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C O N T I N U I N G E D U C AT I O N U N I T CHALLENGES FOR THE When Native Species Meet Their Alien Relatives BUILT ENVIRONMENT By William M. Fountain and James R. Lempke The arborist will be able to h differentiate among terms that describe species based on their area of origin and current location h describe characteristics of non-native plants that may be suitable for the built environment h explain conditions that may cause a non-native plant to become invasive CEUs for this article apply to Certified Arborist, Utility Specialist, Municipal Specialist, Tree/Worker Climber, and the BCMA management category. The new, the different, the exotic. initially arrived on foreign shores. Some, like the zebra mussel, emerald ash borer, and common rat would sneak across oceans on ships as people traveled to distant shores and traded with other cultures. Some would come as welcomed friends—at least initially. Johnsongrass and crabgrass were initially introduced as summer forages for North American livestock. Kudzu, a well-mannered roadside plant in Japan, escaped the ornamental garden and rapidly “ate” its way across hundreds of thousands of acres in the southeastern United States. On the other hand, the tulip, native to Asia Minor, remains relatively harmless and requires considerable effort to perpetuate in our gardens. What is the difference? Why are some species devastating, while others are relatively benign? That answer is complicated and lies in geologic history, ecosystem balance, and the botanical characteristics of each species. Species Characteristics for Native/Non-native Plants The statement, “All natives are always good, and all non-natives are Mention any of these ear-catching words and you are sure to attract always bad,” is indefensible. Before we can discuss questions of why attention. There is a natural human attraction to things that we think some plants are invasive while others are not, several terms must are uniquely different or better than what we currently possess. The be defined. lust for new plant species is one facet of this that has both enriched our lives and provided life-altering challenges—and is a commonality across generations and cultures. Through the centuries, the return of sailors and travelers was eagerly awaited by people who wanted to see the exotic treasures and oddities they brought home from distant places. Early European plant explorers rushed into areas marked as Terra Incognita (unknown land) on maps of the Americas and Asia. Their primary objective was to return with unique additions for the herbarium, ornamental garden, and dinner table. Along the way, a very limited number of these species would become problematic and remain so for generations to come. Where people have traveled, diseases and pests have always followed. These exotic invaders have rarely Kudzu (Pueraria montana var. lobata), native to Japan, is a climbing, coiling vine that smothers and kills other plant species, and it now covers hundreds of thousands of acres in the United States. appeared to be threatening as they 12 www.isa-arbor.com ARBORIST • NEWS JAMES R. ALLISON, GEORGIA DEPARTMENT OF NATURAL RESOURCES, BUGWOOD.ORG LEARNING OBJECTIVES C O N T I N U I N G E D U C AT I O N U N I T A native or indigenous organism is any species (plant, animal, or microorganism) that was part of a natural community prior to the intervention of humans. Just because a species is native does not mean that it will always remain a part of that specific biological community. All species that do not evolve will eventually become extinct. For example, dinosaurs were once native to every continent. Their environment changed, they could not adapt, and they became extinct. Birds are believed to be their only living descendants. Conversely, a non-native is an organism that is not originally from an area. Non-native species do not have to be living and reproducing in an area to be classified as a non-native. Quarantine laws seek to restrict many non-native species from areas where they are not currently living and reproducing. An alien species is a non-native that has been moved into a new location by humans. This movement may be intentional or unintentional, but it is always associated with human activities. It does not include the natural migration of an organism by its own efforts or by the forces of nature. Some of these alien species are so closely related to native species that they will interbreed with native species, producing hybrids, or species with a mixture of characteristics from the two different parents. An invasive species is a non-native or an alien species that has begun to reproduce and replace native species. The native species being replaced may be closely related to the invasive species. Invasive species may replace native species by encroaching on habitat currently occupied by native species, leaving the native species with a diminishing area in which to grow and reproduce. It may also involve a disease pathogen or parasite actually attacking a native species. Invasive species affect the web of life in other ways as well— pollination, herbivory, nesting, feeding, migration, reproduction, and predation of native species all may be affected by the change in species composition following such invasion. Most invasive species are problematic under limited circumstances involving climate or habit. For example, a terrible weed of warmer wetlands might be a benign garden plant, or not even able to survive in cooler or drier habitats. It is important to know this in advance so that any restrictions imposed in a given region will be reasonable and not ignored as overzealous environmentalism. Animals and plants that have migrated on their own or by natural efforts are not considered invasive actions even if they