# Looming extinctions due to invasive species: Irreversible loss of ecological strategy and evolutionary history

## Running title: Functional and phylogenetic extinctions due to biological invasions

Céline Bellard, Camille Bernery, Camille Leclerc

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Céline Bellard, Camille Bernery, Camille Leclerc. Looming extinctions due to invasive species: Irreversible loss of ecological strategy and evolutionary history Running title: Functional and phylogenetic extinctions due to biological invasions. Global Change Biology, 2021, 10.1111/gcb.15771. hal-03312166

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Looming extinctions due to invasive species: Irreversible loss of ecological strategy and
evolutionary history

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**Running title:** Functional and phylogenetic extinctions due to biological invasions

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Céline Bellard¹, Camille Bernery¹ & Camille Leclerc¹,²

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¹ Université Paris-Saclay, CNRS, AgroParisTech, Ecologie Systématique Evolution, 91405,
Orsay, France

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² Current Address: INRAE, University of Aix Marseille, UMR RECOVER,
Aix- en- Provence, France

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ORCID: Céline Bellard https://orcid.org/0000-0003-2012-1160

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Camille Bernery https://orcid.org/0000-0002-6393-1668

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Camille Leclerc https://orcid.org/0000-0001-5830-1787

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* Corresponding author: celine.bellard@universite-paris-saclay.fr +33 1 69 15 79 61

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**Abstract**

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Biological invasions are one of the main drivers of biodiversity decline worldwide. However, many associated extinctions are yet to occur, meaning that the ecological debt caused by invasive species could be considerable for biodiversity. We explore extinction scenarios due to invasive species and investigate whether paying off the current extinction debt will shift the global composition of mammals and birds in terms of ecological strategy and evolutionary history. Current studies mostly focus on the number of species potentially at risk due to invasions without taking into account species characteristics in terms of ecological or phylogenetic properties. We found that 11% of phylogenetic diversity worldwide is represented by invasive-threatened species. Further, 14% of worldwide trait diversity is hosted by invasive-threatened mammals and 40% by invasive-threatened birds, with Neotropical and Oceanian realms being primary risk hotspots. Projected extinctions of invasive-threatened species result in a smaller reduction in ecological strategy space and evolutionary history than expected under randomized extinction scenarios. This can be explained by the strong pattern in the clustering of ecological profiles and families impacted by invasive alien species (IAS). However, our results confirm that IAS are likely to cause the selective loss of species with unique evolutionary and ecological profiles. Our results also suggest a global shift in species composition away from those with large body mass, which mostly feed in the lower foraging strata and have a herbivorous diet (mammals). Our findings demonstrate the potential impact of biological invasions on phylogenetic and trait dimensions of diversity, especially in the Oceanian realm. We therefore call for a more systematic integration of all facets of diversity when investigating the consequences of biological invasions in future studies. This would help to establish spatial prioritizations regarding IAS threats worldwide and anticipate the consequences of losing specific ecological profiles in the invaded community.
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## Introduction

Invasive alien species (IAS) are considered to be the second most important cause of worldwide extinctions (C. Bellard, Genovesi, & Jeschke, 2016; Gurevitch & Padilla, 2004; Maxwell, Fuller, Brooks, & Watson, 2016). Invasive predators alone have contributed to 58% of modern extinctions (Doherty, Glen, Nimmo, Ritchie, & Dickman, 2016). Most of these extinctions occurred on islands, but mainland areas also suffer from the presence of IAS (Céline Bellard, Cassey, & Blackburn, 2016; Clavero, Brotons, Pons, & Sol, 2009). The global threat of species invasions continues to increase with the spread of new emerging invaders (Seebens et al., 2017, 2018).

Over the past decades, several review articles and global analyses have investigated the ecological impact of IAS, ranging from local population declines to global extinctions (Lapiedra, Sol, Traveset, & Vilà, 2015; McCreless et al., 2016). However, most studies focus on the number of species prone to extinctions without taking into account species characteristics in terms of ecological or phylogenetic properties, even though all three facets (taxonomic, phylogenetic, trait) are advocated as essential for conservation (Mazel et al., 2014). Indeed, phylogenetic and trait diversity provide a more direct link to ecosystem properties compared to species diversity (e.g., Sekercioglu, 2006; Cadotte, Cardinale & Oakley, 2008). Extinctions and population declines are not random, and as a result, they are more likely to be associated with specific lineages or ecological profiles (i.e., combinations of morphological, phenological, or behavioral features) (Cooke, Eigenbrod, & Bates, 2019; Davis, Faurby, & Svenning, 2018; Pavoine, Bonsall, Davies, & Masi, 2019).

Previous assessments showed that climate change may lead to uncompensated phylogenetic or functional losses, causing further biotic homogenization (Buisson, Grenouillet, Villéger,
72 Canal, & Laffaille, 2013; Thuiller et al., 2011), although this has not yet been explored in the
73 context of biological invasions. In other words, the manner in which species extinctions due
74 to IAS affect the ecological and evolutionary dimensions of biodiversity mostly remains an
75 open question (but see Lapiedra et al., 2015; Sunday et al., 2015; Longman, Rosenblad &
76 Sax, 2018). This issue is particularly important, as shown by the recent surge in interest
77 among international experts of the Intergovernmental Science-Policy Platform on Biodiversity
78 and Ecosystem Services (IPBES), who advocated that including ecological and phylogenetic
79 components of biodiversity is essential in biodiversity assessments (IPBES, 2019).

