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.     

Alien mink predation induces prolonged declines in archipelago amphibians

Amphibians are undergoing enigmatic global declines variously attributed to a complex web of anthropogenic forces. Alien predators pose a fundamental threat to biodiversity generally that is predicted to be most acute in island ecosystems. While amphibian eggs and tadpoles are vulnerable to aquatic predators, the effect of predators on adult, reproducing frogs, which most influence amphibian population processes, is unknown. Here, we report on the responses of amphibian populations in the outer Finnish Archipelago to a long-term, large-scale removal of American mink (Mustela vison Schreb.), an invasive predator linked to recent biodiversity loss across Europe. Removal increased both the densities and distribution of common frogs (Rana temporaria L.) but not those of common toads (Bufo bufo L.), which appear to escape mink predation because of their unpalatable skin. Importantly, the largest benefits of mink removal to frog recovery were slow to appear as frogs apparently have a delayed maturation in these harsh environments, which means we must be cautious about reliance upon short-term results.     

Blue mussels (Mytilus edulis) in the diet of roach (Rutilus rutilus) in outer archipelago areas of the western Gulf of Finland,Baltic Sea

The blue mussel (Mytilus edulis) is one of the key species in the Baltic Sea ecosystem and it is living at the edge of its range in the western Gulf of Finland. Roach (Rutilus rutilus) is a freshwater fish species that has benefited from recent coastal eutrophication and is at present highly abundant in the outer archipelago of the Gulf of Finland. In 2000 and 2001, a total of 516 roach were sampled for diet analysis in three study areas. Shelled molluscs formed over 95% of the diet of roach, blue mussels being the dominant single species. The proportion of this species in the food of roach in the three study areas ranged between 38–61% for smaller roach (<225 mm) and 39–85% for larger (>225 mm) roach, indicating that blue mussel is a highly important food source for roach in outer archipelago areas of the western Gulf of Finland, in contrast to reports from other parts of the northern Baltic Sea. The scarcity of large blue mussels in mussel beds in the easternmost study area was reflected in the lower proportion of blue mussels in the diet of larger roach. However, the growth of roach was not affected by the availability of blue mussels. The twofold differences observed in the annual growth of roach between warm and cold years demonstrated that temperature is an important factor controlling the growth of roach in the western Gulf of Finland.     

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.     

The origin and distribution of feral mink in Scotland

A result of the post-war boom in mink farming was the escape of mink which are now well established in the wild. Feral mink are known to be present in all Scottish mainland counties, except Sutherland and Caithness, and are also found on the islands of Arran and Lewis. Populations are thriving on most of the main river systems. The existence of suitable habitat and adequate food supplies suggests that mink will continue to increase and spread in Scotland.     

Dynamic larval dispersal can mediate the response of marine metapopulations to multiple climate change impacts

Climate change is having multiple impacts on marine species characterized by sedentary adult and pelagic larval phases, from increasing adult mortality to changes in larval duration and ocean currents. Recent studies have shown impacts of climate change on species persistence through direct effects on individual survival and development, but few have considered the indirect effects mediated by ocean currents and species traits such as pelagic larval duration. We used a density-dependent and stochastic metapopulation model to predict how changes in adult mortality and dynamic connectivity can affect marine metapopulation stability. We analyzed our model with connectivity data simulated from a biophysical ocean model of the northeast Pacific coast forced under current (1998–2007) and future (2068–2077) climate scenarios in combination with scenarios of increasing adult mortality and decreasing larval duration. Our results predict that changes of ocean currents and larval duration mediated by climate change interact in complex and opposing directions to shape local mortality and metapopulation connectivity with synergistic effects on regional metapopulation stability: while species with short larval duration are most sensitive to temperature-driven reduction in larval duration, the response of species with longer larval duration are mostly mediated by changes in both the mean and variance of larval connectivity driven by ocean currents. Our results emphasize the importance of considering the spatiotemporal structure of connectivity in order to predict how the multiple effects of climate change will impact marine populations.     

