Does the disruption of the keystone interaction by
non-native ungulates produce cascading effects on the community?
Network complexity - We found differences on network complexity
in pollination networks (F 1,7 = 5.032, P =
0.029) but only marginal in seed dispersal networks
(F 1,7 = 3.849, P = 0.087) between intact
forest sites with the keystone interaction and sites invaded by
non-native ungulates. The complexity on both pollination and seed
dispersal networks was lower in invaded sites than in sites with the
keystone interaction, exhibiting 23% and 59% fewer motifs
respectively, and hence, fewer direct and indirect ecological
interactions supporting the community (Fig. 4a and 4c). In addition,
both pollination and seed dispersal networks exhibit 12% and 11% fewer
frequency of largest motifs integrated by six species in invaded sites
than in sites with the keystone interaction (Fig 4b and 4d).
Functional redundancy among generalist species - The number of
pollinator and plant species that occupied generalist positions was
twice as high in intact forest sites with the keystone interaction than
in invaded sites (Fig. 5a; Table S3). Moreover, in presence of the
keystone interaction a great number of pollinator species occupied the
most generalist position providing more network stability. On the other
hand, the low functional redundancy among generalist species found in
invaded sites suggests low stability in pollinator networks. This
analysis was not possible in seed dispersal networks because of the
reduced number of frugivorous species.
Ecological importance of the keystone interaction members - We
found differences in the ecological importance of the members of the
keystone interaction between intact forest sites with the keystone
interaction and invaded sites (Fig. 5b). In sites with the keystone
interaction, the number of positions occupied by the mistletoe was at
least three times higher than in invaded sites
(Z 1,7 = -2.998, P = 0.002), suggesting a
greater ecological importance of the mistletoe in the pollination
network. In addition, we caught no individuals of D. gliroides in
any field season in the invaded sites, which indicates that the
marsupial may be ecologically extinct in these sites (Rodriguez-Cabal &
Branch 2011). Consequently, we were unable to assess statistically the
ecological importance of the marsupial between treatments. Furthermore,
we found no differences in the ecological importance of the hummingbird
between treatments (Z 1,7 = -0.715, P =
0.475).