Influential factors and barriers change along the invasion continuum of an alien plant

Vol.: (0123456789) 1 3 Biol Invasions (2023) 25:2977–2991 https://doi.org/10.1007/s10530-023-03087-3 ORIGINAL PAPER Influential factors andbarriers change alongtheinvasion continuum ofanalien plant AnaMontero‑Castaño · MarceloA.Aizen · PabloGonzález‑Moreno · LauraCavallero · MontserratVilà · CarolinaL.Morales Received: 24 September 2022 / Accepted: 9 May 2023 / Published online: 12 June 2023 © The Author(s) 2023 Abstract Upon arrival to a new area, alien species have to overcome a series of biotic and abiotic barriers to survive, reproduce, and spread and thus, succeed along the invasion continuum. Failing to understand the role of the different sets of barriers and factors operating across the stages of the invasion continuum limit our ability to predict invasion dynamics, leading to misinformed management. Here, we explore how the European plant Cytisus scoparius overcomes the survival and reproductive barriers to establish along the roadsides of Nahuel Huapi National Park (Argentina). We evaluate the direct and indirect influence of climatic and landscape factors, species traits and their interaction with patch cover, plant height, and pollinator visitation rates as proxies of population persistence, plant growth, and reproduction, respectively. Cytisus scoparius cover was positively associated with the length of water shores and urban cover, factors that may relate to the arrival of propagules, new introduction events, and high levels of disturbance. Plant height was positively associated with annual precipitation and mean temperature. Visitation rates positively related to shrubland cover and to a lesser extent to slope, two factors that may influence pollinator availability and long-distance detectability, respectively. However, factors positively affecting survival had no effect (in the case of height) or negative effect (in the case of cover) on visitation rates, probably due to the saturation of the pollinator Supplementary Information The online version contains supplementary material available at https:// doi. org/ 10. 1007/ s10530- 023- 03087-3. A.Montero-Castaño(*)· M.Vilà(*) Estación Biológica de Doñana (EBD-CSIC), Avda. Américo Vespucio 26, 41092Sevilla, Spain e-mail: [email protected] M. Vilà e-mail: [email protected] A.Montero-Castaño School ofEnvironmental Sciences, University ofGuelph, 50 Stone Road East, Guelph, ONN1G2W1, Canada M.A.Aizen· C.L.Morales Grupo de Ecología de La Polinización, Instituto de Investigaciones en Biodiversidad y Medioambiente (INIBIOMA), Universidad Nacional del Comahue‐ CONICET, SanCarlosdeBariloche, RíoNegro, Argentina P.González-Moreno Department ofForest Engineering, Laboratory ofDendrochronology, Silviculture andGlobal Change-DendrodatLab-ERSAF, University ofCórdoba, Campus de Rabanales, Crta. IV, km. 396, 14071Córdoba, Spain L.Cavallero Centro Científico Tecnológico CONICET Córdoba, X5000IND, Buenos Aires 1418, Córdoba, Argentina M.Vilà Department ofPlant Biology andEcology, Universidad de Sevilla, Sevilla, Spain 2978 A.Montero-Castaño et al. 1 3 Vol:. (1234567890) pool within large flowering patches. Despite the strong environmental gradients, climatic variables did not seem to influence the cover nor the visitation rates of C. scoparius. The microhabitat provided by roadsides seemed to buffer the climatic variability acting at larger spatial scales. This study shows how the relevance of different barriers across the invasion process can vary due to the characteristics of the species and of the spatial context. Actions directed towards the most limiting barriers and limiting factors could be an efficient way to manage invasions and reduce their impacts. In our system, actions aimed at reducing propagule arrival, soil disturbance and availability of pollinators could limit the survival and reproductive success of C. scoparius. Resumen Con su llegada a una nueva área, las especies exóticas deben superar una serie de barreras bióticas y abióticas para sobrevivir, reproducirse y expandirse y, por tanto, avanzar a lo largo del proceso de invasión. La falta de conocimiento acerca de las barreras y factores más influyentes en las distintas etapas de este proceso limita la capacidad de predecir las dinámicas de invasión y su gestión. Aquí exploramos cómo la planta europea Cytisus scoparius supera barreras para su supervivencia y reproducción y así