{"id":2130,"date":"2013-01-16T13:30:12","date_gmt":"2013-01-16T18:30:12","guid":{"rendered":"http:\/\/www.thenatureofcities.com\/?p=2130"},"modified":"2015-06-01T14:48:50","modified_gmt":"2015-06-01T18:48:50","slug":"our-changing-urban-nature-time-to-embrace-certain-exotic-species","status":"publish","type":"post","link":"https:\/\/www.thenatureofcities.com\/TNOC\/2013\/01\/16\/our-changing-urban-nature-time-to-embrace-certain-exotic-species\/","title":{"rendered":"Our Changing Urban Nature: Time to Embrace Exotic Species? (Or at Least Some of Them)"},"content":{"rendered":"

Cities are melting pots.\u00a0 I expect we understand this metaphor best as it relates to human beings. Cities around the world grow because people keep moving into them.\u00a0 People move from nearby rural areas, from other regions in the same country, or from around the world.\u00a0 When they arrive, they bring their skills, their beliefs, their music, their food, their culture\u2026and we end up with the beautiful mosaic of urban life. \u00a0The increased movement of people and goods around the world, especially with increasingly rapid modes of transportation, is the primary way that plants, animals, and microbes get around too \u2013 either as treasured reminders of home or as hitchhikers.\u00a0 So cities are melting pots of all kinds of life.<\/p>\n

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Violet in a sidewalk crack. Photo: (c) Parushin www.fotosearch.com<\/figcaption><\/figure>\n

Some of the new arrivals (the people and all the other organisms) are welcomed with open arms.\u00a0 But many are greeted warily or with open hostility.\u00a0 Family narratives and history books are full of stories about about the hard times that waves of human immigrants found in their new homes.\u00a0 Non-human immigrants \u2013 often referred to as exotic, introduced, or non-native species \u2013 have received a similarly cold welcome in most places.\u00a0 But the movement of plants and animals across the planet is both ancient and inevitable.\u00a0 It is also increasingly difficult to control. \u00a0We face choices about which movements to resist and which to allow or perhaps even encourage.\u00a0 We also have the opportunity to plan as well as we can to get the most from our changing ecosystems.<\/p>\n

Historical context for biological migration<\/strong><\/p>\n

In the pre-human past, evidence of how organisms moved around the earth is inferred from information stored in fossils (and in genes, though I won\u2019t get into that here).\u00a0 The big charismatic fossils \u2013 the dinosaur skeletons<\/a>, preserved leaves<\/a> \u2013 are well known, but the treasure trove of information about changing distributions comes from the microfossils.\u00a0 Microfossils<\/a> include preserved pollen grains, bacteria, foraminifera, diatoms, and other small things.\u00a0 They tell us not only which organisms lived where in the distant past, but they are useful in reconstructions of ancient climate, dating geological events, and other kinds of historical inference.\u00a0 For example, microfossils preserved in wetland and lake sediments can tell us how terrestrial ecosystems shift as glaciers advance and retreat.<\/p>\n

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Clematis pollen. Photo: (c) STEVE GSCHMEISSNER \/ www.fotosearch.com<\/figcaption><\/figure>\n

For the movement of organisms throughout human pre-history we rely on the field of environmental archaeology<\/a>. \u00a0We can trace how crops followed people from their centers of origin around the world, we can trace which plant and animal species were important to which groups of people, and we can observe the origin of new kinds of life resulting from domestication \u2013 the hard work of generation upon generation of farmers.<\/p>\n

As we get into the historical era, and the origin of cities, we have written records of the migration of people and their associates around the world. \u00a0For example, ancient Greek and Chinese texts discuss the relationships between people and both domestic and wild animals and their movements across the landscape.\u00a0 Another interesting source of historical information about the distributions of organisms are the biological collections in natural history museums and botanical gardens around the world.\u00a0 The oldest of these institutions have collections spanning several hundred years and \u2013 when studied collectively \u2013 give a remarkably vivid picture of our changing biological landscape.<\/p>\n

The reason I give this brief history lesson is to provide some context to what has happened since the middle of the last century.\u00a0 With urbanization well underway by the late nineteenth and early twentieth century and with long-distance transit links becoming more frequent and much faster, people and other organisms began to move like never before in the history of the planet. \u00a0We know from all the sources of evidence that I just outlined that plants and animals have always been moving around \u2013 but fast ships, trains, and airplanes and booming international commerce represented a quantum leap in biological migration.\u00a0 Farmers noticed a greater frequency of new weeds and pathogens, foresters and other natural resource professionals noticed an accelerated spread of new plants and animals, and scientists and conservationists wondered what this meant for the organisms in the lands and waters receiving these newcomers. \u00a0A watershed moment in the study of this movement was a book by the British ecologist Charles Elton<\/a>, the Ecology of Invasions by Animals and Plants<\/a>, published in 1958.<\/p>\n

