Intraspecific interactions shift from competitive to facilitative across a low to high disturbance gradient in a salt marsh

Abstract

Disturbances are ubiquitous among salt marshes, and disturbances such as litter removal may form stressful environmental conditions. The stressgradient hypothesis (SGH) predicts that the relative importance of facilitation and competition between species will vary inversely to each other across abiotic stress gradients. However, how intraspecific interactions vary across disturbance levels, and whether they follow the SGH has not been investigated. To test the SGH with intraspecific interactions within Suaeda salsa individuals, we assessed individual survival in established fully mapped salt marsh plots with two treatments, disturbed, litter removal and control, undisturbed plots. Recently developed spatial statistics were applied to distinguish between random processes, intraspecific facilitation, scramble competition, and contest competition underlying the spatial patterns at different spatial scales, growth stages, and disturbance levels. We found evidence that intraspecific interactions among S. salsa individuals across disturbance levels tended to support the SGH, but this support depended on time and space. Intraspecific interactions were more likely to show positive density dependence in disturbed plots, particularly at the seedling period and when individuals were separated by\9 cm. However, positive density dependence was not detected in undisturbed plots. The protective effects of litter on seedlings, which were lost in undisturbed sites, may have been at least partly simulated at high seedling densities. Mortality in disturbed sites was not random, and facilitation had a more important influence than competition on population dynamics in high disturbance sites. However, the influence of competition on population dynamics, specifically scramble competition was also clearly important.

Remotely sensed phenological heterogeneity of restored wetlands: linking vegetation structure and function

“…Alternatively, greater litter accumulation could be hypothesized to enhance site quality (Lenssen et al, 1999) and protect the reed seedlings in patch interior from wind and water-related disturbance (Zhang and Wang, 2016), which, again, could be tested more explicitly via experimental studies.…”

Interspecific facilitation mediates the outcome of intraspecific interactions across an elevational gradient

“…Thus, even if beneficiaries reach high densities as a result of facilitation by a benefactor, unless the beneficiaries are also able to reproduce, the net effect of the benefactor on the beneficiaries will not be positive. Alternatively, it is possible for interactions among beneficiaries to be facilitative (see, e.g., Goldenheim et al 2008, Saccone et al 2010), where, for example, beneficiary individuals could shield one another from strong winds (Zhang and Wang 2016) and/or low temperatures (Eränen and Kozlov 2008, Zhang and Wang 2016, Zhang and Tielbörger 2019), to a point where the positive effects of intraspecific facilitation outweigh the negative impacts of intraspecific competition (Fig. 1C; Fajardo and McIntire 2011, García‐Cervigón et al 2013).…”

“…Biotic interactions may strongly shape ecological communities by affecting plant fitness, abundance, cover, and survival (Cavieres et al 2007, Kunstler et al 2011, Schöb et al 2013, Zhang and Wang 2016, Svanfeldt et al 2017). The impact of these interactions can vary considerably with both extrinsic (e.g., environmental) and intrinsic (e.g., density dependent) factors (Olsen et al 2016, Svanfeldt et al 2017, Kula et al 2020, Zhang and Tielbörger 2020).…”

“…However, a switch from negative density dependence to positive density dependence (i.e., where survival and fecundity are enhanced at high densities) may be observed as environmental stress increases (see Goldenheim et al 2008, Svanfeldt et al 2017, Zhang and Tielbörger 2020). Indeed, facilitation can occur intraspecifically (Eränen and Kozlov 2008, Goldenheim et al 2008, Fajardo and McIntire 2011, García‐Cervigón et al 2013, Zhang and Wang 2016, Svanfeldt et al 2017), with, for example, individuals of the same plant species mitigating the impact of abiotic stress on one another (thereby benefitting each another) when growing in dense stands (e.g., Goldenheim et al 2008, Zhang and Tielbörger 2019; see also Zhang and Tielbörger 2020). Although many studies have documented the impact of density‐dependent effects on the survival and fecundity of individuals (e.g., Goldenheim et al 2008, Dochtermann and Peacock 2013, Svanfeldt et al 2017), few studies have documented density‐dependent effects within the context of interspecific facilitation (but see, e.g., Tielbörger and Kadmon 2000, Zhang and Tielbörger 2020).…”

Phylogenetic relatedness, ecological strategy, and stress determine interspecific interactions within a salt marsh community

“…Stress plays an important role in shaping plant community assemblages (Bornette and Puijalon 2011;Zhang and Wang 2016), and the outcome and strength of species interactions influencing community assemblages may be related to the intensity of abiotic and biotic stress. The stress gradient hypothesis (SGH) states that the frequency or intensity of facilitative and competitive interactions between species is inversely related to abiotic or biotic stress levels (Bertness and Callaway 1994;He et al 2013;Lortie and Callaway 2006;Silliman et al 2015).…”