establecerse en los bordes de carreteras del Parque Nacional Nahuel Huapi (Argentina). Evaluamos la influencia directa e indirecta de factores climáticos y del paisaje, de las características intrínsecas de la especie, y de su interacción con la cobertura, altura y tasa de visitas de C. scoparius como representantes de su persistencia poblacional, crecimiento y reproducción, respectivamente. La cobertura de C. scoparius resultó estar positivamente relacionada con la longitud de los cursos de agua y con la cobertura urbana, factores que pueden favorecer la llegada de propágulos, nuevos eventos de introducción y altos grados de perturbación. La altura se relacionó positivamente con la precipitación y la temperatura media anuales. La tasa de visitas resultó estar positivamente relacionada con la cobertura de matorral y en menor medida con la pendiente del terreno, factores que pueden estar influenciando, respectivamente, una mayor disponibilidad de polinizadores y la detección a larga distancia. Sin embargo, los factores que afectaron de forma positiva a la supervivencia tuvieron un efecto indirecto nulo (en el caso de la altura) o negativo (en el caso de la cobertura) sobre la tasa de visitas, probablemente debido a la saturación de las comunidades de polinizadores en los parches de floración más grandes. Finalmente, y a pesar de los marcados gradientes ambientales, las variables climáticas parecen no afectar la cobertura ni la tasa de visitas en C. scoparius. Las condiciones microclimáticas en los bordes de carretera parecen amortiguar las condiciones climáticas que actúan a mayores escalas espaciales. Este estudio muestra cómo la importancia relativa de las distintas barreras a lo largo de los procesos de invasión cambia dependiendo de las características de las especies y del contexto espacial. Medidas enfocadas en las barreras y factores limitantes que influyen en la invasión podrían mejorar su gestión y reducir sus impactos. En nuestro sistema de estudio, acciones dirigidas a reducir la llegada de propágulos, la perturbación del suelo y la disponibilidad de polinizadores podrían limitar la supervivencia y éxito reproductivo de C. scoparius. Keywords Establishment· Landscape scale· Linear infrastructures· Non-native species· Pollination· Reproduction· Scotch broom· Survival· Structural equation models Introduction Biological invasions are dynamic, non-equilibrium processes characterised by an increase in local abundance and geographical extent of alien species (Richardson et al. 2000). According to the conceptual framework proposed by Blackburn etal. (2011), alien species need to overcome different abiotic and biotic barriers (Richardson et al. 2000; Pyšek et al. 2004; Richardson and Pyšek 2006, 2012; Divíšek et al. 2018) to be able to arrive, survive, reproduce and spread in a new region, that is, to progress along the introduction-invasion continuum (hereafter, “the continuum”). Depending on the characteristics of the alien species, thenew area, and introduction events, the underlying mechanisms and factors influencing the success of alien species differ along the continuum (Václavík and Meentemeyer 2012), while a given factor can also have different direct or indirect effects. Moreover, the invading species can affect their own performance along the continuum through positive and negative feedbacks by altering their recipient environments (Vilà etal. 2011). 2979 Influential factors andbarriers change alongtheinvasion continuum ofanalien plant 1 3 Vol.: (0123456789) Understanding the transition of alien species along the continuum can contribute to the detection of the most limiting sets of barriers and factors influencing invasion,and thus,guide management decision-mak- ing (Richardson and Pyšek 2012). Such a dynamic approach can be especially interesting in the case of alien species associated with disturbed areas such as infrastructure networks. These usually linear infrastructures can act as corridors facilitating the arrival and spread of aliens into new areas (Dar etal. 2015; Asth et al. 2021). Therefore, exploring the barriers and factors mediating the establishment of the alien species, rather than for instance their arrival or spread, can be more informative and can provide new insights about their invasion success and potential management. The establishment of an alien species in its new area requires the overcoming of survival and reproductive barriers. Overcoming survival