Harms of introduced large herbivores outweigh their benefits, while both are greater on islands and for higher trophic levels

doi:10.21203/rs.3.rs-4959829/v1

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Harms of introduced large herbivores outweigh their benefits, while both are greater on islands and for higher trophic levels

https://doi.org/10.21203/rs.3.rs-4959829/v1

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Introduced species significantly impact native biodiversity worldwide, with extensive research on harms but relatively less focus on benefits. Using the IUCN Environmental Impact Classification for Alien Taxa (EICAT) and EICAT + frameworks, we assessed 2021 negative and positive impacts of introduced large mammalian herbivores globally. Negative impacts were more common and of higher magnitude than positive impacts, i.e. affected populations, not only the performance of individuals. Native species on islands and at higher trophic level experienced greater impacts. Reported impact magnitudes declined over time only for positive impacts. Most positive impacts were caused indirectly through changes in species interactions and ecosystem properties, often following negative impacts on native plants through herbivory and disturbance. We therefore caution against the intentional introduction of large mammalian herbivores for conservation purposes (rewilding, assisted colonization) without rigorous assessment of their impacts on native communities.

Biological sciences/Ecology/Invasive species

Biological sciences/Ecology/Biodiversity

Biological sciences/Ecology/Ecosystem ecology

Human-mediated introductions of species outside their native range, so-called alien species, have encompassed all taxonomic groups and geographic regions¹. A subset of alien species have inflicted substantial harm on native biodiversity, exacerbating its decline alongside other major anthropogenic drivers^1–4; these are commonly referred to as invasive alien species⁵. Not all alien species harm native biodiversity and the magnitude of their impacts varies across different alien populations⁶. However, such context dependence of impacts remains poorly understood^7,8 and for most taxonomic groups it is unclear which species are the most harmful and which biological traits and ecological factors determine impact severity. For instance, while islands are generally more susceptible to invasion⁹ and more vulnerable to anthropogenic pressures than continents^10,11, it is unclear whether alien species impacts on insular biodiversity are consistently more pronounced than those on the mainland. Studies have shown that global extinction risk posed by invasive predators on native species is higher on islands^12,13, although it is yet to be determined if the same pattern holds for other negative impact types and magnitudes (reduction in performance of individuals, reduction in population size, local extinction¹⁴). Similarly, theory predicts that higher trophic levels are more vulnerable to environmental alterations^15–17, but few studies have investigated if invasive alien species cause stronger negative impacts to native species positioned high in the food chain^18,19.

Native species can also benefit from the introduction of alien species^20,21. Yet, positive impacts are less often documented than their negative counterparts²⁰. While most scientists acknowledge the existence of positive impacts²², there is a controversy over whether they are overlooked or their extent has been over- or underemphasized in comparison with negative impacts^23–26. As far as we know, there has been no quantitative, systematically collected and taxonomically controlled study in support of any of these claims, so that the alleged bias of focusing on negative impacts or overstating their magnitude has never been rigorously tested. Additionally, no in-depth investigation on factors that determine positive impacts of alien species on native biodiversity has been conducted. A persistent challenge in this regard has been the lack of a transparent and comprehensive framework for measuring and evaluating negative and positive impacts, and effectively comparing their frequencies and magnitudes^21,27–29.

Here, we employ the International Union for Conservation of Nature’s (IUCN) Environmental Impact Classification for Alien Taxa (EICAT) framework^14,30 and the recently developed EICAT + framework³¹ to systematically assess negative and positive impacts of introduced large mammalian herbivores (LMH; Cetartiodactyla, Perissodactyla, Proboscidea) on native biodiversity on a global scale. While many LMH face dramatic population declines and range contractions because of global change³², there is a growing call to introduce them outside their native range for conservation purposes, such as rewilding, assisted colonization and climate change mitigation^33–35. The EICAT(+) frameworks adopt a conservation perspective centered on native biodiversity³⁶ and classify alien species’ impacts by their direction, i.e. distinguishing whether they pose harms or offer benefits to local populations of native species, and by quantifying their magnitude. For this study, assessed impact magnitudes have been further categorized as "weak" or "strong" based on whether they involved individual-level or population-level changes. Additionally, the mechanisms through which these impacts were caused have been classified as "direct" or "indirect" (mechanism type). Finally, each impact has been attributed a confidence (low, medium or high) to express the uncertainty associated with the accuracy of the assigned impact magnitude. The combined use of EICAT(+) enables us to conduct a standardized, comparable and taxonomically controlled bidirectional impact assessment needed to address the above controversy. Furthermore, we used the assessed impact data to investigate to what extent insularity and trophic position shape the magnitude of both negative and positive impacts experienced by native species.

