Abstract
The Earth has been beset by many crises during its history, and yet comparing the ecological impacts of these mass extinctions has been difficult. Key questions concern the kinds of species that go extinct and survive, how communities rebuild in the post-extinction recovery phase, and especially how the scaling of events affects these processes. Here, we explore ecological impacts of terrestrial and freshwater ecosystems in three mass extinctions through the mid-Phanerozoic, a span of 121 million years (295–174 Ma). This critical duration encompasses the largest mass extinction of all time, the Permian–Triassic (P–Tr) and is flanked by two smaller crises, the Guadalupian–Lopingian (G–L) and Triassic–Jurassic (T–J) mass extinctions. Palaeocommunity dynamics modelling of 14 terrestrial and freshwater communities through a long sedimentary succession from the lower Permian to the lower Jurassic in northern Xinjiang, northwest China, shows that the P–Tr mass extinction differed from the other two in two ways: (i) ecological recovery from this extinction was prolonged and the three post-extinction communities in the Early Triassic showed low stability and highly variable and unpredictable responses to perturbation primarily following the huge losses of species, guilds and trophic space; and (ii) the G–L and T–J extinctions were each preceded by low-stability communities, but post-extinction recovery was rapid. Our results confirm the uniqueness of the P–Tr mass extinction and shed light on the trophic structure and ecological dynamics of terrestrial and freshwater ecosystems across the three mid-Phanerozoic extinctions, and how complex communities respond to environmental stress and how communities recovered after the crisis. Comparisons with the coeval communities from the Karoo Basin, South Africa show that geographically and compositionally different communities of terrestrial ecosystems were affected in much the same way by the P–Tr extinction.
Highlights
There have been many mass extinctions in the history of life, and these all seem to have had unique features
The P–Tr extinction damaged ecosystems more severely than the other crises during the Palaeozoic–Mesozoic transition. The differences among these extinction recovery dynamics are consistent with a recent hypothesis claiming that ecosystem compositional and dynamic stabilities are contingent upon histories of functional coevolution among clades, and that only extinctions of magnitudes sufficient to remove guilds are likely to result in post-extinction ecosystem replacement or re-organization [47]
Our results indicate three significant decreases in palaeocommunity stability in Xinjiang, coinciding with the G–L, P–Tr and T–J extinctions
Summary
There have been many mass extinctions in the history of life, and these all seem to have had unique features. The species-level food webs vary in the distribution of interspecific trophic links, which we stochastically generated by applying mixed exponential–power-law link distributions uniformly to all 14 communities, consistent with the hyperbolic distributions typical of modern food webs [7,26] and other complex networks [28] This method of stochastically assigning links using an empirically derived power-law distribution allows our model to capture the structure and spatiotemporal variations of a real food web in the ensemble. In almost all cases examined, a point of perturbation is reached where secondary extinction increases dramatically This is defined as a collapse threshold, and reductions of primary production beyond this threshold are expected to change the community significantly, reducing richness and altering the composition. Network structure properties were calculated using custom code written by P.D.R. in Julia [32]
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