Dynamic impacts of feral mink predation on vole metapopulations in the outer archipelago of the Baltic Sea

Predation impacts by introduced predators are predicted to be most intense in island ecosystems, and also variable depending on environmental conditions, but large-scale experimental field testing is rare. In this study we examine the factors that determine the distribution and abundance of vole metapopulations preyed upon by feral American mink Mustela vison in the outer Finnish archipelago of the Baltic Sea. Specifically, we follow the dynamics of field voles Microtus agrestis and bank voles Clethrionomys glareolus on 40 small islands under variable rainfall as part of a large-scale mink removal experiment. For both vole species occupancy rates were negatively influenced by island isolation, as were extinction events for field voles. High summer rainfall in 1998 corresponded to large vole populations where mink were absent, populations that then crashed in 1999 and 2000 when below average rains fell during the summer breeding season. Where mink were present however, vole abundance remained more constant between years with no boom-bust apparent. We conclude that weather and predation may drive vole abundance whereas habitat patchiness and metapopulation processes more strongly drive vole distributions. There may also be potential for interaction between these factors: because feral mink prevent rapid vole population growth after good summer rains, and vole dispersal is influenced by population size, feral mink may be changing vole dispersal patterns to disrupt the natural metapopulation dynamic. Hence this indirect impact of mink could lead to gradual erosion of vole populations in the outer archipelago by reducing recolonisation processes.     

Impact of released Eurasian otters on a population of American mink: a test using an experimental approach

Competition is considered to be a major evolutionary driving force within assemblages of related and morphologically similar species, but it is notoriously difficult to test and quantify especially amongst species that cannot be easily manipulated in a laboratory or in the field. By exploiting a re-introduction of Eurasian otters ( Lutra lutra ) in the Upper Thames catchment (UK) in 1999 we performed an experiment to test whether there was evidence of competition between otters and American mink ( Mustela vison ). Mink and otters are semi-aquatic mustelids belonging to the same guild. Otters are expected to be the dominant competitor because they are larger and better adapted at exploiting aquatic resources. Our hypothesis was that mink declines when the density of otters increases. We measured the effect of competition at the population level, by observing whether mink distribution and densities changed in an area of 1353 km² in association with the arrival of its putative competitor. We estimated distribution and densities by means of sign surveys and trapping. The results showed that otters were associated with a significant and rapid reduction in the densities of mink, while mink occupancy remained approximately the same in an area of 2464 km² that was used as a control. We observed that the spatial distribution of the mink population throughout its decline was influenced by the yearly cycle of mink activities, with areas being temporarily re-colonized during the dispersal season. Mink is an invasive species in the UK threatening the survival of some native species. Our findings suggest that the re-establishment of otter populations is likely to lead to a decline of mink that may, in turn, be beneficial to native species threatened by this invasive.     

Abundance and distribution of endangered Franciscana dolphins in Argentine waters and conservation implications

This is the first study in Argentine waters on the abundance of the threatened Franciscana dolphin, Pontoporia blainvillei . During 2003–2004 we carried out 17 aerial surveys using line transect sampling methodology. We observed 101 Franciscanas in 71 sightings. In northern areas density was estimated at 0.106 individual/km². Density was lower in southern areas (0.055/km²) and declined with depth beyond 30-m isobaths (0.05/km²). A correction factor for submerged dolphins was applied to density and then extrapolated to the strip between the coastline and the 30-m isobath. Abundance in the northern area was estimated at 8,279 (4,904–13,960) individuals, while in the southern area it was estimated at 5,896 (1,928–17,999) individuals. Considering an annual mortality of about 500–800 individuals, about 3.5%–5.6% of the stock may be removed each year by the fishery and over the 2% recommended by the International Whaling Commission (IWC) and may not be sustainable by the population. Higher densities in coastal areas make Franciscanas more vulnerable to coastal fishing camps, which increased mortality in recent years. A remarkable finding was that while density decreases to the south, values of catch per unit effort (CPUE) increases, indicating different catchability of dolphins between areas.     

Habitat-mediated impact of alien mink predation on common frog densities in the outer archipelago of the Baltic Sea

Alien predators have been recognised as one possible cause for amphibian declines around the world, but little is known of habitat-mediated predation impacts especially on adult amphibians. We studied common frog Rana temporaria under American mink Mustela vison predation in the outer archipelago of the Baltic Sea, south-western Finland. Using egg batches as an index of breeding frog female numbers we compared frog numbers and densities between a large, long-term mink-removal area and a comparable control area. Frog numbers in the removal area were at least 2.7-fold higher than those in the control area. In the presence of mink, frog densities increased with the amount of vegetation cover on the islands, indicating that mink predation affected frog densities especially on less-vegetated islands. An opposite trend appeared to be true for frogs in the mink-removal area, where other predators like snakes could induce a decline of frog densities on more vegetated islands. Shrub or grass vegetation seems to provide frogs shelter against alien mink predation. Our result highlights the importance of landscape-level habitat management as a conservation tool for amphibian populations.     

Voles on small islands: effects of food limitation and alien predation

Ecosystems of three trophic levels may be bottom-up (by food-plant availability) and/or top-down (by predators) limited. Top-down control might be of greater consequence when the predation impact comes from an alien predator. We conducted a replicated two-factor experiment with field voles (Microtus agrestis) during 2004-2005 on small islands of the outer archipelago of the Baltic Sea, south-west Finland, manipulating both predation impact by introduced American mink (Mustela vison) and winter food supply. In autumn 2004, we live-trapped voles on five islands from which mink had been consistently removed, and on four islands where mink were present, and provided half of these islands with 1.8 kg oats per vole. Body mass of female voles increased as a response to supplementary food, whereas both food supplementation and mink removal increased the body mass of male voles in subsequent spring. During winter, there was a positive effect of supplementary food, but in the subsequent summer, possible positive long-term impacts of food supplementation on field voles were not detected. Mink removal appeared not to affect density estimates of field voles during the winter and summer immediately after food addition. Trapping data from 2004 to 2005 and 2007 suggested, however, that in two out of three summers densities of voles were significantly higher in the absence than in the presence of mink. We conclude that vole populations on small islands in the archipelago of the Baltic Sea are mainly bottom-up limited during winter (outside the growing season of food plants), when food availability is low, and limited by mink predation during summer which slows population growth during the reproductive season of voles.     

Preliminary estimates of the population density of the Sokoke scops owl Otus ireneae Ripley in the East Usambara lowlands, Tanzania

Preliminary population density estimates are presented for a recently discovered population of the Sokoke scops owl Otus ireneae in the lowlands of the East Usambara mountains, Tanzania. Calling birds were mapped at two sites totalling 6·3 km². Approximate densities were 3–4 territories/km² in Kwamgumi Forest Reserve and <1·5 territories/km² in Manga Forest Reserve, a much more heavily logged site. Densities in prime habitat in Arabuko‐Sokoke forest, Kenya, the only other locality for the species, exceed seven territories/km². The total population in the East Usambaras cannot be calculated, but it is probably markedly smaller than in Arabuko‐Sokoke. Suggestions for more precise surveys are made.     

Distribution and nesting density of the Philippine Eagle Pithecophaga jefferyi on Mindanao Island,Philippines: what do we know after 100 years?

The Philippine Eagle Pithecophaga jefferyi , first discovered in 1896, is one of the world's most endangered eagles. It has been reported primarily from only four main islands of the Philippine archipelago. We have studied it extensively for the past three decades. Using data from 1991 to 1998 as best representing the current status of the species on the island of Mindanao, we estimated the mean nearest-neighbour distances between breeding pairs, with remarkably little variation, to be 12.74 km ( n  = 13 nests plus six pairs without located nests, se = ±0.86 km, range = 8.3–17.5 km). Forest cover within circular plots based on nearest-neighbour pairs, in conjunction with estimates of remaining suitable forest habitat (approximately 14 000 km²), yield estimates of the maximum number of breeding pairs on Mindanao ranging from 82 to 233, depending on how the forest cover is factored into the estimates.     

Ranges of males of the Great Hornbill Buceros bicornis, Brown Hornbill Ptilolaemus tickelli and Wreathed Hornbill Rhyticeros undulatus in Khao Yai National Park, Thailand

Ranges of individual males of three hornbill species were determined by radio telemetry during the breeding and non-breeding seasons in Khao Yai National Park, Thailand. Two Great Hornbills Buceros bicornis and two Brown Hornbills Ptilolaemus tickelli were studied in both 1988 and 1989, and two Wreathed Hornbills Rhyticeros undulatus were studied in 1989, 1990 and 1991.
In the breeding season, the home range of the Great Hornbill was 3.7 km², similar to that of the Brown Hornbill (4.3 km²), while the Wreathed Hornbill occupied the largest home range (10.0 km²). In the non-breeding season, the range size of the Wreathed Hornbill (28.0 km²) was greater than that of the Great Hornbill (14.7 km²). Differences in range sizes of different species may be related to differences in diet and breeding strategy. Ranges overlapped within and between the species, and this has implications for the estimation of the minimum area required for the conservation of hornbills within the Khao Yai National Park.     

