence that the species is rapidly invading much of its horticultural range, especially in the Eastern United States (Vincent 2005). It appears to be hybridizing and reproducing in the wild, spreading rapidly throughout the Commonwealth of Kentucky, taking over grasslands and forests and disrupting native plant communities (Gassett et al. 2008).
A new invader in Kentucky
Callery pear varieties are listed as one of the most problematic terrestrial nuisance species by the Kentucky Department of Fish and Wildlife Resources (Gassett et al. 2008). The Kentucky Terrestrial Nuisance Species Management Plan defines terrestrial nuisance species as “non-native species that threaten the diversity abundance of native terrestrial species or the ecological stability of ecosystems, or commercial, agricultural, or recreational activities dependent on such ecosystems,” (Gassett et al. 2008). To address the issue of terrestrial nuisance species, the Kentucky Department of Fish and Wildlife Resources developed a plan to manage the problems associated with these invaders. The plan identifies problems caused by terrestrial nuisance species and provides management systems to address current problems and prevent future issues. The Callery pear is a species of high concern due to its rapid proliferation into fields, woodland, and forest ecosystems. According to the Kentucky Exotic Pest Plant Council, Callery pear is classified as a severe threat, a classification that includes “exotic plant species which possess characteristics of invasive species and spread easily into native plant communities and displace native vegetation” and includes species that are currently or could become widespread in Kentucky (p. 2, KY EPPC, 2013). The Callery pear has several traits that make it a prime example of an invasive species, including its ability to cross-pollinate and rapidly spread into natural areas. Because of these traits, Callery pear is threatening “the ecological stability of ecosystems, or commercial, agricultural, or recreational activities dependent on such ecosystems” in Kentucky (Gassett et al. 2008).
Ecosystem-level impacts
The invasion of Callery Pear has created a variety of ecological problems in Kentucky and poses a significant threat to terrestrial ecosystems in the state. Exotic invasive plants, such as the Callery pear, can disrupt plant communities and ecosystems, and displace native plants (p. 2, KY EPPC, 2013). Invasive plant species compete with native, threatened, and endangered plants for space, nutrients, sunlight, and even interactions with pollinators, changing plant populations and decreasing plant biodiversity. At least 250 nonnative plants have or are expected to eventually invade Kentucky (Jones 2005). Recent studies show that the southern United States is an area of high concern due to the spread of nonnative invasive plant species and their displacement of native species (Oswalt & Oswalt 2011). Invasions by nonnative plants result in the loss of habitat and the loss of forage for herbivores. Invasions can even have far-reaching impacts on ecosystem services for humans, including agricultural and forestry disruptions.
Southern forests are experiencing invasions of nonnative plants that reduce forest productivity, degrade biodiversity, and impact wildlife habitats (Miller et al. 2015). There are at least 56 nonnative plants or groups of plants that are aggressively invading the forests of the 13 Southern States and additional non-native plants that represent a growing concern (Miller et al. 2015). Invasive plants currently represent one of the largest challenges facing natural resource management as they can alter entire ecosystems, degrade ecosystem services, water, and soil quality, and destroy wildlife forage and habitat (Pearson & Ortega 2009).
Invasions by nonnative plants can affect the pollination of native plants. Areas with invasive plants show higher species richness of visiting pollinators, a greater abundance of visitors, and higher flower visitation, however, pollen transport tends to be dominated by pollen grains from invasive plants (Lopezaraiza-Mikel et al. 2007). Additionally, generalized native pollinators provide a means of integration for nonnative plants into native plant-pollinator visitation systems (Lopezaraiza-Mikel et al. 2007). In addition to these impacts, invasive shrubs and trees can decrease species richness, alter nutrient cycling, change soil nutrient levels and chemistry, compete with native plants for pollinators, restrict wildlife use, spread disease, impact fire regime, and interbreed with native plants (Boyce 2010).
