# Natural range expansion promotes stress resistance as a component of dispersal syndromes in non-native insects

Charly Géron, Stéphane Derocles, Hoël Hotte, David Renault

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Charly Géron, Stéphane Derocles, Hoël Hotte, David Renault. Natural range expansion promotes stress resistance as a component of dispersal syndromes in non-native insects. Oikos, 2025, pp.e10797. 10.1111/oik.10797. hal-05025312

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OIKOS

Research article

Natural range expansion promotes stress resistance as a
component of dispersal syndromes in non-native insects

Charly Géron ID, Stéphane A. P. Derocles ID<fnref n="1" />, Hoël Hotte ID<fnref n="1" />,<fnref n="2" /> and David Renault ID<fnref n="1" />

**Correspondence:** Charly Géron (charly.geron@gmail.com)

Oikos

2025: e10797

doi: 10.1111/oik.10797

Subject Editor: Allan Edelsparre

Editor-in-Chief: Pedro Peres-Neto

Accepted 19 January 2025

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Global changes and human activities have increased the likelihood of transport of non-native insect species all around the globe. When established, the spread of organisms leads to the spatial sorting of the populations, progressively contributing to the selection of individuals with enhanced dispersal performance at the edges of the invaded range. During dispersal, propagules are also subjected to contrasting environmental conditions that can be stressful. These include temperature extremes and variations, desiccation and the quantity and quality of food, which can impose physiological constraints. As a consequence, higher stress tolerance and higher dispersal capacities may be promoted at the range edge. However, only few studies have examined the differences in stress resistance of non-native insect populations along their invasion gradient. *Merizodus soledadinus* is a non-native insect species invading low-elevation coastal areas of the subantarctic Kerguelen Islands, where its quick spread highly impacts the native fauna. On the Kerguelen Islands, its invasion history is precisely known. This offers a unique opportunity to study its stress resistance in relation to the residence time. In this study, we investigated the effects of the residence time of populations of *M. soledadinus* on its resistance to heat, desiccation, food deprivation and the combination of these three stresses in the laboratory. We found that desiccation and multiple stress treatments caused the highest mortality rates. Populations close to the range edge showed a longer survival compared to populations with longer residence times. However, the dynamics of survival were different: core populations experienced a steady decline in survival, while range edge populations initially experienced a slow decline in mortality followed by rapid mortality. This suggests greater stress resistance for individuals from populations close to the invasion front, potentially explaining the intense expansion of *M. soledadinus* on Kerguelen Islands.

Keywords: biological invasion, desiccation, invasion front, residence time, starvation, temperature

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<footnote n="1">¹University of Rennes, CNRS, ECOBIO (Écosystèmes, Biodiversité, Évolution), UMR 6553, Rennes, France</footnote>

<footnote n="2">²Nematology Unit, Plant Health Laboratory, ANSES, Le Rheu, France</footnote>
Introduction

Global trade, transportation and mass tourism have grown steadily over the last few decades, enabling many organisms, in particular insects, to cross geographic boundaries of several orders of magnitude (Cuthbert et al. 2023, Davenport and Davenport 2006). In parallel, the unstopping degradation of terrestrial and aquatic habitats by human activities and climate change is threatening, or even eradicating native species, facilitating the establishment of non-native species by making communities increasingly unsaturated (Hulme 2017, Rolls et al. 2017, Worischka et al. 2023). Once established, populations of non-native insects proliferate before they start to expand their range, accentuating their ecological (Renault et al. 2022) and economic burdens (Cuthbert et al. 2021, Renault et al. 2021, Ahmed et al. 2023).

Spread of non-natives can result from human-mediated translocations, such as long-distance jumps often assisted by anthropogenic activities as reported in the Asian tiger mosquito *Aedes albopictus* (Diptera: Culicidae) whose invasion routes are correlated with road networks (Eritja et al. 2017). Local propagation can also be based on the movements of organisms (active dispersal) (Hui and Richardson 2017, Heckwolf et al. 2021, Quaresma et al. 2022). In the latter process, spread relies on the dispersal capacities of the individuals and differs from other forms of movement (e.g. migration and routine/home movements; Renault 2020, Alzate and Onstein 2022). During range expansion, the spread dynamics of non-native insects often accelerate at the invasion front, suggesting that dispersal-related traits may be positively selected (Travis and Dytham 2002, Perkins et al. 2013, Chuang and Peterson 2016). The lower genetic diversity generally displayed by non-native insect populations, combined with assortative mating, has been shown to additionally increase the evolution of dispersal ability at range edges (Holt et al. 2005, Ochocki and Miller 2017), a phenomenon known as the spatial sorting of the populations (Phillips and Perkins 2019). This additionally contributes to increased population differentiation along invasion gradients (Sherpa and Després 2021).

The selection of dispersive phenotypes at the range edges has been demonstrated for *Rhinella marina* (Anura: Bufonidae) (Travis and Dytham 2002, Phillips et al. 2008), and higher dispersal and activity levels were also reported from range edge populations in comparison to core populations in non-native insects (Hill et al. 2011, Lieven et al. 2014). Dispersal syndromes result from divergent selection of resident and dispersing morphs which are not experiencing the same environmental pressure (Clobert et al. 2012), and can reflect a tradeoff among the morphological and physiological traits driving the resulting specialized phenotype. The range of environmental pressures makes transience potentially costly, adding up another factor empowering the dispersal syndrome. While comparisons among dispersing and resident morphs were previously performed (Legrand et al. 2016, reviewed by Renault 2020), very few studies have examined the differences in stress resistance of

invasive non-native insects between edge and core populations (Engell Dahl et al. 2019).

