Along the Altitudinal Gradient of Crete, Greece: Species Richness, Activity and Altitudinal Range M
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Journal of Biogeography (J. Biogeogr.) (2005) 32, 813–831 ORIGINAL The distribution of ground spiders ARTICLE (Araneae, Gnaphosidae) along the altitudinal gradient of Crete, Greece: species richness, activity and altitudinal range M. Chatzaki1*, P. Lymberakis1, G. Markakis2 and M. Mylonas1,3 1Natural History Museum of Crete, University ABSTRACT of Crete, Irakleio, 2Technological Education Aim To study the altitudinal variation of ground spiders (Araneae, Gnaphosidae) Institute of Crete, Irakleio and 3Department of Biology, University of Crete, Irakleio, Greece of Crete, Greece, as far as species composition, species richness, activity and range of distribution are concerned. Location Altitudinal zones (0–2400 m) along the three main mountain massifs of the island of Crete. Methods Thirty-three sampling sites were located from 0 to 2400 m a.s.l. on Crete, and sampled using pitfall traps. Material from the high-activity period of Gnapho- sidae (mid-spring to mid-autumn) was analysed. Sampling sites were divided into five altitudinal zones of 500 m each. Statistical analysis involved univariate statistics (anova) and multivariate statistics, such as multidimensional scaling (MDS) and cluster analysis (UPGMA) using binomial data of species presence or absence. Results Species richness declines with altitude and follows a hump-shaped pattern. The activity pattern of the family, as a whole, is not correlated with altitude and is highly species-specific. In the highest zone, both species richness and activity decline dramatically. The altitudinal range of species distribution increases with altitude. On the Cretan summits live highly tolerant lowland species and isolated residents of the high mountains of Crete. Two different patterns of community structure are recorded. Main conclusions Communities of Gnaphosidae on Crete present two distinct structures following the altitudinal gradient, these being separated by a transitional zone between 1600 and 2000 m. This study supports previous results which show a hump-shaped decline in species richness of Gnaphosidae along altitudinal gradients, leading to a peak at 400–700 m, where an optimum of environmental factors exists. This makes this zone the meeting point of the often opportunistic lowland species with the older and most permanent residents of the island. Rapoport’s rule on the positive correlation of the altitudinal range of species distributions with altitude is also supported. The high activity recorded for the species that persist on the high mountains of Crete is indicative of a tolerant arachnofauna, and is considered to result from relaxation of competitive interactions with other species. This is related to a reduction in species numbers, shortening of the activity period on high mountains and the unique presence of high mountain species that thrive only there. As shown in our study, strategies to cope with altitude are species-specific. Therefore, there cannot exist one single model to describe how animals react to *Correspondence: M. Chatzaki, Natural the change in altitude, even under the same environmental conditions. History Museum of Crete, University of Keywords Crete, PO Box 2208, 71409 Irakleio, Crete, Greece. Activity, altitudinal gradient, altitudinal range, Crete, Gnaphosidae, ground E-mail: [email protected] spiders, Mediterranean, mountain ecology, pitfall traps, species richness. ª 2005 Blackwell Publishing Ltd www.blackwellpublishing.com/jbi doi:10.1111/j.1365-2699.2004.01189.x 813 M. Chatzaki et al. intermediate elevation (Bosmans et al., 1986; McCoy, 1990; INTRODUCTION Colwell & Hurtt, 1994; Rahbek, 1997; Fleishman et al., 1998; The effect of altitude on biodiversity has been a topic of great Sa´nchez-Cordero, 2001; Grytnes & Vetaas, 2002; Sanders, interest for many earlier and contemporary biogeographers. 2002) (for a detailed review see also Lomolino, 2001 and During the nineteenth century latitudinal and elevational Sanders, 2002). In insects, both patterns have been observed gradients in diversity were considered direct responses to (Sanders, 2002 and references therein). According to McCoy climatic changes and energy interactions in the environment (1990), the latter pattern is more pronounced in predomin- (see historical review in Lomolino, 2001). These were later antly, or totally, herbivorous insects, such as Coleoptera, interpreted as the species-energy theory by Wright (1983). Homoptera and Hemiptera. Recent researchers connected mountain ecology with the Apart from species richness, changes that occur in the species–area relationship of island biogeography (MacArthur, abundance of a species along altitudinal gradients are often 1972), because of the similar