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COMMUNICATING ABOUT INVASIVE SPECIES

Communicating about Invasive Species: How “Driver” and “Passenger” Models Influence Public

Willingness to Take Action

P. Sol HartUniversity of Michigan, Ann Arbor, USA

Brendon M. H. LarsonUniversity of Waterloo, Canada

Correspondence should be address to: Sol Hart, Communication Studies, University of

Michigan, 5370 North Quad, 105 South State Street, Ann Arbor, MI 48109. Email:

[email protected]. Telephone: (734) 764-0420. Fax: (734) 764-3288.

Keywords. attribution of responsibility, conservation management, invasive species, public action, risk perception

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Communicating about Invasive Species: How “Driver” and “Passenger” Models Influence Public Willingness to Take Action

Abstract

Invasive species may be viewed as “passengers” that spread in response to environmental

changes rather than “drivers” of ecological impacts. To date, however, there has been no

examination of how these alternative models affect public risk perception, sense of

responsibility, and willingness to take action. We report on an experimental study of how these

models affected respondents’ (N=456) willingness to take action to address two invasive species:

tamarisk and garlic mustard. We found that the traditional driver model, compared to the

passenger model, increased perception of risk to humans and the environment, both of which

contributed to willingness to take action. The driver model, however, also decreased personal

causal responsibility, though only when human responsibility for introduction was not

mentioned. Our findings suggest that these alternative models create trade-offs for

communication that necessitate contextual framing that attends to audience sense of risk and

responsibility.

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Communicating about Invasive Species: How “Driver” and “Passenger” Models Influence Public Willingness to Take Action

Introduction

Ecologists increasingly recognize that invasive species may be understood as “passengers” that

spread in response to concurrent environmental changes, rather than “drivers” that directly cause

ecological impacts (Didham et al. 2005; Macdougall & Turkington 2005; Wilson & Pinno 2013).

Ecologists also debate whether the origin of species (native vs. non-native) is pertinent for

judging them or whether we should instead focus on their impacts (Davis et al. 2011; Simberloff

2011). To date, however, there has been no examination of how these alternative models affect

the public’s risk perception of invasive species or their attribution of personal responsibility, both

of which may affect willingness to take action. Yet if the public’s willingness to take action is

affected, it has critical implications for conservation management efforts because the recruitment

and retention of volunteers is an important tool in monitoring and managing invasive species

(Cohn 2008; Crall et al. 2011; Beirne & Lambin 2013).

To examine this issue, we draw on theory from the field of science communication.

When science communicators design public outreach materials they offer interpretive packages

that give meaning to an issue by presenting “a central organizing idea… for making sense of

relevant events, suggesting what is at issue” (Gamson & Modigliani 1989). These choices

“frame” an issue and “promote a particular problem definition, causal interpretation, moral

evaluation, and/or treatment recommendation” (Entman 1993). Scholars increasingly recognize

the effects of framing in environmental management (Allan 2007; Larson 2011; Moore & Moore

2013), and in the case of climate change, there is now evidence that alternative frames can shift

public perception and willingness to take action (Nisbet 2009; Hart & Nisbet 2012). It has been

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proposed that different models of invasive species will have similar effects (Larson 2005;

Keulartz & van der Weele 2008), although we are unaware of any empirical tests of these

propositions. To help fill this research gap, we report on an experimental study of how the driver

and passenger models affect citizen perceptions and willingness to take action to address

invasive species.

To assess the impact of the driver and passenger models on predisposition to take action,

we draw from research identifying two primary causal pathways: the perception of risk and the

attribution of personal responsibility (Figure 1; Story & Forsyth 2008). Selge, Fischer, and Van

der Wal (2011), for example, found that people’s perceptions of both the potential harm of an

invasive species and human responsibility for its spread affect their preferred interventions. Both

the Extended Parallel Process Model (Witte 1992) and Protection Motivation Theory (Maddux &

Rogers 1983) hold that elevated risk perceptions can promote action to address a risk as long as

individuals do not feel hopeless about a situation; multiple studies have confirmed this result in

environmental contexts (Lévy-Leboyer et al. 1996; Hart et al. 2011). In addition, it is advisable

to examine the perceived risks to humans and to the environment separately because they may

differentially affect predispositions for behavior (Schultz 2000).