are problematic for humans. A species that is termed “invasive” is attributed to human action in transporting these organisms, be the actions intentional or unintentional. The armadillo and opossum are examples of animals considered problematic that were originally native to South America. When the isthmus (Panama) joined the two Americas a short three million years ago, these strange-looking, slow-moving animals ambled their way northward into North America. An exotic is something that is recognized as new, different, and appealing to humans. The perceived beauty or interest in an exotic species often results in their introduction without regard for their potential to become an invasive species. A recent survey (Hulme) of the 100 most problematic invasive species of plants in the world attributed more than half to “escapes” from cultivation in botanical gardens. An escape is an exotic species that was or is cultivated but has been able to reproduce from cultivated sources. If escapes become part of wild populations, the species is said to be naturalized. A naturalized species is a non-native that has been introduced into a new environment, where it is now able to grow and reproduce in its new location without human assistance. Naturalized species are always non-native. For various reasons, only a relatively small percentage of the introduced species are capable of becoming naturalized, and only a small percentage of this group is capable of reaching population levels or has characteristics that are considered problematic to humans or native species. Unless there is an overriding issue, naturalized species that are not producing a significant negative impact on their environment are generally not considered to be invasive. Like invasive alien species, aggressive native species have a natural tendency to spread, forming dense thickets. These natives may approach a true monoculture and cover extensive areas. They may or may not be offensive to humans. If these species are in their natural (native) habitat, they are not considered to be invasive even if they are viewed as problematic to humans. By definition they are native, and this is where and how they grow in their natural systems. In eastern North America, poison ivy (Toxicodendron radicans), black locust (Robinia pseudoacacia), and native cane (Arundinaria gigantea) are all known for forming dense thickets, making them unsuitable for small residential properties. Early travelers avoided areas where these were the dominant species. Nevertheless, they are native to eastern North America. This is their natural habitat, and the JOSEPH O'BRIEN, USDA FOREST SERVICE, BUGWOOD.ORG Invasivespecies affect the web of life. Chestnut blight canker on American chestnut (Castanea dentata). D AUGUST 2011 www.isa-arbor.com 13 New Species in the Urban Environment Biological populations of plants and animals growing in natural systems are constantly in a balancing act. Although it is usually too slow to see in a human life span, evolution continues, climates change, and continents drift, permanently altering the environment. Species go extinct and new species evolve as a result of biotic and abiotic pressures on populations. In order to be able to recognize what is native, we must look at a specific point in time. Anything that appears as a result of human activity after that point in time is non-native. If a species arrives unassisted by human activity, it is also considered native even though it is recognized as a recent arrival. It is unrealistic to expect to find all local native species in a limited area; because not only do we have to look at a specific point in time, we must also look at relatively large geographic areas. The least confusing designation is to cite plants native to one or multiple states, provinces, or geographic regions of a country. Very little in life is absolute. Before we label all non-natives as bad, remember what you ate for your last meal. Even if grown locally, the odds are that the majority of the plant products you consume and wear are non-natives. Non-natives can be beneficial, even essential to our lives. Most notable are our food crops. Our diets would be both meager and limited in variety if we were forced to live exclusively on edible plants native to our specific regions. Agricultural production of non-native foods allows most of us to live in cities. These high-population centers help preserve natural areas in a more natural state than if human populations were uniformly spread around the world. Cities, or the built environment, only remotely resemble their pre-human character. Soils have been altered and hydrology changed. Reflected light and drying winds make it an alien environment for most native species. Human stresses ensure that plants do not survive as long in the built environment as they would in their native homes. Species that survive in altered urban and suburban environments are often species associated with flood plains or less hospitable climates. Some urban-tolerant species are native species selected IAN TRUEMAN, UNIVERSITY OF WOLVERHAMPTON, BUGWOOD.ORG C O N T I N U I N G E D U C AT I O N U N I T web of life that has evolved in such areas over millennia depends on them. The small, suburban lots and the residents now reside where these species once formed dense stands, and are now the ones who are out of their element (e.g., non-native). The tree of heaven (Ailanthus altissima), native to China, is an invasive species in Europe and the United States that grows quickly. The tree also releases allelopathic chemicals that can suppress the growth of other species. Photo location: Poland. 