Modern perspectives on exotic species<\/strong><\/p>\n

Elton\u2019s book not only started a new field of science called invasion ecology, it also shifted the language about this phenomenon in a militaristic direction<\/a>. \u00a0These new arrivals were \u201cinvaders\u201d that would do harm to the receiving communities.\u00a0 The modern discourse in invasion biology frequently invokes military terminology: enemies battle each other, managers mount eradication efforts, ecosystems become overrun.<\/p>\n

This may motivate action against non-native species to potentially good effect such as recruiting volunteers to manage parkland<\/a>, but it also runs the risk of alienating some portion of the public.\u00a0 Some will find the talk of war distasteful, but others may question why scientists and managers are using violent terminology (and actual violence) against wild animals and plants.\u00a0 For people who take a \u201clive and let live\u201d attitude towards wildlife, efforts against exotic species can be construed as xenophobic efforts against nature.<\/p>\n

Another serious issue with this language is that it suggests a war, but in many cases that war will not be won.\u00a0 Successful efforts to manage invasive species are almost always chronic rather than episodic.\u00a0 The best outcome for the manager is usually to keep the invader at bay; it is never really defeated. \u00a0Cessation of the management effort will generally lead to reinvasion, unless all individuals of the invading species, including dormant propagules, are removed or there is a change in the environment that disfavors the invader. \u00a0In cases where outright victory is unlikely (which is most of them), language about fighting to put things back the way they were is probably less useful than language about adapting to a changed environment.<\/p>\n

Another challenge with the militaristic language on invasions is that it has led to negative feelings about introduced species generally among much of the public.\u00a0 Even my environmentally-aware students and neighbors often seem disappointed when I tell them that the beautiful plant they were admiring is from some other part of the world. These negative feelings are often misplaced \u2013 the vast majority of new introductions don\u2019t lead to any serious environmental damage. \u00a0Scientists and practitioners know this and are focused on the exotics that cause the most damage \u2013 but a less well-informed public concerned about nature my tend to associate all exotics with damage.<\/p>\n

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A grass flowering next to an abandoned building\u00a0 Credit: CasaDeQueso from flickr.com<\/figcaption><\/figure>\n

Positive contributions of exotic species\u00a0<\/strong><\/p>\n

Many of these introduced species thrive in habitats with frequent human disturbance \u2013 like so many areas in cities.\u00a0 They may live side-by-side with indigenous species that also tolerate the disturbance, but their presence isn\u2019t meaningfully detracting from their neighbors.\u00a0 And these new arrivals may be adding something \u2013 a bit of shade, an extra splash of green or color in a concrete landscape, a morsel for the next link in the food chain.\u00a0 To anthropomorphize a bit, they are newcomers that are willing to settle down in rough neighborhood and do their part to improve the community.\u00a0 There\u2019s something beautiful about life grabbing hold and bearing fruit in the crack of a sidewalk.<\/p>\n

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A zebra mussel-encrusted current meter near Michigan City, IN. Lake Michigan, June 1999.
Photo: NOAA <\/a><\/figcaption><\/figure>\n

The motivation behind both the military language and the negative public perceptions comes from examples of some truly damaging invasions.\u00a0 Zebra mussels <\/a>in North American lakes have added hundreds of millions of dollars to the cost of managing freshwater infrastructure, introduced rats on islands <\/a>have driven many kinds of birds extinct, introduced woody plants like Myrica<\/em> in Hawaii <\/a>and pines in the South African fynbos <\/a>have fundamentally changed those ecosystems, and introduced pathogens have devastated populations of plants, wildlife, and people throughout history.\u00a0 These invaders have compromised critical functions of the invaded ecosystems, eliminated other species, and borne tremendous costs to human communities.<\/p>\n

I don\u2019t think anyone can win an argument that says all exotic species are ok.\u00a0 However, the relatively small number of clearly damaging examples has led to a bias against non-native species generally and to some misguided efforts at controlling dubiously harmful species.<\/p>\n

A group of scientists, many of whom had studied invasive species for much of their careers, published a short paper in Nature<\/em> in 2011<\/a> urging conservationists to shift their emphasis from where a species originated (native vs. exotic) to the specific functions of those species in their new habitats.\u00a0 The authors make the point that the world\u2019s ecosystems are changing rapidly in response to climate change, altered nutrient inputs, and urbanization, so the idea that non-native species should be managed to protect status quo<\/em> communities of native species is increasingly obsolete.\u00a0 Many invasive species management programs are very expensive and \u2013 in the long run \u2013 unlikely to succeed.\u00a0 Not only is it nearly impossible to \u201cput the genie back in the bottle\u201d, but species composition will shift with environmental changes, just as it always has.<\/p>\n