Under the assumption that the introduction of species outside their native range mostly disrupts established ecological dynamics, we hypothesize that (1) negative impacts caused by introduced LMH on native species occur at a higher frequency and with greater impact magnitude compared to positive impacts. We name this hypothesis the "Harm Dominance Hypothesis". We also hypothesize that (2) both negative and positive impacts of introduced LMH are greater in magnitude on islands and on native species positioned higher in the trophic chain. The hypothesis regarding the influence of insularity and trophic position in amplifying negative impacts stems from circumstantial evidence from previous research^19,37. Conversely, the hypothesis of greater positive impacts on islands and higher trophic level stems from the rarely tested assumptions that introduced species can restore functions of extinct insular species^34,38, or serve as an important novel food resource for native consumers positioned directly above in the trophic chain^39,40. Finally, we hypothesize that (3) due to their salience, negative and positive impacts of higher magnitude (strong impacts) have been identified first, and thus reported impact magnitude would decline over time.

Large mammalian herbivores as a study system

Large mammalian herbivores (LMH) have important effects on terrestrial ecosystems by causing disturbances, consuming low-nutrient vegetation and dispersing plant propagules and nutrients⁴¹. They have an intermediate position within the food chain and were frequently introduced to both insular and continental sites^6,42. Among the currently recognized 286 modern species of LMH (Cetartiodactyla, Perissodactyla, Proboscidea), including wild and domesticated forms⁴³, 66 species from six families have established alien populations according to the IUCN Global Register of Introduced and Invasive Species (http://www.griis.org). The impacts of their introductions outside their native ranges are controversial^44,45, but a systematic comparison of their positive and negative impacts on native species is currently lacking.

Impact assessment frameworks

As measurable changes (decreases or increases) to ecosystem attributes⁸, ecological impacts can be classified based on their direction, i.e. be distinguished between negative and positive impacts²¹. The interpretation of such impacts, however, might entail subjectivity because it depends on the selection of the ecosystem attributes that are measured — whether they are e.g. species, populations, individuals, genes, or abiotic ecosystem attributes — a choice guided by study purpose, feasibility and convention, but also by values and interests³¹. Values and interests also play a role in determining whether impacts are perceived as detrimental or beneficial to nature or people^20,21, thereby making it challenging to reach a consensus regarding the interpretation of impact direction^22,29. Here, we take a conservation perspective centered on native biodiversity³⁶ and classify alien species’ impacts discerning whether they pose detriments or offer benefits to local populations of native species. This approach has also been used in the recent global Thematic Assessment Report on Invasive Alien Species and their Control of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services⁴⁶. Detrimental and beneficial impacts are measured as decreasing and increasing changes to specific attributes of local populations of native species, such as performance of individuals, population size and area of occupancy, thus aligning with arithmetically defined negative and positive impacts^21,31. To classify the magnitude of these negative and positive impacts, we have used the IUCN EICAT and EICAT + frameworks ^30,31,47. Both frameworks use a semi-quantitative five-tier classification of impact magnitudes (ranging from “Minimal” to “Massive”) based on a common set of attributes (Table I). While impacts classified as “Minimal” and “Minor“ distinguish whether or not changes in the performance of native individuals are detected, the other categories relate to changes at the population level. Specifically, “Moderate” impacts are assigned for changes in population size, while “Major" and ”Massive” impacts relate to changes in the area of occupancy through extinction or re-establishment/extinction prevention of a local population (Table 1). Following the approach adopted by the IUCN³⁰, which classifies negative impacts on populations of native species (Moderate, Major, Massive) as “harmful”, in contrast to impacts that do not involve changes in population size (Minimal, Minor), we further categorized the magnitude of both positive and negative impacts as either “strong” or “weak” (Table 1). This dichotomous variable is referred to as “impact magnitude” hereafter.

Table 1. Criteria used to assess impact magnitudes in EICAT and EICAT+ and to categorize impact magnitude as “weak” or “strong”^30,31.

IUCN EICAT	EICAT+	Detailed criteria for defining negative/positive impacts as measurable decreases/increases in native species attributes	Impact magnitude (level of organization)
Levels of impact magnitude for negative impacts	Levels of impact magnitude for positive impacts		Impact magnitude (level of organization)
Minimal Concern (MC)	Minimal positive impact (ML+)	The alien taxon causes negligible decreases/increases in the performance of native individuals (i.e. their capacity to survive, gather resources, grow, or reproduce).	Weak (individual level)
Minor impact (MN)	Minor positive impact (MN+)	The alien taxon causes decreases/increases in the performance of native individuals, but no decrease/increase in the native population size.	Weak (individual level)
Moderate impact (MO)	Moderate positive impact (MO+)	The alien taxon causes a decrease/increase in the native population size, but no decrease/increase in the area of occupancy (through extinction/re-establishment or extinction prevention of a local population).	Strong (population level)
Major impact (MR)	Major positive impact (MR+)	The alien taxon causes a “reversible” decrease/increase in the area of occupancy (through extinction/re-establishment or extinction prevention of a local population). Reversible impacts are those which disappear after the removal of the alien taxon.
Massive impact (MV)	Massive positive impact (MV+)	The alien taxon causes an “irreversible” decrease/increase in the area of occupancy (through extinction/re-establishment or extinction prevention of a local population). Irreversible impacts are those which do not disappear after the removal of the alien taxon.