80 Moreover, taxonomic diversity often fails to capture phylogenetic and ecological diversity
81 (Devictor et al., 2010; Leclerc, Villéger, Marino, & Bellard, 2020; Mazel et al., 2014); it is
82 thus highly important to consider multiple dimensions of diversity when providing
83 recommendations on spatial conservation priorities (Brum et al., 2017; Pollock, Thuiller, &
84 Jetz, 2017). Indeed, biodiversity is a multifaceted concept, and the emerging consensus is that
85 a macro-ecological approach is needed to understand the impact of IAS worldwide. With
86 increasing information about the phylogenetic and life history traits of species, it is now
87 possible to conduct such an assessment and provide a clear picture of the consequences of
88 IAS on multiple dimensions of diversity.

89 A critical aspect of biodiversity assessments is thus to identify and characterize the diversity
90 elements most likely to go extinct, thus inferring the future state of biodiversity. Here, we aim
91 to provide such an assessment of at-risk biodiversity related to biological invasions with a
92 focus on the trait and phylogenetic diversity of birds (n = 8,113) and mammals (n = 4,804).
93 Trait diversity (TrD) represents how species are distributed in a multidimensional niche space
94 and provides a way to assess the ecological strategy adopted by species. We measured the
95 amount of trait space occupied by invasive-threatened species compared to the global pool
96 worldwide following the methodology of Villéger, Mason, and Mouillot (2008) based on five
ecological traits (Table 1). In other words, for TrD, we calculate how much of the total trait space is occupied by invasive-threatened mammals and birds. Phylogenetic diversity (PD) captures the evolutionary history of species (Pavoine et al., 2011) and potentially the species contribution to feature diversity (Faith, 1992). We measured the amount of evolutionary history represented by invasive-threatened species at a global scale for both birds and mammals. We expect that species losses due to IAS may result in the loss of ecosystem functions and/or adaptive features, which are indispensable in a changing environment. We also investigate the contribution of invasive-threatened birds and mammals to the total PD and TrD in different realms. Because all invasive-threatened species are unlikely to go extinct in the near future, we further investigate extinction scenarios related to IAS worldwide. We specifically explore the potential level of phylogenetic and ecological erosion in IAS extinction scenarios over the next 50 and 100 years as well as under randomized extinction scenarios. Finally, we determine the ecological profiles and species families most likely to be lost because of IAS.

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## 112 Methods

### 113 Species studied

We used the IUCN Red List of Species to identify the species considered to be the most threatened by IAS (Version 3, accessed in August 2019). We considered species classified as critically endangered (CR), endangered (EN), vulnerable (VU), near-threatened (NT), or least concern (LC) by the IUCN Red List. We focused on the threatened species defined by the IUCN (CR, EN, VU) for which IAS are listed as a threat (hereafter, invasive-threatened) and excluded NT and LC species, which only represent “potentially threatened” species for two reasons. Therefore, we filtered out species for which IAS is listed as a future threat or when
the threat is due to a problematic species/disease (including viral or prion) of unknown origin
or native. First, we want to focus our analyses on the threatened species (CR, EN, VU) facing
imminent extinctions so as to be conservative in our study following the methodology of
Toussaint et al. (2016). Second, IUCN only assigns a specific extinction probability to VU,
EN, and CR. No quantitative analyses of the probability of extinction are required for NT or
LC, while the latter category does not specify the major threat faced, which will add
uncertainty to our analyses of the IAS threat (IUCN, 2012). Moreover, we focused our
analysis on the most comprehensively assessed vertebrate groups by the IUCN: birds and
mammals (Meiri & Chapple, 2016). This resulted in a total of 207 invasive-threatened
mammals out of 5,708 mammals assessed by the IUCN Red List and 499 invasive-threatened
birds out of 10,965 birds (IUCN, 2017). Note that it is impossible to disentangle the effects of
IAS from other threats. IAS threat is mostly associated with biological resource use and
agriculture. In fact, the vast majority of species threatened by IAS are also likely to be
threatened by other threats with an average of 3.9 threats co-occurring for IAS-threatened
birds and mammals; thus, IAS-threatened species are species for which IAS is listed as a
threat, although other threats might also occur. In fact, 837 mammals and 736 birds are
threatened by other threats. We also used the mechanisms documented for each invasive-
threatened species based on the IUCN information (e.g., competition, hybridization, reduced
reproductive success, species mortality, ecosystem degradation) to assess the contribution of
each mechanism to the potential loss of PD and TrD. We considered the top three
mechanisms for both birds and mammals, which represent more than 80% of all mechanisms
documented for each taxon. We considered species mortality, competition, and ecosystem
degradation for mammals and ecosystem degradation, species mortality, and reduction in
reproductive success for birds. We considered each mechanism when cited alone or in
association with another mechanism. Therefore, our indicator of mechanisms represents the
documented occurrences of a given mechanism for each species. We also used the realm
associated with each species extracted from the IUCN database to conduct analyses at the
realm scale (i.e., Afrotropical, Australasian, Indomalayan, Neartic, Neotropical, Oceanian,
and Paleartic).

