  The dynamic network of relationships among corporations underlies cascading
economic failures including the current economic crisis, and can be inferred
from correlations in market value fluctuations. We analyze the time dependence
of the network of correlations to reveal the changing relationships among the
financial, technology, and basic materials sectors with rising and falling
markets and resource constraints. The financial sector links otherwise weakly
coupled economic sectors, particularly during economic declines. Such links
increase economic risk and the extent of cascading failures. Our results
suggest that firewalls between financial services for different sectors would
reduce systemic risk without hampering economic growth.
