Campaigns Against Invasive Species, part two

Happy American Wetlands Month!

One of the biggest threats to wetland ecosystems is, of course, invasive species. In last week’s post I shared a selection of videos that were produced by a variety of organizations to inform the public about invasive species. Many such videos specifically address invasives in wetlands and waterways. Here are a few of those videos.

Invasive Species of Idaho reminds you to “Clean, Drain, Dry” to avoid transporting aquatic hitchhikers:

Purple loosestrife is a “very wicked plant:”

Commander Ben vs. the Saltcedar Bandits:

Michigan’s Department of Environmental Quality urges hunters not to use Phragmites australis to make duck blinds:

More information about Phragmites by National Geographic:

Texas Parks and Wildlife and the Attack of the Zebra Mussels:

The story of Eurasian milfoil told by students at George Williams College:

Water hyacinth – another “very wicked plant”:

Water hyacinth invasion in Africa:

Attack of the Killer Algae by TED-Ed:

Campaigns Against Invasive Species, part one

I have been posting almost exclusively about invasive species for four months now. If you have made it this far, I salute you. It is neither the most exciting nor the most encouraging topic, but it is the journey I am on (for whatever reason), and I am pleased to have you along.

In the battle against invasive species, citizen awareness and participation is imperative. The public and private sectors can try as they may, but if individual citizens are acting in ways that help introduce or spread invasives, then much of this effort can be for naught. Thus, campaigns to educate the public are regularly launched.

One popular way to spread the word is through video. Often, the goal of these videos is to both educate and entertain. Some achieve this better than others, while some are downright dull or simply baffling. Speculating on the effectiveness of these videos is not the purpose of this post. Rather, I just thought I would take a break from the usual text heavy posts and share a few videos that I found interesting and/or entertaining. If you have a favorite invasive species video, please share it in the comment section below.

Invasive species explained:

Introducing Bob Noxious from Invasive Species of Idaho:

And here is the particularly creepy, Vin Vasive, from USDA APHIS:

Invaders! in British Columbia:

In Namibia, “Cacti must die!”:

Eco Sapien and the story of Japanese knotweed in the UK:

What happened when American minks, brought to Europe for the fur trade, escaped into natural areas?:

Michigan’s Department of Environmental Quality explains how invasive species spread:

Pennsylvania’s Wild Resource Conservation Program teaches kids about invasive species:

MinuteEarth‘s take on invasive species:

Also, check out these five TEDx talks:

Screening for Invasive Plants at Botanical Gardens and Arboreta

As discussed in last week’s post, many of the invasive species that we find in our natural areas were first introduced to North America via the horticulture trade. As awareness of this phenomenon grows, steps are being taken by the horticulture industry to address this issue. The concluding remarks by Sarah Reichard and Peter White in their 2001 article in BioScience describe some recommended actions. One of them involves the leadership role that botanical gardens can play by both stopping the introduction and spread of invasive species and by presenting or promoting public education programs.

Reichard and White offer North Carolina Botanical Garden as an example, citing their “Chapel Hill Challenge,” which urges botanical gardens to “do no harm to plant diversity and natural areas.” Reichard and White also encourage botanical gardens and nurseries to adopt a code of conservation ethics addressing invasive species and other conservation issues. Codes of conduct for invasive species have since been developed for the botanical garden community and are endorsed by the American Public Gardens Association.

 

Botanical gardens that adopt this code have a number of responsibilities, one of which is to “establish an invasive plant assessment procedure,” preferably one that predicts the risks of plant species that are new to the gardens. In other words, botanical gardens are encouraged to screen the plants that are currently in their collections, as well as plants that are being added, to determine whether these plants currently exhibit invasive behavior or have the potential to become invasive. Many botanical gardens now have such programs in place, and while they may not be able to predict all invasions, they are a step in the right direction.

In an article published in Weed Technology (2004), staff members at Chicago Botanic Garden (CBG) describe the process they went through to determine a screening process that would work for them. CBG has an active plant exploration program, collecting plants in Asia, Europe, and other parts of North America. Apart from adding plants to their collection, one of the goals of this program is to find plants with horticulture potential and, through their Ornamental Plant Development department, prepare these plants to be introduced to the nursery industry in the Chicago region. As their concern about invasive species has grown, CBG (guided by a robust Invasive Plant Policy) has expanded and strengthened its screening process.