The passenger and driver models may also affect predispositions for action through the

pathway of attribution of personal responsibility, which has two components: causal and

treatment responsibility (Iyengar 1994; Stern et al. 1999). Causal responsibility refers to the

belief that an individual is responsible for causing a problem, which may increase treatment

responsibility, the belief that an individual is responsible for fixing a problem. Previous research

has found a positive association between attribution of personal responsibility and willingness to

take action across a variety of environmental issues (Story & Forsyth 2008).

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Overall, we expect the driver model, compared to the passenger model, to increase risk

perceptions and thus to strongly motivate action, because it emphasizes the agency of invasive

species in causing ecological disruption. However, by the same token we expect that these

aspects of agency in the driver model have the potential to lower the impetus to action by

lowering attribution of personal responsibility. Accordingly, we expect the passenger model,

which emphasizes the role of both humans and other actors in ecological disruption, to increase

attribution of responsibility compared to the driver model, even as it lowers risk perception.

We also explore whether mentioning human introduction of an invasive species

influences the relative effect of the driver and passenger models through attribution of

responsibility. Human introduction suggests that humans are at fault for causing the problem, so

we expect that mentioning it will lower the difference between the effect of adopting a driver or

passenger model on perceived causal responsibility.

Method

Study Design

We adopted a between-subject random assignment 2 (driver vs. passenger model) x 2 (mention

of human introduction of invasive species vs. no mention of human introduction) experimental

design to identify how these different models for invasive species may affect willingness to take

personal action through the causal pathways of attribution of responsibility, perceived risk to

humans, and perceived risk to the environment. To improve the generalizability of the study,

each participant was asked about one of two invasive species, tamarisk/salt cedar (Tamarix

species) or garlic mustard (Alliaria petiolata), both of which are amenable to descriptions under

a passenger or driver model (see Stromberg et al. 2009; Davis et al. 2012, respectively). Thus,

there were 8 conditions (four experimental conditions X two invasive species). 57 participants

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were randomly assigned to each condition for a total sample of 456. We recruited undergraduate

students from a university in the northeastern United States for the study; such students are

frequently recruited as volunteers for invasive species management programs (Ryan et al. 2001;

Krasny & Lee 2002; Cohn 2008; Delaney et al. 2008). Students were randomly recruited from

multiple public areas on campus and then directed to a private area for the study. After listwise

deletion of participants who did not complete the questionnaire, 436 participants remained for

the analysis. APA ethical guidelines were followed and experimental participants did not receive

compensation for completing the experiment.

After signing a consent form, participants were first provided with an information sheet

that described one of the two invasive species under one of the varied conditions described above

(full text of conditions is provided in Appendix 1). While the information sheets were

constructed for the experiment, we consulted with ecologists to ensure their scientific accuracy.

The driver model conditions focused on plant characteristics that cause ecological change

whereas those for the passenger model focused on characteristics that allowed the plant to thrive

under conditions of ecological change.

Independent Control Variables

Sociodemographics. Sociodemographics were measured by asking participants their age

(x=20.4; SD=2.77) and gender (Male = 0, Female = 1, x=0.6, SD=0.5). There was little variance

in age, so it was dropped from the analysis.

Core ecological beliefs. Participants were asked about their core ecological beliefs using

the scale adopted by Stedman, Davidson, and Wellstead (2004). The ecological core beliefs scale

is a 7-item subset of the new environmental paradigm (NEP) scale developed by Dunlap and Van

Liere (1978), and asks participants how much they agree with statements such as “The balance of

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nature is very delicate and easily upset by human activities.” Answers were aggregated into a

mean scale (x=4.88, SD=1.05, Cronbach’s =0.811)

Invasive species type. A dummy variable was coded to indicate whether the participant

was in a tamarisk or garlic mustard condition with garlic mustard as the reference group; this was

then used to control for species type.

