Resource conversion: a generalizable mechanism for resource‐mediated positive species interactions

Abstract

Positive species interactions are ubiquitous in natural communities, but the mechanisms through which they operate are poorly understood. One proposed mechanism is resource conversion – the conversion by a benefactor species of a resource from a resource state that is inaccessible to a potential beneficiary species into a resource state that is accessible. Such conversion often occurs as a byproduct of resource consumption, and sometimes in exchange for non‐resource benefits to the benefactor species. At least five known classes of interactions, including both facilitative and mutualistic ones, may be classified as resource conversion interactions. We formulated a generalizable mathematical model for resource conversion interactions and examined two model variants that represent processing chain and nurse plant interactions. We examined the conditions under which these conformed to the stress‐gradient hypothesis (SGH), which predicts increased interaction benefits in more stressful environments. These yielded four key insights: 1) resource conversion interactions can be positive (towards the resource recipient) only when facilitator‐mediated resource conversion is more efficient than the baseline, spontaneous, facilitator‐independent resource conversion; 2) the sign of resource conversion interaction outcomes never switches (e.g. from net positive to net negative) with changing levels of resource availability, when all other parameters are kept constant; 3) processing chain interactions at equilibrium can never be positive in a manner that conforms to the SGH; 4) nurse plant interactions can be positive and conform to the SGH, although the manner in which they do depends largely on how resource stress is defined, and the environmental supply rate of surface soil moisture. The first two insights are likely to be generalizable across all resource conversion interactions. The general agreement of the model with empirical studies suggest that resource conversion is the mechanism underlying the aforementioned interactions, and an ecologically meaningful way of classifying these previously unassociated positive species interactions.

Toward spatio‐temporal delineation of positive interactions in ecology

“…Despite the progress that has been made by directly testing the SGH, literature also suggests that the mechanism of positive interactions (e.g., habitat modification) and traits of the organisms that are facilitating or being facilitated may be equally important as environmental context in explaining positive interaction strength across space and time (Albertson & Allen, 2015;Cameron, Coulson, & Marshall, 2019;Hastings et al, 2007;Lam & Chisholm, 2020;Schöb, Armas, Guler, Prieto, & Pugnaire, 2013;Subalusky & Post, 2019).…”

“…We hypothesize that the extent of positive interaction from the facilitator to beneficiaries will scale spatio-temporally based on the mechanism of stress amelioration (Table 1; Figure 1a,b). The primary mechanisms of facilitation that reduce stress on the beneficiary are (a) physical habitat modifications that ameliorate biophysical stress (e.g., desiccation, temperature extremes) or provide predator refuge, and (b) resource modification by which organisms increase availability of resources (Jones et al, 1994;Lam & Chisholm, 2020). Habitat modifications by ecosystem engineers are often fixed in space; however, the positive effects conferred by these modifications (e.g., amelioration of environmental stress) can extend further than the bounds of the structure itself, and persist temporally beyond the life of the engineer (Hastings et al, 2007;Jones et al, 1994).…”

“…Since its formalization, there has been support for both the simplicity and utility of the SGH (Bruno et al., 2003; Callaway et al., 2002; He, Bertness, & Altieri, 2013), however see counterexamples in extremely stressful environments (He & Bertness, 2014; Maestre, Callaway, Valladares, & Lortie, 2009). Despite the progress that has been made by directly testing the SGH, literature also suggests that the mechanism of positive interactions (e.g., habitat modification) and traits of the organisms that are facilitating or being facilitated may be equally important as environmental context in explaining positive interaction strength across space and time (Albertson & Allen, 2015; Cameron, Coulson, & Marshall, 2019; Hastings et al., 2007; Lam & Chisholm, 2020; Schöb, Armas, Guler, Prieto, & Pugnaire, 2013; Subalusky & Post, 2019).…”

“…We hypothesize that the extent of positive interaction from the facilitator to beneficiaries will scale spatio‐temporally based on the mechanism of stress amelioration (Table 1; Figure 1a,b). The primary mechanisms of facilitation that reduce stress on the beneficiary are (a) physical habitat modifications that ameliorate biophysical stress (e.g., desiccation, temperature extremes) or provide predator refuge, and (b) resource modification by which organisms increase availability of resources (Jones et al., 1994; Lam & Chisholm, 2020). Improved description and prediction of positive interaction extent may lead researchers to better design quantitative metrics to characterize positive interactions when they result from these distinct mechanisms (Figure 1a,b).…”

“…Resource limitation is a common ecological stressor and understanding the spatio‐temporal extent of benefit from organism‐mediated resource modification is an important source of variation to consider in delineating positive interactions (Figure 1b; Lam & Chisholm, 2020; Tilman, 1985). Organism‐mediated resource modification can occur through modification of resource state, form, and transport (Jones et al., 1994; Lam & Chisholm, 2020; Tilman, 1985). For example, bioturbation through the stirring up of sediments can increase availability and transport of food particles and nutrients otherwise unavailable to consumers (Moore, 2006; Silknetter et al., 2020).…”