Ecological Role and Interactions
Seed dispersal by Didelphid marsupials has been widely reported in tropical forests (Cáceres, 2002; Santori, De Moraes, & Cerqueira, 1995, 2004). A few marsupial species have also been involved in seed dispersal interactions in Australia (Ballardie & Whelan, 1986; Bass, 1990; Dennis, 2003) and even in New Zealand, where they are non-native (Dungan, O’Cain, Lopez, & Norton, 2002; Williams, Karl, Bannister, & Lee, 2000). The high incidence of frugivory and seed dispersal onDromiciops is remarkable among American and Australian marsupials, posing interesting questions about the coevolutionary processes that shaped the temperate rainforest’s native flora. Recent work in Madagascar has uncovered comparable inter-dependence between mistletoes and mouse lemurs (Cheirogaleidae). As with Dromiciops , these small mammals are active throughout the canopy and act as principal dispersers of mistletoe seeds in their habitats. They also undergo prolonged periods of torpor/hibernation during periods of low resource availability (Génin & Rambeloarivony, 2018 and references therein).
Mistletoes are shrubby stem-parasitic plants with more than 1600 species for which dispersal represents a critical link in their life cycle (Mathiasen, Nickrent, Shaw, & Watson, 2008; Nickrent et al., 2010; Norton & Carpenter, 1998). Most of these plants depend on animal vectors for transporting their seeds from the parent plant to the branches of competent host plants. Mistletoes produce ripe green fruits within the South American temperate rainforests, which are not easily detected by birds (as they depend on chromatic contrast). Nevertheless,Dromiciops are nocturnal and locate their food primarily by scent, hearing, and vision (Amico et al., 2011). Together with their capacity for colour vision at the ultraviolet-infrared spectrum (the trichromacy, discussed before), permit them to be excellent foragers at night. Seed passage through Dromiciops digestive tract is critical for T. corymbosus germination (Amico & Aizen, 2000; Amico, Sasal, Vidal-Russell, Aizen, & Morales, 2017), as revealed by experimental germination trials (close to 100% of successful germination; Amico et al., 2017). Furthermore, seed establishment is strongly favoured by Dromiciops ’ climbing behaviour, defecating seeds within suitable hosts and at adequate branch sizes (Amico et al., 2017), in turn impacting positively on the mistletoe regeneration rate in the forest (Amico et al., 2017; García, Rodríguez-Cabal, & Amico, 2009). Consequently, T. corymbosus abundance and distribution is spatially correlated with the presence of Dromiciops , both at different spatial scales (Fontúrbel et al., 2017a; García et al., 2009; Rodríguez-Cabal & Branch, 2011).
The cascade of ecological services provided by D. gliroidesextends to the whole forest community in different ways, one being the important relationship between the mistletoe T. corymbosus and the hummingbird Sephanoides sephaniodes for pollination (Aizen, 2003). This hummingbird is responsible for pollinating several species of the highly endemic woody flora in this biome (Aizen, Vázquez, & Smith-Ramírez, 2002; Armesto, León-Lobos, & Arroyo, 1996). In contrast to most temperate forests where hummingbirds migrate to warmer climates in winter, this species is resident (Aizen & Ezcurra, 1998) whenT. corymbosus is present, serving as its principal food during this period. Hence, the mutualistic relationship between the marsupial and the mistletoe determines the distribution of the plant and may have broader evolutionary consequences. The marsupial might have allowed the mistletoe T. corymbosus to retain green colouration in mature fruits, a condition to which it is preadapted by a slower ripening process in temperate forest populations (Amico et al., 2011).