In order to do this, CBG first evaluated three common weed risk assessment models. The models were modified slightly in order to adapt them to the Chicago region. Forty exotic species (20 known invasives and 20 known non-invasives) were selected for testing. Each invasive was matched with a noninvasive from the same genus, family, or growth form in order to “minimize ‘noise’ associated with phylogenetic differences.” The selected species also included an even distribution of forbs, vines, shrubs, and trees.

Weed risk assessment models are used to quickly determine the potential of a plant species to become invasive by asking a series of questions about the plant’s attributes and life history traits, as well as its native climate and geography. A plant species can be accepted, rejected, or require further evaluation depending on how the questions are answered. For example, if a plant is known to be invasive elsewhere and/or if it displays traits commonly found in other invasive species, it receives a high score and is either rejected or evaluated further. Such models offer a quick and affordable way to weed out incoming invasives; however, they are not likely to spot every potential invasive species, and they may also lead to the rejection of species that ultimately would not have become invasive.

After testing the three models, CBG settled on the IOWA-modified Reichard and Hamilton model “because it was extensively tested in a climatic zone reasonably analogous to … Illinois,” and because it is easy to use and limits the possibility of a plant being falsely accepted or rejected. The selected model was then tested on 208 plants that were collected in the Republic of Georgia. Because few details were known about some of the plants, many of the questions posed by the model could not be answered. This lead CBG to modify their model to allow for such plants to be grown out in quarantined garden plots. This way pertinent information can be gathered, such as “duration to maturity; self-compatibility; fruit type and potential methods of seed or fruit dispersal; seed production, viability, and longevity in the field; and vegetative spread.” CBG believes that evaluations such as this will help them modify their model over time and give them more confidence in their screening efforts.

More about botanical gardens and invasive species: Botanic Gardens Conservation International – Invasive Alien Species

More about weed risk assessment models: Weed Risk Assessment – A way forward or a waste of time? by Philip E. Hulme

Horticulture’s Role in the Spread of Invasive Plants

I live in the city of Boise – a bustling metropolis by Idaho’s standards. It is located in the high desert of the Intermountain Northwest in a region called the sagebrush steppe. Our summers are hot and dry, and our native flora reflects this.

When I leave my apartment I am greeted by a flowering quince (Chaenomeles sp.). At this time of year it is in full bloom and looking amazing. It originated in East Asia. To my left I see a tree of heaven (Ailanthus altissima), a common urban tree that came to America from China via Europe. To my right there is a row of Norway maples (Acer platanoides), another popular urban tree. As its common name suggests, it is a European species that is distributed across large portions of eastern and central Europe. None of these plants are native to the sagebrush steppe, nor would they survive the harsh conditions without supplemental irrigation. All are horticultural introductions.

Tree of Heaven (Ailanthus altissima) – photo credit: wikimedia commons

But there is another thing that at least two of these species have in common. Tree of heaven and Norway maple are considered invasive species in North America due to their propensity to spread into natural areas and disrupt native ecosystems. They also have a reputation of being pesky urban weeds.

My experience isn’t unique. Yards across North America are planted largely with species that are not native to this continent, and while most species stay where we plant them, a significant portion of them have leaped out of our tidy landscapes and disseminated themselves across natural areas, earning them the title invasive species.

In a paper published in BioScience (2001), Sarah Hayden Reichard and Peter White discuss the role that horticulture has played in introducing invasive species to the United States. Humans have a long history of moving plants from one part of the world to another for food, fuel, and fiber. However, collecting plants from around the world and organizing them into gardens for aesthetic purposes is, by comparison, a more recent thing. Species used for ornamental horticulture are what Reichard and White are concerned about.

As an introduction, Reichard and White offer a quick history of the beginnings of ornamental horticulture in the United States. This period is summed up well in an article by Richard Mack and Mark Lonsdale in the same issue of BioScience:

As colonists became more secure in their new environments, they began to import ornamental species from their homelands and elsewhere, in simultaneous quests for both familiar and unfamiliar plants. These plant importations sprang from deep-seated or primal aspects of human behavior shared by people in former colonies and homelands alike. … Many needed to be reassured with familiar plants from home, and they also had seemingly antithetical desires to experience novel, exotic ornamental plants.

Today, plant explorations continue throughout the world, often with the goal of introducing new plant species to the horticulture trade, and avid gardeners remain eager to find something new and interesting to add to their yards. There is nothing inherently wrong with this. Nor is there anything inherently wrong with filling our yards with exotic plants. The trouble comes when these plants escape cultivation and cause problems in neighboring ecosystems. Bringing awareness to this darker side of ornamental horticulture is what Reichard and White endeavor to do.

