Changes In Adult Survival Research Articles

Characterization of elasticity patterns of North American freshwater fishes

We applied elasticity analysis to 88 North American freshwater fishes to assess the relative impacts of changes in the vital rates on asymptotic population growth. Variance in vital rates was summarized for four distinct functional groups: (i) species with population growth rates strongly sensitive to perturbations in adult survival; (ii) species with population growth rates sensitive to perturbations in overall survival; (iii) species with population growth rates most sensitive to perturbations in juvenile survival; and (iv) species with population growth rates sensitive to perturbations in juvenile survival and fecundity. The results of the present study also showed that (a) elasticity patterns cannot be inferred in a straightforward manner from trade-offs between life-history traits, (b) the sensitivity of a population's growth rate to changes in adult survival and fecundity can be predicted empirically from life span and age at maturity, respectively, (c) elasticities are highly conserved among genera within the same taxonomic family, and (d) there are key divergences between elasticity patterns of freshwater fish and other vertebrate taxa.

Modelling populations of long-lived birds of prey for conservation: A study of imperial eagles ( Aquila heliaca) in Kazakhstan

Eagle populations worldwide are in decline and their demography is generally poorly understood. We use novel sensitivity analysis of stochastic simulation models to analyse the demography of the worlds highest-density and longest-studied population of eastern imperial eagles ( Aquila heliaca), at the Naurzum National Nature Reserve in Kazakhstan. Single variable perturbation (a simple elasticity-type analysis) showed that population growth was most sensitive to changes in adult survival but provided no information on how interactions between parameters may influence population growth. Multiple-variable perturbations (a more comprehensive elasticity-type analysis) suggested that population growth is relatively more sensitive to adult survival than is indicated by single-variable perturbation but also that when adult survival is within a biologically reasonable range, other parameters are still highly consequential to model outputs. For Naurzum’s imperial eagles, and for other structured populations of vertebrates, effective conservation and management likely requires an approach that addresses the importance of simultaneous variation in multiple vital rates including both survivorship and reproductive output.

Albatross Populations In Peril: A Population Trajectory For Black-Browed Albatrosses At South Georgia

Simulation modeling was used to reconstruct Black-browed Albatross (Diomedea melanophris) population trends. Close approximations to observed data were accomplished by annually varying survival rates, reproductive success, and probabilities of returning to breed given success in previous years. The temporal shift in annual values coincided with the start of longline fishing at South Georgia and potential changes in krill abundance. We used 23 years of demographic data from long-term studies of a breeding colony of this species at Bird Island, South Georgia, to validate our model. When we used annual parameter estimates for survival, reproductive success, and probabilities of returning to breed given success in previous years, our model trajectory closely followed the observed changes in breeding population size over time. Population growth rate was below replacement (lambda < 1) in most years and was most sensitive to changes in adult survival. This supports the recent IUCN uplisting of this species from "Vulnerable" to "Endangered." Comparison of pre-1988 and post-1988 demography (before and after the inception of a longline fishery in the breeding area) reveals a decrease in lambda from 0.963 to 0.910. A life table response experiment (LTRE) showed that this decline in lambda was caused mostly by declines in survival of adults. If 1988-1998 demographic rates are maintained, the model predicts a 98% chance of a population of fewer than 25 pairs within 78 years. For this population to recover to a status under which it could be "delisted," a 10% increase in survival of all age classes would be needed.

Declining winter survival and fitness implications associated with latitudinal distribution in Norwegian Greylag Geese Anser anser.

The Norwegian Greylag Goose Anser anser population has been increasing steadily over the past few decades, causing increasing nuisance in terms of agricultural crop damage. This, in combination with the importance of Greylags as a hunting target, has called for demographic estimates for the population to assist in management decisions. To this end, we analysed long‐term mark–recapture data using Cormack–Jolly‐Seber models embedded in program MARK to obtain survival estimates for the population. No sex‐specific difference, or age effect on survival after juveniles had completed their first migration (3 months of age), was evident. Mean first‐year survival was reported as 0.485 and annual survival of older birds as 0.700. On a monthly basis, survival in Greylags during summer and winter was very similar over the study period. A significant linear decline in winter survival from 0.909 to 0.807 was, however, apparent during the study period. Over the second half of the study (1994–2002), summer survival was about 3% lower than in the first half (1986–94) but no linear relationship was evident. We found a significant inverse relationship between Greylag survival during summer and latitudinal distribution in Norway. A similar relationship was evident between survival and annual bag numbers. The changes in adult survival observed in this study are likely to have had a substantial impact on the growth rate of the Norwegian Greylag population.

