Diversity in the Desert


Robichaux, R. H. & D. A. Yetman The Tropical Deciduous Forest of Alamos: Biodiversity of a Threatened Ecosystem in Mexico. , editors. 2000 . University of Arizona Press , Tucson . 259 pp. $45.00. ISBN 0-8165-1922-6 .

The tropical deciduous forests of northwestern Mexico, along with the people who live there and know the forests best, stand at a turning point… Although one might not realize this from TV programs, rain forest is not the only important or threatened forest type in the tropics.

Martin & Yetman ( 1998)

In the United States, the television and newspaper weather maps, as well as serious vegetation maps, are cut off with a blank void beyond the Mexican border. It is as if nobody in America would want to connect with an image of a “lifeless” desert beneath the parching, subsiding air in the global circulation cell that jettisons its moisture on “climb-out” from the equatorial belt. Water that saturates the desert air at “lift-off” never escapes its equatorial origin. Rather it falls back into tropical rainforests before heading northward. In such a functional “rain shadow” only 150 km south of the border in Mexico, a tropical deciduous forest with 100 species of tropical trees representing 36 families and 48 species of orchids seems unlikely. Yet Military Macaws (Ara militaris), Lilac-crowned Parrots (Amazona finschi), jaguars (Panthera onca), mouse-opossums (Marmosa canescens), vampires bats (Desmodus rotundus), armadillos (Dasypus novemcinctus), Mexican beaded lizards (Heloderma horridum), Boa constrictors (Constrictor constrictor), and barking and leaf frogs (respectively, Eleutherodactylus augusti and Pachymedusa dacnicolor) are all part of the biodiversity of the tropical deciduous forest ( TDF ).

The TDF is the major plant formation that occurs between tropical rainforest and tropical thorn forest in the Whittaker diagram (1975) that appears in most ecology and conservation texts (e.g., Meffe & Carroll 1997. The TDF exists at 27–33° north only because the Hadley cell's subsidence there is overwhelmed seasonally by monsoon-like input from the Gulf of Mexico. This produces a dramatic annual cycling between a dry, leafless, thorn-scrub-like environment and a summer rainy season when the canopy becomes wet and leafy, more like rainforest than thorn forest. For the moment, such wonders persist in eastern Sonora, but only where the terrain is rugged and roads are absent. Elsewhere, TDF is the most endangered tropical ecosystem, because of deforestation, lack of protected areas, and—in Sonora—massive introduction of African buffel-grass (Pennisetum ciliare), an invasion obsessively promoted by the government and ranchers (Martin et al. 1998; Martínez-Yrízar et al., chapter 2, this volume).

Each chapter of The Tropical Deciduous Forest of Alamos offers discussions to read and savor that will provide an excellent source of supplementary reading for courses. The discussions cumulatively occupy half the book's pages. Examples demonstrate several textbook characteristics of conservation biology (e.g., a “crisis discipline,” hindered by uncertainty about “quality, depth, and breadth of information available,” including “humans as part of the play,” needing multidisciplinary approaches; Meffe & Carroll 1997. The half of the book's pages not in discussion form add up to a TDF information handbook, containing a superb set of annotated checklists of vascular plants, ethnobotanically useful trees and columnar cacti, trees of the Mayo region (cross-referencing scientific, Mayo, and Spanish names), traditional crop assemblage and native crop species, and indigenous names for native wild and domesticated crops and vertebrates (except fishes).

Martin and Yetman set the scene impressively (chapter 1), pulling together the climate, TDF vegetation, fauna, human cultures, utilization of natural products, sociology, and economics. In chapter 2, Martínez-Yrízar, Búrquez, and Maass discuss climate, TDF structure, and ecosystem function, basing this last discussion on comparison of TDF in the vicinities of San Javier (northern distributional limit, 28.6° north), Alamos (27° north), and Chamela (near the coast at 19.5° north). In chapter 3, “Vegetation, Flora, and Seasons of the Río Cuchujaqui, a Tropical Deciduous Forest near Alamos, Sonora,” Van Devender, Sanders, Wilson, and Meyer analyze the comparative phytogeography of the TDF on the Cuchujaqui and in seven other floras of northwestern Mexico and southern Arizona. One focus of this book is the overlaps or commonalities among regions, which yields a sense of continuity as well as contrast.

