Global shifts towards positive species interactions with increasing environmental stress

Abstract

The study of positive species interactions is a rapidly evolving field in ecology. Despite decades of research, controversy has emerged as to whether positive and negative interactions predictably shift with increasing environmental stress as hypothesised by the stress-gradient hypothesis (SGH). Here, we provide a synthesis of 727 tests of the SGH in plant communities across the globe to examine its generality across a variety of ecological factors. Our results show that plant interactions change with stress through an outright shift to facilitation (survival) or a reduction in competition (growth and reproduction). In a limited number of cases, plant interactions do not respond to stress, but they never shift towards competition with stress. These findings are consistent across stress types, plant growth forms, life histories, origins (invasive vs. native), climates, ecosystems and methodologies, though the magnitude of the shifts towards facilitation with stress is dependent on these factors. We suggest that future studies should employ standardised definitions and protocols to test the SGH, take a multi-factorial approach that considers variables such as plant traits in addition to stress, and apply the SGH to better understand how species and communities will respond to environmental change.

Facilitation shifts paradigms and can amplify coastal restoration efforts

“…Indeed, in The Netherlands, interactions shifted from facilitative to competitive with decreasing stress from low to high elevation. This agrees with the stress-gradient hypothesis that the importance of positive species interactions increases with increasing physical stress (30,32). The applied implications for these findings are that clumping designs are likely to enhance restoration designs in high stress areas, such as the low intertidal for marshes, but could be counterproductive in more benign habitats, such as in the high intertidal.…”

“…In natural communities, positive interactions have been shown to be more prevalent in arid and tropical regions where drought and heat stresses are often high (32). Moreover, natural, self-organized patchiness is most prevalent in ecosystems in dry (51,52), cold (52), or intertidal regions (52,53), where there are significant environmental constraints on organism growth and establishment (54).…”

“…All but one of the organizations surveyed plant propagules at restoration sites with, rather than without, spacing. This focus on planting designs that minimize the potential for negative interactions among outplants has persisted despite over two decades of ecological research showing positive interactions (e.g., mutualisms, facilitations) play a critical role in controlling the structure and function of ecological communities, especially under conditions of elevated physical stress (30)(31)(32) where neighboring plants can ameliorate physical stress for each other, including anoxic stress and wave-induced erosion stress in seagrasses, marshes, and mangroves (33)(34)(35). Indeed, a global metaanalysis of over 700 studies revealed that positive species interactions are most important for organism success and community persistence and recovery as physical stress increases (32), a scenario that mirrors intense abiotic stress conditions that are found on bare substrate after a coastal ecosystem has been degraded, which then is often targeted for restoration efforts (29,31).…”

Extreme drought stress shifts net facilitation to neutral interactions between shrubs and sub‐canopy plants in an arid desert

“…In contrast to negative plant-plant interactions, which can reduce diversity and cause species extinctions (Hautier et al 2009), positive interactions can ensure productivity and maintain diversity and community structure in stressful environments (Cavieres et al 2007, Butterfield 2009, Isabell et al 2009. A large amount of evidence shows that both facilitation and competition can occur simultaneously and that their balance can vary dramatically in accordance with environmental conditions (Dickie et al 2005, He et al 2013). An increasing number of studies have attempted to integrate facilitation into empirical and theoretical research (Callaway et al 2002, Brooker et al 2008, Filazzola and Lortie 2014 and ecological restorations (Gómez-Aparicio et al 2004, but see Noumi et al 2015).…”

“…As a result, the SGH has been refined to indicate that positive interactions prevail under moderate environmental stress but are weakened, or even reversed under higher stress levels (Michalet et al 2006, Holmgren and Scheffer 2010, Malkinson and Tielbörger 2010, Liancourt et al 2017, O'Brien et al 2017. As suggested by a variety of ecologists (Butterfield 2009, Armas et al 2011, He et al 2013, Michalet et al 2014, Soliveres et al 2015, Liancourt et al 2017, the apparently opposing views of the original and recent versions of the SGH may stem from inter-study differences in the nature of stress gradients (resource or non-resource stress), life-history of response species (stress tolerant or competitively inferior), scale of study focus (community or species level), component of stress factors (single or multiple stresses), or whether the observed gradient is complete. Even though a large number of studies from various ecosystems have explored the SGH, empirical tests of the hypothesis in extreme environments remains insufficient (but see Armas et al 2011, de Bello et al 2011, Castanho et al 2015, Lopez et al 2016, Liancourt et al 2017.…”