The Causes
Horticulture and nonnative plants
Horticultural practices have been a significant contributor to invasive plant species worldwide. Nurseries are a major factor in the introduction of non-native plants (Harris et al. 2009). A large percentage of invasive species are introduced intentionally as horticulture plants, and exotic nonnative plants sold in nurseries can become invasive, displace natural plants, and disrupt ecosystems (Ranney 2004). The nursery industry has been a significant contributor of invasives in the United States with 85% of invasive woody plants being introduced for ornamental use in landscaping (Ranney 2004). Recurrent introductions of nonnative plant species have markedly hastened invasions (Harris et al. 2009). Urban and suburban landscaping is the primary driver of the Callery pear invasion in Kentucky. These invasive trees are heavily used along roadways, strip malls, and even schools and government buildings across the state. Most of the invasive shrubs and trees in Kentucky, including Callery pear, are known to be invasive yet can be purchased at local nurseries or online because there are no federal regulations or state laws to prevent the import, sale, or purchase of these invasive plants (Boyce 2010).
Figure 3. Map identifying those states that list Callery pear on their invasive species list or law.
Native origin and family membership can be used as an indicator of the potential to become an invasive species in the future (Harris et al. 2009). Species that belong to certain families and come from certain parts of the world have a higher probability of being introduced to new areas through horticultural use. These repeated introductions can increase the likelihood that a nonnative plant will become invasive. According to Harris et al., who studied 462 horticultural plants carried by nurseries across Kentucky, native origin and family membership are effective indicators in the identification of potentially invasive species. Species from Asia are carried most often by nurseries and also represent the greatest potential for invasions (Harris et al. 2009). Because of the absence of regulations and laws regarding the sale and purchase of invasive species, there is also a lack of education in the nursery industry and a lack of information available to the general public on the importance of avoiding currently invasive and nonnative plant species with the potential to become invasive.
Urbanization increases the prevalence of invasive plants
Invasions of nonnative species in urban areas posed a multitude of risks. Urban ecosystems can be “hotspots” for invasive species due to human activities including purposeful introduction through horticulture, through the disturbance of natural areas, and transport networks including roadways, rail systems, and waterways (Gaertner et al. 2017). Most urban ecosystems have effective networks of pollinators and small animals that act as seed dispersers (Gaertner et al. 2017). Invasions in urban areas can negatively impact the ecosystem services upon which humans depend, and can act as vectors of diseases that impact plants, animals, and even humans (Gaertner et al. 2017). Invasions can also lead to the homogenization of communities, an issue with which urban ecosystems already struggle.
The population density of humans often positively correlates with species richness, but this species richness is not limited to native species (Gaertner et al. 2017). In Kentucky, the richness of invasive shrub species increases with the population of the county, highlighting the need for management systems specific to urban areas (Boyce 2010). The “most heavily populated areas of the state are serving as sources of invasive shrubs,” and most have escaped cultivation (Boyce 2010). Further, the prevalence of invasive species is underreported in most of Kentucky (Boyce 2010). Much of the nonnative plant introductions were done with the intent to “create, augment, or restore ecosystem services,” however, some of the nonnative plant species introduced for these purposes have escaped cultivation and are now causing negative impacts on ecosystem services (Gaertner et al. 2017). The majority of plants that we use for horticulture, agriculture, and forestry are not native to North America and a small portion of these introduced plants escape cultivation and become invasive (Reichard & White 2009). These invasions are not without consequence. Invasive plants, animals, and fungi are the second leading cause of the endangerment of native plant species after habitat loss and degradation (Reichard & White 2009).
Horticultural practices leading to hybridization of Callery pear through cross-pollination
The Callery pear is an ornamental tree originating in China that has begun to appear in disturbed areas across the United States (Culley & Hardiman 2007). The species possesses a variety of traits that make it a prime example of invasive species, including the ability of sterile cultivars to cross-pollinate and spread into natural areas (Culley & Hardiman 2007). This hybridization played a role in the evolution of invasiveness in this species in terms of its initial establishment and the endurance of invasive genotypes (Hardiman & Culley 2010). Invasive spread is known to be instigated by the cross-pollination of genetically diverse individuals, primarily from different varieties planted in residential landscaping and commercial areas (Culley and Hardiman 2009). Caulley and Hardiman identify intraspecific hybridization between genetically distinct populations of sterile Callery pear and wild pear varieties as a significant impetus for the development of invasiveness. Hybridization is a powerful but generally overlooked process in the evolution of invasiveness (Culley & Hardiman 2009).
