Self-organization and complex dynamics of regenerating vegetation in an arid ecosystem: 82 years of recovery after grazing V. Lawley a, * , L. Parrott b , M. Lewis a , R. Sinclair a , B. Ostendorf a a Landscape Science, School of Earth and Environmental Sciences, The University of Adelaide, Adelaide, SA 5005, Australia b Complex Systems Laboratory, Département de géographie, Université de Montréal, C.P. 6128 succursale “centre-ville”, Montréal, Québec, Canada H3C 3J7. article info Article history: Received 17 November 2011 Received in revised form 5 August 2012 Accepted 21 August 2012 Available online 16 October 2012 Keywords: Community assembly Ecological restoration Long-term ecological monitoring Self-organization Semi-arid ecosystems abstract Understanding the relative contributions of internal dynamics versus external factors in the process of community assembly is important for establishing guidelines for conservation and restoration of native vegetation. The role of internal dynamics and external factors in the process of community assembly at the local scale is a poorly understood issue in ecology, especially in highly variable environments. We analyse an 82-year spatiotemporal record of vegetation recovering from a history of overgrazing within a semi-arid environment to investigate the relative contribution of internal and external factors on community assembly. Community composition and spatial structure were used as indicators of change over time in four sites within a vegetation reserve, which were subjected to the same environmental constraints, climate and grazing history. The four sites follow remarkably different, asynchronous trajectories characterized by periods of stability interrupted by episodic change. The high variability between sites suggests that initial communities are internally reinforced through random chance events, directing them on different pathways of assembly and self-organization; hence external factors may play a less significant role in long-term community assembly at the local scale than previously believed. These results have important implications for rangeland restoration and conservation in many of the world’s semi-arid regions. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Natural communities are temporally and spatially variable. At continental scales evolutionary history and biophysical conditions influence species assemblages (Virtanen et al., 2006), whereas at the local scale community assembly is influenced by external variables and internal dynamics (Irvine et al., 2009). External variables include precipitation, topography and other elements of habitat, whilst internal variables include population dynamics and species interactions, such as dispersal, competition and other positive and negative feedback mechanisms. These processes operate and interact at multiple levels over time producing emer- gent properties and patterns within ecological systems (Levin, 1998). The high complexity of these interrelationships confounds the interpretation of how ecosystems behave and respond to change. Understanding the relative influences of internal versus external factors is therefore essential for management, developing complex models, and for predicting the effects of climate change on natural systems (Brown and Lawson, 2010). Within semi-arid environments, low erratic rainfall and soil nutrients are limiting factors and therefore are often found to be key drivers of the system (Ward et al., 2000). Rainfall drives vegetation recruitment predominantly through episodic events (Wiegand et al., 1995), but also through slow continuous processes of recruitment and mortality (Watson et al., 1997). Nutrients are also limiting, particularly in Australia (Orians and Milewski, 2007), and redistribution through runoff and wind erosion create a patchy distribution of this resource (Schlesinger et al., 1999). The concen- tration of both water and nutrients provides suitable sites for vegetation establishment, and this is believed to cause the heterogeneous pattern characteristic of semi-arid vegetation (Ludwig and Tongway, 1995). Internally within semi-arid communities, species have evolved traits and feedback mechanisms to cope with a variable environ- ment and these arguably act as strong driving forces. One of the most well known in arid environments is the positive feedback between plant density and water infiltration (HilleRisLambers et al., 2001). Another process is the “nurse plant syndrome” (Niering et al., 1963), where seedlings are more likely to establish near adult plants because of the protection from abiotic factors and herbivory, particularly in resource limited environments (Francisco * Corresponding author. Tel.: þ61 4 03885240. E-mail address: valerie.lawley@gmail.com (V. Lawley). Contents lists available at SciVerse ScienceDirect Journal of Arid Environments journal homepage: www.elsevier.com/locate/jaridenv 0140-1963/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jaridenv.2012.08.014 Journal of Arid Environments 88 (2013) 156e164