Thor Harald Ringsby

Senescence rates are determined by ranking on the fast-slow life-history continuum

Comparative analyses of survival senescence by using life tables have identified generalizations including the observation that mammals senesce faster than similar-sized birds. These generalizations have been challenged because of limitations of life-table approaches and the growing appreciation that senescence is more than an increasing probability of death. Without using life tables, we examine senescence rates in annual individual fitness using 20 individual-based data sets of terrestrial vertebrates with contrasting life histories and body size. We find that senescence is widespread in the wild and equally likely to occur in survival and reproduction. Additionally, mammals senesce faster than birds because they have a faster life history for a given body size. By allowing us to disentangle the effects of two major fitness components our methods allow an assessment of the robustness of the prevalent life-table approach. Focusing on one aspect of life history - survival or recruitment - can provide reliable information on overall senescence.

Life History Variation, Population Processes and Priorities in Species Conservation: Towards a Reunion of Research Paradigms

We argue that a relationship between life history variation and population processes may form the foundation for developing a theory for variation in population growth rate. An examination of the distribution of 104 European bird species in relation to their clutch size and adult survival rate showed three different clusters. First, there is a large group of species which lay a large number of eggs and have low adult survival rate. The second cluster consists of species with very high survival rates and a clutch size of only one egg. The third group is characterized by species with high survival rates but still a relatively large clutch size. From these clusters of life history characteristics we argue that the species can be classified according to the quality of their survival and breeding habitats, respectively. The high-reproductive species live in favourable breeding habitats, but poor survival habitat. In contrast, the survival habitat of the survivorship species are very good, but the breeding habitats are poor. The bet-hedging species live in favourable breeding and survival habitats, but the annual variation in the quality of the breeding habitats is very large, favouring the evolution of a larger clutch size than in the survivorship species. In order to examine the effects of these patterns of covariation between life history traits on population dynamics we calculated the sensitivity and elasticity of population growth rate to a change in age-specific fecundity or mortality rates for one species from each of the three life history types. These analyses showed that population growth rates of high-reproductive species were more sensitive and elastic to changes in the fecundity among the younger age-classes, compared to the species from the two other groups. Furthermore, elasticity to variation in mortality rates was higher than to variation in fecundity rates in all three species. To provide a further link between life history variation and population dynamics the results from key-factor analyses of population fluctuations in birds and mammals were reviewed. In most altricial birds, the key-factor appears during the non-breeding season. In contrast, in precocial birds key-factors from the breeding season explained a higher proportion of the variance in the total losses than the losses during the non-breeding season. In the majority of the cases density-dependence was found in the losses during the non-breeding season. According to the Allee-effect, we would expect that the population growth rate should decrease with density at low population sizes. No evidence was found for the presence of an Allee-effect in the studies of 11 bird species which were reduced to very low population levels during the study period. We suggest however that such an Allee-effect still may be important due to a reduction in the defence efficiency among predators or parasites, reduction in mating efficiency, or reduction in the foraging efficiency at low population densities. These results may have some important implications for overall priorities in the development of strategies for conserving species diversity. In particular, we focus on the securing of survival habitats for especially longlived species outside the breeding season.

Factors affecting juvenile survival in House Sparrow Passer domesticus

We studied how variation in different reproductive traits (hatching date, clutch number and clutch size) in addition to mass, size and condition of the fledgling influenced its probability of survival until recruitment in a House Sparrow population living on islands off the coast of northern Norway in 1993 and 1994. Twenty-three and 21% of the fledglings were recorded alive in the population the year after hatching, respectively. Most mortality occurred just after fledging. In both years larger-sized fledglings survived better than smaller ones. Juvenile survival was independent of clutch size. Multiple logistic regression analyses showed that in both years juvenile survival increased with body size (expressed as tarsus length). In addition, in 1993 fledglings in high body condition survived better, whereas in 1994 hatching day explained a significant proportion of the variation in juvenile survival. However, this last year both body condition and size increased significantly with hatching date, suggesting that juvenile survival rate may have been dependent on body condition also in this year. These results demonstrate that losses of juveniles during the non-breeding season are strongly influenced by factors during the breeding season that affect the size and condition of the fledglings.

Phenotypic correlates and consequences of dispersal in a metapopulation of house sparrows Passer domesticus

1. We examine causes and consequences of natal dispersal within a metapopulation of house sparrows Passer domesticus in an archipelago in Northern Norway where a large proportion of the individuals is colour-ringed. 2. Less than 10% of the fledglings dispersed, i.e. left their natal island. 3. Dispersal was female biased and almost exclusively performed by juveniles. 4. The probability of natal dispersal was not related either to the body condition or the body mass of the juvenile. Similarly, neither clutch size nor hatching date explained a significant proportion of the variance in the probability of dispersal. 5. The probability of male natal dispersal was related to the rank of the fledgling in the size-hierarchy within the brood. Low ranking individuals that hatched early in the season were more likely to disperse. 6. In both sexes, the survival of dispersers at the island of establishment was higher than among the residents on that island. Similarly, dispersers survived better than adults that remained on their island of birth. 7. These results suggest that dispersal may be an adaptive strategy to avoid poor conditions in the natal area.

ASYNCHRONOUS SPATIOTEMPORAL DEMOGRAPHY OF A HOUSE SPARROW METAPOPULATION IN A CORRELATED ENVIRONMENT

One important question in ecology is how large-scale regional phenomena affect the demography and dynamics of local populations. The purpose of the present study was to examine how climate affected spatial and temporal variation in demography within an insular metapopulation of House Sparrows (Passer domesticus). Daily weather conditions during the breeding season were highly correlated within the study region, and because the onset of breeding differed significantly among the five study islands, fledgling success was also affected. Thus, local recruitment was reduced by the occurrence of bad weather during the nestling period, and the asynchronous breeding phenology among the islands led to a significant island × year interaction in recruitment rate. Interaction between climate and the local environment provides a mechanism by which spatial synchrony in population dynamics can be reduced even in strongly spatially autocorrelated environments.