When the impact of a species could not be classified due to insufficient data, the species was classified as Data Deficient (DD) and not used for statistical analysis. Note that cases assigned as Data Deficient (DD) differ from Minimal impacts (MC/ML+), where the study design would have allowed discovering impacts, but none were found. For each impact classified by both EICAT and EICAT + we assigned a specific impact mechanism, with ten mechanisms for EICAT and eight for EICAT + ^30,31 (Supplementary Table 1). Mechanisms were classified as either direct (when the alien species directly affected a native species, e.g. through competing for common resources or serving as food) or indirect (when the impact on the native species is indirect through changing another species or ecosystem property, e.g. transmitting a disease or suppressing a dominant competitor). To express the uncertainty associated with the accuracy of the assigned impact magnitudes, the assessor included a confidence level (Low, Medium, High) according to how likely the assigned impact magnitude reflects the true impact. For further information on assigning confidence levels see IUCN^30,47, and Volery et al.⁴⁸.

Collection of impact reports

We followed the search protocol described by Evans et al.⁴⁹ to collect the data and built upon the work of Volery et al.⁶ by incorporating positive impacts of alien LMH on native species. The data sources were obtained by conducting a search using the following terms (‘invasive’ OR ‘invasive species’ OR ‘introduced species’ OR ‘introduced’ OR ‘alien’ OR ‘non-native’ OR ‘non-indigenous’ OR ‘feral’ OR ‘exotic’ OR ‘positive impact’ OR ‘ beneficial’ OR ‘benefit’ OR ‘positive effect’ AND ‘[scientific name of the alien species]’) in the online database Google Scholar (https://scholar.google.com) including articles published in scientific journals as well as grey literature, such as conference abstracts, governmental papers, and private sector research. Similar to Volery et al.⁶, a literature review was performed for all 66 alien LMH species. Data sources containing observed impacts of an alien LMH on a native population were selected based on the evaluation of the title, abstract, and content of the first 100 records found. Additionally, we followed up all references to other data sources with observed impacts in the selected papers until no additional impact records were found. The references gathered for negative impacts by Volery and coworkers⁶ were cross-checked to identify any additional positive impacts. Only observed impacts were included for classification, while potential or inferred impacts were not considered, in line with the guidelines of the frameworks used^30,31,47.

Each impact observation recorded refers to a specific alien LMH species at a specific location and year, along with one impacted native species, the assigned impact magnitude, mechanism (direct or indirect; see Supplementary Table 1), and associated confidence level. Additional information, such as the reference of the impact observation, year of impact (publication year when no specific year was given in the report), taxonomy and trophic level of the impacted native species (decomposer, producer, primary consumer, and secondary consumer/omnivore), geographical details (including precise coordinates, country's sub-unit such as district, state, region or county, country, continent, mainland or island), assessor ID (i.e. the person sourcing and assessing impact observations under EICAT/+), assessment date, and reviewer ID (i.e. the person reviewing the data assessed under EICAT/+), were included as supplementary details. In line with IUCN recommendations^30,47, all negative impacts were cross-checked for consistency in impact magnitude and confidence by at least one reviewer⁶. All positive impacts were scored by Z.B.M and cross-checked by G.V.

Statistical analysis

All analyses were conducted in R (version 4.3.2). A paired t-test was conducted to investigate whether the number of negative impacts consistently differed from that of positive impacts in species exhibiting bidirectional impact observations. Pairwise z-tests were conducted to investigate whether the proportion of impact observations assigned with low, medium, and high confidence differs between negative and positive impacts at each level of impact magnitude. Generalized linear mixed-effect models (GLMMs) with binomial error distribution were built using the package “lme4” (version 1.1.35.1)⁵⁰ to test the effects of multiple predictors on impact magnitude, or more precisely the probability of an alien LMH species causing a strong impact on native biodiversity. The response variable was the impact magnitude coded as “0” for “weak” and “1” for “strong” impacts, for both positive and negative impacts. The predictors coded as fixed effects were impact direction (positive vs. negative), the trophic level of the impacted species (four categories), the impact location (island vs. mainland), reporting impact year (scaled to 0 mean and 1SD⁵¹), mechanism types (direct vs. indirect), and all 2-way interactions of the previous variables with impact direction. The Report ID and alien species name were included as random effects to account for pseudoreplication resulting from multiple observations from the same report and/or the same alien species. To address taxonomic uncertainty, alien LMH subspecies were elevated to the species level (i.e. they had a distinct name in the analysis) only if they were found to impact native conspecifics. For instance, a few observations focused on the impacts of two alien subspecies of Cervus elaphus (C. e. germanicus and C. e. hippelaphus) on native conspecifics through hybridization. We treated these observations as distinct from those attributed to Cervus elaphus and that were investigated at the species level (Fig. 1).