When moving across environments, the environmental conditions along the dispersal pathways also contribute to the filtering of dispersing individuals: the contrasting conditions to which propagules (individuals released into a region to which they are not native) are subjected likely select those individuals with the highest level of stress resistance (West-Eberhard 2003, Weldon et al. 2018). The selection pressure may be even higher for non-native organisms which disperse over unfamiliar landscapes which can be challenging their physiological homeostasis. In particular, extreme temperatures, thermal variations, relative humidity, variations in food quality and/or food limitation can represent stressful conditions across dispersal pathways (Jackson et al. 2009, Hill et al. 2011). Those stressors can be experienced concurrently, thus making integrated stress resistance a significant factor of invasive success (Renault et al. 2015, Tarusikirwa et al. 2022). In invasion contexts, it is thus expected that insects from range edge populations should outcompete their relatives from core populations in terms of stress resistance, reflecting the selection pressures they experienced during dispersal events (Hill et al. 1999, Renault et al. 2013, Simon et al. 2019).

Sub-Antarctic islands are cold environments situated between 45°S and 54°S, and encompass most of the terrestrial biotas at mid and high latitudes of these locations (Bergstrom and Chown 1999). These environments are characterized by a strong gradient of abiotic conditions according to distance from the ocean and altitude, and therefore by a high degree of landscape fragmentation (Clobert et al. 2009, Moon et al. 2017). Consequently, the relatively low species diversity present in sub-Antarctic islands is largely found in low-elevation coastal environments where thermal conditions are more favorable and food resources are present (Lebouvier et al. 2011). Several non-native insects have been introduced in the region (Frenot et al. 2005, Chown et al. 2008), and nowadays, the number of non-native arthropods can even be higher than native ones. The sub-Antarctic Kerguelen Islands are no exception and present a relative large number of non-native insect species (Frenot et al. 2005, Lebouvier et al. 2011). For example, the non-native flightless carabid beetle *Merizodus soledadinus* (Coleoptera: Trechidae) has been introduced from the Falkland Islands in 1913 at a single site of the Kerguelen Islands (Port Couvreux: 49°17′04.9″S, 69°41′41″E) (Jeannel 1940, Lebouvier et al. 2020). *Merizodus soledadinus* has now colonized the eastern part of the Kerguelen Islands, along low-elevation coasts and more recently along low-elevation watercourses (Renault et al. 2015, Lebouvier et al. 2020). In these areas, prey densities in the form of fly maggots are relatively high, and thermal and humidity conditions are favorable to *M. soledadinus* (Renault et al. 2015, Laparie and Renault 2016, Ouisse et al. 2016, 2017). Geographic expansion of *M. soledadinus* is continuously monitored using standardized methods, which allows the documentation of the establishment period of the populations in the different colonized localities, and therefore the residence time of the species (Lebouvier et al. 2020). Since
2005, the geographic expansion of *M. soledadinus* has been
accelerating at the Kerguelen Islands, likely assisted by global
warming and milder winter temperatures. Strict biosecurity
measures are now taken to counteract its arrival to sites that
are not yet colonized (Lebouvier et al. 2011, 2020). The spe-
cies has a strong impact on native invertebrate species such as
the endemic flies *Anatalanta aptera* (Diptera: Sphaeroceridae)
and *Calycopteryx moseleyi* (Diptera: Micropezidae), which
have locally disappeared from areas where *M. soledadinus* is
present in high densities (Lebouvier et al. 2020). The ongo-
ing expansion of the species on the Kerguelen Islands thus
makes it an ideal system-model for testing the existence of
directional environmental filtering on the insects' phenotypic
features along invasion and environmental gradients.

In this study, we investigated whether the residence time
affects the stress resistance of *M. soledadinus*. The effects of
starvation, desiccation, heat exposure and the combination
of these three stresses on the survival of *M. soledadinus* were
tested in six non-native populations established from four
years to a century on the Kerguelen Islands. We hypothe-
sized that 1) desiccation, and by extension the combination
of stresses (due to potential additive effects of the stresses),
would represent the most stressful conditions for the insects,
as humidity has been reported to be one of the most impor-
tant stressing factors affecting the survival of *M. soledadinus*
(Hidalgo et al. 2013, Ouisse et al. 2016), 2) stress tolerance
would differ according to the residence time of the popula-
tions, with individuals from populations closer to the inva-
sion front (i.e. shorter residence times) displaying higher
tolerance to stressful conditions; and 3) differences in stress
resistance among populations across the invasion gradient
would be visible when studying each stress treatment inde-
pendently, with for example a highest starvation resistance for
populations with short residence times as dispersing insects
are more likely to be exposed to food limitation or depriva-
tion as they move away from the source population.

Methods

Insect collection

Adults of *M. soledadinus* were hand-collected from six dis-
tinct localities of the Kerguelen Islands (48°25′–50°S,
68°27′–70°35′E) during the austral summer 2015–2016:
Port Couvreux (the unique site of introduction of the bee-
tle), Cataractes, Anse du Pacha, Pointe Suzanne, Isthme Bas
and Val Studer (all localities are at least 8 km apart, with
the furthest being more than 50 km apart, Fig. 1). The aus-
tral summer (from September to April) corresponds to the
maximum period of emergence for *M. soledadinus* on the
Kerguelen Islands (Ouisse et al. 2017). We collected adults
in December 2015–January 2016, to maximize the collec-
tion of newly emerged individuals. All of the localities where
specimens of *M. soledadinus* were collected are coastal, except
Val Studer which lies around 7.2 km from the coastline.
All sampling sites host large densities of *M. soledadinus*, for

which the year of establishment is known, and therefore the
time elapsed since colonization (hereafter ‘residence time’,
Lebouvier et al. 2020). At the time of collection, popula-
tions of *M. soledadinus* were established for 103 years at Port
Couvreux, 36 years at Cataractes, 21 years at Anse du Pacha
and Port-aux-Français, 10 years at Pointe Suzanne, five years
at Isthme Bas, and four years at Val Studer (Fig. 1) (Source:
project 136 - SUBANTECO, French polar institute, also see
Lebouvier et al. 2020). Adult longevity of *M. soledadinus* is
eight months on average (Ouisse et al. 2017). This beetle spe-
cies can produce up to two generations per year. The number
of generations since the establishment of the insect in each
locality is thus expected to vary from a minimum of six (for
the population with the shortest residence time: Val Studer)
to 154 (for the population with the longest residence time:
Port Couvreux).

