for food due to their dominance of the forest floor. Predators Adult red crabs have no natural predators on Christmas Island. The yellow crazy ant, an invasive species accidentally introduced to Christmas Island and Australia from Africa, is believed to have killed 10–15 million red crabs (one-quarter to one-third of the total population) in recent years. In total (including killed), the ants are believed to have displaced 15–20 million red crabs on Christmas Island. During their larval stage, millions of red crab larvae are eaten by fish and large filter-feeders such as manta rays and whale sharks which visit Christmas Island during the red crab breeding season. Coconut crabs (alternatively known as robber crabs) have also been filmed on Christmas Island preying on red crabs. Early inhabitants of Christmas Island rarely mentioned these crabs. It is possible that their current large population size was caused by the extinction of the endemic Maclear's rat, Rattus macleari in 1903, which may have limited the crab's population. Population Surveys have found a density of 0.09–0.57 adult red crabs per square metre, equalling an estimated total population of 43.7 million on Christmas Island. Less information is available for the population in the Cocos (Keeling) Islands, but numbers there are relatively low. Based on genetic evidence, it appears that the Cocos (Keeling) red crabs are relatively recent immigrants from Christmas Island, and for conservation purposes the two can be managed as a single population. Relationship with humans During their annual breeding migration, red crabs will often have to cross several roads to get to their breeding grounds and then back to the forest. As a result, thousands of red crabs are crushed by vehicles and sometimes cause accidents due to their tough exoskeletons which are capable of puncturing tires. To ensure the safety of both the crabs and humans, local park rangers work to ensure that crabs can safely make their journey from the centre of the island to the sea; along heavily travelled roads, they set up aluminium barriers whose purpose is to funnel the crabs towards small underpasses so that they can safely traverse the roads. Other infrastructure to assist the crab migration includes a five-metre-high "crab bridge". In recent years, the human inhabitants of Christmas Island have become more tolerant and respectful of the crabs during their annual migration and are now more cautious while driving, which helps to minimise crab casualties.