Thread by @tomdiserens

The ecological impact of wolf reintroduction to Britain. A wolf ecologist’s view🧵.

Wolf reintroduction to Britain is again a hot topic in the media. So I want to share what we know about the wolf’s ecological impacts in a model European system, Białowieża Primeval Forest.

The knowledge learned during our research may help you decide whether the benefits of having wolves around outweigh their costs.

In this thread you will find out about the following:

How large carnivores suppress herbivore abundances in Białowieża Primeval Forest.

How the presence of wolves modifies herbivore behaviour.

How wolves affect forest regeneration.

Just for the record, I’m based in Białowieża and recently submitted my PhD thesis on wolf ecology. My colleagues at @mriBialowieza have been studying wolves for three decades and have perhaps published more original research articles on wolf ecology than any other team in Europe.

You can find my publications here scholar.google.com/citations?user…

Białowieża Primeval Forest, as the best-preserved lowland woodland in temperate Europe, serves as a reference system for temperate forests elsewhere in Europe. Studies here can help reveal what ecological interactions may manifest elsewhere as species recolonise/are reintroduced.

If you want to know more about why this forest is special, see this paper: mdpi.com/1999-4907/10/1…

A caveat: Large carnivore impacts are context dependent. Impacts that manifest in one area may not appear in another.

However, the knowledge from Białowieża Forest does give insight into what you can expect should wolves be reintroduced to the Scottish Highlands.

Let’s start with some basic information. How do wolves shape ecosystems?

They do so by impacting herbivores (like deer) and smaller carnivores (like the fox, badger) through two pathways:

1) By killing prey, they lower prey abundances.

2) By instilling fear in prey, wolves influence prey behaviour.
As prey attempt to reduce their probability of being killed or disturbed, they can alter, e.g., their space-use, activity times or vigilance.

But wolves not only affect their prey; they can also affect the species their prey eat. This is what ecologists term a trophic cascade.

For instance, trees can be affected as a consequence of wolves changing the abundances and behaviour of deer.

In Białowieża Forest we have found evidence for both direct and indirect impacts of wolves.

We have also found that these effects appear to cascade down the food chain to plants, which regenerate better in areas of high wolf activity.

Read on for more detail.

Wolf effect 1) In Białowieża Forest, wolves reduce the population sizes of herbivores.

Long term monitoring of both ungulate and herbivore abundances suggests when wolf densities are high, herbivore populations are lowered by 10%.

This was a correlative study, and so it is difficult to prove cause and effect. But it’s the best data we’ve got.

Reference: If people want to see this data I can find it next time I’m at work. It's paper only.

Implications: It is reasonable to expect that ecologically functional densities of wolves elsewhere in Europe will have similar suppressive effects on herbivore population abundances.

Wolf effect 2) In Białowieża Forest, red deer avoid areas of high wolf activity (but other herbivores do not)

My colleagues looked into whether different herbivore species avoid areas of high wolf activity.

The attached maps show the space use patterns of wolves and red deer; shaded red are areas of high activity of the given speciesd, shaded yellow are areas of low activity. Notice how red deer and wolf space use patterns are the opposite of each other.

The spatial distributions of moose, bison, roe deer or wild boar were unrelated to wolf space use patterns. This is likely because wolves predate these species relatively infrequently and so they have no need to avoid wolves.

In Białowieża Forest, the redistribution of red deer means vegetation in different parts of the Białowieża Forest landscape is subject to different levels of herbivore pressure.

Consequently, browsing intolerant plant species may grow better in areas of high wolf activity. Indeed, my colleagues found the proportion of a common browsing-tolerant tree increased and the proportion of a browsing-intolerant tree decreased with increased herbivory pressure.

Implications: In the Scottish Highlands, wolves may likewise alter deer spatial distributions, herbivore browsing regimes and therefore forest regeneration.

Reference: elifesciences.org/articles/44937

Wolf effect 3) Wolves affect the time red deer spend vigilant.

My colleagues tested whether red deer and wild boar increase their vigilance when they smell a wolf scat nearby. Vigilance is where the animal has its head raised up looking for threats, indicating how scared it is.

My colleagues found red deer were far more vigilant when there was a wolf scat nearby than when there wasn’t. Whereas wild boar couldn’t have cared less.
Reference: https://t.co/PpDJHp1gYLdoi.org/10.1371/journa…

Implications: The more an animal has to be vigilant, the less time it can spend feeding. Wolf reintroduction to Sctoland should lower red deer fitness and put a downard pressure on red deer numbers. It may also reduce red deer foraging in areas with a high risk of wolf predation.

Wolf effect 4) Tree logs magnify the effect of wolves on deer behaviour.

In another experiment my colleagues explored whether herbivores increase their vigilance when near tree logs.

Tree logs are an escape impediment for herbivores, logs pose an escape impediment to herbivores, which may therefore avoid the vicinity of such barriers to reduce their chances of being killed by wolves

Red deer avoided coming close to tree logs and were more vigilant when logs were present nearby. The strength of these effects depended on how intensively wolves used the local area; deer perceived tree logs as more risky when wolves were more often present in a given area.

