Conditional outcome of ecosystem engineering: A case study on tussocks of the salt marsh pioneer Spartina anglica Thorsten Balke a, b, ⁎, Paul C. Klaassen a , Angus Garbutt c , Daphne van der Wal a , Peter M.J. Herman a , Tjeerd J. Bouma a, b a Royal Netherlands Institute for Sea Research (NIOZ-Yerseke; former NIOO-KNAW), PO Box 140, 4400 AC Yerseke, The Netherlands b Deltares, PO Box 177, 2600MH Delft, The Netherlands c NERC Centre for Ecology and Hydrology (CEH), Environment Centre Wales, Deiniol Road, Bangor, LL57 2UP, Wales, UK abstract article info Article history: Received 5 December 2011 Received in revised form 27 February 2012 Accepted 1 March 2012 Available online 8 March 2012 Keywords: Cordgrass Tidal flat Context dependency Biogeomorphology Salt marsh Pioneer zone The salt marsh grass Spartina anglica is an important habitat-modifying ecosystem engineering agent that fa- cilitates large-scale salt marsh formation by enhancing sediment accretion. It dominates many European tidal environments and is invasive in many other parts of the world. We question (1) to what extent the ecosys- tem engineering ability of patchy Spartina vegetation depends on large-scale abiotic processes, and (2) whether tussock shape provides an indicator for future lateral salt marsh development. Analysing the topog- raphy of 83 individual tussocks in contrasting environments revealed that there are 6 clearly distinguishable tussock shapes, and that the classical example of a sediment-accumulating dome-shaped tussock only occurs under a limited set of abiotic conditions. The outcome of habitat modification by S. anglica is shown to be conditional, depending on large-scale morphodynamics and sediment grain size. Resulting tussock shape provides a clear indication for the long-term development of the pioneer zone. Understanding of the condi- tional outcome of ecosystem engineering is highly relevant in this era of climate change and ongoing anthro- pogenic influences on coastal ecosystems. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Since the introduction by Jones et al. (1994), the concept of eco- system engineering has become an important research area which has yielded important insights in to the structure and functioning of ecosystems (Wright et al., 2006). Whereas allogenic ecosystem engi- neers are biotic agents that actively modify their environment (e.g., dam building beavers), autogenic ecosystem engineers change their environment via the interaction between their physical structures and the abiotic environment (Jones et al., 1994) as present in many bio-geomorphic systems (e.g. salt marshes, dunes, rivers). Clearly, habitat modification by autogenic ecosystem engineering depends both on organism structure and on prevailing abiotic conditions also described as context dependency (Jones et al., 2010). Some recent studies elucidate the role of physical characteristics of the structures for habitat modification (e.g., Bouma et al., 2005; Peralta et al., 2008). However, to our knowledge relatively little work has been done on how the effect of autogenic ecosystem engineering varies with abiotic forcing (Jones et al., 1997; Wright et al., 2006; Hastings et al., 2007) although it is mentioned e.g. in the context of ‘ecohydrol- ogy’ for coastal wetlands (Wolanski et al., 2004). Norkko et al. (2006) do, however, clearly show that ecosystem engineering effects may differ between sites with contrasting abiotic conditions. We pose the hypothesis that the outcome of autogenic ecosystem engineering in biogeomorphic systems is dependent on large-scale abiotic pro- cesses, with positive and negative effects on habitat modification. The tidal salt marsh grass Spartina anglica is a well-described ex- ample of an autogenic ecosystem engineering agent, which can occur under a broad range of abiotic conditions (Castellanos et al., 1994; Widdows et al., 2008). It is a dominant species in the dynamic pioneer zone of European salt marshes and can alter the salt marsh habitat by trapping large amounts of sediment. Spartina was widely planted for land claim and coastal protection in the 20th century and is currently regarded as a threatening invasive species in many places (Nehring and Hesse, 2008). The ecosystem engineering effect of Spartina is most obvious at the seaward edge of the salt marsh, where Spartina is often described as forming dome- or hummock- shaped tussocks due to enhanced sediment accretion (Castellanos et al., 1994; Sanchez et al., 2001; Ward et al., 2003) similar to Puccinellia hummocks (Langlois et al., 2003). We aim to elucidate the impor- tance of abiotic processes as modulator of the outcomes of autogenic ecosystem engineering, by analysing the topography of Spartina tus- socks in contrasting environments. We hypothesize that conditional outcome of ecosystem engineer- ing by Spartina is reflected in tussock shape and size, so that tussock shape may be a useful indicator of the future pioneer zone develop- ment in a specific area. This hypothesis was studied in a number of Geomorphology 153–154 (2012) 232–238 ⁎ Corresponding author at: Deltares, P.O. Box 177, 2600 MH Delft, The Netherlands. E-mail address: thorsten.balke@deltares.nl (T. Balke). 0169-555X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2012.03.002 Contents lists available at SciVerse ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph