Every once in awhile, I read a paper that surprises me. Today, I read one of those papers, and it surprised me because it analyzes a phenomenon that is so obvious that I wonder why no one ever thought of studying it in a systematic and rigorous way before. I am referring to a paper that was just published by a team of researchers who found that particular behaviors can damage and even reverse the recovery of endangered species under certain circumstances. In this situation, the team documented the negative effects that can occur when individuals of a social species engage in frequent battles for limited reproductive opportunities.
As you are aware, it is typical for males to fight over reproductive opportunities but these battles benefit the individual “winners” while often having negative effects upon the losers, and even for population as a whole when that population is severely limited. Why? These aggressive interactions can interrupt the breeding output of established breeding pairs. Further, it is also likely that nonbreeding individuals can decrease breeding output by harassing and interfering with breeding adults.
This makes sense, but what actually happens in a real-life conservation situation that has been designed and carefully managed to save an endangered species? A team of biologists led by Andrés López-Sepulcre from the University of California, Riverside, wondered if this situation could be documented and studied in a very rare bird species, the Seychelles magpie-robin, Copsychus sechellarum.
The Seychelles magpie-robin has the dubious honor of being named one of the world’s most endangered birds in 1988, after its total population had reached a nadir of 7 pairs, all of which were restricted to just one island in the Seychelles archipelago in the Indian Ocean. These birds are monogamous, they breed aseasonally, and they rely on subordinate “helpers” to assist the territory-holding breeding parents with chick rearing duties.
Because of their small population size and the collapse of their range to just one island, it was possible to observe and document social and behavioral interactions for every individual. Biologists carefully observed these birds from June 1988 to December 2004 and developed a detailed database that catalogued these observations. This database is a monthly register of every individual in the population, specifically following its breeding and social status. All individuals in the population were identified by unique color combinations of leg bands (rings, refer to the photograph at the top) so their location and dominance status could be noted each month. All new birds were either banded while they were nestlings or, if the nest was inaccessible, soon after they fledged. All birds were seen and marked within 2 months of initially being observed, and only six birds moved between neighboring islands in the archipelago.
To do this work, the team first used the database to describe the monthly population fluctuations of the Seychelles Magpie-Robin found on all four of the islands that it now inhabits (figure 1);
Figure 1: Time series of Seychelles magpie robins’ monthly abundances on the four islands it inhabits. Since January 2000, data for Frégate — and hence total population — was gathered biannually only. The arrow indicates the date when the species’ recovery program was fully implemented. (Inset) A Seychelles magpie robin (photo: A. López-Sepulcre).
DOI: 10.1111/j.1365-2656.2008.01475.x [larger view].
Next, López-Sepulcre and his colleagues analyzed aggressive interactions between adult birds of both sexes and found that socially dominant individuals were involved in more aggressive interactions when same-sex subordinates occupied the same territory. Further, a dominant bird was more likely to lose its position to one of those same-sex subordinates (figure 2);
Figure 2: Effect of adult subordinates on dominant individuals. The presence of same-sex subordinates is associated with (a) higher levels of aggression in dominants, and (b) higher chances of losing their dominant position. Note the similarity in the pattern of results derived from short-term behavioral observations (a) and long-term individual-history data (b). Bars indicate standard errors. Sample size n indicates the number of dominant individuals that contributed to that category (note that a same individual can contribute to several categories if its state changed over time).
It is important to note that territory disputes are not simple cut-and-dried events, but instead, occur over a period of time, regardless of their ultimate outcome. These aggressive interactions are sufficient to distract the adult territory-holders’ attention and energies away from breeding, thereby reducing their reproductive success (both males and females of this monogamous species compete with each other for breeding territory — see video of aggressive behavior in this species, below);
A Seychelles Magpie-Robin, Copsychus sechellarum, engaged in a territotial dispute, holding out her wings to show off the large white patches and singing. There’s actually two birds singing on this video.
There’s only around 200 [actually, there’s 149 birds] of these birds left, just over 20 on the island of Aride, where this bird was filmed.
The researchers noted that even though territory takeovers clearly have a negative impact upon short-term breeding output, this does not exclude the possibility of longer-term benefits since territory takeovers could bring in new pairs with potentially higher reproductive success. So López-Sepulcre and his colleagues looked at this effect by comparing the period of time that elapsed between the last breeding attempt of the old pair (immediately before a territory turnover) with the first breeding attempt for the replacement pair (figure 3);
Figure 3: Effect of territory takeovers on breeding frequency. (a) The left half of the graph shows that the interval between two successful breeding attempts is significantly longer when there is a pair takeover in between. This delay in breeding is also significantly longer than the time between the settling of a new territory by a pair and their first successful breeding attempt, as indicated by the right half of the graph. Bars indicate standard errors. (b) Schematic representation of a territory’s timeline illustrating the three first types of intervals considered in the figure above. The fourth type includes intervals where there was a takeover before the production of the first fledging of the territory.
The found that a territory takeover increased the time interval until the appearance of the first fledgling produced by the newly established dominant birds. The team then used a mathematical model to detemrine the overall effect upon population growth and found a significant negative effect (figure 4);
Figure 4: Simulated times to recovery of the Seychelles magpie robin. Cumulative probability of the Seychelles magpie robin meeting the criteria for being downlisted from Critically Endangered, as calculated by simulation of 1000 populations. The solid line corresponds to the control scenario. The dashed line represents the conflict-free scenario. The x-axis starts at the time of full implementation of the species’ recovery program. The actual date when the species met the IUCN criteria to be downlisted is indicated by the solid circle and the end of the data by the vertical dotted line. Asterisks indicate a significant difference with respect to the control scenario at the 0.05 (*) and 0.001 (***) levels.
DOI: 10.1111/j.1365-2656.2008.01475.x [larger view].
According to these data, the resulting delay in breeding translates into an average delay of 33% for the population to lose its “Critically Endangered” status, despite the fact that the study population is intensively managed and is showing a healthy rate of recovery.
This paper is very interesting because it apparently is the first study that documents the effects of very small population sizes on social behaviors that have the potential to negatively affect species recovery. This finding has important implications for predicting the time, expense and effort necessary for successful conservation efforts.
This study is also interesting because it reminds us that evolution favors behaviors that maximize the relative fitness of individuals, even when those same behaviors directly damage the absolute fitness of the population. Further, following these data to their logical conclusion, the ultimate consequence of natural selection can favor behaviors that are detrimental to the population, leading to the theoretical prediction that natural selection can actually increase the likelihood of extinction. Certainly, this study is timely because it is in line with the current worldwide economic disaster, which resulted from the behaviors of a few selfish individuals who were maximizing their relative fitness while simultaneously damaging the absolute fitness for millions, or even billions, of people.
Andrés López-Sepulcre, Ken Norris, Hanna Kokko (2008). Reproductive conflict delays the recovery of an endangered social species Journal of Animal Ecology DOI: 10.1111/j.1365-2656.2008.01475.x.