Chance, Variation and the Nature of Causality in Ecological Communities

Abstract

Chance is pervasive in nature. Erratic events such as storms and fires can cause major damage to an ecosystem. Rare successful long distance dispersal events like a viable seed landing in just the right habitat can form the stepping stone for range expansion of a plant species. Illustrated with two examples we argue that in ecology chance events are scale-dependent. We show how random stochastic variation in species interactions may result in relative stability at a higher community level. In other systems the reverse may take place, in which deterministic interactions result in unpredictable chaotic dynamics. Analysing the processes and dynamics at these different scales has led to an increasing mechanistic understanding of the variation in ecological communities in space and time. Unambiguous identification of cause and effect relations from this work is of the greatest importance, as many ecosystems in the world are not amenable to experimentation. This work should form the scientific basis for identifying the threats to ecosystems and defining proper conservation and mitigation measures.

Plant community evenness responds to spatial plant–soil feedback heterogeneity primarily through the diversity of soil conditioning

“…Field observations show that the spatiotemporal patterns of species replacement in late‐successional grasslands are consistent with the model of PSF‐mediated coexistence (De Kroon & Jongejans, ; Herben, Krahulec, Hadincová, & Kováfiová, ; Van der Maarel & Sykes, ), suggesting that the same mechanisms may be in operation. However, an open question is how plants condition soils spatially in the field.…”

“…In addition, the range of competitive abilities among species was smaller on mixed‐conditioned soils, and this led to more equalized shoot biomass production among species and thus higher community evenness. Although we cannot demonstrate this directly here, our data underscore the idea that PSFs, through their frequency‐dependent effects on plant performance, may mediate competitive intransitivity among species (De Kroon & Jongejans, ; De Kroon et al., ; Soliveres et al., ), which is thought to influence dynamics of many natural communities (Soliveres et al., ). Alternatively, PSFs may only lead to equalized competitive abilities, which would falsify PSF as stabilizing mechanism of plant diversity.…”

Temporal carry‐over effects in sequential plant–soil feedbacks

“…to be unpredictable (De Kroon and Jongejans 2016). Our data suggests that multiple generations of soil conditioning may lead to species replacement patterns that without this historical perspective appear unpredictable.…”

“…It has been shown using fine‐scale, long‐term monitoring plots that in natural grasslands plant species replace each other rapidly within local patches (Van der Maarel and Sykes , Herben et al ). However, which species replaces which often appears to be unpredictable (De Kroon and Jongejans ). Our data suggests that multiple generations of soil conditioning may lead to species replacement patterns that without this historical perspective appear unpredictable.…”

The steering role of plant-soil interactions in natural community dynamics and nature restoration

“…In soils conditioned by multiple species (both heterogeneous and mixed-conditioned uniform soils) PSF effects were less pronounced and this led to more equalized competitive abilities among species and higher diversity. Together this underscores the idea that PSFs, through their density dependent effects on plant performance, may mediate competitive intransitivity among species (De Kroon et al 2012;Soliveres et al 2015;De Kroon & Jongejans 2016), which is thought to influence dynamics of many natural communities (Soliveres et al 2015). As a next step, pairwise competition experiments among all species in the community are needed to definitively demonstrate PSF-mediated intransitivity of competitive abilities (Petraitis 1979, Laird and Schamp 2006, Jing et al 2015.…”