Mediating variables

Attribution of responsibility. Attribution of personal causal responsibility was assessed

by asking participants how responsible “individual people like me” are for causing the

tamarisk/garlic mustard problem (not at all responsible=1, very responsible=7, x=3.11,

SD=1.87). Attribution of personal treatment responsibility was assessed by asking participants

how responsible “individual people like me” are for addressing (fixing) the tamarisk/garlic

mustard problem (not at all responsible=1, very responsible=7, x=3.75, SD=1.83).

Risk perception. To assess respondents’ perception of risk that tamarisk/garlic mustard

poses to humans, we asked them to evaluate the severity of their threat to a) “people like me,” b)

“local communities near rivers that have tamarisk” (or “near woodlands that have garlic

mustard”), and c) “the U.S. as a whole.” To assess their perception of risk that the species poses

to the environment, we asked them to evaluate the severity of their threat to a) “biodiversity,” b)

rivers in the southwestern U.S. (or “woodlands in the northeastern US”), c) “plants,” and d) “the

U.S. as a whole.” For each item respondents answered on a seven-point scale ranging from 1 (not

at all a threat) to 7 (very serious threat); the answers to these questions were then aggregated into

two mean scales (humans: x=3.23, SD=1.46, Cronbach’s =0.816; environment: x=4.72,

SD=1.40, Cronbach’s =0.769).

Dependent Variable

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Willingness to address invasive species. To assess the participants’ willingness to take

action to address invasive species, we asked them the following question: “assuming you lived in

a region where tamarisk/garlic mustard occurred, how likely would you be to participate in any

of the following activities that could help remove tamarisk from along rivers” (or “garlic mustard

from woodlands)?” The five activities were a) attend a local public meeting, b) contact a local

public official, c) volunteer with a local citizens’ or environmental group, d) participate on a

local committee or task force, and e) uproot it while hiking along a river” (or “in a woodland”).

For each item, participants answered on a seven-point scale ranging from 1 (not at all likely) to 7

(very likely); answers were then aggregated into a single mean scale (x=2.89, SD=1.55,

Cronbach’s =0.898).

Data Analysis

We first used ANOVA to investigate the representation of control variables for participants

across conditions. We then used an ordinary least squares regression-based path analytic

framework with the SPSS PROCESS macro developed by Hayes (2013) to examine the direct

and indirect effect of these alternative models on willingness to take action through three causal

pathways: i) perception of risks to humans, ii) perception of risks to the environment, and iii)

personal causal responsibility (see Figure 1).

The PROCESS macro generates a bootstrapped confidence interval for each of the three

causal pathways while controlling for the control variables and other variables in the conceptual

framework; the bootstrap analysis was conducted with 10,000 iterations and bias-corrected

estimates, based on the recommendations of Hayes (2013). The bootstrapping approach towards

indirect effects adopted here is generally considered superior to the Sobel test or causal steps

approach (Hayes 2013). In addition, results from the PROCESS macro do not significantly differ

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from results of structural equation modeling (Preacher & Hayes 2008; Hayes 2013) and the

inferential test process for coefficients adopted by the PROCESS macro, which is based on the t

distribution, is more appropriate for smaller samples than the process typically adopted by SEM

programs (Hayes 2013).

Results

A one-way ANOVA found no differences between conditions for gender (F7,429 = 0.679, p =

0.69) or environmental values (F7,429 = 1.445, p = 0.19). The regression analysis found that

individuals were more willing to take action to address tamarisk then garlic mustard

(unstandardized regression coefficient b = -0.356, p < 0.001); tamarisk and garlic mustard

conditions were combined for subsequent analysis with a dummy variable included to control for

this difference.

Looking first to risk perceptions, the regression analysis found that the driver model

increased perception of the risk that the invasive species posed to humans (b = 0.495, p < 0.001)

and the perception of risk to humans had a positive influence on willingness to take action (b =

0.180, p < 0.01) (see Table 1).