Reference:

Implications: Tree logs in areas of high wolf activity create fine-scale patches of fear with reduced deer browsing pressure, thereby enhancing chances for successful tree recruitment.doi.org/10.1093/beheco…

Next in the list of effects: Wolves improve forest regeneration

As some deer are killed, and others change their behaviour to avoid foraging in risky places, trees can be relieved from browsing pressure. Read on for details.

Wolf effect 5) Herbivore browsing on trees is lower in areas of high wolf activity

My colleagues investigated whether herbivore browsing was lower in areas intensively used by wolves. They found tree browsing intensity was 8% lower in wolf core areas compared to outside them.

Herbivore browsing was even lower when tree logs were nearby

The same study also looked at whether the presence of tree logs modifies the intensity herbivores browse trees. In areas of high wolf activity, when tree logs were nearby, browsing was 20% lower.

This shows that a higher presence of wolves is not the sole factor determining the wolf’s effect on forest regeneration: fine-scale risk factors such as tree logs also play an important role.

In areas of low wolf activity, browsing was only reduced by 7% in the vicinity of tree logs.

Reference: doi.org/10.1111/j.1600…

Implications: I think it is reasonable to predict that if wolves are reintroduced to Scotland, the proportion of trees growing out of herbivore control will be higher where wolf activity is high and where escape impediments like logs are present.

Wolf effect 6) The presence of fallen trees increases the positive impact of wolves on forest regeneration

Another paper explored the effect of tree logs in more detail.

The study found, in low-risk areas, tree logs reduced browsing intensity only in small patches (approx. 4–6 m from logs), whereas in high-risk areas browsing intensity was reduced up to at least 16 m from tree logs.

Moreover, the magnitude of these effects varied depending on the level of wolf predation risk, with browsing intensity being most reduced around tree logs in areas with high wolf risk (up to 37%) and the smallest in low-risk areas (< 20%).

Reference: doi.org/10.1007/s10021…

Implications: These studies suggest that to maximise the wolf’s ecological role we should combine wolf presence with a diverse landscape structure that contains escape impediments. Managed plantation forests often lack such habitat features.

Wolf effect 7) Wolf activity level alters the diet of red deer (the wolf’s main prey) but not bison (rarely preyed upon)

My colleagues found that red deer in areas of wolf high activity ate more deciduous tree species and less forbs than in low wolf-use areas.

CAVEAT: These effects of the wolf on the red deer diet were only observed in Białowieża National Park (strictly protected old-growth forest). They did not occur in the managed forest, where occur extensive tracts of monoculture plantations and hunting.

Reference: doi.org/10.1016/j.fore…

Implications: The paper doesn’t mention what effect this may have on plant regeneration. But we could speculate that landscape level variation in deer food preferences will cause some plants to regenerate better where wolf activity is high.

As the effect couldn’t be seen in more managed forest, wolves may play a limited role in shaping the deer diet in areas with high human impact.

Summary of wolf ecosystem impacts in Białowieża Primeval Forest:
· Herbivore abundances are reduced by 10% when large carnivore abundances are high.
· Red deer, particularly females, avoid using parts of the landscape where wolf activity is high.

· Other herbivore species used the landscape irrespective of wolf activity.
· Herbivore browsing is reduced by 8% in areas of high wolf activity
· Herbivore browsing is reduced by 20% in the vicinity of tree logs in areas of high wolf activity.

· Areas with high wolf activity and tree logs seem to have more trees growing above the browsing height of deer, indicating improved forest regeneration.

· The redistribution of red deer may lead to distinct herbivory regimes, allowing browsing intolerant tree species to grow better in high wolf risk areas

What are the implications of our research for other parts of Europe? That’s a tough question to answer.

Remember, the wolf’s impacts observed in one study system may not appear in another.

European landscapes differ from each other immensely: the vast open landscapes in the Scottish Highlands with huge herbivore densities are very different from the relatively natural closed-canopy forests of Białowieża.

The wolf’s impacts on herbivores or forest regeneration may be overriden or modified by this differing context.

To illustrate, deer in Scotland may behave differently to those in Białowieża Forest. In Scotland deer are often famished due to intense competition for food. They may therefore have no choice but to forage in areas of high wolf activity, thus dampening the effects wolves have on the behaviour of herbivores. Will wolves shape forest regeneration in such a context? I can’t say.

In conclusion, the recovery of Europe’s large carnivores has raised hopes that restored ecosystem structure and function will help mitigate the problems large herbivores cause.

But context dependency makes me hesitant to make any specific predictions as to the effects wolves could have in Scotland or elsewhere.

Wolves will kill some deer, thus lowering their abundances to an extent. Wolves will also scare deer, changing their behaviour. But the magnitude and nature of these effects is difficult to predict with any accuracy.

In another thread I will document what we know about how wolves shape the distributions and abundances of their smaller carnivore competitions (the topic of my PhD).

If there is interest, perhaps I will write further threads on what we know about the threat wolves pose to livestock and humans, and the ways we can mitigate those threats.

I could also write a thread on how bears shape ecosystems.

Let me know if you’re interested in these topics.

If you enjoyed this thread, please scroll back to the top and retweet the initial post. You can also find me on Instagram where I regularly post photos from Europe’s last primeval forest instagram.com/tomdiserens/

If you enjoyed this thread, please scroll back to the top and retweet the initial post. You can also find me on Instagram where I regularly post photos from Europe’s last primeval forest instagram.com/tomdiserens/