Fox predation on cyclic field vole populations in Britain

The diet of the red fox Vulpes vulpes L. was studied during three winter periods in spruce pklantations in Britain, during which time the cyclic field vole Microtus agrestis L. populations varied in abundance. Field voles and roe deer Capreolus capreolus L. were the two main prey species in the diet of the red fox. The contribution of lagomorphs to fox diet never exceeded 35% and species of small mammal other than field voles were of minor importance. The contribution of field voles was dependent on vole density. The non-linear density dependent relationship with a rather abrupt increase of field voles in fox did when vole density exceeded ca 100 voles ha⁻¹ was consistent with a prey-switching response. The contribution of field voles to fox diet during the low phase of population cycles was lower in Kielder Forest than in other ecosystems with cyclic vole populations. The number of foxes killed annually by forestry rangers was consistent with the evidence from other studies that foxes preying on cyclic small rodents might show a delayed numerical response to changes in vole abundance. Estimates of the maximum predation rate of the fox alone (200–290 voles ha⁻¹ of vole habitat year⁻¹) was well above a previously predicted value for the whole generalist predator community in Kielder Forest. Our data on the functional response of red foxes and estimates of their predation rates suggest that foxes should have a strong stabilising impact on vole populations, yet voles show characteristic 3-4 yr cycles.     

Notes on the food of feral mink Mustela vison in England and Wales

There is some apprehension about the possible effect of the introduced American mink Mustela vison upon the indigenous British fauna, either as a predator exerting new and excessive pressure upon native prey species, or as a carnivore competing for prey resources against native carnivores. Concern is also felt by preservers of game, and by keepers of small domestic animals (poultry and rabbits) about the mink's potential as a pest. To investigate the food habits of the mink in England and Wales the alimentary canals of 1165 trapped animals were examined, yielding 204 samples for analysis. The resulting information provides a general picture of the mink's diet, including seasonal variations. Comparisons were possible with similar work carried out in other countries, and with the diets of other British carnivores. Tentative conclusions were reached regarding the broad pattern of food relationships between the mink and other British animals, wild and domestic.     

Patches and pulses as fundamental characteristics for matching ecological and cultural diversity: the Baltic Sea archipelago

Focusing on the Baltic archipelago, we address the questions: to what extent are the rhythms of natural and social systems compatible and under which criteria can we make them coincide? Existing mismatches between resource availability and human demand are identified as well as human attempts to dampen ecosystem fluctuations. By means of examples from forestry and fisheries, we illustrate how changes in property rights and technology have altered the diversity and resilience of the archipelago system. Our results suggest that intermediate scale processes of years up to a century are most critical for bringing natural and cultural systems in concordance. The time frame relevant to management and policy in the archipelago seems to correlate with eutrophication processes and the regrowth of forests. In fisheries, a shift from traditional to recreational fisheries has created fishery patterns badly adapted to the dynamics of the coastal ecosystem in disregard of traditional ecological knowledge. A multipurpose and adaptive management of natural resources is advocated as the most appropriate approach for promoting ecological and cultural diversity in the Baltic archipelago. Existing mismatches between the two have to be addressed by governing institutions at many hierarchical levels.     

Ecological impacts of feral pigs in the Hawaiian Islands

The foraging habits of exotic ungulate species can directly and indirectly affect native plant and animal distribution and abundance patterns. Most of the studies on feral pig interactions with other biota in the Hawaiian Islands have been published as difficult to access reports to governmental and nongovernmental organizations, graduate student theses, and a few in peer reviewed journals. In this paper we discuss the origins of pig introductions to Hawaii, their feralization process, population expansion, and interactions with native and non-native biota. We also consider the environmental effects triggered by pigs on local ecosystems and biotic communities. Feral pig activities can reduce the abundance of native plant species, enhance conditions for the establishment of invasive non-indigenous plants, and perhaps indirectly negatively impact native forest bird species. Pig foraging and traveling patterns may also lead to physical alteration of ecosystems by increasing soil erosion that may lead to watershed degradation. However, much remains to be learned about the strength and significance of aforementioned interactions and their long-term effects on Hawaiian biota and ecosystems due to some confounding events. Elucidating the dynamics and long-term ecological effects generated by pigs is a crucial step towards increasing our understanding of and more effectively managing biotic interactions.     