The regression analysis also found that the driver model increased perception of the risk

that the invasive species posed to the environment (b = 0.311, p < 0.05), which, in turn, increased

willingness to take action (b = 0.132, p < 0.05) (see Table 1).

Looking next to attribution of personal responsibility, the analysis revealed that the effect

of the driver model was conditional on whether or not human introduction of invasive species

was mentioned. When human introduction was not mentioned, the driver model suppressed

perceptions of causal responsibility compared to the passenger model (b = -0.648, p < 0.01),

whereas when human introduction was mentioned, the influence of the driver model was similar

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to the passenger model on individual causal responsibility (b = 0.139, p = 0.56; see Table 2,

Figure 2). Individual causal responsibility, in turn, had a positive influence on attribution of

individual treatment responsibility (b = 0.500, p < 0.001), which had a positive influence on

willingness to take action (b = 0.140, p < 0.01) (see Table 2).

The analysis of indirect effects through bootstrapped mediation tests corroborated the

aforementioned results. If the lower (LCI) and upper (UCI) 95% confidence intervals for the

indirect effects are either both above zero or both below zero there is a significant indirect effect

through the mediator(s). The driver condition had a significant positive mediated influence on

willingness to take action through the perceived risk to humans (LCI = 0.023, UCI = 0.200) and

through the perceived risk to the environment (LCI = 0.006, UCI = 0.113). When human

introduction was not mentioned, the driver model had a significant negative mediated influence

on willingness to take action through attribution of personal responsibility (LCI = -0.110, UCI =

-0.011); this indirect influence did not appear, however, when human introduction was

mentioned (LCI = -0.020, UCI = 0.060).

Discussion

To the best of our knowledge, this is the first study to examine how the use of the driver or

passenger models of invasive species in public outreach affects willingness to take action. As

volunteers often serve a critical function in conservation efforts, this study will encourage

conservation practitioners to consider and to evaluate the public impact of framing outreach

materials in different ways. Our results offer a nuanced view into how these two models may

affect willingness to take action through the mediated causal pathways of public perceptions of

risk and attribution of responsibility. The driver model, compared to the passenger model,

encouraged willingness to take action to address invasive species by raising perceptions of the

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risk posed to humans and the environment. The driver model, however, also demonstrated the

potential to suppress willingness to take action to address invasive species by lowering

perceptions of individual responsibility, although this suppressive effect appears to be mitigated

by the inclusion of a statement of human responsibility for introducing a species. We thus

conclude that outreach materials will increase the predisposition to action when a driver model of

invasive species is paired with an explicit statement about their human introduction.

Of course, this is only one study, and caution should be taken when generalizing. In

particular, our study focused on predispositions to engage in behavior to address invasive species

but did not measure actual behavior (cf. Prinbeck et al. 2011). In addition, while undergraduate

students offer useful information for how potential student volunteers are likely to respond to

information about invasive species, they are not representative of the general U.S. or even

broader populations. Thus, future studies may build from this research to include observed

behavioral responses among a more heterogeneous population; for example, personal

characteristics may impact the mental models of both the lay public and scientists (e.g., ideology,

disciplinary background, see Corley et al. 2009; Besley & Nisbet 2013) and affect whether they

support or oppose alternative intervention strategies.

The unstandardized coefficients for the effects in our study were somewhat small. We

suspect that this results from the use of a single exposure to the stimulus in our experiment,

which provides a conservative estimate of effects that might occur with a more comprehensive

communication campaign in which individuals are exposed to a message multiple times. The

results may have also been affected by the specific species and language adopted for the driver

and passenger model descriptions; we note, for example, that in general individuals were more

willing to take action to address tamarisk than garlic mustard. In addition, is also possible that

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individuals may have interpreted the “driver” model to have a more negative tone (Slovic et al.

2004), or perceived other, unaccounted differences between conditions, which could have also

impacted the observed results. While we used two different species that varied in discussion of

species relevant impacts to improve generalizability and reduce the chance that language choice

in a single condition would overly affect the results, future studies may focus in on more fine

scale implications of language choices.

