Traffic flattens billions of frogs every year

Amphibians are run over by cars more often than other vertebrates. Per road kilometer, an average 250 amphibian individuals die every year because of traffic. According to this calculation, over 113.5 million frogs die annually on the Finnish road network (454 000 km). In Brazil, one of the world’s amphibian hot spots, traffic annually kills 9 420 frogs on each road kilometer. This means a total of over 16 billion frogs lost due to traffic.


Roads built near wetlands are the most significant cause of frog mortality on all continents, but particularly in Europe. No relief is in sight for this problem, because traffic amounts are increasing every year throughout the world.


Fast-moving frog species are somewhat fortunate because their traffic mortality is quite low on roads with little traffic (24–40 cars per hour). Up to 94% of fast-moving frogs survive when crossing a road. Slow-moving species, such as the common toad (Bufo bufo), are not that lucky. Only half of common toads survive to the other side of a road. On busier roads (60 cars in an hour) over 90% of common toads are run over by a car.

A dead common toad (Bufo bufo) hit by a car. © Mia Vehkaoja

Amphibians suffer from both direct and indirect negative effects of road networks and traffic. Mortality is a direct cause, whereas isolation is an indirect cause. Amphibians migrate according to seasons: during spring to their breeding grounds and during autumn to their wintering grounds. These migrations make amphibians vulnerable to traffic mortality. Season migrations occur particularly in the temperate zone, such as in Europe, where traffic has become the greatest threat to amphibian survival in certain places.


The traffic mortality of frogs decreases population sizes and reduces migration, which lead to a decreasing gene flow between populations and the disappearance of genetic diversity. Smaller populations are at greater risk of going extinct.


Historically thousands of kilometers of roads have been built through wetlands, which leads to the disappearance, isolation and depletion of wetland habitats. Roads also influence the cycle and function of water systems. Road construction has drained and polluted wetlands all over the world.


Conservation actions should concentrate not only on restricting road construction laws and regulations, but on preventing frogs from accessing roads by installing culverts and fences. According to a French study, the combination of culverts and fences is the most efficient way for saving frogs from traffic mortality. But this is just one study, and unfortunately we still know too little about which methods are best for amphibian conservation.


A requiem for birds killed by alien predators

A small pond in the Finnish countryside is filled with squeaking, when several goldeneye (Bucephala clangula) and wigeon (Mareca penelope) broods are foraging. Suddenly a goldeneye hen alerts and flies cackling across the pond. The figure of an American mink (Neovison vison) appears on the water surface, which leads to an emergency escape of the duck broods. The American mink is an efficient predator, which does not belong in Finnish nature. Nonetheless it has already occupied the whole country. Alien species in Finland and other countries are a serious threat to birds.

Wetland ecology group_University of Helsinki_duck_goldeneye_wigeon_wetland

Duck broods escape after a watchful goldeneye female alerts after spotting an American mink.

A fur farming runaway became a nuisance

The American mink is, as its name reveals, an American species, which was brought to North Europe at the beginning of the 20th century. Minks escaped from farms and were successful in Europe. They were also introduced to nature on the Russian side. Rapidly American minks occupied all of Fennoscandia.

The American mink utilizes various wetlands, lakes and archipelagos, where it predates birds and limits their populations. Minks don’t only eat birds, but also their eggs. The American mink is especially harmful in Fennoscandian archipelagos, because birds are not adapted to such predators. Certain bird species, such as the black guillemot (Cepphus grylle), are especially threatened by mink predation. Traditionally Fennoscandia has not had such predators. The European mink (Mustela lutreola), which is now extinct from many of its historical areas, did not occupy the archipelago in the same way as its American cousin does.

Wetland ecology group_University of Helsinki_American mink_alien predator_wetland

The American mink is currently a common wetland species in Fennoscandia.

Raccoon dog ended up on EU’s black list

The raccoon dog (Nyctereutes procyonoides) is an Asian species that was introduced to the European parts of the Soviet Union to be hunted for its fur. From the Soviet the raccoon dogs spread west. Soon raccoon dogs occupied all of Finland, and are now also reaching Sweden. Currently Finnish hunters are working to prevent raccoon dogs from going over the Swedish border. The raccoon dog was recently classified as an invasive alien species by the European Union. One reason for this is its influence on birds.

Wetland ecology group_University of Helsinki_supikoira_raccoon dog_vieraslaji

Raccoon dog destroys an artificial duck nest. Game camera photo.