Models with all different combinations of the fixed effects were fitted by maximum likelihood with the Laplace approximation. Models were ranked based on their corrected Akaike Information Criterion (AICc). For the best-fitting models (ΔAICc < 6⁵², we estimated the relative importance (sum of Akaike weights 𝜔⁵³) of each variable and interaction (fixed effects) in the set of models to identify predictors with adequate explanatory power (> 0.5). The evidence ratio of each model i (𝜔₁/𝜔_i, where 𝜔₁ is the Akaike weight of the best-fitting model) was also calculated to estimate the likelihood of each model being the most appropriate representation of the underlying data. While higher ratios indicate stronger evidence in favor of the best-fitting model, lower ratios suggest comparatively stronger support for alternative models. The most supported model, used for pairwise comparisons and graphical representations, was chosen as the model retaining all predictors with adequate explanatory power (sum of weights > 0.5) among the models considered equally plausible (evidence ratio < 2). To verify that the assumptions of the selected model were not violated, we used the R package “DHARMa” (version 0.4.6⁵⁴) to examine the normality of the residuals with a QQ-plot and test for the presence of overdispersion and outliers (p = 0.376, p = 0.380).

Generalized linear mixed-effect models (GLMMs) with binomial error distribution were additionally constructed to test whether weak and strong impacts were assigned with different levels of confidence, and to examine how the assignment of confidence varies by direction and over the years. By using magnitude as a response variable, the predictors coded as fixed effects were impact direction (positive vs. negative), confidence (three categories: low, medium and high), reporting impact year (scaled to 0 mean and 1SD), and all 2-way interactions of the previous variables. The Report ID and alien species name were included as random effects to account for pseudoreplication resulting from multiple observations from the same report and/or the same alien species. We hypothesized that strong impacts are assigned with higher confidence than weak impacts, while we did not expect any difference in confidence when assigning impact magnitude between positive and negative impacts. We also hypothesized that strong impacts assigned with high confidence are reported first. As a consequence, we expect that impact magnitude would decline more quickly in impacts classified with high confidence in comparison with those assigned with lower degrees of confidence.

Tukey’s pairwise multiple comparisons were performed to identify significant differences among levels of categorical variables using the emmeans() and emmip() functions from the package “emmeans” (version 1.8.6). Figures were plotted using the packages “ggplot2” (version 3.4.2) and “sjPlot” (version 2.8.15).

Frequency of negative and positive impacts

We found 315 reports describing 1615 negative and 406 positive impacts for native species that could be classified under EICAT or EICAT+, from 29 of the 66 listed alien LMHspecies. Negative and positive impacts were caused by 27 and 21 LMH species, respectively (Fig. 1). About two thirds of alien LMH species (19 out of 29) caused simultaneously both negative and positive impacts, although for species having bidirectional impacts, we detected 3.7 times more negative than positive impact observations overall (1488 vs. 399, Fig. 1). When comparing these LMH species individually, the trend remained largely consistent (paired t-test: n = 19, t = 4.4; p < 0.001), with records of negative impacts (average = 78.3 ± 64 SD) outnumbering positive impacts (average = 20.8 ± 17.3 SD) in all species except one (Bos taurus, Fig. 1).

Species having exclusively negative impacts in their alien ranges were the Aoudad (Ammotragus lervia), the American bison (Bison bison), the Wapiti (Cervus canadensis), the Asian elephant (Elephas maximus), the Guanaco (Lama guanicoe), the Gemsbok (Oryx gazella), the Mouflon (Ovis orientalis), and the Javan deer (Rusa timorensis). Species (and subspecies) for which only positive impacts have been reported were the Indian hog deer (Axis porcinus), the Nilgai (Boselaphus tragocamelus), the red deer C. elaphus germanicus and C. elaphus hippelaphus (Fig. 1), although in all cases the number of impact observations was small (five or lower).

A great majority (n = 27; 93%) of alien LMH species for which impacts are reported caused strong impacts (positive or negative). Almost all the 27 species with negative impacts caused strong impacts (96%, n = 26). By contrast, among the 21 species causing positive impacts, only 71% (n = 15) caused strong impacts (Fig. 1). Observations of negative impacts outnumbered those of positive impacts across all levels of magnitude (Fig. 1, Supplementary Fig. 1) and confidence (Supplementary Fig. 2).

The predominant impact magnitude observed was Moderate (MO and MO+, Supplementary Fig. 1.), with native population decline documented at 52% (840/1615) and native population increase at 45% (184/406). Across the five impact magnitude levels, confidence was mostly categorized as Low and Medium, while a High confidence was less frequently assigned (Supplementary Fig. 2). This trend was consistent for both negative and positive impacts, except for cases where alien LMH increased the size of native populations (MO+). These cases were assigned with significantly higher confidence compared to instances of native population decreases (MO) (z-test, z = -4.1, p < 0.001, Supplementary Fig. 2).

