7.4 Sustainable management of tropical environments

Sustainable agroforestry, Santa Rosa rainforest, Mexico

Revised

About 30,000 Popoluca Indians live in southern Mexico. The Popoluca of Santa Rosa farm the Mexican rainforest, using a form of agriculture known as the milpa system (Table 7.1), which resembles shifting cultivation but which mimics the natural rainforest: l This is a labour-intensive form of agriculture, using fallow. l It is a diverse form of polyculture with over 200 species cultivated, including maize, beans, cucurbits, papaya, squash, water melons, tomatoes, pineapples, chayotes, oregano, coffee and chili. l Coffee is sold for cash. l Two crops are planted annually. l Fields are usually dug with digging sticks although a few households use ploughs.

The variety of plants found in a natural rainforest is mirrored by using shifting cultivation of crops with different requirements. For example, lemon trees, peppervine and spearmint are heliophytes – light seeking – and prefer open conditions not shade. Coffee, by contrast, is a sciophyte – preferring shade – while the mango tree requires damp conditions.

The close associations that are found in natural conditions are also seen in the milpa system. For example, maize and beans go well together, as maize extracts nutrients from the soil whereas beans return them. Tree trunks and small trees are left because they are useful for returning nutrients to the soil and preventing soil erosion. They are also used as a source of material for housing , hunting spears and medicines. As in a rainforest, the crops are multi layered, with tree, shrub and herb layers. This increases NPP, because photosynthesis is taking place on at least three levels, and soil erosion is reduced, as no soil or space is left bare. In all, 244 species of plant are used in the farming system. Most of the crops are self seeding, which reduces the cost of inputs. The Popolucas show a high level of ecological knowledge in managing the forest.

Sustainable agroforestry involves farming forests without destroying the natural environment or putting excessive strain on resources.

Table 7.1 A comparison between the milpa system and the new forms of agriculture

NPP Work (labour) Inputs Crops

Yield (compared with inputs) Reliability of farming system Economics Income Carrying capacity Milpa system

High, stable High Few Polyculture (244 species used)

200% Quite stable Mainly subsistence None/little Several families/4ha plus livestock

Tobacco plantations or ranching

Declining Higher and increasing Very high: 2.5–3 tonnes fertiliser/ha/pa Monoculture (risk of disease, poor yield, loss of demand and/or overproduction) 140% if lucky High-risk operation Commercial More 1 family on a plantation (200 ha); ranching – 1 ha of good land/cow, 20 ha of poor land/cow

Animals

l Animals farmed include chickens, pigs and turkeys. l These are used as a source of food, or for bartering and selling, and their waste is used as fertliser. l Rivers and lakes are used for fishing and catching turtles. l Deer, boar, rabbits and some birds are hunted with arrows and spears. l It is not entirely a subsistence lifestyle since wood, fruit, turtles and other animals are traded for some seeds, mainly maize.

Pressures on the Popolucas

About 90% of Mexico’s rainforest has been cut down in recent decades, largely for new forms of agriculture. This is partly a response to Mexico’s huge international debt and attempts by the government to increase agricultural exports and reduce imports. The main new forms of farming are: l cattle ranching for export l plantations for cash crops, such as tobacco

However, these new methods are not necessarily suited to the physical and economic environment: l Tobacco needs protection from too much sunlight and excess moisture and the soil needs to be very fertile. l The cleared rainforest is frequently left bare and this leads to soil erosion.

Unlike the milpa system, the new systems are very labour intensive. l Pineapple, sugar cane and tobacco plantations require large inputs of fertiliser and pesticides. Inputs are expensive and the costs are rising rapidly.

Ranching prevents the natural succession of vegetation, because of a lack of seed from nearby forests and the grazing effects of cattle: l Grasses and a few legumes become dominant. l One hectare of rainforest supports about 200 species of trees and up to 10,000 individual plants. By contrast, one hectare of rangeland supports just one cow and one or two types of grass. But it is profitable, in the short-term, because land is available, and it is supported by Mexican government.

The Mexican rainforest can be described as a ‘desert covered by trees’. Under natural conditions it is very dynamic, but its resilience depends on the level of disturbance.

Typical mistake

The Popolucas are not completely isolated – they trade and barter with other Mexicans. So they are not purely subsistence farmers.

Now test yourself

Tested

18 Compare the Popoluca’s methods of farming with the natural tropical rainforest ecosystem. What lessons can be learnt from this? 19 The tropical rainforest is a ‘desert covered by trees’. What does this mean?

Answers on p.218

Exam-style questions

1 (a) Compare and contrast the climate of humid tropical environments with that of seasonally humid environments. (b) Explain how different tropical climates can affect human activities. 2 (a) Outline the characteristics of vegetation in (i) the tropical rainforest and (ii) the savanna. In what ways are they adapted to their environments? (b) Comment on the role of human activities in modifying the ecosystem of either tropical rainforests or savannas.

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