Increased homogenisation (decreased beta-diversity) among biological assemblages is often interpreted as being caused by already-widespread species increasing, but individual species contributions are poorly understood. Here, we consider species contributions to beta-diversity over decades to millennia, within different taxonomic groups and environments. Species occurring in around half of sites provided the greatest contributions to differentiation at a given time, but not through time. They had a slightly homogenising effect if they declined below 0.5 or increased above 0.5 (in strongly nested communities, changes above 0.5 have little impact on differentiation). The most widespread species (>0.75 of sites) contributed little to beta-diversity change. In contrast, localised species (initially in <0.25 of sites) contributed most to both homogenisation (when declining) and differentiation (when increasing). Conservation interventions to increase localised species occurrence would do more to limit homogenisation than attempts to control already-widespread species or prevent others becoming widespread.