A theory of stochastic harvesting in stochastic environments

We investigate how model populations respond to stochastic harvesting in a stochastic environment. In particular, we show that the effects of variable harvesting on the variance in population density and yield depend critically on the autocorrelation of environmental noise and on whether the endogenous dynamics of the population display over- or undercompensation to density. These factors interact in complicated ways; harvesting shifts the slope of the renewal function, and the net effect of this shift will depend on the sign and magnitude of the other influences. For example, when environmental noise exhibits a positive auto correlation, the relative importance of a variable harvest to the variance in density increases with overcompensation but decreases with undercompensation. For a fixed harvesting level, an increasing level of autocorrelation in environmental noise will decrease the relative variation in population density when overcompensation would otherwise occur. These and other intricate interactions have important ramifications for the interpretation of time series data when no prior knowledge of demographic or environmental details exists, These effects are important whenever the harvesting rate is sufficiently high or variable, conditions likely to occur in many systems, whether the harvesting is caused by commercial exploitation or by any other strong agent of density-independent mortality

Timing of spring migration in birds: long-term trends, North Atlantic Oscillation and the significance of different migration routes

We studied long-term trends and the yearly variation in mean spring passage time in 36 passerine bird species trapped at Ottenby Bird Observatory in south-eastern Sweden. Between the years 1952-2002, data were available for 22-45 years depending on species. Most long-distance migrant species passed progressively earlier over the study period (range: 2.5 days earlier to 0.7 days later per 10 years, with an average of 0.9 days earlier per 10 years). The annual variation in timing of migration in most species, regardless of migration distance, correlated negatively with the winter index of the North Atlantic Oscillation (NAO), a large-scale climate phenomenon influencing the climate in the North Atlantic region. Birds passed earlier after mild and humid winters., corresponding to the high phase of the NAO. This corroborates the pattern round at a nearby migration site with a comparable dataset (Helgoland, 600 km WSW of Ottenby). However, short/medium-distance migrant species at Otterby, in contrast to the situation at Helgoland, have shown no general trend of earlier passage in recent years. This was probably a consequence of the shorter study period at Ottenby, which included only the last 22-32 years (41 years at Helgoland), when the NAO showed no significant trend. At the species-specific level, the long-term trends in passage time were similar at the two sites, and there was some congruence to the extent that a given species was affected by NAO. Long-distance migrants wintering south and south-east of the breeding grounds showed some of the strongest changes in long-term trends (passing progressively earlier) at Ottenby, and for some of these species passage time varied negatively with NAO. Obviously, and contrary to previous suggestions, variations in NAO also influence birds migrating through eastern Europe, although the direct or indirect mechanisms through which this is achieved are unknown

Individual heterogeneity and senescence in Silvereyes on Heron Island

Individual heterogeneity and correlations between life history traits play a fundamental role in life history evolution and population dynamics. Unobserved individual heterogeneity in survival can be a nuisance for estimation of age effects at the individual level by causing bias due to mortality selection. We jointly analyze survival and breeding output from successful breeding attempts in an island population of Silvereyes (Zosterops lateralis chlorocephalus) by fitting models that incorporate age effects and individual heterogeneity via random effects. The number of offspring produced increased with age of parents in their first years of life but then eventually declined with age. A similar pattern was found for the probability of successful breeding. Annual survival declined with age even when individual heterogeneity was not accounted for. The rate of senescence in survival, however, depends on the variance of individual heterogeneity and vice versa; hence, both cannot be simultaneously estimated with precision. Model selection supported individual heterogeneity in breeding performance, but we found no correlation between individual heterogeneity in survival and breeding performance. We argue that individual random effects, unless unambiguously identified, should be treated as statistical nuisance or taken as a starting point in a search for mechanisms rather than given direct biological interpretation

