Sociality and cooperative rearing may have evolved to increase direct fitness when conditions are challenging to reproduction and/or to reduce environmentally induced variance in fecundity. Examination of these hypotheses comes mostly from studies on singularly breeding birds where reproduction is monopolized by a male-female adult pair. Instead, little is known about plurally breeding species where most group members breed and rear their offspring communally. We used data from an 8-year field study to explore the relationship between the ecology and per capita offspring production and survival (2 components of reproductive success and direct fitness) of the plurally breeding rodent Octodon degus. We determined how mean and variance in food abundance, precipitation levels, degu density, soil hardness, predation risk, and thermal conditions modulated the effects of group size and number of breeding females (potential for breeding cooperation) on reproductive success. The effect of number of females per group on the per capita number of offspring produced was more positive during years with lower mean food and degu density. More positive effects of group size (on per capita number of offspring produced and on per capita surviving offspring) and of the number of females (on per capita number of offspring produced) occurred during years with decreasing mean precipitation levels. Thus, the hypothesis that group living and communal rearing are more beneficial (or less costly) under low mean habitat conditions is supported. In contrast, the social effects on reproductive success seem insensitive to variance in ecological conditions.