Abstract

Mesofauna taxa fill key trophic positions in soil food webs, even in terrestrial–marine boundary habitats characterized by frequent natural disturbances. Salt marshes represent such boundary habitats, characterized by frequent inundations increasing from the terrestrial upper to the marine pioneer zone. Despite the high abundance of soil mesofauna in salt marshes and their important function by facilitating energy and carbon flows, the structure, trophic ecology and habitat-related diet shifts of mesofauna species in natural salt marsh habitats is virtually unknown. Therefore, we investigated the effects of natural disturbance (inundation frequency) on community structure, food web complexity and resource use of soil mesofauna using stable isotope analysis (15N, 13C) in three salt marsh zones. In this intertidal habitat, the pioneer zone is exposed to inundations twice a day, but lower and upper salt marshes are less frequently inundated based on shore height. The mesofauna comprised 86 species / taxa dominated by Collembola, Oribatida and Mesostigmata. Shifts in environmental disturbances influenced the structure of food webs, diversity and density declined strongly from the land to the sea pointing to the importance of increasing levels of inundation frequency. Accordingly, the reduced diversity and density was associated by a simplification of the food web in the pioneer zone as compared to the less inundated lower and upper salt marsh with a higher number of trophic levels. Strong variations in δ15N signatures demonstrated that mesofauna species are feeding at multiple trophic levels. Primary decomposers were low and most mesofauna species functioned as secondary decomposers or predators including second order predators or scavengers. The results document that major decomposer taxa, such as Collembola and Oribatida, are more diverse than previously assumed and predominantly dwell on autochthonous resources of the respective salt marsh zone. The results further suggest that Mesostigmata mostly adopt an intraguild predation lifestyle. The high trophic position of a large number of predators suggests that intraguild predation is of significant importance in salt marsh food webs. Presumably, intraguild predation contributes to stabilizing the salt marsh food web against disturbances.

Highlights

  • Salt marshes are widespread along the European coasts and cover 20% of the area of the North Atlantic Wadden Sea [1]

  • Density of the soil mesofauna significantly declined from the upper salt marsh to the lower salt marsh and pioneer zone (F2,15 = 13.26, p < 0.001; Fig 3A)

  • Strong changes in community structure and food web complexity along the studied salt marsh gradient suggest that the occurrence of the majority of species in salt marshes is related to inundation frequency

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Summary

Introduction

Salt marshes are widespread along the European coasts and cover 20% of the area of the North Atlantic Wadden Sea [1]. The exposure to frequent inundations and the clear elevational gradient with the upper salt marsh, lower salt marsh and pioneer zone make them to ideal model systems to study, how physical and biological disturbance factors interact to create pattern in natural communities [2]. Salt marshes provide important ecosystem services such as biomass production, supply of food sources, nitrogen and carbon cycling, they are heterogeneous habitats, and plants and animals must cope with increasing abiotic disturbance due to increasing frequency of inundations towards the pioneer zone [3,4,5,6,7]. Similar to the plant based zonation, the organic material of the soil differs between zones, depending on decomposition rates, the amount of deposition, and its origin from either marine or terrestrial resources. C3 and C4 material differ in their photosynthetic pathway resulting in distinct stable carbon isotopic signals allowing identification of their contribution as basal resources of food webs [16,17,18,19]

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