Feedback loops drive ecological succession: towards a unified conceptual framework

Abstract

The core principle shared by most theories and models of succession is that, following a major disturbance, plant–environment feedback dynamics drive a directional change in the plant community. The most commonly studied feedback loops are those in which the regrowth of the plant community causes changes to the abiotic (e.g. soil nutrients) or biotic (e.g. dispersers) environment, which differentially affect species availability or performance. This, in turn, leads to shifts in the species composition of the plant community. However, there are many other PE feedback loops that potentially drive succession, each of which can be considered a model of succession.While plant–environment feedback loops in principle generate predictable successional trajectories, succession is generally observed to be highly variable. Factors contributing to this variability are the stochastic processes involved in feedback dynamics, such as individual mortality and seed dispersal, and extrinsic causes of succession, which are not affected by changes in the plant community but do affect species performance or availability. Both can lead to variation in the identity of dominant species within communities. This, in turn, leads to further contingencies if these species differ in their effect on their environment (priority effects). Predictability and variability are thus intrinsically linked features of ecological succession.We present a new conceptual framework of ecological succession that integrates the propositions discussed above. This framework defines seven general causes: landscape context, disturbance and land‐use, biotic factors, abiotic factors, species availability, species performance, and the plant community. When involved in a feedback loop, these general causes drive succession and when not, they are extrinsic causes that create variability in successional trajectories and dynamics. The proposed framework provides a guide for linking these general causes into causal pathways that represent specific models of succession.Our framework represents a systematic approach to identifying the main feedback processes and causes of variation at different successional stages. It can be used for systematic comparisons among study sites and along environmental gradients, to conceptualise studies, and to guide the formulation of research questions and design of field studies. Mapping an extensive field study onto our conceptual framework revealed that the pathways representing the study's empirical outcomes and conceptual model had important differences, underlining the need to move beyond the conceptual models that currently dominate in specific fields and to find ways to examine the importance of and interactions among alternative causal pathways of succession. To further this aim, we argue for integrating long‐term studies across environmental and anthropogenic gradients, combined with controlled experiments and dynamic modelling.

A comprehensive framework for vegetation succession

“…These biotic and abiotic changes cause feedback loops (Figure 3, Box 5, dotted lines) affecting different parts of the successional process, thus fueling, retarding, or inhibiting successional change (van Breugel et al, 2024). For example, changes in the animal community directly impact plant-animal interactions (arrow 24), changes in the microbial community directly affect the magnitude and nature of plant-microbe interactions (arrow 25), and changes in soil and microclimate modify resources, conditions, and signals (arrows 26) and, hence, plant performance.…”

“…Species availability is determined by propagule dispersal to the patch (Figure 3, arrow 8) and by the propagules in the patch that have survived the disturbance event (arrow 9). The order of species colonization is important, as it may determine the sequence of facilitative, neutral, or inhibitory actions, and such priority effects of arrival may therefore determine subsequent successional pathways, and the heterogeneity in successional pathways observed within a site (Fukami, 2015;Pickett et al, 2011;van Breugel et al, 2024).…”

“…This paves the way for the establishment and dominance of later successional plant species (Van der Putten et al, 1993). Such pathogen-mediated plant-soil feedbacks are very common, and given the specificity of these relationships, this may lead to many different successional pathways (Fukami, 2015;van Breugel et al, 2024).…”

“…These feedback loops can be species specific. For example, plant species modify the microclimate and soil in different ways, due to their difference in canopy and root architecture (Mejía-Domínguez et al, 2011), and differences in resource uptake and loss (van Breugel et al, 2024). As plant-animal interactions and especially plant-microbe interactions can be very species specific, this gives rise to a multitude of different successional pathways for plant, animal, and microbial communities.…”

“…Succession has been studied for over a century, and since the pioneering work of Clements (1916), significant conceptual and empirical advances have been made, which have been summarized in excellent reviews that have focused on succession in general (Glenn-Lewin et al, 1992), successional theories (Pickett et al, 2011;Poorter et al, 2023), empirical evidence (Connell & Slatyer, 1977;Drury & Nisbet, 1973), feedback loops and variation in successional pathways (van Breugel et al, 2024), primary succession (Walker & Del Moral, 2003), temperate old-field succession , tropical grassland succession (Peterson & Carson, 2008), and different disturbance types across the world (Prach & Walker, 2020). Over time, there has been a gradual development in our thinking of succession.…”