Population dynamics of obligate cooperators

Abstract

Obligate cooperative breeding species demonstrate a high rate of group extinction, which may be due to the existence of a critical number of helpers below which the group cannot subsist. Through a simple model, we study the population dynamics of obligate cooperative breeding species, taking into account the existence of a lower threshold below which the instantaneous growth rate becomes negative. The model successively incorporates (i) a distinction between species that need helpers for reproduction, survival or both, (ii) the existence of a migration rate accounting for dispersal, and (iii) stochastic mortality to simulate the e¡ects of random catastrophic events. Our results suggest that the need for a minimum number of helpers increases the risk of extinction for obligate cooperative breeding species. The constraint imposed by this threshold is higher when helpers are needed for reproduction only or for both reproduction and survival. By driving them below this lower threshold, stochastic mortality of lower amplitude and/or lower frequency than for non-cooperative breeders may be su¤cient to cause the extinction of obligate cooperative breeding groups. Migration may have a bu¡ering e¡ect only for groups where immigration is higher than emigration; otherwise (when immigrants from nearby groups are not available) it lowers the di¡erence between actual group size and critical threshold, thereby constituting a higher constraint.

Dynamics of a small re-introduced population of wild dogs over 25 years: Allee effects and the implications of sociality for endangered species’ recovery

“…Creel and Creel 2002), manifested in the disappearance of a large number of dispersing animals from the park, although wild dogs can disperse over considerable distances (Fuller et al 1992b). Moreover, Courchamp et al (1999b) showed that when emigration exceeds immigration, migration is a disadvantageous process for groups of cooperators, making it more probable for groups to drop below the critical threshold size. In the HiP wild dog population, young adults were the most common dispersers and dispersal was most likely following the recruitment of sexually mature individuals, suggesting a role of dispersal in avoiding reproductive suppression (Creel and Creel 2002).…”

“…It has been suggested that at low densities, migration may be a regulatory mechanism in the population dynamics of cooperative breeders, as migration between groups through dispersal may act as a buffer, limiting the erosion of the group (Courchamp et al 1999b). However, immigration may prevent the group from falling below the critical threshold size only when individuals are always available from other groups nearby, particularly as inverse density dependence in wild dogs was demonstrated to result in fewer dispersers and a lower colonisation rate of empty patches (Courchamp et al 2000b).…”

“…As an Allee effect can be generated by a shortage of interactions among conspecifics at low density, the degree of sociality of a species might reflect the degree of severity of the Allee effect to which it is subject. Allee effects thus potentially have particularly serious impacts on the population dynamics of obligately cooperative breeding species, which rely on the presence of a minimum group size (or number of helpers) for raising their young (Creel 1998;Courchamp et al 1999b). In addition, constraints imposed by natural enemies, such as predators or kleptoparasites, have the potential to push cooperator groups closer to or below this critical group size threshold (Courchamp et al 2000a;Creel et al 2001).…”

Inbreeding, heterozygosity and fitness in a reintroduced population of endangered African wild dogs (Lycaon pictus)

“…For the purpose of this study, we considered fitness of wild dogs to be largely reliant on the ability of an individual to find a breeding pack (McNutt 1996), be dominant (Creel and Creel 2002), produce offspring and survive (Buettner et al 2007). We included inclusive reproductive success in our analyses because wild dogs are mostly obligate cooperative breeders (Courchamp et al 1999), and we sought a precise measure of lifetime reproduction through each individual's contribution via actual breeding and/or helping to raise related kin. Finally, regardless of outcome on the relatedness issues described above, we predicted that this study of a major population of an endangered species would be important because it (1) begins a substantial database useful for long-term genetic and fitness monitoring of a reintroduced wild carnivore and (2) provides objective data informative to conservation managers on minimal best practices to sustain genetic diversity and future population health.…”

The sociality–health–fitness nexus: synthesis, conclusions and future directions

“…This effect has been posited as a reason for added challenges in conserving social wild animals. Obligate cooperative breeding species, such as the African wild dog and Ethiopian wolf, might be especially vulnerable to disease-mediated extinctions involving the Allee effect [129,130].…”