In a study correlating carnivore density with study area size, Smallwood & Schonewald (1996) suggested that for larger areas, biologists extrapolate from small, high-density study sites to many unstudied low density sites, but that this approach is invalid because of heterogeneous distribution of animals across the landscape. For wolves (Canis lupus), however, size of study areas may also be a consequence of density. Linear regression of data from 18 recent field studies of wolves indicates that study area size is correlated less strongly to mean numbers of wolves counted in an area (r2=0·24) than to wolf density (r2=0·85). When parameters representing biological features of each study area (average wolf pack territory size, ungulate biomass density index and predominant ungulate species), as well as logistical constraints of the study (number of packs), were made available for inclusion through a stepwise procedure, mean numbers of wolves counted were related (r2=0·94) to number of packs, ungulate biomass and deer, while density was related to both study area size and number of wolves counted (r2=0·91). Study area size was found to be strongly predicted (r2=0·96) by ungulate biomass, wolf territory size and wolf numbers. These results clearly reinforce the idea that biological parameters play an essential role in determining animal numbers and density. It also follows, as emphasized by Smallwood & Schonewald (1996, in press), that extrapolation of census or survey information to larger areas needs to be done carefully to account for actual variation in animal density.

Branching (birth–death) models are used to project the time to extinction for bird and mammal orders. While such projections are very coarse, they provide information on how long fundamental units of biodiversity, as represented by such higher taxa as orders, will persist at specified rates of species extinction. The results of a basic decay model indicate that, if all threatened species in each order go extinct within the next 100 years, and this species extinction rate continues indefinitely, then half the species in most orders will disappear in a few hundred years or less. This result is robust and is in agreement with other estimates. A more detailed branching model indicates that most bird and mammal orders will become 100% extinct in a few thousand years if current extinction trends are extrapolated. This projection is also relatively robust, as it is consistently produced using a number of different methods and estimates of future species extinction rates. Mammal orders generally have substantially shorter half-lives and time to 100% extinction than bird orders because of a combination of fewer species per order and higher species extinction rates.

A serious concern with the increasing use of translocation and reintroduction in animal conservation programs is the potential for disease transmission between captive and free-ranging populations. As part of an experimental headstarting program, 45 juvenile Cuban iguanas, Cyclura nubila, were artificially incubated, maintained in captivity for six to 18 months, and subsequently released into natural areas on the US Naval Base at Guantánamo Bay, Cuba. Prior to release, all animals underwent physical examinations, hematological analyses, plasma biochemical determinations, and blood and fecal parasite screening. Comparable data were collected from free-ranging juveniles to establish normal baseline values. Although all hematological parameters were within expected ranges for healthy reptiles, captive juveniles exhibited higher total leukocyte counts and percent lymphocytes, but lower percent heterophils, than free-ranging juveniles. All biochemical values other than lactate dehydrogenase and creatine phosphokinase were within normal ranges. Although higher than reported in other species, these enzymes did not differ in captive and free-ranging juveniles. Free-ranging juveniles significantly exceeded captive juveniles in levels of potassium, alkaline phosphatase, lactate dehydrogenase, total protein, cholesterol and triglycerides, whereas the reverse was true for carbon dioxide and glucose. Although free-ranging juveniles exhibited oxyurids and coccidian oocysts in their feces, captive juveniles were negative for the presence of these parasites. Electron microscopy confirmed that a high percentage of both captive and free-ranging juveniles possessed erythrocytes infected with the piroplasm Sauroplasma. Implications of these results for successful repatriation of captive-reared iguanas are discussed.

As effective population size (Ne) decreases, genetic factors may become relatively important to a population's or species' persistence. Conservation biologists should be aware of anything that can potentially cause a sudden reduction in Ne. I used simple models to illustrate how certain types of female mating preferences combined with certain types of male traits may lead to a sudden and substantial decrease in Ne. Specifically, if and when there is a sudden ‘downward’ shift in the expression of condition-dependent male traits, females using fixed-threshold mate choice criteria might find fewer acceptable males. While mechanisms of female choice remain elusive, a variety of sexually selected traits may be condition dependent. Because the expression of condition-dependent traits is likely to be impacted by natural or human-induced environmental changes, behavioral and conservation biologists should pay special attention to them around the mating season. Armed with knowledge of condition-dependent male traits, it may be possible to minimize the impact on condition-dependent traits while planning translocations or reintroductions.

We used a demographic model to calculate a series of annual Ne/Nestimates ((Ne/N)S) for a population of the endangered San Joaquin kit fox (Vulpes macrotis mutica) and to identify the demographic parameters explaining significant portions of the variation in (Ne/N)S estimates. Demographic data were collected between 1981 and 1995 from adult kit foxes located on the Naval Petroleum Reserves in California. Demographic parameters were estimated by determining the sex of captured adult foxes and monitoring the survival and reproduction of adult foxes fitted with radiocollars. Individual demographic parameters explaining significant portions (P < 0.10) of the variation in (Ne/N)S estimates was determined using forward stepwise multiple regression. Estimates of (Ne/N)S were highly variable, ranging from 0.22 to 1.15, (x = 0.67, SEM = 0.07, n = 14), with a harmonic mean of 0.55. Forward stepwise multiple regression analysis indicated that mean annual female survival (P = 0.002) and mean annual female reproductive success (P = 0.004) explained significant portions of the variation in estimates of (Ne/N)S, while mean annual sex ratio (P = 0.159), mean annual litter size (P = 0.406) and mean annual male survival (P = 0.922) did not. Long-term kit fox conservation strategies will probably have to account for wide variation in (Ne/N)S ratios and focus on maintaining or expanding the size and connectivity of populations.

