Opposing trends in survival and recruitment slow the recovery of a historically overexploited fishery

Quantifying temporal variation in demographic rates is a central goal of population ecology. In this study, we analyzed a multidecadal age-structured time series of Arctic charr (Salvelinus alpinus) abundance in Lake Mývatn, Iceland, to infer the time-varying demographic response of the population to reduced harvest in the wake of the fishery's collapse. Our analysis shows that while survival probability of adults increased following the alleviation of harvesting pressure, per capita recruitment consistently declined over most of the study period, until the final three years when it began to increase. The countervailing demographic trends resulted in only limited directional change in the total population size and population growth rate. Rather, the population dynamics were dominated by large interannual variability and a shift towards an older age distribution. Our results are indicative of a slow recovery of the population after its collapse, despite the rising number of adults following relaxed harvest. This underscores the potential for heterogeneous demographic responses to management efforts due to the complex ecological context in which such efforts take place.

Assessing the Viability of the Sarasota Bay Community of Bottlenose Dolphins

Population models, such as those used for Population Viability Analysis (PVA), are valuable for projecting trends, assessing threats, guiding environmental resource management, and planning species conservation measures. However, rarely are the needed data on all aspects of the life history available for cetacean species, because they are long-lived and difficult to study in their aquatic habitats. We present a detailed assessment of population dynamics for the long-term resident Sarasota Bay common bottlenose dolphin (Tursiops truncatus) community. Model parameters were estimated from 27 years of nearly complete monitoring, allowing calculation of age-specific and sex-specific mortality and reproductive rates, uncertainty in parameter values, fluctuation in demographic rates over time, and intrinsic uncertainty in the population trajectory resulting from stochastic processes. Using the Vortex PVA model, we projected mean population growth and quantified causes of variation and uncertainty in growth. The ability of the model to simulate the dynamics of the population was confirmed by comparing model projections to observed census trends from 1993 to 2020. When the simulation treated all losses as deaths and included observed immigration, the model projects a long-term mean annual population growth of 2.1%. Variance in annual growth across years of the simulation (SD = 3.1%) was due more to environmental variation and intrinsic demographic stochasticity than to uncertainty in estimates of mean demographic rates. Population growth was most sensitive to uncertainty and annual variation in reproduction of peak breeding age females and in calf and juvenile mortality, while adult survival varied little over time. We examined potential threats to the population, including increased anthropogenic mortality and impacts of red tides, and tested resilience to catastrophic events. Due to its life history characteristics, the population was projected to be demographically stable at smaller sizes than commonly assumed for Minimum Viable Population of mammals, but it is expected to recover only slowly from any catastrophic events, such as disease outbreaks and spills of oil or other toxins. The analyses indicate that well-studied populations of small cetaceans might typically experience slower growth rates (about 2%) than has been assumed in calculations of Potential Biological Removal used by management agencies to determine limits to incidental take of marine mammals. The loss of an additional one dolphin per year was found to cause significant harm to this population of about 150 to 175 animals. Beyond the significance for the specific population, demographic analyses of the Sarasota Bay dolphins provide a template for examining viability of other populations of small cetaceans.

Measuring habitat quality for waterbirds: a review

Quantifying habitat quality is dependent on measuring a site’s relative contribution to population growth rate. This is challenging for studies of waterbirds, whose high mobility can decouple demographic rates from local habitat conditions and make sustained monitoring of individuals near-impossible. To overcome these challenges, biologists have used many direct and indirect proxies of waterbird habitat quality. However, consensus on what methods are most appropriate for a given scenario is lacking. We undertook a structured literature review of the methods used to quantify waterbird habitat quality, and provide a synthesis of the context-dependent strengths and limitations of those methods. Our structured search of the Web of Science database returned a sample of 398 studies, upon which our review was based. The reviewed studies assessed habitat quality by either measuring habitat attributes (e.g., food abundance, water quality, vegetation structure), or measuring attributes of the waterbirds themselves (e.g., demographic parameters, body condition, behaviour, distribution). Measuring habitat attributes, although they are only indirectly related to demographic rates, has the advantage of being unaffected by waterbird behavioural stochasticity. Conversely, waterbird-derived measures (e.g., body condition, peck rates) may be more directly related to demographic rates than habitat variables, but may be subject to greater stochastic variation (e.g., behavioural change due to presence of conspecifics). Therefore, caution is needed to ensure that the measured variable does influence waterbird demographic rates. This assumption was usually based on ecological theory rather than empirical evidence. Our review highlighted that there is no single best, universally applicable method to quantify waterbird habitat quality. Individual project specifics (e.g., time frame, spatial scale, funding) will influence the choice of variables measured. Where possible, practitioners should measure variables most directly related to demographic rates. Generally, measuring multiple variables yields a better chance of accurately capturing the relationship between habitat characteristics and demographic rates.

