  Online communities are becoming increasingly important as platforms for
large-scale human cooperation. These communities allow users seeking and
sharing professional skills to solve problems collaboratively. To investigate
how users cooperate to complete a large number of knowledge-producing tasks, we
analyze StackExchange, one of the largest question and answer systems in the
world. We construct attention networks to model the growth of 110 communities
in the StackExchange system and quantify individual answering strategies using
the linking dynamics of attention networks. We identify two types of users
taking different strategies. One strategy (type A) aims at performing
maintenance by doing simple tasks, while the other strategy (type B) aims
investing time in doing challenging tasks. We find that the number of type A
needs to be twice as big as type B users for a sustainable growth of
communities.