barriers depends on the conditions that allow for the species population persistence and vegetative growth, the former depending on environmental factors but also on the characteristics of the introduction event (e.g. propagule pressure) (Blackburn et al. 2011). Overcoming reproductive barriers and the corresponding influential factors, highly depends on the reproductive system of the alien species (Montero-Castaño and Traveset 2020). Here, we study the establishment of the European Cytisus scoparius (Scotch broom), a pollinatordependent alien plant (Stokes et al. 2006) invading roadsides in southern South America along wide environmental gradients (Potter et al. 2009). Specifically, we test a series of hypotheses regarding its performance overcoming (1) survival and (2) reproductive barriers. Regarding (1) its survival, we expect C. scoparius patch cover to be positively associated with urban cover and length of river-or lake-shores, which are expected to provide suitable environmental conditions and disturbance regimes, while acting as sources of propagules and introduction events. We also expect the cover and the height to be positively associated with annual precipitation and annual mean temperature. Precipitation and temperature gradients in the study area fall within the lowest part of the wide annual precipitation (500 to > 3000 mm) and temperature ranges (optimal temperature during the growing season ranging from 18 to 23°C) where C. scoparius is found in its native distribution area in Europe (Potter etal. 2009). Therefore, increasing precipitation and temperature in the study area are expected to relate positively with the two dimensions of population survival, persistence and growth (C. scoparius cover and height, respectively). Regarding (2) its reproduction, we expect pollinator visitation rates to C. scoparius to be positively associated with the cover of open and mid-succes- sional habitats, to annual mean temperature and mean slope of the terrain. Open and mid-successional habitats host diverse and abundant pollinator communities (Steffan-Dewenter and Tscharntke 2002; Steffan-Dewenter and Westphal 2008; Wu et al. 2019). Increasing temperatures usually benefit foraging capacities of pollinators. While terrain slope could favour the attraction to C. scoparius patches because steep patches might be better detected visually by pollinators at long distances. On the contrary, we expect pollinator visitation rates to be negatively associated with patches with individuals of C. scoparius with reddish flowers and to patch height and cover. Most insect pollinators are able to discriminate colours in the ultraviolet to the yellow-orange spectrum (Kevan and Baker 1983) and are differently attracted to them (Bradshaw and Schemske 2003; Nuttman etal. 2006; Papiorek etal. 2016). Cytisus scoparius yellow flowers have an ultraviolet pattern invisible to the human eye that is more attractive to pollinators than the UV pattern of reddish flowers (Gavini and Farji-Brener 2015; Vidal and Farji-Brener 2020). Patch height and cover determine the floral abundance. In the study area, C. scoparius usually forms large monospecific stands with massive flower blooms (Cavallero etal. 2018). Therefore, at such high floral densities, we expected to find a negative relationship of both patch cover and height with visitation rate due to saturation of the pollinator pool. Consequently, we also expect the climatic variables annual precipitation and annual mean temperature, as well as urban cover and length of water shores, influencing patch growth to be indirect- and negatively associated to pollinator visitation rate. 2980 A.Montero-Castaño et al. 1 3 Vol:. (1234567890) Material andmethods Study system The study was conducted in Nahuel Huapi National Park, in NW Patagonia, Argentina (41° 06′ S–41° 36′ S, Fig.1). The National Park covers strong environmental gradients along short distances. In the ca. 60 × 120 km2 covered in this study, the altitudinal gradient of the Andes ranges from 500m above sea level at the bottom of glacial valleys, to 3500m at the top of glacier-covered mountains. The rain shadow caused by the Andean range creates a strong west–east precipitation gradient where annual precipitation drops from > 4000 mm at the temperate Valdivian rainforest at the foothills, to < 500mm at the Patagonian steppe. Finally, an urbanisation gradient with several small to medium size cities lies