Abstract
Central to ecology and ecosystem management, succession theory aims to mechanistically explain and predict the assembly and development of ecological communities. Yet processes at lower hierarchical levels, e.g. at the species and functional group level, are rarely mechanistically linked to the under-investigated system-level processes which drive changes in ecosystem properties and functioning and are comparable across ecosystems. As a model system for secondary succession, seasonal plankton succession during the growing season is readily observable and largely driven autogenically. We used a long-term dataset from large, deep Lake Constance comprising biomasses, auto- and heterotrophic production, food quality, functional diversity, and mass-balanced food webs of the energy and nutrient flows between functional guilds of plankton and partly fish. Extracting population- and system-level indices from this dataset, we tested current hypotheses about the directionality of successional progress which are rooted in ecosystem theory, the metabolic theory of ecology, quantitative food web theory, thermodynamics, and information theory. Our results indicate that successional progress in Lake Constance is quantifiable, passing through predictable stages. Mean body mass, functional diversity, predator-prey weight ratios, trophic positions, system residence times of carbon and nutrients, and the complexity of the energy flow patterns increased during succession. In contrast, both the mass-specific metabolic activity and the system export decreased, while the succession rate exhibited a bimodal pattern. The weighted connectance introduced here represents a suitable index for assessing the evenness and interconnectedness of energy flows during succession. Diverging from earlier predictions, ascendency and eco-exergy did not increase during succession. Linking aspects of functional diversity to metabolic theory and food web complexity, we reconcile previously disjoint bodies of ecological theory to form a complete picture of successional progress within a pelagic food web. This comprehensive synthesis may be used as a benchmark for quantifying successional progress in other ecosystems.
Highlights
Coping with global environmental change demands an improved understanding of ecological succession for ecosystembased management and restoration [1,2]
Our results indicate that successional progress within the plankton community of Lake Constance (LC) was quantifiable by generalizable system-level indices (Table 4), passing through predictable stages from early to late succession
By drawing on highly resolved long-term empirical data and by establishing crossreferences between these indices, we shed light onto the mechanisms driving changes in community composition at the functional group level and how these changes are reflected at the system level
Summary
Coping with global environmental change demands an improved understanding of ecological succession for ecosystembased management and restoration [1,2]. The annually repeated seasonal succession of temperate plankton communities is readily observable [9], spanning 30–100 generations of small organisms dispersed in a nearly homogeneous medium. It is ideally suited as a model system of secondary succession because community assembly during the growing season is largely driven by autogenic processes, passing through characteristic stages in just a few months [10,11,12]. This enables to unravel consequences of abiotic forcing in concert with biotic mechanisms [13] such as predator-prey interactions and competition over comparatively short time scales [14,15]
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