NEW WORLDS - The New Wild: Why Invasive Species will be Natures Salvation - page 22

made up an estimated 80 percent of the lake's fish population. Edward O. Wilson called the loss of so
many of these primitive fish “the most catastrophic extinction episode of recent history.” 32 But some
question whether the appetite of the Nile perch is to blame for their demise.
Dirk Verschuren of the University of Ghent suggests that the real damage was done by pollution.
The demise of the cichlids coincided with the accumulation of polluting nutrients in the lake that both
fed the water hyacinth and exhausted oxygen supplies in the lake bottom. That process was under way
before the first perch were dropped into the lake, and it intensified subsequently. The deep waters were
occasionally devoid of oxygen in the 1960s and regularly so by the 1980s, which is when both water hy-
acinth and Nile perch started taking over the lake. Fighting for breath and forced to leave their lakebed
lairs, the cichlids became easy meat for the perch—which duly exploded in numbers while eating their
way through the indigenous fish stocks, says Verschuren. 33
There has been a partial revival in cichlid numbers—and some increase in species surveyed
too—since the early 1990s, in the aftermath of the lake's flushing. Much remains unclear, but it seems
likely that it was the demise of the cichlids that allowed the takeover by the Nile perch rather than the
other way around. 34 The lesson from Lake Victoria is that in two highly publicized cases, the bursts of
activity by alien species were made a scapegoat for much wider problems of pollution and environment-
al decline in the lake's catchment. The aliens took advantage of the environmental crisis, but they did
not cause it. Demonizing the alien species obscured this and prevented effective action to halt the prob-
lem.
Africa has been the victim of poor science and outrageously ill-considered policies on alien species fois-
ted on it from outside. Its environment often seems to be a pawn in wider political games, many of them
resulting in policy U-turns. How else can we explain the changing perceptions of mesquite, or Prosopis
juliflora , the thorny evergreen bush found throughout the deserts of the southwestern United States and
northern Mexico? The bush copes with drought by sinking roots tens of yards down to find water. That
makes it a good colonist in arid lands. But anyone reading the international literature on the plant will
be confused about whether we should love it or loathe it. 35
The United Nations and many international development agencies once called mesquite a “wonder
tree.” They planted it across the dry, famine-prone lands of sub-Saharan Africa from the 1970s right up
to the late 1990s. They promised that, by tapping water from deep underground and surviving where
other plants could not, it would bring life back to areas hit by spreading deserts in Ethiopia, Sudan, and
elsewhere. It would give locals an alternative source of firewood, while its nitrogen-fixing properties
improved farm soils and its gum treated minor infections. Birds would nest in it. Bees would be attrac-
ted to its nectar. Its virtues seemed endless. 36
One of the bush's most feverish and persistent advocates was the UN Environment Programme
(UNEP), which wanted to halt desertification in Africa. As recently as 1997 it said that its mesquite-
planting projects in Sudan and elsewhere were “helping communities protect their environment” by halt-
ing the desert's advance and “bringing new areas into cultivation.” 37
But minds changed. A decade later UNEP was warning against what it called “mesquite mania.” In
a topic on invasive plants in Africa, Arne Witt, the Nairobi-based UNEP consultant on invasive spe-
cies in Africa, said that “most introduced Prosopis species have turned out to be relentlessly aggress-
ive invaders . . . rampant across Morocco, Algeria, Mali, Burkina Faso, Niger, Chad, Sudan, Somalia,
Ethiopia, Kenya and South Africa.” Early benefits had been “overshadowed by their negative, invasive