The exact cause of the decline of the Olmec culture is uncertain. Between 400 and 350 BC, the population in the eastern half of the Olmec heartland dropped precipitously. This depopulation was probably the result of serious environmental changes that rendered the region unsuited for large groups of farmers, in particular changes to the riverine environment that the Olmec depended upon for agriculture, hunting and gathering, and transportation. These changes may have been triggered by tectonic upheavals or subsidence, or the silting up of rivers due to agricultural practices. Within a few hundred years of the abandonment of the last Olmec cities, successor cultures became firmly established. The Tres Zapotes site, on the western edge of the Olmec heartland, continued to be occupied well past 400 BC, but without the hallmarks of the Olmec culture. This post-Olmec culture, often labeled Epi-Olmec, has features similar to those found at Izapa, some 550 km (330 miles) to the southeast.