Top-down control causes collapse of a self-organized landscape on intertidal mudflats

Background/Question/Methods

During spring, intertidal flats can exhibit strikingly regular spatial patterns of diatom-covered hummocks alternating with almost bare hollows. These patterns are a seasonal phenomenon, being most prominent during April and May, followed by a relatively sudden collapse in June to a homogeneous tidal flat characterized by a much more erosive regime. This rapid shift co-occurs with a gradual increase in herbivore numbers.

We hypothesize that 1) the formation of this geomorphic landscape is caused by a strong interaction between benthic diatoms and sediment dynamics, inducing spatial self-organization, and 2) that herbivore grazing later in the season disrupts diatom-silt interaction and leads to a collapse of this self-organized landscape.

We present a combined empirical and mathematical study to test the first hypothesis. We determined how the sediment erosion threshold varied with diatom cover and elevation. Our results were incorporated into a mathematical model to investigate whether the proposed mechanism could explain the formation of the observed patterns. We further tested whether herbivores could control landscape formation, in a field experiment by removing herbivores from the sediment.

Results/Conclusions

Our mathematical model confirmed that the interaction between sedimentation, diatom growth and water redistribution could the formation of regular patterns on the intertidal mudflat. Our experiments confirmed the hypothesis that the collapse of this self-organized landscape is driven by benthic herbivores that disrupt the interaction between diatoms and sediment dynamics. Our results show that top-down control by herbivores affects the formation of self-organized, geomorphological landscapes by shifting the system to a more erosive regime. Moreover, our study provides a clear example of sudden shifts occurring in a system characterized by self-organized spatial patterns.