within the matrix of natural habitats protected by the National Park. This amplitude of environmental gradients determines a wide variety of habitats and associated vegetation, including alien plants, which represent 25% of the vascular Flora of the Park (Ezcurra and Brion Fig. 1 Location and sampling design along national and provincial roads showing the environmental gradients of the study area. Temperature, urbanisation, and precipitation gradients are represented by colour bars (orange, grey and blue, respectively) where darker colours represent approximately higher values of the corresponding variable. Dots show the 108 study patches, which were located along these roadsides by stopping every 2km and selecting the closest patch of C. scoparius with at least 10 flowering individuals 2981 Influential factors andbarriers change alongtheinvasion continuum ofanalien plant 1 3 Vol.: (0123456789) 2005). Among them, Cytisus scoparius (L.) Link, (Scotch broom) is one of the most widespread invasive plant species, also reaching the highest local abundances (Cavallero et al. 2018). This perennial legume shrub is native from Europe and has become invasive in many temperate regions worldwide (Bossard and Rejmanek 1994; Downey et al. 2000; Sheppard et al. 2002; Suzuki 2003; Simpson etal. 2005; Potter etal. 2009; Cordero etal. 2016). In invaded regions, this species spreads along roadsides, water shores and disturbed areas where it forms dense stands (Fig.2a, b), excluding native plants and altering community structure (Bossard and Rejmanek 1994; Wearne and Morgan 2004). Scotch broom was introduced in the region in the twentieth century— probably as an ornamental plant—and since 2021 is listed as a major “Invasive Alien Species” in Argentina (Boletín Oficial de la República Argentina 2021). Cytisus scoparius lacks vegetative propagation; thus, population persistence, growth and spread depend entirely on seed production (Parker 1997, 2000). At reproductive maturity (3–5 years), each plant can produce several thousands of papilionate flowers that bloom during ca. 4–8 weeks in spring (Parker 2000; Stout 2000). Flowers are nectarless but rich in pollen and are mainly yellow, although some individuals produce reddish flowers (Fig.2c, d) (Gavini and Farji-Brener 2015). Insect visitors must be large enough to push the keel down and split the petals, releasing the style and anthers, which then spring up to contact the back of the pollinator in an explosive motion. In this area, Bombus dahlbomii, Apis mellifera and more recently, the invasive Bombus terrestris, have been reported as effective pollinators of this alien plant (Morales and Aizen 2002; Aizen et al. 2008; Gavini and Farji-Brener 2015). Once a flower is "tripped", it does not return to its original configuration (Parker 1997) and can be distinguished from virgin unvisited flowers (Fig.2c, d). Thus, the proportion of tripped flowers is a reliable estimator of pollinators visitation rate at the plant and patch levels (Cavallero etal. 2018), allowing rapid estimations Fig. 2 Cytisus scoparius invading a roadsides and b disturbed areas in the study area in Nahuel Huapi National Park (Argentina). Examples of individuals with c yellow and d reddish flowers being visited by invasive Bombus terrestris. Tripped and untripped flowers are pointed with red and white arrows, respectively 2982 A.Montero-Castaño et al. 1 3 Vol:. (1234567890) of visitation rates for large sample sizes. Moreover, the proportion of tripped flowers is closely related to fruit and seed production, as reproduction in C. scoparius is severely limited by visitation frequency, which in turn reflects pollinator availability (Cavallero etal. 2018). In the study area, while untripped flowers set no fruit, 36% of tripped flowers set fruits, and fruit set is significantly and positively related to seed output (Cavallero et al. 2018). In species lacking vegetative reproduction, seed output is only the first step for overcoming reproductive barriers. Other posterior barriers such as seed predation and seedling mortality, must be overcome as well. However, in the case of C. scoparius, as seed production is massive and seedling mortality is negligible (Sheppard etal. 2002), very high seed predation rates would be needed to significantly reduce its reproductive success (Paynter et al. 2010). Thus, because of strong pollinator dependence and severe pollination limitation, we consider the