Environmental variability directly affects the prevalence of divorce in monogamous albatrosses

In many socially monogamous species, divorce is a strategy used to correct for sub-optimal partnerships and is informed by measures of previous breeding performance. The environment affects the productivity and survival of populations, thus indirectly affecting divorce via changes in demographic rates. However, whether environmental fluctuations directly modulate the prevalence of divorce in a population remains poorly understood. Here, using a longitudinal dataset on the long-lived black-browed albatross ( Thalassarche melanophris ) as a model organism, we test the hypothesis that environmental variability directly affects divorce. We found that divorce rate varied across years (1% to 8%). Individuals were more likely to divorce after breeding failures. However, regardless of previous breeding performance, the probability of divorce was directly affected by the environment, increasing in years with warm sea surface temperature anomalies (SSTA). Furthermore, our state-space models show that warm SSTA increased the probability of switching mates in females in successful relationships. For the first time, to our knowledge, we document the disruptive effects of challenging environmental conditions on the breeding processes of a monogamous population, potentially mediated by higher reproductive costs, changes in phenology and physiological stress. Environmentally driven divorce may therefore represent an overlooked consequence of global change.

Population Demography

This chapter reviews population measurements and the demographic and epidemiological transitions and how these may change over time. Knowledge of the population age and sex structure and distribution are essential to estimate those people at most risk and for estimating population access to services and programmes. Sources of population information are presented and factors highlighted for the quality of population data. Definitions of demographic rates and life expectancy, population growth, census procedures, death certification, and demographic surveillance are all outlined.

Growth and Space-use of Eastern Red-backed Salamanders (Plethodon cinereus) in Mature and Regenerating Forests

Movement and demographic rates are critical to the persistence of populations in space and time. Despite their importance, estimates of these processes are often derived from a limited number of populations spanning broad habitat or environmental gradients. With increasing appreciation of the role fine-scale environmental variation in microgeographic adaptation, there is need and value to assessing within-site variation in movement, growth, and demographic rates. In this study, we analyze three years of spatial capture-recapture data collected from a mixed-use deciduous forest site in central Ohio, USA. Study plots were situated in mature forest on a slope and in successional forest on a ridge but were separated by less than 100-m distance. Our data showed that the density of salamanders was less on ridges, which corresponded with greater distance between nearest neighbors, less overlap in core use areas, greater space-use, and greater shifts in activity centers when compared to salamander occupying the slope habitat. However, these differences were moderate. In contrast, we estimated growth rates of salamanders occupying the ridge to be significantly greater than salamander on the slope. These differences result in ridge salamanders reaching maturity more than one year earlier than slope salamanders, increasing their lifetime fecundity by as much as 43%. The patterns we observed in space use and growth are likely the result of density-dependent processes, reflecting differences in resource availability or quality. Our study highlights how fine-scale, within-site, variation can shape population demographics. As research into the demographic and population consequences of climate change and habitat loss and alteration continue, future research should take care to acknowledge the role that fine-scale variation may play, especially for organisms with small home ranges or limited vagility.

Spatial Variation in Demographic Processes and the Potential Role of Hybridization to Future Persistence

CONTEXTSpatial variation in life history traits plays a crucial role in the structure and dynamics of populations. The demographic responses of local populations to fine-scale habitat heterogeneity have consequences for species at a broader scale and responses vary across spatial scales. Yet, the specific nature of such relationships is unclear across taxa.OBJECTIVESWe evaluated the spatial variation in demographic traits of cryptic terrestrial salamanders across the broad scale environmental gradient of elevation (i.e. temperature) and the fine-scale gradient of stream distance (i.e. moisture).METHODSUsing a 4-years of spatial mark-recapture and count data, we implemented a spatially explicit Integrated Population Model to understand demographic rates across scales. We also investigated how hybridization, which occurs in between lungless salamanders at mid-elevations, may influence demographic rates.RESULTSWe found that high elevation animals grow faster and move more, especially far from streams likely as a result of increased temperatures. Survival was highest but recruitment rates were lowest at low elevations and significantly declined with distance to stream. We also found that hybrid animals at low elevations had higher survival probabilities.CONCLUSIONSOur study reveals nuanced spatial variation in demographic rates that differ in magnitude depending on the scale at which they are assessed. Our results also show animals exhibit demographic compensation across abiotic gradients, underscoring the need for further conservation and management efforts to implement spatially explicit and dynamic strategies to match the demographic variation of species and populations of species separated across space.

Spatial demography

Demographic methods can be used to study the spatial response of individuals and populations to current global changes. The first mechanism underlying range shifts is a change in the spatial distribution of births and deaths. The spatial regression of demographic rates with geostatistical and spatially explicit models documents the intrinsic growth rate across the range of a population. The population distribution is expected to shift towards areas with the largest intrinsic growth rate, both mechanistically and because these areas are attractive to dispersing individuals. The second mechanism is indeed movement, including emigration away from places that recently became inhospitable and immigration into newly available locations. The analysis of dispersal fluxes using movement data, or indirectly by comparing the observed and intrinsic growth rates in integrated population models, documents these fluxes. Combining these two mechanisms in integral projection models or in individual-based simulations is expected to yield major advances in predictive spatial ecology, that is, mechanistic species distribution models.