Skogliga grunddata

Skogliga grunddata är digitala raster-kartor som beskriver tillståndet i skog och mark. De skogliga variablerna är i huvudsak framtagna genom en sambearbetning av laserdata från Lantmäteriets nationella laserskanning och provytor från Riksskogstaxeringen. De digitala kartorna som beskriver skogsmarken är till stor hjälp för privata skogsägare, skogstjänstemän, myndigheter, m fl. Data är dels tillgängligt för gratis nedladdning, men det finns även möjligheter att titta på kartorna med hjälp av interaktiva webverktyg. Lantmäteriets laserskanning påbörjades 2009 och fortgår än idag. Endast delar av fjällkedjan återstår. Grundtanken med laserskanningen var att få en bra höjdmodell över landet. En bra höjdmodell ger stor nytta vid all samhällsplanering, men är även till hjälp vid klimatanpassning. Riksskogstaxeringen samlar årligen in data om Sveriges skogstillstånd med permanenta och tillfälliga provytor. Dessa provytor är koordinatsatta och är väl spridda över hela landet. I detta projekt har provytor från Riksskogstaxeringen kombinerats med laserdata för att ta fram rasterkartor med uppgifter om virkesvolym, grundyta, medelhöjd, medeldiameter och trädbiomassa

Testing for effects of climate change on competitive relationships and coexistence between two bird species

Nils Christian Stenseth et al.© 2015 The Author(s) Published by the Royal Society. Climate change is expected to have profound ecological effects, yet shifts in competitive abilities among species are rarely studied in this context. Blue tits (Cyanistes caeruleus) and great tits (Parus major) compete for food and roosting sites, yet coexist across much of their range. Climate change might thus change the competitive relationships and coexistence between these two species. Analysing four of the highest-quality, long-term datasets available on these species across Europe, we extend the textbook example of coexistence between competing species to include the dynamic effects of long-term climate variation. Using threshold time-series statistical modelling, we demonstrate that long-term climate variation affects species demography through different influences on density-dependent and density-independent processes. The competitive interaction between blue tits and great tits has shifted in one of the studied sites, creating conditions that alter the relative equilibrium densities between the two species, potentially disrupting longterm coexistence. Our analyses show that long-term climate change can, but does not always, generate local differences in the equilibrium conditions of spatially structured species assemblages. We demonstrate how long-term data can be used to better understand whether (and how), for instance, climate change might change the relationships between coexisting species. However, the studied populations are rather robust against competitive exclusion.Peer Reviewe

Timing of spring migration in birds: long-term trends, North Atlantic Oscillation and the significance of different migration routes

We studied long-term trends and the yearly variation in mean spring passage time in 36 passerine bird species trapped at Ottenby Bird Observatory in south-eastern Sweden. Between the years 1952-2002, data were available for 22-45 years depending on species. Most long-distance migrant species passed progressively earlier over the study period (range: 2.5 days earlier to 0.7 days later per 10 years, with an average of 0.9 days earlier per 10 years). The annual variation in timing of migration in most species, regardless of migration distance, correlated negatively with the winter index of the North Atlantic Oscillation (NAO), a large-scale climate phenomenon influencing the climate in the North Atlantic region. Birds passed earlier after mild and humid winters., corresponding to the high phase of the NAO. This corroborates the pattern round at a nearby migration site with a comparable dataset (Helgoland, 600 km WSW of Ottenby). However, short/medium-distance migrant species at Otterby, in contrast to the situation at Helgoland, have shown no general trend of earlier passage in recent years. This was probably a consequence of the shorter study period at Ottenby, which included only the last 22-32 years (41 years at Helgoland), when the NAO showed no significant trend. At the species-specific level, the long-term trends in passage time were similar at the two sites, and there was some congruence to the extent that a given species was affected by NAO. Long-distance migrants wintering south and south-east of the breeding grounds showed some of the strongest changes in long-term trends (passing progressively earlier) at Ottenby, and for some of these species passage time varied negatively with NAO. Obviously, and contrary to previous suggestions, variations in NAO also influence birds migrating through eastern Europe, although the direct or indirect mechanisms through which this is achieved are unknown