Management of Habitat Fragments in a Tropical Dry Forest: Growth

Daniel H. Janzen2 Tropical conservation biology is inescapably the biology of habitat fragments and has been focused on habitat decay. Habitat restoration is primarily the initiation. growth. and coalescence of habitat fragments. Management of a tropical wildland will become the art and science of arresting the decomposition of habitat fragments and promoting their growth and coalescence. Forces that determine accumulation of structure and species are significantly within human control. Today's management actions will determine the nature of wildland habitats for centuries to come. Tropical dry forest is the most threatened of all the major lowland tropical forest habitats. simply because it has always occupied some of the lands most easily farmed in the tropics. and because it is so susceptible to fire. When dry forest and fields are abandoned and therefore allowed to return to dry forest. there are two principal kinds offorest initiation (assuming that there are nearby seed sources). a. When large pastures are downwind of a relatively intact forest. the initial invasion is primarily by individuals of large wind-dispersed trees that will persist and characterize the site for hundreds of years. However. these tree species are a minority of the total flora. Such forests of wind-dispersed trees are relatively inhospitable to animals. highly deciduous, and relatively species-poor. b. When there is any kind of attraction for animals in an abandoned open area, they may perch in it qr rest below it while crossing the open area. This results in accumulation of an entirely vertebrate-dispersed forest patch. Such patches may grow and coalesce to form a forest type as artificial as is a wind-generated forest. Vertebrate-generated forests contain more food items of interest to animals, are more species rich, and are more evergreen than are wind-generated forests. As large areas of abandoned low-grade farm and ranch land are returned to dry forest, the manager of national parks or other wildlands is confronted with the difficult decision of just which of the above, or other, forest types is to be promoted. The same will apply to rainforest when its restoration becomes a focus of concern. Tropical conservation biology is inescapably the biology of habitat fragments. There are two kinds of fragments. First, much of what is worthy of conservation has already been broken into decomposing habitat fragments that are refugia and remnants. Even a large national park that is a solid block of pristine forest is a fragment. The biology of the decomposition process of these fragments is of intense contemporary interest to conservation planners and managers (e.g., Lovejoy et aI., 1986; Diamond, 1986; Janzen, 1986a, c; Wilcove et aI., 1986; Uhl & Buschbacher, 1985; Newmark, 1987). Second, habitat restoration is primarily the initiation and coalescence of growing habitat fragments. Management of a tropical wildland therefore becomes the art and science of arresting decomposition of habitat fragments and promoting their growth and coalescence. In such an arena, today's management actions will determine the nature of wildland habitats for centuries to come; forces that determine accumulation of structure and species are significantly within human control. Here I examine the biology of habitat initiation and growth in a Costa Rican tropical dry forest. Dry forest is the most threatened 9f all the major lowland tropical forest habitats. It once covered more than half of the world's tropics (e.g., Brown & Lugo, 1982; Murphy & Lugo, 1986) but now supports a diverse array of breadbaskets, cotton fields, and pastures. In Pacific Mesoamerica, for example, less than 0.1 % of the original tropical dry forest, which once covered an area the size of France (equal to five Guatemalas in area), has conservation status, and there I This study was supported by NSF BSR 83-07887, BSR 84-03531, BSR 83-08388, and DEB 80-11558, and by the Servicio de Parques Nacionales de Costa Rica. The manuscript has been constructively reviewed by W Hallwachs. 2 Department of Biology, University of Pennsylvania, Philadelphia, Pennsylvania 19104, U.S.A. ANN. MISSOURI BOT. GARD. 75: 105-116. 1988. 106 Annals of the Missouri Botanical Garden TABLE l. Monthly precipitation (rounded to the nearest mm) in the administration area of Santa Rosa National Park, Guanacaste Province, Costa Rica (data collected by park rangers and extracted from the Meteorology Institute in San Jose) . Year Jan Feb Mar Apr May Jun 1980 1 0 5 0 184 175 1981 0 1 1 11 353 582 1982 16 2 0 41 919 129 1983 2 0 22 4 21 18