Maintenance of the insect populations

More than 500 adults were collected from each locality and directly placed into sealed plastic boxes before being brought back to the laboratory of the research station (Port-aux-Français, Kerguelen Islands) within 24-48 hours after sampling. At the laboratory, insects were transferred to plastic boxes (18 × 12 × 7 cm) at standardized densities of approximately 100 individuals per box for each population. The bottom of all plastic boxes was covered with filter paper moistened with tap water. Insects were fed ad libitum with larvae of the invasive non-native fly *Fucellia maritima* (Diptera: Anthomyiidae), and they were provided drinkable water by the addition of 2 ml microtubes filled with tap water. The microtube caps were kept open, and the opening of the tubes was plugged with cotton for a slow diffusion of the water. All rearing boxes were maintained during seven days in a walk-in climatic chamber at 8°C, photoperiod 12:12h (light:dark).

Range expansion and evolution of stress tolerance

Transparent Petri dishes (ø 9 × 1.5 cm) were used for all sur-
vival assays described below.

Assessment of rearing conditions

We used insects from the population of Port-aux-Français
to monitor the survival of the insects under controlled –
supposedly non-stressful – conditions. We decided to use
insects from Port-aux-Français as the longevity of this popu-
lation was formerly assessed by Ouisse et al. (2017) under
laboratory conditions, thus allowing to determine if lifespan
remained in a similar time range. Moreover, this population
is not used in the stress treatments. In total, ten Petri dishes
were prepared, each containing 10 individuals of *M. soledadi-
nus* (n = 100 *M. soledadinus* individuals).

The Petri dishes were maintained at 8°C in the walk-in
climatic chamber (photoperiod 12:12h (light:dark)) corre-
sponding to the mean monthly temperature of the austral
summer on the Kerguelen Islands (Supporting information;
Lebouvier et al. 2011). The bottom of each Petri dish was
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Figure 1. Locations of the populations of *M. soledadinus* in the Kerguelen Islands are represented by black dots, with increasing dot size corresponding to increasing residence time: 103 years at Port Couvreux, 36 years at Cataractes, 21 years at Anse du Pacha and Port-aux-Français, 10 years at Pointe Suzanne, five years at Isthme Bas and 4 years at Val Studer. The background map is either (a) OpenStreetMap or (b) above sea level elevation (m a.s.l). The maps of the Kerguelen Islands were created by the authors. The OpenStreetMap background map comes from OpenStreetMap (OpenStreetMap Contributors, 2023); OpenStreetMap is open data (Open Data Commons Open Database License, ODbL, and license CC BY-SA 2.0 for the documentation). The m a.s.l. elevation background map was adapted and acquired from NASA Shuttle Radar Topography Mission (SRTM) (2013) Global, which is distributed by OpenTopography. OpenTopography data are free of all copyright restrictions and made fully and freely available for both non-commercial and commercial uses. QGIS 3.28.8 LTR was used to generate the map (www.qgis.org).
covered with moistened filter paper and a microtube filled
with tap water was added to provide drinkable water and
suitable humidity conditions for *M. soledadinus* individuals.
Every 10 days, the filter paper and Eppendorf of the Petri
dishes were checked to ensure the presence of drinkable water
and suitable humidity. The filter paper was changed every two
months. Once a month, adults of *M. soledadinus* were fed
with larvae of *F. maritima*. The survival of adult *M. soledadi-
nus* under the rearing conditions was monitored fortnightly.

The longevity of the individuals of *M. soledadinus* of
Port-aux-Français, calculated for the 10 replicates, averaged
705 days. Longevity ranged from a minimum of 190 days
and a maximum of 884 days (Supporting information). A
similar lifespan was reported for this species by Ouisse et al.
(2017), and is also consistent with former observations in
other ground beetle species (Lövei and Sunderland 1996),
attesting that our laboratory conditions did not lead to a
reduced survival.

Abiotic stress

The tolerance of *M. soledadinus* individuals from the six pop-
ulations (Port Couvreux, Cataractes, Anse du Pacha, Pointe
Suzanne, Isthme Bas and Val Studer) to four experimental
stress was examined: heat (1), desiccation (2), starvation (3)
and exposure to multiple stress which combined heat, des-
iccation and starvation (4). For each experimental stress,
10 Petri dishes, each containing 10 insects of one of the six
populations, were prepared (with few exceptions, 9–12 Petri
dishes replicates, n=248 Petri dishes in total, Supporting
information). The experimental stress started at the end of a
7-day maintenance period following field capture.

1) Heat stress treatment. Petri dishes were transferred to an incubator (Panasonic, MIR154) that allowed an accurate control of the exposure duration at each desired experimental temperature: temperature remained at 8°C for six hours before gradually increasing with one-hour increments to 10, 12, 16°C, and with two-hour increment to 20°C, and finally peaking at 28°C for two hours. Then, the temperature gradually decreased on an hourly basis to 20, 16, 12, 10 and finally 8°C which was kept constant for 6 h (Supporting information). This daily variation of the temperature, and the resulting heat stress, represented an intense warm event for the insects. So far, the maximum mean air temperature measured at 2 m above the ground level on the Kerguelen Islands is 25.8°C (Lebouvier et al. 2011, project 136 - SUBANTECO, French polar institute). A 12:12h (light/dark) photoperiod was applied. To reproduce a diurnal increase in temperature, the periods during which the temperature started to increase above 8°C coincided with the sunrise. Humidity conditions and food supply of the insects were similar to those used for the assessment of the rearing conditions (Supporting information).

