Global Change Biology (2006) 12, 1985–1992, doi: 10.1111/j.1365-2486.2006.01236.x

Influence of climate changes on communities in space and time: the case of assemblages along an alpine glacier foreland

MAURO GOBBI, DIEGO FONTANETO andFIORENZA DE BERNARDI Dipartimento di Biologia, Universita` degli Studi di Milano, Via Celoria 26, I-20133 Milano, Italy

Abstract The impact of global warming in space and time is described for species assemblages of wandering along the alpine glacier foreland of the Forni Valley (Northern Italy). We tested the effect of environmental variables (e.g. elevation, age of glacier retreat, vegetation cover, debris cover) on species richness and on species composition of spiders. Age of glacier retreat was the only significant variable influencing spider species assemblages in the valley. A spatially structured distribution of species and species assemblages along the chronosequence of glacier retreat was evidenced. The threshold abruptly differentiating two groups of species richness and species composition fell between sites deglaciated 100 and 155 years before the analysis. Latitudinal shifts towards the poles in species ranges at the global scale in response to climatic changes are known, and an altitudinal shift in species range should be expected for spiders at the local scale of the Forni Valley. Such a shift is present in spider species assemblages, although not as an expected gradual change in species richness and composition, but with a threshold effect after one century of glacier retreat. We discuss our results in the light of plausible future scenarios due to global warming, the consequence of further glacier retreats onto spiders, and caveats for monitoring studies. Keywords: altitudinal shift, Araneae, Forni Glacier, glacier retreats, global change, species richness, threshold effect.

Received 16 February 2006; revised version received 3 May 2006 and accept 11 May 2006

as species assemblages are determined by the interac- Introduction tions within the assemblages themselves and between In the last century, the climate of the Earth has been the organisms and the abiotic and biotic components of characterized by an increase in mean surface tempera- the ecosystem. No clear idea of how climatic changes ture of approximately 0.6 0.2 1C (IPCC, 2001) with may influence richness and composition of biotic com- two main periods of heating, the former between 1910 munities is available yet and more studies are needed. and 1945 and the latter from 1976 to nowadays (Jones Analysis of historical datasets of more than 1700 species et al., 1999). Climate warming of the last century is of living organisms, both plants and (Parmesan known to have caused problems in conservation biol- & Yohe, 2003) suggests a significant range shift toward ogy because of shifts in species ranges and distribution, the poles and a significant advance of spring events. But enhancements of extinction risks and changes in repro- historical datasets are generally scarce, especially for ductive biology, phenology, fitness and population invertebrates. dynamics of many species (cf. Parmesan, 1996; Langen- In order to deal with analyses of the effect of climate berg & Aldhous, 2000; McCarty, 2001; Thomas et al., changes on present species assemblages in inverte- 2004). brates, suggestions may come from the study of Global change may, therefore, influence local species the current situation in habitats that produce great assemblages and species richness (Walther et al., 2002), shifts with long-lasting effects in relation to climate changes of low intensity, where species assemblages Correspondence: Mauro Gobbi, tel. 1 39 0250314722, are forced to change in response to those environmental fax 1 39 0250314713, e-mail: [email protected] changes. r 2006 The Authors Journal compilation r 2006 Blackwell Publishing Ltd 1985 1986 M. GOBBI et al.

