View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by RERO DOC Digital Library Hydrobiologia (2006) 565:121–133 Ó Springer 2006 R.S.E.W. Leuven, A.M.J. Ragas, A.J.M. Smits & G. van der Velde (eds), Living Rivers: Trends and Challenges in Science and Management DOI 10.1007/s10750-005-1909-3 Amphibian diversity and nestedness in a dynamic floodplain river (Tagliamento, NE-Italy) K. Tockner1,2,*, I. Klaus1, C. Baumgartner3 & J.V. Ward1 1Department of Limnology, EAWAG/ETH, 8600 Du¨bendorf, Switzerland 2Institute of Ecosystem Studies (IES), Box AB, Millbrook, 12545 NY, USA 3Alluvial Zone National Park, 2304, Orth/Donau, Austria (*Author for correspondence: E-mail: [email protected]) Key words: braided river, Bufo bufo, conservation, disturbance, indicator species, island, large wood, population, parafluvial pond, Rana latastei, restoration Abstract Amphibian distribution and assemblage structure were investigated along the last morphologically intact river corridor in Central Europe (Tagliamento). Thirteen taxa were identified with Rana latastei and Bufo bufo being the predominant species. In the main study reach, a 2 km2 dynamic island-braided floodplain in the middle section of the river, 130 water bodies were delineated that were situated either in the active floodplain (82 sites) or in the adjacent riparian forest (48 sites). Results demonstrated that the active floodplain increased appreciably the available habitat for amphibians, despite frequent disturbances by floods or droughts. Amphibian richness within a given habitat was significantly correlated with distance from vegetated islands, fish density, and water temperature. In the active floodplain, species distribution was highly predictable, exhibiting nearly perfect nestedness, suggesting that selective colonisation and extinction processes predominated. The degree of nestedness was much higher than in the adjacent riparian forest or in regulated floodplains in Central Europe. Results clearly emphasise that amphibians can exploit the entire hydrodynamic gradient, except the main channel. In the active floodplain, vegetated islands and large woody debris are important, directly and indirectly, in maintaining both habitat and amphibian diversity and density in this gravel-bed river. Introduction landscape-corridor function place floodplains high on the conservation agenda (Ward et al., 1999a; Globally, amphibian populations have declined Hughes & Rood, 2001; Tockner & Stanford, over the past several decades and continue to do so 2002). However, today they are among the most (Houlahan et al., 2000; Stuart et al., 2004). Possi- endangered ecosystems worldwide. In Europe, for ble underlying causes of the decline are changes in example, more than 90% of the former floodplains climate, increased exposure to UV-B radiation, either disappeared or they are functionally extinct increased prevalence of diseases, acidification, (Tockner et al., 2006). Amphibians are generally water pollution, habitat fragmentation, and habi- considered as ‘indicators’ of stable floodplain tat loss (Leuven et al., 1986; Alford & Richards, ponds with a low degree of hydrological connec- 1999; Kiesecker et al., 2001). Therefore, many tivity, or as indicators of temporary waters lacking amphibian species are listed as threatened or fish predators (e.g., Waringer-Lo¨ schenkohl & endangered, regionally and globally (Beebee, 1996; Waringer, 1990; Joly & Morand, 1994; Morand & No¨ llert & No¨ llert, 1992). Joly, 1995; Wellborn et al., 1996; Skelly, 1997; Natural floodplains are highly dynamic envi- Tockner et al., 1999; Kuhn et al., 2001). Limited ronments with floods as the primary agent of dis- information is available, however, about amphib- turbance. Their high species diversity and ian populations in dynamic gravel-bed rivers, since 122 such rivers were never the main focus of amphib- is a 7th order river, characterised by a flashy ian research (e.g., Beebee, 1996). One reason was hydrological regime, with highest discharges dur- that nearly all formerly dynamic floodplain rivers ing spring and autumn. The main-stem corridor in developed countries were regulated during the covers about 150 km2. The corridor is character- last two centuries (Petts et al., 1989; Nilsson et al., ised by a high number of vegetated islands within 2005; Tockner & Stanford, 2002). Further, flood the active zone (652 islands >0.01 ha), numerous prone gravel-bed rivers were not expected to pro- gravel bars (952), a considerable habitat diversity vide hospitable habitats for amphibians. Kuhn and a continuous riparian woodland along the (1993), however, demonstrated that species such as margins of the active channel (Tockner & Ward, Bufo bufo exhibited a pronounced reproduction 1999). For a complete description of the catchment plasticity that allowed this species to exploit and longitudinal geomorphic features see Ward dynamic and ephemeral habitats in gravel-bed rivers. et al. (1999b), Arscott et al. (2000, 2002), Gurnell Understanding patterns and processes in natu- et al. (2001), and Tockner et al. (2003). ral river corridors is a prerequisite for