They're known as the big five: Africa's elephants, lions, rhinos, leopards and buffalos. Once the prize-animals hunters would aim to bag while in Africa, now they are THE animals everyone must see on safari when visiting the continent.

But it's getting harder to get a glimpse of big mammals, like these, and soon it might even be impossible, according to a recent article published in the Oxford Journal, BioScience. Large mammals are at a higher risk of becoming extinct than their smaller counterparts, and, if nothing changes, many could die out before the start of the 22nd century, say the conservation scientists who penned the piece.

"Under a business-as-usual scenario, conservation scientists will soon be busy writing obituaries for species and subspecies of megafauna as they vanish from the planet," they wrote.

Around 59 percent of the world's largest carnivores and 60 percent of the world's largest herbivores are already threatened with extinction, according to the article, which cites the International Union for Conservation of Nature's (IUCN) Red List. The situation is particularly dire in sub-Saharan Africa and Asia, where there remains the biggest and most diverse populations of large mammals.

Call for action

David Whyte Macdonald, a director of the Wildlife Conservation Research Unit at the UK's University of Oxford, and Sarah Durant, senior research fellow at the Zoological Society of London, are among more than 40 conservationists, who have put their name to the article.

The scientists also wrote a declaration vowing to acknowledge the declining population of most terrestrial megafauna species and urging 'individuals, governments, corporations and nongovernmental organizations to stop practices that are harmful to these species and actively engage in helping to reverse the decline in megafauna.'

Deforestation, agriculture, livestock and human encroachment particularly impact large mammals, which are vulnerable due to their need for large areas with low population densities. Overhunting and trafficking of animals for their meat and body parts, which are used in traditional medicine or for ornaments, like ivory, also play a big part in threatening populations.

Impact on ecotourism

As well as having a potentially catastrophic impact on the numbers of these large mammals that remain, scientists warn that the disappearance of some of these iconic creatures could also affect 'economic and social services' like ecotourism.

Tourism makes up 9 percent of global GDP, and one in 11 jobs. Ecotourism accounts for around a quarter of tourism and is continuing to grow.

"Ecotourism is the fastest growing subsector of tourism in developing countries and megafauna are a major draw for these tourists," according to the article.

The declaration from the scientists calls on the international community to 'take necessary action to prevent mass extinction of the world's megafauna and other species.'

"Successfully conserving megafauna requires bold social, political and financial commitments from nations around the world," wrote the authors. "Through understanding the value and importance of local human needs and by combining international financial support with a coordinated multilateral approach to conservation, it may be possible to rescue megafauna from the brink of extinction."