proportion of tripped flowers as an appropriate proxy of reproductive success. Field sampling design To embrace the widest environmental gradients of the study area while avoiding their covariation, we established three transects that run along different major national and provincial roads in different directions (RN 40, RP 237, RP 77 and RP 82). Overall, we covered an annual mean precipitation gradient ranging from 797 to 1785mm; an annual mean temperature gradient ranging from 6.8 to 9.7 °C, corresponding to a ca. 500m altitudinal gradient (Fig.1); and an urbanisation gradient covering natural areas and the cities of Villa La Angostura, Dina Huapi and San Carlos de Bariloche and their surroundings. Along these roadsides, we selected a total of 108 patches of C. scoparius. Every 2km at a randomly selected side of the road, we located the closest patch lying on the roadside (i.e., 0–2m from the border, depending on the conditions of the roadside) of C. scoparius with at least 10 flowering individuals. Mean (± SE) distance between consecutive patches was 1.79 ± 0.04 km (Fig.1). Field sampling took place during the 2010 flowering peak, from October 15 to December 13. To avoid any phenological bias, we sampled patches at a similar blooming stage, when individual plants had most of their flowers open (i.e., there were < 10% preanthesis flower buds and no well-developed fruits). Variables We measured insitu or calculated from aerial photos or digital thematic maps the set of patch, landscape and climatic variables that we expected to explain the success of C. scoparius in overcoming survival and reproductive barriers (see Fig.3). a. Variables measured in situ. For each study patch, we estimated visitation rate, height and flower colour. First, in 10 randomly selected C. scoparius individual plants we counted the proportion of tripped flowers over the total number of open flowers in three randomly selected branches of ca. 30 cm. We then pooled together the per branch and per individual flower counts and estimated the visitation rate per patch as a proxy of C. scoparius success overcoming reproductive barriers. Visitation rate estimated this way positively influences fruit set and seed output in this species (Cavallero etal. 2018), and therefore is a good proxy of the overall reproductive success. Second, we measured the height (m) of each of the 10 individuals to estimate the average patch as a proxy of growth, one of the dimensions of population survival (Blackburn et al. 2011). Third, we recorded patch colour as a categorical variable with two levels: yellow (i.e.; all individuals in the study patch had yellow flowers) and bicolour (i.e., there were individuals in the study patch with reddish flowers; there were no patches with only reddish flowers). b. Patch cover calculated from aerial photographs. We built a Cover Map from aerial colour photos taken during C. scoparius blooming peak in 2010 to estimate C. scoparius cover. We took advantage of the mass blooming and brightyellow flowering display of C. scoparius, which form patches that can be easily distinguished from other associated vegetation or background structures from a long distance (see Cavallero et al. 2018). Aerial photographs were taken in two cloud-free days (22 and 25 November 2010) near noon (11:00–13:15) to minimise shadow effects. Aerial photographs were georeferenced using ArcGIS 9.2 and the Esri World Imagery basemap, and then combined into image mosaics. The software ENVI 4.4 was used for the automated identification of C. scoparius patches. 2983 Influential factors andbarriers change alongtheinvasion continuum ofanalien plant 1 3 Vol.: (0123456789) Patch cover was quantified as the percentage of cover of C. scoparius in an area of 100m radius from the centroid of the study patch where insitu variables were measured (see above). The selection of this radius was a compromise between an area large enough to maximise the variability of C. scoparius cover, and an area where a representative in situ sampling of the previously mentioned variables were feasible. In the study area, C. scoparius patches are usually large, thus, a smaller sampling radius would have implied most of the patches having covers close to 100%, impeding a proper exploration of C. scoparius cover as a response variable. On the other hand, sampling a larger radius in the field would have required sampling more than 10 individuals per site in order to