CHAPTER 34: INVASIVE SPECIES AND ECOLOGICAL RESTORATION
J. C. Dudney1,†, L. M. Hallett1, E. N. Spotswood1, and K. N. Suding2
1Department of Environmental Science Policy and Management, University of California at Berkeley,
Berkeley, CA 94720
2Department of Ecology and Evolutionary Biology and Institute of Arctic and Alpine Research,
University of Colorado, Boulder, CO 80309
†Corresponding author email: firstname.lastname@example.org
Exotic species invasions have dramatically increased over the past century (Pyšek and Richardson 2010),
and are considered one of the greatest threats to endangered and vulnerable species worldwide
(D’Antonio and Vitousek 1992). Some exotic species that become invasive in a new geographic range
severely disrupt reproductive mutualisms (Traveset and Richardson 2006), reduce ecosystem services
(Peltzer et al. 2010), and decrease biodiversity (Gaertner et al., 2009). Although conservationists and
restoration ecologists typically prioritize invasive species management, several issues preclude success. In
many settings, it is impractical if not impossible to restore systems to pre-disturbed states, especially with
increasing external forcings, such as climate change, development, and nitrogen eutrophication (Seastedt
et al. 2008). There is also growing evidence that exotic species can serve important functional roles within
ecosystems (Davis et al. 2011). Given these constraints, it is becoming increasingly difficult to develop
practical invasive species restoration goals.
A number of conceptual frameworks for invasive species control have been proposed, including
integrated pest management that emphasizes the use of multiple control methods (Flint 2012), adaptive
management for broader conservation planning, as well as approaches that focus on managing multiple
species and multiple landscapes simultaneously (Green et al. 2013). An increasingly common theme is to
integrate monetary value with other ecological and social information (Larson et al. 2011) by viewing
invasive species management within an ecosystem services or biodiversity offsetting context. In this
chapter, we focus specially on invasive species management within a restoration setting, highlighting
important factors to consider at the site level (Figure 1). Throughout the three main stages of management
– assessment, prioritization, and control – we review key factors and present case studies to illustrate
practical implementations of each stage. We conclude with a discussion of invasive species management
The assessment stage is a helpful precursor of invasive species prioritization for restoration sites. Often
successful management outcomes are contingent on strategic goal setting, using detailed, site-specific
information about the social and ecological constraints on the system. For example, controlling invasive
species is very costly (Larson et al. 2011), and management plans benefit from comprehensive budgeting,
even when it changes project goals to incorporate follow-up species control and monitoring. In addition,
the public may respond negatively to removal efforts of exotic species, especially if they are culturally
valued (Shackelford et al. 2013). Expanding local partnerships and developing public outreach programs
are costly, but often important components of control efforts. Involving the public can also increase a
project’s sustainability, especially when ongoing management actions are limited financially. Finally,
recognizing ecological knowledge gaps at the site helps managers decide whether site-focused field
experiments or additional research are needed.
Taking a step back and considering the state of the restoration site first before examining the individual
invasive species can also improve long-term management strategies (Hobbs and Humphries 1995).
Gathering information about the current or historic cause of habitat degradation, the number and
distribution of invasive species at the site, and the interaction of the restoration site within the surrounding
landscapes can strengthen goal setting. As managers begin to compile this information, it may be possible
to place the site along a continuum from historical to altered (after Hobbs et al. 2009) (Figure 2).
Although often applied more broadly to restoration sites, we present one possible framework across this
continuum specifically focused on invasive species that could help simplify the often-daunting task of
invasive species prioritization (Figure 2).
1.1 Historical site. Historical sites are devoid of exotic or invasive species and the structure and function
of the ecosystem reflect prior conditions. For example, fire frequencies match historical records, trophic
levels are unaltered, and/or human impacts, such as road construction or nitrogen deposition, are
nonexistent or very limited. Keeping historical sites free of exotic species is the primary management goal
and conducting effective weed risk analysis and large-scale monitoring plans for early detection can be
effective management strategies. Few places, if any, on earth remain free of human influence, but those
that retain their historical heritage may persist with effective management. Such ecosystems are typically
remote and/or high in elevation. The Atacama Desert in Chile, for instance, is still considered relatively
undisturbed and exotic species free, though scientists predict that increased CO2 will change the abiotic
environment and encourage exotic plant invasions in deserts (Smith et al. 2000). Because invasive species
are spreading rapidly worldwide, emphasizing management of more pristine areas may be an important
goal for conservation and restoration in the future.
