Karin C. Harding

A Unifying Framework for Metapopulation Dynamics

Many biologically important processes, such as genetic differentiation, the spread of disease, and population stability, are affected by the (natural or enforced) subdivision of populations into networks of smaller, partly isolated, subunits. Such “metapopulations” can have extremely complex dynamics. We present a new general model that uses only two functions to capture, at the metapopulation scale, the main behavior of metapopulations. We show how complex, structured metapopulation models can be translated into our generalized framework. The metapopulation dynamics arising from some important biological processes are illustrated: the rescue effect, the Allee effect, and what we term the “antirescue effect.” The antirescue effect captures instances where high migration rates are deleterious to population persistence, a phenomenon that has been largely ignored in metapopulation conservation theory. Management regimes that ignore a significant antirescue effect will be inadequate and may actually increase extinction risk. Further, consequences of territoriality and conspecific attraction on metapopulation‐level dynamics are investigated. The new, simplified framework can incorporate knowledge from epidemiology, genetics, and population biology in a phenomenological way. It opens up new possibilities to identify and analyze the factors that are important for the evolution and persistence of the many spatially subdivided species.

Status of Baltic grey seals: Population assessment and extinction risk

The grey seal ( Halichoerus grypus ) population in the Baltic Sea is recovering after a century of bounty hunting and 3 decades of low fertility rates caused by environmental pollution. A conservative estimate of the population size in 2003 was 19,400 animals, and available data suggest an annual rate of increase of 7.5% since 1990. The growing population has led to increased interactions with the fishery, and demands are being raised for the re-introduction of the hunt. We provide a demographic analysis and a risk assessment of the population, and make recommendations on how to decrease the risk of over exploitation. Although hunting increases the risk of quasi-extinction, the risk can be significantly reduced by the choice of a cautious hunting regime. The least hazardous regimes allow no hunting below a ‘security level’ in population size. Obviously, to implement such a hunting regime detailed knowledge of the population size and growth rate is required. It is not possible to estimate “true” risks for quasi-extinction, but we used an approach where the relative difference for different scenarios can be compared. With a security level at 5,000 females, the population quasi-extinction risk increases 50 fold at an annual hunt of 500 females compared with a scenario with no hunting. The risk of quasi-extinction is very sensitive to declines in the mean growth rate and to increased variance in growth rate. The variance in the population estimates over the last 14 years imply that it would take 9 years to detect a decline from 1.075 to 1.027 in the rate of population increase. We also show how the age composition of killed animals influences the impact of the hunt. The overall recommendation is that hunting should be kept to a minimum, carefully documented and accompanied by close population monitoring.

Ecology and Distribution of the Isopod Genus Idotea in the Baltic Sea: Key Species in a Changing Environment

ABSTRACT Marine isopods of the genus Idotea [I. balthica (Pallas, 1772), I. chelipes (Pallas, 1766), and I. granulosa Rathke, 1843] are common meso-grazers that enter deep into the Baltic Sea and here appear to live at their physiological limit, determined by salinity and temperature tolerance. We review available data on distribution and community ecology to assess the functional role of Idotea in the Baltic Sea and how global change may affect essential ecological interactions. Data from the last 150 years suggest an on-going shift southward for I. chelipes and I. granulosa that may be caused by a changing climate. Several studies report local extinctions and mass abundances, which may be caused by a changing food web from over-fishing and eutrophication. The three species of Idotea have clear habitat segregation in the Baltic Sea, where salinity, temperature and vegetation are the main dimensions. Idotea spp. have a central role as grazers and in communities dominated by the perennial macrophytes Fucus spp. and Zostera marina and attain impressive feeding rates on a range of epiphytes/filamentous algae (top-down effect). Idotea can have both a direct negative grazing effect on macrophytes but also an indirect positive effect by removing epiphytes. The relative role of nutritional value and chemical defence for food preference is yet unclear for Idotea. Baltic idoteids are also important prey for several fish (bottom-up effect) and fish predation may have increased following over-fishing of piscivorous fish. It is concluded that Idotea is a key taxon in the Baltic Sea food web, where guilds often contain few dominant species. Changes in population dynamics of Idotea, as a function of human generated global change, may have large-scale consequences for ecosystem functions in a future Baltic Sea, e.g. the extent of vegetation cover in the coastal zone.

Age- and Sex-Specific Mortality Patterns in an Emerging Wildlife Epidemic: The Phocine Distemper in European Harbour Seals

Analyses of the dynamics of diseases in wild populations typically assume all individuals to be identical. However, profound effects on the long-term impact on the host population can be expected if the disease has age and sex dependent dynamics. The Phocine Distemper Virus (PDV) caused two mass mortalities in European harbour seals in 1988 and in 2002. We show the mortality patterns were highly age specific on both occasions, where young of the year and adult (>4 yrs) animals suffered extremely high mortality, and sub-adult seals (1–3 yrs) of both sexes experienced low mortality. Consequently, genetic differences cannot have played a main role explaining why some seals survived and some did not in the study region, since parents had higher mortality levels than their progeny. Furthermore, there was a conspicuous absence of animals older than 14 years among the victims in 2002, which strongly indicates that the survivors from the previous disease outbreak in 1988 had acquired and maintained immunity to PDV. These specific mortality patterns imply that contact rates and susceptibility to the disease are strongly age and sex dependent variables, underlining the need for structured epidemic models for wildlife diseases. Detailed data can thus provide crucial information about a number of vital parameters such as functional herd immunity. One of many future challenges in understanding the epidemiology of the PDV and other wildlife diseases is to reveal how immune system responses differ among animals in different stages during their life cycle. The influence of such underlying mechanisms may also explain the limited evidence for abrupt disease thresholds in wild populations.

