Landscape context modifies the rate and distribution of predation around habitat restoration sites

Highlights

• The position of restoration sites in landscapes modifies animal abundance.

• It is not clear how site positioning affects ecological functioning.

• Predation was significantly higher at restored oyster reefs than at nearby controls.

• Predation was highest at reefs isolated from nearby mangroves and seagrasses.

• Strategically placing restoration sites is vital in optimising restoration outcomes.

Abstract

The rate and distribution of ecological functions is modified by how species respond to the composition of landscapes. Extensive loss of habitats has led to habitat restoration becoming an important management tool, however, it is not clear where restoration sites should be located in heterogeneous landscapes to maximise outcomes for ecological functions. We used restored oyster reefs, and the guild of predators associated with them, as a model system to test whether, and how ecological functioning is modified by the spatial context of restoration sites in marine landscapes (i.e. seascapes). We measured predation rates and surveyed predators using videoed deployments of ‘squidpops’ (dried squid tethered using fishing line) at multiple restored oyster reefs and nearby control sites in Queensland, Australia. Sites were located in different spatial contexts in a seascape composed of a mosaic of habitat types. Predation rates at restored oyster reefs were double those at control sites. Seascape context was important in modifying these predation rates; consumption near reefs was significantly lower when reefs were close to seagrass and mangroves. By contrast, higher rates were observed on reefs surrounded by non-vegetated seafloor, far from seagrass and mangroves. In addition, the distance over which predation extendeds into the surrounding unvegetated areas was greater on reefs father from vegetation. Strategically placing restoration sites in heterogeneous landscapes can maximise the effects of habitat restoration for ecosystem functioning and modify the distance over which these effects extent into surrounding seascape.

Introduction

The maintenance of ecosystem condition is contingent upon the preservation of ecological functions that enable ecosystems to resist or recover from disturbance (Risser, 1995; Decker et al., 2017). The distribution of many ecological functions in landscapes correlates with the presence or abundance of functionally important species (Brose and Hillebrand, 2016). These functionally important species are under threat from human activities in many settings (Vitousek et al., 1997). For example, habitat loss and degradation has resulted in the loss of functionally important species (e.g. herbivores and predators) in marine (Waycott et al., 2009), freshwater (Quesnelle et al., 2013) and terrestrial ecosystems (Kormann et al., 2016). This can have knock-on effects for the rates and distributions of key ecological functions in both disturbed habitat patches (Valiente-Banuet et al., 2015), and in surrounding landscapes (Tylianakis et al., 2010). Rehabilitating or restoring degraded ecosystems is an increasingly important management intervention in all modified landscapes (Aerts and Honnay, 2011; Cosentino et al., 2014; Bouley et al., 2018). Whilst habitat restoration has in many settings been shown to increase the rates of key ecological functions (Frainer et al., 2018), it remains uncommon for restoration projects to explicitly target the restoration of mobile animals that perform important ecological functions (Gilby et al., 2018a).

The position of restoration sites in landscapes plays a pivotal role in shaping the assemblages of animals which colonise restored habitats, and the rates of ecological functions that animals provide (Bell et al., 1997; Jones and Davidson, 2016; Gilby et al., 2018a; Laszlo et al., 2018). Restoring habitats at sites with high connectivity to nearby ecosystems, which provide alternative habitats or source populations for animals, can enhance recruitment into restored habitats (Pullinger and Johnson, 2010; zu Ermgassen et al., 2016; Volk et al., 2018). For example, restoring corridors between forest patches increases faunal abundance by facilitating species movement and settlement (Tewksbury et al., 2002; Lees and Peres, 2008). Similarly, restoring habitat patches in locations with connections to many habitat patches of different types, might serve to enhance the abundance and diversity of animals that use multiple habitats during their lives (Micheli and Peterson, 1999; Nagelkerken et al., 2015). Whilst the principles of landscape ecology are regularly suggested as important considerations in restoration plans, they are rarely implemented when selecting possible sites for restoration activities, with only 12% of restoration sites globally having been placed strategically in landscapes to enhance possible effects on animals (Gilby et al., 2018a). Consequently, empirical data that can be used to test the functional effectiveness of restoration in different landscape contexts is limited. Most studies that have examined possible landscape effects on habitat restoration have focused on changes in animal abundance, however, the abundance of animals does not always correlate with the functions they perform (Bullock et al., 2011; Gamfeldt and Roger, 2017). Quantifying the effects of restoration in different landscape contexts and determining whether these changes in species abundance proliferate to differences in key ecological functions is, therefore, pivotal for optimising the design and placement of restoration efforts (Gilby et al., 2018a).

