Four centuries ago, the African island of Mauritius was still covered with tropical rainforest. That quickly changed when the Dutch settled on the island in 1638. The clearing that started then eventually led to an additional problem that islands with little vegetation inevitably face:soil erosion.
In 1598, Dutch merchantmen on their way to Indonesia discovered a volcanic island, which they named after the then stadtholder Prince Maurits:Mauritius. The uninhabited island was strategically located for merchantmen east of Madagascar, halfway between South Africa and Indonesia. Mauritius was an ideal stopover before the great journey to the east, to stock up on fresh food and water. In 1638 the island was formally claimed by the VOC and the Dutch settled there. In 1710 the island was no longer profitable and was abandoned.
Before the Dutch settlers took over the island in 1638, Mauritius was covered with tropical rainforest. Now more than 98 percent has been cleared, which means that soils are less protected against heavy rainfall. Nutrients and agrochemicals wash into rivers and threaten the coral reef surrounding the island. An erosion model shows how natural and human factors accelerate this process.
To make the new colony profitable, the Dutch soon started cutting and trading ebony. Large areas of tropical rainforest gave way to sugar cane that thrived on the fertile soils. But the erosion, which washed away a lot of soil, increased considerably as a result. Under the influence of global economic developments, land use in Mauritius has changed again in the last twenty years. Soil erosion will continue unabated unless measures are taken.
Heavy rainfall
Mauritius is ravaged by tropical cyclones every year, accompanied by heavy rainfall with peak intensities sometimes exceeding 80 millimeters per hour. The rain falls almost directly on the unprotected, fertile soil, which washes away quickly from the hard volcanic subsoil, especially on steep slopes. Meters thick nutrient-rich tropical soils have developed on the more than 100,000-year-old lava packs.
The erosion not only affects the soil fertility of agricultural land, it also has major consequences for the water quality in and around Mauritius – a danger that has only recently been recognized. When nutrients and pesticides flow down slopes from agricultural lands, they can wreak havoc there and disrupt marine ecosystems in lagoons. Mauritius is surrounded by a vast 300 km reef ecosystem 2 . The great biodiversity in this ecosystem, with 600 fish species and more than 200 crab species, is coming under great pressure.
Erosion model
Because almost half of Mauritius is in agricultural use and large areas become fallow after harvest, erosion in Mauritius is a major problem. For this reason, the Universities of Amsterdam and Mauritius have inventoried the areas most vulnerable to erosion. For this they used an erosion model within a geographic information system (GIS, see box) with which the potential erosion was calculated per grid cell of 100 by 100 meters for the entire island.
The model takes into account natural and human factors. The natural factors are rainfall, topography and soil type. The rainfall varies widely across the island, from 600 mm/year on the (west) coast to as much as 4000 mm/year on the central plateau. Moreover, precipitation is unevenly distributed throughout the year. Seventy percent of the annual amount of rain falls between November and April. One of the human factors is the way the land is worked. Farmers can work in such a way that the soil is not disturbed as much and therefore does not wash away so easily. The force with which raindrops fall on the soil can also be reduced:a dense foliage and plant roots below the surface retain the soil particles.
The interaction between human and natural factors is especially important. The risk of erosion is greatest when there is a lot of rain and the ground cover is minimal. Correct crop selection and appropriate cultivation methods can limit the influence of rainfall. Vegetables are harvested every year, leaving the land fallow. With sugar cane, the soil is disturbed less often because the cane stems can be harvested for seven years while the roots remain in the ground. This applies even more to tea and banana plantations.
Historic land use
Dutch settlers started cutting down rainforest from 1638 and founded sugar cane plantations almost everywhere. The French settlers continued this policy. The areas that were deforested between 1773 and 1835 appear to be the least prone to erosion. They are mainly located in the northern part of the island where there is little rainfall.
In 1810 the British conquered the island from the French. Because the forest in the low-lying areas had already been cleared, the British settlers moved into the inland mountains. The areas that were deforested between 1835 and 1872 are very sensitive to erosion. They are located on the central plateau (with a lot of precipitation) and on the steep parts in the east of the island. The parts that were deforested in recent decades (1872-2005) also appear to be very sensitive to erosion due to their location on steep slopes.
Model calculations
From the model calculations we know how great the potential soil erosion was when the entire island was still covered with rainforest. It has also been determined how great the potential erosion will be when the entire island is deforested. If all the forest is replaced by sugar cane, the potential soil erosion will increase by almost half. In other words, if all of Mauritius were covered with sugar cane, the island would be one and a half times more prone to erosion than it was before humans came to live there.
However, the relationship between land use change and the amount of soil erosion is not linear. In an area with steep slopes and heavy precipitation, the effect is much greater than in a flat area with little rainfall.
Now and in the future
Currently, forty percent of the island's surface is occupied by sugar cane fields. Various scenarios have been worked out in which sugar cane is converted to other land use. A scenario in which a part changes into an urban area is very likely.
A scenario in which sugar cane is replaced by other crops is also an obvious one. Many small-scale sugar farmers will stop growing sugar cane in the near future. Their plots are located on steep slopes that are very prone to erosion. Some will switch to food crops, with the amount of soil erosion depending on the type of crop. Other farmers will leave their land. There the soil erosion will first increase, after which the soil will become overgrown again and the erosion will decrease. The development of soil erosion therefore strongly depends on the choices that landowners make in the future.
This article was previously published in Geography. The authors are affiliated with the Institute for Interdisciplinary Studies, Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam &University of Mauritius.