1.2 Intact site. Within an intact site, the vast majority of the biota is native and the structure and function
of the system remains unaltered. Natural disturbance regimes may have shifted, such as fire frequencies
and nitrogen deposition, but these anthropogenic changes have not severely disrupted the biota or
ecosystem processes. Traditional restoration goals of rehabilitating historic biodiversity can be feasible, as
the number of different invaders on the landscape is limited compared to more degraded sites. Redwood
forests in northern California are an example of intact ecosystems. Though fire regimes have been altered
and human traffic is on the rise, only a few exotic species threaten the landscape, such as New Zealand
mudsnails, Potamopyrgus antipodarum, and barred owls, Strix varia.
In an intact system, it may be realistic to curtail the spread of all invasive species to some degree. Alpine
zones, for example, are characterized by rapidly changing, steep environmental gradients. As a result,
they have dramatically reduced rates of species invasion compared to lower elevations (Baret et al. 2006).
However, propagule pressure from lower elevations, continued human disturbance, and climate change
will likely facilitate species invasions in the future (Pauchard and Alaback, 2004). Proactive management
to keep exotic species from spreading into higher elevations can be more cost-effective compared to post-
invasion control. Wilderness-wide management plans may still be impractical, however, and carefully
targeting areas where invasive species are likely to spread may be more feasible.
1.3 Modified site. A modified site is one that contains a mixture of invasive and native species, though
retains much of its original structure and function. These sites are typically influenced by human
disturbance, such as urbanization and globalization, and restoration is generally more costly or in some
cases impossible. For example, the Great Lakes of North America have been invaded by over 180 exotic
species within the last two centuries, some of which endanger underwater flora communities as well as
commercial fisheries (Vander Zanden and Olden 2008). Despite the strong presence of invasive species,
much of the ecosystem functioning in the Great Lakes region remains intact, suggesting that timely and
strategic management could increase native species composition.
A feasible management goal for a modified site may be to remove problematic organisms and accept the
existence of ubiquitous or naturalized species that improve the ecosystem or provide important services.
Problematic species include those that disrupt ecosystem functioning, cause further degradation, or alter
important characteristics, such as vegetation structure or aesthetic qualities. Species that facilitate invasion
of other exotics are also important management targets in modified systems. For example, nitrogen
enrichment following the invasion of nitrogen fixing plants has been well-documented (Scherer-Lorenzen
et al. 2008). In South Africa, the invasion of Acacia saligna led to a secondary increase in the exotic,
nitrophilous Ehrharta calycina. Understanding which species have higher impacts on ecosystem health
can be an important strategy to improve restoration outcomes in sites with abundant distributions and
numbers of invasive species.
1.4 Altered site. An altered site is one that is dominated by exotic and invasive species. The degree to
which ecosystem structure and function resembles historic conditions varies depending on the severity
and type of disturbance. Restoration is often very costly or infeasible and novel approaches for
management are advised (Hobbs and Humphries 1995). The goals of invasive species management within
an altered site should be highly strategic and pragmatic, focusing on species impacts and their interactions
with the broader landscape. Targeting problematic invasive species that reduce ecosystem functioning,
such as water retention or carbon cycling, may be a feasible goal even in highly disturbed sites. For
example, the giant reed, Arundo donax threatens riparian areas in California by increasing flammability in
historically fire retardant areas (Coffman et al. 2010). Reducing fire risk is important for the overall
functioning of the ecosystem, a goal that is less about origin and more about the species’ impacts. In
addition, monitoring for exotic species’ introductions in highly trafficked and disturbed sites could be an
important goal when considering the site’s effects on surrounding landscapes, as well as opportunities to
prevent further degradation.
1.5 Mosaic site. In many cases, the ecology of restoration sites is highly variable and the invasive species
distributions are fragmented across the landscape. Many restoration sites may contain habitats across
multiple states of degradation from historical to altered. In such cases, it may be useful to consider the
landscape as a mosaic of different states that each require goal-setting reflective of this variability.
Considering the altered ecosystem within a mosaic of urban and rural landscapes may also help managers
develop more effective management goals. For example, invasive fruit bearing species in Hawaii are often
spread by frugivorous birds (Simberloff and Holle 1999) and to protect surrounding islands from
invasion, it may still be more cost-effective in the long-term to control a highly altered site in order to
protect another less modified site.
