Fine-scale responses of mobile invertebrates and mesopredatory fish to habitat configuration

Highlights

• Patches of macroalgal mimics were deployed to understand responses of biodiversity to habitat configuration.

• After one month, invertebrate and fish communities were quantified.

• Patch edges had the highest epifaunal abundance, where fish were least commonly observed.

Abstract

As habitat-forming species continue to decline globally, it is important to understand how associated communities respond to habitat loss and fragmentation. Changes in the density and spatial configuration of habitat have important consequences for associated communities. However, tests of these factors are often confounded by morphological variation of habitat-formers, which can be resolved by using standardised habitat-mimics. Furthermore, few studies have incorporated the role of predators in mediating the observed effects. To test whether predators mediate the abundance of invertebrates among algal habitats of varying configuration (isolated vs patches, and positions within patches), we placed macroalgal mimics into subtidal estuarine habitats for one month to sample epifaunal communities. At the same time, we conducted underwater video surveys of fish communities to quantify fish communities and their feeding behaviour among the artificial habitats. Isolated habitats did not differ from patch habitats, however, patch edges had the highest epifaunal abundance, where fish were least commonly observed. Observed fish feeding was highest in the middle of patches and increased fish observations and feeding in habitats with reduced epifaunal communities suggest that mesopredatory fish are mediating epifauna in patches, with predation pressure altered by the spatial configuration of the habitat. This contrasts to previous studies that focus on predators that congregate outside patches and suggest that fragmentation leads to reduced invertebrate abundance at habitat edges in contrast to centres. However, this study highlights that in habitat patches housing small mesopredators that also benefit from the increased structure, the centre of the patch experiences higher predation and therefore fewer epifauna in contrast to patch edges and individual algal mimics.

Introduction

Foundation species (sensu Dayton 1972) such as oysters, seagrass, and algae create complex habitats that support high biodiversity in coastal marine systems (McLeod et al., 2019; Gribben et al., 2019; Lloyd et al., 2020). However, anthropogenic stressors (e.g., pollution, urbanisation and boating activities) have frequently resulted in large-scale loss and fragmentation of these critical habitats (Wilcove et al., 1986; Halpern et al., 2008; Wahl et al., 2015; Wernberg et al., 2016). Fragmentation of habitat-forming species alters the spatial configuration of remaining patches, resulting in more habitat edges and more isolated habitats (Fischer and Lindenmayer, 2007). These fragmented patches can be more exposed to biotic (e.g., predation and herbivory) and abiotic factors (e.g., wind or waves) that affect associated communities. For example, this fragmentation can result in a decrease in the amount of suitable habitat, reducing the density of individuals within the habitat (i.e., the number of individual habitat-formers; Fahrig 2013). Thus, understanding how communities and species interactions respond to increasing habitat fragmentation and the subsequent changes to habitat configuration is important for their conservation and management.

Variation in faunal responses to altered habitat configuration may result from several processes occurring within and between patches. For example, within seagrass meadows, invertebrate abundance can increase with increasing distance to the edge of the patch and the strength of this effect can differ depending on the canopy density (Arie Vonk et al., 2010). In contrast, in artificial seagrass patches, the number of invertebrates decreased from patch edges to patch centres (Macreadie et al., 2010a). Variation in the habitat-formers themselves due to fragmentation may therefore alter how associated communities are affected. The connectivity among patches for associated fauna can also determine the composition of communities. For example, inter-patch distances can affect the ability of species to move between them, often resulting in small, isolated patches housing altered epifaunal communities (Roberts and Poore 2006). As fragmentation transforms larger patches into multiple smaller, isolated patches, altered habitat configuration may lead to fewer individuals inhabiting patches, with further reductions in isolated patches. However, the extent to which the distance between patches affects connectivity can be determined by the quality or suitability of neighbouring habitats (Ricketts 2001). For example, invertebrates associated with the brown alga Sargassum were more similar in patches bordering conspecifics than those neighbouring patches of the green alga Caulerpa filiformis (Lanham et al., 2015).

The distribution of predators and their prey may be associated with habitat characteristics that are altered by habitat fragmentation. Habitat density, morphology and connectivity all influence the invertebrate prey community (Horinouchi 2007; Chen et al., 2020), in turn altering the distribution of predatory fish across the seascape. The response of fish to habitat configuration can occur over fine scales, with smaller fish in larger abundances in dense patch centres and more larger fish at less dense patch edges (Yeager and Hovel 2017). In fragmented seagrass habitats, the composition of the fish community differs between patch edges and centres and predatory fish are generally more common at edges, while smaller species are more common in sheltered patch centres (Laurel et al., 2003; Smith et al., 2011; Ferrari et al., 2018). Isolated patches can act as a predation refuge for epifauna if habitats are disconnected from small, mesopredatory fish (Lanham et al., 2020). Increased habitat density can reduce the foraging success of predators reducing fish abundance in dense patch centres (Grabowski 2004). Increased structural morphology of habitat-forming species can also reduce the foraging success of fish species by providing more surface area for invertebrate prey to seek refuge (Bartholomew et al., 2000). However, an increase in habitat density can increase the time spent foraging, and foraging success of mesopredatory fish due to reduced escape attempts by prey (Wellenreuther and Connell 2002; Hovel et al., 2016). We can therefore expect to see shifts in the abundance and feeding behaviour of fish depending on access to invertebrate prey, which is likely to be higher at habitat edges due to lower density of individual habitat-formers.

In this study, we test whether small, mesopredatory fish mediate variation in epifaunal abundance among algal habitat patches that differ in spatial context. Specifically, this study aimed to test 1) whether epifaunal communities differ among algal mimics in isolation and three regions of patch habitat (patch centre, middle, and edges), and 2) whether fish abundance, residency and bites differed among the same habitats. To control for variation in algal morphology, we used artificial algal units (Jelbart et al., 2006; Arponen and Boström 2012) that mimic the natural macroalgae present at the study location, Sirophysalis trinodis (as per data collected by Lanham et al., 2020).