Experimental Analysis of the Impact of Foxes on Freshwater Turtle Populations

Abstract: Common demographic parameters such as survival and fecundity are regularly estimated in freshwater turtles, but large‐scale manipulative and comparative experiments are rare. The resilience of a population to stage‐specific predation is dependent on density‐dependent inversity or compensation. Some species are also more susceptible to predation because of their life‐history strategies. Nest predation by introduced red foxes ( Vulpes vulpes) on Australian freshwater turtles is a major source of mortality, but its full impact has yet to be evaluated. Freshwater turtles are long lived, and a population decline resulting from nest predation by foxes may take a long time to become evident. We evaluated the impact of foxes on the population growth and patterns of survival of two widely distributed Australian freshwater turtles in the Murray River, where nest predation rates have previously been reported at above 95%. We estimated densities and survivorship of Emydura macquarii and Chelodina expansa from the nest to adult stage. We then conducted a fox removal program (before‐after control‐impact design) to determine the impact of foxes on populations of these species. We found a contrast in life‐history strategies between the two sympatric species. C. expansa was five times less abundant than E. macquarii. A type III survivorship curve described the life history of E. macquarii because nest predation and adult survival rates were &gt;0.9. Densities of C. expansa nests were low and situated away from shore. Consequently, nest predation rates were half those of E. macquarii. Projection matrices predicted that E. macquarii populations may be declining slowly but are likely to respond to changes in adult survival, which has the greatest elasticity value. Foxes destroyed a proportion of nesting adult females each year; however, there appears to be no density‐dependent compensation in adult survival, and foxes potentially regulate these populations. E. macquarii populations had low productivity and relied on a standing crop of turtles for population maintenance, whereas C. expansa had relatively more juveniles in the population and was less reliant on adult survival. Despite its vulnerable status, C. expansa is relatively more stable than E. macquarii, primarily because there were proportionally more juveniles in the population. Management options focusing on reducing adult mortality in both species will be most effective.

Estimating predation on breeding European storm‐petrels ( Hydrobates pelagicus ) by yellow‐legged gulls ( Larus michahellis )

Several hypotheses about changes in adult survival of European storm-petrels Hydrobates pelagicus and predation by yellow-legged gulls Larus michahellis at Benidorm Island (western Mediterranean) during 1993–2003 were assessed. Two approaches were used: analysis of pellets containing remains of predated petrels from two caves, and multistate capture–recapture models using recaptures of breeders together with recoveries only from one of these caves. These models were used to estimate separately adult storm-petrel survival and a minimum probability of being killed by gulls. At least 11–14% of the storm-petrels killed had been ringed as breeders. Local adult survival varied greatly among years, from 0.701 (SE=0.095) to extremely high values (0.951, SE=0.031) for such a small species (0.833 on average for the whole period). Nevertheless, it was not possible to assess whether this variation was influenced by inter-annual differences in gull predation. The peak of predation occurred in June, when maximum numbers of prospecting petrels visit the colonies. Predation was much higher at the cave that received stronger illumination from Benidorm city. Minimum predation probabilities estimated from capture–recapture varied with time from 0.022 (SE=0.011) to 0.040 (SE=0.016), and increased significantly after the powering of light received from the city in 1997. In these years, predation by yellow-legged gulls accounted for a large percentage of mortality of storm-petrels at Benidorm Island (up to 33%). Both methodological approaches confirmed that predation was carried out by specialist gulls, and that neither the size of the gull colony nor its food availability influenced predation on petrels. Management might reduce predation rate by removing gulls that nest in the caves, since these individuals tend to kill most storm-petrels.

Is the reintroduced white stork ( Ciconia ciconia) population in Switzerland self-sustainable?