Some horticultural practices used with woody ornamental trees and shrubs, such as grafting, may intensify their potential for invasiveness (Culley et al. 2011). If the rootstock can sprout and flower, it may have the potential to cross-pollinate, leading to fruit production in an otherwise sterile species (Culley et al. 2011). This horticultural practice of grafting has contributed to the emergence of wild populations of Callery pear in natural areas. Grafting can incorporate two different genotypes into a single individual, which if allowed to sprout and flower, can cross with the grafted part of itself or with neighboring trees (Culley et al. 2011). This contributes to the formation of invasive populations if both genotypes are able to cross-pollinate and yield viable seeds which are dispersed into wild areas. This cross-pollination of rootstock has occurred under natural conditions and is evidenced by the identification of rootstock genotypes in invasive populations (Culley et al. 2011). Because Callery pear is so popular, there is already a wide variety of genotypes available which can contribute to the formation of wild and invasive populations in the coming years (Culley et al. 2011).
Invasive traits
Callery pears have several traits that make them the perfect invaders. These ornamental trees were cultivated to be sterile, meaning that they are not supposed to be able to reproduce. However, they have cross-pollinated with other varieties and even wild pear trees to create hybrids that are capable of producing viable seeds.
Callery pear produces an abundance of fruit with small seeds that are transported easily by birds and small mammals (Miller et al. 2015). It is also widely invading over a large range due to its cold hardiness and ability to grow quickly in a variety of environments (Miller et al. 2015). The ability of Callery pear seeds to survive long term and geminate is identified as a survival characteristic common to many invasive plants (Serota & Culley 2019). Serota and Culley examined the long-term survival and viability of Callery pear seeds and concluded that even after 11 years, a significant percentage of seeds (45-87%) were still viable and were able to generate seedlings that would survive after germination. These results indicate an additional challenge to the management and eradication of Callery pear. Because seedlings have high survival rates and grow quickly, they outpace native plants and trees and quickly shade them, allowing Callery pears to take over a variety of ecosystems (Serota & Culley 2019). The wild varieties often form large thorns, which protect them from foraging herbivores allowing them to beat out other plants and resulting in the formation of Callery pear monocultures after cross-pollination among ornamental cultivars and rootstock or wild varieties (Serota & Culley 2019).
Figure 4. Invasive traits of Callery pear: (a) fruit, (b) seeds, and (c) thorns.
Facilitation of the spread of nonnative plants via generalist native pollinators
Generalized native pollinators provide a means of integration for nonnative plants into native plant-pollinator visitation systems (Lopezaraiza-Mikel et al. 2007). Native pollinators will often change their foraging behavior to include the invasive species in their diets (Lopezaraiza-Mikel et al. 2007). Through this change in foraging preferences, the number of flower visits and quality of the visit as determined by the amount of pollen deposited can be affected (Lopezaraiza-Mikel et al. 2007). While the introduction of an invasive plant species can initially promote the visitation of native plant species by pollinators, pollen from invasive plants eventually comes to dominate the system, interfering with native plant pollination and negating any positive effect (Lopezaraiza-Mikel et al. 2007). While specialist pollinators tend to continue to visit native plants, generalized pollinators will include nonnative plants in their diets, providing a pathway for invasive species to integrate into pollination networks and facilitating their spread into natural areas (Lopezaraiza-Mikel et al. 2007).
Solutions
Plans for detecting and managing invasives and preserving biodiversity
Due to the invasion of aggressive nonnative plants that have increased exponentially in recent decades, environmentally sound approaches to prevention, detection, and control of invasive plants are needed (Westbrooks 1998). An additional 250 species from outside of the state are soon expected to spread to Kentucky (Jones 2005). Once established, these nonnative plants can cause serious ecological and economic damage, which highlights the importance of prevention and early detection of invasive species (Westbrooks 1998). In order to minimize costs and effectively control an invasion, proactive management is required at an early stage (Gaertner et al. 2017).
