Hatch size, somatic growth rate and size-dependent survival in the endangered ploughshare tortoise

Successful conservation requires a good understanding of both the direct and indirect causes of any decline in population size. Harvests of wild populations often target the largest, oldest individuals that have the greatest economic value. If these individuals contribute disproportionately more to recruitment than conspecifics, the harvest will cause a greater reduction in population viability than initially anticipated. The ploughshare tortoise, endemic to Madagascar, has been reduced to <600 individuals due to exploitation and habitat loss. A trial release of captive-bred juveniles has been carried out but the relative suitability of individuals for release, in terms of body size, was not considered. Using data from a long-term mark-recapture study of an unharvested wild population of ploughshare tortoises, we found that larger juveniles were significantly more likely to survive their first year of life than smaller tortoises. Juveniles that survived beyond their first year of life generally grew at the same rate, but had a significantly larger hatch size (mean = 41.7 mm), compared with juveniles that did not survive (mean = 39.3 mm). The conservation implications of these results are that release of captive-bred ploughshare tortoises is likely to be more successful if larger individuals are released. Previous studies have reported larger female ploughshare tortoises laying larger eggs which hatch larger juveniles. This study shows individuals with a larger hatch size retain their size advantage over smaller conspecifics and are more likely to survive their first year of life. This suggests a harvest that targets the largest females in the ploughshare tortoise population could be highly detrimental to population viability.     

Distribution, status, and conservation of radiated tortoises (Geochelone radiata) in Madagascar

The radiated tortoise, Geochelone radiata, one of Madagascar’s four endemic tortoises, occupies a narrow band of xeric spiny forest along the island’s southwest coast. Traditionally avoided by indigenous tribes, these tortoises are now routinely harvested for food. An accurate assessment of human exploitation remains problematic, however, hindered by limited, dated statistics available on tortoise populations. To update the radiated tortoise’s status and distribution, we established a series of line transects at seven localities across its range and implemented a mark-recapture study at one of these localities (Cap Sainte Marie). Tortoises currently range from south of Tulear to east of Cap Sainte Marie, at density estimates spanning 27-5744 tortoises/km². The mark-recapture estimate for Cap Sainte Marie (1905-2105 tortoises/km²) was substantially higher than its transect estimate (654 tortoises/km²) though comparable to actual tortoise captures (1438) there. Thus, our transect density values probably err as underestimates, and from these data, we calculate a conservative total population size of 12 million radiated tortoises. We also examined mitochondrial DNA sequences (ND4 gene) for two individuals/locality in a preliminary assessment of genetic variation across the species’ range. Only two ND4 haplotypes were recovered, the more common haplotype representing 13 of the 14 individuals. We offer several conservation recommendations in light of our survey results.     

An empirical evaluation of the area and isolation paradigm of metapopulation dynamics

Much of metapopulation theory assumes that the persistence of individual populations in a metapopulation, and persistence of the metapopulation as a whole, is best modeled by the area of habitat patches and their isolation. Estimates of isolation typically include a measure of geographic distance and a measure of either population size or patch area. This “area and isolation paradigm” assumes a functional relationship between the area of a patch and its extinction probability, and between isolation of a patch and its colonization probability. Although these assumptions are fundamental to use of incidence function models of metapopulation dynamics, the assumptions have been validated in only a small number of studies. We tested the ability of area and isolation to predict extinction and colonization patterns using multiple-year occupancy data for 10 species from three taxonomic groups (butterflies, amphibians, and birds). We examined 13 potential models of metapopulation dynamics. All models included four basic parameters: occupancy during the first year of the survey, probability of extinction, probability of colonization, and single-visit detection probability. In eight models, each parameter was either constant or time-dependent. Five models included a patch-level covariate of extinction probability (patch area or population size), colonization probability (connectivity, the inverse of isolation), or both. Extinction patterns generally were predicted more effectively as a function of local population size than as a function of patch area, a constant probability of extinction, or a time-dependent probability of extinction. In most cases, inclusion of connectivity as a patch-level covariate did not improve predictions of colonization patterns. We estimated single-visit detection probabilities for all species in our analyses, thus providing evidence-based guidelines for the refinement of future monitoring protocols.     