14 from the extremes of their natural range. Others tolerant of urban pressures are from distant continents. An important criterion for inclusion in the urban landscape is that introduced species do not become invasive or problematic in the human or natural environment. Why do some non-native plant species become invasive while others do not? There is no single reason. Rather, there are complex environmental and biological causes that are compounded with the anatomical characteristics of the introduced plant. Some non-native species are not readily adapted to their new environment. While they may be able to become naturalized, environmental conditions (e.g., moisture, soil drainage, temperature, length of the growing season, pollinators) may limit growth rates or ability to reproduce and become a dominant species. These species just have difficulty surviving. Non-native species with the potential to reproduce rapidly are often introduced without the diseases and insects that have traditionally kept populations under control. Host plants and the diseases and insects that feed on them always live in a delicate balance. A pathogen that kills its host may die. The absence of primary predators in its new home sometimes results in a host plant growing and reproducing with wild abandon; that is, until the local pathogens and pests adapt to being able to consume the recent invasive species as a new food source. Plants develop new defenses against existing pathogens and pests with time. This delicate balancing act occurs rather gradually as viewed through human eyes. Some species are able to out-compete native species by production of allelopathic chemicals, which suppress the growth of other species. Bush honeysuckle (Lonicera maackii and L. tatarica), introduced from Asia in the late 19th century, reduces competition from natives by producing chemicals in its foliage. The bush honeysuckle also gains an upper hand in the competition by leafing out very early in the spring and retaining green foliage later into autumn than its competitors. This helps it gradually outgrow and shade out other species. Invasive Insects and Diseases It is important to understand that it is not just plants that become invasive. Insects and diseases can also become invasive when they are moved to areas where they are not native. The hemlock woolly adelgid (Adelges tsugae) was unintentionally introduced into eastern North America from southern Japan. The Canadian hemlock (Tsuga canadensis) is highly susceptible (has no resistance) to this insect. Resistance differs from immunity in that a species that is resistant to predation can sustain some feeding. A species that is immune will not be susceptible to any feeding from the pest. In its native Japan, the adelgid population has been kept under control by a combination of host plant resistance and natural predators. Disaster struck the Appalachian forests of North America in the early 1900s when a fungus, later named chestnut blight (Cryphonectria parasitica), is thought to have been accidently introduced into the northeastern United States on either timber or live trees brought from Asia. Within a few decades, 4 billion native chestnut trees (Castanea dentata), a quarter of the canopy cover in the Appalachian range, were dead or dying. Species of chestnuts from Japan and eastern Asia are resistant to the fungal disease. Another fungal disease, later named Dutch elm disease (Ophiostoma ulmi), began killing elms in Europe by 1910. By 1928, it had traveled on logs from Europe to North America. This fungal disease is thought to have originally come from Asia given how Asian species of elm www.isa-arbor.com ARBORIST • NEWS Ecosystem Balance slightly different environmental conditions. Insects and diseases began to feed on their host plants. By chance, some of these host plants had naturally occurring defensive compounds that resulted in slightly less predation. This gave minimally resistant host plants a slight edge in reproduction over others in the same species. Eventually, resistance to feeding increased. At the same time that the host plant was increasing its ability to defend itself, the insect or disease was also evolving an ability to tolerate the plant’s defensive strategy. While this may sound wasteful, a host that does not suffer some predation is going to become less vigorous. Over time, insects, diseases, and host plants develop a delicate ecological balance where each is able to tolerate (and benefit) from the presence of the other. This balance between a host and its pathogens and predatory insects worked well until inquisitive humans began traveling the globe, moving different species from one ecosystem to another. The most significant of these disasters have occurred since the mid 19th century and can only be expected to continue with globalization of world economies. These problems stemming from invasive species are of our own making. Solutions are rarely simple and vary with the characteristics of the host and pest: • Quarantines work when their need is understood and when they are reasonable (and thus respected) and are fairly but consistently enforced. • Infestations discovered before they become widespread can often be eradicated. • Selective breeding for resistance or introduction of resistant, non-native species is another management technique. • Programs involving prescribed use of pesticides and/or cultural techniques have contained some pests at manageable levels. • Over time, doing nothing is often the most expensive option. Provenance—the reference to where an organism evolved or is considered to be native—is important in determining the potential for a species to become problematic to native species. References to where a species is native must reference geographic place names. Researchers recognize that many organisms are not bound by artificial lines, be they state/province or the national boundaries found on our maps. Birds and other animals migrate with predictable regularity between continents. They