EVOLUTIONARY DYNAMICS OF A SEXUAL ORNAMENT IN THE HOUSE SPARROW (PASSER DOMESTICUS): THE ROLE OF INDIRECT SELECTION WITHIN AND BETWEEN SEXES

Abstract The relative contribution of sexual and natural selection to evolution of sexual ornaments has rarely been quantified under natural conditions. In this study we used a long-term dataset of house sparrows in which parents and offspring were matched genetically to estimate the within- and across-sex genetic basis for variation and covariation among morphological traits. By applying two-sex multivariate “animal models” to estimate genetic parameters, we estimated evolutionary changes in a male sexual ornament, badge size, from the contribution of direct and indirect selection on correlated traits within males and females, after accounting for overlapping generations and age-structure. Indirect natural selection on genetically correlated traits in males and females was the major force causing evolutionary change in the male ornament. Thus, natural selection on female morphology may cause indirect evolutionary changes in male ornaments. We observed however no directional phenotypic change in the ornament size of one-year-old males during the study period. On the other hand, changes were recorded in other morphological characters of both sexes. Our analyses of evolutionary dynamics in sexual characters require application of appropriate two-sex models to account for how selection on correlated traits in both sexes affects the evolutionary outcome of sexual selection.

EFFECTIVE SIZE OF FLUCTUATING POPULATIONS WITH TWO SEXES AND OVERLAPPING GENERATIONS

Abstract We derive formulas that can be applied to estimate the effective population size Ne for organisms with two sexes reproducing once a year and having constant adult mean vital rates independent of age. Temporal fluctuations in population size are generated by demographic and environmental stochasticity. For populations with even sex ratio at birth, no deterministic population growth and identical mean vital rates for both sexes, the key parameter determining Ne is simply the mean value of the demographic variance for males and females considered separately. In this case Crow and Kimuras generalization of Wrights formula for Ne with two sexes, in terms of the effective population sizes for each sex, is applicable even for fluctuating populations with different stochasticity in vital rates for males and females. If the mean vital rates are different for the sexes then a simple linear combination of the demographic variances determines Ne, further extending Wrights formula. For long-lived species an expression is derived for Ne involving the generation times for both sexes. In the general case with nonzero population growth and uneven sex ratio of newborns, we use the model to investigate numerically the effects of different population parameters on Ne. We also estimate the ratio of effective to actual population size in six populations of house sparrows on islands off the coast of northern Norway. This ratio showed large interisland variation because of demographic differences among the populations. Finally, we calculate how Ne in a growing house sparrow population will change over time.

A Parametric Model for Estimation of Dispersal Patterns Applied to Five Passerine Spatially Structured Populations

Natal dispersal capture‐recapture data from five fragmented populations of house sparrows, great tits, and blue tits were analyzed using maximum likelihood methods. A new two‐parametric distribution was constructed that includes four distributions previously used as special cases in the literature. Dispersal standard deviations were estimated at 22.9 km for the house sparrows and ranged from 0.66 to 4.4 km for the tits. Female great tits and blue tits dispersed consistently further than males. Estimates of the shape parameter of the dispersal distribution ranged from 0.66 to 2.27, indicating strong to moderately leptokurtic dispersal displacements. There were significant effects of density on local immigration rates and a consistent tendency for immigration rates to depend underproportionally on local densities. Potential implications of the shape of the dispersal distribution for the spread of invading organisms were investigated and compared with previous results. It is shown that the wave speed, for a given dispersal standard deviation, depends only to some extent on leptokurtosis, provided that the intrinsic growth rate of the population is moderate or small. In estimating the dispersal standard deviation, however, incorrect assumptions about the degree of leptokurtosis can lead to a large bias in estimation and predictions.

Spatial heterogeneity in the effects of climate and density-dependence on dispersal in a house sparrow metapopulation

Dispersal plays a key role in the response of populations to climate change and habitat fragmentation. Here, we use data from a long-term metapopulation study of a non-migratory bird, the house sparrow (Passer domesticus), to examine the influence of increasing spring temperature and density-dependence on natal dispersal rates and how these relationships depend on spatial variation in habitat quality. The effects of spring temperature and population size on dispersal rate depended on the habitat quality. Dispersal rate increased with temperature and population size on poor-quality islands without farms, where house sparrows were more exposed to temporal fluctuations in weather conditions and food availability. By contrast, dispersal rate was independent of spring temperature and population size on high-quality islands with farms, where house sparrows had access to food and shelter all the year around. This illustrates large spatial heterogeneity within the metapopulation in how population density and environmental fluctuations affect the dispersal process.

Dispersal of introduced house sparrows Passer domesticus: an experiment

An important issue concerning the introduction of non-indigenous organisms into local populations is the potential of the introduced individuals to spread and interfere both demographically and genetically with the local population. Accordingly, the potential of spatial dispersal among introduced individuals compared with local individuals is a key parameter to understand the spatial and temporal dynamics of populations after an introduction event. In addition, if the variance in dispersal rate and distance is linked to individual characteristics, this may further affect the population dynamics. We conducted a large-scale experiment where we introduced 123 house sparrows from a distant population into 18 local populations without changing population density or sex ratio. Introduced individuals dispersed more frequently and over longer distances than residents. Furthermore, females had higher probability of dispersal than males. In females, there was also a positive relationship between the wing length and the probability of dispersal and dispersal distance. These results suggest that the distribution and frequency of introduced individuals may be predicted by their sex ratio as well as their phenotypic characteristics.