Overall, alien LMH caused negative impacts mostly through direct mechanisms (direct = 78%, denoted by black labels in Fig. 2), while the opposite trend was observed for positive impacts (indirect = 84%, denoted by green labels in Fig. 2, Supplementary Table 1). The most frequently recorded mechanism for negative impacts was direct “grazing, herbivory, or browsing” (n = 981 impacts), followed by indirect “chemical, physical, or structural impacts on ecosystems” (n = 314 impacts), and direct “bio-fouling or other direct physical disturbances” (n = 296 impacts). Conversely, “indirect impact through interactions with other taxa” (n = 275 impacts) was the predominant mechanism through which positive impacts were caused, followed by indirect “chemical/physical/structural impact on the ecosystem” (n = 74 impacts).

Negative impacts of alien LMH were more frequently documented on islands (68% of all reports of negative impacts), whereas positive impacts showed a more even distribution (51.5% from islands, 48.5% from mainland) (Fig. 3B). Impacts from alien LMH affected four trophic levels: decomposers, producers, primary consumers, and secondary consumers. The trophic level most frequently impacted, both negatively (74%) and positively (59%), was producers (Fig. 3D).

Predictors of impact magnitude

Among the 512 models obtained, each representing different combinations of variables and their interactions with impact direction, 11 exhibited a ΔAICc < 6 in relation to the best model (Table 2). Thus, we performed model averaging and estimated the relative importance (sum of weights) of each factor and interaction within the selected set of models (Table 3).

Table 2

Degrees of freedom (df), values of AICc (corrected Akaike Information Criterion), ΔAICc, Akaike weight and evidence ratio (w1/w, where 𝜔1 is the best-fitting model) of 12 best-fitting generalized linear mixed-effect models (ΔAICc < 6) predicting impact magnitude of alien LMH. In the first column, the letter ‘D’ stands for Direction, ‘L’ for Location, ‘M’ for Mechanism types, ‘Y’ for Year, and ‘TL’ for Trophic level, while the symbol “*” indicates an interaction between two variables. Note that all models retain “Alien species name” and “Report ID” as random effects.
Fixed effects	df	AICc	ΔAICc	weight	Evidence ratio
D, L, M, TL, Y, DL, DY (best-fitting model)	12	1690.14	0	0.24	-
D, L, M, TL, Y, DY*	11	1690.46	0.31	0.2	1.18
D, L, TL, Y, DL, DY, DTL*	14	1690.86	0.71	0.17	1.39
D, L, M, TL, Y, DL, DM, DY*	13	1692.13	1.98	0.09	2.66
D, L, M, TL, Y, DL, DY, DTL*	15	1692.43	2.28	0.08	3.00
D, L, M, TL, Y, DM, DY	12	1692.49	2.34	0.07	3.22
D, L, Y, TL, DY, DTL	13	1693.77	3.63	0.04	6.05
D, L, M, TL, Y, DL, DM, DY, DTL	16	1693.94	3.80	0.04	6.31
D, L, M, Y, DL, DY	9	1694.73	4.59	0.02	10.26
D, L, TL, Y, DY*	10	1694.76	4.62	0.02	10.29
D, L, M, TL, Y, DY, DTL	14	1695.25	5.11	0.02	12.53
D, L, M, Y, DY*	8	1695.47	5.32	0.02	14.94

Table 3

The relative importance, indicated by the sum of weights, of each predictor (variables and interaction between variables) along with the number of models in which they appear, as assessed among the 12 best-fitting generalized linear mixed-effect models (ΔAICc < 6) predicting impact magnitude of alien LMH. In the column header, the letter ‘D’ stands for Direction, ‘L’ for Location, ‘M’ for Mechanism types, ‘Y’ for Year, and ‘TL’ for Trophic level, while the symbol “*” indicates an interaction between two variables.
Predictor	D	L	Y	D*Y	TL	M	D*L	D*TL	D*M
Relative importance	1	1	1	1	0.96	0.77	0.63	0.33	0.2
# models containing the predictor	12	12	12	12	10	9	6	5	3

After averaging models across the 12 best-fitting candidates, predictors (both factors and interactions) demonstrating sufficient explanatory power (relative importance > 0.5, Table 3) aligned with those featured in the model characterized by the lowest AICc (best-fitting model, Table 2). We therefore selected the best-fitting model (Tables 2, 4) as the most supported model for further analyses, pairwise comparisons and data visualizations.

