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The dynamic evolution of ecological systems in which predators and prey compete for survival has been investigated by applying suitable mathematical models. Dynamic systems theory provides a useful way to model interspecies competition and thus the evolution of predator and prey populations. This kind of mathematical framework has been shown to be well suited to describe evolution of economical systems as well, where instead of predators and prey there are consumers and resources. Maintenance and testing activities absorb the most relevant part of the total life-cycle cost of software. Such economic relevance strongly suggests to investigate the maintenance and testing processes in order to find new models allowing software engineers to better estimate, plan and manage costs and activities. We show how dynamic systems theory could be usefully applied to maintenance and testing context, namely to model the dynamic evolution of the effort. When programmers start trying to recognize and correct code defects, while the number of residual defects decreases, the effort spent to find out any new defect has an initial increase, followed by a decline, in a similar way as prey and predator populations do. The feasibility of this approach is supported by the experimental data about two real world software projects.