Seasonal Update - April 2019

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From summer to autumn is probably the best description of this year’s transition into spring. Following the record-breaking warm temperatures that saw out February, storm “Freya” brought heavy rain and gale force winds to much of the UK on the first Sunday of last month, albeit with temperatures still a few Celsius above average for the time of year. Freya was followed in rapid succession by “Gareth” and “Hannah”, the latter of which brought gales to the whole of the UK along with a month’s-worth of rain in only 24 hours for parts of Wales, leading to widespread flooding.

The unusually windy weather that plagued Britain for two weeks almost without surcease was a result of the jet stream being lower than normal in the North Atlantic, bringing low pressure systems that would normally have skirted further north at this time of year. Fortunately, this pattern changed during the second half of March, for most of England, Wales and Ireland, at least. High pressure started to build in from the Atlantic on the middle Sunday, resulting in more settled and milder conditions that lasted for the remainder of the month.

Elsewhere, Mozambique, Zimbabwe and Malawi were devastated by cyclone “Idai” last month, which brought 106mph (177kmph) winds and heavy rain that resulted in the death of at least a thousand people. This natural disaster followed months of drought in the region that means agriculture is no longer viable for many in Mozambique. Some scientists see increasingly frequent extreme weather such as that in South Africa and Australia as the tipping point where humans are forced to tackle climate change. Indeed, the United Nations have recently announced, in no small part in response to continued walkouts by school children across the world angry at the lack of action on climate change, they will hold a Climate Summit later this year, describing 2019 as “a critical year, the “last chance” for the international community to take effective action on climate change”.

Children protesting about government inaction on climate change in London during February 2019. Similar rallies were held across the world in March. - Credit: David Holt (CC BY 2.0)

Individually, it may seem like we can only have an infinitesimal impact on the environment, but it’s important not to confuse “small” with “pointless”. Lots of small changes, when made by lots of individuals, can have a huge impact. Recycling is a good place to start, and most of this can be done at home with limited impact on our daily lives. One problem with recycling is that there’s significant variation in what different councils recycle, particularly when it comes to plastics. You can find out what your local council recycles via the Recycle Now website and, if you’re keen to get involved, DEFRA are asking people to take part in their recycling consultation, which runs until mid-May. For some helpful tips on reducing your environmental impact, check out the Instructables website and the WWF habits blog. If you’re fortunate enough to have a garden, have a look at the Wildlife Trusts Gardening for Wildlife website and the RHS’s Encouraging wildlife to your garden blog.

As usual, the Wildlife Trusts have a series of nature-themed events up and down the country, as do the RSPB. If you feel like being proactive this month and live near the coast, Surfers Against Sewage is looking for people to organise cleans of their local beaches – details here. The Forestry Commission is running a series of events this month (full list here).

Interested in the wildlife to be found this month? Check out my Wildlife Watching - April page.

Discoveries of the Month

Habitat heterogeneity crucial to helping water voles evade mink

When was the last time you saw a water vole? A friend recently asked me this question and it was with more than a little regret that I answered: nearly three years ago. More disheartening yet is that this lack of sightings is not for want of trying, and local sites that were once reliable for these riparian rodents now seem bereft - an increasingly common observation in Britain. Indeed, while we believe that water voles (Arvicola amphibius) were among the most common mammals in the country during the Roman occupation, they are now increasingly rare, with data from the Environment Agency and Wildlife Trusts suggesting numbers have fallen by some 90% since the 1970s. There appear to be two main drivers behind this decline.

A water vole (Arvicola amphibius) feeding among the reeds. These small mammals are highly susceptible to predation by the introduced mink. - Credit: Marc Baldwin

The first issue facing voles is one of habitat degradation, by which we mean both pollution and habitat fragmentation, which means there’s simply less suitable water vole habitat than there used to be. The second, and perhaps more pervasive, threat is that of the American mink (Neovison vison); a small carnivore released into Britain’s waterways during the 1930s. Mink are a particular problem because they’re sleek enough to pursue voles into their burrows, which larger (native) predators such as otters cannot.

Despite often being mentioned independently, it should not be assumed that mink predation and habitat loss are mutually exclusive. In 1998, Guillermo Barreto, David Macdonald and the late Rob Strachan proposed a “tightrope hypothesis” to explain the relationship between water voles and mink. Essentially, they suggested that on many of our waterways, mink predation was exacerbated, possibly even precipitated by, habitat loss and fragmentation. In other words, how vulnerable a water vole population is to predation by mink varies depending on how the suitable habitat is configured. A subsequent study, conducted in Belarus, found that while mink suppressed local water vole populations in river habitats, wetland areas free of mink were key in sustaining regional populations despite being isolated. Now a recent study in Poland lends further support to these early theories.