“Thomas Jefferson, an avid horticulturist, also introduced several species. He may have been the first person to introduce Cytisus scoparius (Scotch broom) as an ornamental species; that plant is now an invasive species in many parts of North America.” — Reichard and White (2001) [photo credit: www.eol.org]

Major players involved in the global movement of horticultural specimens include botanical gardens and arboreta, nurseries, garden clubs and horticultural societies, and the seed trade industry. The motives for transporting species vary among the groups, as do their roles in addressing the invasive species issue. Many botanical gardens have extensive plant exploration programs, which today are often more conservation focused than they were in the past; however, some of the species acquired during these explorations are released to the public, often without certainty that they won’t spread.

Even though most nurseries don’t have active plant exploration programs, they may acquire plants from nurseries or other institutions that do. For business reasons, plants may be sold before they have been properly screened for invasive-ness. Some retail nurseries make an effort to not sell plants that are known invasives in their regions. However, there are plenty of mail order nurseries that may not be aware of or may simply ignore the fact that they are shipping plants to regions where they are invasive. Seed exchanges between garden clubs and botanical societies, as well as the seed trade industry, are also responsible for shipping species to areas where they are currently or may become invasive.

“Uninformed people sometimes dump their aquarium water and plants into local water sources, and many of the aquarium plants survive and multiply. Hydrilla verticillata, a very aggressive aquatic weed in the South, was probably introduced to provide a domestic source of this plant for the aquarium trade.” — Reichard and White (2001) [photo credit: wikimedia commons]

Plant exploration will continue, and many new plants will be introduced to the public through the horticulture trade. Rules and regulations help restrict some plant movement, but in a capitalist society such restrictions will ultimately be, as Reichard and White write, “a compromise between ideal invasive plant exclusion and trade facilitation.” Plants can be screened for invasivibility, but it is difficult to know if, when, and where a species may become invasive. Furthermore, given enough time, a species that appeared to stay put can suddenly start to spread (or could have been spreading all along unnoticed).

Reichard and White acknowledge that “the burden of finding a solution to the problems posed by invasive plants does not necessarily fall on the shoulders of [the horticulture] industry.” Various groups from broad disciplines will have to come to together to work towards a solution. Reichard and White offer some suggestions for working together. For example, invasive species biologists can share their research with the horticulture industry which can, in turn, communicate this information to the public through garden writers and speakers. Botanical gardens can take a leadership role by vowing to “first do no harm to plant diversity and natural areas” and by providing public education about the issue.

Efforts can be made to ban the sale of problematic plants and to encourage proper screening of new introductions, but public demand for certain plants may remain. So, “better communication from ecologists to the public about which species are causing problems will discourage people from buying them.” Involving the public in eradication efforts can also help raise awareness, as people can see first hand that plants in their yards have invaded the wild.

When Alien Plants Invade – The Four Stages of Invasion, part two

In a review published in New Phytologist (2007), Kathleen Theoharides and Jeffrey Dukes examine four stages of invasion as they relate to alien (i.e. introduced or non-indigenous) plant species. In part one we discussed transport and colonization, in which species must survive being transported long distances and then take root and reach maturity in an unfamiliar location. Alien plant species don’t become invasive until they have reached the last two stages: establishment and landscape spread. Removal of the species upon reaching these stages is no easy task. Luckily, introduced species have a few barriers to overcome before this point.

An established population is one that is self-sustaining and expanding. Environmental conditions may be a limiting factor, as they were during colonization, but the main constraints at this stage are “biotic filters.” Theoharides and Dukes define these as “barriers to invasion created by the actions or presence of living organisms.” Through competition for various resources, as well as herbivory and disease, neighboring organisms affect the survival, growth, and reproduction of introduced plant species. Thus, “traits that enhance competitive performance, reduce niche overlap between [introduced species] and natives, or increase enemy resistance may be most important during establishment… Other advantageous traits include secondary chemical compounds that deter herbivores, ‘novel weapons’ such as root exudates that negatively impact other plants, fast growth, and high fecundity.”

Plants compete for light, moisture, and soil nutrients, as well as for pollinators and seed dispersers. Competition inhibits the establishment of invaders when neighboring plants consume available resources more efficiently. Introduced plants risk being outcompeted by plants that are of the same functional type (plants that are “morphologically, phenologically, and physiologically similar”). They also risk competition by a single dominant species (or group of similar species) or by “an assemblage of species with different traits.” As a general rule, plant communities with greater diversity are more resistant to invasion.

