Mistletoes & Futility Of Models Predicting Complex Systems

Can you give an example where these changes are happening now?

My colleague, Francisco Fontúrbel, works in southern Chile. Where mistletoe is around, because it’s a reliable nectar source, the southernmost hummingbird (Sephanoides sephaniodes) becomes resident. They pollinate the mistletoes, but they also pollinate all sorts of other plants. After drought, mistletoes die, and those hummingbirds become migrants: They pack up, they follow the nectar further and further north. One study showed mistletoe deaths doubling in the dry year of 2015, and visits from hummingbirds dropped.

When the hummingbirds leave, the local plants don’t have pollinators anymore. This is predicted to trigger a community-wide cascade of extinctions, although that hasn’t been documented yet.

In Australia, large-scale research shows that mistletoe is super-important during drought as a sort of last-ditch nectar resource. But then, that same work shows that drought kills many mistletoes: In the summer of 2009, for example, there was a prolonged heat wave in Melbourne, including the hottest day ever recorded — and nearly 90 percent of a monitored set of mistletoes died. That caused a crash in bird numbers and insect-eating animals.

It’s not across the board. Some tropical systems, some temperate forest systems, are not showing those early warnings of system failure, these mistletoe deaths. But in many arid zones, and in some southern forests at higher latitudes, we’re already seeing food webs breaking down. We don’t want to ring the alarm bells and say the sky is falling, but it’s not looking good.

Are there any models yet to show where this may lead in the future?

No. There’s just so much complexity in terms of the interplay between the mistletoe plant’s natural enemies, pollinators and the host’s seed-dispersal mechanisms. We don’t have a handle on those interactions. We can do really quick-and-dirty models, but it’s just guesswork. They’re not nearly detailed enough to come up with meaningful predictions.

This seems to be a big problem I hear from many scientists: With biodiversity loss and climate change, there are so many unknowns and so many interactions, we just don’t know how badly things can go wrong, or how quickly.

Yeah, that’s right.

- Mistletoes in a warming world