2) Desiccation stress treatment. No source of humidity and no drinkable water were provided to *M. soledadinus* individuals. The feeding regime, temperature and photoperiod

were the same as those used for the assessment of the rearing conditions (Supporting information).

3) Starvation stress treatment. For this treatment, *M. soledadinus* individuals were deprived from food for three weeks before the start of the experiment to ensure that the beetles had empty guts (Laparie et al. 2012) and that the food stress started at the beginning of the assay. The insects were then starved until death but given drinking water as previously explained (moistened filter paper and microtube filled with tap water). Humidity level was weekly checked. Temperature and photoperiod were the same to those used for the assessment of the rearing conditions (Supporting information).

4) Multiple stress treatment. This treatment corresponded to the combination of heat, desiccation and starvation stress treatments. The temperature and photoperiod conditions corresponded to those applied to the heat treatment (Supporting information). Humidity conditions were the same as used in the desiccation treatment. The feeding regime was the same as the one applied to the starving insects, including the three-week deprivation period before the experiment started. During these three weeks, water was provided to the individuals with the same setup as previously explained (moistened filter paper and microtube filled with tap water), and humidity level was weekly checked (Supporting information).

**Assessment of survival**

The survival of adult *M. soledadinus* was monitored:

1) every 12 hours for Petri dishes exposed to the desiccation and multiple stress treatments, as they were expected to be the most stressful conditions for the beetles (Ouisse et al. 2016),

2) once a day for the heat and starvation stress treatments.

During these checks, hereafter referred to as ‘experiment
time points’, the status of all *M. soledadinus* individuals was
determined with a binocular magnifier installed in the walk-
in climatic chamber (8°C): individuals were classified as alive,
comatose or dead. Alive individuals were moving and were
standing on their legs. Individuals in a coma were no longer
on their legs but displayed movements of their appendages
(antennas, mandibles, legs); individuals in a coma were cat-
egorized as alive individuals. Dead individuals were on their
backs with their legs folded, and exhibited no movement of
appendages even if stimulated with a paintbrush. At each
experiment time point, the individuals identified as dead,
and if necessary, larvae of *M. soledadinus* were removed from
the Petri dishes. For each Petri dish, the observations ended
when all adults had died. Therefore, the duration of the
experiment was different between each Petri dish. Predation
events among *M. soledadinus* conspecifics were observed by
individuals found dead and which had been partially attacked
(e.g. missing or damaged body parts). Predation events were
observed in low numbers (mostly once per Petri dish), only
for the longest stress treatments: starvation and heat stress treatments, and for all the studied populations (Supporting information).

## Statistical analyses

All statistical analyses were performed in R ver. 4.3.0 (www.r-project.org).

### Proportion of dead *M. soledadinus*

At each experiment time point, the number of dead and alive individuals was counted for each Petri dish. We tested the effects of the duration of the experiment (continuous covariable), the residence time (continuous) and the stress treatment (categorical) on the proportion of dead individuals. These effects were first tested across the four stress treatments, then on each stress treatment independently. We used a binomial denominator written as a vector: number of successes, number of failures (Crawley 2013), corresponding respectively to the number of dead individuals and the number of living individuals.

First, across the stress treatments, the effects of residence time (explanatory variable, continuous), the duration of the experiment (explanatory variable, continuous) and the stress treatment (explanatory variable, categorical with desiccation stress treatment taken as a reference because these conditions have been found to impact the most the survival of *M. soledadinus* Hidalgo et al. 2013, Ouisse et al. 2016) on the proportion of dead individuals (response variable, continuous) were analyzed using a generalized linear mixed model with a binomial family and a logit link (R package 'lme4', Le Bourg 2014, Bates et al. 2015). Petri dish identity nested within source population was included as a nested random intercept to account for repeated measurement of survival in each Petri dish across the duration of the experiment. The fit of the model with the nested random intercept was tested with the Akaike information criterion (AIC) and decreased with more than two units compared to the AIC of the model without the nested random factor (Galwey 2006). We calculated the pseudo-R-squared for this model (R package 'MuMln', Barton 2009). Then, for each stress treatment independently, the effects of the duration of the experiment (continuous), the residence time (continuous) on the proportion of dead *M. soledadinus* (response variable) were analyzed using a generalized linear mixed model with a binomial family and a logit link (R package 'lme4', Bates et al. 2015). For each of the four models, we included the second order interaction between the residence time and the duration of the experiment to investigate how populations with different residence times could differ in stress resistance at different points in the experiment. Petri dish identity nested within source population was included as a nested random intercept to account for repeated measurement of survival in each Petri dish across the duration of the experiment. For each of the four models, the fit of the model with the nested random intercept was tested with the AIC and decreased with more than two units compared to the AIC of the model without the nested random

factor (Galwey 2006). We calculated the pseudo-R-squared for the model of each stress treatment (R package 'MuMln', Barton 2009).

### Lethal times

For each stress treatment, population and across replicates, we calculated the mean (in days) and variance (in days²) of the lethal times 50 and 90 (the time until which fifty percent and ninety percent of individuals die, respectively (R package 'Survival', Therneau and Grambsch 2000, Therneau 2024). These metrics were used because each individual inside the Petri dishes was not followed individually. For each of the stress treatments and response variables separately (mean and variance of lethal times 50 and 90), the relationships between the response variable (continuous) and residence time (explanatory variable, continuous) were analyzed using generalized linear models with a Gamma family and a logit link (R package 'lme4', Bates et al. 2015).

## Results

### Range expansion and survival capacities to a range of stressors

The four experimental treatments differentially affected the mean survival time of the insects (Table 1): 3.99 ± 0.53 days for the multiple stress, 9.54 ± 1.79 days for the desiccation stress, 117.49 ± 36.23 days for the starvation stress and 309.83 ± 99.96 days for the heat stress (ranging from a minimum of 2 to a maximum of 4 days for the multiple stress, from 4 to 10.5 days for the desiccation stress, from 38 to 202 days for the starvation stress and from 68 to 456 days for the heat stress) (Fig. 2). Mortality of adult *M. soledadinus* augmented when the duration of the experiment increased (Table 1).

