Across the world’s ecosystems, from the boreal forests of North America to the savannas of Africa, a host of animals and plants constantly interact: predators fell prey, insects devour plants, epiphytes perched high in their host trees draw moisture from the air. All of these interactions influence animal and plant populations in myriad ways, but researchers are still trying to understand the complex dynamics. A working group to be held January 29-31 will look at the consequences of the changes in interactions between species—especially how these changes lead to irreversible transitions in the structure and composition of ecological communities.