Reports Ecology, 96(10), 2015, pp. 2575–2582 Ó 2015 by the Ecological Society of America Positive interactions expand habitat use and the realized niches of sympatric species SINEAD M. CROTTY1 AND MARK D. BERTNESS Department of Ecology and Evolutionary Biology, Brown University, Providence, Rhode Island 02912 USA Abstract. Niche theory, the oldest, most established community assembly model, predicts that in sympatry, the realized niche will contract due to negative interspecific interactions, but fails to recognize the effects of positive interactions on community assembly. The stress gradient hypothesis predicts that positive interactions expand realized niches in stressful habitats. We tested the predictions of the stress gradient hypothesis in a cobble beach model system across both physical and biological stress gradients. We transplanted seven common littoral species within, adjacent to, and below Spartina alterniflora cordgrass stands in control, cage control, predator exclusion cage, shade, and shaded predator exclusion cage treatments to test the hypothesis that cordgrass expands intertidal organism habitats. On cobble beaches, cordgrass ameliorates physical and predation stresses, expanding the distribution and realized niches of species to habitats in which they cannot live without facilitation, suggesting that niche theory and species distribution models should be amended to accommodate the role of positive interactions in community assembly. Key words: cobble beach; facilitation; foundation species; Narragansett Bay, Rhode Island, USA; niche theory; Spartina alterniflora; species distribution model; stress gradient hypothesis. INTRODUCTION Niche theory dictates that species distributions are largely shaped by interspecific competition as well as by physical and resource constraints (Grinnell 1917, Elton 1927). Textbook niche theory operationally defines the fundamental niche as the abiotic conditions and habitats that a species can occupy in allopatry, and the realized niche as the conditions and habitats that a species occupies in sympatry. In theory, the niche is defined abstractly by habitat and resource variables as an ndimensional hypervolume (Hutchinson 1957). However, in practice, the niche is commonly operationally defined and typically measured correlatively and/or experimentally as habitat overlap with and without neighbors (e.g., MacArthur 1958, Connell 1961), the original Grinnellian concept of the niche as a species address (Trainor and Schmitz 2014). Recent treatments of niche theory have attempted to reduce its dependence on competition, but still consider the realized niche to be by definition smaller than the fundamental niche (Rodriguez-Cabal et al. 2012). Manuscript received 6 February 2015; revised 13 May 2015; accepted 19 May 2015. Corresponding Editor: B. J. Grewell. 1 E-mail: sinead_crotty@brown.edu Biogeographers and species distribution modelers have more aggressively contested the veracity of niche theory, questioning its value (Vannette and Fukami 2014), broadening its definition and application (Kearney 2006, Soberón and Nakamura 2009, Kylafis and Loreau 2011), and proposing to abandon it altogether (Chase and Leibold 2003). Positive interactions are particularly problematic for niche theory (Bruno and Bertness 2001). Foundation species, for example, provide group benefits, ameliorate physical and biotic stresses, expand species distributions, and increase the size of fundamental and realized niches (Bruno et al. 2003). Positive interspecific interactions exist on a continuum where the net interspecific interaction may be positive or negative depending on environmental conditions (Stachowicz 2001). The contextual nature of these interactions makes their incorporation into advanced conceptual definitions or species distribution models (SDMs) especially challenging. Some studies have acknowledged the role of facilitators in community assembly and have incorporated them into definitions of the fundamental niche (Sax et al. 2013). In the context of SDMs, this would require that any habitat suitable for a facilitator be incorporated into the fundamental niche of its co-occurring 2575 Reports 2576 SINEAD M. CROTTY AND MARK D. BERTNESS species, but would ignore the areas where the net interspecific interaction is negative. SDMs generated using this definition would overestimate the fundamental niche of the focal species. Critics argue that distinctions between certain types of sympatric interactions are artificial and detract from the usefulness of niche models (Araújo and Guisan 2006). Alternatively, maintaining the