To prevent the spread of invasive species in southern forests, methods for early detection and monitoring are critical, especially in terms of emerging invasions and invasive plant populations that have the potential to threaten native species and ecosystems (Oswalt & Oswalt 2011). By the time an economically valuable nonnative species becomes well established, has been found to pose a risk, and is identified as an invasive species, eradication will likely be very costly or even impossible (Gaertner et al. 2017). While the Kentucky Department of Fish and Wildlife Resources includes Callery pear in the Kentucky Terrestrial Nuisance Species Management Plan, this plan lacks a critical element - the acknowledgment of the role that urban and suburban areas play in this invasion (Gassett et al. 2008). When considering the development of detection and management plans, urban areas should be the focus as they are a major source of introductions of invasive species (Gaertner et al. 2017).
Urban-specific management systems to eliminate major sources of Callery pear invasions
Urbanization has increased dramatically across the globe, with major implications for biodiversity and conservation (Gaertner et al. 2017). There are additional challenges in managing invasive species in urban ecosystems. Because urban invasions differ from those in other contexts, special management systems and policies regarding invasive species are needed in urban ecosystems (Gaertner et al. 2017). Urban-specific management systems may be necessary to eliminate major sources of Callery pear infestations because the nursery industry continues to introduce Callery pear and other nonnative plants to urban and suburban areas.
These urban areas then serve as a launching point for new invasions into rural and natural areas of the state, introducing invasive plants and even spreading forest pests and pathogens, and threatening valuable forest resources (Gaertner et al. 2017). Invasions by nonnative plants in urban areas can also improve conditions for other nonnative invaders in urban ecosystems, further threatening native plants and increasing the impact of urbanization (Gaertner et al. 2017). Therefore, cities must develop plans to manage the sale of invasives, and identify and control their spread. Cities must also educate private landowners regarding both the human and ecological problems posed by nonnative invasive plants and provide rewards and alternatives for residents who are willing to remove invasive species and replace them with beneficial native species.
Tree Removal and/or Replacement
Tree removal and replacement programs have been popular and successful across the midwest and south. Tree replacement programs that offer free native tree replacements for anyone who removes Callery pear off their property or tree bounties that offer prizes could slow the spread of these invasive trees in urban and suburban areas while providing an avenue for public education. Tree bounties and tree replacement programs could give out native ornamental trees, such as eastern redbud, flowering dogwood varieties, and serviceberry, as replacements for Callery pears.
Removal of existing Callery pear trees in both urban and wild areas is recommended to prevent the escalation of this invasion (Miller et al. 2015). In wild areas, full removal is recommended to aid in the recovery of ecosystems and native plant communities (Miller 2003). Because the extent of the invasion is so severe and because Callery pear is resistant to traditional mitigation and control methods, the forestry service often has to resort to bulldozing stands of trees to ensure full removal (Miller 2003). Removal programs in the wild should focus on cutting down Callery pear along with restoring native plant species disrupted by the spread of this invasive species (Miller et al. 2015).
There is a solution for areas suffering from extensive Callery pear invasions where full removal would be difficult or impossible. Pelikan et al. determined the best pattern for the removal of Callery pear trees to suppress population regrowth based on gene dispersal. The removal of invasive Callery pear based on gene dispersal can outperform traditional methods of removal in terms of suppressing and preventing the return of invasive plant populations (Pelikan et al. 2015).
Restoration of native vegetation to support ecosystem recovery
The restoration of native, rare, and endangered plants and ecosystems will likely be necessary once Callery pears have been removed from an area. Restoration of impacted plants may pose challenges because while nearly half of the federally listed threatened or endangered species are plants, they receive a very small percentage of funding for conservation (Schemske et al. 1994). Current research on endangered plants can be evaluated for its usefulness in conservation and can be used to identify biological information necessary to develop recovery plans for endangered plants (Schemske et al. 1994). Existing recovery plans for 98 endangered or threatened plant species can be reviewed to develop research recommendations, evaluate criteria for identifying species for protection, and suggest guidelines for recovery plans (Schemske et al. 1994). Additionally, historical changes in flora and plant communities and current threats to native plants show the need for plans for preserving plant biodiversity and future studies and research of Kentucky plants (Jones 2005).