A significantly higher proportion of dead beetles was found for the populations with longer residence times in the model across the stress treatments (Table 1). The survival of adult *M. soledadinus* exposed to multiple stress did not differ from survival of their relatives subjected to desiccation (Table 1).

<table><caption>Table 1. Estimates, standard errors, Z values, and model parameters for the generalized linear mixed models testing for the drivers of the proportion of dead *M. soledadinus* (transformed data (log-link function), Methods for more details about the statistical analyses) across stress treatments (with the desiccation stress treatment taken as reference). Significant effects are in bold. The conditional pseudo-R² (corresponding to the variance explained by the entire model) was equal to 0.91, while the marginal pseudo-R² (corresponding to the variance explained by the fixed effects only) was equal to 0.84.</caption><thead><tr><th></th><th>Estimate</th><th>SE</th><th>Z value</th><th>p-value</th></tr></thead><tbody><tr><td>Intercept</td><td>-0.78</td><td>0.17</td><td>-4.66</td><td>&lt; 0.001</td></tr><tr><td>Experiment time point</td><td>0.02</td><td>&lt; 0.001</td><td>120.18</td><td>&lt; 0.001</td></tr><tr><td>Residence time</td><td>0.01</td><td>0.003</td><td>2.66</td><td>0.01</td></tr><tr><td>Starvation stress</td><td>-0.99</td><td>0.08</td><td>-11.91</td><td>&lt; 0.001</td></tr><tr><td>Multiple stress</td><td>-0.01</td><td>0.09</td><td>-0.17</td><td>0.87</td></tr><tr><td>Heat stress</td><td>-1.81</td><td>0.08</td><td>-21.44</td><td>&lt; 0.001</td></tr></tbody></table>
<figure><img src="image_7.png" /><figcaption>Figure 2. Survival curves of *M. soledadinus* in function of the stress treatments across all six populations. Confidence intervals are added for each stress treatment and for each experiment time point, but hardly visible due to the scale (long experimental time).</figcaption></figure>

Conversely, the survival was significantly longer for the bees exposed to the starvation and heat stress treatments in comparison to the desiccation stress treatment (Table 1).

## Survival patterns for each experimental treatment

For the heat stress, the duration of the experiment and its interaction with residence time significantly explained the variation of the proportion of dead *M. soledadinus* (Table 2). For example, individuals from the Port Couvreux population survived the longest with a survival probability that decreased with the lowest slope (Fig. 3a, Table 2). For the other populations with long residence times (Cataractes, Anse du Pacha), the proportion of dead individuals seemed to increase more steadily (Fig. 3a, Table 2). On the contrary, individuals from populations with short residence times (Val Studer, Isthme Bas, Pointe Suzanne) exhibited better survival rates at the beginning of the experiment, but their survival dropped more quickly after the 150-day of the experiment (Fig. 3a).

For the desiccation stress, the duration of the experiment, the residence time, and their interaction significantly explained the variation of the proportion of dead *M. soledadinus* (Table 3). The proportion of dead individuals increased more steadily for individuals from populations with long residence times (Port Couvreux, Cataractes, Anse du Pacha, Fig. 3b, Table 3). The proportion of dead individuals increased later in the experiment, but more quickly for individuals from populations with a short residence time (Val Studer, Isthme Bas, Pointe Suzanne, Fig. 3b, Table 3). A contrasting survival length was observed for individuals from populations with short residence times (Val Studer, Isthme Bas, Pointe Suzanne), with individuals from the population of Isthme Bas population having the shortest survival duration under desiccating conditions (Fig. 3b).

For the starvation stress, the duration of the experiment and its interaction with residence time significantly explained the variation of the proportion of dead *M. soledadinus* (Table 4). The proportion of dead individuals increased at

<table><caption>Table 2. Estimates, standard errors, Z values, and model parameters for the generalized linear mixed models testing for the drivers of the proportion of dead *M. soledadinus* (transformed data (log-link function), Methods for more details about the statistical analyses) for the heat stress treatment. Significant effects are in bold. The conditional pseudo-R² (corresponding to the variance explained by the entire model) was equal to 0.96, while the marginal pseudo-R² (corresponding to the variance explained by the fixed effects only) was equal to 0.73.</caption><thead><tr><th></th><th>Estimate</th><th>SE</th><th>Z value</th><th>p-value</th></tr></thead><tbody><tr><td>Intercept</td><td>-4.60</td><td>0.7</td><td>-6.543</td><td>&lt; 0.001</td></tr><tr><td>Residence time</td><td>0.02</td><td>0.002</td><td>1.32</td><td>0.188</td></tr><tr><td>Experiment time point</td><td>0.05</td><td>&lt; 0.001</td><td>54.152</td><td>&lt; 0.001</td></tr><tr><td>Residence time ×<br>Experiment time point</td><td>&lt; 0.001</td><td>&lt; 0.001</td><td>3.717</td><td>&lt; 0.001</td></tr></tbody></table>

higher rates for individuals from populations with longer residence times (Port Couvreux, Cataractes, Anse du Pacha, Fig. 3c, Table 4). Individuals from populations with short residence times seemed to survive longer and have a strong drop in their survival from the 115-day duration of the experiment (populations from Val Studer and Pointe Suzanne, but not for the population of Isthme Bas, Fig. 3c).

Finally, for the multiple stress, the duration of the experiment and the residence time significantly explained the variation of the proportion of dead *M. soledadinus* (Table 5). The proportion of dead individuals increased with increasing duration of the experiment (Fig. 3d, Table 5). All individuals from all populations seemed to have a very low survival from day-3 onward (Fig. 3d).

