Pär Forslund

Age-related reproductive success in the barnacle goose

1. We studied age-related reproductive success in a recently established breeding colony of barnacle goose Branta leucopsis on the island of Gotland, Sweden.2. Associations of age and breeding experience with several reproductive parameters were investigated by comparing the success of individuals of 2, 3, 4 and >=5 years of age. Also, we measured the within-individual changes in reproductive parameters as the individuals grew older.3. Associations between survival and reproductive success were investigated by comparing the reproductive success of 2, 3 and 4-year-old birds that returned or did not return, respectively, in the subsequent breeding season.4. The reproductive success increased up to an age of 4-5 years. Thus, clutch size, number of hatched young and number of fledged young increased, and hatching date was advanced with increasing age. This was due to the fact that individuals increased in reproductive success as they grew older.5. A possibly higher probability of survival among individuals that perform well in reproduction could not explain the higher reproductive success in older age-classes as compared to young age-classes of geese, because there were no associations between survival and reproductive success, and very few individuals did actually disappear between any two breeding seasons.6. Path analysis suggested age effects only at earlier stages of reproduction, i. e. timing of breeding and clutch size. These characters, in turn, were associated with number of fledged young. These findings were further supported by measurements of hatching success and rearing success, which did not seem to increase with age.7. Increased breeding experience was associated with early hatching date and larger clutch size in males, and with larger clutch size in females. This was concluded from path analysis and from comparisons of individuals of the same age but with different breeding experience.8. The increase in reproductive success with age in the barnacle goose is probably a result of age-related skills in individuals and the direct effects of these skills on reproductive success, but possibly also because of increased reproductive effort with age owing to these age-related skills.

BODY SIZE DECLINES DESPITE POSITIVE DIRECTIONAL SELECTION ON HERITABLE SIZE TRAITS IN A BARNACLE GOOSE POPULATION

Analyses of more than 2000 marked barnacle geese (Branta leucopsis) in the largest Baltic colony, Sweden, showed that structurally large females generally produced larger clutches and larger eggs, hatched their broods earlier in the season, and produced more and heavier young than smaller females. In males, the corresponding relationships between reproductive parameters and structural body size were weaker or nonsignificant. Because structural body size traits have previously been found to be significantly heritable and positively genetically correlated, an increase in mean structural body size of individuals as a response to selection might have been expected. By contrast, we found that the mean adult head length and mean adult tarsus length decreased significantly in the largest colony by approximately 0.7 and 0.5 standard deviations, respectively, in both males and females during the 13‐year study period. Environmental factors, such as the amount of rain in different years, were found to affect the availability of high‐quality food for growing geese. As a consequence of this temporal variability in the availability of high‐quality food, the mean adult structural body size of different cohorts differed by up to 1.3 standard deviations. Comparisons of mean body size of cohorts born in different colonies suggest that the most likely explanation for the body‐size decline in the main study colony is that a density‐dependent process, which mainly was in effect during the very early phase of colony growth, negatively affected juvenile growth and final size. We conclude that large environmental effects on growth and final structural body size easily can mask microevolutionary responses to selection. Analyses of environmental causes underlying temporal and spatial body size variation should always be considered in the reconstruction and prediction of evolutionary changes in natural populations.

Habitat-specific population growth of a farmland bird.

Background To assess population persistence of species living in heterogeneous landscapes, the effects of habitat on reproduction and survival have to be investigated. Methodology/Principal Findings We used a matrix population model to estimate habitat-specific population growth rates for a population of northern wheatears Oenanthe oenanthe breeding in farmland consisting of a mosaic of distinct habitat (land use) types. Based on extensive long-term data on reproduction and survival, habitats characterised by tall field layers (spring- and autumn-sown crop fields, ungrazed grasslands) displayed negative stochastic population growth rates (log λs: −0.332, −0.429, −0.168, respectively), that were markedly lower than growth rates of habitats characterised by permanently short field layers (pastures grazed by cattle or horses, and farmyards, log λs: −0.056, +0.081, −0.059). Although habitats differed with respect to reproductive performance, differences in habitat-specific population growth were largely due to differences in adult and first-year survival rates, as shown by a life table response experiment (LTRE). Conclusions/Significance Our results show that estimation of survival rates is important for realistic assessments of habitat quality. Results also indicate that grazed grasslands and farmyards may act as source habitats, whereas crop fields and ungrazed grasslands with tall field layers may act as sink habitats. We suggest that the strong decline of northern wheatears in Swedish farmland may be linked to the corresponding observed loss of high quality breeding habitat, i.e. grazed semi-natural grasslands.

