DOI: 10.5061/dryad.p5hqbzksf
title: Connectivity among thermal habitats buffers the effects of warm climate on life-history traits and population dynamics

description: 1. Contemporary climate change affects population dynamics, but its
 influence varies with landscape structure. It is still unclear whether
 landscape fragmentation buffers or enhances the effects of climate on
 population size and on the age and body size of individuals composing
 these populations. 2. This study aims to investigate the impacts of warm
 climates on lizard life-history traits and population dynamics in habitats
 varying in their connectivity. 3. We monitored common lizard (Zootoca
 vivipara) populations for three years in an experimental system in which
 both climatic conditions and connectivity among habitats were
 simultaneously manipulated. We considered two climatic treatments (i.e.,
 present-day climate and warm climate (+1.4°C than present-day climate))
 and two connectivity treatments (i.e., a connected treatment in which
 individuals could move from one climate to the other and an isolated
 treatment in which movement between climates was not possible). We
 monitored survival, reproduction, growth, dispersal, age and body size of
 each individual in the system as well as population density through time.
 4. We found that the influence of warm climates on the life-history traits
 and population dynamics depended on the connectivity among thermal
 habitats. Populations in warm climates were i) composed of younger
 individuals only when isolated; ii) larger in population size only in
 connected habitats; and iii) composed of larger age-specific individuals
 independently of the landscape configuration. The connectivity among
 habitats altered population responses to climate warming likely through
 asymmetries in the flow and phenotype of dispersers between thermal
 habitats. 5. Our results demonstrate that landscape fragmentation can
 drastically change the dynamics and persistence of populations facing
 climate change.
description: <em>Experiments</em>
 Starting in 2015, we performed
 a three-year-long experiment on the common lizard (<em>Zootoca
 vivipara</em>) using the Metatron, an experimental system comprising
 48 interconnected semi-natural mesocosms (100 m<sup>2</sup>
 each (Legrand et al.,
 2012), Fig. 1b) connected
 by 19-meter-long corridors which allowed us to simultaneously manipulate
 temperature (with the help of automatic shutters) and connectivity (by
 opening or closing the doors which separate the mesocosm from the
 corridor, see details in Fig. 1 and Appendix A, ethics permit number
 APAFIS#19523-201902281559649 v3). 
 We created two climatic treatments, a present-day
 and a warm climate, by automatically closing the shutters at ambient
 temperature thresholds of
 either 28°C or 38°C
 (Bestion, Teyssier, et
 al., 2015).
 Given that mesocosms are
 intrinsically warmer than outside, present-day climates matched thermal
 conditions near the Metatron (meteorological station of Saint-Girons
 Antichan (Bestion,
 Teyssier, et al., 2015)).
 As our treatments depended on outdoor climatic conditions,
 the generated climate
 regimes follow day-to-day fluctuations in a coordinated manner so that
 daily fluctuation and seasonality are efficiently reproduced.
 The climate manipulations
 were active and efficient during the summer daytime, and the difference
 between treatments varied with the weather (Fig. 1c). Consequently, the
 warm climate treatment was
 on average 1.4 and 2.6°C warmer (mean and maximal summer daily
 temperatures) than the
 present-day climate treatment and the summer temperature slightly differed among
 years (Fig. 1c). The warm climate treatment matched the scenario SSP5-8.5 for 2041-2060 and SSP2-4.5,
 SSP3-7.0 and SSP5-8.5 for 2081-2100 (Masson-Delmotte, V., P. Zhai, A. Pirani et al.,
 2021). Of the 48 mesocosms constituting the Metatron,
 we used 16 to create eight
 pairs of mesocosms that combined a present-day climate
 mesocosm and a warm climate mesocosm with two connectivity treatments, either opening or
 closing the connecting corridors (Fig. 1a). For four pairs, lizards could move
 between climate treatments
 (connected treatment),
 while movement was prevented for the four remaining
 pairs (isolated
 treatment). In the
 connected treatments, corridors were opened from early March to
 mid-October each year, except in 2017 when corridors opened in late March,
 spanning the entire period of lizard activity. 
 <em>Population
 initialization</em> In 2015, 240 adults and 306 juveniles were
 released into 16 mesocosms. The individuals were descendants of lizards
 captured in the Cevennes, France, in 2010 and 2013, maintained in the
 Metatron (Ariège, France) for several experiments (Bestion et al., 2017, 2019; Bestion, Teyssier,
 et al., 2015) and
 intermixed regularly before the present experiment to prevent high levels
 of inbreeding. Each
 mesocosm was initially populated in early July 2015 with adult females,
 males and juveniles, with a later addition of 4 adults (2 males, 2
 females) in September 2015. The population size after this initialization
 period was 10 females, 5 males and 19 ± 1 juveniles, matching densities observed in
 natural populations. The individuals added in September were not
 considered in the analyses
 for the first year. At population initialization only, we split clutches
 among different mesocosms, and the different treatments
 to enhance genetic
 diversity within populations and released juveniles without their mother to
 prevent kin competition. All the lizards present in the system,
 individually tagged at
 birth by toe clipping (see the ethics statement section,
 ethics permit number
 APAFIS#15897-2018070615164391 v3), were therefore of known age.
