Lower food intake due to domestic grazing reduces colony size and worsens the body condition of reproductive females of harvester ants

Abstract

Feeding flexibility is believed to reduce species vulnerability to local extinction associated with environmental changes that cause food shortage, but to what extent such behavioural response is enough to cope with global change is scarcely known in insects. The harvester ant Pogonomyrmex mendozanus modifies its feeding behaviour and widens its diet in response to seed scarcity under grazing and reduces the colony food intake. We tested whether lower food intake, in particular of carbohydrate-rich grass seed, reduces worker survival rate, worsens the body condition of workers and reproductive individuals, and reduces the colony size of P. mendozanus. Our goal was to identify likely links between low quality nutrition and low colony density under heavy grazing previously reported. The reduction of food intake did not affect the body condition of workers, but it was associated with a significant decrease in the colony size and body fat content of queens and gynes. A starvation experiment showed that worker survival did not differ between grazing conditions, but seed deprivation increased worker mortality under all conditions. Implications for insect conservation: The reduced colony size and deterioration of body condition of reproductive females could impair the survival and reproductive success of P. mendozanus under grazing. These mechanisms may explain the severe decline in colony density in the grazed habitats. In order to protect harvester ants, we highlight the importance of native grass seeds as key resources for population maintenance.