The migratory white stork ( Ciconia ciconia) became extinct in Switzerland in 1950. A reintroduction project with intensive management (translocation, prevention of migration, artificial feeding) started in 1948, and 175 pairs were breeding in 2000. For the period 1973–2000 we estimated annual survival rates and fledging success to estimate the population growth rate by a stochastic matrix projection model. Compared to other populations, adult survival rate (0.86, with 95% CI: 0.81–0.89) was very high and little variable over time, juvenile survival (0.37, CI: 0.31–0.43) was comparable to other populations whereas the average fledging success was low (1.65) but strongly variable over time. The population growth rate was positive, indicating that the population is self-sustainable at the moment. The growth of the white stork population was largely favoured by the high adult survival which more than compensates for the low fledging success. The population growth rate is particularly sensitive to changes in adult survival, but it would be very difficult to improve this further with management. However, maintenance of the high adult survival is crucial and an improvement in fledging success seems important for the long-term persistence of the white stork population in Switzerland. Fledging success depends on habitat quality, and thus restoration of breeding habitats should be the main management activity in the near future.

Variable age structure and apparent density dependence in survival of adult ungulates.

Large herbivores have strongly age-structured populations. Because recruitment often decreases as population density increases, in unexploited populations the proportion of older adults may increase with density. Because survival senescence is typical of ungulates, ignoring density-dependent changes in age structure could lead to apparent density-dependence in adult survival. To test for density dependence in adult survival, we used data from three populations that underwent considerable changes in density. Bighorn sheep (Ovis canadensis) on Ram Mountain, Alberta, ranged from 94 to 232, mountain goats (Oreamnos americanus) on Caw Ridge, Alberta, varied from 81 to 147, and estimates of roe deer (Capreolus capreolus) older than 1year at Chizé, France, ranged from 157 to 569. We used recent developments of capture-mark-recapture modelling to assess the response of adult survival to changes in density when age structure was and was not taken into account. Survival rates were 10-15% higher during the prime-age stage than during the senescent stage for all sex-species combinations. When adults were pooled into a single age class there was an apparent negative effect of density on female survival in bighorns and roe deer, and negative trends for female mountain goats, male roe deer and male bighorn sheep. When age class was taken into account, there were no significant effects of density on adult survival. Except for male mountain goats, the strength of density dependence was lower when age was taken into account. In ungulate populations, age structure is an important determinant of adult survival. Most reports of density dependence in adult survival may have been confounded by changes in age structure.

Population modelling of European shags ( Phalacrocorax aristotelis) at their southern limit: conservation implications

The European shag ( Phalacrocorax aristotelis) population at Cı́es Islands (the most important breeding area in its southern limit) increased rapidly from 1986 to 1992, and afterwards the population suffered a slight decline. This study analyzed population data obtained from ringing recoveries and reproductive monitoring between 1993 and 1997. The reproductive success was highly variable and associated with adverse weather events. Adult survival rate was very low compared with other colonies, probably due to high accidental capture in gill-nets. In recent years, the fishing effort with gill-nets increased in the study area. Sensitivity analysis of parameters showed that the population is more affected by changes in adult survival than in reproductive success. When dynamic simulations were run with an increase in shag mortality of 5% above the present level, population extinction occurred in all simulations. In contrast, when a reduction of mortality of 5% was introduced in the simulations, the population increased in all cases. The main lines of action to study and protect this population should be: (1) ringing schemes to obtain better estimates of survival variability; (2) studies on the interaction of feeding areas and fishing vessels; (3) regulations on gill-netting; and (4) the incorporation of population models as an adaptive management tool to synthesize assessment work and management scenarios.

THE EFFECTS OF NEIGHBORS ON THE DEMOGRAPHY OF A DOMINANT DESERT SHRUB (AMBROSIA DUMOSA)