## Lethal times 50 and 90

We did not observe any significant general trends between residence time and variations in the mean and variance of lethal times 50 and 90 analyzed for each stress treatment separately (but see the slight positive significant influence of increasing residence time only on the variance of lethal times 90 for the desiccation treatment; see the Supporting information for mean and variance values of lethal times 50 and 90 for each population and each treatment, and for the results of the generalized linear mixed models testing for the drivers of the mean or the variance of the lethal times 50 or 90 of *M. soledadinus* for each stress treatment separately).

## Discussion

Our study aimed to determine whether the resistance of *M. soledadinus* to heat, desiccation, starvation and the combination of these three stresses differs according to the residence time of the populations. We found that the type of stress and the duration of the experiment had a very significant impact on the survival time of adults of *M. soledadinus*. In the stress conditions, the longest survival time was recorded for heat-exposed insects, while the shortest survival time was measured for insects subjected to desiccating conditions and multiple stress. Interestingly, the residence
<figure><img src="image_8.png" /><figcaption>Figure 3. Survival curves of *M. soledadinus* in function of the studied populations for each of the stress treatments: (a) heat stress treatment, (b) desiccation stress treatment, (c) starvation stress treatment, (d) multiple stress treatment. Note that the x-axes differ between each panel. Confidence intervals are added for each population and for each experiment time point. Each population is represented with a different color and symbol. The color gradient represents the residence time, with the lightest color for the shortest residence time and the darkest for the longest.</figcaption></figure>

time of the populations differentially affected the ability of adult *M. soledadinus* to survive stress exposure: individuals from populations with long residence times tended to have lower survival rates, and a steadier decline in survival when exposed to desiccating conditions, starvation and heat stress. Conversely, individuals from the populations with short residence times, i.e. collected closer from the range

<table><caption>Table 3. Estimates, standard errors, Z values, and model parameters for the generalized linear mixed models testing for the drivers of the proportion of dead *M. soledadinus* (transformed data (log-link function), Methods for more details about the statistical analyses) for the desiccation stress treatment. Significant effects are in bold. The conditional pseudo-R² (corresponding to the variance explained by the entire model) was equal to 0.97, while the marginal pseudo-R² (corresponding to the variance explained by the fixed effects only) was equal to 0.73.</caption><thead><tr><th></th><th>Estimate</th><th>SE</th><th>Z value</th><th>p-value</th></tr></thead><tbody><tr><td>Intercept</td><td>-5.89</td><td>0.33</td><td>-18.90</td><td>&lt; 0.001</td></tr><tr><td>Residence time</td><td>0.04</td><td>0.007</td><td>5.77</td><td>0.03</td></tr><tr><td>Experiment time point</td><td>0.95</td><td>0.03</td><td>33.01</td><td>&lt; 0.001</td></tr><tr><td>Residence time x<br>Experiment time point</td><td>&lt; -0.01</td><td>&lt; 0.001</td><td>-4.80</td><td>&lt; 0.001</td></tr></tbody></table>

edges of the invaded range, outlived their relatives from core populations by exhibiting longer survival durations. They however exhibited a more rapid decline in survival later in the experiment.

The impacts of stress on insects highly depends on stress intensity and duration, with mild and moderate stress being potentially detrimental over prolonged exposure periods.

<table><caption>Table 4. Estimates, standard errors, Z values, and model parameters for the generalized linear mixed models testing for the drivers of the proportion of dead *M. soledadinus* (transformed data (log-link function), Methods for more details about the statistical analyses) for the starvation stress treatment. Significant effects are in bold. The conditional pseudo-R² (corresponding to the variance explained by the entire model) was equal to 0.97, while the marginal pseudo-R² (corresponding to the variance explained by the fixed effects only) was equal to 0.80.</caption><thead><tr><th></th><th>Estimate</th><th>SE</th><th>Z value</th><th>p-value</th></tr></thead><tbody><tr><td>Intercept</td><td>-4.60</td><td>0.27</td><td>-6.55</td><td>&lt; 0.001</td></tr><tr><td>Residence time</td><td>0.02</td><td>0.006</td><td>1.33</td><td>0.19</td></tr><tr><td>Experiment time point</td><td>0.06</td><td>0.001</td><td>53.86</td><td>&lt; 0.001</td></tr><tr><td>Residence time x<br>Experiment time point</td><td>&lt; 0.001</td><td>&lt; 0.001</td><td>3.73</td><td>&lt; 0.001</td></tr></tbody></table>
<table><caption>Table 5. Estimates, standard errors, Z values, and model parameters for the generalized linear mixed models testing for the drivers of the proportion of dead *M. soledadinus* (transformed data (log-link function), Methods for more details about the statistical analyses) for the multiple stress treatment. Significant effects are in bold. The conditional pseudo-R² (corresponding to the variance explained by the entire model) was equal to 0.97, while the marginal pseudo-R² (corresponding to the variance explained by the fixed effects only) was equal to 0.89.</caption><thead><tr><th></th><th>Estimate</th><th>SE</th><th>Z value</th><th>p-value</th></tr></thead><tbody><tr><td>Intercept</td><td>-5.91</td><td>0.31</td><td>-19.354</td><td>&lt;0.001</td></tr><tr><td>Residence time</td><td>0.01</td><td>0.01</td><td>2.10</td><td>0.04</td></tr><tr><td>Experiment time point</td><td>2.86</td><td>0.09</td><td>32.84</td><td>&lt;0.001</td></tr><tr><td>Residence time ×<br>Experiment time point</td><td>&lt; -0.01</td><td>&lt;0.01</td><td>-1.05</td><td>0.29</td></tr></tbody></table>

Consistently, survival of *M. soledadinus* was reduced in all treatments when the duration of the exposure to the chronic stress increased, evidencing the progressive disruption of physiological functions (Gefen et al. 2006, Everatt et al. 2013, McCue et al. 2017). The physiological compensatory capacity and the physiological costs of compensatory responses challenge the survival of insects subjected to prolonged exposure to mild and moderate stress. Moreover, stress progressively becomes bioenergetically unbearable as the energy-limited stress tolerance, which assumes that maintenance takes priority over other functions (Sokolova 2013), meaning that all physiological damages cannot be repaired, ultimately resulting in the death of individuals.

