Regime Shifts in an Early Triassic Subtropical Ecosystem

Elke Schneebeli-Hermann

doi:10.3389/feart.2020.588696

Abstract

The Early Triassic was one of the most remarkable time intervals in Earth History. To begin with, life on Earth had to face one of the largest subaerial volcanic degassing, the Siberian Traps, followed by a plethora of accompanying environmental hazards with pronounced and repeated climatic changes. These changes not only led to repeated and, for several marine nektonic clades, intense extinction events but also to significant changes in terrestrial ecosystems. The Early Triassic terrestrial ecosystems of the southern subtropical region (Pakistan) are not necessarily marked by abrupt extinction events but by extreme shifts in composition. Modern ecological theories describe such shifts as catastrophic regime shifts. Here, the applicability of modern ecological theories to these past events is tested. Abrupt shifts in ecosystems can occur when protracted changing abiotic drivers (e.g. climate) reach critical points (thresholds or tipping points) sometimes accentuated by stochastic events. Early Triassic terrestrial plant ecosystem changes stand out from the longer term paleobotanical records because changes of similar magnitude have not been observed for many millions of years before and after the Early Triassic. To date, these changes have been attributed to repeated severe environmental perturbations, but here an alternative explanation is tested: the initial environmental perturbations around the Permian–Triassic boundary interval are regarded here as a main cause for a massive loss in terrestrial ecosystem resilience with the effect that comparatively small-scale perturbations in the following ∼5 Ma lead to abrupt regime shifts in terrestrial ecosystems.

Highlights

Sediments – the Earth history archive – reveal their mysteries only bit by bit
The search for the culprit for this massive extinction of marine communities has included a long list of suspects, such as sea level regression (Holser et al, 1989), massive volcanic activity (Renne and Basu, 1991; Renne et al, 1995), ocean acidification caused by volcanic degassing (Heydari et al, 2008), global warming caused by CO2 (Wignall, 2001; Kidder and Worsley, 2004; Kiehl and Shields, 2005), oceanic anoxia (Wignall and Hallam, 1992; Wignall and Hallam, 1993; Wignall and Twitchett, 1996; Wignall and Twitchett, 2002; Grice et al, 2005; Kump et al, 2005), hypercapnia (Knoll et al, 1996) and an extra-terrestrial impact (Becker et al, 2004)
Responses of ecosystems to these incremental changes can differ from gradual or linear change to abrupt changes when certain thresholds are reached (Figure 1 e.g. geography for condition I) to a response with hysteresis (Figure 1 geography of condition III). In the latter, changing environmental conditions drive the system towards conditions II and close to switch point 1 (SP1), which represents a threshold (Figure 1) from where the system drops abruptly from one regime into the other

Summary

Introduction

Sediments – the Earth history archive – reveal their mysteries only bit by bit. The narrative of momentous and complex events in the remote past of system Earth significantly changes as knowledge accumulates. The research history of the biotic and abiotic events around the Permian–Triassic boundary and the subsequent Early Triassic exemplifies how complex the story can get. About 52% of marine invertebrate and vertebrate families and ∼81% species went extinct across the Permian–Triassic boundary (Raup and Sepskoski, 1982; Stanley, 2016). The plethora of possible triggers for the mass extinction

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Conclusion