DOI: 10.5061/dryad.44j0zpcqq
title: Population dynamics of a communally rearing mammal is driven by population but not group-level Allee effects

description: Theoretical and some empirical evidence suggest population dynamics of
 cooperative breeders (i.e., species with groups including non-reproductive
 individuals that raise the offspring of dominant breeders) are more likely
 to exhibit Allee effects at the level of social groups rather than at the
 population level. However, the extent to which this population dynamics is
 similar in species where breeding is plural, and group members communally
 rear their offspring remains unclear. Such species may still be subject to
 demographic Allee effects at the population-level. Using a 15-year
 dataset, we examined population and group-level dynamics of communal
 rearing and colonial Octodon degus to determine whether population- and
 group-level Allee effects influence population dynamics. We tested whether
 these effects are contingent on food availability, and whether group size
 is decoupled from population density, i.e., implying group- but not
 population-level Allee effects. We recorded (i) population-level Allee
 effects on per capita population growth rate (i.e., demographic) and on
 per female fecundity rate (i.e., component), (ii) no group-level Allee
 effects on group per female fecundity, and (iii) that Allee effects
 detected are more likely whenever food availability is scarce. We further
 verified that group size is coupled to population density (iv). Our study
 highlighted how food-mediated cooperation through a colonial setting
 underlies Allee effects at the population level, and that group-living
 does not buffer degus against population-level Allee effects. Thus, our
 findings provide a plausible mechanism underpinning risk of local
 extinction in these rodents and potentially, in other plurally breeding
 and colonial species.
description: # Population dynamics of a communally rearing mammal is driven by
 population but not group-level Allee effects
 [https://doi.org/10.5061/dryad.44j0zpcqq](https://doi.org/10.5061/dryad.44j0zpcqq) ## Description of the data and file structure To quantify degu density we established 2 grids approximately 150 m apart to quantify degu density during June (early Austral winter) of each study year. The grids were characterized by a similar distribution of grasses, forbs, and shrubs (Hayes et al., 2007) and covered 0.18 ha (30 x 60 m; grid 1) and 0.25 ha (50 x 50 m; grid 2), respectively.  In June of each year, we set metal live traps (similar to Sherman live traps in design) baited with rolled oats at fixed stations located at 5-m intervals, resulting in 91 traps (a 7 x 13 array) on grid 1 and 121 traps (a 11 x 11 array) on grid 2. We opened traps for 5 days during the morning (08:00 h) prior to emergence of degus from burrows and closed after 3 h. We used data from these two grids to calculate mean (degus ha1) degu population density from 2005 through 2019. To quantify food abundance (i.e., food availability) we collected samples of green herbs at burrow systems in September (spring) of each year at the spatial scale of burrows used by individual degus. Food availability typically peaked at this time of year in our study population, a time that matches offspring rearing in degus. We placed a 250 x 250 mm quadrant and removed the aboveground parts of all green herbs found at randomly selected locations (north, south, east, or west directions) at 3 m and 9 m from the center of burrow systems. We stored these samples inside 2 kg capacity paper bags and oven-dried at 60 °C for 72 h to determine its dry mass (biomass in g). Data from 3 and 9 m sampling points were averaged per burrow system and standardized to g per m2 for subsequent analysis. We estimated per capita fecundity rate through assigning degu offspring to the main burrow systems used by female members of each social group during lactation. Here, we considered the location of first capture during early spring, which corresponds with the period of lactation. The per capita number of offspring weaned by females within each social group was calculated by dividing the total number of offspring captured at burrow systems shared by same group females during lactation by the number of females within the group To estimate density of groups (number of social groups per ha) we used Ranges 9 v2.02 program ([https://www.anatrack.com/ranges_home](https://www.anatrack.com/ranges_home); Kenward, 2001) to calculate total area monitored with traps during each study year. We calculated these areas (in ha) based on 95% fixed Kernels after including every trap location within a study year (e.g., Hayes et al., 2007). We used “selected cores of 95%”, “fixed kernel type”, and “location density only” for contours options. **Description:** Variables examined are properly identified with self-explanatory text labels.   ##### Variables * Per capita population rate * Percapita fecundity rate  * Group per capita fecundity rate * Population density in winter (degus/ha) * Spring group size (number of adults) * Group density (groups/ha) * Spring food abundance (g/m2) ## Code/software Microsoft Excel