Land use and disturbance effects on the dynamics of natural ecosystems of the Monte Desert: Implications for their management

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Abstract

The complex interactions between human activity and natural processes determine non-linear dynamics in ecosystems that can difficult their management. Human settlements in arid lands contribute to the modification of disturbance regimes, including the introduction of new disturbances and the elimination of others. In consequence, they can alter the functional mechanisms that allow systems to overcome limiting factors, leading to desertification. In this revision, we evaluated the effects of the changes on disturbance regimes produced by the different forms of land transformation on the structure and function of ecosystems of the Monte Biogeographical Province, in Argentinean arid west. Two approaches were used: the analysis of land use history and the analysis of the effects of the main disturbances on the dynamics of different communities. We concluded that throughout the history of the Monte Desert, the joint action of natural and anthropic agents has resulted in complex dynamics that lead most area of the Monte to a moderate to severe status of desertification. The modification of the disturbance regime had strong consequences for several aspects of the dynamics of communities, such as species composition and diversity, water dynamics, soil conditions, trophic structure and productivity of Monte Desert ecosystems. However, disturbance regimes could be managed to promote favorable transitions in ecosystems and, therefore, could be a tool for optimizing productivity of agro-ecosystems, and recovering and conserving natural ecosystems.

Introduction

The complex interactions between human activity and natural processes determine non-linear dynamics in ecosystems, with thresholds, feedbacks, time lags and legacy effects. When this complexity is not understood, surprising results can difficult the management or conservation of ecosystems (Liu et al., 2007). In arid and semiarid ecosystems, changes of disturbance regimes, including the introduction of new disturbances and the elimination of others, can alter the functional mechanisms (e.g. positive feedbacks between species) that allow systems to overcome limiting factors. In such cases, land degradation processes are difficult to revert (Solé, 2007). This loss of resilience, called desertification, reduces the potential land productivity, and consequently the life quality of local people (Vogel and Smith, 2002). Traditionally, the re-establishment of historical abiotic conditions and the protection of ecosystems against disturbances has been suggested as a necessary management tool to halt land degradation and to promote the natural recovery of degraded areas (succesional-based management) (Suding et al., 2004). However, several studies indicate that the effects of excluding disturbances are not easy to predict. Therefore, greater knowledge of interacting factors conducing succession and threshold occurrence are needed to generate effective models applicable for management and restoration of natural systems (Bestelmeyer, 2006; Briske et al., 2005).

Different frameworks have been proposed to assess the management of these complex systems based on state and transitions, thresholds and rangeland health models (Briske et al., 2005; Herrick et al., 2006; Westoby et al., 1989). These models understand the dynamics of ecosystems as a set of possible stable states and transitions among states, and interpret the ecosystem changes as a consequence of the interactions among several factors, including disturbance regime, weather variability, soil conditions, and management. From a decision-making point of view, the knowledge of the possible states and transitions allows the recognition of opportunities to achieve favorable, or avoid hazardous, transitions (Bestelmeyer et al., 2003; Briske et al., 2005).

The Monte Biogeographical Province (so-called ‘Monte’ or ‘Monte Desert’, Fig. 1) occupies approximately 460 000 km2 of the Argentinean arid west (Cabrera, 1976; Rundel et al., 2007). Climatically, it is an arid to semiarid region, with mean annual precipitation ranging from 30 to 350 mm, and temperatures ranging from mean maximum of 25.2 °C and mean minimum of 10.2 °C in its northern part, to 20.4 and 7.3 °C in the southern portion. Five natural areas have been identified for the Monte according to the endemic assemblages (Roig-Juñent et al., 2001). Most of the ecological studies have been performed in the Northern (25–30°S), Central (30–37°S) and Southern Monte (37–43°S), the three most extensive natural areas.

Several areas of the Monte present a moderate to severe degree of desertification, and human activities and their associated disturbances have been suggested as the main causes of degradation processes (Roig et al., 1991; del Valle et al., 1998). The main disturbances in this region are fires, occurring from immemorial times to present, and domestic grazing and tree cutting, which started about two centuries ago as a consequence of colonization and economic development (Abraham and Prieto, 1981; Defossé et al., 2003; Rostagno et al., 2006).

We proposed that the current spatial patterns of Monte communities, their present land use and conservation status is the result of the complex interactions between natural conditions and different land use histories. In order to understand these complex interactions, we evaluated the effects of the changes on disturbance regimes produced by the different forms of land transformation on the present structure and function of the Monte ecosystems. We performed this analysis using two main approaches: (a) The analysis of land use history and its relation with the temporal and spatial pattern of main disturbances of the Monte; and (b) The analysis of the effects of the main disturbances on the dynamics of the different communities, focusing on the structural and functional changes conducting successional processes and transitions among different community states. We expected that this analysis will allow us to identify typical pathways of dryland change, to determine indicators of rangeland conditions, risk situations and management opportunities, and to detect knowledge gaps for developing dynamic models of the Monte ecosystems.

Section snippets

Brief history of main changes in land use

Historical studies pointed out the different human settlement periods in the area and allowed us to determine the principal disturbances occurring in the Monte and to understand the main changes in their regimes (Fig. 2). In the Central Monte, there are evidences of land occupation by groups of hunters–gatherers from at least the last 10,000 years (Lagiglia, 1994). When the Spaniards arrived (ca. 1530), the region was settled following a riparian occupation pattern, with a relatively low impact

Fire effects on ecosystem dynamics

During 2004 and 2005, 1338 and 1521 fire events were detected, respectively in the Monte (Fig. 3). The highest density of fire events occurred in areas with annual precipitation between 200 and 300 mm and the lowest density in area with annual rainfall between 100 and 200 mm (Table 2). Among vegetal communities defined by Eva et al. (2004) closed shrubland steppes (Larrea shrublands) appeared as the physiognomic group most affected by fire events (nearly 60% of the fire events occurred in this

Implications for the sustainable management of natural areas

The severe ecological situation showed by the desertification maps (Roig et al., 1991; del Valle et al., 1998) calls for the development of a general strategy to overcome the desertification process. This strategy should consider the biological potential of ecosystems (e.g. forage or wood productivity), the response of ecosystem processes to disturbance regimes, and the importance of landscape diversity and environmental heterogeneity in the managing of natural resources.

Adoption of state and

Acknowledgments

This review was supported by the National Agency for Scientific and Technological Promotion of Argentina (PICT 01-11050 and PICT 13-15034), CONICET (PIP 5953; PIP 6413 and PIA 6387/97), and GOFC-GOLD, Maryland University, RedLaTif (AQL2004).

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