3 Modelling ecological changes

For the purpose of exploring alternative ecological futures for tropical Latin America it was necessary to choose a land classification system capable of including the ecological characteristics of the region and its potential and limitations, and adaptable to the type and quality of the available information. The life-zone approach (Holdridge,1967) was considered appropriate, subdivided into specific categories (e.g. savannas, mangrove forests) according to the criterion of actual vegetation.

The spatial extent of the tropical zone was adjusted by taking into account the ecological and productive characteristics of the different areas (Brown and Lugo,1980; Winograd, 1989b); as a consequence, some ecological units exceed the geographical limits (23°27' South to 23°27' North) of the strict definition of the tropics. A total of twelve tropical and subtropical life-zones were identified for the region, aggregated into seven major zones for the purposes of the present paper (figs. 2.1 and 2.2).

Simulation models were implemented (GallopÍn and Gross, 1989; Winograd, 1989a) for each of the twelve zones. Each zone is modelled as a set of compartments representing different ecological categories or conditions and with different structural, functional, and productive characteristics. The following seven categories were defined:

¹ "Natural": virgin areas, and areas with past alteration but currently similar to the original ecosystems; ² "Altered": denotes a mosaic of patches of land under production coexisting with patches of original and secondary vegetation, and areas with slight to moderate soil erosion; ³ "Agricultural": annual, permanent, and nontraditional (i.e. coca, marijuana) crop areas, including fallow from permanent agriculture;³ (4) "Grazing": ranching areas in natural or artificial pastures; (5) "Plantations": reforested areas used for forestry and watershed protection; (6) "Wastelands": unproductive lands irreversibly transformed by extreme soil erosion and desertification (natural deserts are not included here); and (7) "Urban": urbanized areas (mainly cities).

Every year, land shifts from one category to others according to the intensity and nature of the human activities (defined by an assumed scenario) and of the natural processes occurring on it (fig. 2.3). Simulations span the period 1980-2030. A simple compartment model was used. Each compartment represents the surface of a land category, for each life-zone, and it changes according to the following equation:

where S = surface of a given land category (Km²); Inflows = surface of land of other categories converted into the considered category in a given year (Km²/year); 0utflows = surface of land of the considered category converted into other categories (including itself) in a given year (Km²/year); Smax = maximum potential surface of the category (Km²); 1= set of all land categories. The scenario yearly defines the process generating the transformations (human activities or natural regeneration; see fig. 2.3) for each category and life-zone, specifying the portion of the category affected by the activity and the rates of conversion to other categories. The scenario is exogenously defined, taking into account the current situation, the assumed rate of growth of the activity, and the availability of land. Models were run under both a reference and a sustainable development scenario (see next section).

While the simulation models, in their present state, do not calculate production, but only the surfaces of land under different categories and production systems, the estimates of production are based upon the expected improvements in agricultural yields (compatible with the historic changes within the region and probably underestimating the future increases).