carry pests, pathogens, and seeds on their bodies and in their waste products. Seeds are also blown long distances and across borders by wind. These means of travel are recognized as natural methods of dispersion. They differ from human dissemination in that they are more gradual and have occurred over millennia, giving natural systems a chance to adapt. It also frequently differs in that chance introductions might be extremely limited in scope, and thus unlikely to succeed, whereas plants or animals introduced by humans will be more numerous. This vastly increases the odds that one of these individuals will find the perfect niche in which to reproduce and begin the process of invasion. It also increases the odds that natural variation within the large population of individuals will include the equivalent of the perfect genetic storm of site compatibility and vigor. Such has been the case with many of the plants introduced for conservation purposes. Species that are almost identical, but are separated geographically, are referred to as disjunct species. Disjunct Cities, or the built environment, only remotely resemble their pre-human character. Soils have been altered species of plants began to co-evolve and hydrology changed. Reflected light and drying winds make it an alien environment for most native with similar, but slightly different, species. Human stresses ensure that plants do not survive as long in the built environment as they would in their native homes. insects and pathogens and under D AUGUST 2011 www.isa-arbor.com 15 C O N T I N U I N G E D U C AT I O N U N I T Species lost are lost forever. are resistant to the disease. Because species in North America and Europe had never developed a tolerance for this disease, they had no resistance and died by the thousands. Later, two other closely related species of the fungus were introduced. One was from western Himalaya (O. himalulmi) and another (O. novo-ulmi) was thought to be a hybrid of the other two fungal species or—some believe—from China. While North American customs officials look vigilantly for any biological organism that may be coming in from another country, most of the problematic disease and insect pests of North America are the result of introductions from Asia and Europe. The flow of invasive diseases and insects has moved from each of these continents to the other two. Relatively few plant pests have come from South America, Africa, and Australia. The reason is provenance. C O N T I N U I N G E D U C AT I O N U N I T While the problems created by invasive species and species extinction will always find a natural balance, this occurs more in a geologic time frame rather than over the simple course of multiple human life spans. The loss of a single species sends ripples through the ecosystem as other life forms dependant upon it are forced to adapt or enter the fossil record. Species lost are lost forever. What are our obligations as green industry professionals? 1. Recommend and install only plant species known to present no ecological problems in the area and habitat in question. The acquisition of this knowledge is an ongoing process. 2. Work to rectify problems of the past by practicing environmental stewardship and restoration. The green industry is responsible for the majority of the most significant invasive species causing problems today. 3. Learn and practice the principals of Plant Health Care. The urban environment is a biologically dynamic web of life. 4. Educate the public on the value of species diversity. 5. Match the plant to the site. 6. Install and manage plants in accordance with industryrecognized Best Management Practices. 7. Work with engineers and design professionals for sustainable green infrastructure. Trees and other urban plants only provide maximum benefits when they are capable of becoming a mature portion of a safe and sustainable urban environment. Selected References Borrell, B. 2011. A Friend to Aliens. Scientific American (February 2011) 304(2):74–77. Burdick, A. 2005. Out of Eden: An Odyssey of Ecological Invasion. Farrah, Straus, & Giroux: New York, NY. Davis, M.A. 2009. Invasion Biology. Oxford University Press. Dawson, W., A.S. Mndolwa, D. F.R.P. Burslem, P.E. Hulme. 2008. Assessing the risks of plant invasions arising from collections in tropical botanical gardens. Biodiversity and Conservation 17(8):1979–1995. Marris, E. Rambunctious Garden: Saving Nature in a Post-Wild World. Bloomsbury (in press). Pollan, M. 1994. How To Make a Pond. New York Times Magazine July 24, 1994. <http://michaelpollan.com/articles-archive/ how-to-make-a-pond/> Sternberg, G., and J. Wilson. 2004. Native Trees of North American Landscapes. Timber Press. William M. Fountain is Extension Professor of Arboriculture and Landscape Management, University of Kentucky. James R. Lempke is Curator of Native Plants and Natural Ecosystems (retired), The State Botanic Garden of Kentucky. CEU TEST QUESTIONS Take your quiz online! Go to www.isa-arbor.com/certification/ceus.aspx and click on “Arborist News Quizzes Online.” If you need a login and password, send an e-mail to [email protected]. To receive continuing education unit (CEU) credit (1.0 CEU) for home study of this article, after you have read it, darken the appropriate circles on the answer form of the insert card in this issue of Arborist News. (A photocopy of the answer form is not acceptable.) A passing score for this test is 16 correct answers. Next, complete the registration information, including your certification number, on the answer form and send it to ISA, P.O. Box 3129, Champaign, IL 61826-3129. Answer forms for this test, Challenges for the Built Environment: When Native Species Meet Their Alien Relatives, may be sent for the next 12 months. If you do not pass the quiz, ISA will send you a retake answer sheet. You may take the quiz as often as necessary to pass. If you pass, you will not be notified; rather, you will see the credit on your CEU report (available online). Processing CEUs takes 4 to 6 weeks. CEUs for this article apply to Certified Arborist, Utility Specialist, Municipal Specialist, Tree/Worker Climber, and the BCMA management category. 