The audience is asking questions to Vicente Guallart.[Photo provided to chinadaily.com.cn]

When asked about how cities are supposed to be planned to respond to increasing populations, Guallart believes that it is not a problem for Europe.

"European countries have aging societies with a stable demographic structure, so population is less of a worry in city planning there," says Guallart.

Yu has a positive attitude when it comes to tacking this problem.

In his eyes, urbanization means improving living standards, and he says people tend to give birth to fewer children when they get rich. Therefore, as long as cities develop to some extent, the population will gros in a slow pace.

Since Sep 27, an exhibition on the theme "ecological urbanism" is on at Cervantes Institute. The exhibition, which runs until December 2, features works by Guallart and Yu showcasing their ideas about city development and ecology protection.