Large scale risk-assessment of wind-farms on population viability of a globally endangered long-lived raptor

Wind-farms receive public and governmental support as an alternative energy source mitigating air pollution. However, they can have adverse effects on wildlife, particularly through collision with turbines. Research on wind-farm effects has focused on estimating mortality rates, behavioural changes or interspecific differences in vulnerability. Studies dealing with their effects on endangered or rare species populations are notably scarce. We tested the hypothesis that wind-farms increase extinction probability of long-lived species through increments in mortality rates. For this purpose, we evaluate potential consequences of wind-farms on the population dynamics of a globally endangered long-lived raptor in an area where the species maintains its greatest stronghold and wind-farms are rapidly increasing. Nearly one-third of all breeding territories of our model species are in wind-farm risk zones. Our intensive survey shows that wind-farms decrease survival rates of this species differently depending on individual breeding status. Consistent with population monitoring, population projections showed that all subpopulations and the meta-population are decreasing. However, population sizes and, therefore, time to extinction significantly decreased when wind-farm mortality was included in models. Our results represent a qualitative warning exercise showing how very low reductions in survival of territorial and non-territorial birds associated with wind-farms can strongly impact population viability of long-lived species. This highlights the need for examining long-term impacts of wind-farms rather than focusing on short-term mortality, as is often promoted by power companies and some wildlife agencies. Unlike other non-natural causes of mortality difficult to eradicate or control, wind-farm fatalities can be lowered by powering down or removing risky turbines and/or farms, and by placing them outside areas critical for endangered birds.     

Factors influencing survival and long-term population viability of New Zealand long-tailed bats (Chalinolobus tuberculatus): Implications for conservation

A population viability analysis is important for the management of endangered populations and requires the estimation of survival parameters. The long-tailed bat (Chalinolobus tuberculatus) is one of only two native terrestrial mammals currently found in New Zealand and is classed as vulnerable. Its viability in temperate beech (Nothofagus) forest, Eglinton Valley, Fiordland, New Zealand was estimated using mark-recapture data collected between 1993 and 2003 using the Program MARK. Survival was estimated based on a total of 5286 captures representing 1026 individuals. Overall annual survival varied between 0.34 and 0.83 but varied significantly among three sub-populations and with sex and age. Females generally had a higher survival rate compared to males; and adults had higher survival relative to juveniles. Survival of all bats was lower in years when the number of introduced mammalian predators was high and when the winter temperature was warmer than average. High numbers of introduced predators occurred during three of the 10 years in the study. Climate change may mean that the conditions that promote high predator numbers may occur more frequently. A preliminary population viability analysis using a projection matrix on the overall adult female population showed an average 5% decline per year (λ = 0.95). Increased predator control targeting a range of predators is required in years when their numbers are high in order to halt the decline of this population of long-tailed bats. Population estimates using minimum number alive estimates supported the population estimates derived from Program MARK and a population viability analysis using matrices.     

Dynamics of a black bear population within a desert metapopulation

Understanding metapopulation dynamics in large carnivores with naturally fragmented populations is difficult because of the large temporal and spatial context of such dynamics. We coupled a long-term database of visitor sighting records with an intensive 3-year telemetry study to describe population dynamics of recolonization by black bears (Ursus americanus) of Big Bend National Park in Texas during 1988-2002. This population, which occurs within a metapopulation in western Texas and northern Mexico, increased from a single pair of known breeding-age animals in 1988 to 29 bears (including 6 females of breeding age) in March 2000 (λ = 1.25/year). A migration and dispersal event in August-December 2000 reduced the population to 2 adult females and as few as 5-7 individuals. One-way movement distances from the study area during this event averaged 76 km for females (n = 7) and 92 km for males (n = 4), and 3 animals conducted migrations of at least 154, 178, and 214 km, respectively. Our observations exemplify the importance of stochastic events on demographics of small populations and highlight the potential scale of bear movement among montane islands of southwestern North America. They also provide insight into the use of dispersal data in parameterizing metapopulation models for large carnivores.     