properly represent the patch, which was not feasible due to logistic limitations. Patch cover, together with patch height, was used as a proxy of C. scoparius success in overcoming survival barriers. Fig. 3 Causal model proposed (grey, black and brown cells and arrows) and optimised (grey and black cells and arrows) through a backward stepwise process based on the AIC. The key barriers (blue dotted lines) for the establishment of alien species according to Blackburn et al. (2011), Pyšek et al. (2004) and Richardson et al. (2000) are shown. One-headed arrows depict significant (p-value < 0.05) causal relationships. Positive, negative and non-significant effects are represented by solid, dashed and grey lines, respectively. Numbers over the arrows represent the standardised path coefficients and p-values (*p-value: 0.05 <> 0.06, **p-value: 0.01 <> 0.001, *** p-value: < 0.001). Arrow widths are proportional to the magnitude of these path coefficients. Values in parentheses are the strength of indirect effects on visitation rate of urban cover and annual mean temperature via patch cover. For each partial model (see Eqs.1–3) the R2 is given under the corresponding response variable. Fisher’s C goodness-of-fit test and the AIC of the optimised model are given in the text 2984 A.Montero-Castaño et al. 1 3 Vol:. (1234567890) c. Variables calculated from digital thematic maps. The rest of the variables in Fig.3 (urban cover, annual precipitation, annual mean temperature, shrubland cover and mean slope) were obtained from digital thematic maps with ArcGIS 9.2. To calculate urban cover (%), we used the Land cover Map based on 2003 Landsat 30 m resolution images (Table 1). Because in the study region urbanisation is among the most expanding land uses, we updated the land cover map by manually digitising new urban areas based on 2011 Google maps images. The length of river or lakeshore in the surrounding landscape of study patches was calculated from the shapefile of Water streams (Table1). Annual precipitation and annual mean temperature were obtained for the centroid of the study patches from the World- Clim dataset (Table 1). Shrubland cover was calculated from the Land cover Map of Nahuel Huapi National Park by pooling together all the different open and mid-successional covers distinguished in the map. Specifically, the classes named (in Spanish) alto andino, matorral, estepa y pradera, pastizal húmedo, pastizal antropizado, Table 1 Predictor variables included in the proposed causal model (Fig.3) and the processes they are expected to influence The variables patch cover and height were also included in the model as proxies of C. scoparius population growth and persistence, respectively, representing the population success in overcoming survival barriers. Visitation rate was included only as a proxy of C. scoparius success in overcoming reproductive barriers. The sources and references for each variable are provided together with the radii at which the variables were calculated: 0 (i.e., the value of the variable in the centroid of the patch), 100, 250, 500, 750 and 1000m. The mean ± SE and ranges of variation of each variable included in the model are also given. For those variables calculated at several radii, the values at the 750m consensus radius are the ones given *1 Updated based on Google Earth 2011 n.a.: not applicable Variable Process they influence Source Reference Radii (m) Mean ± SE (min, max) Urban cover (%) Propagule arrival and disturbance Land cover Map of Nahuel Huapi N.P.*1 Gowda etal. (2014) 100, 250, 500, 750, 1000 14.64 ± 2.10 (0, 89.20) Water shore length (m) Propagule arrival and disturbance Water streams shapefile Instituto Geográfico Nacional de la República Argentina 100, 250, 500, 750, 1000 1806.76 ± 106.74 (0, 4938.16) Annual precipitation (mm) Plant growth WorldClim Fick and Hijmans (2017) 01129.11 ± 17.67 (797, 1785) Pollinator foraging activity Annual mean temperature (°C*10) Plant growth WorldClim Fick and Hijmans (2017) 083.12 ± 0.50 (68, 97) Pollinator foraging activity Shrubland cover (%) Abundance pollinators Land cover Map of Nahuel Huapi N.P Gowda etal. (2014) 100, 250, 500, 750, 1000 31.12 ± 3.17 (0, 100) Slope (°) Attractiveness floral rewards Digital elevation model http:// aster web. jpl. nasa. gov/ gdem. asp 100, 250, 500, 750, 1000 11.20 ± 0.84 (0, 44.59) Patch cover (%) Attractiveness floral rewards Aerial photo n.a 100 1.48 ± 0.18 (0.01, 10.04) Patch