historical definition of the realized niche as the culmination of interspecific interactions (Soberón and Nakamura 2009), along with careful incorporation of positive interactions, may improve these models without sacrificing simplicity. In contrast to niche theory, the stress gradient hypothesis (SGH; Bertness and Callaway 1994) predicts that positive intra- and interspecific species interactions are common in physically and biologically stressful habitats, driven by habitat amelioration and associational defenses. The SGH has been widely tested by plant ecologists, typically with single species along physical stress gradients (He et al. 2013). Consumer pressure gradients and associational defenses have received little attention, as has the generality of the SGH with animals, even though it was originally proposed based on sessile animal interactions (Bertness 1989). The SGH predicts that at physical and biological extremes the distribution and abundance of a species will expand in sympatry due to positive interactions (Bertness and Callaway 1994). Incorporating facilitations into niche theory leads to the paradoxical prediction that a species’ realized niche will be larger than its fundamental niche in high-stress environments (Bruno et al. 2003). Thus, incorporating the SGH into niche theory leads to novel, testable predictions. We conducted experimental studies on New England cobble beaches to test the prediction that positive interactions in stressful habitats expand realized niches. We hypothesize that Spartina alterniflora cordgrass, the foundation species (Dayton 1975) that builds and maintains New England salt marshes, increases the realized niches of marine fauna and flora by ameliorating limiting physical and biological stresses. We operationally define realized niche space as habitat occupied in sympatry, as is common in empirical niche studies of extant and extinct assemblages (e.g., Benson et al. 2014, Price et al. 2014). The realized niche is therefore either a portion of the fundamental niche and reflects competitive and/or consumer exclusion, is larger than the fundamental niche and reflects habitat amelioration, or is potentially equal to the fundamental niche and reflects associational defenses (see Bruno et al. 2003, Rodriguez-Cabal et al. 2012, Stachowicz 2012). We expand the generality of the SGH to an entire intertidal assemblage and simultaneously consider niche relationships between plants and animals along both physical and biological stress gradients to test the relationship between SGH and niche theory. MATERIALS Ecology, Vol. 96, No. 10 AND METHODS Species abundance To examine the relationship between the abundance and distribution of the numerically dominant fauna and flora of cobble beaches and cordgrass, we quantified species abundance in cordgrass stands (high vegetated zone), cobble habitats at the same elevation as cordgrass stands (high bare zone), and cobble habitats at elevations 1 m horizontally below where cordgrass occurs (low bare zone). We quantified cover of organisms at 2-m intervals in the high vegetated, high bare, and low bare zones (N ¼ 10 sites) along random 50-m transects. Points were sampled with a 50 3 50 cm quadrat subdivided into 100 5 3 5 cm points (N ¼ 8 replicates/site). Transect data (pooled organism percent cover/quadrat) were transformed as necessary to meet the assumptions of parametric statistics and analyzed among zones with ANOVA; results of this and all other analyses were considered significant at P , 0.05. Transplant experiment To elucidate mechanisms driving species distributions on cobble beaches, we ran a factorial experiment manipulating physical stress with shades and large predators with cages in the high vegetated, high bare, and low bare zones at Providence Point in the Narragansett Bay National Estuarine Research Reserve, Rhode Island, USA. We installed 0.5 3 0.5 m plots at .3-m intervals with five treatments: controls, predator exclusion cages (0.5 3 0.5 3 0.5 m of 1 3 1 cm plasticcoated wire mesh), cage controls (exclusion cages lacking two sides), shades (cage controls covered with two layers of 0.5 3 0.5 cm plastic mesh), and shaded cages (exclusion cages covered with two layers of 0.5 3 0.5 cm plastic mesh). Treatments were replicated eight times per zone. Surface photosynthetically active radiation (PAR) was reduced by 76%, 75%, 14%, and 11% in shaded cage, shade, cage, and cage control treatments, respectively. Cages eliminated access by pelagic predators, but not benthic predators such as green crabs (Carcinus maenus) or oyster drills (Urosalpinx cinerea) that crawled