Table 4

Results from the most supported generalized linear mixed-effect model (with Alien species name and Report ID as random effect) testing the overall effects of Direction, Location, Mechanism type, Trophic level and the interaction between Direction and Location, as well as Year, on the probability of an alien LMH species causing a strong impact on native biodiversity. Asterisks indicate significant differences from the reference levels. ***p < .001; **p < .01; *p < .05.
Predictor	Chi-square	df	p
Direction	4.24	1	0.040*
Location	17.11	1	< 0.001***
Mechanism type	9.32	3	0.002**
Year	0.88	1	0.350
Trophic level	10.28	1	0.016*
Direction * Location	2.46	1	0.117
Direction * Year	13.70	1	< 0.001***

Globally, alien LMH species exhibited a higher probability to cause strong negative impacts than positive impacts (Table 4, Fig. 3A, Supplementary Table 2). Moreover, both negative and positive impacts were stronger in insular locations compared to mainland locations (Table 4, Fig. 4A, Supplementary Table 2). Over the years, we detected a non-significant overall decrease in the probability of causing strong impacts, with a steeper decline in the magnitude of positive compared to negative impacts (Table 4, Fig. 4B). Regardless of impact direction, alien LMH species were more likely to cause strong impacts on insular than on mainland locations (Fig. 3B, Supplementary Table 2), through indirect compared to direct impact mechanisms (Fig. 3C, Supplementary Table 2), and on secondary consumers compared to primary consumers (Fig. 3D, Supplementary Table 2). Conversely, the effects on other trophic levels (producers and decomposers) were indistinguishable (Fig. 3D, Supplementary Table 2).

Confidence in assigning impact magnitude

The complete model including Confidence, Direction, Year and their 2-way interactions as predictors of impact magnitude strongly outperformed all simpler models (ΔAICc = 6.2 from the second-best model). According to this model (Table 5), strong impacts were assigned with higher confidence than weak impacts, regardless of impact direction (Fig. 5A, Supplementary Table 2), but the rise in confidence with impact magnitude was steeper in positive than in negative impacts (Fig. 5B, Supplementary Table 2). Moreover, the probability of causing strong impacts decreased significantly faster over the years for impacts classified with high and medium confidence than for those classified with low confidence (Fig. 5C).

Table V. Results from the most supported generalized linear mixed-effect model (with Alien species name and Report ID as random effect) testing the overall effects of Direction, Confidence, Year and the interaction between them on the probability of an alien LMH species causing a strong impact on native biodiversity. Asterisks indicate significant differences from the reference levels. ***p < .001; **p < .01; *p < .05.

Predictor	Chi-square	df	p
Direction	3.35	1	0.067
Year	4.16	1	0.041*
Confidence	25.04	2	< 0.001***
Direction * Year	14.55	1	< 0.001***
Direction * Confidence	31.07	2	< 0.001***
Confidence * Year	10.2	2	0.006**

The introduction of large mammalian herbivores outside their native range has both harmed and benefited local native biodiversity, but negative consequences have largely surpassed positive outcomes, both in frequency and magnitude. The novelty of this study lies in its comprehensive comparison of the negative and positive impacts of alien species, alongside an analysis of the factors determining their magnitude. By leveraging the methodological advances of the EICAT(+) frameworks, we systematically tested hypotheses that were previously only supported anecdotally for negative impacts and never tested for positive impacts. This enabled us to provide a rigorous and detailed examination of how alien species impact native biodiversity, demonstrating that the magnitude of both their negative and positive impacts is influenced by common factors such as insularity and trophic position.

Negative impacts are more numerous, larger and often precede positive impacts

The observed negative impacts of alien LMH disproportionately outnumber positive impacts on a global scale. This overall pattern does not arise solely from a few highly impactful taxa but remains consistent when examining species individually (Fig. 1), and when focusing on mechanisms through which both negative and positive impacts can be caused (Fig. 2, Supplementary Table 1). Our finding that only 20% of all impacts of alien LMH are positive (406/2021) aligns with findings from other systematic searches for positive and negative impacts. The recent IPBES report classified 15% of documented alien species impacts on nature from all taxonomic groups as positive²⁰, while a study by Chen and coworkers⁵⁵ on alien freshwater megafish found only 3% positive environmental impacts. Notably, however, temporal reporting of positive impacts is not increasing faster than that of negative impacts. In fact, the number of positive impact observations has remained quite stable over the last decades (Supplementary Fig. 1.), despite the recent popularity of literature emphasizing the necessity to acknowledge positive impacts of alien species for conservation purposes^25,56–62. Moreover, the magnitude of reported positive impacts decreases faster over the years than negative impacts (Fig. 4B), suggesting that the greater number and severity of negative impacts are not due to reporting bias but reflect an inherent asymmetry in how alien LMH affect native biodiversity.