DIVORCE RATES IN BIRDS: PREDICTIONS FROM AN OPTIMIZATION MODEL

We present a theoretical investigation of the dependence of divorce rate on longevity, variation in quality between mates, and the costs of divorce or mate retention. We model the decision of a female on whether to divorce her mate after breeding. This decision is based on his quality. We show that it is optimal for the female to divorce if the reproductive success in breeding with the mate next year is less than the average annual future success if she divorces. Under this strategy the females mean annual divorce rate declines with age, while her mean annual reproduction success rises We investigated the annual divorce rate, averaged over the lifetime of the female. Costs caused this divorce rate to increase with variance of male quality. When divorce resulted in a loss in reproductive success in the following year, this divorce cost reduced divorce in short-lived species far more than in long-lived species. The reverse was true for mortality costs. A mate retention cost of overwinter mate death increased divorce in short-lived species dramatically, while long-lived species were little affected Female choosiness increased with longevity, but this did not necessarily increase divorce rates averaged over the females life. The selection pressure against the suboptimal strategies of obligate divorce and obligate mate retention increased strongly with longevity.

Population dynamics of the barnacle goose Branta leucopsis in the Baltic area: density-dependent effects on reproduction

1. The three largest breeding colonies of barnacle geese Branta leucopsis in the Baltic area were studied from their natural establishments in 1971, 1982 and 1987, respectively, to 1993. The number of breeding pairs increased from one pair in 1971 to 1550, 225 and 150 pairs in the largest, second largest, and third largest colonies, respectively, in 1993. 2. Data on clutch size and number of hatched and fledged young were collected from the beginning or the middle of the 1980s. In the largest colony most data on reproductive success were obtained from observations of individually marked geese. 3. Mean clutch size differed among years and colonies. However, there was no trend for decreasing mean clutch size with increasing density in any of the colonies. Hatching success did not decrease with increasing density in the largest colony. 4. The production of fledged young per breeding pair declined drastically in the largest colony as numbers of breeding pairs rose. This decline was mainly caused by an increasing proportion of the breeding pairs failing to produce any fledged young at all. 5. The relative contribution of the smaller colonies to the total production of fledged young in the Baltic area increased considerably over the study years. In 1993, colony 2 hosted only about 13% of the number of breeding pairs in colonies 1 and 2 combined, but it produced about 38% of the fledged young. 6. Our results point out the need to study populations of colonially nesting geese on a colony by colony basis in order to detect density-dependent effects on reproduction.

Male-male competition and large size mating advantage in European earwigs, Forficula auricularia.

European earwigs are sexually dimorphic in forceps shape and length. Male forceps are thought to be weapons in male contests for access to females, but recent findings suggest that females choose males on the basis of their forceps length. I investigated sexual selection on forceps length and body size and the occurrence of male-male competition. When I controlled for forceps length experimentally and statistically, relatively heavy males had greater copulation success than relatively light males. When I controlled for body size, males with relatively longer forceps had no tendency for greater copulation success than males with shorter forceps. Relatively heavy males more often took over copulations from smaller males than vice versa. Male contests were important for the outcome of mate competition, as males commonly interrupted and took over copulations. My results therefore suggest that intrasexual selection is significant in competition for copulations in male earwigs, and acts on body size. This contrasts with previous findings, which have shown intersexual selection on forceps length to be important. However, both modes of sexual selection may be acting through a two-stage process, where male-male competition first determines which males have access to females, and then through female choice among available males. Morphological measurements supported the conclusion that forceps length and body size are male secondary sexual characters, as these characters had large variance and skewed distributions in males, but were normally distributed in females. Copyright 2000 The Association for the Study of Animal Behaviour.

An experimental investigation into status-dependent male dimorphism in the European earwig, Forficula auricularia

Discrete alternative reproductive phenotypes are probably often due to individuals adopting alternative tactics with unequal fitnesses in conditional strategies with status-dependent selection. One tactic is believed to be favoured below a status switch point and another tactic above it, owing to different fitness functions of the tactics. Males of the European earwig are dimorphic. Macrolabic males are large (high status) and have long forceps, and brachylabic males are small (low status) with short forceps. I tested whether fitness functions, measured as mating success, of the two morphs differed with regard to forceps length and body weight. Macrolabic males with longer forceps than a competing male had higher mating success but brachylabic males benefited by being heavier than their competitor. Thus, different selection regimes were acting on the two morphs, suggesting that their fitness functions differed in relation to status. These observations and those of previous studies, showing that morph expression is environmentally determined and is associated with body size and that the morphs have unequal fitness, provide support for the hypothesis that male dimorphism in this species is a conditional strategy that has evolved under status-dependent selection. The differential investment in forceps growth that characterizes the morphs was manifested in a steeper allometric relation between forceps length and body size in macrolabic than brachylabic males. Behavioural observations showed that males of both morphs engaged in direct contests for females. Copyright 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.