In addition to the direct effects examined here, framing effects have the potential to shape

opinions through patterns of negotiated meaning that arise through interactions between various

members of the public. We thus propose future research that examines additional dependent

variables such as conversation dynamics to integrate the present research with literature

examining how issue framing may impact discursive processes between scientists, journalists,

managers, and the public (Price et al. 2005; Nisbet 2009; Nisbet & Scheufele 2009). Such

inquiry would also touch more directly on broader ethical questions related to the form of

communication and its implications for different audiences (e.g. Warner & Kinslow 2013).

Our study provides evidence that adopting different models for communicating about

invasive species is likely to impact public perceptions and willingness to take action. However,

we have analyzed “passenger” and “driver” as alternative communication models, when it may

ultimately be a scientific question whether one or the other is appropriate in a given context.

Thus, our findings are not intended as a prescription of how conservation practitioners ought to

design messages about invasive species to reach out to the public and recruit volunteers, but

instead seek to provide insight into the potential effects of alternative messages. It is likely that

the insights from the present study are not limited to communicating about invasive species, but

may apply in additional domains of environmental management.

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Acknowledgments

We appreciate comments on the alternative descriptions of tamarisk and garlic mustard from

Matt Chew and Mark Davis (respectively), and helpful comments from several anonymous

reviewers and editors.

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Table 1. Results from mediation analysis of the passenger and driver models through perception of risk to humans and perception of risk to the environment. Predictor bEquation predicting mediator 1 (Risk to Humans)

Intercept 0.651*Gender (Female) 0.347*Environmental Values 0.193*Causal Responsibility 0.102*Treatment Responsibility 0.200***Invasive Species (Garlic Mustard) 0.176Message Condition (Human Introduction) 0.047Message Condition (Driver Model) 0.495***

Equation predicting mediator 2 (Risk to Environment)Intercept 2.05***Gender (Female) 0.243Environmental Values 0.309***Causal Responsibility 0.092*Treatment Responsibility 0.123**Invasive Species (Garlic Mustard) 0.157Message Condition (Human Introduction) 0.054Message Condition (Driver Model) 0.311*

Equation predicting dependent variable (Willingness to Take Action)Intercept -0.006Gender (Female) 0.062Environmental Values 0.301***Causal Responsibility -0.012Treatment Responsibility 0.141**Invasive Species (Garlic Mustard) -0.356**Message Condition (Human Introduction) 0.010Message Condition (Driver Model) -0.198Risk to Environment 0.132*Risk to Humans 0.180**

Note: *** p < 0.001; ** p < 0.01; * p < 0.05. Unstandardized coefficients are reported.

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Table 2. Results from moderated-mediation analysis of the passenger and driver models through attribution of causal and treatment responsibility.Predictor bEquation predicting mediator (Causal Responsibility)

Intercept 2.012***Gender (Female) -0.172Environmental Values 0.001Threat to Humans 0.290***Threat to Environment 0.135Invasive Species (Garlic Mustard) -0.656***Message Condition (Human Introduction) 0.158Message Condition Interaction (Human Intro X Driver) 0.786*Message Condition (Driver Model) With no mention of human introduction -0.648** With mention of human introduction 0.139

Equation predicting mediator (Treatment Responsibility)Intercept 0.390Gender (Female) -0.038Environmental Values 0.189**Threat to Humans 0.217***Threat to Environment -0.055Invasive Species (Garlic Mustard) -0.055Message Condition (Human Introduction) -0.079Message Condition Interaction (Human Intro X Driver) -0.094Message Condition (Driver Model) 0.061Causal Responsibility 0.500***

Equation predicting dependent variable (Willingness to Take Action)Intercept -0.020Gender (Female) 0.062Environmental Values 0.302***Threat to Humans 0.175**Threat to Environment 0.132*Invasive Species (Garlic Mustard) -0.356**Message Condition (Human Introduction) 0.028Message Condition Interaction (Human Intro X Driver) -0.038Message Condition (Driver Model) -0.179Causal Responsibility -0.012Treatment Responsibility 0.140**

Note: *** p < 0.001; ** p < 0.01; * p < 0.05. Unstandardized coefficients are reported. For the equation predicting the mediator causal responsibility, two driver model coefficients are provided to demonstrate how the impact of the driver model varies depending on the whether human introduction of species is mentioned or not. For the other equations in the table (predicting treatment responsibility and willingness to take action), there was no significant difference between the two versions of the coding for the human introduction variable, and the reported driver coefficient is with human introduction not mentioned.