We used patch-specific matrix models to test the influence of neighboring plants on the demography of Ambrosia dumosa, a dominant perennial shrub of the Colorado Desert in southern California. In the desert literature, the presence or absence of neighboring plants is reported to influence plant growth and survival and has long been associated with plant–plant interactions that range from extreme competition to facilitation. Here we consider the less addressed question of how neighbor-defined population subsets contribute differently to overall population dynamics. Demographic data collected from >6000 individual plants from a permanently mapped hectare over 10 yr were used to divide this A. dumosa population into subsets that were defined by the presence or absence of neighbors. By partitioning the population in this way, we documented differences in population growth, elasticity structure, and stable size structure among the distinct patch types, and evaluated the contributions of each patch type to overall population dynamics. Population growth was consistently higher for population subsets of plants that were isolated throughout their ontogeny, compared to those with close neighbors throughout their ontogeny. Further, overall population growth was proportionally more sensitive to perturbations involving isolated adults, despite the projected persistence of plants with close neighbors. In short, changes in survival of isolated adults had a greater influence on population growth than changes in survival of adults with neighbors. We used life table response experiments (LTREs) to test for spatiotemporal effects of neighbors. The LTRE was consistent with the elasticity analyses in showing that dynamics among adult members made the greatest contribution to the observed differences in population growth among the neighbor-defined population subsets during each census period. Neighbor effects among adults were greater and more important than temporal variation in drought levels in decreasing population growth relative to a pooled 10-yr reference matrix that ignored neighbor effects. Although neighbor effects contributed greatly to differences in population growth among the models projected, the LTRE revealed that, relative to the reference matrix, transitions among juvenile size classes decreased population growth for the predominantly drought free 1984–1989 census interval and increased population growth for the 1989–1994 interval, which included four winters of extreme drought. We hypothesize that higher mortality during periods of high rainfall was due to increased competition among plants, especially in favor of adults at the expense of juveniles. The drought period was also characterized by increased recruitment of new adults, probably reflecting diminished competition from adults for well-established juveniles capable of growing into reproductive condition after elimination of smaller juveniles during 1984–1989. Our habitat-specific partitioning of this population revealed dramatic differences in the demographic behavior of this population and showed that neighbor status is a structuring force in this plant community. Neighbor effects were shown to be dependent on plant size and suggest that conspicuous clumped distributions of adults reflect leftover individuals with diminished demographic influence remaining in a population that is largely driven by isolated adults.

Age‐specific breeding probability in black brant: effects of population density

SummaryWe estimated age‐specific probability of breeding for black brant (Branta bernicla nigricansLawrence) (hereafter brant) at the Tutakoke River Colony, Alaska, during a series of years in which local breeding density and juvenile mortality both increased. We use these analyses to examine demographic responses to increased population density.Estimates of breeding probability were based on Kendall &amp; Nichols (1995) full‐likelihood modification of Pollock’s (1982) robust design applied to observations of uniquely colour‐banded brant.Breeding probability varied with female age but not across years, indicating lack of response to increased population density. Annual probability of breeding increased from 0·67 ± 0·042 for 2‐year‐old females to 0·90 ± 0·024 for females 5 years old or older.Annual breeding probability of males ≥ 5 years old was 0·78 ± 0·03. Lower breeding probability of males compared to females probably reflects dispersal of males whose mates died.Lower juvenile survival appears to be a general response to increased density in large herbivorous birds and mammals, while changes in adult survival and reproductive investment may be more variable.

Population Cycles in Small Mammals: The Role of Age at Sexual Maturity

Several hypotheses have been proposed to explain cyclic fluctuations in abundance of some small mammal populations. These hypotheses have been controversial, however, and there is no consensus among biologists as to why population cycles occur In a demographically based model, we tested the potential influence of phase-specific changes in life history traits (age at maturity, fertility, juvenile survival and adult survival) on population cycles. Our demographic model considers, and is logically consistent with, the empirical pattern of population characteristics during a cycle The essence of the model is that phase-specific changes in age at maturity, abetted secondarily by changes in juvenile survival, result in cyclic fluctuations in population size. Changes in adult survival and fertility may play a minor role, but they are neither necessary nor sufficient by themselves to generate population cycles. Phase-specific changes in age at maturity might be related to primary changes in the quality of the ecological and social environment that permit particularly high densities.

ESTIMATES OF ANNUAL SURVIVAL PROBABILITIES FOR ADULT FLORIDA MANATEES (TRICHECHUS MANATUS LATIROSTRIS)

The population dynamics of large, long-lived mammals are particularly sensitive to changes in adult survival. Understanding factors affecting survival patterns is therefore critical for developing and testing theories of population dynamics and for developing management strategies aimed at preventing declines or extinction in such taxa. Few studies have used modern analytical approaches for analyzing variation and testing hypotheses about survival probabilities in large mammals. This paper reports a detailed analysis of annual adult survival in the Florida manatee (Trichechus manatus latirostris), an endangered marine mammal, based on a mark–recapture approach. Natural and boat-inflicted scars distinctively “marked” individual manatees that were cataloged in a computer-based photographic system. Photo-documented resightings provided “recaptures.” Using open population models, annual adult-survival probabilities were estimated for manatees observed in winter in three areas of Florida: Blue Spring, Crystal ...