Between 2011 and 2014, a team of researchers led by Marcin Brzeziński at the University of Warsaw trapped, tagged and released 275 water voles along the Mazurian Lakeland in north-east Poland. Nearly 500 mink scats were also collected throughout the year, between March 2012 and August 2013, and analysed to assess what these mustelids were eating. The researchers found that the local mink ate a varied diet, including fish, amphibians and mammals. In particular, the percentage of the diet composed of rodents varied from about 8% in the winter to nearly 64% in the summer, with water voles being the most frequently consumed rodent species in almost all seasons. Interestingly, however, despite the high prevalence of water voles in the diet of the mink, voles were still abundant on the study site, particularly in the midfield ponds. The scientists think that the complex structure of aquatic habitats in the Mazurian Lakeland, i.e. the presence of lakes, rivers, midfield and forest ponds, ditches, canals, and marshes, etc., provide isolated refuges from the mink. In their paper to Mammalian Biology, Brzeziński and his team note:

The high number of suitable habitat patches in the postglacial landscape can sustain a water vole metapopulation even if some local populations are strongly affected by mink predation or are even temporarily exterminated …”

An American mink (Neovison vison). - Credit: Marc Baldwin

In other words, habitat diversity is an important aspect of helping water vole populations escape even high levels of mink predation.

Reference: Brzeziński, M. et al. (2018). Water vole Arvicola amphibius population under the impact of the American mink Neovison vison: Are small midfield ponds safe refuges against this invasive predator? Mammal. Biol. 93: 182-188.

Fallow deer relax in when in a big group

Fallow deer (Dama dama) are among the most familiar deer species in Britain, despite not being a native cervid. They’re widely distributed in England, more patchily so in the north and Scotland, and arguably the most popular deer species in captivity. Indeed, originating from Turkey and Iran, the fallow deer was introduced to Britain by the Normans during the 11th century and is still kept widely in deer parks and farms in Britain. Those in our countryside today appear to be descendants of mediaeval introductions or escapees from parks that fell into disrepair during the World Wars.

While fallow are not considered an invasive species, they can cause damage to forestry and agriculture if numbers grow sufficiently large and they, along with all other deer species in Britain, require some degree of human management in the absence of large carnivores. This species is, however, one of the most difficult to cull, readily adapting to culling pressure and becoming elusive. New research from the Mediterranean suggests that both group size, sex and age has a big influence on the alertness and vigilance behaviour in this species, which may have management implications.

Adult male Fallow deer (Dama dama) in a bachelor group on the New Forest. New research suggests that mature bucks are more relaxed when in a herd. - Credit: Marc Baldwin

A team of researchers led by Ilaria Pecorella at the University of Turin studied the feeding and vigilance behaviour of fallow deer in a meadow of the Maremma Regional Park in central Italy between November and March in two periods; 2006-2008 and 2012-2013. The deer were watched through binoculars and scopes so as not to disturb them and different age classes and sexes were compared. This species is a social one and often found in groups of 20 or more, which presents a behavioural conflict. On the one hand, there’s “safety in numbers”, with more eyes on the lookout for threats and so any predation risk is diluted. Conversely, however, larger numbers can be a strong attraction for predators and with lots of mouths to feed, competition for food is higher than for small groups or individuals.

Pecorella and her team found that females spent less time feeding than males and adult males in particular spent the most time eating, which is not unexpected given that they’ll have lost the most weight during the autumn rut. More interestingly, however, was the researchers’ observation that, while young males were more vigilant as group size increased, adult bucks and does were less vigilant. Young bucks and females were also more alert, while adult bucks were much less so.

Young male Fallow deer are less relaxed in herds than when singly or in pairs, which appears to reflect greater intra-group aggression in this age class. - Credit: Marc Baldwin

In the park, fallow deer are prey of the only large carnivore present, wolf-dog hybrids, and in their paper to Acta Ethologica earlier this year, the researchers suggest that adult bucks, being some 60% larger than adult females, are less vulnerable to predation and can afford to be less vigilant. Females were also still accompanied by their fawns from the previous year and this may contribute to their increased alertness over the bucks. Young bucks were frequently observed being aggressive towards one another and the authors suggest that this may explain the increased vigilance in this age class – keeping an eye out for other aggressive youngsters. The researchers conclude that:

“… both antipredatory and social factors could explain sex/age differences of vigilance in fallow deer. Most likely, females tended to reduce the predation risk for themselves and offspring through a comparatively greater duration and frequency of vigilance postures than males. In males, social factors, such as competitive interactions and dominance hierarchies in large groups, may explain differences of feeding and vigilance rates among individuals of different age classes.”

In practical terms, this means does may be more difficult to stalk even outside the calf-dependency season if the youngsters are still in tow, but are generally less vigilant when in a large herd. Conversely, young males are less vigilant when on their own or in small groups, while adult bucks are most relaxed when feeding in large groups outside of the rut.

Reference: Pecorella, I. et al. (2019). Sex/age differences in foraging, vigilance and alertness in a social herbivore. Acta Ethol. 22: 1-8.

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