“In forests of the northeastern USA, Alliaria petiolata, an herbaceous mustard species, contains a type of phytotoxic glucosinolate that appears to disrupt the mutualism between arbuscular mycorrhizal fungi and hardwood canopy trees.” – – Theoharides and Dukes (2007) [photo credit: eol.org]

Two hypotheses postulate the success of some plant invaders in establishing themselves: the enemy release hypothesis and the evolution of increased competitive ability hypothesis. In the first hypothesis, plant species – having been removed from their native habitat – are freed from their natural enemies and are thus able to allocate more resources to growth and reproduction. The second hypothesis states that, in light of “reduced enemy pressure,” introduced species quickly evolve to allocate resources “from enemy defense to faster growth.” Escape from herbivory and diseases, however, is likely not the only factor in the success of invaders, and much still depends on the competitiveness of the plant and the availability of key resources.

After introduced plants become established, a lag phase generally occurs before landscape spread. This can be a result of a lack of genetic variation, a dearth of suitable habitat, unfavorable environmental conditions, or some combination of the three. New introductions may occur, and the population may continue to adapt and expand. Suitable habitat may be made available, and environmental conditions may shift. In time, landscape spread becomes a possibility.

Landscape spread occurs when multiple populations of a species are connected via long-distance dispersal. At this “metacommunity scale,” populations of an introduced plant species interact across a large area, with each population in a different stage of colonization and establishment. This means that transport, colonization, and establishment are all at play during the landscape spread stage.

Abutilon theophrasti (velvetleaf) was originally introduced before 1700 in the USA. This species has only recently become an aggressive invader as a result of the evolution of different life-history strategies based on the nature of competition in its new environment.” — Theoharides and Dukes (2007) [photo credit: wikimedia commons]

Dispersal ability and habitat connectivity are key factors in determining the success of an introduced plant species during landscape spread. Long-distance dispersal can occur via wind, water, or animals. Species that depend on animals to spread their seeds rely on specific animals to be present. The seeds of Prunus serotina (black cherry), for example, are dispersed by birds. So, landscape spread is reliant on birds and “roosting trees” where the birds can perch and defacate the seeds. In many cases, “humans also play a large role in intraregional dispersal.”

Habitats vary across the landscape due to a combination of numerous geological and biological processes. The disturbance regime – “the frequency, spatial extent, severity, and intensity of killing events over time” – also helps determine landscape patterns. Natural disturbances, such as fire, weather, and natural disasters, are differentiated from disturbances caused by human activity. Large scale development and disturbance of natural areas by humans disrupts the natural disturbance regime and alters historical landscape patterns. As the authors write, “alterations of the disturbance regime that increase resource availability or change landscape patterns can promote non-indigenous plant species spread by creating favorable patches for colonization and establishment.”

Fragmented landscapes consisting of small patches of natural areas dispersed among large areas of human development are particularly prone to invasion by introduced plant species for many reasons, including increased influx of propagules and a high degree of edge effects (habitat edges have environmental conditions that are generally more prone to invasion than habitat interiors).

Habitat patches can be connected via corridors. It is through these corridors that dispersal can occur between populations in a metacommunity. Corridors connect populations of both introduced and native plant species. However, “native plants often require wide undisturbed corridors of intact habitat, while [introduced plant species] may disperse best through strips of human-disturbed habitat or ‘disturbance corridors.'” The environmental conditions in disturbance corridors and the presence of dispersal agents (including humans and domesticated animals) help facilitate the connectivity of populations of introduced plant species and promote the colonization and establishment of new populations.

In their abstract, Theoharides and Dukes write, “both research and management programs may benefit from employing multiscale and stage approaches to studying and controlling invasion.” With their conclusion they provide a list of potential management strategies for each stage, and they advise employing “natural filters in order to prevent invasion succees.” Examples include reducing habitat fragmentation and edge effects, promoting intact native communities, reducing human disturbances, promoting natural disturbance regimes, and minimizing disturbance corridors.

More Posts about Invasive Species:

When Alien Plants Invade – The Four Stages of Invasion, part one

As humans move around the globe, they are regularly accompanied by plants. Some plant species are intentional guests, while others are interlopers. This steady movement of plants from one region to another results in plants being introduced to areas where they are not native. In this regard, they are aliens. Some of these alien species will take up permanent residence and, as a result, can disrupt ecosystems, compete with native plant species, and cause economic damage. This earns them the title “invasive”. But not all introduced plant species achieve this. In fact, many will find themselves in a new region but will be unable to colonize. Others will colonize but not become fully established. Still others become established but will not spread. In all cases there are factors at play that either aid or limit an introduced plant species in becoming invasive.