Creating priority lists for invasive species is a useful management strategy, especially for modified,
altered, or mosaic sites where invasive species are often well established. Various classification trees have
been developed to assist managers and policy makers with species prioritization (Randall et al. 2008;
Skurka Darin et al. 2011). Lists are often created using multiple criteria, such as impact, potential for
spread, and feasibility of control. Although prioritization lists are useful for regional management
strategies, the high variability among restoration sites, as well as their corresponding social and ecological
constraints, often necessitates site-specific decision-making. In addition, numerous factors affect
management efficacy and it is often difficult to identify all the social and ecological constraints of a
system. For the scope of this chapter, we highlight three key factors that significantly affect management
outcomes and warrant careful consideration when developing species prioritization lists at a restoration
2.1 Non-target ecosystem impacts. Management success is often contingent on understanding the
potential for non-target ecosystem impacts following invasive species removal (D’Antonio and Meyerson
2002). For example, has the invader been incorporated into trophic levels or do native species rely on the
exotic for habitat? The endangered California clapper rail is an archetype of non-target impacts, as its
population dramatically declined following removal of invasive Spartina in the San Francisco Bay Area.
When managers realized that the bird was using invasive Spartina as habitat, they responded by slowing
exotic removal until native habitat was restored (Buckley and Han 2014). The exotic legume, Ulex
europaeus, in New Zealand also played an important role in ecosystem function. While lowering native
biodiversity, it simultaneously provided essential microsites for native forest recovery following livestock
removal in pastureland (Norton 2009). The interactions between invasive species and the local ecology at
restoration sites can pose difficult choices for managers. Though easy solutions may be elusive, it is still
important to consider the negative and positive impacts of invasive species when developing prioritization
The act of invasive species control can also have non-target effects on native species diversity and
ecosystem function. For example, much of the Galapagos’ humid highlands have been invaded by the
exotic ground cover, Tradescantia flumiesis. When managers used chemical control to contain the
population, however, the compounds also reduced native species cover, thus providing a window of
opportunity for other invasive species to spread (Hallett et al. 2013). Disturbance due to mechanical
control can also facilitate invasion by other exotic species. Although topsoil removal has been touted as
an effective tool to reduce annual grass propagule pressure, as it can modify site hydrology (Holl et al.
2014), reduce the native seed bank, and alter the microbial community (Buisson et al. 2006). Thus,
assessing and weighing additional ecosystem effects of removal strategies is an important step when
deciding how to manage invasive species at a restoration site.
2.2 Recovery constraints. Invasive species may also alter ecosystem attributes and functions (e.g.,
geomorphology, hydrology, microbial communities, and disturbance regimes) that impede native species
recovery (Corbin and Antonio 2011). These changes may persist on the landscape following removal and
favor invasive over native species. For example, coastal dunes dominated by the invasive, Acacia
longifolia, have greater litter accumulation, available nitrogen and microbial biomass. These legacies
intensify with time following invasion (Marchante et al. 2008), creating a positive feedback that favors A.
longifolia over other native species. Invasive species establishment may also directly inhibit native
species. Garlic mustard, Alliaria petiolata, a widespread non-native species in North American forests,
produces a long-lasting compound that impedes symbiotic associations between mychorrizae and native
plants (Perry et al. 2005). In addition, invasive species can impede management goals for ecosystem
function and native species recovery, particularly when they alter disturbance regimes. Invasive annual
grasses in the Western United States, for instance, have increased fire frequency which threatens public
safety and prevents native species recovery (Brooks et al. 2004). Understanding the recovery constraints
affecting native species re-establishment following invasive species removal will likely improve decision-
making and help managers identify which species are more feasible to control.
2.3 Re-invasion risk. Invasive species removal efforts may be rendered ineffective if managers cannot
prevent subsequent re-invasion. Re-invasion is particularly likely if sites are located near a source
population, highlighting that knowledge of species distributions within the surrounding landscape is
integral for management success (Brudvig 2011). Depending on jurisdiction and cost, one option for sites
in invaded landscapes is to scale-up invasive control efforts (Holl et al. 2003). Alternatively, managers
may opt to prioritize removal in sites located far from source populations (Heller and Hobbs 2014). Life
history of the invasive species may also affect prioritization decisions. For example, re-invasion may be
unavoidable for sites located near populations of highly fecund (Larios, Aicher, and Suding 2013) or
widely dispersing invasive species (Osawa et al. 2013).