under edges because of irregular cobble surface. We did not manipulate the impact of cobble movement disturbance, as our experiment was performed in the summer when cobble disturbance is minimal (Bruno 2000). Instead, we experimentally examined summer (May–August) distributions relative to thermal and consumer stress gradients. To quantify physical stresses, evaporative water loss, rock temperature, and water flow were measured. Evaporative water loss, a proxy for desiccation, was measured with 5 3 5 3 1.5 cm sponges in each replicate placed on 1-cm mesh hardware cloth platforms. Sponges were saturated with fresh water, weighed, and deployed as the midday tide was receding. Sponges were reweighed every 30 min and total water loss was October 2015 FACILITATION EXPANDS REALIZED NICHES chi-square tests with Bonferroni-corrected P values for pairwise comparisons. RESULTS Species abundance Cobble beach organisms had 19 times higher percent cover in high vegetated zones (100%) than high bare zones (5%), and more than 2.5 times higher percent cover than in low bare zones (39%). Cordgrass stands on cobble beaches had 13 associated species, while low bare and high bare habitats had seven and three associated species, respectively. Physical stress Physical parameters differed markedly between vegetated and bare zones, generally revealing that cordgrass ameliorated physical stresses on cobble beaches (Fig. 1). During low tide exposure, the low bare zone (1-h exposure) and the high vegetated zone (4-h exposure) experienced less total desiccation stress (evaporative water loss) than the high bare zone (4-h exposure; Tukey HSD, P , 0.01). At similar elevations, cordgrass cut evaporative water loss threefold. There were no significant (P , 0.05) treatment effects in the low bare or high vegetated zones. In the high bare zone, controls experienced higher desiccation stress than shaded treatments (Tukey HSD, P , 0.01), with other treatments experiencing intermediate levels (Fig. 1a). Mean maximum daily temperature was lower in the high vegetated and low bare than in the high bare zone (Tukey HSD, P , 0.01). There were no treatment effects in the high vegetated zone. In the low bare zone, unshaded treatments had higher temperatures than both shaded treatments (Tukey HSD, P , 0.01). In the high bare zone, controls had higher temperatures than cages and cage controls, which had higher temperatures than shaded treatments (Tukey HSD, P , 0.01; Fig. 1b). Chalk dissolution, a time-integrated measure of flow, was highest in low bare, intermediate in high bare, and lowest in the high vegetated zone (Tukey HSD, P , 0.01). There were no treatment effects in any zone (Fig. 1c). Transplant experiment Common cobble beach organisms transplanted to experimental plots performed better in cordgrass stands than on cobble without cordgrass (see Plate 1). In general, when transplanted to cobble at the same elevation without cordgrass (high bare zone), heat and desiccation mortality was common, but when transplanted to elevations below cobble beach cordgrass (low bare zone), mortality due to predation was more common. More Enteromorpha survived in the high vegetated (70% alive) than the low bare (0%; v2 ¼ 43.08, P , 0.001) or high bare zones (5%; v2 ¼ 36.05, P , 0.001; Fig. 2a). Fucus survival was also highest in the high vegetated (70%) as compared with the low bare (20%; v2 Reports calculated for each plot. Surface air temperature was measured every 5 min for 8 weeks with data loggers attached to cobbles with epoxy (N ¼ 2 loggers/ treatment/zone). Flow was measured by erosion of magnesium calcite chalk blocks glued to galvanized hardware cloth pinned to the surface in each plot. Physical parameter data were transformed as necessary and analyzed with a zone 3 treatment ANOVA. To test the hypothesis that cordgrass facilitates intertidal fauna and flora, we transplanted seven common littoral organisms into all replicates and scored their mortality and/or survivorship from May to August 2014. We transplanted two algal species, Enteromorpha linza and Fucus vesiculosus, the barnacle Semibalanus balanoides, two mussel species, Mytilus edulis and Geukensia demissa, the herbivorous snail Littorina littorea, and the Asian shore crab Hemigrapsus sanguineus (all species will be referred to by generic name hereafter). Cobbles of standardized size with high Enteromorpha cover were collected from a common source and transplanted to each plot. Cobbles were photographed initially and after 48 h. Fucus was attached to large cobbles with epoxy and transplanted to all plots. Fucus was photographed initially