In a review published in New Phytologist (2007), Kathleen Theoharides and Jeffrey Dukes examine four stages of invasion (transport, colonization, establishment, and landscape spread) and some of the “filters” that occur in each stage that help determine whether or not an alien plant species will become invasive. In their introduction they clarify, “these stages are not discrete, and filters will likely affect more than one stage,” but by analyzing each of the stages we can better determine how and why some introduced species are successful at becoming invasive while others are not. Generalities derived from this investigation can “be used to predict the outcome of invasion events, or to explore mechanisms responsible for deviations from these generalizations.”

Theoharides, K. A. and Dukes, J. S. (2007), Plant invasion across space and time: factors affecting nonindigenous species success during four stages of invasion. New Phytologist, 176: 256–273. doi:10.1111/j.1469-8137.2007.02207.x

In part one, we will look at the first two stages of invasion: transport and colonization.

Species have always moved around from region to region by various means. However, as Theoharides and Dukes write, “current species movements are happening faster than before and from more distant regions, primarily as a result of global commerce and travel.” When it comes to human-mediated dispersal, many plants may never be transported by humans, while others simply won’t survive the journey. Species that are widespread may have a better chance of being transported because they are more likely to make contact with humans. Transporting high numbers of propagules (i.e. seeds, spores, cuttings) generally increases the likelihood that a species will survive the journey.

The invasion of Phalaris arundinacea (reed canary grass) was facilitated by multiple introduction events from a variety of sources within the native European range.” — Theoharides, K. A. and Dukes, J. S. (2007) [photo credit: eol.org]

Plants are transported by humans for many reasons. Sometimes its accidental, but often it is purposeful for either utilitarian or aesthetic purposes. Plants provide us with food, fuel, forage, building materials, clothing, and medicine. Over millennia, we have selected suites of species that are ideal for such purposes, and we have carried them with us into new regions or brought them home from other parts of the world. Not all of these species are well-behaved in their new homes, and many have become invasive. These species are given an advantage because they have been selected for traits like cold hardiness, disease resistance, and high yield. When they are transported, they are brought to locations with similar climates. “Climate matching, combined with intentional cultivation, greatly increases the likelihood that [these] species will escape cultivation.”

Surviving transportation is not a guarantee that alien plants will successfully colonize a new area. Myriad environmental conditions and biological processes stand in their way. Much depends on propagule pressure – “the combined measure of the number of individuals reaching a new area in any one release event and the number of discrete release events.” Where propagule pressure is high, colonization is more likely. Repeated introductions across a large area offer the species a greater chance of finding itself in a suitable location as well as a greater level of genetic variation. Disturbed environments with less competitors and increased resources (i.e. light, moisture, soil nutrients) are often easier to colonize than locations with a high level of biodiversity and fewer available resources.

“Populations of Salix babylonica (weeping willow) in New Zealand may have invaded from a single cutting.” — Theoharides, K. A. and Dukes, J. S. (2007) [photo credit: eol.org]

Climate is one of the main filters of colonization, yet plant species have still managed to colonize regions with very different climates compared to what they’re used to, while other plant species have been unsuccessful in colonizing regions with similar climates. Plant species that originate from wide geographic ranges tend to have “broader climatic tolerances” – a trait that along with phenotypic plasticity and a high level of genetic variability can enable a species to adapt to new and challenging environments. Other advantageous traits include “fast growth, self-compatibility, a short juvenile period, and seeds that germinate without a pre-treatment.”

If and when colonization is achieved, establishment is no guarantee. “In order for a plant to establish itself it must continue to increase from low density over the long term.” Small numbers of plants may successfully reproduce, but environmental factors, genetic issues, and biological competition may still stand in their way. Species that invade disturbed sites where resource availability is temporarily high, may soon find themselves in a resource-limited situation. As a result, their populations may dwindle.

With transport and colonization accomplished, establishment is the next goal. Establishment and landscape spread will be covered in part two.

