The rapid expansion of the human population in recent years has resulted in the natural habitats of many animals being altered substantially.  This has occurred without sufficient time for natural selection to have yet had a significant effect on adjusting the behaviour of many animals, for instance in response to newly introduced species. We make use of signal detection theory, within a wider theoretical framework of state-dependent modelling, to consider the effect of rapid changes.   We allow thresholds for action to be a function of an animal’s reserves, and show how those optimal thresholds can be calculated.  We term this framework, ‘state-dependent detection theory’.  By altering the environment, we show various effects of animals using those formerly adaptive traits (the previously evolved thresholds).  Introducing novel animals which appear dangerous – but are actually safe – can have catastrophic consequences for a species.  We show that following some forms of HIREC, even though individuals may have a similar (or even increased) expected lifespan, they may reproduce far less often, leading to a collapse in the population.  Although the effect of HIREC is often difficult to predict, we suggest that state-dependent detection theory is a useful route ahead.