Determining the effects of habitat management and climate on the population trends of a declining steppe bird

The Little Bustard Tetrax tetrax is one of the most threatened steppe bird species in Europe, due mainly to agricultural intensification. Despite the relative importance of the Iberian population (approximately 50% of the global population) little is known about its dynamics and trends, especially in core distribution areas. This study evaluates the influences of meteorological factors and land management on the oscillations and medium-term trends of two Little Bustard populations in Central Spain. During 2001–2007, surveys of breeding male and female Little Bustards were carried out in two central Spanish locations: Valdetorres, in Special Protection Area (SPA) no. 139 (1600 ha), and Campo Real, in Important Bird Area (IBA) no. 075 (1150 ha). Densities were 3.3–4.0 and 1.1–2.1 males/km² in Campo Real, and 1.8–2.2 and 0.6–1.3 females/km² in Valdetorres. The sex ratio was biased towards males in both cases. Both populations declined during 2001–2007, especially in Valdetorres (60%). Variation in habitat composition did not explain variation in the numbers of males. Both populations were influenced by total precipitation in the preceding October–May period. Results suggest that the Little Bustard may be sensitive to future climate trends in Europe. Finally, different simulated demographic scenarios suggest that low female survival and productivity may be the immediate cause of the decline in Little Bustard populations, which is consistent with their sensitivity to climatic conditions.     

Population studies on feral American mink Mustela vison in Ireland

Populations of feral American mink Mustela vison were investigated in several study areas in Ireland, principally by capture-recapture techniques. The populations were found to be essentially self-regulating, with stress, weight loss and mortality evident during the winter and spring. The highest mean density recorded, in terms of animals per unit length of river or lake shore, was 1.4 individuals km⁻¹ with a peak density of 2.3 individuals km⁻¹, but population densities were considerably lower in most areas. Trappability and population structure, in age, sex and residency classes, varied between habitats, but population turnover was high, leading to complete replacement of individuals in two populations monitored for two years. The availability of the preferred prey, crayfish Austropotamobius pallipes appeared to be a major factor determining population size and preferred breeding areas. Control methods are either unnecessary or are likely to fail in most situations, and may be counterproductive.     

Brazilian Tapir Density in the Pantanal: A Comparison of Systematic Camera-Trapping and Line-Transect Surveys

The density of Brazilian tapirs ( Tapirus terrestris) was studied in the northeastern part of the Pantanal wetlands of Brazil using two simultaneous and independent methods: (1) systematic camera trapping combined with capture–recapture analysis, with camera traps spaced 1 km apart and distributed over 54 km²; and (2) line-transect sampling using an array of 12 linear transects, from 3.8 to 7.2 km long, covering the principal open and forest habitat types across the entire 1063 km² SESC Pantanal Reserve. The two methods yielded conservative density estimates of 0.58 ± 0.11 tapirs/km² (camera trapping) and 0.55 (95% CI 0.30–1.01) tapirs/km² (line transects). The study suggests that certain Pantanal habitats and sites can sustain relatively high population densities of tapirs when these animals are protected from hunting. Further testing of the camera-trapping methodology as applied to tapirs is required, particularly focusing on extending the survey period. As it represents a relatively rapid method for estimating population density, in comparison to line-transect surveys, and as it generates information simultaneously on multiple species that are conservation priorities, we recommend that camera-trapping surveys be applied more widely across a variety of Pantanal habitats and land-use categories in order to confirm the value of the vast 140,000 km² wilderness region for this vulnerable species.     