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Figure 1. Conceptual Framework for testing.

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Figure 2. Driver Model vs. Mention of Human Introduction on Attribution of Causal Responsibility. Note: Estimates in this figure are calculated with covariates set to their sample means.

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Appendix 1Text used for experimental conditions.

Condition 1: Tamarisk as a Driver of environmental change, including mention of human introduction

Tamarisk is an ‘Old World’ shrub that is highly tolerant of drought and salt. It has pretty pink flowers and attracts honeybees. Humans brought it to the US beginning around 1800 and began planting it as an ornamental, then later to control erosion on beaches, stream banks, and dry farmlands.

Tamarisk often forms dense stands that seem to leave little room for other plants. It has become extremely common along most rivers in the American southwest, replacing larger trees that once lived there, and it has caused major environmental changes.

Tamarisk became established due in part to: 1) its long tap roots that give it access to shallow ground water most plants can’t reach, 2) adaptations allowing it to use salty water that would damage other plants, and 3), re-sprouting from live roots after fires that usually kill many of the trees it competes with.

Due to the characteristics of tamarisk that allow it to spread along rivers in the American southwest and displace other species, many ecologists consider this plant to be a primary cause or “driver” of changes in these ecosystems.

Condition 2: Tamarisk as a Passenger in environmental change, including mention of human introduction

Tamarisk is an ‘Old World’ shrub that is highly tolerant of drought and salt. It has pretty pink flowers and attracts honeybees. Humans brought it to the US beginning around 1800 and began planting it as an ornamental, then later to control erosion on beaches, stream banks, and dry farmlands.

Once tamarisk is established along a river, it can become very abundant. Sometimes it replaces the tree species once common along rivers in the American southwest, and it represents the onset of significant environmental changes.

Tamarisk became established due in part to: 1) dammed and diverted southwestern rivers providing water flows similar to those where tamarisk came from, 2) its utilization of salty irrigation runoff and wastewater that would damage other plants, and 3) its ability to colonize areas disturbed or cleared by human activity (e.g., for dam construction or clearing for agriculture.

Tamarisk is a product of changes in riparian environments; it largely spreads in response to the activities of humans. Thus, many ecologists consider this plant to be a “passenger” that responds to these broader environmental changes.

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415416417418419420421422423424425426427428429430431432433434435436437438439440441442443444445446447448449450451452453454455456457458459460


Condition 3: Tamarisk as a Driver of environmental change, not including mention of human introduction

Tamarisk is an ‘Old World’ shrub that is highly tolerant of drought and salt. It has pretty pink flowers and attracts honeybees.

Tamarisk often forms dense stands that seem to leave little room for other plants. It has become extremely common along most rivers in the American southwest, replacing larger trees that once lived there, and it has caused major environmental changes.

Tamarisk became established due in part to: 1) its long tap roots that give it access to shallow ground water most plants can’t reach, 2) adaptations allowing it to use salty water that would damage other plants, and 3), re-sprouting from live roots after fires that usually kill many of the trees it competes with.

Due to the characteristics of tamarisk that allow it to spread along rivers in the American southwest and displace other species, many ecologists consider this plant to be a primary cause or “driver” of changes in these ecosystems.

Condition 4: Tamarisk as a Passenger in environmental change, not including mention of human introduction

Tamarisk is an ‘Old World’ shrub that is highly tolerant of drought and salt. It has pretty pink flowers and attracts honeybees.

Once tamarisk is established along a river, it can become very abundant. Sometimes it replaces the tree species once common along rivers in the American southwest, and it represents the onset of significant environmental changes.

