Interactive effects of environmental stress and inbreeding on reproductive traits in a wild bird population

Abstract

Summary1. Conservation biologists are concerned about the interactive effects of environmental stress and inbreeding because such interactions could affect the dynamics and extinction risk of small and isolated populations, but few studies have tested for these interactions in nature. 2. We used data from the long-term population study of song sparrows Melospiza melodia on Mandarte Island to examine the joint effects of inbreeding and environmental stress on four fitness traits that are known to be affected by the inbreeding level of adult birds: hatching success, laying date, male mating success and fledgling survival. 3. We found that inbreeding depression interacted with environmental stress to reduce hatching success in the nests of inbred females during periods of rain. 4. For laying date, we found equivocal support for an interaction between parental inbreeding and environmental stress. In this case, however, inbred females experienced less inbreeding depression in more stressful, cooler years. 5. For two other traits, we found no evidence that the strength of inbreeding depression varied with environmental stress. First, mated males fathered fewer nests per season if inbred or if the ratio of males to females in the population was high, but inbreeding depression did not depend on sex ratio. Second, fledglings survived poorly during rainy periods and if their father was inbred, but the effects of paternal inbreeding and rain did not interact. 6. Thus, even for a single species, interactions between the inbreeding level and environmental stress may not occur in all traits affected by inbreeding depression, and interactions that do occur will not always act synergistically to further decrease fitness.

The strength of the association between heterozygosity and probability of interannual local recruitment increases with environmental harshness in blue tits

“…However, the direction and magnitude of HFC differed among years and they were only significantly explained by multilocus heterozygosity calculated at all markers or the panel of neutral markers, but not by genetic diversity estimated at the subset of putatively functional loci. We also found a highly significant positive relationship between selection differentials ( S ) for heterozygosity and annual accumulated precipitation, which indicates that interannual differences in HFC may be explained by the different environmental conditions to which individuals are exposed each year (Armbruster & Reed, 2005; Fox & Reed, 2011; Marr et al., 2006). Hence, our results reinforce the expectation of stronger selection under adverse environmental conditions in wild populations (Endler, 1986).…”

“…Several studies have found that the association between genetic diversity (or inbreeding) and different components of fitness become stronger under stressful conditions (Armbruster & Reed, 2005; Fox & Reed, 2011), but most evidence on this respect comes from experimental approaches and only a few studies have analyzed such interactions in wild populations (Forcada & Hoffman, 2014; Marr et al., 2006; Szulkin & Sheldon, 2007). Here, we quantified the relationship between heterozygosity and fitness along an environmental continuum, comprising harsh (wet–cool) and benign (dry–warm) years.…”

“…The strength of the association between heterozygosity and fitness is expected to depend on factors such as the intensity of selection and the level of ID in the population (Armbruster & Reed, 2005; Marr et al., 2006; Fox et al. 2011).…”

“…In addition, we examine the relationship between environmental variability and the strength of HFC. As with other small passerines, the blue tit has a high metabolic rate and a limited capacity to deposit long‐term body reserves (McNab, 2002), factors that are likely to increase its susceptibility to environmental stress imposed by food shortage or severe weather conditions (Marr et al., 2006). For these reasons, our study system is particularly amenable to study HFC and their interactions with environment.…”

Heterozygosity–fitness correlations in blue tit nestlings (Cyanistis caeruleus) under contrasting rearing conditions

“…The inconsistency between studies may be partially explained by the fact that HFCs could become meaningful only under particular environmental conditions; (Chapman et al 2009) these may be revealed in competitive environments and appear non-significant under favorable conditions. Indeed, environmental factors such as thermal stress, limited food availability, parasite presence and harsh weather conditions have been found to strengthen the association between heterozygosity and fitness with more pronounced correlations arising under stressful environmental conditions (Lesbarreres et al 2005;Da Silva et al 2006;Marr et al 2006;Fox and Reed 2011;Voegeli et al 2012;Forcada and Hoffman 2014;Ferrer et al 2016). However, this avenue of research has received limited attention to date and has rarely been tested in experimental studies.…”