Meffe and Carroll's (1997 ) “third guiding principle of conservation biology,” that “the human presence must be included…” is driven home with the wealth of knowledge provided by local people in two thorough and absorbing chapters. In chapter 4, Yetman, Van Devender, López Estudillo, and Guerrero, along with eight consultants from local communities, combine knowledge of flora and human culture and economy to convey understanding of their intertwining and their relevance to current socioeconomic problems, such as the drug trade. In a book otherwise sparsely illustrated, this fascinating information is enhanced by Van Devender's gallery-quality photographs of the consultants and of crafts made from native wood (chapters 1 and 4).

In chapter 5, “Crop Diversity among Indigenous Farming Cultures in the Tropical Deciduous Forest,” Burns, Drees, Nabhan, and Nelson point out the high crop-species richness among indigenous farmers of the northern Sierra Madre Occidental. They make an interesting correlation between natural habitat heterogeneity, adaptation of folk varieties to local microclimates, and the resulting crop diversity. Their argument lies in contrast to the assumed wide adaptability of commercial corn varieties, advertised on fences across the midwestern United States. The chapter authors' closing contention is that “[the] need for supporting and protecting the lifestyles of traditional agriculturalists is just as compelling as the need for wilderness protection” ( p. 166).

Schwalbe and Lowe (chapter 6) include not only a list of the 79 species of amphibians and reptiles of the Sierra de Alamo, but also a commentary on and list of 81 species of mammals. Several species from each of these classes reach their extreme northern or northwestern distributional limits there. They point out that the herpetofaunal species diversity of the region is 16 times that of Arizona and 130 times that of the United States. Conservation threats to this herpetofauna are common to other segments of the biota: growth of human populations, habitat destruction for conversion to buffel-grass pastures, and—on a more local scale—tourist accommodations, collection of long-lived species for market and hobby trades, drug plantations, and birding stops.

Birding stops? In chapter 7, Russell describes “Birds of the Tropical Deciduous Forest of the Alamos, Sonora, Area.” Here, many original or potential TDF areas are now repeatedly degraded open areas—such as crop fields, pastures, fencerows, and recently abandoned fields—or secondary growth. Many of the 152 bird species of the Alamos may use more than one of these anthropogenic open areas or two or more of the natural habitat categories. Matters are further complicated when one considers the resident versus transient status of the avifauna. Also covered in this chapter are those birds found in riparian forests only, such as the rare and poorly known Orange-billed Nightingale-Thrush (Catharus aurantiirostris), not noted as far north as Sonora before 1995.

The uncertainty of conservation in the tropical deciduous forest is manifested at different levels. First, the absence of data on species' densities derived from long-term monitoring makes it difficult to detect species declines. Second, for those species whose numbers have declined or that are rarely sighted, it is difficult to identify the cause of their decline. But, such methodological difficulties do not diminish the understanding that clearing of TDF has spurred the decline of many native species. Overall, a universal handicap in conservation planning is the deficit of background information at all levels. Conservation of the TDF is no exception: it is “among the least known tropical ecosystems.” This fact is emphasized in chapter 2, which joins two other works as a significant “fill-in” effort (the revised, expanded edition of Gentry's Río Mayo Plants[Martin et al. 1998] and The Trees of Sonora[Felger et al. 2001]). As a nonbotanist with limited travel experience in eastern Sonora, I found these three works synergistic in facilitating an appreciation of North American TDF distribution and ecology. Thus, the perspectives and information in this new book will be valued by ecologists, biogeographers, conservation biologists, anthropologists, and prospective ecotourists to the southwestern United States and Mexico.