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## 151 Trait diversity analysis

To quantify TrD associated with invasive-threatened species, we used the five following
traits: body size, habitat breadth, foraging strata, main diet, and period of activity (Table 1 for
modalities), which are associated with the key ecological strategies of species (e.g., carcass
removal, seed dispersal, soil fertility, pollination; Sekercioglu, 2010; Hevia et al., 2017; see
also Appendix S3). All traits were extracted from the Elton trait database (Wilman et al.,
2014) except for habitat types, which were taken from the IUCN habitat classification scheme
(version 3.1). Main diet category for mammals is based on the majority of diet consumed by
each species, with a 50% threshold (following Wilman et al. (2014)). For instance, the mixed
herbivore category represents all species whose diet consists of more than 50% of plant
material, seeds, fleshy fruits, and nectar. The omnivore (mixed) category includes all species
that are 50% mixed herbivore and 50% mixed animal. Foraging strata category for birds is
based on the location of the different items consumed by each species with a 50% threshold
(following Wilman et al. (2014)). If the foraging strata include several categories, we kept the
foraging strata represented by more than 50% of the whole foraging strata; and if there is no
main foraging stratum represented by more than 50%, it is designated as "multiple". Body mass
category for birds and mammals is based on their respective quantiles with the very large
category representing the largest 20% of species. Overall, we extracted trait data for 4,804
mammals, including 186 invasive-threatened mammals, and 8,113 birds, including 360
170 invasive-threatened birds (Fig. S2). The two continuous variables (i.e., main diet and body size) were categorized as discrete variables following the method of Leclerc *et al.* (2020) (see also Table 1 for details).

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174 To compute and investigate TrD for each invasive-threatened species assemblage, we
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176 (i) First, we grouped species as ecological entities, which represent groups of species
177 sharing the same trait values (Table 1), methodology that has been successfully
178 applied in functional analyses (Keyel & Wiegand, 2016; David Mouillot et al.,
179 2014). This step allowed us to calculate trait vulnerability, which is related to the
180 number of species included in each ecological entity. Trait vulnerability is high
181 when ecological entities have a small number of species and low when ecological
182 entities host a high number of species, which indicates high redundancy.

183 (ii) To build the trait space, we also calculated pairwise trait distances using the
184 Gower metric, which allowed us to deal with data of mixed types (Pavoine, Vallet,
185 Dufour, Gachet, & Daniel, 2009). We then used principal coordinate analysis
186 (PCoA) on the trait distance matrix to build a multidimensional trait space, where
187 the position of ecological entities corresponds to their differences. We selected the
188 best functional space by choosing the number of PCoA axes that provided the most
189 faithful representation of the initial functional trait values. In other terms, we
190 selected the number of axes that minimized the mean squared-deviation metric
191 (i.e., average deviation between Euclidean distance and Gower distance; Maire *et
192 al.*, 2015), which allowed us to compute the indices in a reasonable timeframe. In
193 this study, we selected five axes for mammals, which represent an average error of
194 3.6% (average deviation between original and transformation data) and three axes
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for birds, which had an average error of 5.6%, indicating functional spaces of high
qualities.

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(iii) Based on these trait spaces for birds and mammals, we calculated TrD (more commonly known as functional richness following the definition of Villéger *et al.* (2008)), which represents the volume of trait space occupied by invasive-threatened species within the PCoA compared to the total pool of species. This metric is widely applied in functional studies (Leclerc et al., 2020; David Mouillot et al., 2014; Toussaint et al., 2016). It has been demonstrated as the best performing index and is highly correlated to other functional diversity measures (Mouchet, Villéger, Mason, & Mouillot, 2010). This step was also conducted at the realm scale.

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(iv) We also performed statistical analyses to compare the trait modalities associated with invasive-threatened species and those associated with either non-invasive-threatened species worldwide and species threatened by threats other than IAS (i.e., other-threatened species). Specifically, we compared the trait distributions of invasive-threatened species with a random sample (with replacement) of an equal number of species that are not invasive-threatened within the global pool (999 repetitions). We also compared the trait distributions of invasive-threatened species with a random sample of an equal number of other-threatened species within the global pool. These two comparisons allowed us to disentangle whether the trait modalities were specific to the invasive threat or whether there were related to threatened species (Table S1).

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To test the significance of the results, we applied Chi-squared tests when the number of
species in each modality was ≥5 for all the samples. Otherwise, a Fisher's exact test was

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used. If at least 95% of tests had a p-value <0.05, the two distributions were considered to
significantly differ. We repeated the same analyses for both mammals and birds.

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**Phylogenetic diversity analyses**

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To compute PD hosted by invasive-threatened species, we used the PHYLACINE 1.2
complete phylogeny by Faurby *et al.* (2018) for mammals. This built phylogeny relies on the
morphological and genetic data of 5,831 known mammal species that have lived since the last
interglacial period. It also contains 1,000 trees that represent the uncertainties in topology and
branch lengths. We also verified species synonyms using the rl_synonyms() functions in the
rredList package (Chamberlain, 2019), and we pruned the original phylogenetic tree
(n = 5,831) to the species of the IUCN Red List included in the tree for mammals. In total,
5,529 species, including 205 invasive-threatened species, were included in the phylogenetic
trees (Fig. S1 for the sample size of each analysis). For birds, we used the tree version (V2.iii,
Ericson backbone) with the complete phylogeny of Jetz *et al.* (2014) and extracted 1,000
trees. The built phylogeny combines the relaxed clock molecular trees of well-supported avian
clades with a fossil-calibrated backbone with representatives from each clade. Similar to
mammals, we also searched for synonyms and pruned the original phylogenetic tree to 8,113
birds, including 360 invasive-threatened species (Fig. S1). Then, to measure PD associated
with invasive-threatened species for both mammals and birds, we used the Faith Index (using
the pd() function of the picante package; Kembel *et al.*, 2010) and calculated the average PD
across the 1,000 trees. Ultimately, we calculated the percentage for the invasive-threatened
species PD value compared to the total PD value of the sample in order to compare between
taxa. We also conducted PD analysis for each realm.

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243 **Null model analyses for both phylogenetic and trait diversity measures**

244 For both mammals and birds, we tested whether the observed values of TrD were significantly 245 different from the null hypothesis that species are randomly distributed into ecological 246 entities. We used null models here to examine if TrD values are related to the number of 247 species or to the ecological profile of species. In each invasive-threatened sample, we 248 simulated a random assignment of species to ecological entities, while ensuring that each 249 ecological entity has at least one species. We simulated 999 random assemblages, while 250 keeping the number of species and ecological entities constant.

