Chapter 1, pt3: Porto Praya

Modern-day aerial image of Praia, the capital of Cape Verde. Licensed under Public Domain via Wikimedia Commons. 

On the 16th of January 1832 we anchored at Porto Praya, in St. Jago, the chief island of the Cape de Verd archipelago.

The neighbourhood of Porto Praya, viewed from the sea, wears a desolate aspect. The volcanic fires of a past age, and the scorching heat of a tropical sun, have in most places rendered the soil unfit for vegetation. The country rises in successive steps of table-land, interspersed with some truncate conical hills, and the horizon is bounded by an irregular chain of more lofty mountains. The scene, as beheld through the hazy atmosphere of this climate, is one of great interest; if, indeed, a person, fresh from sea, and who has just walked, for the first time, in a grove of cocoa-nut trees, can be a judge of anything but his own happiness. The island would generally be considered as very uninteresting, but to any one accustomed only to an English landscape, the novel aspect of an utterly sterile land possesses a grandeur which more vegetation might spoil. A single green leaf can scarcely be discovered over wide tracts of the lava plains; yet flocks of goats, together with a few cows, contrive to exist. It rains very seldom, but during a short portion of the year heavy torrents fall, and immediately afterwards a light vegetation springs out of every crevice. This soon withers; and upon such naturally formed hay the animals live. It had not now rained for an entire year. When the island was discovered, the immediate neighbourhood of Porto Praya was clothed with trees, the reckless destruction of which has caused here, as at St. Helena, and at some of the Canary islands, almost entire sterility. 

Cape Verde is an archipelago of ten volcanic islands, arising from what geologists refer to as a "hotspot" — an area where, according to the leading hypothesis, a plume of hot mantle material is rising from the core of the Earth, leading to the production of magma and its eruption onto the Earth's surface. The Cape Verde archipelago is shaped something like a sideways V, and the largest of the islands, located on the southern limb of the V, is Santiago (or St. Jago), where Darwin landed in 1832. At the time, Cape Verde was a Portuguese possession. It is now a sovereign nation, having gained its independence in 1975 after more than 500 years under Portuguese control.

Satellite view of Cape Verde in October 2010. Image by NASA.

Cape Verde was uninhabited when it was first discovered by European explorers in 1456. Alvise Cadamosto, a Venetian slave trader and explorer, was among a party who landed on Santiago to take on water in May of that year. It would have been near the end of the dry season, but Cadamosto noted that the island "appeared well-wooded" and that his sailors had found the mouth of a river from which to collect fresh water. Diogo Gomes, a Portuguese explorer who visited the island a few years later, had the following to say:

...on the shore we found many strange birds and stream-lets of fresh water. ... There were also an abundance of figs, but they do not grow on the trees in the same manner as in our parts, for our figs grow near the leaf, but these all over the bark from the foot of the tree to the top. These trees grow in great numbers, and there was great quantity of grass there.

Ripening fruits of the Cape fig (Ficus sur), which is found on Cape Verde and throughout much of Africa. Photograph by JMK - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons.

Explorers like Gomes may have been somewhat exaggerating the wonders of the newly discovered islands in order to tempt Portuguese settlers to colonize them; however, it seems likely that at the time of their discovery, the islands of Cape Verde were dominated by a well wooded savanna ecosystem. Today, like in Darwin's time, the islands tell a different story. Santiago's mountainous interior remains relatively green and fertile, but the lower "table-lands" described by Darwin are dusty and dry.

Satellite image of Santiago, with Praia in the bottom right corner. The island is dominated by a dry, brown landscape with only a few faint traces of green. Image by NASA.

Why had the environment of Cape Verde changed so much since the 15th century? To Darwin and his contemporaries, this was a familiar story. As he mentions in his account, other Atlantic islands settled by Europeans had experienced similar environmental degradation. Darwin ascribed Cape Verde's "desolate aspect" quite simply to deforestation — but that likely isn't the whole story.

A herd of goats on the Cape Verde island of Fogo. Image by Cayambe - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons.

A 1996 paper by Per A. Lindskog and Benoît Delaite conducted a detailed review of the environmental history of Cape Verde. Rather than attributing Cape Verde's environmental degradation to deforestation alone, Lindskog and Delaite point out a more complex set of factors. While humans would undoubtedly have felled trees for use as construction material and firewood, they also introduced another agent of environmental change to the islands: goats.

"Cape Verdean goat" by David Trainer - Flickr. Licensed under CC BY-SA 2.0 via Wikimedia Commons.

It's hard to overstate the impact that the introduction of livestock can have on a landscape that has never seen ungulates before. Cape Verde, in fact, was mammal-free before European colonization, with the exception of bats. Within a few years of the islands' discovery, Europeans had brought livestock — including goats, donkeys, and even rabbits — to the archipelago. The goats in particular went feral, multiplying even on uninhabited islands into sizable herds. By the early 16th century, Portugal was receiving regular shipments of goat and cattle skins from the islands, and a traveler visiting the islands around 1545 noted reports that tens of thousands of goat skins were produced every year.

Overgrazing by goats likely was one of the key factors in the environmental degradation of Cape Verde. Unlike cattle, goats pull up plants' roots as they graze, gaining as much sustenance as possible. In addition, large herds of goats trampled the newly denuded landscape, accelerating the process of erosion. On steep-sloped volcanic islands like those of Cape Verde, bare soil can be rapidly swept away by wind or water, making it harder for plants to reestablish themselves.

A view of inner (and greener) Santiago, showing the importance of plant cover in preventing erosion on steep volcanic slopes. Image via Wikimedia Commons.

Scientists have even been able to quantify the rate of erosion on Cape Verde. Estimates vary widely by location and time period, but a useful index is how long it would take for a centimeter of topsoil to erode from the land surface. In different areas of Santiago, this could take as long as 65,000 years or as little as 3 years. One study even found that on the island of São Nicolau, nearly a centimeter of topsoil eroded during two rain events in a 12-day period in 1981.

Although erosion rates vary widely, the vast majority of them are much faster than the soil can be replaced. Based on a study of soil formation rates in basalt in a comparable semi-arid environment (Queensland, Australia), it takes over 30,000 years for a single centimeter of soil to form from basalt bedrock. That means that in the 560 years since Cape Verde's discovery, less than two millimeters of topsoil has formed.

Darwin may not have known the numbers behind Cape Verde's environmental degradation, but he seems to have grasped the situation immediately — a "sterile" landscape, dotted with flocks of goats, subject to infrequent but heavy torrents of rain; once wooded and fertile, now characterized by "wide tracts of lava plains" with scarcely a green leaf in sight.