|The Cretaceous fragmentation of West Gondwana set in motion a grand experiment in biogeography. Just as the first angiosperms began to challenge the araucarias, ferns, and cycads, a shared African-South American biome was pulled apart and forced to radiate in isolation. Interpreting the results of this seemingly simple vicariant event is complicated, however, by questions about the timing of continental isolation, the role of postdrift dispersal, and the effect of geological and climatic changes since the separation. In nineteen substantive essays, "Biological Relationships between Africa and South America" interprets this biogeographical puzzle in light of recent research on West Gondwana plate movements and post-Cretaceous climatic change.
The four sections in this collection cover plate tectonics, plants, animals, and ecosystems. The material begins with an overview of biological relationships between the two continents by Peter Goldblatt. W. C. Pitman II, S. Cande, J. LaBrecque, and J. Pindell provide a useful synthesis of the mechanics and timing of the West Gondwana breakup. In their model, Africa initially acts as two semi-independent plates; southern South America and Africa gradually drift apart, while Africa north of the Niger delta remains firmly attached to Brazil. Magnetic lineations indicate that the continents were completely separated 90 million years ago, and these authors suggest a date probably 5 to 10 million years earlier. Fine tuning these dates is crucial to understanding the biogeographical relations of the two continents, as this was precisely when flowering plants were beginning their explosive divergence.
The plant section includes reviews of the paleofloras of Africa and South America, based on micro- and macrofossil evidence, as well as essays on two extant families. The latter are especially relevant because one of the families, Annonaceae, is believed to have evolved before the continental division, whereas Asteraceae appeared after it. George E. Schatz and Annick Le Thomas's work with Annonaceae suggest that biogeographical patterns are most plausibly explained by a common Gondwana ancestry followed by tremendous postdrift diversification. However, because the earliest Annonaceae fossils date from 70 million years ago, well after plate separation, the evidence of vicariance still teeters on fragmentary, incomplete fossil records. A cladistic analysis of six Asteraceae groups by Kare Bremer indicates that South America and the Pacific region, but not Africa, were ancestral areas for the family. Because it first appeared in the Tertiary, the cosmopolitan distribution of Asteraceae owes more to its inherent areas of seed dispersal than to continental drifting and fragmentation.
Among the essays on animal distributions, vicariance is infrequently invoked as a causal mechanism. Francois Vuilleumier and Allison V. Andors propose that most families of birds common to the two continents arrived in South America by long-distance dispersal. According to Larry G. Marshall and Thierry Sempere, less vagile mammals, such as primates and caviomorph rodents, likewise arrived considerably after the separation, in these cases via island hopping. John G. Lundberg suggests that lungfish biogeography is parsimoniously explained by a simple vicariant event but that five of the other major freshwater fish groups may owe their disjunct African-South American distributions to postdrift dispersal across the Atlantic, as all have or once had marine ancestors. He warns that "large biotas have complex, piecemeal histories" and that bio-geographical explanations based on single vicariant events are likely to be wrong. John G. Maisey amplifies this point: his work on Brazil leads him to conclude that most fossil evidence is uninformative, because it relates to endemic genera or cosmopolitan families, and that many faunal distributions which suggest vicariance lack early fossil evidence from both continents. Postbreakup migration of taxa across the Atlantic by means of island hopping or drifting, according to Maisey, is consistent with both the fossil and the geological records.
The section on ecosystems offers some provocative essays. Alwyn H. Gentry questions the odd-man-out status of Africa by showing that the floras of the two continents are remarkably similar at the community level. Though regionally lacking in tree and liana species, lowland African plots are nevertheless dominated by virtually the same families as those found in South America and share upward of one-third of their genera. By adjusting for differences of soil and precipitation, Gentry finds that African communities are as speciose as their South American cousins. Some of the reported floristic differences between the two continents may be illusory: he notes that many neotropical genera "masquerade" under different names in Africa.
Several authors emphasize the role of enhanced speciation in South America as opposed to extinction in Africa as a principal cause of species-richness differences. Terming the Pleistocene refugial theory intellectually barren, Paul Colinvaux offers a disturbance-vicariance hypothesis to explain elevated Amazonian species richness. Pleistocene cooling rather than desiccation resulted in creative intermingling of previously separated South American forest elements. Diversification was facilitated by large species ranges coupled with the isolating influence of lowland Amazonia's large and dynamic river system. According to Colinvaux, relatively limited lowland habitats of Africa would have favored extinction rather than speciation. Similarly, work on ant-plant relations by Doyle McKey and Diane W. Davidson suggests that relatively diverse physiography and climate in South America enhanced speciation. In addition to moisture differences between the two continents, Gentry invokes differences in reproductive ecology as potential speciation factors. Some African seed dispersers, such as elephants, contributed to long-distance dispersal and gene flow and limit the possibility of reproductive isolation. In South America, pollination systems exhibit a greater degree of host specificity and hence species radiation than they do in Africa.
The only shortcoming of this otherwise well balanced and timely collection is the complete omission of humans as biogeographical factors. Much of the biotic similarity exhibited between Africa and South America can be attributed to 500 years of human-mediated plant and animal dispersal and colonization. For the contributors to this volume, humans complicate rather than clarify the biogeographical picture. That there is no room for the role of people in "Biological Relationships between Africa and South America" underscores the fundamental difference between how biologists and geographers view the scope of biogeography.