Eutrophication in the Baltic Sea

Fish consumption is increasing globally. Overfishing puts pressure on fisheries, but aquaculture provides an alternative to satisfy the growing need for seafood. However, nutrient emissions from aquaculture contribute to eutrophication, and raising fish from the top of the food chain is inefficient. Here we use the approach of industrial ecology and report ImPACT decomposition analysis of the drivers of nutrient emissions to the Baltic Sea from rainbow trout aquaculture in Finland during 1980?2007. During this period, the nitrogen load studied increased markedly and was 522 tonnes in 2007. The phosphorus load quadrupled and then returned to its original level of about 65 tonnes. The Finnish population increased slightly, while the average affluence level increased significantly. Total salmonid consumption increased substantially during the period. The increasing percentage of imported salmonids and improvements in domestic aquaculture technology ended the period of strong growth of emissions in the 1980s. Decreasing the nutrient load through reductions in salmonid consumption in the future is unlikely, due to health benefits and consumer preferences. Replacing domestic production with import of salmonids raises questions regarding outsourcing of the environmental impact, and regarding rural development in Finland. Major improvements in production technology are not in sight. New perspectives on rainbow trout aquaculture may be needed, including using feed from the Baltic Sea, thus closing the nutrient cycle or changing consumption and production to herbivorous fish species.     

The food of feral mink (Mustela vison) in Scotland

Feral American mink ( Mustela vison ) thrive in Scotland. The potential impact of the mink on the native fauna was investigated through its food. Fifty-five stomachs and 33 scats were analysed by identifying the indigestible remains. Fish formed the main prey (49% occurrence) and birds and mammals 28% and 23% respectively. Most of the fish eaten were Salmonidae (brown trout and salmon). No remains of eels were found.
Mink may compete with otters for food, but the smaller prey taken by the mink and its greater use of land separates it from the otter. Mink seem to be occupying a vacant feeding niche in Scotland.     

Population dynamics of calanoid copepods and the implications of their predation by clupeid fish in the Central Baltic Sea

Population dynamics of major Baltic calanoid copepod speciesin the Gotland Basin during the last two decades were characterizedby a decline of Pseudocalanus elongatus associated with decliningsalinities, and an increase of Temora longicornis and Acartiaspp. potentially due to warmer conditions. Additionally thisstudy investigated the effect of predation by the major planktivorousfish species herring (Clupea harengus) and sprat (Sprattus sprattus)for the period 1977–1996 in the Gotland Basin (CentralBaltic Sea). Examination of consumption by these fish speciesin relation to copepod production estimates showed a switchby herring from consuming mainly CV/VI of P. elongatus and T.longicornis, to preying on CII of the latter copepod. This switchwas potentially due to increased competition with the drasticallyincreased sprat stock since the late 1980s. Further, an increasedpredation pressure by sprat on CV/CVI of both copepod speciesin spring resulted in higher copepod mortality rates. In consequence,based on these results we suggest that the increase in the spratstock since the late 1980s contributed to a decline of P. elongatus,and additionally prevented an even more pronounced temperature-drivenincrease in the T. longicornis stock, as was observed for Acartiaspp., which was not significantly consumed.     

The diet of the feral mink (Mustela vison) in southwest Britain

Mink diet in three study areas in southwest Britain was studied by faeces (scat) analysis. In these areas mink ate a wide range of prey, mainly fish, birds and mammals. Proportions of prey in the diet were determined largely by availability, but were also affected by prey behaviour, the physical nature of the habitat and overall prey abundance. Seasonal variation in predation was recorded on moorhens and fish. Changes in rod and net fishing success on the river Teign could not be correlated with the arrival of the mink. Changes in fish populations in Slapton Ley appeared to have causes other than mink. It is concluded that, in the areas studied, domestic stock make up a very small proportion of the mink's diet, and that the wild prey species are not suffering any obvious population depletion.     