The mean survival time of starving adult *M. soledadinus* was consistent with former investigations on the species (101.6 days ± 0.43, Laparie et al. 2012). Ouisse et al. (2016) reported that food-deprived adults could survive for about 105 days, which is similar to values reported from other predatory ground beetles (Knapp 2016). In prey-poor or food-deprived habitats, many insects adopt a sit and wait strategy whereby locomotor and metabolic activities are reduced (Hervant and Renault 2002), allowing body reserves to be spared and increasing starvation resistance (Rion and Kawecki 2007). Such a behavioral response was already suggested in *M. soledadinus* as evidenced by the significant decrease of most metabolic compounds of the primary metabolism formerly measured in starving adults, and could explain the long starvation resistance found in our study (Hidalgo et al. 2013).

When experimentally heat-exposed, specimens of *M. soledadinus* survived around 309 days, thus confirming the high thermal plasticity of this species. In this insect of Australian origin, and for individuals from populations from the Kerguelen Islands, adults can survive prolonged exposures at constant 20°C, and fall into heat coma at temperature extremes as high as 38°C (Lalouette et al. 2012, Laparie and Renault 2016). This thermal extreme is well above maximum temperatures recorded on the Kerguelen Islands (maximum monthly temperature reached 25.8°C, data source: Meteo France, Lebouvier et al. 2011), and above the highest temperature applied in the heat treatment. Moreover, the fluctuating thermal regime, with temperatures oscillating between 8 and 28°C, likely allowed the beetle to recover

at cooler temperatures, as formerly noted in other insects (Lalouette et al. 2012, Chen et al. 2018). The relatively high survival capacity of adult *M. soledadinus* to temperature variations is consistent with the recent work performed by Renault et al. (2022) who suggested that climate warming on the Kerguelen Islands should further assist the invasion dynamics of the species. This finding also matches the ‘thermal sensitivity hypothesis’ which states that invertebrates from high latitudes, as is the case of *M. soledadinus*, are more tolerant to temperature fluctuations than their counterparts from lower latitudes (Deutsch et al. 2008, Everatt et al. 2013).

The desiccation and multiple stress treatments represented the most stressful conditions for *M. soledadinus*, independent of the residence time of the populations. The high sensitivity of the insect to low humidity conditions is in line with previous studies (individuals’ survival ranged from 22 to 42 hours at relative humidity of 30% and 70%, respectively; Ouisse et al. 2016). Even if survival did not significantly differ from the desiccation treatment, exposure to multiple stress (starvation, heat and desiccation) resulted in the lowest survival and fastest death of all adults of *M. soledadinus*. The combination of multiple stressors often leads to additive or synergistic effects, and tend to accelerate mortality rates (Holmstrup et al. 2010, Piiroinen et al. 2013). The combination of desiccation and starvation was formerly examined by Hidalgo et al. (2013) who reported comparable survival durations to the ones we measured in the multiple stress experiment. The survival of *M. soledadinus* was also lowered when insects were maintained under dry (desiccating) conditions and high temperatures (Todd and Block 1997). In general, insects from Polar and Austral regions are highly sensitive to variations in relative humidity (Everatt et al. 2015). In *M. soledadinus*, it has been suggested that insects could have increased predation rates to maintain body water content when exposed to desiccation (Ouisse et al. 2016). This additional feeding increases the catabolism of lipid and carbohydrate reserves, which in turn increases the amount of water produced internally (Ben-Yosef et al. 2021). Yet, a favorable effect of higher predation could not be observed in our multiple stress treatment since starvation was concomitant with desiccation.

Differences in the level of stress resistance was formerly reported among populations of invasive non-native insects (Weldon et al. 2018). However, few studies have compared how insect species sampled along invasion gradients perform when facing different types of environmental stress (Weldon et al. 2018, but see Kolbe et al. 2014, Engell Dahl et al. 2019). Here, the detailed knowledge of the invasion history of *M. soledadinus* on the Kerguelen Islands makes it possible to have accurate knowledge of the residence time of the different populations: populations from the invasion fronts (range edge populations with short residence times) and core populations (long residence times) (Lebouvier et al. 2020). Residence time significantly influenced the survival patterns of *M. soledadinus* in interaction with the duration of the experiment for desiccation, heat exposure and starvation. Specifically, populations with short residence times (closer to the invasion front) performed better under stressful
conditions, and their survival seemed to decrease quicker
after a breakpoint time corresponding to about half the maxi-
mum duration of the stress treatment. Conversely, individu-
als from populations with long residence times appeared to
have a lower stress resistance and a more steadily decrease in
survival. These effects were the most visible for the desicca-
tion and starvation stress conditions and could reflect the
selection pressures *M. soledadinus* experience during dispersal
events such as variations in humidity or food availability and/
or quality (Hill et al. 1999, Renault et al. 2013, Simon et al.
2019). The multiple stress treatment led to a very short sur-
vival of all individuals which might have prevented us from
detecting differences among populations, while the heat
treatment was probably not sufficiently stressful to generate
large survival differences among the populations.