Development of non-invasive horticultural replacements
The development of non-invasive horticultural alternatives to replace popular Callery pear varieties is a promising potential solution to halting the invasion in Kentucky (Ranney 2004). While there are not many simple solutions to eradicate plants that have naturalized, modern methods can prevent further introduction. Biotechnologies can be used to develop non-invasive cultivars utilizing genetic selection, traditional breeding, and genetic engineering to prevent sexual reproduction (Ranney 2004). By utilizing this technology, economically important plants can continue to be grown while minimizing their potential for invasiveness (Ranney 2004). The development of genetically sterile cultivars will likely only slow down and not entirely eliminate the invasion of Callery pear (Culley et al. 2011). However, the invasiveness of Callery pear has provided an important lesson that can be applied to horticultural practices to prevent future invasions of woody shrubs and trees. The compatibility of and potential for cross-pollination between grafted portions must be considered as a catalyst for invasiveness (Culley et al. 2011).
State and Federal restrictions and legislation banning Callery pear
Public policy restricting the import, sale, and purchase of Callery pear could prevent future invasions. Solutions to the Callery pear invasion include careful screening and selection of plants to avoid species with invasive properties, and halting sales of varieties including the popular Bradford pear (Ranney 2004). To prevent economic harm, native plant species could be promoted in place of invasives.
Sales of ornamental trees could focus on flowering native trees of similar size and aesthetic, such as redbud, serviceberry, or dogwood varieties.
Legislation restricting the sale of Callery pear is a reasonable solution given its classification as a severe threat to native plants and ecosystems in Kentucky. Such legislation has been used across the US to ban or “blacklist” the sale of invasive or nuisance species (Ranney 2004). Recently, the Connecticut legislature passed a bill “prohibiting the importing, moving, selling, purchasing, transplanting, cultivating, or distributing of 81 different plants with penalties of up to $100 per individual plant,” (Ranney 2004). While Kentucky does have noxious-plant laws, only one invasive shrub, Rosa multiflora, is included on the list (Boyce 2010). Therefore, vendors and consumers are unlikely to know that they are selling and buying invasive species (Boyce 2010). Stronger regulations at both the state and federal levels could help control the spread of invasive plant species (Boyce 2010).
Discussion
The invasion of Callery Pear has created a variety of ecological problems in Kentucky. The invasive plants compete with native, threatened, and endangered plants for space, nutrients, sunlight, and even interactions with pollinators, changing plant populations and decreasing plant biodiversity. Invasions by nonnative plants result in the loss of habitat and the loss of forage for herbivores. Invasions can even have far-reaching impacts on ecosystem services for humans, including agricultural and forestry disruptions. The impact of this invasive species in Kentucky has been significant.
Commonly used horticultural practices have led to the evolution of the Callery pear from sterile cultivar to hybridized invasive species. Callery pears are one of the most common and widespread ornamental trees used in urban and suburban landscaping, facilitating their invasion into disturbed areas, fields, roadsides, and even forests.
Many solutions exist which have the potential to prevent further invasions, or even fully eradicate the invasive Callery pear in Kentucky, and restore natural ecosystems and plant communities, however, this problem is growing exponentially larger with each passing season. The expansion of this invader is already visibly evident across the state, especially in the spring when the trees are in bloom. While removal of invasive Callery pear and restoration of native plant species are critical components of restoring Kentucky ecosystems, legislation is the best solution to curtail the sale of this species and slow the invasion. The longer Kentucky waits to take decisive action to end the invasion of Callery pear, the worse the ecological impacts will be, requiring increasingly more complex solutions.
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Map of the suggested horticultural range for Callery pear cultivars in the United States (shown in gray). States marked with an “X” are those in which invasive populations of wild Callery pear in natural areas have been collected or observed.
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Invasive traits of Callery pear: (a) fruit, (b) seeds, and (c) thorns.
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