Humans have fundamentally transformed many coastal seascapes (i.e. marine landscapes) via the combined effects of urbanisation, poor water quality, dredging and fishing, and these changes have resulted in the loss or degradation of many marine ecosystems (Halpern et al., 2008). Consequently, the restoration of coastal ecosystems has become an important focus in marine spatial planning (Barbier et al., 2011), and enhancing the abundance of animals (especially fishes and large crustaceans) and ecosystem functioning is a primary objective for many restoration projects (Baggett et al., 2015; zu Ermgassen et al., 2016). Oyster reefs are a highly threatened but restorable ecosystem (Beck et al., 2011), consequently oyster restoration projects are now expanding rapidly in number globally (Alleway et al., 2015). Whilst oyster reefs are restored for multiple purposes (e.g. shoreline stabilisation, water quality, return of lost habitats), and provide important habitats for many coastal fish species and are often restored to augment fish abundance and diversity (Baggett et al., 2015), rarely are they restored explicitly to enhance the ecological function performed by mobile fish and crustaceans (Gilby et al., 2018c). Oyster restoration can have positive effects on fish assemblages over what were previously unstructured sediments (Harding and Mann, 1999; Peterson et al., 2003; Grabowski et al., 2005), however, the possible benefits of oyster restoration for ecological functions have rarely been tested with empirical data (Smyth et al., 2015; Gilby et al., 2018c). The landscape context of oyster reefs can modify the composition of fish assemblages, both over reefs and in surrounding temperate (Micheli and Peterson, 1999; Grabowski et al., 2005), and subtropical (Gilby et al., 2018b) seascapes, but there is no data to describe whether these effects also modify the spatial distribution of ecological functions (Gilby et al., 2018c).

Restoration projects often seek to enhance the condition of ecosystems and the diversity or abundance of animals that use these ecosystems as habitat (Jones and Davidson, 2016; Middendorp et al., 2016). Many restoration efforts also aim to promote ecological functions, but the potential functional effects of restoration are rarely measured or monitored. This study quantified the effects of oyster reef restoration on the rate and distribution of predation in an estuarine seascape. Predation is an important ecological function that helps to maintain community structure in all ecosystems (Ritchie and Johnson, 2009; Estes et al., 2010; Ripple and Beschta, 2012). Quantifying rates of predation around habitat restoration projects is important because predation is significantly, and quickly, modified by the rapid colonisation of predators to restored coastal ecosystems (Harding, 1999; Micheli and Peterson, 1999; Peterson et al., 2003) and predators are sensitive to ecosystem changes as they rely on prey availability to survive and reproduce, and so are good indicator species for this purpose (González-Tokman and Martínez-Garza, 2015; Gilby et al., 2017a). We aimed to determine: 1) the degree to which oyster reef restoration enhances the function of predation at restoration sites; 2) the distance over which predation extends into the seascape surrounding restored oyster reefs, 3) how the seascape context of restored oyster reefs modifies their effects on ecological functions, and 4) the identity of the species performing the function. We surveyed rates of predation at six restored oyster reefs, and in the seascape surrounding each reef, which differed in terms of their proximity to nearby seagrass meadows and mangrove forests. We hypothesised that oyster reef restoration would enhance predation rates both on reefs and in the surrounding seascape (relative to nearby control sites) and expected that these functional effects of restoration would depend on the spatial context of oyster reefs relative to other habitats (e.g. seagrass, mangroves) that provide high-relief and structurally complex habitats for fish.