Reciprocal altruism is paradoxical; theoretically the more one is trusted, the better the outcomes from one-shot prisoner's dilemmas, although for individuals the best outcomes are when trust is not reciprocated. Most real life games are not one-shot, but iterated where trust develops through past actions (Cárdenas and Ostrom, 2004). Furthermore, in social-ecological systems outcomes are driven by the biophysical context. In rangelands, our focus, low levels of biophysical variation limit the returns from trusting others and vice versa (McAllister et al., in press). Regardless of context, individuals who are too trusting always loose out. We explore trust and cooperation using agistment of livestock in Australian rangelands as an example, which is a human response to variation in rangeland resources in time and space. Agistment interactions are essentially iterated interactions, where livestock is transferred between pastoral enterprises in a commercial arrangement. The interaction occurs between a pastoralist with a shortage of forage (whether induced by rainfall deficiencies or management practices) and another who have an excess. Agistment may facilitate stock movements between pastoralists when it is not possible to maintain an economically viable herd in the long-term on a single management unit and where attempts to do so can lead to the loss of income or capital (livestock or landscape function) (Goodhue and McCarthy, 2000). We use the model of McAllister et al. (in press), which combines a landscape, with variable resourse disbribution in time and space, and humans, who have the ability to build networks for facilitating agistment. Our results show that fostering a climate of trust is critical in cooperative action. However, from an individual's point of view, one can be worse off if too much trust is placed in others (Figure 1). Even though if the need arises, trust is generally likely to develop as part of a informal institution, non-cooperative action experienced by otherwise trusting individuals implies that the informal institutions may be insufficient and formal polices which support those of an informal nature may be effective in improving outcomes from cooperative actions in rangelands.