In long-established populations, local environmental
parameters and biotic factors may lead to local adaptations.
The most successful non-native insects are often charac-
terized by higher evolution propensities (Comeault et al.
2020), and local adaptation may contribute to align bio-
logical performance of individuals to the conditions of their
environment (Engell Dahl et al. 2019). For example, con-
ditions at Port Couvreux where *M. soledadinus* have been
present the longest might be considered as highly degraded
in comparison to more recently invaded localities on the
Kerguelen Islands (Laparie et al. 2010, Lebouvier et al.
2020). Here, compared with areas where the beetle has
been present for shorter periods of time, the density of *M.
soledadinus* is much higher, while the densities of native
prey species are much lower (Lebouvier et al. 2020). This
means that *M. soledadinus* individuals from the population
of Port Couvreux population could be facing frequent food
limitations. Thus, even if this was not assessed in our study,
adaptation to local conditions might be a possible explana-
tion of the greater resistance to starvation of the individu-
als from the population of Port Couvreux in comparison
to individuals from the two other populations with a high
residence time.

Range expansion can be associated with directional selec-
tion toward a dispersing morph optimum, which then may
end up in dispersal-associated traits and success in new
habitats being more prominent at the invasion front. As
expansion is also associated with serial founder effects and
assortative mating, spatial sorting is likely to be strengthened
in the invasion wave (Hui and Richardson 2017, Ochocki
and Miller 2017, Phillips and Perkins 2019). This is particu-
larly true for the flightless *M. soledadinus* whose geographic
spread on the Kerguelen Islands occurred step-by-step, as evi-
denced by the low genetic divergence reported from spatially
close populations (over gradients < 1000 m; Lebouvier et al.
2011, Ouisse 2017). As defined by Novoa et al. (2020) the
‘combination of pathways, alien species traits, and character-
istics of the recipient ecosystem collectively results in predict-
able dynamics and impacts…’, and this drives the invasion
dynamics and impacts by selecting propagules with special-
ized phenotypes. Here, the higher level of stress resistance

of insects from populations closer to the range edge on the
Kerguelen Islands could be a confirmation of the crucial role
of environmental filtering on dispersing individuals, and the
stronger environmental selection they experience in compari-
son with core populations (Iacarella et al. 2015, Renault et al.
2018). Because of the long lifespan *M. soledadinus*, it has not
been possible to rear new generations under common garden
controlled rearing conditions. Thus, with our experimental
design, we cannot determine if the differences in stress resis-
tance result from plasticity, selection and/or parental effects
(Amarillo-Suárez and Fox 2006, Jardeleza et al. 2022). The
differences in stress resistance we observed may thus be linked
to parental effects, since we did not control this parameter
and it has already been reported as an important factor for
other insects (Bonte et al. 2012, Hoffmann et al. 2013,
Sgro et al. 2016).

Moving away from the source population, adults *M. soledadinus* were likely exposed to a range of environmental selective pressures making transience potentially costly, and in turn shaping the level of stress resistance of insects from range edge populations (Engell Dahl et al. 2019). This phenomenon can be further amplified by landscape fragmentation (Clobert et al. 2009), which is particularly pronounced in sub-Antarctic Islands (Moon et al. 2017). As a result, population from invasion fronts may be composed of insects having specialized phenotypes making them more resilient to environmental extremes (Renault et al. 2018, Manfredini et al. 2019). The relatively high potential of *M. soledadinus* to overcome stressful conditions is thus consistent with this assumption, and help explain its wide distribution on the Kerguelen Islands. Its distribution has increased exponentially in recent years and may no longer be limited to low-elevation coastal areas (Lebouvier et al. 2020).

While dispersal polymorphism has been tested in a range
of insect species (Renault et al. 2018, Renault 2020), dif-
ferential abilities to handle environmental stressors along
an invasion gradient has been surprisingly less examined.
Here, by using the spread of the non-native carabid beetle
*M. soledadinus* on the Kerguelen Islands, which offers unique
opportunities to examine if range expansion contributes to
directional environmental filtering of insects along invasion
gradients, we found that stress resistance seems to be posi-
tively selected for individuals from populations close to the
invasion front. Stress resistance is often associated with physi-
ological and life-history traits, and this should be examined
in future studies (Bonte et al. 2012, Sulmon et al. 2015).
The present results have several implications for our knowl-
edge of invasion processes, giving potential explanations for
the intensifications of expansion dynamics of some highly
successful species. The consideration of differences in stress
resistance in slowly *versus* rapidly expanding non-native spe-
cies would be particularly interesting. Indeed, stress resistance
may not have the same pattern of co-variation in slowly and
rapidly expanding specimens, and/or the strengths of envi-
ronmental filtering may differ between the two situations,
making selection less intense in slowly expending invaders.
*Acknowledgements* – Civil volunteers recruited by the French polar institute and put at the disposal of the projects supported by the French polar Institute are warmly thanked.

*Funding* – This work was funded by the ASICS project (ANR-20-EBI5-0004, BiodivERsA, BiodivClim call 2019–2020), the French Polar Institute (Project 136-SUBANTECO), Zone Atelier CNRS Antarctique et Terres Australes (ZATA, Long-term research on biodiversity in Antarctic and sub-Antarctic ecosystems), and the long-term observatory of the biodiversity supported by CNRS Ecologie and Environnement ‘SEE-Life’.

## Author contributions

**Charly Géron:** Data curation (equal); Formal analysis (lead); Validation (lead); Visualization (lead); Writing - original draft (lead); Writing - review and editing (equal). **Stéphane A. P. Derocles:** Formal analysis (supporting); Validation (supporting); Visualization (supporting); Writing - review and editing (equal). **Hoel Hotte:** Conceptualization (supporting); Data curation (lead); Writing - review and editing (equal). **David Renault:** Conceptualization (lead); Data curation (lead); Funding acquisition (lead); Investigation (lead); Methodology (lead); Project administration (lead); Supervision (equal); Validation (equal); Writing - review and editing (equal).

## Data availability statement

Data are available from the Dryad Digital Repository: https://doi.org/10.5061/dryad.05qfttfbc (Géron et al. 2025).

The R scripts used for the statistical analyses are available on Figshare: https://doi.org/10.6084/m9.figshare.28352705.v1.

## Supporting information

The Supporting information associated with this article is available with the online version.

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