conditions prevailing for both similar to changes along its geographical range (Whittaker, types of ecosystems (small area, isolation, restricted spatial 1952; Hagvar, 1976; Claridge & Singhrao, 1978; Randall, 1982). heterogeneity). The negative effect of latitude on species Abundance appears to be higher at the centre of a species range richness and latitudinal range, or Rapoport’s rule (Stevens, and lower near the edges (Brown, 1984; Brussard, 1984; Brown 1989) has also been correlated with the same phenomenon et al., 1996). The pattern of abundance of a species along along altitudinal gradients (Stevens, 1992; Brown et al., 1996). altitudinal gradients must be highly species-specific, as it is The latter is explained as a result of the wider ecological related to many factors such as responses to climate changes, to tolerances of organisms at higher elevations, a crucial charac- food quantity and quality, to natural pressure of enemies and teristic which they have to possess in order to withstand the to interspecific competition (see Lawton et al., 1987 for wider climatic fluctuations to which they are exposed. Lawton detailed citations). et al. (1987) ascribed the effect of elevation on species richness Evidence for the value of Rapoport’s rule along elevation to the following reasons: (1) reduction in productivity with gradients emerges from surveys of plants, mammals, reptiles elevation; (2) reduction in the total area; (3) reduction in and insects (Stevens, 1992). More detailed surveys verify this resource diversity; and (4) harshness and unpredictability of rule for butterflies (Fleishman et al., 1998), grasshoppers the conditions prevailing at higher elevations. (Claridge & Singhrao, 1978), ants (Sanders, 2002) and partially Two more phenomena have been related to the negative isopods (Sfenthourakis, 1992). According to Brown et al. effect of altitude on species richness, the ‘mid-domain effect’ (1996), Rapoport’s rule is closely related to general factors (Colwell & Lees, 2000) or ‘ecotone effect’ (Lomolino, 2001), which limit the range of distributions along geographical or i.e. the peak in species richness at mid elevations, due to the ecological gradients, i.e. increasing physical stress in one increasing overlap of species ranges towards the centre of a direction, and increasing numbers and impacts of biological domain or minor peaks at transitions between elevational enemies in the other. Therefore, it is a very dynamic, species- communities, and the ‘rescue effect’ (Brown & Kodric-Brown, specific phenomenon, partly depending on global climatic 1977), i.e. the reduced likelihood of a population at higher changes and on human activities. elevations to be rescued by individuals dispersing from other Concerning spiders, not many detailed studies have been zones when compared with populations at lower elevations. carried out focusing on the relationship between species Stevens (1992) proposed the Rapoport-rescue hypothesis, richness and altitude or other of the above questions on a which is the extension of the previous idea to species level, species level. Maurer & Ha¨nggi (1991) presented the altitudinal suggesting that species richness is inflated in lower latitudes/ variation of spider species in Switzerland, reporting a more or altitudes by the emigration of high-altitude species at the less linear decline and an abrupt decrease in the number of margins of their ranges due to wider tolerance, while taxa from species above the timberline (only 7% of the total number of lower elevations cannot expand their upper limit of elevational species of the country occur above 2300 m). Although very few range. In total, this would mean that extinction rates of species species occupy the whole altitudinal range of the Swiss Alps increase with elevation and so does isolation, in contrast to (21 species), about half of the total arachnofauna have wide immigration rates, which decrease with elevation (Stevens, altitudinal range, their distribution extending from the valleys 1992; Lomolino, 2001). to the timberline. Referring to the invertebrate diversity Although the negative effect of altitude on diversity is (including spiders) at high altitudes of the Central Alps, broadly documented (Lawton et al., 1987; McCoy, 1990; Meyer & Thaler (1995) reported a gradual decline of species Stevens, 1992; Brown et al., 1996; Lomolino, 2001; Sanders, within the main life zones from 1800 to 3500 m and a stepwise 2002), the pattern of diversity decline is still controversial. decline of species at the main borders. Bosmans et al. (1986) One group of scientists favours a monotonic decrease in studied the spider communities along an altitudinal gradient in species richness with increasing elevation (Claridge & the French and Spanish Pyre´ne´es (700–2475 m), concentrating Singhrao, 1978; Lawton et al., 1987;