The alpine glacier forelands provide such a habitat, years of glacier advances and retreats stretching over a with a spatial and temporal scale for understanding the length of 2.5 km. The terminal moraine delimiting the nature of the relationships between animals, vegetation, glacier foreland was deposed in 1850 while the other and the abiotic components (Ettema & Wardle, 2002). moraines are dated 1904, 1926 and 1980. Using various With their well-known chronology of glacial recession, sources, including reports, photographs, iconography glacier forelands have been used as a unique model and records, we were able to determine the position of system to study the action of climate changes on living the glacier in 1943, 1953 and 2000. Age of deglaciation soil communities (Matthews, 1992; Bardgett et al., 2005). along the proglacial area of the valley is confirmed by Studies on colonization of recently deglaciated valleys previous glaciological, geo-morphological and licheno- are already available for plants (e.g. Grabherr et al., metric studies of the glacier retreat in the area (Pelfini, 1994; Caccianiga & Andreis, 2004; Caccianiga et al., 1992; Pelfini & Smiraglia, 1992). 2006) and invertebrates (e.g. Hodkinson et al., 1998, 2004; Kaufmann, 2001, 2002; Gobbi et al., 2006), but the mechanisms driving community composition in these valleys are not clear yet. Sampling stations Newly exposed land surface as glacier foreland are firstly colonized by heterotrophic organisms (Hodkinson Sites from A to G were chosen in areas with dated age of et al., 2002): animal consumers as predators or omnivores glacier retreat, from 1850 to 2000; site H was located on are supported by allochthonous income of food, as the glacier; sites J and K were located on areas never organic particles and invertebrates coming with the reached by the glacier since the little ice age, thus, older upward winds (Coulson et al., 2003). We chose species than 155 years (Fig. 1). To perform the analyses, we assemblages of wandering spider (Arachnida, Araneae) conventionally define their age as 10 000 years. Together as a model because wandering spiders are the most with the age gradient and the elevation gradient along active top predators in this habitat. Moreover, they the valley bottom from A to G, there is also an environ- represent the earliest stages of primary community mental gradient of decreasing vegetation cover and, assembly in recently deglaciated areas like newly ex- oppositely, increasing stony debris cover from A to G, posed moraines with nonclimax vegetation and low with A completely covered by vegetation and G with no productivity (Thaler, 1996; Hodkinson et al., 2001). vegetation cover. Sites J and K, located above the valley Goal of our paper is to describe the spatial succession bottom, are high alpine grassland, almost completely of species assemblages of aboveground spiders. We covered by herbaceous vegetation. Sampling season analyse the spatial patterns of species assemblages of spanned across periods without snow cover, from July spiders in relation to an environmental gradient due to to September both in 2004 and 2005, and time of Holocene glacier retreat in the Forni Valley in Italian exposition of the traps was 20 days. Alps. We analyse both species richness and species During summer 2004 the right side of the valley was composition along areas with a spatial and temporal analysed from A to F (sites identified by number 1 gradient of glacier retreat. Then, we discuss our results following the letter), together with samples H and K, in the light of plausible future scenarios due to global while during summer 2005 the left side of the valley warming and the consequence of further glacier retreats was analysed (sites with number 2 following the letter), onto spiders, and caveats for monitoring studies. together with samples G and J (Fig. 2). We used a total of 96 pitfall traps, located along transects of six traps, 10 m distant each one from the following one, for each Materials and methods collecting station. Pitfall trapping is a standardized method to sample wandering epigean invertebrates, Study area able to define the species richness and abundance of a The study area is the Forni Valley (461250N, 101340E). It habitat (Southwood, 1978). We collected different in- is located in the Italian Middle-Eastern Alps (Marazzi, vertebrate taxa, but in this paper we focus on spiders. 2005) and it is characterized by a glacier foreland We used plastic glasses (7 cm of diameter and 8 cm of (elevation from 2150 to 2500 m) with a well-preserved height) baited with a standard mixture of wine vinegar chronosequence of Holocene Forni Glacier fluctuation. and salt. This solution does not influence the capture The tree line cannot be clearly identified (M. Caccianiga, because it is not attractive (C. Pesarini, personal com- personal communication), with individuals of Larix munication), but only preservative for the animals fall- decidua, Picea abies and Pinus cembra colonizing the en in them. Trap size was the same used in other entire proglacial area (Rossi et al., 2001). The chronose- sampling areas in glacier forelands (R. Kaufmann, quence along the bottom of the valley represents 155 personal communication).

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Fig. 1 Geographic location of the sampling sites. Continuous white lines represent positions of dated moraines, while broken white lines represent dated positions of the glacier front.

