Implications of the “Homogecene Debt”


Biotic Homogenization. Lockwood, J. L., and M. L. McKinney , editors . 2001 . Kluwer Academic/Plenum Publishers , New York . 289 pp. $65.00. ISBN 0-306-46542-6 .

“Extinction is forever,” the saying goes, and certainly the list of books about extinction sometimes seems to go on forever. This thought-provoking book argues that extinction-based thinking slightly misses the point and that biotic homogenization, the willful or thoughtless replacement of local biotas with an invasive “McBiota,” is the strongest effect that people are having on the planet. Minimizing homogenization, the editors argue, has been a neglected part of the strategy to conserve biodiversity. Biotic homogenization is defined as both a pattern of change and a process through which change occurs. The definition based on pattern seems more satisfying because a range of processes can lead to the same end point. Newcomer species can make room for themselves or fill gaps made by people. There is already plenty of awareness of the role these processes play in causing extinctions—introduced species and habitat loss are two of the “evil quartet”—but the strong focus on the loss of differences among places is what sets this book apart. It foretells of the Homogecene, the next geological epoch, in which people will cease to care about the impoverished and globalized flora and fauna. Biodiversity may inspire people to care about species and habitats, but biomonotony will not.

The first stated aim of the book is to identify taxa and regions where homogenization has greatest effect. The attempt, however, is compromised by the skewed focus of the book toward countries where most universities are, rather than where most current threats are, and to a lesser extent by a focus on birds. Part of the problem with focusing on the G8 nations is the massive signal-of-extinction filters, highlighted in the editors' initial chapter, that hamper clear discernment of other processes. The second aim, identifying lessons to be learned from past homogenizations, is hampered by the relatively sparse treatment of paleontological evidence. Roy and Kauffman's case study of Californian marine and terrestrial molluscs is the only chapter focused fully on the fossil record, and the review it contains is ( as in some of the other case studies ) rather brief.

The book makes a better stab at the third aim, a start on turning patterns into ways of minimizing homogenization. McKinney and Lockwood suggest a general need for more ecological awareness in planning urbanization and the conservation of open space within towns. The chapters cover many axes of homogenization—spatial, taxonomic, character, climatic, and genetic, for instance—and often suggest factors affecting the rate of progression along each axis. Webb et al. show that, in birds, rarity shows less phylogenetic clumping than does extinction risk, highlighting that what is happening now is probably not merely an intensification of “natural” extinction. Bennett and coworkers distinguish between avian families for which high extinction rates may be a fact of life ( e.g., parrots ) and those for which it is a new phenomenon ( e.g., kiwis ). Their finding that species-rich clades tend to be generalists links nicely with Brooks's result that generalism is a better predictor of a species' success as an invader than abundance in native range. In the only chapter about insects, Vazquez and Simberloff show that recorded insect invasions in the United States are taxonomically clumped, but that the data are too patchy to reveal exactly why. Lomolino et al. highlight how human effects are “downsizing nature” by selectively removing large species, large ecosystems, and even large habitat patches. They eloquently invoke the woolly mammoth as a metaphor for biodiversity: a species once capable of inspiring awe was reduced to a shadow of its former self before humans finally overwhelmed its final refuge. Climate change threatens even those places where people don't go, and Green et al. indicate how warming will lead to a probable reduction in latitudinal temperature gradients—homogenization on the largest scale—and species loss.

My favorite chapter, by Daehler and Carino, examines the creep of genetic homogeneity by analyzing rates of hybridization between native and alien plants in Great Britain and Hawaii. In Britain the prevalence of hybridization is predicted most strongly by the number of introduced congeners. Data are inevitably much less solid for Hawaii, but hybridization seems to be more common, perhaps because isolating mechanisms are generally less well developed in remote island floras than on continental islands and the mainland. This combination of a study of the best data set available, that of Britain, with a study of a flora in deep crisis, that of Hawaii, is compelling and again raises the question of why so few of the other chapters have anything to say specifically about the most biotically rich parts of the globe.

The last aim—prediction of what the Homogecene will be like—provides perhaps the best justification for the focus on developed countries. Business districts of towns in different parts of the country don't just look alike architecturally, they are biotically alike too. Blair shows that the butterfly community of a Californian business district is more similar to that of an Ohio business district than that of a Californian preserve just a handful of kilometers away. On a smaller scale, regional differences in aquatic faunas have been eroded significantly by human actions in both Tennessee ( Duncan and Lockwood ) and California ( Marchetti et al. ). Maurer et al.'s study of the dynamics of range changes in North American birds shows that the effects of habitat alteration are nonlinear. Change ( i.e., expansion for invaders and contraction for some natives ) is often initially slow but can accelerate rapidly. This may imply the existence of a “homogenization debt” analogous to extinction debt.

Perhaps because of a wish to maintain diversity, there is no sign of a strong editorial hand shoe-horning contributions into any framework. Such a framework might in any event be premature, but the order of the chapters seems more or less arbitrary and the lack of a concluding chapter is a shame. The chapters are a mixture of reviews, global analyses, and local ( sometimes very local ) case studies. Some are excellent, some less so, but each approach provides an important insight, so the diversity just about pays off. But what did the publishers do for their money? The cover is nice, but the production values aren't otherwise very high, and there is little sign of a copyeditor having been employed.

The book does a very good job of conveying the rate at which the McBiota is spreading, and several authors sound a warning of how tragic this will be. Their warnings are mostly based in aesthetics: “our human appetite for variety will be cheated.” This worries me. Our human appetite for variety seems no match for our human appetite for what we know—our human appetite for McDonald's and Budweiser, chain stores and blockbuster sequels. Not everyone shares the view that diversity is worth saving for its own sake, and I would have liked to see more about the ecosystem consequences of homogenization over ecological and evolutionary time scales. Are biomonotonous ecosystems as functional as the biodiverse systems they replace? Are they more or less resistant and resilient in the face of global changes? If aesthetics alone stand in the way of biomonotony, then much of biodiversity will go the way of regional accents and the uniqueness of every high street, because aesthetics aren't enough.