What is the cause of the significant decrease in the rhino population?

Historically, Overhunting had a great impact in the declive of the 3 main rhinoceros species. Then it was called 'sport hunting'.

After conservationist laws were inforced, in the xx century, the main menace is poaching for rhino horns. But also there is a significative problem of habitat loss which is more important for island species, although it impacts all the rest.

Rhino horns are used as dagger handles in parts of the Middle East, and it is believed that rhino horns have healing properties in East Asia. Poachers sell rhino horns to criminal enterprises to satisfy growing demand in East Asia. Ivory horns are used as aphrodisiacs and hang-over cures. Rhino horn wine is also popular, and these horns are viewed as highly valued gifts.

In spite that I firmly believe that the oriental demand is the problem, is important to mention that speculation and illegal rhino hornes traffic is carried out in and outside Oriental Asia:

Texas emerging as center for illegal trade of black rhino horns

Vietnam's illegal trade in rhino horn - BBC News 

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There are 5 different species of rhinoceros in the world: Black and White rhinos living in Africa, Indian or Greater one horned (India), Sumatran rhino and Javan rhino.

In the case of the lesser known species, the causes of its decrease is loss of habitat.

Javan rhino: estimated 35 to 45 individuals left in a single population in Ujung Kulon National Park, in Java. Local conservationists, supported by Save the Rhino, are working hard to increase the habitat for this species since it is believed that the current habitat cannot support any more rhinos, and Rhino Protection Units have been set up to monitor and protect both the remaining Javan and Sumatran rhinos.

Sumatran rhino: There are fewer than 100 Sumatran rhinos left in the wild, and efforts are now being invested in captive breeding in an attempt to boost the population. There is a least one female calf born in captivity, Suci, in the Cincinnati zoo.

By the way, do you notice the long fur in the ears, hind part of the head and shoulders?. It is an extraordinary animal, isn't it?.

Although the hornes of these two magnificent species are used in traditional medicine, and also have suffered of overhunting, the key reason of its decrease is habitat loss.

Indian rhino: There are over 3000 Indian rhinos, but this number pales when it is compared with the poplation in the XIX century. For a long time sport hunting was a common practice, reports from the middle of the 19th century claim that some British officers Assan shot more than 200 rhinos. In the early 1900s, the species had declined to near extinction.

Poaching for rhinoceros horn became the single most important reason for the decline of the Indian rhino after conservation measures were put in place from the beginning of the 20th century, when legal hunting ended. From 1980 to 1993, 692 rhinos were poached in India. In India's Laokhowa, 41 rhinos were killed in 1983, virtually the entire population of the sanctuary.

Black rhino and White rhino: Game hunting before conservationist laws had its share in rhino decrease:

Today the menace comes from poaching for hornes, without any doubt:

Large-scale poaching of the now critically endangered black rhino resulted in a dramatic 96% decline from 65,000 individuals in 1970 to just 2,300 in 1993. Thanks to the persistent efforts of conservation programmes across Africa black rhino numbers have risen since the early 1990s to a current popul

ation of 5,055.

The overwhelming rhino conservation success story is that of the southern white rhino. With numbers as low as 50 left in the wild in the early 1900s, this subspecies of rhino has now increased to over 20,000 and has become the most populous of all the rhino species. The population is continuing to increase every year, however there are concerns that the unprecedented rise in rhino poaching since 2008 may bring this species back into decline if the poaching is not reduced.