251 We also randomized the phylogenetic information between the invasive-threatened species for 252 both mammals and birds. For this purpose, we considered a constrained null model that 253 randomizes the names of taxa to the phylogeny. Thus, it randomizes which species are most 254 closely related to each other, although it does not alter the actual branch lengths or their 255 distributions. The rationale for this null model is to investigate if the species at risk of 256 extinction due to IAS are more closely related than expected randomly given their number. 257 The randomization was repeated 999 times over 1,000 trees each time.

258 More specifically, we calculated the deviation from the null expectation by computing the 259 standardized effect size (SES) and associated p-value. SES represents the difference between 260 the observed values of TrD or PD and the mean of predicted values by the null model divided 261 by the standard deviation of predicted values. The significance of the difference between 262 observed values and null expectations was tested using a bilateral test at a level of 5%. In 263 short, an observed value is considered significantly different from null expectations if the 264 observed value is in the top or bottom 2.5% of the null expectation distribution.

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266 **Projected extinction scenarios**
We considered two extinction scenarios based on the IUCN Red List categories over the next 267 50 and 100 years. The IUCN Red List designed probabilities of extinctions for the three 268 threatened categories that are evaluated under criterion E: Prob(ext)CR = 0.5 in 10 years, 269 Prob(ext)EN = 0.2 in 20 years, and Prob(ext)VU = 0.1 in 100 years (Mooers, Faith, & 270 Maddison, 2008). Based on these probabilities of extinction, in the first scenario (50 years), 271 we assigned a probability of extinction for all species (even if there are not evaluated under 272 criterion E) of 97% for CR invasive-threatened species, 42% for EN, and 5% for VU 273 following Mooers et al. (2008) and Cooke et al. (2019). Similarly, the probability of 274 extinction under the second scenario (100 years) was 99% for CR invasive-threatened species, 275 66.7% for EN, and 10% for VU following Mooers et al. (2008). The randomized extinction 276 scenario assigned an equivalent number of species that were predicted to go extinct (from the 277 global pool) over the next 50 and 100 years but randomly with respect to species identity and 278 traits (Cooke et al., 2019). All three extinction scenarios (100 years, 50 years, and random) 279 were each repeated 999 times. Finally, we compared the potential level of phylogenetic and 280 ecological erosion obtained under both scenarios with that obtained from a randomized 281 extinction scenario. 282

We used R version 3.6.1 to conduct the analyses and the ggplot2 package to plot the figures 283 (Wickham, 2016). 284

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## 286 Results

287 *Trait and phylogenetic diversity hosted by invasive-threatened species worldwide and within* 288 **realms.** Our results showed that invasive-threatened birds occupy 40% of the total trait space 289 (TrD) for birds worldwide. By contrast, invasive-threatened mammals represented only 14% 290 of the total trait space for mammals (Table 2). The contribution of invasive-threatened species
291 to the total PD worldwide was 10.2% for mammals and 11.4% for birds. In all cases, the PD
292 values for invasive-threatened mammals and birds were significantly lower than expected
293 given the number of species (standardized effect size (SES = -7.294 and P > 0.99 for
294 mammals, and SES = -4.542 and P > 0.99 for birds)). Similarly, TrD values were significantly
295 lower than expected given the number of species (SES = -4.177 and P > 0.99 for mammals
296 and SES = -3.672 and P > 0.99 for birds). Both diversity metrics were thus more clustered
297 than expected under the null scenario.

298 We also calculated diversity measures for the top three mechanisms associated with invasive-
299 threatened species (Fig. 1). We found that the contribution of invasive-threatened birds
300 worldwide to TrD and PD was largely driven by ecosystem degradation (36.8% for TrD and
301 11.1% for PD) and mortality induced by IAS. The reduction in reproduction success was also
302 an important mechanism associated with the potential loss of TrD and PD in birds worldwide.
303 Conversely, the contribution of invasive-threatened mammals to TrD and PD worldwide was
304 mostly driven by mortality (12.05% for TrD and 8.8% for PD) induced by IAS, while
305 competition and ecosystem degradation were less likely to lead to mammal TrD losses.

306 By analyzing the diversity represented by invasive-threatened birds and mammals for each
307 realm, we found clear spatial differences (Fig. 2). The contribution of invasive-threatened
308 birds located in the Oceanian realm to the total bird TrD worldwide is nearly 30%. The
309 contributions of invasive-threatened birds in the Afrotropical and Neotropical realms to total
310 bird TrD was also high with 20% and 17%, respectively, whereas the contribution of
311 invasive-threatened mammals in the Neotropical realm to total mammal TrD was only 4%.
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313 We found that all the realms showed low PD values for invasive-threatened mammals (0.7%-
314 4.2%) and birds (0.93%-5.4%). In addition, we found an average of 1.4 invasive-threatened
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for mammals and 1.2 for birds within realms, while we estimated 6.8 species per ecological

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entity in the rest of the species sample for mammals and 13.7 for birds.

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*Extinction scenarios of invasive-threatened species compared to random scenarios.* Because all the invasive-threatened species are unlikely to go extinct in the near future, we considered extinction scenarios following the methodology of Cooke *et al.* (2019). On average, 6.5% to 7.7% of worldwide PD associated with invasive-threatened mammals is expected to be lost over the next 50-100 years, while the percentage is slightly higher under the randomized extinction scenarios for mammals (7.7% to 9.3%). Loss of PD worldwide for birds is projected to be of a similar order of magnitude with 6.1% to 7.2%, which is also slightly lower compared to randomized extinction scenarios (6.5% to 7.8%). The potential loss is more pronounced for TrD, which may reach 5.8% to 7.8% for mammals (11.3% to 15.0% under the randomized extinction scenarios) and 27.4% to 30.6% for birds worldwide (32.5% to 36.3% under the randomized extinction scenarios) over the next 50-100 years due to IAS.

