Habitat Cascades: The Conceptual Context and Global Relevance of Facilitation Cascades via Habitat Formation and Modification

Abstract

The importance of positive interactions is increasingly acknowledged in contemporary ecology. Most research has focused on direct positive effects of one species on another. However, there is recent evidence that indirect positive effects in the form of facilitation cascades can also structure species abundances and biodiversity. Here we conceptualize a specific type of facilitation cascade-the habitat cascade. The habitat cascade is defined as indirect positive effects on focal organisms mediated by successive facilitation in the form of biogenic formation or modification of habitat. Based on a literature review, we demonstrate that habitat cascades are a general phenomenon that enhances species abundance and diversity in forests, salt marshes, seagrass meadows, and seaweed beds. Habitat cascades are characterized by a hierarchy of facilitative interactions in which a basal habitat former (typically a large primary producer, e.g., a tree) creates living space for an intermediate habitat former (e.g., an epiphyte) that in turn creates living space for the focal organisms (e.g., spiders, beetles, and mites). We then present new data on a habitat cascade common to soft-bottom estuaries in which a relatively small invertebrate provides basal habitat for larger intermediate seaweeds that, in turn, generate habitat for focal invertebrates and epiphytes. We propose that indirect positive effects on focal organisms will be strongest when the intermediate habitat former is larger and different in form and function from the basal habitat former. We also discuss how humans create, modify, and destroy habitat cascades via global habitat destruction, climatic change, over-harvesting, pollution, or transfer of invasive species. Finally, we outline future directions for research that will lead to a better understanding of habitat cascades.

A sixth‐level habitat cascade increases biodiversity in an intertidal estuary

“…A habitat cascade is defined as an indirect positive effect on inhabitants (organisms found associated with habitat‐forming species) mediated by sequential formation or modification of biogenic habitat (Thomsen et al., 2010). For example, large first‐order habitat‐forming trees provide structural support to smaller second‐order habitat‐formers, such as orchids, mistletoes, nest epiphytes, and lichens, thereby indirectly facilitating bird and invertebrate inhabitants (Angelini & Silliman, 2014; Cruz‐Angon & Greenberg, 2005; Pettersson et al., 1995; Watson & Herring, 2012).…”

“…Habitat cascades should occur where second‐order habitat‐formers are common and embedded within, entangled around, or attached to first‐order habitat‐formers, for example, along intertidal (Thomsen et al., 2016) and subtidal (Bell et al., 2014) rocky shores, in forests (Angelini & Silliman, 2014; Watson & Herring, 2012), seagrass beds (Edgar & Robertson, 1992; Gartner et al., 2013), mangroves (Bishop, Fraser, & Gribben, 2013; Bishop et al., 2012), salt marshes (Altieri et al., 2007; Angelini et al., 2015), and estuaries (Thomsen et al., 2010). …”

“…The strength of a habitat cascade should increase with the (1) Amount (abundance or size) of the second‐order habitat‐former, (2) Difference in form and function between the second and first‐order habitat‐formers, and (3) Affinity of the inhabitants for the second‐order habitat‐former, ranging from specialist (obligate) to generalist (facultative) affinities (Thomsen et al., 2010). These three factors (here referred to as the “ADA” model) are likely to operate simultaneously and should therefore be studied in concert.…”

Large-scale facilitation of a sessile community by an invasive habitat-forming snail

“…However, the traditional perception of nature, where negative interactions are most important, still dominates invasion ecology (Rodriguez 2006;Bulleri 2009). A key mechanism whereby marine invaders can have positive impacts on local species is via habitat formation (Crooks 2002;Bulleri 2009;Thomsen et al 2010a), a process that is particularly important for shell-forming species (epibiosis, reviewed in Gutierrez et al 2003;Wahl 2009). Many studies have documented facilitation from invasive marine shell-formers, typically focusing on specific positively affected species (e.g.…”

“…However, we are not aware of studies that have quantified community facilitation across multiple habitats and throughout an invaded system. Such large-scale community facilitation is most likely to occur when an invader provides a novel function or greatly increases the availability of a limiting resource and is abundant in the major habitats within the invaded system (Crooks 2002;Thomsen et al 2010a).…”