This paper generated significant opposition from the conservation community and from natural resource professionals who had personal experience managing damaging invasions.\u00a0 Many of the counter-points from this community are also quite valid \u2013 these professionals do focus on the damaging species rather than lumping all non-natives together, we have an obligation to attempt to stop extinction from damages wrought by humans, and many invasive-species management projects have been successful, at least at local scales and over short time periods.<\/p>\n

Exotics in the city<\/strong><\/p>\n

The broader point I take from this debate for cities is that we should engage in a critical discussion about our goals in managing urban vegetation and wildlife.\u00a0 Cities already represent a significant change from the indigenous landscape \u2013 altered land cover, climate, hydrology, chemistry, soils, disturbance regimes, and a suite of other factors.\u00a0 And as much as the physical environment of cities has already changed, in most parts of the world it will change even more \u2013 and possibly more rapidly \u2013 as climate change affects the timing and intensity of storms, sea levels rise, and growing urban populations increase demands for food, water, and energy.\u00a0 Add to all this physical change the immigration of new plants and animals connected by global trade and human migration and you realize we are facing a future where it would be foolish to believe that our urban environments should look or function exactly like they have in the past.<\/p>\n

This is not to say that I think we should give up on preserving native species or managing for native-dominated systems in urban landscapes. \u00a0There are many good reasons to favor natives where feasible:\u00a0native species may provide goods or services that we value more than exotics, species that have coexisted for long periods form complex networks that be more stable or higher functioning, we have an ethical responsibility to care for the land and its inhabitants, and there is real value the connection between people and familiar environments. \u00a0Even in cases where it is hard work to protect native species, the benefits may outweigh the costs.<\/p>\n

However, this calculus isn\u2019t always easy.<\/p>\n

Two examples from New York City<\/strong><\/p>\n

In New York City, one of the major invasive control programs involves removing exotic vines, largely porcelain berry (Ampelopsis brevipedunculata<\/em><\/a>) and Oriental bittersweet (Celastrus orbiculatus<\/em><\/a>), from forested parkland.\u00a0 These vines, both from temperate Asia, grow vigorously in canopy gaps and prevent the recruitment and growth of trees that would, in the absence of the vines, grow up and close the gap.\u00a0 Forest ecologists from the NYC Department of Parks and Recreation\u00a0Natural Resources Group <\/a>have mapped vine-dominated canopy gaps through time and found that forest succession is arrested by the presence of abundant exotic vines: vine-dominated patches tends to stay vine-dominated patches.<\/p>\n

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A canopy gap dominated by exotic vines in New York City. Photo: Tim Wenskus<\/figcaption><\/figure>\n

As part of a citywide effort to add forest canopy<\/a>, Parks has invested significant time and money to remove these vines and plant young trees.\u00a0 They manage these reforestation sites for years after the tree planting, primarily by removing vines that have regrown.\u00a0 Their expectation (and hope) is that some of the young trees will grow and close the canopy gap, reduce light to the understory, and inhibit the regrowth of the vines.\u00a0 Over time, urban forest canopy will increase, light and soil resources will be captured by trees and thus be unavailable to the vines, and Parks can scale back their maintenance.\u00a0 This intervention could shift the system from vine-dominated patches to tree-dominated patches and improve some services: more carbon storage, better stormwater management, and improved visitor access in the closed-canopy forest, but with some loss of the fruit resource the vines provided to wildlife. \u00a0On the whole, this would improve the parks and could be a case of effective long-term management of exotic species.<\/p>\n

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A stand of Japanese knotweed along the Bronx River, New York City. Photo by Matt Palmer<\/figcaption><\/figure>\n

As a second example, Japanese knotweed (Fallopia japonica<\/em><\/a>, synonyms Reynoutria japonica<\/em> and Polygonum cuspidatum<\/em>) is an herbaceous perennial originally from Asia and now common to riparian wetlands, roadsides, and waste places throughout New York City.\u00a0 Knotweed can grow in very dense stands that displace other kinds of vegetation \u2013 a stand of knotweed is often just knotweed.\u00a0 The banks of the Bronx River support very large populations of knotweed and other exotic species which the NYC Parks department is beginning to manage<\/a>.\u00a0 The primary goal of this management is to improve conditions for native plants and wildlife in and along the river. They accomplish this through the physical removal of knotweed (excavating rhizomes from soil \u2013 backbreaking work) or by spraying with herbicide.\u00a0 Removal is followed by replanting native shrubs and herbaceous plants.\u00a0 The expected return for this effort is the recovery of high diversity native vegetation, which will support both terrestrial and aquatic wildlife<\/a>.<\/p>\n

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Underneath a stand of Japanese knotweed \u2013 more knotweed and knotweed litter. Photo: Jacoba Charles.<\/figcaption><\/figure>\n

