Out now: a new Atlas of Dutch Mammals that charts the distribution, behaviour, ecology and population trends of more than one hundred species. There's been no shortage of developments in the twenty years since the last atlas, says one of the editors, NIOO-researcher Kamiel Spoelstra.


De nieuwe Zoogdieratlas

Kamiel Spoelstra mainly edited, organised and wrote material for the chapter in the atlas about bats in the Netherlands. Two of the other chapters are about marine mammals and predators. 

Improved techniques

About these three groups in particular, says Spoelstra, a wealth of new information is available now that just wasn't there yet twenty years ago. 

"The tools and methods we have for observing mammals have improved vastly. We now use camera traps, for instance, for species that would otherwise be very hard to observe. It's a technique that has only been in general use for the past six or seven years."

More sensitive bat detectors have greatly expanded our knowledge about the distribution of bats. And radio telemetry makes it easier to locate their dwelling places. "You capture a bat in the forest at night, fit it with a radio-tag and the next day all you have to do is walk around."

Strong comeback

But it's not just the tools and methods that are different now. The distribution patterns of many species of mammals have also changed so dramatically that an update of the atlas seemed long overdue.

Although it is difficult to draw conclusions about these changes beyond the wellbeing of individual species, it at least seems clear that there hasn't been an overall decline in mammal biodiversity in the Netherlands.

Some species are still at risk of disappearing - including the hamster. But that's not a new trend. A number of other species, on the other hand, have been making a surprisingly strong comeback.

Reintroduction programmes

These include several species of bats - including Geoffroy's bat - that had long been in decline. Recovery has in some cases been "nothing short of spectacular", says Spoelstra. Although there isn't always a clear explanation.

The success of species that have been reintroduced into former habitats is of course easier to explain, although not all reintroduction programmes have been equally successful. Otters are still at risk, while beavers are now thriving again after having been absent from the Netherlands for more than 150 years.

"There have been many policies in the past twenty years aimed specifically at conservation and protection", says Spoelstra. Bats have also benefited. "Winter quarters have been constructed especially for them, and in some cases where bat colonies were discovered inside buildings, those buildings were actually bought by the authorities."

Newcomers & returnees

Another interesting trend in the new Dutch Mammal Atlas is the arrival of a number of relatively new and unusual species. These include rare species of whales that entered Dutch waters as vagrants and were sighted just once. "Our only criterion", says Spoelstra, "was that they had to reach the Netherlands under their own steam, as it were."

That means an incident where a greater noctule - Europe's largest bat - was discovered at a flower auction in the province of North Holland probably doesn't count. But in other cases, the return of species long absent from the Netherlands, or the discovery of animals in unexpected locations, may indeed say something significant.

One example would be the much publicized return of the wolf to the Netherlands. Another the recent discovery of a seal in the centre of Utrecht, which it entered by swimming up one of the city's canals. That may have been a one-off, says Spoelstra, but "there have been more examples during the atlas period of seals entering bodies of fresh water." 

Climate change

Such changes in distribution are by definition difficult to interpret: "there are too many factors we just don't know." And while climate change offers a possible explanation for at least some changes - such as the increasingly northern locations where some bat species are now found - Spoelstra warns against drawing broad conclusions.

"I don't think a reliabe connection between changing distribution patterns and climate change has been established anywhere yet." Even the bats that seem to be heading north, says Spoelstra, are in fact only returning to areas that used to be their habitats before they began to decline.

But even without any clear-cut explanations, the mammal atlas will no doubt be a treasure trove for ecologists."The distribution of species", says Spoelstra, "is core ecological information. When I analyze bat data, knowing what species are found in a particular area will at least let me eliminate some of the more exotic options the software may suggest."
 

  • Atlas van de Nederlandse Zoogdieren (Atlas of Mammals in the Netherlands), editors: Sim Broekhuizen, Kamiel Spoelstra, Johan B.M. Thissen, Kees J. Canters, Jan C. Buys. KNNV Uitgeverij 2016