Determinants of avian species richness at different spatial scales

ABSTRACT. Studies of factors influencing avian biodiversity yield very different results depending on the spatial scale at which species richness is calculated. Ecological studies at small spatial scales (plot size 0.0025–0.4 km²) emphasize the importance of habitat diversity, whereas biogeographical studies at large spatial scales (quadrat size 400–50,000 km²) emphasize variables related to available energy such as temperature. In order to bridge the gap between those two approaches the bird atlas data set of Lake Constance was used to study factors determining avian species diversity at the intermediate spatial scales of landscapes (quadrat size 4–36 km²). At these spatial scales bird species richness was influenced by habitat diversity and not by variables related to available energy probably because, at the landscape scale, variation in available energy is small. Changing quadrat size between 4 and 36 km², but keeping the geographical extension of the study constant resulted in profound changes in the degree to which the amount of different habitat types was correlated with species richness. This suggests that high species diversity is achieved by different management regimes depending on the spatial scale at which species richness is calculated. However, generally, avian species diversity seems to be determined by spatial heterogeneity at the corresponding spatial scale. Thus, protecting the diversity of landscapes and ecosystems appears to ensure also high levels of species diversity.     

Wolf-avoidance strategies of moose

Two possible anti-predator strategies of prey were suggested by the distribution of moose at Isle Royale, a 544 km² island with numerous offshore islets. Aerial surveys in winter indicated that moose density on small islets (<1.5 km²) averaged 415% greater than on the main island, with about 11% of the calf population existing on the small islets which comprise less than 2% of the total land area. On small islets and shorelines, protection from wolves (in the form of escape terrain) is provided by nearby water in summer and coniferous cover in winter. Calves on islands appear to have a higher survival rate. In summer, cow moose with calves were seen more commonly in camps with human activity (and no wolves) than away from camps, suggesting that people provided a refuge from wolf predation for this unhunted moose population. These subtle patterns of habitat selection by moose are attributed to the intensive natural selection pressure of wolf predation.     

Feral donkey (<Emphasis Type="Italic">Equus asinus</Emphasis>) eradications in the Galápagos

Introduced herbivores are major drivers of ecosystem change and biodiversity loss, particularly on islands. Tools and techniques now exist to routinely remove introduced herbivores from islands, providing a powerful conservation tool. Here, we summarize the few documented feral donkey removals on islands worldwide, and report on the removal of populations from the Galápagos archipelago, Ecuador. After decades of sporadic control programs on Santiago Island and Alcedo Volcano, Isabela Island, donkey populations were removed from both areas, concurrent with a goat eradication program. Both ground and aerial hunting programs were utilized. The latter method was highly efficient; donkeys were removed from Santiago Island with less than 80 h of aerial hunting. Given the clear impacts of introduced herbivores on islands worldwide, feral donkey populations should be routinely removed from islands.     

Feral cat diet and impact on sooty terns at Juan de Nova Island, Mozambique Channel

Feral cat Felis catus predation on seabirds has been well documented; however, details regarding shifts in feral cat diet in relation to seabird availability, seabird predation rate and impact on seabird population dynamics are scarce. Here, we present data documenting a seasonal shift in feral cat diet at Juan de Nova Island, Mozambique Channel. We also quantify sooty tern Sterna fuscata predation by feral cats and examine the impact on sooty terns over both the short term (by removing individual cats from sub-colonies) and over the longer term by highlighting their influence on population growth rate ( λ ) using a deterministic matrix model. Cat diet shifted dramatically from insects, rats and mice outside the tern breeding season to primarily terns when terns were breeding. The predation rate of sooty terns at Juan de Nova was estimated at 5.94 terns cat⁻¹ day⁻¹, with a proportion of these (22%) being killed without being consumed ('surplus kills'). When only one cat was removed from each sub-colony, tern predation declined tenfold in the short term. From our matrix model, the annual growth rate for sooty terns was 1.01 in the absence of cat predation. It remained above one until a predation impact equivalent to approximately three times the estimated cat density (12.04 per km²) was incorporated. Our results demonstrate that cats preferentially predate and have an impact on breeding sooty terns at Juan de Nova, and that an increase in cat density could lead to negative effects on population growth, despite the large breeding tern population.     

White rhinoceros range size in the south-western Kruger National Park

White rhinoceros range size was ascertained telemetrically in the south-western Kruger National Park. The mean annual range size of territorial males was 9.86 km² compared to 22.83 km² for adult females. White rhinoceros females' summer wet season range was larger (21.44 km²) than the winter dry season range (11.64 km²). It is argued that abundant field-water during the wet season enables animals to range further from permanent water supplies and to utilize larger foraging areas. White rhinoceros have core areas in their individual ranges that usually are situated along riverbanks in the preferred grazing regions. These core areas also include some favourite resting spots on high-lying areas. White rhinoceros range sizes in the south-western Kruger National Park were similar to those of other reserves with comparable white rhinoceros densities. In the Umfolozi Game Reserve, which has a higher white rhinoceros density than the Kruger National Park, the individual ranges are much smaller.     