捕食风险对东方田鼠功能反应格局的作用

植食性哺乳动物对食物斑块的选择和利用不仅取决于食物的可利用性,且与觅食环境潜存的各种风险紧密关联。捕食风险是否通过作用于动物觅食活动中的警觉影响其功能反应格局。在新鲜白三叶叶片构成的各类食物密集斑块上,测定东方田鼠觅食行为,建立功能反应模型,检验捕食风险对其功能反应格局的作用。结果发现,捕食风险能显著地延长东方田鼠的觅食决定时间,但其摄入率保持稳定,功能反应构型亦未发生改变,仍为Ⅱ型功能反应;除了对照组个体的采食时间随叶片大小增大无明显变动规律外,处理组个体的采食时间及对照组和处理组个体的处理时间、觅食中断时间均随叶片大小及口量的增大呈线性增高趋势,处理组个体的觅食中断时间明显大于对照组个体的;对照组和处理组个体的采食率均随叶片大小及口量呈非线性渐近递减趋势,但处理组个体的采食率较对照组个体的略有降低。结果揭示,在捕食风险压力下,虽然上述觅食参数变异能潜在地降低摄入率,但个体能通过改变觅食活动中各种警觉行为动作如降低嗅闻和静听监视动作的发生频次,增大视觉监视动作比重,以此缓冲捕食风险压力,维持摄入率。摄入率测定值与模型预测值的线性回归极显著,表明,功能反应模型具有良好的预测性。在可利用植物密集斑块,动物觅食活动中的警觉能缓冲捕食风险压力;动物摄入率是由植物大小调控的口量决定的,且受采食与处理食物竞争及觅食中断的制约;其功能反应仍属Ⅱ型功能反应。     

Genetic variability and structure of the water vole Arvicola amphibius across four metapopulations in northern Norway

Water vole Arvicola amphibius populations have recently experienced severe decline in several European countries as a consequence of both reduction in suitable habitat and the establishment of the alien predator American mink Neovison vison. We used DNA microsatellite markers to describe the genetic structure of 14 island populations of water vole off the coast of northern Norway. We looked at intra‐ and inter‐population levels of genetic variation and examined the effect of distance among pairs of populations on genetic differentiation (isolation by distance). We found a high level of genetic differentiation (measured by F_ST) among populations overall as well as between all pairs of populations. The genetic differentiation between populations was positively correlated with geographic distance between them. A clustering analysis grouped individuals into 7 distinct clusters and showed the presence of 3 immigrants among them. Our results suggest a small geographic scale for evolutionary and population dynamic processes in our water vole populations.     

Experiments on epibenthic predation in the Wadden Sea

Field experiments were designed to evaluate the role of predators in the Wadden Sea, small predators like shore crabs, shrimps and gobies, and large ones like flatfish and birds. Exclosures, maintained in aCorophium volutator bed, an eelgrass bed, in a sandy and a muddy flat, protected the infauna from such epibenthic predation. The resulting changes in the macrofauna were recorded and compared with an unaffected control area. In sandy and muddy flats of the lower intertidal zone, cages (mesh size5 mm) altered abundance and composition of the infauna almost entirely. Nearly all species achieved higher population densities than in the control area, and in addition, the number of species increased as well. In the the scarcely populated mud flat a dense suspension-feeder assemblage emerged, associated with numerous tube-building polychaetes. The sand flat, normally dominated by deposit feeders, also became occupied by a dense suspension-feeder assemblage, mainly cockles. In contrast, narrowly meshed cages had only little effect in the beds of eelgrass and ofCorophium volutator. Both are positioned in the upper intertidal zone. Although a number of species still responded with significant increases in abundance, many remained indifferent or even tended to be less abundant within cages. Cages provided with a 20-mm mesh nylon net, excluding only birds, flatfish and the biggest crabs, increased significantly the survival of large-sized infaunal members. This was only apparent in the upper intertidal zone. It was concluded, that small sized epibenthic predators are the major determinants of the dynamic species abundance pattern of the lower intertidal flats. In the eelgrass bed, the meshwork of rootlets constitutes an important spatial refuge from these predators. The complex habitat structure causes a diversified faunal assemblage. TheCorophium bed in the uppermost intertidal zone is less accessable to predators like crabs, shrimp and gobies. The monotonous appearance of this faunal assemblage is assumed to be the outcome of competitive exclusion and of occasional harsh physical conditions.