and then weekly for 7 weeks. Given the complete loss of both species in the low and high bare zones, algal mortality was reported as a binary variable and analyzed with chi-square tests with Bonferroni-corrected P values for pairwise comparisons. Semibalanus was transplanted at standardized densities into all replicates on cobbles of consistent size. Each cobble was photographed initially and then weekly for 8 weeks to follow percentage survival. Endpoint survivorship was arcsine square root transformed to meet parametric assumptions and analyzed with a zone 3 treatment ANOVA. Mussels were transplanted in predator-permeable aquaculture mesh bags of five individuals each (Mytilus, 20–40 mm; Geukensia, 40–70 mm) pinned to the surface in each plot. After 4 weeks, they were scored for mortality and cause of death. Mortality was observed as shells drilled by Urosalpinx or as intact empty shells as a result of heat and desiccation stress. Mussel mortality among zones and experimental treatments was analyzed using chi-square tests with Bonferroni-corrected P values for pairwise comparisons. Littorina and Hemigrapsus were transplanted on 5–7 cm tethers attached to substrate (N ¼ 4/plot). Juvenile Littorina ,15 mm long were tethered in each plot with access to shelter and left in the field for 4 weeks, after which they were scored for mortality and cause of death (predation by crabs or fish or by desiccation). Hemigrapsus were similarly tethered and scored for mortality after 5 d when more than 50% mortality had occurred. Predation mortality (carapace fragments or clipped tethers) and desiccation mortality (intact crab carapaces on tethers) were easily distinguished. Species mortality counts among zones and treatments were analyzed using 2577 Reports 2578 SINEAD M. CROTTY AND MARK D. BERTNESS Ecology, Vol. 96, No. 10 FIG. 1. Physical parameters in experimental plots; plots were control (C), control cage (CgC), predator exclusion cage (Cg), shade (Sd), and shaded predator exclusion cage (Sd Cg). (a) Evaporative water loss (exposure duration reflects low tidal exposure), (b) daily maximum surface rock temperature, and (c) water flow per day measured by the dissolution of chalk blocks (percentage basis); all means are shown þ SE. Statistically significant differences (P , 0.05) were determined by post hoc testing and are indicated by different lowercase letters above bars in zones where differences were found. ¼ 20.20, P , 0.001) and high bare zones (5%; v2 ¼ 36.05, P , 0.001; Fig. 2b). There were no treatment effects in any zone for either algal species. Semibalanus survival was highest in the high vegetated compared to the high and low bare zones (Tukey HSD, P , 0.01; Fig. 2c). In the high vegetated zone, controls had higher mortality than both shaded treatments (Tukey HSD, P , 0.01), with no other treatment effects. Low bare zone unshaded treatments had higher mortality than shaded treatments (Tukey HSD, P , 0.01). In the high bare zone, controls had the highest mortality, cages and cage controls had intermediate mortality, and shaded treatments suffered the lowest mortality (Tukey HSD, P , 0.01). The high vegetated zone had less Mytilus predation mortality (0%) than the low bare zone (49%; v2 ¼ 118.24, P , 0.001) and less desiccation mortality (23%) than the high bare zone (52%; v2 ¼ 29.75, P , 0.001; Fig. 2d). Over 65% of Mytilus mortality in the low bare zone was caused by Urosalpinx, as evidenced by drilled shells. There were no treatment effects in the high vegetated or the low bare zones, as Urosalpinx was not excluded by cages in the low bare zone. High bare zone controls had higher desiccation mortality than shaded treatments (shades: v2 ¼ 16.31; shaded cages: v2 ¼ 16.98, P , 0.001) but did not differ from cages and cage controls. Geukensia suffered less predation mortality (0%) in the high vegetated than in the low bare zone (13%; v2 ¼ 38.90, P , 0.001) and less desiccation mortality (5%) than the high bare zone (13%; v2 ¼ 7.25, P ¼ 0.007; Fig. 2e). Over 96% of Geukensia mortality in the low bare zone was caused by Urosalpinx predation. There were no significant treatment effects within the high vegetated or low bare zones. In the high bare zone, desiccation mortality was significantly higher in controls than in shaded cages (v2 ¼ 11.5467, P ¼ 0.001), with no other treatment effects. The high vegetated zone had less Littorina predation mortality (1%) than the low bare zone (12%; v2 ¼ 15.07, P , 0.001) and less desiccation mortality (3%) than the high bare zone (19%; v2 ¼ 20.04, P , 0.001; Fig. 2f ). There were no treatment effects within the high