Many studies have linked chemical exposure with testicular abnormalities and reductions in sperm counts; however, the ecological impact in wildlife of reduced male fertility has been difficult to predict. Very few studies have correlated sperm counts with fertilisation success in arthropods. Following a recent investigation indicating reduced sperm counts (20%) in field collected amphipods from an industrially polluted site when compared to reference sites, this study, first aimed to review the relationship between sperm counts and fertilisation success using available data published on arthropods, and second, to predict the long-term (10 year) impacts of reduced fertilisation through the use of an existing amphipod population model. The relationships between sperm counts and fertilisation success were compared between two crustacean (Gammarus duebeni and Armadillidium vulgare) and two insect species (Ostrinia nubialis and Agrotis segetum). All species investigated demonstrated reduced fertility with reduced sperm with the relationship varying between species. Results from this study suggest that relatively small reductions in sperm can have variable and possibly dramatic impacts on fertilisation success for the species examined. For example, based on the species reviewed, a 5% reduction in sperm is predicted to give rise to between 0.4% and 9.1% reduction in female brood size dependent on the species. Modelling the effects of reduced fertilisation using a dynamic amphipod population model revealed that whilst 5% reductions in brood size allow the population to persist after 10 years (albeit at critically low densities), brood size reductions of 10% or greater result in population collapse in less than 6–7 years. Whilst, data are limited at this stage, this study highlights the potential for small scale changes in reproductive fitness to produce effects at the population level. As a consequence, a better understanding of the relationships between male fertility and female fecundity are required to adequately assess effects of contaminants such as endocrine disruptors on reproduction in invertebrates.