Elsevier

Palaeoworld

Volume 16, Issues 1–3, January–September 2007, Pages 9-15
Palaeoworld

Research paper
Increasing returns, ecological feedback and the Early Triassic recovery

https://doi.org/10.1016/j.palwor.2007.05.013Get rights and content

Abstract

Most models of biotic recovery following mass extinction generally invoke logistic growth, with increasing competition due to resource competition eventually causing a decline in the rate of increase in diversity. Yet one of the most interesting aspects of recoveries is the acquisition of new resources and the expansion of diversity. In other words, the carrying capacity of the ecosystem increases, a process not captured by logistic models. Insights from theories of economic growth, particularly of the sources of economic innovation, suggest an important role for the spillover effects of particular types of ecosystem engineering. The positive feedback effects of these spillovers allow the rapid expansion of biodiversity, as is seen during the late Early Triassic phase of biotic recovery.

Introduction

The past decade has seen a considerable increase in studies of post-extinction recoveries, most of them based on detailed studies of the fossil and geochemical record. However, paleontologists have tended to assume that life simply rebounded after the end of whatever disturbances produced a mass extinction. The pre-extinction diversity dynamics then return, at the end of the recovery interval, albeit often with new taxa. Such a view is inherent in Sepkoski's (1984) coupled logistic growth model of Phaneozoic marine diversity, where exponential growth occurs in the interval following a perturbation, slowing as competition increases.
Yet this view fails to acknowledge the dynamics of the recovery process itself, and may inhibit understanding of the ecological and evolutionary dynamics of post-extinction biotic recovery. A more conceptually sophisticated approach to biotic diversification was offered by Valentine (1980) in his tessera model where ecospace was conceptualized as an array of ecological niches and the magnitude of innovation was a function of the extent of open or vacant niches (see also Valentine and Walker, 1986, Herbold and Moyle, 1986). Innovation and diversification were initially high, but large morphological transitions began to decline as the space filled, with smaller-scale transitions continuing as a result of a relatively constant extinction rate. Competition was responsible for the decline in rapid increases in diversity. Each of these approaches follows a long tradition of thinking, stretching back beyond George Gaylord Simpson, which views macroevolutionary diversity increases as scaled-up versions of the competition for resources among individuals within a population.
Such approaches provide a conceptual ecological framework which predicts larger morphological steps early in the process and operationalize the intuitively appealing idea of new opportunities in the aftermath of a mass extinction. However, these scenarios fail to capture the dynamics of biotic recovery, particularly changes in ecological structure of communities and the dominant groups, and the expansion of diversity, biomass and ecological complexity. The challenge for understanding biotic recoveries, and other macroevolutionary transitions more broadly, is to identify the processes involved in the redeployment of resources, the acquisition and partitioning of new resources, and the construction of new ecospace (see, for example, Knoll and Bambach, 2000).
Post extinction biotic recoveries provide a very useful avenue with which to explore these issues. The initial clades are usually well identified, and issues of preservational change are more tractable (although the continuing uncertainties about the origin of icthyosaurs are a cautionary tale). Recent work on biotic recoveries has largely been empirical, focusing on description of ecological patterns, the history of particular clades, or studies of fluctuating geochemical proxies for biological activity (the most recent review of post-extinction recoveries is Erwin (2001) which is now dated by the significant number of new studies of the aftermath of many different biotic crises). Although new data may refine our picture of the patterns of recovery, only new models will allow us to develop a more accurate understanding of the processes of recovery.
This contribution briefly considers the nature of the Early Triassic recovery as a guide to both the nature of the questions about process and the scope and temporal resolution of currently available data. The bulk of the paper then considers the difficulties with past approaches, and sketches an alternative conceptual approach, focusing on how ecosystems capture spillovers from ecosystem engineering (the impact that some species have, both positively and negatively, on other species).

Section snippets

Patterns of Early Triassic recovery

Many aspects of the Early Triassic biotic recovery after the end-Permian mass extinction have long been quite puzzling. For many marine clades there was a long delay, or survival period, before the onset of recovery (Hallam, 1991) through the Early Triassic to the Spathian or Anisian. This is the longest delayed recovery after any of the great mass extinctions (Erwin, 2001) and has led to considerable discussion over whether it reflected continuing environmental perturbation, simply the

The poverty of logistic growth, or why Mendel was wrong

Both Sepkoski's and Valentine's approaches to diversification, like most paleontological explorations of the issue, assume the ubiquity of competition, and the relevance of a logistic model of growth (derived from population dynamics), where the critical parameters are r, the rate of diversification, n, the number of taxa, and K, the total carrying capacity of the system (see also Benton, 1996, Sepkoski, 1996). While logistic growth models have been of great use in understanding the dynamics of

Models of economic growth

Economists have faced similar challenges in trying to understand the sources of economic growth, and while the situations are not strictly analogous, I believe that a brief discussion of the approach and some recent advances may prove useful in understanding the linked problems of evolutionary innovation and biotic recovery. In Solow's early models of economic growth (e.g. 1956) he examined changes in economic output as a function of the amounts of capital and labor; technology was an exogenous

Increasing returns and positive feedback during biotic recovery

I believe that these same distinctions are useful in identifying the drivers of increased biological diversity, particularly when ecospace expands during major evolutionary transitions or post-extinction biotic recoveries. Fig. 3 shows the biological equivalent of Fig. 2, with examples of a variety of biological adaptations and products in different fields. Most biological adaptations and attributes (the equivalent of economic goods) are specific to the species or clade in which they evolved,

Discussion and conclusions

Our conceptual understanding of the processes of biotic recovery from mass extinction has been hampered by an over-reliance upon models driven by competition and resource limitation (carrying capacity). Such reliance has, in my view, blinded us to the less common but perhaps more generative situations where, from this perspective, overall carrying capacity has greatly increased. There are a variety of possible models of increased carrying capacity, from a simple form where, with a certain

Acknowledgements

This paper is dedicated to the memory of my good friend Jin Yugan, with great respect for our many long discussions about events across the Permo-Triassic boundary, and science more generally. This work has been supported by the Santa Fe Institute through grants from the Thaw Charitable Trust and by the Smithsonian Institution through the Charles and Mary Walcott Fund. I appreciate discussions on aspects of the Early Triassic and innovation with Sam Bowring, Sam Bowles, David Krakauer, and

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