Data analysis observed ones, the sampling design may be considered adequate for the study area, and the obtained species Species richness assemblages can be used as reliable data to infer envir- onmental effects on the species compositions them- Spiders were determined to species level and we fol- selves. lowed the checklist of the Fauna Europaea Web Service for the nomenclature (van Helsdingen, 2004). Environmental variables Alpha diversity was evaluated as species richness in each pitfall trap. Gamma diversity was computed as the After testing the accuracy of the sampling design, we cumulative number of species collected across the entire could test the effect of environmental variables on a valley. diversity. The variables we measured were: elevation of As noted by Kaufmann (2001), pitfall traps sampling the site, age of glacier retreat, percentage of vegetation of aboveground invertebrates is not exhaustive as plant cover and of stony debris cover in an area of roughly investigations; it is considered impossible to obtain a 50 m2 around each pitfall trap. Age of glacier retreat complete list of species, as catch numbers are a result along the bottom of the valley was inferred by previous of population density and activity of individuals, geo-morphological studies of the glacier retreat in the more than the true distribution of species (Topping & area (see details given in the Study Area section). All Sunderland, 1992). Therefore, we tested the accuracy of other variables were measured directly in the field. our sampling design estimating the theoretical total All values obtained for the environmental variables species richness according to two indices, based on were transformed to their natural logarithms before the observed data: the Incidence-based Coverage Estimator analysis. A preliminary collinearity diagnostics gave (ICE), and, the Abundance-based Coverage Estimator age, vegetation cover, and debris cover as highly re- (ACE). ICE estimates the overall number of species that lated; thus, we decided to disregard vegetation and may live in the study area, on the basis of the observed debris cover in the following analysis, and to keep only number of species and the frequency of their occurrence age as a variable to test. This choice was done because in the patches, while ACE on the basis of the abun- age of glacier retreat is the only variable of the three dances of individuals found for each species in each related ones that may influence the other two related sample (Colwell & Coddington, 1994; Chazdon et al., variables, with positive influence on the percentage of 1998). If both ICE and ACE give similar numbers to the vegetation cover, and negative influence on the percen- r 2006 The Authors Journal compilation r 2006 Blackwell Publishing Ltd, Global Change Biology, 12, 1985–1992 1988 M. GOBBI et al.

Fig. 2 Species assemblages of spiders in each analysed area. Sampling sites are ordered following age of deglaciation; species are ordered according to the first axis obtained from a correspondence analysis (Leibold & Mikkelson, 2002). Other variables as year of sampling and elevation are given. Species abundance (as percentage on total captures) is also shown, following four ranks of abundance: , rare species, 0–1%; , occasional species, 1.1–2%; , accessory species 2.1–5%; , dominant species 45.1%. tage of debris cover. Therefore, we tested with a regres- in order to exclude the effect of gradual decrease of sion analysis only two variables influencing a diversity: vegetation cover along the valley bottom, due to the age of glacier retreat, and elevation of the sampling site. glacier retreat and inextricably related to the spatial After finding the most significant variable explaining gradient. To evaluate the effect of distance among a diversity, we used Tukey test of Honestly Significant samples on composition of each species assemblages, Difference (Crawley, 2002) to highlight possible homo- we used a Mantel test between (i) the dissimilarity geneous subsets of samples, in relation to the important matrix obtained with Jaccard’s distances, and (ii) the independent variable. matrix of geographic distances. The Mantel test was Then, we tested whether the differences between performed applying 10 000 replicates with Pearson’s groups were due to spatial isolation, and, thus, to the product–moment correlation method. inability of some species to move across the valley. We used Jaccard’s distances between each species assem- Species replacements blage of every trap as a metric to express differences in species composition (b diversity): in the case of ecolo- Most sites were sampled during two sampling seasons, gical distances related to geographic distances between and to analyse species replacements, we merged data each pitfall trap, spatial isolation may be the cause of from all traps in 2 years. We could hypothesise that the observed spatial patterns, while in the case of no during 1 year no differences in species composition relationship, we could infer that individuals may move could be expected, but sampling sites from A to G in across all the valley, with differences in species compo- 2004 and in 2005 were in different sides of the valley. sition due to habitat differences and not to distance. We Therefore, we assessed the reliability of this merged considered only samples with vegetation cover 460%, data, testing whether (i) a diversity and (ii) species

