PerspectiveEcology

The Forgotten Megafauna

See allHide authors and affiliations

Science  03 Apr 2009:
Vol. 324, Issue 5923, pp. 42-43
DOI: 10.1126/science.1172393

Large terrestrial vertebrates—called megafauna—play key roles in ecosystem dynamics by feeding on plants and by maintaining habitat heterogeneity (1). A global wave of megafauna extinctions occurred 50,000 to 10,000 years ago, when many large continental mammals were lost (25). Classical definitions of megafauna are based on such continental mammals and are variously given as animals larger than 44 kg (6) or above 1000 kg (7). Here, we argue that the megafauna concept should be extended beyond an absolute animal size to be context-dependent. In any given ecosystem, the largest vertebrates have ecosystem impacts that are similar on a relative scale to those of the largest vertebrates in another ecosystem: One ecosystem's mesofauna is another ecosystem's megafauna.

An ecosystem function that clearly illustrates this argument is animal-mediated seed dispersal. Here, the link between animal body mass and ecosystem function is straightforward: The larger the fruit-eating animal (frugivore), the larger the fruits it can consume. Thus, extinction-mediated “ecological shrinkage”—the loss of species interactions—in community-level seed dispersal roughly scales with frugivore body mass.

Scientists have argued that in continental Central and South America, the extinction of the classic mammalian megafauna—such as giant ground sloths and gomphotheres—caused disruption of seed dispersal for large fruits (4, 5). However, on islands, the extinction of large birds and reptiles in the past two or three millennia has led to similar disruptions (8, 9). In both locations the demographic and genetic consequences of large-vertebrate extinctions for plants are likely similar—for example, disruption of long-distance gene flow or changes in species composition (10, 11). Yet, by the classic definitions, large insular vertebrates would not be considered megafauna.

To illustrate our point, we have examined tropical and subtropical faunas from three kinds of ecosystems: continental, continental islands, and oceanic islands. For each fauna, we compared the body masses of the largest extant frugivorous vertebrate—mammal, bird, or reptile—to the largest that has gone extinct since the late Pleistocene. On continents, the body masses of extant frugivores are an order of magnitude lower than those of extinct frugivores; in contrast, in some continental and oceanic islands, body masses of extant animals are two or even three orders of magnitude lower than those of their extinct predecessors (see the supporting online material).

For instance, the largest frugivores in South America were gomphoteres (7580 kg), whereas the largest living frugivores are the tapirs (300 kg). On the continental island of Madagascar, the role of largest frugivore has passed from the elephant bird (450 kg) to the radiated tortoise (10 kg). On Mauritius, giant tortoises weighing up to 100 kg were the largest native frugivores; today, the title goes to a fruitbat weighing only 0.54 kg. Thus, the loss of the island giants—tortoises, lizards, and flightless birds such as the dodo that were once found on many islands—has, in relative terms, led to a greater megafaunal downsizing than the extinction of even the largest gomphotheres in South America (see the figure).

Scaling the megafauna.

The magnitude of loss of frugivorous megafauna is currently most dramatic on islands, as illustrated by the smaller drawn sizes of the giant ground sloth and the gomphothere from South America, compared with the elephant bird in Madagascar and the giant tortoise of Mauritius. However, many continental regions are poised to catch up.

CREDIT: MEGAFAUNA ARTWORK BY CARL BUELL

Moreover, in the relatively species-poor and simple island ecosystems, cascading effects of megafaunal loss may manifest themselves faster and with more devastating results than in more complex continental ecosystems. For example, lost megafaunal seed-dispersal interactions on islands will not be compensated for by surviving frugivores, because they are too small. In continental ecosystems, there is a higher functional redundancy, with medium- and even small-sized species capable of filling at least part of the megafaunal niche (12).

Anthropogenic impacts are set to cause further extinctions among large vertebrates, with dramatic consequences for ecosystem dynamics (11). If all currently threatened vertebrate frugivores were to go extinct, the relative ecological shrinkage in many continental ecosystems would equal that of islands. For instance, if all threatened frugivores in South America were to go extinct, the largest remaining frugivore would be the howler monkey, weighing 9 kg—a factor of 700 less than the giant ground sloth. Even some islands stand to suffer further losses; in Mauritius, the largest nonthreatened native frugivore is the gray white-eye, a bird weighing a mere 0.009 kg (see the supporting online material).

An extended megafauna concept has the potential to promote synergy between otherwise disparate research and conservation foci, and to facilitate broader syntheses of ecosystem-level effects of extinctions of the largest vertebrates and the resulting ecological shrinkage. It is high time to more fully understand and ameliorate the recent and ongoing losses of all “the hugest, and fiercest, and strangest forms” (13).

Supporting Online Material

www.sciencemag.org/cgi/content/full/324/5923/42/DC1

Fig. S1

Table S1

References

References and Notes

  1. 1.
  2. 2.
  3. 3.
  4. 4.
  5. 5.
  6. 6.
  7. 7.
  8. 8.
  9. 9.
  10. 10.
  11. 11.
  12. 12.
  13. 13.
  14. 14.
View Abstract

Stay Connected to Science

Navigate This Article