 We measured their body size (snout-vent length) at
 release and ensured that
 there were no differences in age structure and body size between
 treatments (p-values &gt; 0.63 for the effects of climate,
 connectivity treatments and their interaction).
 <em>Population
 monitorin</em>g In May, from 2016 to 2018, before the females
 started laying eggs, we closed the corridors to recapture all the
 individuals and brought them to the laboratory. They were identified,
 measured for body size and mass and maintained in individual terraria
 (18x35x22 cm for adult females and gravid one-year-old females and
 15.5x25x15 cm for males and non-gravid one-year-old females). Terraria
 contained a 3 cm sterilized litter layer, a petri dish with water, a piece
 of absorbent paper, and a cardboard and a plastic tube as a shelter. A
 light bulb (25 W) and an ultraviolet lamp (Zoomed Reptisun 5.0 UVB 36 W)
 provided heat for thermoregulation and light 6 h per day (from 9:00 to
 12:00 and from 14:00 to 17:00). Lizards were lightly sprayed with water
 three times a day (in the morning, at mid-day, and in the evening) and
 offered two crickets (<em>Acheta domestica</em>) daily.
 Females laid eggs in their terrarium and the juveniles were isolated from
 their mother directly after parturition. They were marked by toe-clipping,
 measured for snout-vent length (nearest mm), weighed with an electronic
 scale (Thermo Fisher, 0.01g) and a tail tip of 0.3 cm was collected
 for paternity analysis
 (ethics permit number
 APAFIS#19523-201902281559649 v3). 
 These captures allowed (i)
 monitoring population size, mean age and body size of individuals that
 composed the populations through time,(ii) measuring clutch sizes (i.e.
 number of viable offspring), yearly survival probability, body growth
 (i.e., difference in snout-vent length from one year compared to the
 previous one), and (iii), in connected treatments, the dispersal status of
 each individual every experimental year. Dispersers (respectively,
 residents) are defined as individuals recaptured after one year in a
 different (respectively, the same) mesocosm than the previous year. The
 dispersal probability observed in this experiment was similar to the one
 of a previous experiment (Legrand et al., 2012) with similar experimental conditions and in
 which movements were recorded daily, meaning that our one-year movement
 monitoring provided a good indication of individual dispersal status.
 Nevertheless, seasonal back-and-forth movements might have occurred, as it
 has been previously estimated that 47% of individuals that moved at least
 once moved a second time in a course of a year (Legrand et al., 2012). Early July, all males, females and their clutch
 were released into the Metatron into their population of origin. We
 released adult and juvenile individuals back into the
 mesocosm where they or their mother were captured.
 To avoid stress-induced
 dispersal, we closed the corridors for 7 days after release and opened
 them later. Over the course of the experiment, two populations went
 extinct in 2016 (one of each climate treatment in the isolated treatment)
 and one in 2017 (from present-day climate in the isolated treatment).
 Extinctions were likely
 due to demographic fluctuations, as extinctions occurred only in isolated
 populations where population rescue from neighbouring populations was not
 possible. In 2016, the two
 extinct populations were reinitialized with the same density, age and sex
 structure and phenotypic composition as in 2015 using lizards from stock
 mesocosms that were not the subject of any previous climatic experiment.
 In 2017, the extinct population was not
 reinitialized. <em>Statistical
 analyses</em> General statistical
 procedure We analyzed the additive and interactive effects
 of climate and habitat connectivity first on life-history traits, then on
 individuals’ mean age and body size and finally on population size. We
 further investigated the influence of climate in each connectivity
 condition, by separately running the models for each connectivity
 treatment. All the models are summarized in Tables S1 & S2. All
 continuous variables were centered and scaled in all
 analyses. We used generalized/linear mixed models with
 random intercepts to account for the non-independency of the data points.
 We proceeded in two steps. First, we built full models with all fixed
 variables and random effects, and the random structure of each model was
 selected by AIC (Zuur et
 al., 2009). Second, we
 built models with the selected random effect(s) and with all possible
 combinations of fixed effects, including one without any fixed effects
 (i.e., null model), and ranked them by AIC. We obtained conditional
 estimates, standard errors, z-values, relative importance (RI), and
 p-values of all variables that feated in those models that had a delta AIC
 of less than 2 from the best model using a model averaging procedure
 (Burnham et al.,
 2011). Models containing
 all variables present in the averaged best models were run to calculate
 the conditional (R<sup>2</sup><sub>c</sub>, effect
 of the fixed and random effects) and marginal
 (R<sup>2</sup><sub>m</sub>, effect of the fixed
 effect) R<sup>2</sup> (Nakagawa et al., 2017). When the best model structure was that without
 random effect, the adjusted R<sup>2</sup> was calculated,
 except for models with zero-inflated distribution, where the
 pseudo-R<sup>2</sup> was calculated.