r 2006 The Authors Journal compilation r 2006 Blackwell Publishing Ltd, Global Change Biology, 12, 1985–1992 SPIDER ASSEMBLAGES ALONG A GLACIER FORELAND 1989 composition of each overall site, with coupled sampling in 2004 and in 2005 were more similar to each other than to the other samples. We used (i) correlation test be- tween coupled a diversity of each site in 2004 and in 2005, merging all data from each traps for every site in each year, and (ii) ANOSIM test, based on Jaccard’s index of similarity, for similarity of species assemblages from all merged data for each site. To analyse species replacements between each site, we performed the analysis of meta-community struc- ture proposed by Leibold & Mikkelson (2002). We analysed 10 different species assemblages, from A to K, that were considered the communities for the meta- community analysis. The probability of obtaining the observed spatial turnover between communities was calculated using a Monte Carlo simulation. We gener- Fig. 3 Alpha diversity of spiders in relation to age of deglacia- tion of the sites (in logarithmic scale). The bold line represents ated 200 random matrices of species presence following the overall regression line; the dashed line represents the regres- a Null-Model subject only to the constraint that the sion line, with a discontinuity point at the threshold between matrices have the same number of rows, columns, and 100 and 155 years. presences of the analysed metacommunity, and that no row (5site) or column (5 species) has only absences Table 1 Model coefficients of a linear regression analysis, (Leibold & Mikkelson, 2002). We tested the significance with a diversity as dependent variable, both for species of differences in the number of species replacements assemblages in sites older than 150 years (ANOVA test: among communities, between the observed and the F2,21 5 0.158, P 5 0.855), and younger than 150 years (ANOVA simulated matrices using a two-tailed Z-test. Signifi- test: F2,69 5 0.193, P 5 0.825) cantly lower rates of species replacements than tP expected ( 5 low spatial turnover) represent nested structures, with hierarchical structures among commu- Model, older than 150 years nities. Higher rates than expected are known as anti- (constant) 0.546 0.591 nested structures, and provide evidence of groups of ln (age of glacier retreat) 0.556 0.584 species that are mutually exclusive. ln (elevation) 0.562 0.580 Model, younger than 150 years ICE and ACE estimator of species richness were (constant) 0.025 0.980 computed using Estimates 7 (Colwell, 2004); all other r ln (age of glacier retreat) 0.162 0.872 analyses were performed using Microsoft Excel , the ln (elevation) 0.018 0.986 Excel macro written by Leibold & Mikkelson (2002), SPSSr and vegan package in Rr. from the glacier foreland to areas that were left degla- ciated 100 years before, while a diversity immediately Results jumps to higher values in areas deglaciated 155 years Out of the 96 pitfall traps we analysed, 273 spiders were ago and remains constant in areas never reached by the collected. The spider species assemblages in the valley glaciers since the last glacial period (Fig. 3). The homo- gave a g diversity of 17 species (Fig. 2). Both ICE and geneous subsets displayed by Tukey’s HSD test confirm ACE estimators of species richness produced exactly 17 this situation: no changes in a diversity from glacier species, confirming the accuracy of the sampling de- foreland to 100 years, and then an immediate increase sign, notwithstanding the relatively low number of after 155 years. Dividing the samples at the cut-point individuals. Testing the effect of elevation and of age between 100 and 155 years before, the two subsets of glacier retreat on spider a diversity with a linear revealed no influence of age of glacier retreat or of regression analysis (ANOVA test: F2,93 5 22.62, elevation on a diversity (Table 1, Fig. 3). Po0.0001), elevation could be ruled out as an explain- No relationship was shown between b diversity and ing factor (t 5 0.618, P 5 0.538), and age of glacier retreat geographic distance (Mantel test: R 5 0.0433, P 5 0.246). was confirmed as the most important factor influencing We can, thus, rule out spatial isolation as an explanation species richness (t 5 6.589, Po0.001). The effect of age of of the abrupt differences found between sites younger glacier retreat on a diversity does not show a gradual and older than the cut-point between 100 and 155 years, trend: a diversity remains on values between 0 and 1 as spiders may move across the entire valley, and r 2006 The Authors Journal compilation r 2006 Blackwell Publishing Ltd, Global Change Biology, 12, 1985–1992 1990 M. GOBBI et al. distant species assemblages are more similar to each found along the chronosequence of glacier retreat in other, in relation to age of glacial retreat and not in the valley, with a major change in species composition relation to geographic distance. exactly at the threshold between 100 and 155 years, as Correlation of a diversity between sampling in 2004 for species richness. and 2005 for each site was complete (Spearman’s