Ecosystem assembly rules: the interplay of green and brown webs during salt marsh succession

Current theories about vegetation succession and food web assembly are poorly compatible, as food webs are generally viewed to be static, and succession is usually analyzed without the inclusion of higher trophic levels. In this study we present results from a detailed analysis of ecosystem assembly rules over a chronosequence of 100 years of salt marsh succession. First, using 13 yearlong observations on vegetation and soil parameters in different successional stages, we show that the space-for-time substitution is valid for this chronosequence. We then quantify biomass changes for all dominant invertebrate and vertebrate species across all main trophic groups of plants and animals. All invertebrate and vertebrate species were assigned to a trophic group according to feeding preference, and changes in trophic group abundance were quantified for seven different successional stages of the ecosystem. We found changes from a marine-fueled, decomposer-based (brown) food web in early stages to a more terrestrial, plant-based, herbivore-driven (green) food web in intermediate succession stages, and finally to a decomposer-based, terrestrial-driven food web in the latest stages. These changes were accompanied not only by an increase in live plant biomass and a leveling toward late succession but also by a constant increase in the amount of dead plant biomass over succession. Our results show that the structure and dynamics of salt marsh food webs cannot be understood except in light of vegetation succession, and vice versa.