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An herbivorous cichlid fish Cichlasoma ( Theraps) nicaraguense has been reported to perform brood care for the fry of a piscivorous cichlid Cichlasoma ( Nandopsis) dovii in a Nicaraguan lake. It was suggested that interspecific brood care (IBC) evolved to propagate C. dovii as a predator of another herbivorous fish Neetroplus nematopus, which is superior to C. nicaraguense in competition for breeding territory. However, it is still unclear how IBC is maintained in the face of cheaters that do not provide IBC. In this study, to elucidate the conditions for maintenance of IBC, we developed a spatially structured model, assuming a trade-off between reproduction and IBC (i.e., intra- and interspecific competition) and behavioral plasticity in IBC. The model predicted the following maintenance conditions: the probability of expressing (or suppressing) IBC in response to competitor invasion (or exclusion) is low, or, if the probability is high, then IBC is less effective for exclusion of competitors. Furthermore, we also predicted that IBC can be maintained without spatial structure if it is plastic behavior. Our model contributes to understanding of maintenance of IBC by providing empirically testable predictions. We discuss how this model is applicable to more general relationships, such as those in competitive or mutualistic systems. [ABSTRACT FROM AUTHOR]