Sites left by glacier cover after more than 100 years r 5 1.0, P 5 0.000); ANOSIM test confirmed that species show constant high values of a diversity of inverte- assemblages did not change in composition between the brates, maybe because of habitat maturity and stability two sampling seasons and that samplings on each side (Kaufmann, 2002). We can hypothesize that in the Forni of the valley may be considered similar (R 5 0.6667, Valley the threshold between 100 and 155 years marks P 5 0.001). the boundary of unstable habitats from stable ones, The spatial turnover in species distribution in sites was characterised by climax vegetation and by high values significantly higher than expected by random chance of productivity which influence spider species richness (P 5 0.0016), with an anti-nested pattern of the analysed (Kinzig et al., 2002; Death & Zimmermann, 2005). meta-community. Therefore, there are species which The existence of ecological thresholds is an old but presences are mutually exclusive. Looking in detail at understudied and underconsidered topic in conserva- the distribution of species across the valley (Fig. 2), the tion biology (Huggett, 2005). The ecological threshold situation of mutually exclusive species can be described we found between 100 and 155 years, may represent a for its biological meaning. Some species, ordered in the problem for monitoring community response to global right side of the matrix, are found only in areas where changes in temperature. No effect may be seen on glaciers retreated more than 155 years ago, while other community composition for long time, because of the species, as rossica and Thanatus formicinus live latency of the response of the biota to the abiotic change only in soils left by the glacier between 60 and 25 years (Knick & Rotenberry, 2000). before, and one species, Pardosa saturatior, is present and Some species are able to live on recently exposed soils dominant only nearby and on the glacier. younger than 100 years: C. mediocris, T. formicinus, and P. saturatior, Coelotes mediocris, endemic of the alpine grass- lands of the Alpine region, is considered one of the first Discussion species able to recolonize ice-free areas from short In the alpine environment two of the main factors distances (Zingerle, 1998, 1999). Its pioneering habits influencing the local changes in aboveground species seems to justify its presence in sites D and E, ice free richness are the altitudinal gradient (Meyer & Thaler, from 50 and 60 years, and site J, the old alpine grass- 1995; Thaler, 1996) and the age of community stabiliza- land, probably its refuge area in the valley. Maybe its tion (Kaufmann, 2002; Bardgett et al., 2005). present distribution in the valley bottom is reflecting its Glacier forelands show both these factors tightly re- recolonization of the valley from the upper alpine lated: glacier retreats ( 5 age of stabilization) follow a grassland. Thanatus formicinus needs soils with high vertical succession ( 5 altitudinal gradient). With our humidity (C. Pesarini, personal communication), and sampling design, we were able to disentangle this rela- it was found in site F, always damp because of waters tionship, and to highlight age of glacier retreat as the from glacier fusion. Pardosa saturatior is a euryzonal only relevant factor. In our study area, elevation had no ripicolous species linked to bare ground close to gla- influence on species richness and species composition. ciers (Thaler, 1996) and was found in recently degla- Age of glacier retreat corresponds to glacier history, ciated soils (5-year old), always damp, and on the and can be considered the principal force driving glacier surface. debris-vegetation cover (Caccianiga & Andreis, 2004), Considering that the increment of global temperature and species richness of spiders. How does species of 0.6 0.2 1C in the last century (IPCC, 2001) produced richness change along the glacier foreland? Thaler great changes in the glacier length and amplitude of ice (1996) described a decrease in species richness and cover of soil, this small increase in temperature pro- abundance of spiders with elevation, and suggested duced long-lasting lag effects on species assemblages of that in alpine valleys a relationship between altitudinal spiders, which a diversity and species composition did gradient and spider a diversity is present, with an not recover to higher values, like those of the higher abrupt change in richness between habitats and a prairies (sites J and K), for at least 100 years as the gradual decrease with elevation, within habitat. We glacier left the soil open to biotic colonization. The found exactly this stepwise reduction in species rich- valley revealed two different situations: one group of ness in relationship to soil age, with a threshold be- species that quickly move along the glacier, and one tween 100 and 155 years. A spatially structured group of species that did not move for at least one distribution of species and species assemblages was century. Considering the future trend of temperature,

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