Two genetically distinct units of the Chinese sika deer (cervus nippon): analyses of mitochondrial DNA variation

Chinese sika deer, Cervus nippon, are currently threatened by habitat loss, fragmentation and human hunting, which has led to the extinction of three subspecies in the wild. The remaining subspecies subsist in the narrow regions of Jilin, Heilongjiang, Sichuan, Gansu, Jiangxi, Anhui, and Zhejiang provinces. In order to design effective conservation strategies for the Chinese sika deer, we have investigated genetic diversity, population structure and gene flow in the Chinese sika deer populations by analyzing ≈995 base pairs of the mitochondrial DNA (mtDNA) control region in 59 individuals sampled from the northeast of China, Sichuan, Jiangxi and Zhejiang. Chinese sika deer exhibited low mtDNA diversity and high gene flow among the four populations, and showed no strong geographical structure. The analyses of mtDNA variation among individuals of sika deer identified only two main phylogenetic groups even though three subspecies were sampled. These data singles out the Zhejiang population as being highly genetically distinct and worthy of separate conservation consideration. Therefore, it is recommended that a breeding program for the Zhejiang population be established.     

Incorporating parametric uncertainty into population viability analysis models

Uncertainty in parameter estimates from sampling variation or expert judgment can introduce substantial uncertainty into ecological predictions based on those estimates. However, in standard population viability analyses, one of the most widely used tools for managing plant, fish and wildlife populations, parametric uncertainty is often ignored in or discarded from model projections. We present a method for explicitly incorporating this source of uncertainty into population models to fully account for risk in management and decision contexts. Our method involves a two-step simulation process where parametric uncertainty is incorporated into the replication loop of the model and temporal variance is incorporated into the loop for time steps in the model. Using the piping plover, a federally threatened shorebird in the USA and Canada, as an example, we compare abundance projections and extinction probabilities from simulations that exclude and include parametric uncertainty. Although final abundance was very low for all sets of simulations, estimated extinction risk was much greater for the simulation that incorporated parametric uncertainty in the replication loop. Decisions about species conservation (e.g., listing, delisting, and jeopardy) might differ greatly depending on the treatment of parametric uncertainty in population models.     

Time and burial depth influencing the viability and bacterial colonization of sclerotia of Sclerotinia sclerotiorum

Sclerotia are the primary over wintering inoculum of Sclerotinia sclerotiorum (Lib.) de Bary. The effects of tillage on the primary inoculum are not well understood. The purpose of this research was to study sclerotial viability over time and between burial depths in soil, to identify bacteria colonizing and degrading the sclerotia, and determine whether these bacteria may be utilized as biological control agents. Correlation analysis indicated that a significant negative relationship existed between sclerotial viability and elapsed temporal factors (R²=−0.68, P<0.0001), and depth of burial (R²=−0.58, P<0.0001). After twelve months, sclerotia on the soil surface had the highest viability (57.5%), followed by those at the 5 cm depth (12.5%), and only 2.5% of those placed at the 10 cm depth remained viable. A significant negative relationship between sclerotial viability and bacterial populations also existed (R²=−0.60, P<0.0001). Two hundred and sixty-eight bacteria were isolated from sclerotia, 29 of which showed strong in vitro antagonism to the mycelial growth of S. sclerotiorum. Biodiversity of the inhibitory bacterial isolates was minimal on sclerotia from the soil surface and within all depths sampled at three months (i.e. in January). All burial depths within the April and July sampling dates produced bacterial diversities that were distinct from each other.     