Research conducted at the Helsinki University shows that raccoon dog density and predation pressure on artificial nests correlates on wetlands. Other studies have found raccoon dogs to destroy both pheasant and duck nests. Thus raccoon dog removal around wetlands is an important way to protect birds.

And then there were no Stephens Island wrens left

A cat named Tibbles carried little birds to a light house yard on an island of New Zealand in the late 1800s. The birds were Stephens Island wrens (Xenicus lyalli). The cat hunted at least 15, after which apparently none were left. Rats and cats might already have killed the wren populations from the other islands. The Stephens Island wren is not the only victim of cats. Australian researchers have counted that domestic cats have killed at least 20 native species. In the USA cats are estimated to kill 3,7 milliard (3.7 billion) birds annually. Most birds are killed by unowned cats. Cats therefore appear to kill more birds than any other anthropogenic cause in the USA. Worldwide cats have killed 33 animal species to extinction.

Wetland ecology group_University of Helsinki_cat_alien predator_wetland

A cat stalking a duck brood in a wetland. In this case cutting the vegetation saved the brood.

The raccoon is occupying Europe

In addition to the American mink, the raccoon (Procyon lotor) has also arrived in Europe from North America. The raccoon was also brought Europe to be farmed for its fur. This highly adaptable animal has succeeded well in Europe, and is now common for example in Germany and France. The population size in Germany is already evaluated at over a million. Raccoons are spreading north, and are currently settled in Denmark and individuals have also been found in Sweden. Compared to raccoon dogs, raccoons also live successfully in cities. But just like raccoon dogs, raccoons are also well adapted to the wetland environment, and are thus harmful to waterbirds.

Raccoons reproduce effectively, and therefore their extirpation is impossible once a population has been established. This is why efforts need to be focused on stopping the species from spreading. The raccoon is classified as an invasive alien species in the EU, so farming them or having one as a pet is illegal.

Read more:

Väänänen, V.-M. 2007: The effect of raccoon dog Nyctereutes procyonoides removal on waterbird breeding success. Suomen Riista (PDF)

PHYS.ORG 29.1. 2014: Cats in US kill billions of birds, mammals, study finds

Loss, S.R. et al. 2013: The impact of free-ranging domestic cats on wildlife of the United States. Nature Communications

Marra, P. & Santella C. 2017. Cat Wars. The Obituary of the Stephens Island Wren

Spiegel Online 3.8.2012. Germany Overrun by Hordes of Masked Omnivores

NOBANIS Invasive Alien fact sheets, Raccoon (PDF)

Increased geese populations occupy pastures and city lawns in Fennoscandia

Many geese populations in Fennoscandia are increasing rapidly, and geese have become more visible in human-inhabited landscapes. Currently geese utilize agricultural lands and even urban lawns. High geese brood densities have a significant impact on their environments due to increasing grazing pressure.

Greylag geese graze on pastures and hay lands, preferring short vegetation to high ones. Geese grazing also keeps vegetation short. Geese trimming a lawn in Reykjavik, the capital of Iceland.

Geese broods prefer pastures near shores

A newly published Swedish study revealed that greylag geese broods are rather picky when selecting farmland fields for grazing. The most used fields were pasturelands near water. Goslings preferred shorter vegetation, assumingly due to its higher quality and the open landscape views in case of predators. Grazing geese also keep the vegetation short.

Broods tend to prefer grazing areas near shores, from where they can easily reach the safety of water when threatened. Grazing geese broods are suggested to pose a fairly small risk of agricultural conflicts due to their preference for near-shore pastures (instead of crop fields for example). However, extremely high grazing pressure by geese can reduce plant biomass, thus affecting livestock grazing. In arctic areas, such as Greenland and Svalbard, geese grazing is observed to be the reason for decreased hay and decreased seed counts in soil.

In contrast to broods that prefer near-shore areas, non-breeding geese can cause conflicts with agriculture, due to their grazing in crop fields. Non-breeding birds that are able to fly can utilize areas further from water, and according to a Swedish study, they also graze also on crop and vegetable fields in addition to pastures. Large flocks preferred typically open and flat with no or few trees or shrubs.

The two differing patterns shown by broods and adults means that geese managers should consider the two behavioural strategies when planning geese management.

Barnacle geese grazing among Helsinki University research cattle. Breeding geese flocks have e.g. destroyed some the university’s research fields and caused high expenses.