 We provided both RI and
 p-value, and chose to discuss the influence of variables when their RI,
 p-value and visual pattern on the figures were consistent (i.e. high RI,
 low p-value and clear visual pattern), without fixing absolute thresholds.
 Accordingly, when a treatment was retained in the best average model with
 low RI and high p-value, its effect was interpreted as
 weak. All analyses were performed using R version
 4.0.5 (R Core Team,
 2021) and necessitate lme4
 (Bates et al.,
 2015), glmmADMB
 (Fournier et al., 2012; Skaug et al., n.d.), performance
 (Lüdecke et al.,
 2021), MuMIn
 (Barton,
 2020), DHARMa
 (Hartig,
 2021), and emmeans
 (Lenth,
 2021) R
 packages. <em>Life-history
 traits</em> All individuals older than one year were
 considered adults and analyzed together, while younger individuals
 (hereafter named juveniles) were analyzed separately. We analyzed the
 clutch size, yearly survival probability and body growth rate (Table S1).
 The clutch sizes were analyzed separately for each sex because (i) the
 reproductive strategy of each sex may differ with respect to the
 experimental treatments and (ii) to avoid testing for four-way
 interactions (i.e., climate*connectivity*time*sex). The clutch sizes of
 males were obtained via paternity analyses and all males' results are
 presented in appendix B. We used generalized mixed models with a binomial
 distribution for survival, with zero-inflated Poisson distribution for the
 clutch size and linear mixed models for body growth. All models included
 climate treatments, connectivity treatments, and time as a continuous
 variable and their three-way interaction. The models for the clutch size
 of juveniles did not converge with the three-way interaction due to a low
 number of reproductive juveniles and so this interaction was removed.
 Models
 further included covariates known to influence life-history traits: body
 size (e.g. Cotto et al.,
 2015) for all analyses,
 sex (e.g. Bestion,
 Teyssier, et al., 2015)
 for survival and body growth analyses, and birth date (e.g.
 Bestion, Teyssier, et al.,
 2015) in Julian days in
 the analyses on juveniles. Random intercepts included mesocosm identity,
 individual identity for analyses on adults, and family identity for the
 analyses on juveniles as siblings were not
 independent. In the connected treatment, juvenile and adult
 dispersal statuses (i.e., disperser or resident, N=116 for juveniles and
 N=113 for adults) were analyzed (Table S1). The models included climate
 treatments, time, their interaction, body size and the interaction between
 body size and climate treatments, as body size strongly influences
 dispersal decisions, costs and benefits (Cote et al., 2007). Random structure only included mesocosm
 identity because models did not converge when individual identity was
 included, due to the low number of dispersers. Note that the mesocosm
 identity of an individual could change from one year to the other if the
 given individual dispersed.
 <em>Mean age, body size and population
 size</em> We used generalized mixed models with Poisson
 distribution to analyze population size (N=47) and mean age of individuals
 composing the populations (N=617), and linear mixed models for
 individuals’ mean body size (N=617). For all models, the fixed effects
 were climate treatment, connectivity treatment, number of years since
 population initialization (hereafter referred to as “time”) and their
 three-way interaction. The fixed time effect estimates the temporal
 pattern of population dynamics, rather than controlling for temporal
 autocorrelation in our data. Nevertheless, we checked for potential
 temporal autocorrelation in the residuals of the final models (i.e.
 Durbin-Watson test and ACF plot). All tests revealed no autocorrelation in
 the residuals. All the data at time 0 (i.e. before climate treatment) were
 excluded from the analyses and when a population was reinitialized after
 extinction, time was set at 0. Models for population size also included
 mesocosm identity as a random intercept to take into account the repeated
 model structure (Table S2). For individuals’ mean age and body size (Table
 S2), the models were run at the individual level and included mesocosm and
 individual identities as random intercepts to account for the
 non-independency of individuals of the same population and the multiple
 occurrences of individuals over time. We also included sex and population
 density as fixed effects to control for the indirect effects of the
 treatments on age and body size structure through their effects on sex
 ratio and density. The age of the individuals was also included in the
 body size analysis to disentangle the direct effect of climate on body
 size from its indirect effect through age structure changes. The same
 model without age was additionally run (Table S7).
  Ethics statement:
 The Station d’Ecologie
 Théorique et Expérimentale has a national agreement for use of animals in
 the laboratory (number B09583), and our experiments are made in accordance
 with French ethics regulations (Ethics permits number
 APAFIS#15897-2018070615164391 v3 for toe clipping and
 APAFIS#19523-201902281559649 v3 for other experimental procedures,
 including the maintenance of lizards in the Metatron). The lizards were
 initially captured in the wild under license numbers 2010-189-16 DREAL and
 2013-274-0002.
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