Artificial dispersal of endangered epiphytic lichens: a tool for conservation in boreal forest landscapes

Due to increasing fragmentation of the boreal forests of Fennoscandia, a number of epiphytic lichens are now becoming threatened. Since these species typically are limited by a poor ability of dispersal, one possible but largely unexplored strategy for conservation is to disperse lichen material artificially into suitable habitats. Therefore, the objective of this study was to evaluate survival and vitality in lichen fragments from Evernia divaricata (L.) Ach. and Ramalina dilacerata (Hoffm.) Hoffm. after transplantation into three different stand types situated in northern Sweden, using different fragment sizes and modes of transplantation. After one year, survival ranged between 85% and 97.5%, and new growth occurred at all transplantation sites. The study has shown that transplantations of small fragments might constitute a resource-efficient option for establishment of new populations of endangered lichen species, or for enlarging their populations at the present sites of occurrence. In addition, the mode of transplantation was of importance for fragment vitality.     

Population viability of Cape mountain zebra in Gamka Mountain Nature Reserve, South Africa: the influence of habitat and fire

The small Cape mountain zebra population in Gamka Mountain Nature Reserve represents a third of the entire gene pool of this endangered species and is thus vital for it’s conservation. Presently, management of this population is largely hands off, with the belief that it will grow to levels which will allow it to form a source for the mixing of mountain zebra stocks in the future. The growth of this population however, has been slow and we investigated the influence of habitat and fire on this growth. Firstly, we used a diffusion model to perform a population viability analysis. This analysis indicated that the population had a low probability of attaining quasi-extinction in the next 50 years (G = 0.0032). However, our findings indicated that less than 30% of the reserve was suitable for mountain zebra and that the preferred habitat would have to be burnt at unnaturally short intervals to sustain the present growth. We therefore argue that the risk of quasi-extinction to this population is greater than predicted and suggest that management options need to be implemented to reduce this risk. These options include; translocation to another protected area; acquisition of adjacent land; burning preferred habitat at unnaturally short intervals; forming a conservancy with adjacent landowners; leasing cultivated land for pasture. We suggest that only the latter two options are likely to stimulate mountain zebra population growth in the short term and that these should receive immediate attention.     

Squirrel poxvirus: Landscape scale strategies for managing disease threat

Squirrel poxvirus (SQPV) is a well-documented example of pathogen-mediated competition between an invasive species, the grey squirrel (Sciurus carolinensis), and a native species, the Eurasian red squirrel (Sciurus vulgaris). SQPV causes disease with high mortality in red squirrels but appears non-pathogenic in grey squirrels. Not all populations of introduced grey squirrels carry the virus, notably those in Scotland and Italy, and the rate of red squirrel replacement by grey squirrels is some twenty times faster in those areas where grey squirrels carry the virus. Here we develop strategies to manage the SQPV disease threat to red squirrels by reference to the largest, designated red squirrel refuge site in England, Kielder Forest (50 000 ha). Using modelling techniques, we identify four main corridors within the buffer zone by which grey squirrels will reach Kielder, initially within two years and in large numbers within 10 years. Assuming that greys will not settle within Kielder because of the unfavourable nature of the spruce habitat, we predict that SQPV disease will burn out at the edges of the forest, although many red squirrels will die. This burn-out is unlikely to be the scenario in other refuge areas where the habitat is more favourable to greys. We conclude that the conservation of red squirrels will depend on minimising contact between red and grey squirrel populations, and we advocate monitoring grey squirrels in corridors within buffer zones around refuge areas, and removing them when detected.     

Recovering marbled murrelets via corvid management: A population viability analysis approach

The expansion of human activities into rural areas and natural landscapes has resulted in widespread increases in the abundance of synanthropic species that threaten rarer native species. Quantitative assessments of how much impacts need to be reduced to reach acceptable levels of risk to the affected species are rarely conducted prior to the implementation of control measures, and it is perhaps not surprising that many efforts have not yielded the desired outcome. Here, we used matrix-based population viability analysis models to show that reducing predation by rapidly growing corvid populations on marbled murrelet (Brachyramphus marmoratus) nests likely constitutes an effective means for recovering a declining murrelet population in central California. For example, a modest 40% reduction in predation reduced extinction risk dramatically from 95.8% to 4.6% over 100 years and a 60% reduction resulted in a stable population (λ = 1) when the proportion of breeders, renesting rates, and corvid predation rates were assumed to be 0.77, 0.13, and 0.69, respectively. However, nest predation would only need to be reduced by 40% to produce a stable population if corvid management was coupled with a modest increase in after-hatch-year survival from 0.896 to 0.910. Corvid control resulted in greater gains in murrelet population size when the maximum number of breeders was allowed to increase over the projection period, as might be expected if the amount of old-growth nesting habitat increased over time, but extinction risk was insensitive to the presence of a carrying capacity. Approximately half of known murrelet nests in central California are within 1 km of heavily used campgrounds in a single state park, indicating that significant gains in viability could be achieved by targeting efforts in small areas providing corvid food subsidies. Risk assessments such as ours can provide quantitative prioritization rationale for efforts intended to mitigate the impacts of synanthropic species on threatened species.     

