Conservation is often complicated.
Lions have suffered dramatic population and geographic range collapse owing to human activities. They now occur in less than 30% of their historic range and total numbers have fallen by 70% to 33,000 since 1980. Retaliatory killing in the wake of livestock depredations, habitat loss and fragmentation, and prey loss are the main factors responsible for these declines. As bad as things are, the problem is worsening.
Dr. Craig Packer (U. Minnesota) organized a host of scientists and managers who studied lion populations across 11 African nations and 42 protected areas, including me and my colleagues. Each of us furnished population trends over at least 8 years and details of their management budgets; some areas were enclosed by fences (black symbols) and others were not (red symbols).
Reserves varied in how closely lion populations approached the reserve's carrying capacity (horizontal red line in each plot, estimated by ecological models) and their population trends. Generally, fenced reserves (black labels) held more stable populations and these were closer to carrying capacity than unfenced populations (red labels). The Taita (Kenya) reserve documents the population studied by MacArthur Curator Bruce Patterson and his colleagues and volunteers.
Depressed growth should only characterize populations above the carrying capacity (100%), as was the case for the fenced reserves (line). The unfenced reserves were exposed to a variety of density-independent factors affecting growth.
Fenced reserves were far more likely to maintain lion populations over the medium and long-term. This analysis shows the percentage of populations expected to persist at > 10% of their carrying capacity into the future.
The most remarkable finding was the effect of expenditures needed to manage the reserves. $500 per km2 per year permitted a fenced reserve to maintain lions near 80% of capacity. Four times(!) as much money was needed to maintain an unfenced population at only half its capacity.
There are many reasons to be unhappy with these results, but the study’s conclusions seem inescapable. Despite their detrimental effects on gene flow and migratory species, fences are effective in containing large predators--protecting surrounding communities from their depredations. They also exclude human and livestock encroachment and resource extraction (via bushmeat and grazing) from reserves. Fencing reserves requires substantial capital investment and entails additional management expenditures to ensure outbreeding, etc. However, it may represent the only means to protect a growing African population from large and dangerous predators, and vice versa!
To read more about this work, read our recent article:
To learn more about Bruce Patterson's research program, visit his webpages: