Ecological consequences of Late Quaternary extinctions of megafauna

Abstract

Large herbivorous vertebrates have strong interactions with vegetation, affecting the structure, composition and dynamics of plant communities in many ways. Living large herbivores are a small remnant of the assemblages of giants that existed in most terrestrial ecosystems 50,000 years ago. The extinction of so many large herbivores may well have triggered large changes in plant communities. In several parts of the world, palaeoecological studies suggest that extinct megafauna once maintained vegetation openness, and in wooded landscapes created mosaics of different structural types of vegetation with high habitat and species diversity. Following megafaunal extinction, these habitats reverted to more dense and uniform formations. Megafaunal extinction also led to changes in fire regimes and increased fire frequency due to accumulation of uncropped plant material, but there is a great deal of variation in post-extinction changes in fire. Plant communities that once interacted with extinct large herbivores still contain many species with obsolete defences against browsing and non-functional adaptations for seed dispersal. Such plants may be in decline, and, as a result, many plant communities may be in various stages of a process of relaxation from megafauna-conditioned to megafauna-naive states. Understanding the past role of giant herbivores provides fundamental insight into the history, dynamics and conservation of contemporary plant communities.

Figure 1

Body mass distributions of herbivorous mammals in (a) North America, (b) South America, (c) Australia and (d) Africa, distinguishing species that were present in the Late Pleistocene but went extinct before the historical period (filled bars) from survivors into the historical period (open bars). Data adapted from Smith et al. (2003) except for extinct Australian mammals, which are from Johnson (2006).

Figure 2

Time of arrival of modern Homo sapiens in different parts of the world in relation to the time of megafaunal extinction, against the background of changes in global temperature through the Last Glacial cycle. The line shows the relationship expected if human arrival and megafauna extinction were simultaneous. Regions are (1) New Zealand (Worthy & Holdaway 2002; Wilmshurst et al. 2008), (2) Madagascar (Burney et al. 2003, 2004), (3) northeast Siberia (Pitul'ko 2001; Stuart et al. 2004), (4) North America (Solow et al. 2006), (5) southern Europe (Stuart 2005; Roebroeks 2008), (6) Tasmania (Turney et al. 2008) and (7) mainland Australia (Gillespie et al. 2006; Johnson 2006). Temperature is indicated by deuterium ratios in Antarctic ice, from EPICA (2004); high values indicate high temperatures.