Demography and conservation of western European Bonelli's eagle Hieraaetus fasciatus populations

The basic demographic parameters of six Bonelli's eagle Hieraaetus fasciatus populations in Spain and France were calculated from field data obtained in the last 15 years. Average annual productivity ranged from 0.36 to 1.24 young/pair and average annual adult survival from 84% to 96%. Preadult survival (from fledging to recruitment) was estimated at 10%. All the populations were declining at annual rates ranging from −7.3 to −1.1%. Although no statistically significant disagreement was observed between the predictions of a Leslie matrix model fitted to the population parameters and the real trends, some populations declined faster and others more slowly than expected. These differences were interpreted as a result of differential emigration, recruitment rates or preadult survival not accounted for by the model. Since the intrinsic population growth rate was about four times less sensitive to changes in preadult survival than to changes in adult survival, and about ten times less sensitive to changes in fecundity and predispersal survival, conservation efforts must be primarily directed towards increasing adult and preadult survival. Power line casualties and direct persecution must be eliminated in order to reduce mortality. Priorities on research should address dispersal, mortality and habitat relationships involving these two parameters. Monitoring should be extended to other populations.

A Demographic Model for a Population of the Endangered Lesser Kestrel in Southern Spain

1. The lesser kestrel (Falco naumanni) has experienced a dramatic decline in the last 20 years in the Western Palearctic. To help in making decisions for the recovery of the species, a matrix projection model has been developed using demographic data from an intensively monitored population in southern Spain in 1988-93. Survival rates were estimated using the Jolly-Seber modelling approach and Program SURGE. 2. The growth rate of the population was λ = 0.959 ± 0.04. The upper 95% confidence limit of λ is 1.0398, and thus our estimate of λ is not significantly different from that of a stable population (i.e. λ = 1). Using the mean value for λ, the probability of extinction for this population, now consisting of 1000 breeding pairs, is 35% in 100 years and 98% in 200 years. 3. A sensitivity analysis indicates that population growth was most sensitive for changes in adult survival, followed by juvenile survival, productivity of fledglings, proportion of adults that attempt breeding and age at first breeding. 4. Adult and juvenile survival do not seem easily amenable to management, but the small improvement (8.5%) that could be reached enforcing protective laws substantially reduce the probability of extinction of the population. 5. Productivity is less than half its potential maximum due to massive nestling mortality. Increasing food availability around the breeding colonies through habitat management (e.g. leaving uncultivated strips around fields and favouring cereal crops), or introducing the species in areas containing suitable habitat may substantially increase productivity. Combining these measures with the achievable improvement (8.5% increase) in adult and juvenile survival maximizes the long-term survival of lesser kestrel populations.

Sustainable Harvesting of Tropical Trees: Demography and Matrix Models of Two Palm Species in Mexico

Thrinax radiata and Coccothrinax readii, two palm species of the dry tropical forest in Quintana Roo, Mexico, had been used in earlier times by the Mayans and adult palms are used extensively today. The overexploitation of these species in the Sian Ka'an Biosphere Reserve has made management and conservation measures necessary. To assist in the preparation of a management plan for sustainable use, we obtained the patterns of survivorship, growth, and reproduction from four populations of Thrinax radiata and one population of Coccothrinax readii. Thrinax were more abundant (≤2977 individuals/ha) than Coccothrinax (≤760 individuals/ha), and abundance and population structures varied depending on habitat conditions and previous harvesting. Flowering, fruiting, and leaf production varied in both species, apparently as a function of rainfall and site. Seedling survival is high in comparison to other tree species (67‐91% in Thrinax). Thrinax attains a height of 3 m at an age of 31‐55 yr and Coccothrinax at an age of 63 yr. Both species start reproducing when 4 m tall. Maximum life spans were estimated at 100‐145 yr for Thrinax and at &gt;145 yr for Coccothrinax. We estimated transition probabilities for each life cycle category of both species and obtained Lefkovitch (1965) matrices to estimate the populations' finite growth rates (λ = 1.05 for Coccothrinax and λ = 1.09‐1.15 for different populations of Thrinax), to generate sensitivity and elasticity matrices under studied conditions, and to simulate various harvesting regimes. Growth rates are very sensitive to changes in adult survival and growth. The long recuperation times in both species (47‐84 yr for the population to recover to 400 adult Thrinax palms per hectare and &gt;100 yr for Coccothrinax) emphasize the importance of being extremely cautious before harvesting additional adult palms. Simulations suggest that only up to 40 Thrinax and no Coccothrinax adult palms per hectare may be removed each year if the population is to be sustained. If palms are harvested on a rotational basis every 2, 3, or 4 yr, the number of palms that may be sustainably harvested increases up to 40‐80 palms/ha in the Thrinax populations and up to 8 palms/ha in the Coccothrinax population. Our simulation results should be taken cautiously because they do not consider year‐to‐year variation.