City geese have found Helsinki’s shore lawns

The barnacle goose is a fairly new species in Helsinki. The species tends to breed in remote arctic areas, but after geese were released from the local zoo in the late 1980s, geese began breeding on the islands and islets of the Helsinki archipelago. The released geese are assumed to have returned to breed, and brought their offspring and other geese with them. Since then the goose population has been growing and occupying shore areas from the islands and mainland. Grazing geese are nowadays a visible element in the city of Helsinki, competing over space with citizens.

Geese densities are rather high on Helsinki shore lawns, where non-flying broods gather to graze. In August juvenile birds can move further from the shoreline to feed. The best seashore lawns tempt dozens of broods. In urban areas lawns are usually a nice buffet table for the geese: they typically prefer plant species used in lawns, and mowing sustains fresh vegetation. Compared to natural lawns, urban lawns can be better for broods.

This geese enclosure has very limited plant diversity, but Potentilla species not preferred by geese are flourishing.


However, geese grazing is affecting plant diversity by decreasing it. Few plant species tend to dominate in the grazed areas, while  the diversity and coverage of species is more balanced in areas with no geese grazing. Good quality lawns benefit broods, because they don’t need to move long distances while grazing. Geese population growth in the Helsinki area has been refracting after reaching ca. 1300 breeding pairs, and one reason is thought to be the limitation of good feeding habitats for broods. Geese already use almost all possible lawns in Helsinki. During dry summers with poor lawn growth geese may be greatly food-limited, which is reflected in the population size. Thus it seems that the barnacle goose population in Helsinki has reached its carrying capacity.

In the Helsinki archipelago barnacle geese nest commonly on rocky islands and islets, where food availability is highly limited. Well-managed city lawns are thus tempting for the broods.

Methods for preventing geese grazing were measured in Helsinki. One possibility is to use plant species that geese don’t prefer, instead of the current species mix that seems to be especially tempting for geese. Another possibility is to fence off areas were geese are not welcome. Goslings cannot fly, and thus cannot reach fenced areas, and they also avoid areas where they have limited visual contact to water.


Read more:

Olsson et al. 2017: Field preference of Greylag geese Anser anser during the breeding season. European Journal of Wildlife Research

Barnacle goose population declined in the Helsinki Metropolitan Area. 2016.

Barnacle goose population remains unchanged despite a good breeding year. 2013.

Niemi et al. 2007: Valkoposkihanhi pääkaupunkiseudulla. Suomen Ympäristö.

Valkoposkihanhien seuranta. 2016.

Four reasons why beaver wetlands are paradise for pin lichens

Beaver activity enhances the occurrence and diversity of pin lichens (Caliciales). Both the number of species and individuals is much higher in beaver-created wetlands than in other types of boreal forest landscapes. There are four reasons behind this:

1. High amounts of deadwood. Pin lichens grow on both living trees and deadwood. Decorticated deadwood in particular is preferred by pin lichens. Beaver-induced flooding kills trees in the riparian zone and produces high amounts of decorticated snags.

Pin lichen on decorticated stump. © Mia Vehkaoja

2. Diversity of deadwood types. Beaver activity produces snags, logs and stumps. Snags are created by the flood, whereas logs and stumps are also produced by beaver gnawing. The diversity of deadwood tree species is also wide, containing both deciduous and coniferous tree species. The diversity of deadwood types maintains a high diversity of pin lichen species.

3. High humidity conditions. High humidity conditions are favorable for many pin lichen species. Old-growth forests are usually the only places in the boreal forest belt that contain high humidity conditions. There the shading of trees creates a beneficial microclimate for pin lichens. Lighting, on the other hand, becomes a limiting factor for pin lichens in old-growth forests. Most snags in beaver wetlands stand in water, where steady and continuously humid conditions are maintained on the deadwood surface.

Snags produced by a beaver flood in Evo (southern Finland). © Mia Vehkaoja

4. Sufficient lighting conditions. Because most of the deadwood in beaver wetlands stands in water, it is concurrently in a very open and sunny environment. Many boreal pin lichens are believed to be cheimophotophytic (cheimoon=winter), meaning that they are able to maintain photosynthesis also during winter at very low temperatures. The algae member of pin lichens requires enough light for photosynthesis. Open beaver wetlands make photosynthesis possible for pin lichens during both summer and winter. Snow also enhances light availability during winter.

More information: Vehkaoja, M., Nummi, P., Rikkinen, J. 2016: Beavers promote calicioid diversity in boreal forest landscapes. Biodiversity and Conservation. 26 (3): 579-591.

It walks and quacks like a mallard, but does it look like one?

