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).