Additionally, a great majority of all alien LMH species studied for their environmental impacts (26 of 29) caused negative population level impacts (Fig. 1), and through disparate mechanisms, such as herbivory, direct physical disturbance, hybridization, interactions with other species, and indirect impacts on ecosystems (Fig. 2, Supplementary Table 1). Almost half of the studied alien LMH species (14), conversely, did not have documented positive impacts at the population level (Fig. 1). Accordingly, positive impacts were overall characterized by lower magnitude than their negative counterparts (Figs. 3A,4A), thus further supporting the "Harm Dominance Hypothesis". Moreover, strong positive impacts were predominantly caused through indirect mechanisms (195 cases out of 202, Fig. 1), while direct mechanisms (i.e. provision of trophic resources and habitat or overcompensation) rarely led to population level impacts (7). Since strong positive impacts on native biodiversity were caused by alien LMH mostly via indirect impacts through interactions with other species (167), and often the latter were species negatively affected by the same alien LMH, we suggest that negative impacts often precede positive ones. Particularly insightful are cases where the same species exhibited both strong negative and strong positive impacts, with the negative impacts consistently outnumbering the positive (Fig. 1, p < 0.001). For instance, the grazing pressure imposed by introduced goats (Capra hircus) on native vegetation caused a decline in the abundance of 66 insular plant species, and three instances of extirpation were also reported (Fig. 1). Conversely, only 13 plants have increased their abundance after the introduction of goats to islands. This positive effect was primarily observed on unpalatable ferns and monocotyledonous species, which benefited from competitive release as the goats preferentially fed on more palatable broadleaf plants⁶³. Similar ecosystem changes from woodlands to grasslands (including ferns) were promoted by widespread alien deer species such as Cervus elaphus^63,64, Cervus nippon⁶⁵ and Muntiacus reevesi⁶⁶. Under some circumstances alien LMH significantly benefit native plant species that are less abundant in native communities by releasing them from their competitors. Such a positive outcome is achieved at the expense of more competitive native species that are suppressed by the same alien LMH (see mechanism “Interaction with other species” in Fig. 2). Thus, many positive impacts generally do not occur directly, but only indirectly, after other native species suffer. If positive impacts are often due to the prior occurrence of negative impacts – which can conversely occur independently of positive impacts through mechanisms such as herbivory or direct disturbance (Fig. 2) – this could partially explain why the number of negative impacts of alien species is larger overall.

Explaining magnitude and direction: islands

In accordance with our predictions, both negative and positive impacts of alien LMH were larger on islands (Fig. 3). The effect of insularity on impact magnitude is particularly evident for negative impacts (Fig. 4A), supporting the hypothesis that insular biodiversity is particularly vulnerable to anthropogenic alterations^10,11. Previous studies have shown that species on islands were driven towards local or global extinction primarily by predatory effects from a few widely introduced mammals such as rats, mongooses, wild boars, and feral cats and dogs^37,67. Predation is the most widely cited mechanism for biodiversity decline on islands^68–71. In our study, most predation events (49 out of 52) were by wild boars (Sus scrofa), with 17 occurring on islands, but only six led to strong impacts, such as declines in two insular lizards, three seabirds, and one rail species. Conversely, most strong negative impacts on islands (60) were caused by other mechanisms, such as direct physical disturbance, and chemical, physical, or structural impacts on ecosystems, and grazing/herbivory/browsing, which also occurred on the mainland. This substantiates the rarely tested assumption that native biodiversity on islands is particularly vulnerable to impacts of alien species, regardless of the mechanisms.

Our findings highlight the unique and sensitive nature of insular ecosystems, where positive impacts of alien species are also higher in magnitude. Alien species can facilitate native biodiversity by restoring functions previously held by recently extinct or extirpated species⁶¹, particularly on islands^34,38. However, our data on alien LMH do not conclusively support the functional replacement hypothesis. For instance, alien wild boars, feral goats, Reeves' muntjacs, and mule deer have facilitated the dispersal of native plants on islands, but their positive impacts were weak, meaning they did not increase plant populations. Only a few strong positive impacts on islands were caused through chemical, physical, and structural impacts on ecosystems (N = 4), epibiosis or other direct habitat provisions (1), overcompensation (1), and provision of trophic resources (1). Alien species had positive population-level impacts mainly through interactions with other species (113), mostly benefiting plants (98) by reducing the grazing or browsing pressure on their direct native competitors. The higher magnitude of positive impacts on islands might be an indirect consequence of the initial decline caused by alien LMH on insular biodiversity.

Explaining magnitude and direction: trophic level

We found that native species at higher trophic levels (secondary consumers) were more impacted by alien LMH than those at lower trophic levels. While there is evidence that top trophic levels are more sensitive to environmental change¹⁷, our study is the first to demonstrate this across multiple terrestrial taxa. Previous studies have mainly explored this relationship within single taxa or taxonomic levels, or only in marine communities. For example, terrestrial alien plants have caused various negative impacts on higher trophic levels^18,19,72,73, but it remains unclear if these impacts are larger, equal to, or smaller than those on native producers⁷⁴. Thomsen and coworkers⁷⁵ found that alien marine producers and consumers negatively impact native species within their trophic level rather than higher ones, mainly through competition and other antagonistic interactions. They also found that introduced species can serve as significant novel food resources for native consumers, benefiting species positioned directly above in the trophic chain. Our results suggest that introduced LMH have severe impacts on high-trophic-level species, mostly through indirect interactions or ecosystem changes. In contrast, direct impacts on species at the same or lower trophic levels through antagonistic interactions like competition, predation, or herbivory have lower impact magnitudes. While species high in the food chain might be particularly vulnerable to alien species, our findings stress the need for community-level studies that include complex indirect interactions beyond direct individual species interactions.