Tamarisk became established due in part to: 1) dammed and diverted southwestern rivers providing water flows similar to those where tamarisk came from, 2) its utilization of salty irrigation runoff and wastewater that would damage other plants, and 3) its ability to colonize areas disturbed or cleared by human activity (e.g., for dam construction or clearing for agriculture.

Tamarisk is a product of changes in riparian environments; it largely spreads in response to the activities of humans. Thus, many ecologists consider this plant to be a “passenger” that responds to these broader environmental changes.

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461462463464465466467468469470471472473474475476477478479480481482483484485486487488489490491492493494495496497498499500501502503504505


Condition 5: Garlic Mustard as Driver of environmental change, including mention of human introduction

Garlic mustard is a biennial flowering plant from Europe and Asia that humans brought to the U.S. in the 1860s. Garlic mustard was introduced into the United States for food, erosion control, and as a culinary herb.

Garlic mustard can take over a forest floor, crowding out other plants, including wildflowers. Garlic mustard grows densely and can displace many herbaceous plants within 10 years.

The success of garlic mustard is due in part to: 1) the absence of its natural enemies (that is, species that eat it) in North America, 2) its ability to self-fertilize, 3) its high production of 15,000 seeds annually, 4) its rapid growth during the second growing season, and 5) the release of chemicals from its roots that harm other plant species. Furthermore, its seeds survive in the soil for several years, which make it more difficult to remove.

Due to the characteristics of garlic mustard that allow it to spread in woodlands in the U.S. and displace other species, ecologists consider this plant to be a primary cause or “driver” of changes in these ecosystems.

Condition 6: Garlic Mustard as Passenger in environmental change, including human introduction

Garlic mustard is a biennial flowering plant from Europe and Asia that humans brought to the U.S. in the 1860s. Garlic mustard was introduced into the United States for food, erosion control, and as a culinary herb.

Once garlic mustard has become established in a forest, it can spread quickly and become one of the most abundant herbs in the forest.

The success of garlic mustard is due in part to: 1) large populations of white-tailed deer, which dislike feeding on garlic mustard relative to other species, and thus encourage its spread, and 2) the compaction of the soil by deer and other animals, including earthworms, which encourages growth of garlic mustard relative to other species.

Garlic mustard is a product of changes in the woodland environment; it spreads in response to the activities of deer, earthworms, humans, and other animals. Thus, ecologists consider this plant to be a “passenger” that responds to these changes.

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Condition 7: Garlic Mustard as Driver of environmental change, not including human introduction

Garlic mustard is a biennial flowering plant. Garlic mustard can take over a forest floor, crowding out other plants, including wildflowers. Garlic mustard grows densely and can displace many herbaceous plants within 10 years.

The success of garlic mustard is due in part to: 1) the absence of its natural enemies (that is, species that eat it) in North America, 2) its ability to self-fertilize, 3) its high production of 15,000 seeds annually, 4) its rapid growth during the second growing season, and 5) the release of chemicals from its roots that harm other plant species. Furthermore, its seeds survive in the soil for several years, which make it more difficult to remove.

Due to the characteristics of garlic mustard that allow it to spread in woodlands in the U.S. and displace other species, ecologists consider this plant to be a primary cause or “driver” of changes in these ecosystems.

Condition 8: Garlic Mustard as Passenger in environmental change, not including human introduction

Garlic mustard is a biennial flowering plant. Once garlic mustard has become established in a forest, it can spread quickly and become one of the most abundant herbs in the forest.

The success of garlic mustard is due in part to: 1) large populations of white-tailed deer, which dislike feeding on garlic mustard relative to other species, and thus encourage its spread, and 2) the compaction of the soil by deer and other animals, including earthworms, which encourages growth of garlic mustard relative to other species.

Garlic mustard is a product of changes in the woodland environment; it spreads in response to the activities of deer, earthworms, humans, and other animals. Thus, ecologists consider this plant to be a “passenger” that responds to these changes.

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