Can Life History Predict the Effect of Demographic Stochasticity on Extinction Risk

Demographic stochasticity is important in determining extinction risks of small populations, but it is largely unknown how its effect depends on the life histories of species. We modeled effects of demographic stochasticity on extinction risk in a broad range of generalized life histories, using matrix models and branching processes. Extinction risks of life histories varied greatly in their sensitivity to demographic stochasticity. Comparing life histories, extinction risk generally increased with increasing fecundity and decreased with higher ages of maturation. Effects of adult survival depended on age of maturation. At lower ages of maturation, extinction risk peaked at intermediate levels of adult survival, but it increased along with adult survival at higher ages of maturation. These differences were largely explained by differences in sensitivities of population growth to perturbations of life-history traits. Juvenile survival rate contributed most to total demographic variance in the majority of life histories. Our general results confirmed earlier findings, suggesting that empirical patterns can be explained by a relatively simple model. Thus, basic life-history information can be used to assign life-history-specific sensitivity to demographic stochasticity. This is of great value when assessing the vulnerability of small populations.

Environmental Gradients Explain Species Richness and Community Composition of Coastal Breeding Birds in the Baltic Sea

Scientifically-based systematic conservation planning for reserve design requires knowledge of species richness patterns and how these are related to environmental gradients. In this study, we explore a large inventory of coastal breeding birds, in total 48 species, sampled in 4646 1 km2 squares which covered a large archipelago in the Baltic Sea on the east coast of Sweden. We analysed how species richness (α diversity) and community composition (β diversity) of two groups of coastal breeding birds (specialists, i.e. obligate coastal breeders; generalists, i.e. facultative coastal breeders) were affected by distance to open sea, land area, shoreline length and archipelago width. The total number of species per square increased with increasing shoreline length, but increasing land area counteracted this effect in specialists. The number of specialist bird species per square increased with decreasing distance to open sea, while the opposite was true for the generalists. Differences in community composition between squares were associated with differences in land area and distance to open sea, both when considering all species pooled and each group separately. Fourteen species were nationally red-listed, and showed similar relationships to the environmental gradients as did all species, specialists and generalists. We suggest that availability of suitable breeding habitats, and probably also proximity to feeding areas, explain much of the observed spatial distributions of coastal birds in this study. Our findings have important implications for systematic conservation planning of coastal breeding birds. In particular, we provide information on where coastal breeding birds occur and which environments they seem to prefer. Small land areas with long shorelines are highly valuable both in general and for red-listed species. Thus, such areas should be prioritized for protection against human disturbance and used by management in reserve selection.

Carcass records of autumn-slaughtered reindeer as indicator of long-term changes in animal condition

This study investigates the possibility of using carcass records from the commercial slaughter of reindeer as indicator of long-term changes in animal condition and, thus, the condition and use of their snow-free pasture. The aim was to assess the suitability of this indicator for use within adaptive management programmes for reindeer husbandry grazing resources. Data comprising measurements of carcass weight, conformation and fatness taken from commercial reindeer slaughter between 1994 and 2007, were analysed in relation to year, slaughter date, herding district, population density, and three categories of animals selected for slaughter. The carcass measures were significantly affected by year, and the effects were strongly correlated among the three animal categories. There were generally positive trends over the 14-year period studied. We identified several factors that should be considered when using carcass data to indicate long-term changes in animal body condition: (i) slaughter date had different effects depending on animal category; (ii) reindeer population density negatively affected female and calf carcasses, but not male carcasses. The effects of herding district were similar for carcasses of calves and females, but differed between females and males. Some of the differences between animal categories may be due to differing timing of slaughter (point i above), by different slaughter selection among districts, or have ecological explanations, e.g. sex differences in range use. Uncertainties in the classification of animals when using skeletal development to discriminate between calf and yearling carcasses, may also add to differences among districts. That population density effects on body condition were detectable together with the similarities in the effects of year and general long-term trends between animal categories support the suggestion that carcass measures can be used to indicate general changes in reindeer body condition and range use.