This is a mallard (Anas platyrhynchos). It is your basic duck, familiar from park wetlands. A mallard quack is also the classic duck sound.

Wetland ecology group_University of Helsinki_duck_mallard

A wintering mallard flock is quite colourful: males have green heads with yellow beaks and both sexes have blue wing spots. A wintering mallard flock is quite colourful: males have green heads with yellow beaks and both sexes have blue wing spots.

Age and season affect plumages

But mallards do not always look like those in the picture above. Males do not always have green heads, nor are females always brownish grey. Depending on the season, and the age and genes of an individual, mallards can look a little different. Downy ducklings resemble the ducklings of all other dabbling duck species. However, they rapidly develop species-specific characters, and young drakes for example develop a hint of green on their head even before all the down has disappeared. In the summertime males briefly change into summer (eclipse) plumage that looks like female plumage. Except that a male beak is still yellow.

Wetland ecology group_University of Helsinki_duck_mallard male_sinisorsa

A young male mallard still has down on his back, while some green is already glittering on his head. Both female and male mallards are brown during summer and autumn. The yellow beak reveals that this individual is a male. © Sari Holopainen A young male mallard still has down on his back, while some green is already glittering on his head. Both female and male mallards are brown during summer and autumn. The yellow beak reveals that this individual is a male.

Beak reveals sex

In addition to normal changes in plumages caused by seasons or growth, weird looking mallards can also be found. Their plumages might be different due to changes in their genes or hormones.

Wetland ecology group_University of Helsinki_duck_mallard female_sinisorsa

Light female mallard.

Various phenotypes are rather typical among animal species. These variations are common in mallards, and peculiar individuals can be found especially in cities. For example, females might be light due to mutations. Mutations can work in several ways causing changes in pigment production or in its appearance traits. Lightly coloured mallards produce pigments, but their colour appearance has changed. If an individual does not produce melanin pigments at all, it becomes a completely white albino.

Colour variations are thought to be typical in mallards in city environments, where predator pressure is lower and thus exceptional individuals survive better. On the other hand, mallard farming has potentially produced weird-looking individuals that have escaped and spread their genes to natural populations.

Wetland ecology group_University of Helsinki_duck_mallard_intersexual

Wetland ecology group_University of Helsinki_duck_mallard_intersexual_male_female

These peculiar mallard males in wintering flocks are actually females. The pictures show intersexual females together with two normal males and a female. Moulting males changing their eclipse plumage into nuptial plumage can look similar, but their beak colour once again reveals the actual sex. These pictures were also taken in the middle of winter, when males have already changed to their nuptial plumage.

The beak has an important role in identifying mallard sexes because males have yellow beaks and females have orange-spotted beaks around the year. The beak can also reveal intersexual females. They are individuals that express both female and male outfit. This can be caused by disturbances in female hormone production, or then an individual has both female and male features. Hormones regulate the outfit, and if large quantities of testosterone are produced, male plumage may result. Beak colouration is not as sensitive to hormonal changes and even though a female displays male characteristics, it will still have a female beak.

Hybrid ducks

Wetland ecology group_University of Helsinki_duck_mallard_teal_sinisorsa_tavi

This common teal x mallard hybrid male was coupled with a normal mallard female and defended it against clearly larger mallard males.

Mallard flocks may also have hybrid individuals. Duck species are close relatives, and can thus mix rather easily. Various species mixes are known, for example mallards can mix with common teals, Eurasian wigeons, northern pintails and black ducks. However, hybrids are quite rare, because each duck species have specific behaviours and characteristics that prevent hybridization. But sometimes these barriers collapse, and hybrid individuals are born. Hybrid individuals express characteristics from both original species. Their habits and characteristics typically do not interest individuals from the original species and therefore might not breed successfully.

Hybridization can cause several problems, which in the worst-case scenario can lead to the extinction of the original species. The hybridizations of mallard and black ducks in North America is becoming more common after shifts in their distribution. Hybridization is now threating black duck populations. Alien mallards can also cause a serious risk for endemic duck species and to their gene pool. For example, the Hawaiian duck (Anas wyvilliana) is unfortunately going extinct because of non-native mallards. Survival of the species now depends on protection actions that target the extirpation of all mallards and hybrids from the islands

Wetland ecology group_University of Helsinki_duck_mallard_intersexual_male_female_sinisorsa

Four naturally different mallards wintering in southern Finland. The normal type male was coupled with a normal female. An intersexual and a light female are in the upper part of the picture.

