In: Steppe Ecosystems ISBN: 978-1-62808-298-2 Editors: M. B. Morales Prieto and J. Traba Diaz © 2013 Nova Science Publishers, Inc. The license for this PDF is unlimited except that no part of this digital document may be reproduced, stored in a retrieval system or transmitted commercially in any form or by any means. The publisher has taken reasonable care in the preparation of this digital document, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained herein. This digital document is sold with the clear understanding that the publisher is not engaged in rendering legal, medical or any other professional services. Chapter 4 DIVERSITY PATTERNS IN THE STEPPE OF ARGENTINEAN SOUTHERN PATAGONIA: ENVIRONMENTAL DRIVERS AND IMPACT OF GRAZING Pablo Luis Peri,1,2, María Vanessa Lencinas,3 Guillermo Martínez Pastur,3 Grant W. Wardell-Johnson4 2 and Romina Lasagno 1Universidad Nacional de la Patagonia Austral (UNPA)-CONICET, Río Gallegos, Santa Cruz, Argentina 2Instituto Nacional de Tecnología Agropecuaria (INTA), Argentina 3Centro Austral de Investigaciones Científicas (CADIC) – CONICET, Argentina 4Curtin Institute for Biodiversity and Climate, Curtin University, Bentley, WA, Australia ABSTRACT The steppe ecosystem, mainly characterised by the presence of tussock, short grasses and shrubs, covers 85% of the total area in Santa Cruz Province and 25% in Tierra del Fuego Island. Most of the land in the Patagonian region has been influenced by domestic livestock grazing for more than 100 years. This has led to a substantial modification of the ecosystem and the original floristic patterns. Erosion and degradation processes have occurred in several areas of Patagonia mainly due to an overestimation of the carrying capacity of these rangelands. In this chapter we review patterns of plant and insect diversity in relation to environmental drivers and grazing impact in the steppe of Argentinian South Patagonia. In Santa Cruz, results from 141 sites indicated significant interactions between grazing and the abiotic environment (mainly water avalilability) on plant diversity. The Corresponding author: Pablo Luis Peri. Universidad Nacional de la Patagonia Austral (UNPA)-CONICET- Instituto Nacional de Tecnología Agropecuaria (INTA). CC 332 (CP 9400), Río Gallegos, Santa Cruz, Argentina. E-mail: [email protected]. 74 Pablo Luis Peri, María Vanessa Lencinas, Guillermo Martínez Pastur et al. complexity of these interactions indicated the need for examining patterns of species turnover at different spatial scales. Analysis of the steppe vegetation patterns from 113 sites along Tierra del Fuego Island demostrated differences related to geographical zones (North, Center, East and South), dominant vegetation types (grasslands, peatlands or shrublands) and disturbance impact (grazing, beavers or burned areas). Because insect diversity of Tierra del Fuego steppes is poorly known, coleopterans were selected as potential indicators of biodiversity using pitfalls traps in the same vegetation survey sites. We found significant changes in ground-active beetle assemblages generated by grazing and livestock activities, both in grassland, peatlands and shrublands. Therefore, this group of insects could be useful indicators of biodiversity conservation and ecosystem management. Keywords: Floristic patterns; grassland; insect diversity; peatlands; plant diversity; shrublands; species richness; water availability INTRODUCTION Argentinian southern Patagonia includes Santa Cruz and Tierra del Fuego Provinces. Santa Cruz Province has an area of 243,943 km2 and extends from latitudes 46º to 52º30’S. Tierra del Fuego Province includes many islands of varied size and the Argentine Antartic Sector. The main island has an area of 20,180 km2, and extends between 52° and 56° S. In Santa Cruz, there are three main ecosystems: the Andes Mountains, the steppe and the valleys. Rainfall decreases from 800-1000 mm to 200 mm/year from west to east across the Andes Mountains, that act as an orographic barrier to moist winds coming from the west. In Santa Cruz, the native forest covers a narrow (100 km wide) and long (1000 km) strip of land. In Tierra del Fuego, the Andes Mountains decrease in height, and twist to the west. Therefore rainfall decreases from north to south, and forests occupy zones where the annual rainfall rises from 400 to 800 mm, south of the steppe. The southern beeches, lenga (Nothofagus pumilio), ñire (N. antarctica) and guindo (N. betuloides) are the dominant forest species in Argentinian southern Patagonia, covering 1,269,796 ha (535,889 ha in Santa Cruz and 733,907 ha in Tierra del Fuego Provinces). The steppe ecosystem, mainly characterised by the