Herd numbers grew from 28 to over 165 between 1968 and 1997 and overgrazing negatively impacted their living environment. To manage population numbers, long-term, non-hormonal contraceptives have been employed, proving 95 percent effective over a seven-year field trial. The contraceptive, which began to be used at a management level in 1995 although it was used in smaller amounts as early as 1989, has also proven effective at improving the health and increasing the life expectancy of older mares through the removal of pregnancy and lactation-related stress. Since 1990, general herd health has improved, early mortality has decreased and older ponies are now found, with many over the age of 20 and some even over 25. No horse has ever been injured during the dart-administered treatments, although there is a 0.2 percent rate of abscess at the injection site, which normally heals within two weeks. Each mare between two and four years old is given contraceptives, and treatment is then withdrawn until she produces a foal. Once she has produced enough foals to be well represented genetically within the herd, she is placed on a yearly treatment plan until her death. After the introduction of the contraceptive, herd numbers continued to rise to a high of 175 in 2001 to 2005, but then dropped significantly to around 130 in 2009. In 2009, a study determined that mitochondrial DNA diversity in the herd was quite low, most likely due to their isolation, but that their nuclear genetic diversity remained at a level similar to that of breeds from the mainland.