A nine-year study of successful breeding in a Bonelli’s eagle population in southeast Spain: a basis for conservation

We analyse the factors influencing breeding success in a healthy population of Bonelli’s eagle (Hieraaetus fasciatus) from south-east Spain, a raptor that has suffered a serious decline in Europe. Between 18 and 33 pairs were annually monitored during the period 1994-2002. Several factors that may affect four breeding parameters were studied, namely human presence, vegetation, relief, climatic factors, intra- and inter-specific relationships, diet, prey abundance, nest building, adult mortality and age of reproduction. A consistently high breeding success was registered during the study period (productivity=1.43, SD=0.11), which was probably the result of high adult survival, adequate prey availability and mild weather conditions. However, a certain vulnerability to the presence of golden eagles (Aquila chrysaetos) and to human disturbance was observed. One interesting result was the reduced survival of young chicks on north-facing cliffs owing to colder conditions, which may partially explain the decline of the Bonelli’s eagle populations along its European distribution limits. We discuss the role of this healthy subpopulation in an Iberian metapopulation context, and propose that the potential interference of golden eagles should be taken into account when designing management strategies for Bonelli’s eagles. Finally, we pinpoint some conservation priorities and the importance of reducing the main causes of adult mortality (i.e. shooting and electrocution) to assure successful reproduction and survival of this species.     

Distribution of non-native trout in Slovenia and their introgression with native trout populations as observed through microsatellite DNA analysis

In Slovenia, the Adriatic basin inhabited by native marble trout (S. marmoratus), and the Danubian basin inhabited by native Danubian lineage of brown trout (S. trutta) have been intensively affected by stocking with non-native trout strains. In order to assess spread of non-native strains and their introgression with native trout, a population study based on five microsatellite loci was applied across ten marble and ten brown trout populations, ranging from allegedly non-introgressed to heavily managed. On the basis of correspondence analysis, which revealed three clear groupings consisting of the Danubian and Atlantic lineages of brown trout and the marble trout, the alleles, characteristic of each grouping were identified and used for estimating genetic composition of each population according to the three possible origins. Among the wild populations, five marble and one brown trout populations were found to be pure; all the others were introgressed with exotic alleles (Atlantic and marmoratus alleles in the Danubian basin and Atlantic and Danubian in the Adriatic basin) that markedly dominate in intensively managed populations. As revealed by non-significant F_IS values, panmixia between native and introduced fishes has for the most part already been reached. Our research showed that it is not only marble trout whose identity is endangered in Slovenia but also the existence of autochthonous Danubian brown trout is critically compromised, which is new information to be taken into account for local trout conservation.     

Impacts of sediment loads, tadpole density, and food type on the growth and development of tadpoles of the spotted tree frog Litoria spenceri: an in-stream experiment

The impact of increased-deposited sediment loads on growth and development of tadpoles of the spotted tree frog, Litoria spenceri, in south-eastern Australia was assessed in conjunction with food type and tadpole density. This investigation was conducted in stream enclosures subjected to varying treatments of tadpole density, substrata and deposited-sediment load. Tadpole growth and development were not significantly affected by food type, but there was a trend toward increased performance on periphyton substrata. Tadpole growth and development were significantly reduced by increases in tadpole density and sediment load. These effects were independent, suggesting that increased sediment load reduced food quality rather than quantity. Activities in catchments that increase sediment loads in streams may adversely affect growth and development of tadpoles of L. spenceri. Increased sediment input into streams may therefore reduce tadpole fitness and recruitment to the terrestrial stage. Disturbance processes that increase stream sediment loads, such as forestry activities and associated road construction, may potentially have contributed to the observed declines of L. spenceri and other lotic anurans in south-eastern Australia.     