Acquired immunity and stochasticity in epidemic intervals impede the evolution of host disease resistance.

Disease can generate intense selection pressure on host populations, but here we show that acquired immunity in a population subject to repeated disease outbreaks can impede the evolution of genetic disease resistance by maintaining susceptible genotypes in the population. Interference between the life‐history schedule of a species and periodicity of the disease has unintuitive effects on selection intensity, and stochasticity in outbreak period further reduces the rate of spread of disease‐resistance alleles. A general age‐structured population genetic model was developed and parameterized using empirical data for phocine distemper virus (PDV) epizootics in harbor seals. Scenarios with acquired immunity had lower levels of epizootic mortality compared with scenarios without acquired immunity for the first PDV outbreaks, but this pattern was reversed after about five disease cycles. Without acquired immunity, evolution of disease resistance was more rapid, and long‐term population size variation is efficiently dampened. Acquired immunity has the potential to significantly influence rapid evolutionary dynamics of a host population in response to age‐structured disease selection and to alter predicted selection intensities compared with epidemiological models that do not consider such feedback. This may have important implications for evolutionary population dynamics in a range of human, agricultural, and wildlife disease settings.

Seasonal Activity Budget of Adult Baltic Ringed Seals

Although ringed seals are important components in oceanic and fresh water ecosystems at high latitudes, little is known about how they exploit these harsh environments. Seasonal activity and diving behaviour of 19 adult Baltic ringed seals were studied by satellite telemetry. We elaborated an activity budget for ten months of the year, extending over the period from moult to the breeding season. Seals from three main regions showed explicit site fidelity and the distributions of animals tagged from different areas did not overlap, suggesting separate stocks. Both the mean duration and the mean depth of dives peaked in June and July. Seals spent 70% (females) to 85% (males) of their time diving in June and July which decreased to 50% in late autumn. Less than one percent of dives exceeded 10 min in females, while 10% of male dives lasted longer than 10 min in June to September. Less than one percent of dives lasted for more than 25 min. Both females and males were most active during day time and hauled out predominantly during the night. Activity patterns during the summer are suggested to be correlated to energy accumulation and prey availability. The information on seasonal activity budget is crucial for developing population energetic models where interactions between ringed seals and other trophic levels can be evaluated.

Mass mortality in harbour seals and harbour porpoises caused by an unknown pathogen

EPIDEMICS caused by phocine distemper virus (pdv) resulted in mass mortalities of European harbour seals ( Phoca vitulina ) in 1988 and 2002 ([Harkonen and others 2006][1]). A third epidemic started in June 2007; the gross pathological changes observed in seals that died in the 2007 outbreak were

Collapse of a marine mammal species driven by human impacts.

Understanding historical roles of species in ecosystems can be crucial for assessing long term human impacts on environments, providing context for management or restoration objectives, and making conservation evaluations of species status. In most cases limited historical abundance data impedes quantitative investigations, but harvested species may have long-term data accessible from hunting records. Here we make use of annual hunting records for Caspian seals (Pusa caspica) dating back to the mid-19th century, and current census data from aerial surveys, to reconstruct historical abundance using a hind-casting model. We estimate the minimum numbers of seals in 1867 to have been 1–1.6 million, but the population declined by at least 90% to around 100,000 individuals by 2005, primarily due to unsustainable hunting throughout the 20th century. This collapse is part of a broader picture of catastrophic ecological change in the Caspian over the 20th Century. Our results combined with fisheries data show that the current biomass of top predators in the Caspian is much reduced compared to historical conditions. The potential for the Caspian and other similar perturbed ecosystems to sustain natural resources of much greater biological and economic value than at present depends on the extent to which a number of anthropogenic impacts can be harnessed.

Increased migration in host-pathogen metapopulations can cause host extinction.

There are at least two potentially counteracting effects of migration in host-pathogen metapopulations. On the one hand increased migration leads to increased colonization of empty habitats by healthy hosts; on the other hand migrants can carry infectious diseases to susceptible populations. Earlier metapopulation models have found that the beneficial effects of increasing migration (reduced infection) are likely to dominate, and a general recommendation for managers of endangered metapopulations has been to increase connectivity between habitat patches. We extend the model framework to simultaneously allow for (1) Allee effects in host colonization rate, (2) spillover of pathogens from a second host species, and (3) differential colonization success by infected and healthy hosts. We find that the dynamics of a host-pathogen system can be highly sensitive to increased migration rates. Allee effects make host populations vulnerable to spillover of pathogens from other hosts, and metapopulation extinction can emerge from seemingly stable situations of endemic coexistence. Increasing connectivity in endangered metapopulations can be a risky management action unless the details of the biology of the host-pathogen system are known.

The scaling of diving time budgets: insights from an optimality approach.

Simple scaling arguments suggest that, among air‐breathing divers, dive duration should scale approximately with mass to the one‐third power. Recent phylogenetic analyses appear to confirm this. The same analyses showed that duration of time spent at the surface between dives has scaling very similar to that of dive duration, with the result that the ratio of dive duration to surface pause duration is approximately mass invariant. This finding runs counter to other arguments found in the diving literature that suggest that surface pause duration should scale more positively with mass, leading to a negative scaling of the dive‐pause ratio. We use a published model of optimal time allocation in the dive cycle to show that optimal decisions can predict approximate mass invariance in the dive‐pause ratio, especially if metabolism scales approximately with mass to the two‐thirds power (as indicated by some recent analyses) and oxygen uptake is assumed to have evolved to supply the body tissues at the required rate. However, emergent scaling rules are sensitive to input parameters, especially to the relationship between the scaling of metabolism and oxygen uptake rate at the surface. Our results illustrate the utility of an optimality approach for developing predictions and identifying key areas for empirical research on the allometry of diving behavior.