global scale. Some evidence, however, suggests that vegetation changes preceded megafaunal extinctions. Pleistocene grazers limited wildfire by consuming biomass in grassland ecosystems; the decline of these grazers increased grassland fire frequency. Some hypothesize that forbs in modern grasslands are adapted to disturbance by large herbivores on former mammoth steppes, and are currently in decline due to the extinction of Pleistocene megafauna. In Europe, evidence from the pollen record suggests that megafauna promoted open vegetation with shifting mosaics of forest and grassland, however this hypothesis is debated. In the Yukon region of Canada, the decline of Mammuthus and Equus may have contributed to the development of woody flora. In Siberia and Beringia, the extinction of mammoths may have contributed to the expansion of Betula (birch) tree cover. In Australia, however, megafaunal extinction may not have caused significant changes in fire regimes or vegetation. Some argue that modern ecosystems can be understood by considering the effects of extinct megafauna. Anachronistic plants Some extant plants have adaptations resulting from interactions with Pleistocene megafauna, including defenses against Pleistocene megaherbivores, and large fruits adapted to dispersal by megaherbivores. Such species are termed anachronistic plants. Members of the plant family Rosaceae (including Malus, apples) evolved large fruits as an adaptation to dispersal by megafaunal herbivores. Megafaunal extinction caused the decline of Cucurbita species which relied on megaherbivores for dispersal. Cucurbita fruits contain cucurbitacins, bitter compounds that are toxic to small herbivores but can be tolerated by megafauna; Cucurbita seeds have been found in fossil mastodon dung. Some argue that plant adaptations to megafauna resulted in traits that allowed for domestication. Human dispersal may have prevented the range contraction of Neotropical species with fruit formerly dispersed by megafauna. Effects on climate Megafaunal extinction may have caused global cooling of the Earth's climate due to reduced methane emissions from megaherbivores and increased woody vegetation associated with reduced trampling and browsing. Smith et al. suggest that megafaunal extinctions in the Americas contributed to the Younger Dryas cooling event. Extinction causes Four theories have been advanced as likely causes of these extinctions: hunting by the spreading humans (or overkill hypothesis, initially developed by geoscientist Paul S. Martin), the change in climate at the end of the last glacial period, disease, and an extraterrestrial impact from an asteroid or comet. These factors are not necessarily exclusive: any or all may have combined to cause the extinctions. Of these, climate change and