The risk of re-invasion may also be high for sites affected by land-use or global change that favors
invasive species over native species. Sites that experience ongoing local disturbance may be at particular
risk of re-invasion by invasive species with “weedy” traits (fast growing, high fecundity) (Larios year). In
addition, global change can alter interactions between native and exotic species (Dukes and Mooney
1998). Invasive species often benefit from increased resource availability (Dukes 2011), and consequently
may be more likely to re-invade areas affected by high nitrogen deposition or increased water availability
due to climate change. Similarly, environmental change can also reduce the efficacy of initial species
removal. Canadian thistle, Cirsium arvense, recovers from herbicide more quickly in areas with elevated
CO2, likely due to increased below-ground investment (Ziska et al. 2004). Distribution modeling can be
useful for predicting areas where invasive-native species interactions may change, and site-level
experimental tests can help managers predict re-invasion risk before implementing large-scale
Invasive species management is typically not a linear process and each stage should inform the others,
enabling managers to adapt and adjust as new information and improved methods are developed. Here we
discuss important factors to consider when deciding which of the four strategies – prevention, eradication,
containment, and observation - would be most effective for each target invasive species (Figure 3). The
literature describing these four strategies is extensive, and the definitions are highly variable and
overlapping (Pyšek and Richardson 2010). For the purpose of this chapter, we use control broadly to
encompass the ideas of prevention, eradication, containment, and observation, and in the sections below
parameterize these four terms with respect to restoration.
3.1 Prevention. Prevention is focused on precluding known or potentially new invasive species from
spreading into un-invaded habitats. Preventative strategies can be the most cost-effective, especially
considering the financial consequences of invasive species establishment (Pimentel et al. 2005). Because
it is difficult to predict invasiveness of exotic species, removing all arriving non-native species where
identified can help curtail the establishment of future invasions (i.e., guilty until proven otherwise). The
two common methods for detecting invasive species are surveys and remote sensing, though significant
limitations persist, such as small or cryptic offspring, dense vegetation structure, dormancy, herbivory and
observer error (Emry et al., 2011). To enhance the efficacy of early detection and mapping techniques,
rigorous, frequent surveys in conjunction with citizen science programs is recommended (Jordan et al.
A relatively new technique, pathway and vector management, can also greatly bolster prevention
strategies (Pyšek and Richardson 2010). By identifying the possible pathways (i.e., ports, roads, and
nurseries) and vectors (i.e., hikers, cattle, big machinery) leading to invasion, managers can improve
monitoring protocols in order to remove incipient invasions (Hulme 2009) or contain expanding
populations. Pathway and vector management strategies, as well other control efforts, are best
implemented with clear plans for monitoring, both to ascertain long-term success rates and to inform
future management practices (Blossey 1999).
3.2 Eradication. The goal of eradication is to eliminate all individuals and/or the seed bank of an invasive
species in a target area. Eradication decisions should be based on the invasive species’ population size and
distribution, and individuals that are farthest from population epicenters should be targeted first because
they are likely less established and more feasible to eradicate (Rejmánek and Pitcairn 2002). Typically,
successful eradications are associated with small, isolated populations less than one hectare in size.
Species that are highly visible with lower fecundity and dormancy rates are possible to eradicate. If one or
more of these criteria are not met, such species are better managed by a containment strategy (Ramsey et
The literature is contradictory in its support of eradication efforts. In many cases, once a species is
established it is almost impossible to remove (Vander Zanden and Olden 2008). Certain situations,
however, may warrant eradication efforts. For instance, various island eradication efforts throughout the
world have succeeded. New Zealand land managers effectively restored native bird diversity to small
islands by removing all invasive predators (Veitch and Clout 2002). In addition, eradications of small
plant populations have also been successful, especially when detected early (Rejmánek and Pitcairn
2002). Eradication is often most efficacious when followed by prevention strategies, as pathways and
vectors may continue to facilitate species spread (Veitch and Clout 2002).
3.3 Containment. The goal of containment is to preclude or slow the spatial expansion of established
invasive species populations. Containment is useful for species that do not meet all criteria for eradication
(i.e., widespread distributions, prohibitively high removal costs, or populations not conducive for
monitoring surveys). Effective containment strategies ideally delineate the invasive species’ distribution
and important habitats threatened by the invasion. Thus managers can identify and remove smaller
satellite populations that are spreading into ecologically valuable areas. Because restorations are often
concentrated on degraded landscapes, containing problematic populations may be the most useful
strategy, as eradications are extremely costly and often ineffective when populations are well established.