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In all cases, the potential loss of both facets of diversity (PD and TrD) induced by IAS is lower than expected given the species richness under the random extinction scenarios (Fig. 3).

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*Profile of species at risk of extinctions.* We further analyzed the phylogenetic and ecological properties of invasive-threatened species for both mammals and birds and compared them to the characteristics of the pool of non-invasive-threatened species (hereafter, global pool) and species threatened by threats other than IAS. We found that the 186 invasive-threatened mammals represent 109 ecological entities (groups of species sharing the same combinations of traits, with 14 ecological entities exclusively represented by invasive-threatened mammals

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and not found in the global pool) (Table S1). Four families were comprised exclusively of species threatened by IAS, although other threats may also be involved: Solenodontidae, Myrmecobiidae, Phascolarctidae, and Thylacomyidae. Muridae and Cricetidae were the most represented families among invasive-threatened mammals as well as among the global pool of mammals. Similarly, the 360 invasive-threatened birds represented 149 ecological entities (including 10 ecological entities exclusively hosted by invasive-threatened birds), which belong to 92 families, including 5 families (i.e., Balaenicipitidae, Strigopidae, Rhynochetidae, Notiomystidae, and Pedionomidae) that were exclusively found in the invasive-threatened pool. The two most represented families within invasive-threatened birds were Procellariidae and Psittacidae, although they were respectively ranked 59th and 4th when considering the global pool of birds.

A closer investigation of the ecological profile of invasive-threatened species revealed that mammals were represented by mostly large to very large species (68%), specialists with only one or two habitats (65%), only nocturnal species (61%), those feeding on the ground (76%), and those with a primarily or exclusively herbivorous diet (62%) (Fig. 4; see also Table S2). We observed some significant differences with the global pool of mammals regarding foraging strata (i.e., species were mostly aerial in the global pool), diet regime (species mostly feed on invertebrates) and body size (species have a very small body size) (Fig. 4; see also Table S4), while significant differences were also detected for the period of activity and habitat breadth when compared to other-threatened species (See Fig. 4 and Table S4). We found a significantly higher percentage of ground-foraging species within invasive-threatened mammals compared to both the global pool and the other-threatened species pool (Fig. 4). By contrast, invasive-threatened mammals were significantly less represented in the arboreal foraging strata compared to the global pool and the other-threatened pool of mammals. Our results also revealed that invasive-threatened species were less likely to be diurnal and less
likely to be specialized to one habitat compared to other-threatened species. In addition, it appears that invasive-threatened mammals were significantly more represented among very large species and significantly less among very small species in comparison with the global pool, although these characteristics were not specific to invasive-threatened species, as other-threatened species were also very large species. Finally, our results also revealed that invertebrate diets were less represented among invasive-threatened mammals compared to the global pool (Fig. 4). This was also the case with other-threatened species, although the difference was not statistically significant.

Invasive-threatened birds were mostly large to very large species (63%) and habitat specialists with three or less habitats (81%); the majority foraged in multiple strata (57%), were active during the day (97%), and mostly fed on animals (41%) (Fig. 5; see also Table S3). Again, invasive-threatened species had similar ecological characteristics compared to the rest of the species pool but with a few notable exceptions (Fig. 5; see also Tables S6-7). For instance, a significantly higher percentage of invasive-threatened birds forages below the water surface compared to both the global pool and the other-threatened species pool, while a significantly lower percentage forages in multiple strata compared to the other samples (Fig. 5). Invasive-threatened species were also significantly less represented among very small species compared to both the global pool and the other-threatened species pool (Fig 4). Moreover, we also observed that invasive-threatened species mostly feed on animals. We also found that invasive-threatened birds were less likely to be habitat specialists compared to other-threatened species and more likely to live in three or four habitats, although we did not detect such differences when comparing these results to the global pool (Table S6-7).

## Discussion
Our results highlight the global contribution of invasive-threatened species to biodiversity worldwide and illustrate the magnitude of the extinction debt related to biological invasions. As such, our results reveal that biological invasion is potentially a major threat to both the phylogenetic and trait diversity of birds and mammals worldwide. The contribution of invasive-threatened birds and mammals to the total PD reaches 11%. Moreover, invasive-threatened birds represent 40% of the total trait space for birds, which is considerable given the number of invasive-threatened birds worldwide (~4.5% of all known birds). Our results confirm that birds are more vulnerable to biological invasions compared to mammals (C. Bellard et al., 2016; Céline Bellard et al., 2016), not only in terms of the number of species at risk of extinction but also in terms of ecological and evolutionary diversity. Given the role of birds for ecosystem services like pollination, seed dispersal, predation, and/or food-web structure (C.H. Sekercioglu, 2010; Cagan H Sekercioglu, Schneider, Fay, & Loarie, 2008), we expect that these losses will have important implications in the near future. Our results contrast with the recent assessment of the IPBES, which ranks IAS as one of the last drivers of change among global change components in the global state of nature (IPBES, 2019). We demonstrate that it is crucial not to overlook biological invasions as a top driver of biodiversity loss given the potential extinction debt. Note that on average, 3.9 threats were associated with invasive-threatened species (Appendix S1). We observed that biological resource use or agriculture and aquaculture are often associated with IAS threats for both birds and mammals (see also Leclerc, Courchamp & Bellard, 2018; Leclerc et al., 2020 for insular ecosystems). In this context, our results suggest that IAS associated with these two threats may lead to an important ecological and evolutionary debt worldwide.