Learning Lessons from Invaded Forests

In 1946, North American beavers were introduced to the archipelago of Tierra del Fuego at the tip of South America in an attempt to start an industry based on beaver fur. Although this industry has not thrived, beavers have multiplied enormously. By cutting trees and building dams, they have transformed forests into meadows and also fostered the spread of introduced ground cover plants. Now numbering in the tens of thousands in both Chilean and Argentinian parts of the archipelago, beavers are the target of a binational campaign to prevent them from spreading to the mainland of these two nations. — Invasive Species: What Everyone Should Know by Daniel Simberloff

Beavers in South America are just one example of the series of effects a species can have when it is placed in a new environment. Prior to the arrival of beavers, there were no species in the area that were functionally equivalent. Thus, through their felling of trees and damning of streams, the beavers introduced novel disturbances that have, among other things, aided the spread of non-native plant species. Ecologists call this an invasional meltdown, wherein invasion by one organism aids the invasion of another, making restoration that much more difficult.

Complicated interactions like this are explored by David Wardle and Duane Peltzer in a paper published last month in Biological Invasions. Organisms from all walks of life have been introduced to forests around the world, and while many introductions have had no discernable impact, others have had significant effects both above and below ground.

The authors selected forest ecosystems for their investigation because “the imprint of different invaders on long-lived tree species can often be observed directly,” even when the invading organisms are doing their work below ground. Moreover, a greater understanding of “the causes and consequences of invasions is essential for reliably predicting large-scale and long-term changes” in forest ecosystems. Forests do not regenerate quickly, so protecting them from major disturbances is important. Learning how forests respond to invasions can teach us how best to address the situation when it occurs.

The authors begin by introducing the various groups of organisms that invade forests and the potential impacts they can have. This is summarized in the graphic below. One main takeaway is that the effects of introduced species vary dramatically depending on their specific attributes or traits and where they fall within the food chain. If, like the beaver, a novel trait is introduced, “interactions between the various aboveground and belowground components, and ultimately the functioning of the ecosystem” can be significantly altered.

Wardle, D.A., and D.A. Peltzer. Impacts of invasive biota in forest ecosystems in an aboveground-belowground context. Biological Invasions (2017).  doi:10.1007/s10530-017-1372-x

After highlighting some of the impacts that invasive species can have above and below ground, the authors discuss basic tenets of invasion biology as they relate to forest ecosystems. Certain ecosystems are more vulnerable to invasions than others, and it is important to understand why. One hypothesis is that ecosystems with a high level of species diversity are more resistant to invasion than those with low species diversity. This is called biotic resistance.  When it comes to introduced plants, soil properties and other environmental factors come in to play. One species of plant may be highly invasive in one forested ecosystem, but completely unsuccessful in another. The combination of factors that help determine this are worth further exploration.

When it comes to restoring invaded forests, simply eliminating invasive species is not always enough. Because of the ecological impacts they can have above and below ground, “invader legacy effects” may persist. As the authors write, this requires “additional interventions to reduce or remove [an invader’s] legacy.” Care also has to be taken to avoid secondary invasions, because as one invasive species is removed another can take its place.

Nitrogen-fixing plants (which, as the authors explain, “feature disproportionately in invasive floras”) offer a prime example of invader legacy effects. Introducing them to forest ecosystems that lack plants with nitrogen fixing capabilities “leads to substantially greater inputs of nitrogen … and enhanced soil fertility.” Native organisms – decomposer and producer alike – are affected. Simply removing the nitrogen fixing plants does not at once remove the legacy they have left. Examples include Morella faya invasions of forest understories in Hawaii and invasions by Acacia species in South Africa and beyond.

“It has been shown that co-invasion by earthworms enhances the effects that the invasive nitrogen fixing shrub Morella faya has on nitrogen accretion and cycling in a Hawaiian forest, by enhancing burial of nitrogen-rich litter.” – D.A. Wardle and D.A. Peltzer (2017) – photo credit: wikimedia commons

The authors conclude with a list of “unresolved issues” for future research. A common theme among at least a couple of their issues is the need for observing invasive species and invaded environments over a long period. Impacts of invasive species tend to “vary across both time and space,” and it is important to be able to predict “whether impacts are likely to amplify or dampen over time.” In short, “focus should shift from resolving the effects of individual invasive species to a broader consideration of their longer term ecosystem effects.”

This paper does not introduce new findings, but it is a decent overview of invasion biology and is worth reading if you are interested in familiarizing yourself with some of the general concepts and hypotheses. It’s also open access, which is a plus. One thing that is clear after reading this is that despite our growing awareness of the impacts of invasive species, there is still much to be learned, particularly regarding how best to respond to them.

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