Species‐specific size vulnerabilities in a competitive arena: Nutrient heterogeneity and soil fertility alter plant competitive size asymmetries

Abstract

The size dependence of competitive interactions is starting to be highlighted as an important driver of species diversity within communities; however, it is still unknown whether all species are equally impacted by size‐asymmetric competition and what resources drive it. Here, we test species‐specific responses to size‐asymmetric competition under various soil environments by manipulating plant size as well as soil fertility, nutrient heterogeneity and the initial suppression of microbial communities within a three‐species community. Competition was primarily size‐asymmetric; however, the degree to which competition was size‐asymmetric was species‐specific, where three out of five soil treatments resulted in size‐asymmetric competition in some species but size‐symmetric competition in others. Overall, soil fertility and nutrient heterogeneity altered the degree of size‐asymmetric competition, while the initial suppression of microbial communities had no effect. Contrary to past predictions, when they had an effect, increased soil fertility and the presence of a high‐quality patch reduced the degree of size‐asymmetric competition, while the presence of a low‐quality patch increased it. This suggests that the role nutrient heterogeneity plays on the degree of size‐asymmetric competition is dependent on the quality and location of the patch relative to an individual's neighbours. These findings challenge the current understanding that competition for soil resources is always size‐symmetric and demonstrate that species within the same community may not experience the same degree of size‐dependent competition. These differential responses suggest we must consider variations in tolerance and suppression between species during size‐dependent plant–plant interactions, which may promote species coexistence as all small individuals may not be negatively impacted during size‐dependent competition. A plain language summary is available for this article.

Problems with models assessing influences of tree size and inter-tree competitive processes on individual tree growth: a cautionary tale

“…Problems with models assessing influences of tree size and inter-tree competitive processes… how, when and how effectively trees may compete with each other (Papaik and Canham 2006;Pretzsch and Biber 2010;Pretzsch et al 2012Pretzsch et al , 2018Craine and Dybzinski 2013;Cordonnier and Kunstler, 2015;Guo et al 2017;Cordonnier et al 2018;Pommerening and Meador 2018;Brown et al 2019;Acquah and Marshall 2020;Wu et al 2020;Orman et al 2021).…”

Vertical size structure is associated with productivity and species diversity in a short‐stature grassland: Evidence for the importance of height variability within herbaceous communities

“…Distinguishing between the two has important implications for the predictability of species loss, as one process is the result of trait‐based exclusion, while the other is random. Under size‐asymmetric competition, larger, fast‐growing species (DeMalach et al., ; Rajaniemi, ) or those more tolerant to size‐asymmetric competition (Brown et al., ) will survive. Under assemblage‐level thinning, on the other hand, each species, through intraspecific competition, competitively excludes its smaller individuals and the coinciding reduction in density promotes species loss due to chance (Stevens & Carson, ).…”

“…However, it is unclear whether light availability is the mechanism driving height inequalities in this system. Another potential mechanism could be a decrease in soil nutrient heterogeneity with increasing productivity, which would also result in a decrease in height inequality with increased productivity either directly by altering individual plant access to nutrients (Casper & Cahill, 1998) or indirectly by altering the degree of size-asymmetric competition (Brown et al, 2019;Rasmussen, Weisbach, Thorup-Kristensen, & Weiner, 2019).…”

“…Conversely, the final or stable state, in theory, is the removal of these smaller individuals as they do not receive enough sunlight (Weiner, ), shifting the vertical size structure of the community such that more individuals are taller and the minimum height to survive in the community is larger. However, the competitive loss of small individuals is not equal among species (Brown, Oppon, & Cahill, ) and an individual's height can be plastic, changing based on its environment (Smith, ; Weinig, ). As such, the relationship between productivity and diversity is likely dependent upon the height inequalities within a community as well as the height plasticity and size‐dependent competitive tolerance of the species within the community.…”

Size‐asymmetric competition among snails disrupts production of human‐infectious Schistosoma mansoni cercariae

“…In addition, parasites may disproportionately infect hosts of certain size or age classes, generating characteristic age or size vs. prevalence curves, because of accumulated exposures, host mortality, or ontogenetic changes in susceptibility (Duerr et al 2003). In virtually all species, larger individuals feed at faster rates than smaller individuals (Kooijman 2010) or preempt access to resources, for example, when larger plants shade smaller ones (Weiner 1990, Brown et al 2019). Thus, larger individuals exert greater competitive effects and smaller individuals are generally much more sensitive to competition.…”