Carrying Capacity and Potential Production of Ungulates for Human Use in a Mexican Tropical Dry Forest

Data are provided on the carrying capacity and potential production for sustainable human use of white-tailed deer ( Odocoileus virginianus ) and collared peccary ( Pecari tajacu ) in a protected tropical dry forest at Chamela on the Pacific coast of Mexico. In this paper, the carrying capacity was defined as the equilibrium density plus the number of animals removed by predators. The equilibrium point was estimated from the density dependent relationship between the finite population growth rate and the current density according to a logistic model. Annual density was estimated using the line transect method. Carrying capacity estimates were 16.5 to 17.2 deer/km² and 9.3–9.5 peccaries/km², representing a combined biomass of 841–874 kg/km². A potential production for human use of 2.1 deer/km² and 4.4 peccaries/km² was estimated employing the model of Robinson and Redford (1991) . The data suggest that, in the protected tropical dry forest of Chamela, the density and biomass of wild ungulates can maintain a similar or greater density and biomass than other Neotropical forests. To obtain an accurate estimation of the maximum sustainable yield ( MSY ), it is necessary to consider predation. From a management point of view, it is important to consider that carrying capacity varies as a function of the rainfall pattern.     

HARBOR PORPOISE (PHOCOENA PHOCOENA) ABUNDANCE IN ALASKA: BRISTOL BAY TO SOUTHEAST ALASKA, 1991-1993

Between 1991 and 1993, Alaska harbor porpoise ( Phocoena phocoena ) abundance was investigated during aerial surveys throughout much of the coastal and offshore waters from Bristol Bay in the eastern Bering Sea to Dixon Entrance in Southeast Alaska. Line-transect methodology was used, and only those observations made during optimal conditions were analyzed. Survey data indicated densities of 4.48 groups/100 km², or approximately 3,531 harbor porpoises (95% C. I. 2,206-5,651) in Bristol Bay and 0.54 groups/100 km², or 136 harbor porpoises (95% C. I. 11-1,645) for Cook Inlet. Efforts off Kodiak Island resulted in densities of 1.85 groups/100 km², or an abundance estimate of 740 (95% C. I. 259-2,115). Surveys off the south side of the Alaska Peninsula found densities of 2.03 groups/100 km² and an abundance estimate of 551 (95% C. I. 423-719). Surveys of offshore waters from Prince William Sound to Dixon Entrance yielded densities of 4.02 groups/100 km² and an abundance estimate of 3,982 (95% C. I. 2,567-6,177). Combining all years and areas yielded an uncorrected density estimate of 3.82 porpoises per 100 km², resulting in an abundance estimate of 8,940 porpoises (CV = 13.8%) with a 95% confidence interval of 6,746-11,848. Using correction factors from other studies to adjust for animals missed by observers, the total number of Alaska harbor porpoises is probably three times this number.     

Movement Patterns and Home Range Of The Bongo (Tragelaphus Eurycerus) In The Rain Forest Of The Dzanga National Park, Central African Republic

Bongos ( Tragelaphus eurycerus Ogilby) were studied for 8 months in the Dzanga National Park, Central African Republic. Tracks were followed and mapped with a compass and a pedometer to study movement patterns and home range. Natural licks were shown to be central points in the home range of the bongos: they visited the licks recurrently to consume soil, but also to forage on grass and herb species, and for social reasons. Forest areas far from licks were used much less than forest areas close to licks. When a lick was visited, distances between two resting sites were longer than in the forest without lick visits, caused by a direct and straight movement from the denser forest areas toward a lick. The study area of about 150 km² was presumably occupied by two groups of bongos. One of them seemed to split temporarily into two subgroups. Groups were not larger than 10–20 individuals. Estimated home ranges were at least 49 km² and 19 km² for the two groups, respectively. Estimated density in the Dzanga National Park was 0.25 bongos per km^2. This study shows the importance of natural licks for the largest social forest antelope, the bongo, and provides information which is important for its future conservation.