1. Which of the following is most likely to become an exotic invasive species in Europe? a. a native species that produces suckers b. a native species propagated by grafting c. species introduced from Asia d. species recognized as wildlife cover 2. The majority of invasive species introduced over the last century can be traced to a. climate change b. the green industry c. landscape architects d. the migration of animals 16 3. An exotic species is one noted for a. aesthetic attractiveness b. ease of cultivation c. growing only on another continent d. unusual growth habit 4. An alien species is one that has been a. crossed with another species b. cultivated for less than 100 years c. able to reproduce from cultivated sources d. introduced from another area by human activity www.isa-arbor.com 5. A naturalized species is one that is a. approved by government plant quarantine officials b. a native species that has been introduced c. an introduced species that reproduces naturally d. a species with no disease or insect problems 6. All non-native species are always bad, and all native species are always good. a. True b. False 7. Non-native species may be more suitable for the built environment than natives because non-natives are often a. easier to transplant and grow b. more attractive and easy to propagate c. more disease and insect resistant d. more adaptable to heat and compaction 8. An American chestnut (Castanea dentata) that has no resistance to infection by chestnut blight is said to be a. disjunct b. immune c. impervious d. susceptible 9. Which of the following might be responsible for keeping a naturalized exotic under control? a. climatic temperature extremes b. lack of appropriate pollinators c. native diseases and/or insects d. all of the above ARBORIST • NEWS 11. Using natives exclusively will guarantee landscapes free of insects and diseases. a. True b. False 12. The task of determining what is native to a specific area necessitates that one consider a specific reference point in time, because a. it determines what was present before people began relocating species b. recent climatic change has eliminated many native species c. significant numbers of species move naturally over long distances each year d. none of the above 13. The difference between an alien species and an exotic species is that exotic species are always a. cultivated b. invasive c. non-native d. similar to a disjunct species 14. In order to be classified as being naturalized, an organism must be a. growing in an area before people were there b. growing and reproducing without human assistance c. missing from the earliest taxonomic records for the area d. native to the site for at least 100 years 15. When invasive pests from other continents are introduced, the prudent long-term approach for management of the pest is to a. apply pesticides to all plants in the area in order to control the pest b. do nothing—natural systems will rapidly acclimate c. eradicate all of the host plants in a 30 mi (50 km) radius d. investigate other organisms and conditions that keep the pest under control where it is native 16. The urban environment differs from natural adjacent systems, because urban plants experience a. confined spaces b. disturbed soils c. higher temperatures d. all of the above 17. When a native is cross-pollinated with a closely-related alien species, the resulting seedlings are called a. cultivars b. disjunct species c. hybrid d. new species 18. Species that are native do not have the potential to become problematic in landscapes. a. True b. False 19. Allelopathic conditions occur when one plant suppresses the growth of another by a. producing chemicals that are toxic b. competing for water and mineral elements c. shading sunlight exposure d. all of the above 20. You should be suspicious that a new, nonnative species might be a potential invasive species if it characteristically a. begins to produce numerous seed at an early age b. leafs out early in spring and retains its leaves late into the fall c. reproduces by both suckers and seed d. all of the above TREE HEALTH • SAFETY • ROPES AND KNOTS • CLIMBING • RIGGING • REMOVAL Tree Climbers’ Guide (3rd Edition) By Sharon Lilly Written specifically from the tree climber’s perspective to learn safe climbing and aerial tree work principles, this publication may be used as a basic text for tree climbers as well as a study guide for the ISA Certified Tree Worker/Climber Specialist exam. Each chapter includes a list of key terms and concludes with a workbook section. The chapter topics, enhanced with more than 200 color illustrations, include: Tree Health and Sciences, Safety, Ropes and Knots, Climbing, Pruning, Rigging, Removal, and Cabling. The guide also contains appendices with answers to the workbook questions, a glossary, and resources listed for further reference. (©2005, softcover, 172 pp., glossary, appendices, index) #P1230 (English) • #P1230S (Spanish) Retail Price: $59.95 ISA Member Price: $49.95 TO ORDER, CALL 1-888-ISA-TREE OR VISIT US ONLINE AT WWW.ISA-ARBOR.COM/STORE TREE SCIENCES • PRUNING • CABLING • AVAILABLE IN ENGLISH OR SPANISH AUGUST 2011 www.isa-arbor.com 17 C O N T I N U I N G E D U C AT I O N U N I T 10. When comparing the close genetic relationship between the American beech (Fagus grandifolia) of eastern North America and the European beech (F. sylvatica), you may describe them as a. co-evolved species b. disjunct species c. exotic species d. potentially invasive species