height (m) Attractiveness floral rewards in situ n.a 100 1.72 ± 0.04 (1.03, 2.80) Patch colour (yellow or reddish) Attractiveness floral rewards in situ n.a 100 n.a Visitation rate (tripped/open flowers) in situ n.a 100 0.47 ± 0.03 (0.08, 0.98) 2985 Influential factors andbarriers change alongtheinvasion continuum ofanalien plant 1 3 Vol.: (0123456789) lenga achaparrada and vacíos, were included as shrubland cover. Finally, we calculated the mean slope in the surrounding area of study patches from the Digital Elevation Model (Table1). When possible, depending on the spatial resolution of the digital maps, we calculated these variables at five radii distances (100, 250, 500, 750 and 1000m) from each patch centroid. For example, the Land Cover Map of Nahuel Huapi National Park has a 30 × 30m resolution. Therefore, shrubland cover was calculated at those five radii distances. However, the resolution of the WorldClim map is ~ 1 km2, allowing the calculation of one value of annual mean temperature and annual precipitation for the centroid of each patch (Table1). For those variables calculated at different radii, we selected a consensus radius of 750m. This radius represented a compromise between maximising significant relationships based on simple regressions, while preventing the covariation among explicative variables at larger spatial scales (see the section Piecewise Structural Equation Modelling below and Supplementary Material). Though the spatial scale of influence of the climatic variables considered here (annual precipitation and annual mean temperature) could be expected to be larger than the 750m radius, due to their marked elevation and environmental gradients in the study area, we expect these variables to be influential even at the spatial extent of this study. Piecewise structural equation modelling To test our hypotheses, we built a causal model with the predicted relationships shown in Fig.3 and conducted a piecewise structural equation model (SEM) with the piecewiseSEM R package (Lefcheck 2016). When needed, we transformed all the predictive and response variables to achieve multivariate normality (see Table S1) and standardised them by subtracting their respective mean and dividing by the standard deviation to make their effects (i.e. coefficients) comparable (Shipley 2000). A full piecewise SEM is decomposed into linear partial models that are analysed individually and allow the incorporation of corrections for potential data structure owing to hierarchical sampling, phylogenetic or spatial correlations (Lefcheck 2016). Our SEM was decomposed in the three linear partial models shown in Eqs.1–3: Due to our regular sampling along roads, we could expect certain spatial autocorrelation in our observations. We explored the potential spatial correlation for the three linear partial models in which our SEM was decomposed (Eqs.1–3) by calculating Moran’s Index for the residuals. Patch cover and visitation rate showed spatial autocorrelation that fitted an exponential and a Gaussian shaped model, respectively (Supplementary Material). Therefore, we included a correlation term in these partial models using the function gls from the nlme package (Pinheiro etal. 2021). To be able to test the largest number of our hypotheses, we selected a final optimised model through a backward stepwise process based on the AIC statistic, choosing the model with the lowest AIC and the largest number of variables included with a difference of ΔAIC > 3 (Shipley 2013; Supplementary Material). To assess model fit, we considered the results of the d-separation test for all the pathways not included in the SEM (i.e., pairs of variables not connected by an arrow) (Supplementary Material). These results are summarised in a Fisher’s C test. Significant results in this test suggest possible path additions, whereas non-signif- icant Fisher’s C test indicates that the pattern predicted by the SEM agrees with that observed and, therefore, the model cannot be rejected. In addition to the path coefficients estimated by the SEM, we calculated the strength of the indirect effects in the SEM (Table 2). All analyses were conducted in R 3.6.2 (R Core Team 2020). (1) patch cover ∼ urban cover + water shore length + annual mean temp. + annual precip. (2) patch height ∼ annual mean temp. + annual precip. (3) visitation rate ∼ patch cover + patch height + slope + annual mean temp. + shrubland cover + patch colour