Intraspecific facilitation: a missing process along increasing stress gradients – insights from simulated shrub populations

Abstract

In recent years many field studies have been conducted to assess the relative importance of facilitation and competition in structuring vegetation communities in different environments. Herein, we present a simulation model which systematically explores the relative importance of intra-specific facilitation and competition between adult shrubs and seedlings for spatial pattern formation. A grid-based simulation model was constructed and calibrated using data collected in the field from Sarcopoterium spinosum populations in Israel to simulate population dynamics along a rainfall gradient. A series of simulation experiments was conducted in which manipulations of seedling survival probabilities were carried out to assess the relative importance of these processes in generating spatial patterns. Increased survival probabilities of first-year shrubs in open areas were used to simulate competition effects, while increased survival probabilities in the vicinity of shrubs were used to simulate facilitation effects.Simulation results were then compared to shrub spatial patterns observed in the field. The results indicate that facilitation is not an important process in generating intra-specific spatial patterns. Rather, in mesic environments with high precipitation, competition is the dominant process generating spatial patterns, resulting in regular spacing of shrubs, similarly to the patterns observed in the field (L(h) values B0). In arid sites, where precipitation values are lower, and stress conditions are higher, the dominant process generating spatial patterns was random mortality due to drought conditions. The resulting spatial patterns in this case are random (L(h) Â0), whereas observed field populations exhibited clumped patterns (L(h) !0). We conclude that as stress conditions increase, the importance of intraspecific neighborhood interactions decrease whereas the importance of environmental factors increase in dictating intra-specific spatial pattern formation. Consequently in mesic environments intra-specific competition among adults determines the emerging patterns, while intraspecific facilitation is a negligible process.

Effects of diversity on diversity: consequences of competition and facilitation

“…Similar to the competition niche axis, if the distance between two types on the facilitation axis is BNB F /2, facilitation is strong; otherwise facilitation is weak. The assumptions of this part of the model are likely unrealistic in their specifics, but there is considerable empirical evidence for positive plantÁsoil feedbacks (Bever 2003) and for facilitation both within species (Sommer 1992, Malkinson andJeltsch 2007) and between species (Bruno et al 2003). The goal here was to incorporate a process that quantitatively represents in a simple way conditions that might generate a scenario in which diversity begets diversity within a single trophic level.…”

A temporal dimension to the stress gradient hypothesis for intraspecific interactions

“…Th e development of the SGH has led to a call to integrate positive interactions into ecological and evolutionary theory (Bruno et al 2003, Michalet et al 2006, Callaway 2007, Brooker et al 2008. Although evidence of negative interactions is ubiquitous both at the inter-and intra-specifi c levels, there is limited evidence of positive species interactions at the intraspecifi c level (Malkinson and Jeltsch 2007). Our study adds to the small number of studies documenting positive interaction at…”

Non‐hierarchical competition among co‐occurring woody seedlings in a resource‐limited environment

“…The increased drought anticipated in the U.S. southwest and other regions (Cayan et al 2010) may alter the frequency and strength of facilitation among plants (Malkinson andJeltsch 2007, L opez et al 2016). This is important because we have observed (1) size-specific differences in drought tolerance, for Ambrosia in particular , and (2) consistent positive spatial associations between juveniles and Ambrosia adults (Miriti 2007) at our reference site.…”