Regulation of population size: evidence from Columbian ground squirrels.

The hypothesis that food resources regulate population size was tested in Columbian ground squirrels, Spermophilus columbianus, from 1981 to 1986 in southwestern Alberta, Canada. Two replicate populations received supplemental food resources from 1981 to 1983, and were subsequently monitored until 1986. Two reference (unmanipulated) populations were monitored throughout the 6 years. During the experiment, dramatic increases in population size of about 500% occurred. After supplementation, spring populations declined by about 20% per year, under conditions that produced stressful shortages of food (as evidenced by significant decreases in body mass of ground squirrels). The demographic process that contributed most to increasing and decreasing experimental populations was production of yearlings, through changes in reproductive success and survival of young. Changes in migration contributed secondarily to changes in population size, and changes in adult survival were least important. While similarities of demographic processes were evident between replicates during population increases, a dramatic decrease in survival of young contributed to population decline at one replicate, and "normal" levels of reproduction and survival were sufficient to produce a decline in the other replicate. Demographic mechanisms may vary, but population regulation of mammalian populations appears strongly dependent on food resources.

THE POPULATION DYNAMICS OF NORTHERN SEA LIONS, 1975‐1985

Abstract: Populations of northern sea lions (Eumetopias jubatus) in the vicinity of Marmot Island, Alaska declined during 1975–1985 at about 5% per year (Merrick et al. 1987). The cause of this decline is not known. A life table for the northern sea lion was calculated assuming that life spans follow a Weibull distribution. Samples of northern sea lions taken in the vicinity of Marmot Island, Alaska during 1975–1978 and 1985–1986 indicate that the average age of females older than 3 yr increased about 1.55 yr (SD = 0.35 yr) while the population was declining at about 5% per year. Fecundity rates decreased by 10% over the same period, but the decrease was not statistically significant (Calkins and Goodwin 1988). Possible causes of the population decline and the change in age structure were examined by writing the Leslie matrix population equation in terms of changes in juvenile and adult survival rates and fecundity, and examining the short–term behavior of the trajectories of the average age of adult females, total number of females, and total number of pups with respect to those changes in the vital parameters. From the observed rate of declines of adults and the changes in average age of adult females and fecundity, estimates of the changes in adult and juvenile survival were calculated; estimates of the standard deviations of these changes were estimated via a bootstrap procedure. One purpose of this exercise is to aid in setting priorities for research for determining the cause of the decline. An explanation for the observed declines in numbers of adult sea lions consistent with the observed fecundity rates, a rate of decrease of 5% in the number of adults, and the corresponding increase in average age (of females age 3 yr and older) was a 10%–20% decrease in the survival of juveniles (age 0‐3 yr) coupled with an insignificant change in adult survival (0.03%, SD = 1%).

Some Factors Affecting the Size of British Grey Seal Populations

SUMMARY (1) Age specific survival and fecundity are evaluated for the rapidly increasing British grey seal population. (2) The effect of changes in these values on the rate of increase of the population are investigated. (3) The only known density dependent mechanism operates on pup survival and has only a small effect on the rate of increase. (4) There is no evidence of a change in age at first pregnancy with increasing population size. (5) Small changes in adult survival have a large effect on the population rate of increase, but no natural mechanisms that directly affect adult survival have been identified. (6) Ultimate population size appears to be set by the number of potential breeding sites.