Benchmarking Successional Progress in a Quantitative Food Web

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.

Food web stability and weighted connectance: the complexity-stability debate revisited

“…However, we could also reason that an increase in weighted connectance might be destabilizing for the food web. An increase in weighted connectance reflects a more even distribution of the fluxes (Ulanowicz 2002;Boit and Gaedke 2014). A more even distribution of fluxes could result in a more even distribution in interaction strengths (which is not necessarily the case, as mentioned above).…”

“…(Hendrix et al 1986), (4) Kjettslinge Experimental Farm Barley field with no fertilizer (B0) and (5) with fertilizer (B120) (Andrén et al 1990), (6) Lovinkhoeve Experimental Farm Integrated Farming (Int) and (7) Convential Farming (Con) (de Ruiter et al 1993), (8) (Neutel et al 2007) We followed the approach of Boit and Gaedke (2014) to determine weighted connectance, which is based on the information theory metrics of Ulanowicz (1997). First, Shannon's formula is used to describe the diversity in flux weights (feeding rates F ij ≠ 0):…”

“…Instead of using the weighted link density as in the analysis of Ulanowicz (1997Ulanowicz ( , 2002, the relation between food web complexity and stability can also be re-examined by looking at weighted connectance (Bersier et al 2002;Boit and Gaedke 2014). Weighted connectance is the number of links in the whole web relative to the total number of links, in which each link is weighed on the basis of the flux rate (in case of food webs) associated with the link.…”

“…Weighted connectance is the number of links in the whole web relative to the total number of links, in which each link is weighed on the basis of the flux rate (in case of food webs) associated with the link. Weighted connectance has been used before in food web studies (Bersier et al 2002;Banasek-Richter et al 2009;Boit and Gaedke 2014), but not yet in the complexity-stability context.…”

“…From a 'complexity begets stability' point of view (MacArthur 1955), we would expect that an increase in weighted connectance (as a measure of complexity) would lead to an increase in food web stability. Boit and Gaedke (2014) found that weighted connectance increased during succession and hypothesized that this could Bimply an insurance effect enhancing response diversity and robustness against disturbances^(p. 19). However, we could also reason that an increase in weighted connectance might be destabilizing for the food web.…”