Because all the species threatened by IAS are unlikely to disappear, we also conducted 50- and 100-year extinction scenarios to provide an initial approximation of the potential cost of the current extinction debt due to biological invasions while considering only the imminent
extinctions. The potential losses might be tremendous with around 7% of PD worldwide for both mammals and birds as well as 6% (mammals) and up to 27% (birds) of TrD worldwide. It is worth noting that we only considered species that are facing imminent extinction according to the IUCN Red List criteria, thus ignoring near-threatened, least concern, or data-deficient species that might be at risk of extinctions in the near future due to IAS such as the Reunion Bulbul (International., 2017). For instance, data-deficient species represent 15% of the entire mammal dataset. Consequently, we potentially underestimated the extinction debt due to IAS in this regard. On the contrary, the potential percentage of diversity predicted to become extinct due to IAS is significantly lower than expected under the projected randomized scenario given the species richness. The main explanation is that invasive-threatened species are clustered in both the phylogenetic and trait space and that the projected diversity losses associated with mammals and birds will not be random. In fact, a selective process will occur across the phylogenetic tree and the ecological strategy space (Thuiller et al., 2011; Yessoufou & Davies, 2016).

We found that both invasive-threatened mammals and birds have a significantly larger body size than the rest of the pool but not when compared to other-threatened mammals. Large body mass has already been highlighted as a life history trait that is strongly associated with increasing extinction risks due to the slow reproductive rate associated with large body mass (Hanna & Cardillo, 2013; Leclerc et al., 2020; Ripple et al., 2017). But note that body mass may hide the effects of other variables linked to reproduction, locomotion or survival. Therefore, the effects of body size could not be direct but reflect the impact of other variables on species extinction risk. We also found that invasive-threatened species were more likely to feed in the lower strata (i.e., ground level or below the water surface) and less likely to feed in the higher strata (i.e., aerial or arboreal) compared to the other-threatened species, and more generally, the global pool. This pattern has recently been highlighted for insular species
(Leclerc et al., 2020). Because most invasive-threatened species are found on islands, one
possible explanation could be related to the naïveté syndrome. Indeed, ground species are
more exposed to the introduction of nocturnal predators and are less capable of changing their
behavior to develop new defenses (Doherty et al., 2016), while higher-strata species may
develop avoidance strategies with the invasive mammals. We also found that invasive-
threatened species live in a limited number of habitats and are more likely to be specialist
species than generalist ones (Foden et al., 2018; Gonzalez-Suarez, Gomez, & Revilla, 2013;
Pacifici et al., 2017), but this tendency was not significantly different compared to the rest of
the species. In fact, we found that other-threatened mammals are more likely to be specialized
to a single habitat than invasive-threatened mammals, which are more likely to live in four
different habitats. We also found that most invasive-threatened mammals are primarily
herbivorous, which confirms the results of a recent study showing that large-bodied
herbivorous mammals are more at risk of extinctions due to global threats, including
biological invasions (Atwood et al., 2020). However, it would appear that the vast majority of
mammals are herbivorous, meaning that this characteristic is not particularly associated with
threatened species. By contrast, we did not observe that invasive-threatened bird species are
more likely to be herbivorous, whereas Atwood *et al.* (2020) detected that large-bodied
herbivorous birds are more at risk of extinctions with biological invasions. In fact, we found a
disproportionately higher number of invasive-threatened birds that feed on animals compared
to the rest of the species. More specifically, our findings suggest that habitat specialists and
species with lower-strata feeding strategies are more likely to be filtered out of the global pool
of mammals and birds. This is confirmed by the higher frequency of birds that feed on
multiple strata in the global pool compared to those that are threatened by IAS. Because these
traits have also been identified as highly vulnerable to other components of global changes,
we can expect that IAS will further affect the global composition of bird and mammal species.
In the near future, we may observe a shift in the global composition of birds and mammals toward species that have a lower body mass. Indeed, the average size of invasive-threatened species is four and eight times larger than the rest of the pool of birds and mammals, respectively. Recently, Cooke et al. (2019) predicted a potential ecological downsizing within mammals and birds due to global changes (Ripple et al., 2017). Forecast shifts in ecological traits could help us to identify the potential ecological consequences of extinctions for community and ecosystem services (D. Mouillot, Graham, Villéger, Mason, & Bellwood, 2013). Moreover, the predicted loss of habitat specialists and species with restricted foraging strata may lead to a shift toward generalist species, which may result in a global process of homogenizing ecological strategies (Clavel, Julliard, & Devictor, 2011; Qian & Ricklefs, 2006; Sébastien Villéger, Blanchet, Beauchard, Oberdorff, & Brosse, 2011). In other words, biological invasions have the potential to disrupt the ecosystem structure and function.