Managing the knotweed long term will be challenging.\u00a0 It spreads quickly by rhizomes and fragments and \u2013 unlike the vine example above \u2013 it is not clear that the establishment of native vegetation will prevent the reinvasion of knotweed.\u00a0 To maintain diverse native communities in this park may require a long-term commitment to removing knotweed.\u00a0 The protection of native plants and wildlife in high-value sites like Bronx River may be worth the effort, but I have lately begun to wonder about the broader set of functions that knotweed may provide.\u00a0 It can grow rapidly in a range of habitats and thrives in poor soils.\u00a0 Stands of knotweed are productive and the roots and rhizomes secure soil on slopes, likely preventing erosion and perhaps trapping sediment from floodwaters.\u00a0 Honeybees collect pollen from its abundant flowers and the young shoots are edible.\u00a0 Research on the ecosystem effects of knotweed invasion in Europe is mixed, showing some positive, some neutral, and some negative changes relative to non-invaded stands.<\/p>\n

When knotweed is displacing high-value riparian vegetation, perhaps we should manage it intensively to protect native biodiversity.\u00a0 But when it is growing on marginal lands and the costs of invasion are lower, perhaps these benefits outweigh the costs.\u00a0 There is so much knotweed in New York City there\u2019s no way we could effectively manage it all, but perhaps we should look at though a utilitarian lens rather than focusing on it\u2019s geographic origin.<\/p>\n

Reconciling with exotic species<\/strong><\/p>\n

The realization that cities are experiencing rapid environmental and biotic change should be forefront in our minds as we choose targets for the living infrastructure of our cities. \u00a0Which biological invasions should we manage and which should we just allow to proceed?\u00a0 For those invasions that we choose to manage, how will know we have succeeded?\u00a0 Or when will we decide to stop trying?\u00a0 When planning a greening program, what is our target in terms of ecosystem structure and function? \u00a0What suite of species, both native and non-native, will get us to that goal?\u00a0 When choosing species for green infrastructure, are we choosing species that will do well in the in city in 2030? \u00a0What about the city in 2100?<\/p>\n

At some level, I am disappointed by the realization of all this change. I like<\/em> the indigenous biological communities of the region where I live. But I also realize that the ecosystems that will replace the preceding system will have their own appeal \u2013 both aesthetically and functionally.\u00a0 I take some hope in the imagery from a post by Stephanie Pincetl<\/a> last summer about the riot of trees from around the world growing in Los Angeles, which sounds pretty great.\u00a0 The recent post by Eric Sanderson<\/a> about learning from (native) species about resilience to storms reminds me that the new systems will still be based on the same template as the historic system.\u00a0 Exotic species only rarely result in massive changes; the more common result is an iterative revision to a functioning system \u2013 forests still grow and rivers still run. \u00a0And perhaps the next version of the system will even benefit from the new arrival in the melting pot.<\/p>\n

Matt Palmer
\nNew York City<\/p>\n","protected":false},"excerpt":{"rendered":"

Cities are melting pots.\u00a0 I expect we understand this metaphor best as it relates to human beings. Cities around the world grow because people keep moving into them.\u00a0 People move from nearby rural areas, from other regions in the same country, or from around the world.\u00a0 When they arrive, they bring their skills, their beliefs, […]<\/p>\n","protected":false},"author":11,"featured_media":6934,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[273,297],"tags":[401,30,29],"coauthors":[157],"class_list":["post-2130","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-essay","category-essay-science-and-tools","tag-biodiversity","tag-invasive-species","tag-what-is-urban-nature"],"_links":{"self":[{"href":"https:\/\/www.thenatureofcities.com\/TNOC\/wp-json\/wp\/v2\/posts\/2130","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.thenatureofcities.com\/TNOC\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.thenatureofcities.com\/TNOC\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.thenatureofcities.com\/TNOC\/wp-json\/wp\/v2\/users\/11"}],"replies":[{"embeddable":true,"href":"https:\/\/www.thenatureofcities.com\/TNOC\/wp-json\/wp\/v2\/comments?post=2130"}],"version-history":[{"count":0,"href":"https:\/\/www.thenatureofcities.com\/TNOC\/wp-json\/wp\/v2\/posts\/2130\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.thenatureofcities.com\/TNOC\/wp-json\/wp\/v2\/media\/6934"}],"wp:attachment":[{"href":"https:\/\/www.thenatureofcities.com\/TNOC\/wp-json\/wp\/v2\/media?parent=2130"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.thenatureofcities.com\/TNOC\/wp-json\/wp\/v2\/categories?post=2130"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.thenatureofcities.com\/TNOC\/wp-json\/wp\/v2\/tags?post=2130"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.thenatureofcities.com\/TNOC\/wp-json\/wp\/v2\/coauthors?post=2130"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}