A risk analysis for the common hamster (Cricetus cricetus)

The common hamster (Cricetus cricetus) has suffered a dramatic decline in Western Europe though the species is characterized by an extremely high reproductive potential. An individual-based model was developed to determine the risk factors which lead to population decline and to investigate management actions for viable populations. Demographic stochasticity was considered by (a) describing reproduction on an individual basis and (b) distinguishing behavioural patterns according to age and sex. Unknown model parameters were determined by comparing field data and simulated patterns in a specific multi-criteria fashion. We conclude that disturbances such as agricultural management and highway construction are most dangerous to the common hamster in autumn. Complex land management is required which should aim at reducing habitat-specific mortality and favouring the safeguarding of adult and subadult females to protect the most sensitive component of the population. According to the model results, large habitat size is not sufficient for survival, and habitat connectivity may be even more important. Late timing of the harvest and following cultivations was most favourable for population survival.     

Conservation implications of the distribution of genetic diversity at different scales: a case study using the marsh fritillary butterfly (Euphydryas aurinia)

In the UK, Euphydryas aurinia exists in fragmented habitat patches, and undergoes population fluctuations as a result of a larval parasitoid. Its range is declining in the UK and conservation is thought to require a landscape approach since populations spread over large areas in some years and contract to core breeding patches in others. We examined populations at a range of geographic scales using allozyme electrophoresis to look for evidence of gene flow and differences in genetic diversity among populations. Nationally, our F_ST value was 0.1542 but between population groups within the suspected colonisation range of the butterfly (ca. 20 km), F_ST values were not significantly different from zero. Genetic diversity in terms of number of alleles and heterozygosity was reasonably high in natural populations (H_e=0.267) but low in an introduced, isolated population. We infer that migration between closely spaced subpopulations (in a metapopulation) maintains a high genetic effective population size (large number of individuals in a population that contribute genes to the next generation) which offsets any local reductions in population numbers due to stochastic extinctions or parasitoid effects. We therefore conclude that effective conservation of the species must seek to provide networks of suitable habitat for groups of subpopulations, rather than maintaining habitat for isolated populations.     

Dead wood threshold values for the three-toed woodpecker presence in boreal and sub-Alpine forest

Predicting species' responses to habitat loss is a significant challenge facing conservation biologists. We examined the response of both European three-toed woodpecker subspecies Picoides tridactylus tridactylus and P. tr. alpinus to different amounts of dead wood in a boreal and a sub-Alpine coniferous forest landscape in central Sweden and Switzerland, respectively. Habitat variables were measured by fieldwork in forests with breeding woodpeckers (n=10+12) and in control forests without breeding woodpeckers (n=10+12) in the same landscape. Logistic regression analyses revealed steep thresholds for the amount of dead standing trees and the probability of three-toed woodpecker presence in both Sweden and Switzerland. The probability of the presence of three-toed woodpeckers increased from 0.10 to 0.95 when snag basal area increased from 0.6 to 1.3 m² ha⁻¹ in Switzerland and from 0.3 to 0.5 m² ha⁻¹ in central Sweden. In Switzerland, a high road network density was negatively correlated to the presence of woodpeckers (r=−0.65, p=0.0007). The higher volumes of dead wood in Switzerland, where population trends are more positive, than in central Sweden, where the population is declining, would suggest that the volumes of dead wood in managed forests in Sweden are too low to sustain three-toed woodpeckers in the long-term. In terms of management implications, we suggest a quantitative target of at least 5% of standing trees in older forests being dead over at least 100 ha large forest areas. This corresponds about to ?1.3 m² ha⁻¹ (basal area) or ?15 m³ ha⁻¹ (volume), still depending on site productivity.