Behold, inbreeding reduces population growth

The population-level significance of inbreeding, especially for endangered populations, has regularly been questioned in the past …

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One reason for the controversy surrounding the issue is that inbreeding effects on individual fitness need not necessarily affect population dynamics. And even if the latter is the case, this is tricky to show, as it requires many replicated populations that differ in levels of inbreeding to be monitored over many generations. To make things worse, or at least more challenging, environmental conditions are expected to modulate inbreeding effects, too. Our study (Bozzuto et al. 2019, Nat. Ecol. Evol. 3: 1359–1364) reveals evidence that inbreeding indeed reduced long-term growth of wild living Alpine ibex populations. So, how did it all start and what’s up next?

Three young male Alpine Ibex. Photo: Iris Biebach

In the 1990ies several Alpine ibex (Capra ibex ibex) populations in Switzerland appeared to be declining. The new millennium was still in its infancy, when a talk of Lukas Keller (University of Zurich) on inbreeding drew the attention of authorities responsible for the management of ibex populations to genetics as one possible cause for the observed population declines. Well, it turned out that Alpine ibex are an ideal study system for understanding the genetic consequences of reintroductions. For one, Alpine ibex were extirpated in the Swiss Alps, with a single remnant population persisting in Northern Italy. This population was crucial for reintroducing the iconic ibex back into the Swiss Alps, but the whole reintroduction history took a genetic toll on the newly established populations. Moreover, the reintroduction history is well documented, not least including sourcing and between-population translocation data as well as yearly population-level census data since the reintroduction. 

And so Lukas Keller and Iris Biebach (starting her PhD at the University of Zurich) launched the genetic project on Alpine ibex populations in Switzerland, financed by the Swiss Federal Office for the Environment. Samples for the genetic analyses were collected with the enormous help of hunting authorities, and Iris and Lukas took additional biopsy samples from a few populations with little or no hunting pressure. To develop microsatellite panels for ibex, existing microsatellites from goat, sheep, and cattle were used. In sum, the resulting data enabled us to estimate genetic diversity indices, explore the influence of history on genetic structure and genetic diversity, and estimate the extent of inbreeding at the population level. 

Biopsy sampling in the Wetterhorn population. Photo: Iris Biebach

Still, the main interest was to quantify the hypothesized influence of inbreeding on population growth, and to achieve this, population-level fitness measures were needed. Claudio Bozzuto (now Wildlife Analysis GmbH, Zurich) and Anthony Ives (University of Wisconsin-Madison) joined the team to explore ibex population dynamics and estimate the intrinsic rate of increase as our main fitness measure, using a nonlinear state-space model. Thanks to the well-documented reintroduction history, some of the 26 census time series spanned almost 100 years, an exceptional situation for wildlife ecologists! Finally, to test for the potential reduction of population growth rates due to inbreeding, Stefanie Muff (now Norwegian University of Science and Technology, Trondheim) joined the team, given her experience in statistical measurement error modelling. 

To make a long and exciting story short: we found evidence for reduced growth rates of Alpine ibex populations caused by inbreeding, especially under harsher environmental conditions. We expect that similar effects be at work in (many?) other wildlife populations experiencing similar ecological conditions. But detecting these effects in our study was a statistical challenge, and documenting similar effects in other wild living populations might remain difficult. To conclude, besides the scientific value of our results these findings are already guiding conservation actions for Alpine ibex: for instance, restocking of highly inbred populations is planned to reduce inbreeding and enhance genetic diversity. And we’ve come full circle.


This post is co-authored by Iris Biebach and Claudio Bozzuto.

Iris Biebach

PostDoc, University Zurich