presence of tussock (Festuca, Stipa), short grasses (Poa, Carex) and shrubs, covers 85% of the total area in Santa Cruz and 25% in Tierra del Fuego. The vegetation physiognomy of the Santa Cruz and Tierra del Fuego steppes is illustrated by Figures 1 and 2, respectively. The main activity in this ecosystem is extensive sheep production, with stocking rates ranging from 0.13 to 0.75 heads ha-1 year-1. The average size of properties is 12,400 ha and the largest station is 179,000 ha. During the last 70 years, a degradation process of the steppe (desertification) has occurred in the central part of Santa Cruz and partialy in the north of Tierra del Fuego, resulting from a combination of overgrazing and drought. Grazing impacts have been particularly profound in high latitude environments such as Patagonia, which has led to substantial ecosystem modification, in particular a significant increase in bare areas and change from the original floristic patterns (Bisigato and Bertiller, 1997). Erosion and degradation processes have occurred in several areas of Patagonia due to an overestimation of the carrying capacity of these rangelands, inadequate distribution of animals in very large and heterogeneous paddocks, and year-long continuous grazing (Golluscio et al., 1998). There are Diversity Patterns in the Steppe of Argentinean Southern Patagonia 75 more than 6.5 million hectares affected by desertification (del Valle et al., 1995), where annual pasture production does not exceed 40 kg dry matter.ha-1. It has also been reported that grazing intensity on these extensively managed grasslands has affected ecosystem C levels. Peri (2011) reported that C stock in grasslands decreased from 130 Mg C. ha-1 under low grazing intensity (0.10 ewe ha-1 yr-1) to 50 Mg C.ha-1 at a heavy stocking rate (0.70 ewe ha-1 yr-1) mainly due to an decline in plant cover and C lost from soil (mainly the organic layer in increasingly bare areas) as a consequence of soil erosion by strong winds. There is some previous information about the distribution and physiognomic floristic description of the Patagonian flora at the regional, province and landscape level (Boelcke et al., 1985; Ares et al., 1990; León et al., 1998). The vegetation of continental Patagonia is in general low in plant cover (10-60%) and dominated by shrubs and perennial grasses (Bertiller and Bisigato, 1998). Precipitation decreases from the western mountains towards the east, and this distinct precipitation gradient substantially influences patterns of vegetation distribution (Jobbágy et al., 1995). Local edaphic and topographic variation in Patagonia is also correlated with local species turnover and structural patterns of vegetation (Soriano, 1983; Bisigato and Bertiller, 2004). Most of the floristic and structural inventories of Patagonian vegetation under different grazing regimes and disturbance intensities have been carried out in the extra- Andean regions such as the steppes of northern Patagonia (Ares et al., 1990; Beeskow et al., 1995; Bisigato and Bertiller, 1997; Aguiar and Sala, 1998). However, the influence of grazing disturbance on the floristic patterns of southern Patagonia remains limited, despite the vast area influenced by grazing. Also, these findings have to be interpreted with caution, as knowledge on interactions between grazing, environmental factors (edaphic and climatic variables) and vegetation type is negligible. An understanding of these interactions is, however, crucial for the design and implementation of viable management options towards the conservation of rangeland biodiversity and function (Ciblis and Coughenour, 2001; Golluscio et al., 1999). In this chapter we review observed patterns of diversity in the steppe of South Patagonia in relation to environmental drivers and grazing impact. Specifically, we present observational data of vegetation diversity in both Santa Cruz and Tierra del Fuego, highlighting the influence of grazing on plant diversity in Santa Cruz and on plant and insect diversity in Tierra del Fuego. PLANT DIVERSITY There are 1,378 vascular plant species recorded from arid and semi-arid Patagonia (Correa, 1971), mainly represented by angiosperms, of which around 30% are endemic and 340 are exotic. Sheep grazing has been shown to reduce vascular plant diversity in several Patagonian ecosystems, both by promoting local extinction of preferred forage plants and by altering the relative abundance of species in the grazed plant communities (Aguiar and Sala, 1998; Bertiller and Bisigato, 1998). Soriano et al. (1995) assembled a list of 76 endangered species in Patagonia, of which a quarter are grasses, under heavy grazing conditions. There are