Species with ecological (habitat, dietary) specialization and low reproductive output may be at particular risk from anthropogenic habitat disturbance. We studied growth, reproduction and diet of the threatened broad-headed snake (Hoplocephalus bungaroides), a small nocturnal elapid from sandstone habitats in south-eastern Australia. Our data suggest that the life-history traits and diets of broad-headed snakes contribute to their vulnerability. Rather than searching actively for prey (like most sympatric elapids), broad-headed snakes wait under rocks to ambush prey. In consequence, these snakes feed infrequently, grow slowly, mature late and reproduce infrequently (compared with other sympatric snake species). Because of an ontogenetic shift in food habits, populations of broad-headed snakes require access to high densities of velvet geckos (in rock outcrops) as well as small mammals (in adjacent woodland). The specialized diet, reliance upon two distinctive habitats, low rate of growth, delayed maturation and low reproductive output may have exacerbated the vulnerability of broad-headed snakes to habitat destruction. All of these traits may be due to the species' reliance upon ambush predation rather than active foraging. A broader analysis supports the hypothesis that foraging mode may contribute to endangerment: a disproportionate number of ‘threatened’ species of Australian elapid snakes rely on ambush predation.

The abundance of foraging bats (Pipistrellus pipistrellus and Myotis daubentonii) and flying insects over two rivers in north-east Scotland (Dee and Ythan), differing eightfold in nitrate levels, were compared by paired samples over three periods in summer 1995, during each of which 10 nights' sampling was undertaken using ultrasonic detectors and insect suction traps. Thirteen out of the 18 pairwise comparisons of bat passes, insect numbers, biomass and diversity were not significantly different. The oligotrophic Dee supported higher numbers of bats than the eutrophic Ythan in June, but this was attributed to an effect of adverse weather conditions affecting the Ythan. In July and August–September, neither bat abundances nor the abundance of Chironomidae and Trichoptera, the main food of the bats, differed significantly between the two rivers, although the Ythan showed higher total insect biomass than the Dee. The Ythan also showed a much higher abundance of small Diptera, mainly Psychodidae and Cecidomyiidae, associated with elevated nitrogen levels in the intensively farmed river valley. Thus a small eutrophic river can support as many bats and as high an insect density as a large oligotrophic one.

Between 1959 and 1968 the entire world population of Seychelles warblers (Acrocephalus sechellensis) comprised only 26–29 individuals entirely confined to Cousin Island (26 ha). Through conservation actions the island population has reached a carrying capacity of ca 320 birds since 1982. The warbler population has significantly lower levels of genetic variation, as determined by multilocus DNA fingerprinting, compared with its mainland relative, the Australian warbler (Acrocephalus australis) and other outbred natural bird populations (mean band sharing coefficients are 0.50, 0.22 and 0.21, respectively), and has a very low annual reproductive rate of 0.28 yearlings per breeding pair. Therefore some degree of inbreeding may have occurred in this population.

The relatively low level of genetic variation is not directly responsible for this low reproductive output since breeding pairs that were transferred to the islands of Aride (68 ha) and Cousine (29 ha) with similar vegetation and climate, but with higher year-round insect food abundance, had significantly higher fecundity and survival. In contrast, no difference in reproductive success was observed between incestuous breeding pairs and non-incestuous breeding pairs.

In addition, annual production of yearlings by pairs on Aride was on average 44 times higher than that by the same pairs on Cousin before the transfer. Annual production of yearlings by pairs on Cousine, that were as inbred as those on Aride, was on average five times higher than that by the same pairs on Cousin before the transfer. Eight years after translocation, the Aride population had grown to over 1000 birds, and six years after translocation, the Cousine population had grown to 186 birds.

The experiments, controlled for group size, breeding partner, breeder age and experience, showed that the differences in reproductive success by warblers on the three islands cannot be due to genetic differences because of similar band-sharing similarities, but seem entirely accounted for by differences in food supply. Although the amount of genetic variation in the Seychelles warblers is relatively low, the demographic parameters do not indicate apparent signs of inbreeding depression in these populations.

White Sands pupfish (Cyprinodon tularosa) are endemic to southern New Mexico and occur in only four localities: Malpais Spring, Salt Creek, Mound Spring and Lost River. Recently reported historical accounts indicate that the latter two populations were derived from translocations. Their limited distribution and complicated history suggest that knowledge of population genetic structure would be useful for the development of a sound conservation strategy. Mitochondrial DNA (mtDNA) sequences for a segment of the control region showed little variation. Variation was observed for microsatellite and allozyme loci with 37% attributable to divergence among populations. The mean genetic distance between Malpais Spring and the other three populations was high (allozymes: Darc = 0.541; microsatellites: Rst = 0.684) compared with the mean distance among the other three populations (Darc= 0.161, Rst = −0.016). There were fixed or nearly fixed differences in allele frequency between the Malpais Spring population and the other three populations at one allozyme locus (hexokinase) and two microsatellite loci (WSP-02 and WSP-11). We suggest the recognition of two evolutionarily significant units (ESUs) for the White Sands pupfish: Malpais Spring and Salt Creek. Our data indicate that the Lost River and Mound Spring populations descended from translocations from the Salt Creek population. Therefore, conservation efforts should focus on the Malpais Spring population.