One of the most prominent findings of this study is that the evolutionary and ecological implications of the extinction debt go far beyond a simple number of lost species. Indeed, species at risk of extinctions due to IAS will impact the ecological and evolutionary composition of future communities. We observed a clear ecological and evolutionary profile of species vulnerable to invasions. While we know that IAS is one of the most important drivers of species extinctions on islands (Bellard et al., 2016a), this is the first time that we documented the need to further investigate the ecological and adaptive consequences of the biological invasion threat worldwide. For this reason, we investigated whether these losses might result in the disappearance of particular profiles and/or lead to a shift in the composition of mammals and birds worldwide. Our results point to the potential disappearance of specific lineages of mammals (e.g., Myrmecobiidae, Thylacomyidae) and birds (e.g., Notiomystidae, Rhynochetidae), because all species in these families are threatened by IAS. This also includes families that are currently represented by a single
species. For instance, the sole member of the Myrmecobiidae family is *Myrmecobius fasciatus*, which now has fewer than 800 individuals in Australia and continues to be threatened by several IAS (Woinarski & Burbidge, 2016). Even if IAS appear to be the primary threat to these species, other threats such as habitat degradation or natural system modifications like fire regime changes are also involved in population decline; indeed, IAS is rarely the sole threat responsible for the increasing risk of extinctions (Leclerc *et al.*, 2018 see also Appendix S1). In fact, IAS is the only threat in less than 8% of cases in our dataset.

The potential implications of our results are multiple. For instance, we expect that the extinction debt due to IAS in terms of PD will reduce future options to adapt in a changing environment. Indeed, PD links evolutionary history to the conservation of feature diversity and potential future options (IPBES, 2019). TrD represents the ecological properties embodied by different species, which are of high concern to comprehend how ecosystems may persist in a changing world. The potential loss of TrD associated with invasive-threatened birds is very high concern for this group and may result in a reduced ability to adapt in the future. Specifically, large bird species that mostly feed in the lower foraging strata should be monitored and benefit from conservation measures. To better understand the threat posed by biological invasions, we also conducted a spatial analysis of TrD and PD threatened by biological invasions, which is a first step when establishing spatial prioritization for research and conservation actions. We found that invasive-threatened bird species located in the Oceanian realm, which is mostly comprised of islands, contribute to about half of the total invasive-threatened bird space for both PD and TrD. Moreover, the Neotropical and Australasian realms also represent hotspots of invasive-threatened mammals regarding TrD and PD. This pattern is very similar to what was observed in previous studies focusing on conservation priorities for birds and mammals (Jetz et al., 2014; Pollock et al., 2017), although it has never been revealed specifically for biological invasions. Therefore, our
results imply that TrD and PD show clear differences across taxonomic groups and realms regarding the biological invasion threat, which should be considered when establishing spatial prioritizations.

Although our study brings potentially important insights into the role played by biological invasions in biodiversity losses, it is important to extend this study to other taxonomic groups before making decisions about conservation planning. The choice of life history traits or the number of modalities and how they are categorized may also affect the results, although our sensitivity analyses of body mass modalities showed that our results are robust (Appendix S2). In addition, TrD or PD could be divided into multiple indicators (richness, divergence, originality, specialization, rarity), all of which give complementary information that is necessary to establish a clear spatial prioritization. Because conservation is mostly undertaken at a local level, this study should be complemented with local assessments of community vulnerability to biological invasions. Moreover, IAS rarely acted alone and were often accompanied by other threats such as overexploitation and agriculture (Appendix S1). To date, it is impossible to disentangle the specific contribution of IAS compared to other threats, because the large majority of threatened species are at risk of extinction due to the actions of simultaneous threats. However, our results suggest that at a minimum, IAS associated with other threats could be an important driver of TrD and PD losses in the near future. We were also able to compare ecological traits that are specifically associated with IAS compared to other threats.

Our study is a first attempt at a global scale to study the potential consequences of IAS on phylogenetic and functional diversity for birds and mammals. This study represents a first step toward integrating the multidimensional nature of diversity. We thus appeal to ecologists to investigate the consequences of biological invasions on multiple indicators of diversity and then transform this knowledge into local conservation initiatives.
540 **Data availability:** All traits used in this study are included in the github repository and can be extracted from the Elton trait database (Wilman et al., 2014). We also used phylogeny for both birds and mammals; the data are freely available (Faurby et al., 2018; Jetz et al., 2014).

543
544 **Code availability:** Original codes for conducting the functional analyses are available at [http://villeger.sebastien.free.fr/Rscripts.html](http://villeger.sebastien.free.fr/Rscripts.html). The R codes used to calculate TrD and PD are available on github (https://github.com/cbellard/FDPD_IAST).

547
548 **Acknowledgements**

549 We are grateful to Sebastien Villeger who provided the R functions on his website to calculate
550 the functional diversity indices. We also thank Franck Courchamp and Anna Turbelin for
551 their comments on an earlier version of this work. More generally, this work was funded by
552 our salaries as French public servants.

553
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746 Does Non-random Extinction Prune the Tree-of-Life? *BT - Biodiversity Conservation*
747 and Phylogenetic Systematics: Preserving our evolutionary heritage in an extinction crisis. In R. Pellens & P. Grandcolas (Eds.) (pp. 57–80). Cham: Springer International

748 crisis. In R. Pellens & P. Grandcolas (Eds.) (pp. 57–80). Cham: Springer International

749 Publishing. https://doi.org/10.1007/978-3-319-22461-9_4

<caption>Table 1: Description of the traits used to measure the trait diversity of birds and mammals as well as their modalities.</caption>

