Mutualism between senita cacti and senita moths: Benefits, costs, and population dynamics

I report the discovery of a pollinating seed-eater mutualism between senita cacti and senita moths, then use it as a model system to investigate the impact of benefits and costs on demography, population dynamics, and evolution of mutualistic populations. I examined the benefits (pollination) and costs (fruit consumption) of senita moths on reproduction of senita cacti, along with other factors affecting cactus reproduction, including diurnal copollinators and water availability for flower production, fruit set, and-fruit production. Senita cacti were found to interact mutualistically with senita moths, all life stages of which were obligately associated with cacti. The benefit of moth pollination to fruit set was found to outweigh the costs their offspring inflicted by consuming fruit. Diurnal co-pollinators were commonly excluded by nocturnal flower closing, and when they were not, they were functionally redundant with senita moths. Immature fruit benefited larvae as a food resource essential for larval survival and development, but cacti imposed a cost on the pollinator population through fruit abortion, which reduced moth recruitment.Functional responses in terms of benefits and costs were introduced to the study of the stability and dynamics of mutualistic populations. For the senita mutualism, and mutualisms in general, these functional responses were shown to be a mechanism inherent to the interaction that can explain growth and stability of populations. The cost of fruit abortion to the moth population was shown to be a mechanism by which the cactus can maximize fruit production and prevent overexploitation by limiting the abundance of moths.Costs may be as important as benefits to the demography and evolution of mutualistic populations. However, results from a 3-way factorial experiment indicated that larval consumption of fruit did not affect fruit production. Instead, fruit production by cacti was limited by water availability and senita moth pollination. Senita moths were effective pollinators because pollen supplementation did not increase fruit set or fruit production