Invasive grasses greatly reduce biodiversity in serpentine grasslands, and various containment strategies
are used to decrease invasive grass densities, such as gazing, fire, and mowing (Weiss 1999). Grass
eradications in these systems are probably impossible because removal techniques often miss small
populations, abutting landscapes continually provide propagules, and human influence, such as increased
nitrogen deposition, is difficult to control. Though containment strategies do not completely remove the
invasive species population, management can still effectively restore native biodiversity to degraded
landscapes when used strategically.
3.4 Observation. Observation strategies may mean that some of the site cannot be restored or that
restoration goals are adjusted to reflect the realities of control for a particular set of invasive species. For
example, it may be more cost-effective and pragmatic to watch that the invasive population does not
spread further and affect nearby areas. In highly disturbed sites dominated by exotic species, future
introductions are likely. These altered sites can provide a frontline of defense against new invasive species
or further spread. While removal may not be cost-effective or feasible, implementing “wait and watch”
monitoring programs can inform managers when to act. In some cases, it is better to focus on
reestablishing native species and increasing habitat resilience to further invasion than attempting to
control a ubiquitous invasive species. For example, managers can control certain invasive fish species by
restoring natural flow regimes below dams (Rahel and Olden 2008) or reduce numbers of shade-intolerant
invasive species in degraded riparian zones by planting native trees. Accepting the presence of an invasive
species and focusing on ecosystem health may be the best course of action in many highly altered sites.
4.0 FUTURE DIRECTIONS
Invasive species management can benefit from harnessing emerging technology to improve detection and
control techniques. Broader advances in technology are already being adapted, and this trend is likely to
continue into the future. For example, the internet will enable broad collaborations among diverse sets of
stakeholders, and big data analytics will drive analyses of larger genetic, remote sensing and citizen
science datasets. Social media and mobile apps will also allow more citizens to become involved in
invasive species detection and research. In addition, spatial data coupled with computing power will
enable spatially explicit simulations, allowing dynamic optimization of scarce dollars. In this section, we
highlight four areas where recent technological advancements have improved or will likely improve
invasive species management strategies.
4.1 Remote sensing. Remote sensing has emerged as a promising technology that can aid in mapping
infested areas and monitoring spread. Currently, remote sensing has been used to create distribution maps
to target the management of early invasion and model future invasion risk (Bradley 2014). The most
common approach is to utilize differences in spectral signatures (typically with hyperspectral data) to
differentiate invasive plants from surrounding vegetation. Textural (or object-based) and phenological
differences can also be effective (Bradley 2014). Obstacles to widespread implementation include
economic expense and poor resolution quality for small patch identification, especially if individuals are
small or early detection is the primary goal (Bradley 2014). The future is likely to see improvements in
satellite technology and reductions in the cost of aerial images, which could expand the utility and
efficacy of remote sensing.
4.2 Citizen science. Available technology has facilitated greater public participation in ecological
research (Newman 2012). By increasing the number of citizen science programs, managers engage larger
audiences that potentially improve early detection and control of invasive species (Dickinson et al. 2012).
Widespread adoption of mobile devices has increased the accuracy of species location information, and
mobile apps are currently experimenting with a broad range of methods to engage the public. Mobile apps
include platforms for learning about the natural world and can assist users with species identification. For
example, the app Leafsnap (http://www.leafsnap.com/) uses image recognition of leaf photos to help with
identification of trees in the eastern United States, while at the same time gathering occurrence and
location data for species of interest. Browser based visualization tools and social media can subsequently
facilitate the dissemination of such user data.
