Ecology

Distribution and habitat
The jaguar has been attested in the fossil record for two million years and it has been an American cat since crossing the Bering Land Bridge during the Pleistocene epoch; the immediate ancestor of modern animals is Panthera onca augusta, which was larger than the contemporary cat. Its present range extends from Mexico, through Central America and into South America, including much of Amazonian Brazil. The countries included in this range are Argentina, Belize, Bolivia, Brazil, Colombia, Costa Rica (particularly on the Osa Peninsula), Ecuador, French Guiana, Guatemala, Guyana, Honduras, Mexico, Nicaragua, Panama, Paraguay, Peru, Suriname, United States and Venezuela. The jaguar is now extinct in El Salvador and Uruguay. It occurs in the 400 km² Cockscomb Basin Wildlife Sanctuary in Belize, the 5,300 km² Sian Ka'an Biosphere Reserve in Mexico, the approximately 15,000 km² Manú National Park in Peru, the approximately 26,000 km² Xingu National Park in Brazil, and numerous other reserves throughout its range.
The inclusion of the United States in the list is based on occasional sightings in the southwest, particularly in Arizona, New Mexico and Texas. In the early 1900s, the jaguar's range extended as far north as the Grand Canyon, and as far west as Southern California. The jaguar is a protected species in the United States under the Endangered Species Act, which has stopped the shooting of the animal for its pelt. In 2004, wildlife officials in Arizona photographed and documented jaguars in the southern part of the state. For any permanent population to thrive, protection from killing, an adequate prey base, and connectivity with Mexican populations are essential. On February 25, 2009 a 118 lb Jaguar was caught, radio-collared and released in an area southwest of Tucson, Arizona; this is farther north than had previously been expected and represents a sign that there may be a permanent breeding population of Jaguars within southern Arizona. It was later confirmed that the animal is indeed the same male individual (known as 'Macho B') that was photographed in 2004 and is now the oldest known Jaguar in the wild (approximately 15 years old.) On Monday March 2, 2009, Macho B, which is the only jaguar spotted in the U.S. in more than a decade, was recaptured and euthanized after he was found to be suffering from kidney failure. Some experts believe that the stress from the repeated captures and sedation caused the weak kidney to fail. The death of Macho B is a significant blow to the recovery of the jaguar.
Completion of the United States–Mexico barrier as currently proposed will reduce the viability of any population currently residing in the United States, by reducing gene flow with Mexican populations, and prevent any further northward expansion for the species.
The historic range of the species included much of the southern half of the United States, and in the south extended much farther to cover most of the South American continent. In total, its northern range has receded 1000 kilometers southward and its southern range 2000 km northward. Ice age fossils of the jaguar, dated between 40,000 and 11,500 years ago, have been discovered in the United States, including some at an important site as far north as Missouri. Fossil evidence shows jaguars of up to 190 kg (420 lb), much larger than the contemporary average for the animal.
The habitat of the cat includes the rain forests of South and Central America, open, seasonally flooded wetlands, and dry grassland terrain. Of these habitats, the jaguar much prefers dense forest; the cat has lost range most rapidly in regions of drier habitat, such as the Argentinian pampas, the arid grasslands of Mexico, and the southwestern United States. The cat will range across tropical, subtropical, and dry deciduous forests (including, historically, oak forests in the United States). The jaguar is strongly associated with water and it often prefers to live by rivers, swamps, and in dense rainforest with thick cover for stalking prey. Jaguars have been found at elevations as high as 3800 m, but they typically avoid montane forest and are not found in the high plateau of central Mexico or in the Andes.

Ecological role
The adult jaguar is an apex predator, meaning that it exists at the top of its food chain and is not preyed on in the wild. The jaguar has also been termed a keystone species, as it is assumed, through controlling the population levels of prey such as herbivorous and granivorous mammals, apex felids maintain the structural integrity of forest systems. However, accurately determining what effect species like the jaguar have on ecosystems is difficult, because data must be compared from regions where the species is absent as well as its current habitats, while controlling for the effects of human activity. It is accepted that mid-sized prey species undergo population increases in the absence of the keystone predators and it has been hypothesized that this has cascading negative effects. However, field work has shown this may be natural variability and that the population increases may not be sustained. Thus, the keystone predator hypothesis is not favoured by all scientists.
The jaguar also has an effect on other predators. The jaguar and the cougar, the next largest feline of the Americas, are often sympatric (related species sharing overlapping territory) and have often been studied in conjunction. Where sympatric with the jaguar, the cougar is smaller than normal and is smaller than the local jaguars. The jaguar tends to take larger prey and the cougar smaller, reducing the latter's size. This situation may be advantageous to the cougar. Its broader prey niche, including its ability to take smaller prey, may give it an advantage over the jaguar in human-altered landscapes; while both are classified as near-threatened species, the cougar has a significantly larger current distribution.