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Friday 1st October 2010
LOSS OF AN APEX PREDATOR
Even though lions have suffered local extinctions in very many locations (North Africa, southern Europe, the Middle East, western Africa) and have undergone drastic population declines in the rest of Africa, there is insufficient “before and after” information to be able to decide what the impact of the loss has meant. In addition, once the lions were gone, many areas in which they used to occur have since been completely converted to livestock and agriculture.
However, there are some parallels that can be used to at least generate a “most likely” indication of what could happen, and in addition, there are some careful studies of the changes in environment that took place after a top predator was re-introduced, many of them unexpected.
The first effect of apex-predator loss is that there is an increase in the so-called mesopredators, the medium-sized ones in the system. A recent paper (L.R. Prugh et al., 2009. The rise of the mesopredator. Bioscience 59 (9):779-791) concludes that the effects of such mesopredators on ecosystems cause major economic and ecological disruptions in both terrestrial and marine ecosystems.
The authors begin by stating that humans have persecuted apex predators in many ecological systems for years, and mention specifically the loss of wolves, bears, and pumas in the USA, lions and leopards in Africa, and jaguars and pumas in southern America. Conflict with humans and livestock is the main reason, and specific eradication programmes to remove all large predators (to increase game for hunters and to eliminate threats to humans and livestock) have occurred especially in the United States. However, rather than resolving the problem, unexpected increases in other species occurred, and it was a failure to consider the real complexities of ecosystems that resulted in subsequent problems. The authors state that apex predators could actually benefit prey populations indirectly by keeping numbers of mesopredators down. Failure to recognize such interactions within the predator guild has led to unforeseen consequences.
Mesopredator “release” is accompanied by some very interesting examples of ecosystem changes as described in the paper – decimation of lion and leopard populations in Africa has led to a population explosion of baboons. These in turn prey on the young of antelopes, cause significant crop damage, raid the nests of many bird species, and even keep children out of school to help protect maize fields from ravenous troops of these monkeys. This cascade of events was totally unanticipated. Similarly, the eradication of sharks led to a release of cow-nosed rays, a major predator of Atlantic Bay Scallops, leading to a collapse of the scallop fishery in many areas.
The authors also state that overabundant mesopredators are often resilient to control programs as they are characterized by high densities, high rates of recruitment, and high rates of dispersal. A good example in a very simple system would be a decision to eliminate the apex predator (cats) from your farmyard – rats would have a heyday, and at least invade your grain stores, your cupboards, eat chicks and ducklings and raid wild bird nests, and probably spread a variety of diseases directly affecting humans. You might be able to call a pest control company and try to bring the rat population under control, but brown rats with a gestation period of 21 days, a litter of up to 14 pups, and five litters per year can overwhelm any human control program. Brown rats have spread to all continents, and there are an estimated 81 million in the UK alone.
APEX PREDATOR RESTORATION
An interesting example of what happens when an apex predator is restored to an ecosystem occurred with the reintroduction of wolves to Yellowstone National Park. Eliminated in 1926, wolves were restored in 1995. What happened? Willow trees, cottonwoods, and aspen trees are growing again, as the elk populations that ate all the seedlings are now wary of entering wooded areas and have been reduced in numbers. Coyote numbers (a mesopredator) have fallen by half, leading to prey increases for red foxes and raptors. Beaver numbers have increased with the return of trees, building more ponds, and creating more diverse habitats. Bird numbers and diversity have increased as well. The study continues, but the results of the reintroduction of wolves are fascinating and largely unexpected, and are based on careful before and after studies.
UNCERTAINTIES IN HIGHLY COMPLEX SYSTEMS
Given those kinds of cascade observations we can at least construct some likely scenarios if lions were to be removed with some important caveats. First, with the exception of Kruger Park in South Africa and possibly the Serengeti in Tanzania, detailed studies of the components of ecosystem complexity have largely not been recorded for protected areas where lions occur. In addition, abiotic factors in Africa are hugely important in determining animal and plant populations. Droughts are frequent, and have a tremendous effect on recruitment of all species. Bush fires are frequent and often intentionally set, destroying saplings and small animal species alike. Also, disease epidemics like rabies and canine distemper can result in local extinctions of many carnivore populations. And finally, the predator/omnivore guild in Africa consists of many more members than the simpler systems in Yellowstone. Just as an example, in my former study area in Botswana, there were at least four species of mongooses; two species of genets; honey badgers; Black-backed and Side-striped jackals, Bat-eared foxes, Cape foxes and African wild dogs; spotted hyenas and aardwolves; African wild cats, servals, caracals, cheetahs, leopards, and lions; and of course baboons. This is not counting raptors, snakes, monitor lizards, crocodiles, etc.
To remove lions from this massive equation and predict a result is not easy, but it can be attempted. The first result would be that large-bodied herbivores – buffalos, hippos, zebras, giraffes, and even elephants would be largely released from predator pressure, though hyenas, for example, could still prey on their newborns. The second result is that medium-sized herbivores could be much more heavily targeted, as populations of African wild dogs, hyenas, cheetahs, and leopards would increase with loss of lions, and this would result in a decimation of impalas for example. Loss in numbers of this predominantly browsing species and highly popular prey item might have in turn an effect on vegetation. Eventually the loss of impalas could lead to increased competition among medium-sized carnivores competing for the same resource. A release of large herbivores from direct predator pressure might result in their populations expanding to the detriment of smaller species, possibly resulting in a depauperate herbivore fauna. In other words another cascade of consequences that should rather be avoided.
COMPLEXITY IS STILL BEYOND PREDICTION
The bottom line is that despite many years of study we still can only underestimate the complexities of many ecosystems. The Bioscience paper indicates that we have not even properly considered the effect of an apex predator on mesopredators, and when the latter take over, considerable and unexpected results can accumulate. Biologists tend to use predictive computer models to anticipate the effects of disruptions, but models are only as robust as the value of the input data, and this is most often incomplete. Best scenario – keep what you have, because when you lose something, consequences are unpredictable and full restoration highly uncertain.
Posted by Pieter Kat at 15:22
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