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A new approach to the ecology of eusocial insects: The Allee Effects in native and invasive ant species

Final Report Summary - TAENIANTS (A new approach to the ecology of eusocial insects: The Allee Effects in native and invasive ant species.)

The Allee effect has been an influential phenomenon for many aspects of basic and applied ecology. Allee effects occur when the per capita population growth rate declines with decreasing population size, due to the loss of mutually beneficial intraspecific interactions. Allee effects are common in social species but it has not been applied to any species of eusocial insects. The present project pursued a pioneering extension of the study of Allee effects to ants, as a first approach to the study of these phenomena in eusocial insects.

During the first year of the project, we carried out preliminary studies with four ant species, and a variety of experimental nests and food resources. We chose two ant species with similar life traits and closely related, one invasive, the Argentine ant Linepithema humble and another non-invasive Tapinoma nigerrimum that grew well under laboratory conditions and that allowed for manipulation of the number of queens and workers. We collected ants from supercolonies in C´ordoba (Spain). For the two species, ants were placed in independent experimental colonies according to a complete two-way factorial design with three categories of queen and worker numbers (8 replicates per treatment). This resulted in a total of 72 nests per species in addition to the trial nests. The experiment lasted from 5 to 10 months depending on colony survival. Several components of fitness where recorded twice a month, including, number of dead queens and workers, brood produced (eggs, larvae, and pupae) produced per nest. We used of cutting-edge and pioneering laboratory methods, including a new set up for keeping ants alive under laboratory conditions for a long period and that allowed the recording of the productivity of nests by a USB Microscope video camera without destroying the nests. During the second year, we continued colony monitoring for surviving colonies and carried out the analysis of video-recordings. This consisted on noting the number of eggs, larvae, and pupae produced and queens and workers alive.

New framework for the study of Allee effects on social species. We found several drawbacks of the current framework for the study of Allee effects in social species. The impact of Allee effects on population dynamics is confounded by the fact that Allee effects operate within cooperative groups rather than across entire populations. These effects are further complicated by the presence of different castes within these cooperative groups or colonies. These aspects are not unique to ant species and they present important drawbacks for the current framework used in studies of Allee effects. During this project we explored the complexities surrounding Allee effects within social species in general, using ants as an extreme example. For this part, we collaborated with specialists on five taxa of social species, including bats (S.D. Gregory, University of Adelaide, Australia), primates (C.B. G´omes, Agro-Paris Tech, Paris), cooperative breeders (E. Angulo, CSIC, Spain), and wasps and bees (J.W. Wenzel, Carnegie Museum, USA) to provide a more adequate framework for the study of Allee effects that incorporates different aspects of social biology, including cooperative grouping, reproductive specialization, inter-group dynamics, and the density and distribution of group sizes within a population.

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