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<table><thead><tr><th>Trait</th><th>Modality (Abbr.)</th><th>Taxa concerned</th></tr></thead><tbody><tr><td rowspan="7">Main diet</td><td>Plant material and seeds (Plant-seeds)</td><td>B, M</td></tr><tr><td>Fleshy fruits and nectar (Fruits-nect)</td><td>B, M</td></tr><tr><td>Invertebrates (Invert)</td><td>B, M</td></tr><tr><td>Vertebrate prey and carrion (Vert)</td><td>B, M</td></tr><tr><td>Omnivore (Mixed)</td><td>B, M</td></tr><tr><td>Mixed herbivore (Main veg)</td><td>B, M</td></tr><tr><td>Mixed animal (Main ani)</td><td>B, M</td></tr><tr><td rowspan="10">Foraging strata</td><td>Water (W)</td><td>B</td></tr><tr><td>Ground-level (G)</td><td>B</td></tr><tr><td>Understory (U)</td><td>B</td></tr><tr><td>Mid-high (Mi)</td><td>B</td></tr><tr><td>Canopy (Ca)</td><td>B</td></tr><tr><td>Aerial (A)</td><td>B</td></tr><tr><td>Multiple strata (Mult)</td><td>B</td></tr><tr><td>Scansorial (S)</td><td>M</td></tr><tr><td>Ground-level (G)</td><td>M</td></tr><tr><td>Marine (M)</td><td>M</td></tr><tr><td>Arboreal (Ar)</td><td>M</td></tr><tr><td>Aerial (A)</td><td>M</td></tr><tr><td rowspan="5">Period of activity</td><td>Crepuscular (C)</td><td>M</td></tr><tr><td>Diurnal (D)</td><td>B, M</td></tr><tr><td>Nocturnal (N)</td><td>B, M</td></tr><tr><td>Crepuscular / Diurnal (CD)</td><td>M</td></tr><tr><td>Crepuscular / Nocturnal (CN)</td><td>M</td></tr><tr><td>Crepuscular / Diurnal / Nocturnal (CDN)</td><td>M</td></tr><tr><td>Habitat breadth</td><td>Number of habitats used</td><td>B, M</td></tr><tr><td rowspan="5">Body mass</td><td>Very small (Vsmall)</td><td>B, M</td></tr><tr><td>Small</td><td>B, M</td></tr><tr><td>Medium</td><td>B, M</td></tr><tr><td>Large</td><td>B, M</td></tr><tr><td>Very large (Vlarge)</td><td>B, M</td></tr></tbody></table>

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<table><caption>Table 2: Trait and phylogenetic diversity observed in worldwide invasive-threatened mammals and birds with their standardized effect size (SES) and p-value.</caption><thead><tr><th>Taxa</th><th>Type of diversity</th><th>Observed (%)</th><th>Expected under null scenario (%)</th><th>SES</th><th colspan="2">p-value</th></tr></thead><tbody><tr><td rowspan="2">Mammals</td><td>Trait</td><td>14.0</td><td>37.9</td><td>-4.177</td><td>&gt; 0.999</td><td>***</td></tr><tr><td>Phylogenetic</td><td>10.2</td><td>14.6</td><td>-7.294</td><td>&gt; 0.999</td><td>***</td></tr><tr><td rowspan="2">Birds</td><td>Trait</td><td>40.1</td><td>67.1</td><td>-3.672</td><td>&gt; 0.999</td><td>***</td></tr><tr><td>Phylogenetic</td><td>11.4</td><td>12.9</td><td>-4.057</td><td>&gt; 0.990</td><td>***</td></tr></tbody></table>

<figure><img src="image_2.png" /><figcaption>Figure 1: Contribution of the mechanisms (i.e., all mechanisms, competition, ecosystem degradation, mortality, and/or reduction in reproduction success) impacting invasive-threatened species to the total bird (or mammal) trait (TrD) and phylogenetic diversity (PD) in percentage. We only considered the top three mechanisms for each taxon. Bar plots represent the observed values of diversity, while the diamonds represent the null models (expected values when species identity is randomized).</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>Figure 2: Contribution of invasive-threatened species located in each realm to the total bird (or mammal) diversity for both trait (TrD) and phylogenetic diversity (PD).</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Figure 3: Contribution of mammals and birds to the total phylogenetic (PD) and trait diversity (TrD) expressed in percentages under 50- and 100-year extinction scenarios. These values represent potential losses of PD and TrD under different extinction scenarios. We considered two samples of species: invasive-threatened species and random species (species taken from the total pool of mammal and bird species worldwide that are not invasive-threatened species). We included jittered points for each of the 999 repetitions; the median and quantiles (0.10 and 0.90) are also shown.</figcaption></figure>

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<figure><img src="image_5.png" /></figure>

783

784 **Figure 4: Percentage of invasive-threatened mammals (in red) for the five traits considered:**

785 A. Foraging strata (with M: marine, G: ground-level, S: scansorial, Ar: arboreal, A: aerial);

786 B. Period of activity (with C: crepuscular, D: diurnal, N: nocturnal, CD: crepuscular-

787 diurnal, CN: crepuscular-nocturnal, CDN: crepuscular-diurnal-nocturnal); C. Main diet; D.

788 Habitat breadth; E. Body size. Their associated modalities are compared to the global pool

789 (black bars with standard deviation errors bars) and other-threatened species (gray bars).

790 See Table 1 for the modality descriptions and abbreviations. * indicates the percentage of

791 trait modalities that are significantly different between invasive-threatened and other

792 mammal species. A difference is significant when 95% of tests have a p-value < 0.05 (see

793 Tables S4-5 for value details).

<figure><img src="image_6.png" /></figure>

794

795 **Figure 5: Percentage of invasive-threatened birds (in red) for the five traits considered:**

796 A. Foraging strata (with M: multiple strata, G: ground-level, W: water, A: aerial, Ca: canopy,

797 Mi: mid-high, U: understory); B. Period of activity; C. Main diet; D. Habitat breadth; E.

798 Body size. Their associated modalities are compared to the global pool (black bars with

799 standard deviation errors bars) and the other-threatened species group (in gray). See Table 1

800 for the modality descriptions and abbreviations. * indicates the percentage of trait modalities

801 that are significantly different between invasive-threatened and other mammal species. A

802 difference is significant when 95% of tests have a p-value < 0.05 (see Tables S6-7 for value

803 details).

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