Habitat connectivity impacts interactions between native and invasive species

While non-native species are commonly managed against by limiting population connectivity, management decisions can get complicated when a non-native species actually indirectly benefits a native one.  In a recent in-depth study that spans three trophic levels, Atobe et al. examine the metacommunity structure of two non-native species (carp and bullfrogs) and one native species (wrinkled frogs) in farm ponds with various connectivity throughout Japan.  In a complex web, they demonstrate that carp indirectly benefit the native frog species, leading to interesting conservation decisions when it comes to managing species diversity in patchy habitats.

Here’s how it works:  Carp eat both frog species, but prefer bullfrogs (as tadpoles).  Bullfrogs eat wrinkled frogs (as adults).  Carp cannot move between ponds, but bullfrogs can disperse in great numbers when ponds have greater connectivity.  In their study, the authors combine both field and laboratory experiments to test how connectivity of ponds where all three species are found affects their population levels.  They found that while ponds with higher connectivity showed a negative direct effect on native frogs due to increased bullfrog densities, these effects were indirectly mitigated by carp through the preferential predation of carp on bullfrogs.  Overall, greater connectivity led to positive indirect effects for a native species by allowing a non-native species to reduce other non-native predators.  All of which leads to interesting conservation and management questions: what is the best way to preserve connectivity in the metacommunity while still limiting the negative effects of non-native species?