a sustainable We investigated the amphibian fauna in conservation and management of their biodiversity six geomorphic reaches along the entire corridor, (e.g., Ward & Tockner, 2001). The Tagliamento with detailed studies in an island-braided reach River in NE-Italy, the last morphologically intact in the middle section of the river. The six river corridor in the Alps, offered the rare oppor- geomorphic reaches are: constrained headwater tunity to investigate amphibian populations under streams (Reach I, 1005–1200 m a.s.l.), headwater natural environmental conditions. In the main island-braided floodplain (Reach II, 705 m a.s.l.), investigation area, an island-braided floodplain, we bar-braided floodplain (Reach III, 200 m a.s.l.), compared amphibian population density, diversity island-braided lowland reach (Reach IV, 180 m and nestedness in the active floodplain (area that a.s.l.), braided-to-meandering transitional flood- extends laterally to the lower limit of persistent plain (Reach V, 19 m a.s.l.) and meandering vegetation and is frequently modified by floods) floodplain (Reach VI, 5 m a.s.l.). and the adjacent riparian forest (periodically The main investigation focused on a 2 km2 inundated by the river). In the present paper, we island-braided floodplain in Reach IV (river-km link habitat heterogeneity with the composition 80; Figs 1 & 2). There, the floodplain was separated and distribution of amphibians and identify indi- into the active area frequently inundated and cator species for different floodplain habitats. In reworked by floods and the adjacent riparian forest addition, we present an empirical example of only inundated during annual floods. Along the left nestedness in the dynamic floodplain system and bank, hillslope forests of Monte Ragogna bordered compare it with more regulated riverine flood- the active floodplain. Along the right bank, the plains. Measure of nestedness provides a quanti- riparian forest extends laterally to a distance of tative indicator of the degree of community ‘order’ about 0.5–1 km. Bare gravel, aquatic habitats and in fragmented systems (e.g., Atmar & Patterson, vegetated islands were the main landscape elements 1993; Patterson & Atmar, 2000). of the active floodplain. The Tagliamento is char- acterised by a dynamic flood regime, with turnover Materials and methods rates of aquatic habitats in Reach IV as high as 50 % during a single flood season (Arscott et al., The Fiume Tagliamento 2002; Van der Nat et al., 2003). The Fiume Tagliamento in NE-Italy (46° N, 12° 30¢ E; Fig. 1) is the last large gravel-bed river Sampling methods and data analyses in the Alps that has escaped intensive river man- agement (Mu¨ ller, 1995; Ward et al., 1999b; Along the mainstem corridor, all water bodies in Tockner et al., 2003). More than 70% of the the six reaches were sampled in March, May, and catchment area (2580 km2) is located within the July 2000, periods without severe floods. In the southern fringe of the Alps, with Mt. Coglians as detailed investigation of the floodplain in Reach the highest peak (2781 m a.s.l.). The Tagliamento IV, the exact location and area of aquatic habitats 123 But A I II SL Fella I IV III Pinzano Udine Tagliamento Pordenone Varmo V 0 25 km VI Latisana Adriatic Sea Water Gravel Riparian vegetation KilometerKilometer Figure 1. Tagliamento catchment, locations of study reaches I–VI, and map of the main study area (Reach IV; mapping date: May 2000). Most ponds investigated are too small to be shown at this scale. The river flows from right to left. 124 distance can be up to 1500 m (C. Baumgartner, unpubl. data). Because it was not possible to distinguish the forms of green frogs expected for the Tagliamento corridor (Rana lessonae, R. klepton esculenta; Gu¨ nther & Plo¨ ttner, 1994, Lapini et al., 1999) from eggs or larvae, they were grouped as a single taxon in all analyses. Maximum water depth (m), specific conductance (lS; portable meter), water temperature (°C) and the presence of fish (four abundance classes based on visual examinations: absent, rare, abundant, dominant) were recorded Figure 2. The main investigation area in the mid-section of during each field campaign. Oxygen (% satura- Reach IV. Active floodplain width is up to 900 m (Photo: tion), sediment composition (relative proportion D. Arscott, May 2000). of silt, sand, fine and coarse gravel, coarse partic- ulate organic matter), density of riparian vegeta- within the active floodplain and the adjacent tion (four cover classes: <10%, 10–25%, 25–75% riparian forest were measured using a differential- and >75%), macrophyte cover (four classes as for GPS (Global Positioning System). Individual riparian vegetation), accumulations of large woody water bodies were marked (130 in all) and debris (LWD) (four cover classes as for riparian numbered in the field. Between 7 March and 15 vegetation), and actual surface area (m2) were July 2000 all water bodies were repeatedly (totally estimated every second visit (four times totally). eight times) visited throughout the amphibian Rank-correlation analyses (Spearman-rank test) breeding period.