vegetated or low bare zones. High bare zone controls had higher desiccation mortality than cage October 2015 FACILITATION EXPANDS REALIZED NICHES 2579 Reports FIG. 2. Control results across high vegetated (HV), high bare (HB), and low bare (LB) zones for mortality (broken down by cause) of (a) Enteromorpha linza, (b) Fucus vesiculosus, (c) Semibalanus balanoides (shown þ SE), (d) Mytilus edulis, (e) Geukensia demissa, and (f ) Littorina littorea. Hemigrapsus sanguineus mortality results are shown across all treatments in the (g) high vegetated, (h) high bare, and (i) low bare zones. Statistically significant differences (P , 0.05) in mortality were determined by post hoc testing and are indicated by different lowercase letters above bars. controls (v2 ¼ 15.32, P , 0.001), cages (v2 ¼ 15.97, P , 0.001), shades (v2 ¼ 21.96, P , 0.001), and shaded cages (v2 ¼ 26.53, P , 0.001), with no other treatment effects. The high vegetated zone had less Hemigrapsus predation mortality (37%) than the low bare (65%; v2 ¼ 22.44, P , 0.001) and less desiccation mortality (6%) than the high bare zone (57%; v2 ¼ 105.85, P , 0.001). There were no treatment effects in the high vegetated zone (Fig. 2g). In the high bare zone, shaded cages had lower desiccation mortality than controls (v2 ¼ 12.03, P ¼ 0.001), cage controls (v2 ¼ 12.62, P , 0.001), and cages (v2 ¼ 12.66, P , 0.001), and did not differ from shaded treatments (Fig. 2h). There was a significant predator exclusion effect in the low bare zone. Controls had higher predation mortality than cages (v2 ¼ 12.57, P , 0.001) and shaded cages (v2 ¼ 11.98, P ¼ 0.001) and did not differ from cage controls or shades (Fig. 2i ). DISCUSSION Our study simultaneously examines facilitation along physical and biotic stress gradients for an entire community of plants and animals, rather than taking the single species, single stress gradient approach that is vulnerable to idiosyncratic, vague results (He and Bertness 2014). Our results reveal that at high elevations, fauna and flora are restricted to cordgrass patches largely due to buffering of thermal and desiccation stress by cordgrass. Shading at high elevations without cordgrass limited the mortality of cordgrass-dependent species. At lower elevations, predation generally limited organisms to cordgrass habitats protected from biotic stress. Reports 2580 SINEAD M. CROTTY AND MARK D. BERTNESS FIG. 3. Conceptual model incorporating positive interactions into niche theory. In contrast to traditional niche theory, this model predicts that the realized niche will be larger than the fundamental niche in physically stressful habitats through habitat amelioration, and will be identical to the fundamental niche in biologically stressful habitats through associational defenses. The areas enclosed by dashed lines represent the fundamental niche space. Gray areas outlined by solid black lines represent realized niche space along physical and biological stress gradients. Shaded and caged treatments rarely increased organism survival in the high vegetated zone. Only one of seven total species experienced enhanced survival in shaded treatments and this effect was smallest in the high vegetated zone. In contrast, shading effects were pervasive in the high bare zone where desiccation stress is highest; five of the seven total species experienced enhanced survival in shaded treatments. Similarly, in the low bare zone, predator exclusion treatments enhanced organism survival when caging effectively excluded predators. These results highlight the mechanisms by which cordgrass facilitates organism survival. Our results support the SGH; in physically and biologically stressful habitats, facilitations driven by habitat amelioration and associational defenses increase habitat use by dependent organisms. This supports the hypothesis that facilitators can expand the distribution of associated species, increasing the size of realized niches relative to fundamental niches. This contradicts a major assumption of niche theory and supports the hypothesis that positive interactions expand species distributions by ameliorating physical and biological stresses. Niche theory Niche theory has been a central model of community assembly for nearly a century, even though it is based on the notion that interspecific competition is the dominant driving force in spatial community assembly (Strong et al. 1984). It has been adapted as decades of experimental studies have improved our understanding of the roles of Ecology, Vol. 96, No. 10 predation, disturbance, propagule supply, dispersal limitation, and mutualism in community