In some cases, citizen science projects are less expensive than traditional scientific research (Gardiner
2012), and even the more expensive and efficient programs have lower costs associated with data
collection and analysis (Goldstein 2014). While technology offers enormous promise for enabling citizen
science, constraints continue to arise. Perhaps the biggest challenge is ensuring data quality (Newman
2012). Volunteers, for example, can make more mistakes in identifying organisms than professionals,
which can lead to inflated species richness estimates (Gardiner 2012). Data quality can be addressed with
the addition of photograph specimens, but this necessitates expert validation and increases project costs
(Gardiner 2012, Newman 2012). Although future advances in technology are needed to increase accuracy
and decrease costs, the future will likely see more widely applicable and effective citizen science
4.3 Genetic tools. Contemporary genetic tools have been increasingly useful in invasive species research
and benefit from advancements in computational and statistical approaches. Understanding how historical
invasions progressed can improve predictive capacity for new invasions, as well as consequent impacts on
host communities. Environmental DNA using barcoding has been successfully applied to aquatic systems
(Dejean et al. 2012), where water samples can help detect incipient fish and amphibian invasions. In
terrestrial systems, genetic tools have quantified dispersal pathways (Medley et al. 2015) and identified
loci contributing to species adaptation during invasion (Vandepitte et al. 2014). In addition,
metagenomics approaches, including Roche/454 pyrosequencing, can create whole community interaction
networks much more easily (Pompanon et al. 2012). These networks (i.e., host-parasite, predator-prey,
and food webs) enable scientists to characterize and measure invasive species impacts at the community
level. Network patterns may also help uncover the conditions that helped communities resist invasion,
improving predictions of when and how invasions occur.
Advancements in computational power, as well as new statistical approaches for analyzing genetic data,
have paralleled the rapid development of genetic tools. Improvements relevant for invasion ecology
include approximate Bayesian computation, which allows invasion routes to be inferred from molecular
and historical data (Keller et al. 2012). Similarly, discriminant analysis of principal components is a
powerful approach to assign individuals to populations, broadening the application of population genetics
to better define invasion pathways. Coupling genetic and computational tools with GIS based landscape
analysis is also improving reconstruction of invasive species dispersal histories. The ability to evaluate
how landscape features and environmental parameters affect dispersal is key to predicting how invasive
species will spread in the future and provides important information for management strategies.
4.4 Collaborations. Invasive species management projects will likely increase collaborations across
agencies and international boundaries. Digital and online resources will also play a greater role in all
stages of weed management, from detection to strategic planning and control. A number of online
databases that aggregate information on the identity, impact, and location of invasive species already
exists. These databases function at the state, national and international levels, including the global
invasive species database (http://www.issg.org/database/welcome/), the National Invasive Species
Information Center in the United States (http://www.invasivespeciesinfo.gov/index.shtml), and the
Delivering Alien Invasive Species for Europe (DAISIE) database (http://www.europe-aliens.org/).
In addition, future collaborative efforts are likely to extend beyond databases and use online platforms to
integrate information from multiple stakeholders. The California Invasive Plant Council is attempting this
approach through an online tool, CalWeedMapper, which consolidates material from various stakeholders
about invasive species distributions, impacts, and management activities in California. The website
provides additional resources as well, such as upcoming conferences and training programs, policy
information, relevant scientific research, and priority areas for research and management. This integrative
approach recognizes that engagement with a broader community of partners is a necessary piece of
Worldwide, it is hard to escape issues of invasive species when embarking on ecological restoration. In
this chapter, we described various strategies to address the challenges that arise with invasive species
management in restoration, highlighting three main stages – assessment, prioritization, and control
(Figure 1). At each stage, we encourage the consideration of how climate and human modifications may
affect what restoration goals can be set and accomplished, including exotic species impacts on local
species interactions and ecosystem functioning. Future management efforts will likely see an emphasis on
more integrative conceptual frameworks and complex optimization tools to guide decision-making.
Increased globalization and climate change are expected to accelerate the rates of invasion, challenging
the ability of managers to keep pace with new invasions as they occur. Funding limitations will also likely
persist, as well as the high costs of invasive species control. Thus, a key to future management efficacy in
ecological restoration will be to build upon and document records of success in assessing, prioritizing, and
controlling invasive species.
Figure 1: Invasive species management framework for restoration sites. Defining the condition
of the restoration site during the assessment stage helps managers decide if they have to
prioritize invasive species or how to control species allotted for management. Though not
necessarily a linear process, by understanding the important factors that affect management
outcomes during the assessment, prioritization and control stages, more site-specific and
achievable goals may be developed.
Figure 2: Modified from Hobbs, Higgs and Harris, (2009). Placing sites into these four categories, historical, intact,
modified and altered, (or identifying a combination of these states at one site), helps managers prioritize invasive
species. For example, if the site is relatively intact and much of the historical biota are still present, then the managers
may decide that more traditional restoration goals of invasive species removal are feasible. However, if the site is
continuously disturbed and highly invaded, perhaps the only thing left to manage are incipient invasions and preventing
new exotic species from dispersing into less degraded areas.
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