Remotely-Sensed Early Warning Signals of a Critical Transition in a Wetland Ecosystem

Sara Alibakhshi,Miina Rautiainen,Babak Naimi,Thomas Groen

doi:10.3390/rs9040352

Abstract

The response of an ecosystem to external drivers may not always be gradual and reversible. Discontinuous and sometimes irreversible changes, called ‘regime shifts’ or ‘critical transitions’, can occur. The likelihood of such shifts is expected to increase for a variety of ecosystems, and it is difficult to predict how close an ecosystem is to a critical transition. Recent modelling studies identified indicators of impending regime shifts that can be used to provide early warning signals of a critical transition. The identification of such transitions crucially depends on the ability to monitor key ecosystem variables, and their success may be limited by lack of appropriate data. Moreover, empirical demonstrations of the actual functioning of these indicators in real-world ecosystems are rare. This paper presents the first study which uses remote sensing data to identify a critical transition in a wetland ecosystem. In this study, we argue that a time series of remote sensing data can help to characterize and determine the timing of a critical transition. This can enhance our abilities to detect and anticipate them. We explored the potentials of remotely sensed vegetation (NDVI), water (MNDWI), and vegetation-water (VWR) indices, obtained from time series of MODIS satellite images to characterize the stability of a wetland ecosystem, Dorge Sangi, near the lake Urmia, Iran, that experienced a regime shift recently. In addition, as a control case, we applied the same methods to another wetland ecosystem in Lake Arpi, Armenia which did not experience a regime shift. We propose a new composite index (MVWR) based on combining vegetation and water indices, which can improve the ability to anticipate a critical transition in a wetland ecosystem. Our results revealed that MVWR in combination with autocorrelation at-lag-1 could successfully provide early warning signals for a critical transition in a wetland ecosystem, and showed a significantly improved performance compared to either vegetation (NDVI) or water (MNDWI) indices alone.

Highlights

Human activities may invoke gradual but profound changes to ecosystems such as coral reefs, lakes, wetlands and forests
We propose our new index Modified Vegetation Water Ratio (MVWR) based on the ratio of rescaled Normalized Difference Vegetation Indices (NDVI) and MNDWI indices calculated on a monthly basis as: MVWR = ln(
The indicators that have been introduced for generating early warning signals may indicate a false alarm due to changes in a stochastic regime of perturbations rather than the actual ecosystem dynamics [1,47], or, most importantly, due to having insufficient or imprecise data

Summary

Introduction

Human activities may invoke gradual but profound changes to ecosystems such as coral reefs, lakes, wetlands and forests. The response of an ecosystem to such changes may not always be gradual, predictable, and reversible, but might instead be nonlinear, abrupt and irreversible [1]. When the resilience of an ecosystem decreases, the ecosystem will be slower in recovering from small disturbances until it is so close to a critical threshold, that is called a ‘tipping point’. Under such a situation, even a small perturbation can invoke a critical transition to another state where returning to the previous state can be difficult and sometimes impossible [3]

Methods

Results

Discussion

Conclusion