organization (Morin 1999). Despite these adaptations, modelers have commonly identified problems with the current state of niche-based SDMs and have proposed that the development of simple, concrete definitions of all niche theory concepts (Araújo and Guisan 2006) and inclusion of biotic interactions (Wisz et al. 2013) may significantly improve their use for conservation and management. The most recent models have attempted to incorporate spatial heterogeneity of trophic interactions and successfully incorporate consumer–resource and predator–prey interactions into SDMs (Trainor and Schmitz 2014). We propose a similar addition of spatially heterogeneous positive interactions using our stress gradient framework (Fig. 3). Stress gradient hypothesis The SGH was proposed at a time when ecologists thought that positive interactions were rare (Connell and Slatyer 1977, Connell 1983). It was offered as a general pedagogical model for plants and animals, predicting that positive interactions would increase under stressful physical conditions due to habitat amelioration and stressful biotic conditions due to associational defenses. Plant ecologists were attracted to the SGH but focused on the physical stress predictions of the model (e.g., Brooker et al. 2008). Little research focused on animals, whole communities, or associational defenses, so the SGH remained poorly tested. Later studies elaborated on the conceptual and practical implications of positive interactions in plant and animal communities (Bruno and Bertness 2001) and the contradictory predictions of the SGH and niche theory (Bruno et al. 2003). In spite of this broadening focus of the SGH, many plant ecologists instead focused on exceptions to the SGH (Maestre et al. 2005, Holmgren and Scheffer 2010). A recent meta-analysis revealed, however, that 20 years of studies on vascular plants found the SGH to be globally robust (He et al. 2013). Incorporating the SGH into niche theory Incorporating positive interactions into niche theory is supported by our data and the inability of niche theory to predict assembly of all but the simplest communities. While niche theory may predict community assembly in low-stress habitats, we predict that realized niches will be larger than or identical to fundamental niches in high-stress habitats due to habitat amelioration and associational defenses. We present a simple graphical model that can be tested in other habitats, species, and whole communities (Fig. 3). The x-axis is the inverse relationship between physical and biological stress documented within and among communities and is a basic assumption of other stress-based October 2015 FACILITATION EXPANDS REALIZED NICHES 2581 community models (Menge and Sutherland 1987). At low physical stress, high predation pressure leads to associational defenses (e.g., Hay 1986). With high predation pressure, competitive exclusion is limited (Paine 1974), and associational defenses allow facilitation-dependent species to live throughout the fundamental niche space. At intermediate stresses, the fundamental niche is larger than the realized niche, reflecting the negative net effects of interspecific competition and predation pressure. In contrast, at high physical stresses, neighbor habitat amelioration in sympatry leads to expansion of the realized niche, potentially making it larger than the fundamental niche. Expansion of the realized niche in relation to the fundamental niche has been proposed in the context of facilitation (Bruno et al. 2003), but never before in the context of a stress gradient. This simple didactic model reflects our experimental results on common cobble beach organisms. When cordgrass is not present at the higher elevations on cobble beaches, physical stress leads to thermal and desiccation death. Habitat amelioration by cordgrass expands the habitat or niche space occupied by other cordgrass-dependent species. At lower elevations, heavy predation pressure limits species survival and forces organisms to live at higher elevations associated with cordgrass for predation refuge. At intermediate stress levels, neither associational defenses nor foundation species stress amelioration occurs, and the realized niche is smaller than the fundamental niche as predicted by current niche theory (Chase and Leibold 2003). Our model is consistent with previous studies on the dynamics of positive interactions and models of the hierarchical organization of biogenic communities built and maintained by foundation species (Bruno and Bertness 2001, Altieri et al. 2010). 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