Alternative Stable States Driven by Density-Dependent Toxicity

Abstract

Many populations are exposed to naturally occurring or synthetic toxicants. An increasing number of studies demonstrate that the toxicity of such compounds is not only dependent on the concentration or load, but also on the biomass or density of exposed organisms. At high biomass, organisms may be able to alleviate adverse effects of the toxicant by actively lowering ambient concentrations through either a joint detoxification mechanism or growth dilution. We show in a conceptual model that this mechanism may potentially lead to alternative stable states if the toxicant is lethal at low densities of organisms, whereas a high density is able to reduce the toxicant concentrations to sublethal levels. We show in an example that this effect may be relevant in real ecosystems. In an earlier published experimental laboratory study, we demonstrated that ammonia toxicity in eelgrass is highly dependent on the eelgrass shoot density. Here, we used the results of these experiments to construct a model describing the complex interactions between the temperate seagrass Zostera marina and potentially lethal ammonia. Analyses of the model show that alternative stable states are indeed present over wide ranges of key-parameter settings, suggesting that the mechanism might be important especially in sheltered, eutrophicated estuaries where mixing of the water layer is poor. We argue that the same mechanism could cause alternative stable states in other biological systems as well.

Toxic effects of increased sediment nutrient and organic matter loading on the seagrass Zostera noltii

“…4). This indicates that the lower shoot density and biomass in the expansion area makes the plants more vulnerable to toxicity, because toxicity cannot be alleviated by joint detoxification or by growth dilution ( Van der Heide et al, 2010b). Both processes are strongly affected by population density, and result in denser vegetation being better able to cope with toxins and showing higher growth rates (Van der Heide et al, 2010b).…”

Understanding seagrass resilience in temperate systems: the importance of timing of the disturbance

“…Understanding timing effects on resilience is clearly particularly important in bi-stable seagrasses ecosystems (Carr et al, 2012(Carr et al, , 2010van der Heide et al, 2010), but may also apply to other bi-stable temperate ecosystems such as, e.g. temperate shallow lakes (Scheffer et al, 2001, Scheffer and; saltmarshes (van Wesenbeeck et al, 2008); temperate reefs (Baskett and Salomon, 2010); and temperate foodplains lakes (Chaparro et al, 2014).…”

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“…The resulting accumulation of ammonia, the end product of ANR, can itself be a significant stressor to plants or, by necessitating protein synthesis to alleviate ammonia stress, can cause depletion of SAV carbohydrate stores (Guerrero et al 1981;Wang et al 2012). In other aquatic ecosystems (ponds, lakes) that are finite in the overall mass of N, potential NO x -N toxicity may be reduced based upon density of SAV (van der Heide et al 2010). However, under the unique lotic conditions of springs (increasing nitrate concentrations…”