It looks like a duck

This white domestic duck is a descendant of a mallard. © Sari Holopainen

This white domestic duck is a descendant of a mallard. © Sari Holopainen

Mallards are commonly farmed, and several different colour variations exist among the domestic breeds. A white duck known by everyone is also a mallard variant. Farmed mallards have sometimes escaped, and now breed with natural mallards. Extraordinary ducks, resembling mallards more or less, are a fairly common sight in Southern and Central European parks. Alien genes in the natural mallard population become more rare in the northern parts of Europe.

Extraordinary ducks in European parks are probably related to mallards: Switzerland, Germany and Sweden. © Sari Holopainen

Extraordinary ducks in European parks are probably related to mallards: Switzerland, Germany and Sweden. © Sari Holopainen

Read more:

Pär Söderquist: Large-Scale Releases of Native Species: the Mallard as a Predictive Model System

Pictures by Harry J. Lehto, intersexual mallards

Pictures by Pekka Sarvela, colour variations

Ducks Unlimited: Waterfowl Hybrids

Asymmetrical competition in boreal lakes

Fish inhabit boreal lakes throughout the year and aquatic invertebrates living in the same lakes belong on the menu of several fish species. Ducks also utilize these same invertebrates for half the year. Even ducks that usually eat plants consume invertebrates; especially females preparing to lay eggs and small ducklings need protein in their diet. Boreal lakes are typically barren and invertebrate-poor. A newly published review article emphasizes the need to carefully deliberate when considering the introduction of fish in wetlands where they do not originally belong or that are established specifically for ducks.

Fish modify the structure of aquatic invertebrate communities, and thus the structure, abundance and diversity of invertebrate communities differ between lakes with and without fish. Fish predate especially large invertebrates that typically are the top predators of invertebrate communities. Therefore the community structure is skewed towards smaller species in lakes with fish.

Invertebrates living among vegetation are better protected from fish predation and thus predation is higher in open water. This means that fish compete especially with those duck species that forage in the open water (e.g. teal), while species foraging among vegetation are less affected (e.g. mallard).

The review article clearly showed that the food competition caused by fish is harmful for breeding ducks. However, the situation is not always clear, because fish and duck abundances are typically limited by the same environmental key factors such as lake productivity. Thus both ducks and fish can be abundant in lakes rich in invertebrate and vegetative food material. But this competitive set-up is emphasized in barren lakes.

Common goldeneye broods prefer fishless lakes. © Sari Holopainen

Common goldeneye broods prefer fishless lakes. © Sari Holopainen

Researchers in Finland introduced fish to lakes that had become fishless due to acidification. Monitoring showed that the lake use by common goldeneye broods declined after these introductions. Pairs on the other hand continued using the lakes as before. The difference is suggested to be caused by the foraging manners of ducks at different life stages. Adult ducks find their food from the lake benthos, while ducklings and fish concentrate on competing over nektonic invertebrates. Competition theory is also supported by other studies: ducklings need to use more time foraging in lakes with fish, and still seem to grow slower than in fishless lakes.

The fish experiment was performed in the opposite way in Sweden, where fish were eradicated from certain lakes. Researchers found that all invertebrate groups became more abundant and goldeneye brood numbers increased.

In Finland lakes have been recovering from acidification and this has positively reflected to fish populations. The recovery of fish might affect the breeding success of ducks in boreal lakes, especially concerning breeding goldeneyes.

The competition between fish and ducks is asymmetric in the sense that fish will affect ducks, but ducks do not affect fish. Fish are present in the lakes year-round and if times are thin, the fish just grow slower. They also affect the invertebrate populations of lakes. Ducklings will also grow slower in bad times, but their mortality increases rapidly if food becomes scarce. The effect of ducks on invertebrates is also milder. Thus fish should not be introduced to wetlands established especially for ducks.

Read more:

Nummi, P., Väänänen, V.-M., Holopainen S. & Pöysä H. 2016. Duck–fish competition in boreal lakes – a review. – Ornis Fennica 93: 67-76.

Gunnarsson, G., Elmberg, J., Sjöberg, K., Pöysä, H. & Nummi, P. 2006. Experimental evidence for density-dependent survival in mallard (Anas platychynchos) ducklings. — Oecologia 149: 203-213

Nummi, P., Väänänen, V-M., Rask, M., Nyberg, K. & Taskinen, K. 2012.  Competitive effects of fish in structurally simple habitats: perch, invertebrates, and goldeneye in small boreal lakes. — Aquatic Sciences 74: 343-350.