Similar considerations may apply to positive impacts. Studies on native predators feeding on alien species^42,76–78, pollinators utilizing alien plant nectar and pollen⁷⁹ and frugivores incorporating alien fruits in their diets⁸⁰ found that alien species can benefit species directly above them in the trophic chain by providing trophic resources^39,40,75. However, among all positive impacts of alien LMH on secondary consumers (114), only nine (8%) were through food provision, with only one having population-level consequences. Instead, alien LMH benefited secondary consumers mostly indirectly through ecosystem changes (53) and interactions with other species (50), leading to strong population-level positive impacts in the majority of cases (60/103). We conclude that species high in the food chain can sometimes benefit from complex trophic cascades or habitat provisioning initiated by alien species introductions, while direct provision of trophic resources plays a minor role in affecting local biodiversity.

Temporal trends of impact magnitude and confidence in its assignment

We did not find support for our hypothesis that regardless of impact direction, strong impacts are reported first and thus impact magnitude would decline over time. Notably, we found that impact magnitude steeply declines over time for positive impacts (Fig. 4B), whereas the decline for negative impacts was much shallower and non-significant (Table 4). This may indicate that strong positive impacts, i.e. those concerning population level changes induced by alien LMH on native species, were identified, and therefore reported, first due to their obvious extent. Conversely, positive impacts having weaker magnitude levels, i.e. involving individuals rather than populations, might have been initially less evident and remained undetected for years.

Alternatively, improved analytical methods might have revealed that positive impacts often affect native individuals without significant population-level consequences. This latter conjecture might be supported by the finding that the magnitude of positive impacts classified with high and medium confidence decreases more steeply than low-confidence impacts. This indicates that population-level positive impacts assigned with greater certainty become scarcer over time in favor of those assigned with analogous levels of confidence but measured at the individual level. It is also worth noting that while weak impacts also encompass Minimal positive impacts (Table 1), they have been reported more often (n = 63 vs 46) and with higher confidence (high/medium = 52% vs 6%) in the last two decades (2000–2019) than in the previous two decades (1980–1999). We anticipate this trend will continue, as our research identified several instances where positive impacts at the individual level might exist. However, the study design or the use of composite biodiversity indicators (such as species richness, diversity, or evenness) did not allow us to conclusively determine the magnitude of these impacts. For example, future studies will likely elucidate to which extent feral donkeys in the Sonoran Desert, which are preyed upon by native cougars⁸¹ and play a role in shaping dryland ecosystems by increasing water availability⁸², benefit native species, but also which other native species might suffer.

Implications for conservation

Large mammalian herbivores have recently been suggested as candidates for restoring ecosystem functions that were lost during the pleistocenic and holocenic human-mediated extinctions^83,84, a strategy often referred to as rewilding. Since many LMH species are also threatened by extinction in their native range³², establishing populations in areas where the species has never occurred in their history might be a viable conservation option (“assisted colonization”^35,85), if the newly introduced species do not significantly harm local communities. Although a recent meta-analysis showed that alien LMH have impacts on vegetation abundance or diversity that are on average no different than those from native species, even on islands⁴⁴, our study clarifies that negative impacts of introduced LMH dominate and are more pronounced on islands, not only for native plants but also for other taxa. Decisions about the introduction or removal of alien large mammalian herbivores (LMH) for conservation purposes, including eradication, rewilding, and assisted colonization, should involve a careful risk assessment considering the local context^86,87. Additionally, these decisions must be clear about the conservation goals and ethical trade-offs⁸⁸. There will be winners and losers in local communities, which can be identified with the EICAT(+) frameworks but may be overlooked when relying on average impacts and community metrics.

Competing interests

The authors declare that they have no competing interests.

Author contribution

Z.B.M., S.B., and G.V. conceived the study. Z.B.M. collected the data and led the data analysis, with supervision by S.B. and G.V. G.V. led the manuscript writing, with significant contributions from Z.B.M. and S.B., and prepared the figures. All authors have read and approved the final version of the manuscript for submission.

Acknowledgements

S.B. and G.V. acknowledge funding by the Belmont Forum BiodivERsA International joint call project InvasiBES and by the Swiss National Science Foundation (31003A_179491 and 31BD30_184114).

Data availability statement

The data that support the findings of this study are provided in the Supplementary Information/SourceData file. Source data are provided with this paper.

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Harms of introduced large herbivores outweigh their benefits, while both are greater on islands and for higher trophic levels

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Abstract

Figures

Introduction

Materials and Methods

Large mammalian herbivores as a study system

Impact assessment frameworks

Collection of impact reports

Statistical analysis

Results

Frequency of negative and positive impacts

Predictors of impact magnitude

Confidence in assigning impact magnitude

Discussion

Negative impacts are more numerous, larger and often precede positive impacts

Explaining magnitude and direction: islands

Explaining magnitude and direction: trophic level

Temporal trends of impact magnitude and confidence in its assignment

Implications for conservation

Declarations

Competing interests

Author contribution

Acknowledgements

Data availability statement

References

Additional Declarations

Supplementary Files

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