Are grey squirrels really bad for the overall habitat in Germany?
As far as I know the grey squirrel introduced in Germany by humans. It starts to slowly oust the common brown squirrel.
Of course I feel bad for the brown squirrel but people around here act like it's the end of the world. They say that it somehow matters for the overall habitat/natural environment. Does the growing number of grey squirrels and the according shrinking number of brown squirrels really effect the complete natural balance?
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Yes, they do damage habitats, namely from bark stripping, and disease. This is why the UK class them as a pest. However it is not limited to grey squirrels as red squirrels also strip bark, but they have much smaller numbers proportionally so their damage is proportional. Lack of grey squirrels therefore does not wholly prevent this damage.
In addition to displacing this native species they frequently cause damage to woodlands by stripping bark from the main stem and branches of trees
This has an effect commercially and natively. This can lead to a loss in food for red squirrels, birds, dormice etc.
Such damage acts as a major disincentive to the planting of broadleaved and coniferous trees for timber as it reduces the value of the final crop. Increasingly wider impacts are being recognised as potentially of major significance to woodland conservation, biodiversity and sustainability. In time damage may lead to a loss of particularly vulnerable species (e.g. beech) within the mature canopy of woodlands and this may be accompanied by loss of associated fungal and invertebrate fauna and their predators. In addition, there may be indirect competition, e.g. for food, between grey squirrels and native fauna such as the red squirrel and common dormouse; or predation by grey squirrels, e.g. on woodland bird populations. Grey squirrels also carry squirrelpox virus, an infection fatal to red squirrels.
The UK has been controlling the population of grey squirrels for many years, and we have some grey free areas, such as Brownsea Island.
Source: The Forestry commissions 2007's grey squirrel PDF
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Excellent answers already; one point no-one has covered is that greys will occasionally eat eggs / chicks of small birds. Wikipedia says there is a lack of evidence on the actual impact of that, but it is something that does have a wider effect and that reds very, very rarely do.
However, there is, in the UK at least, also a reverse viewpoint. The introduced grey squirrels are greater generalists than the reds. Red squirrels thus suffer far more from habitat destruction and climate change than greys.
You often see greys bouncing along the pavements in my town, a city of 250,000 - they can live in parks or gardens and will coexist with humans and eat a wide range of foods; whereas reds are limited to conifer woods.
So another way of looking at it is that humans have depleted the red squirrel population by destruction of their specific habitat requirements, and the greys have filled the gap. The greys get a bad rap as invasive vermin, but we should really be looking at what we did that allowed them to become so dominant.
Britain’s surviving red squirrels feel the pressure (New Scientist)
However, reds do have some natural advantages; they have greater agility, enabling them to avoid pine martens, a major predator.
In neighbouring Ireland, the population of the less nimble greys crashed when pine martens became more abundant, while the reds were unaffected
Pine marten in search of a snack takes a selfie (New Scientist)
In answer to your actual question, in my view there is no straightforward answer. Liam's statement of "any (unnatural/man made) change in an ecosystem should be resisted" is a classic conservation viewpoint, but there are other ones, and once a change such as an introduction of an non-native species has occurred then any attempt to resist risks being an even more dramatic man-made intervention. Also, a species as successful as the greys will spread widely itself - at what point does their population growth into other areas become natural rather than introduced?
And of course, reintroduction attempts such as the UK beaver reintroduction blur the line - a reintroduction is clearly an unnatural change. Should we accept that a snapshot in time when beavers were a native species has gone, and conserve the current moment, or should we try to return to and conserve the past?
Grey squirrels have been in Britain since 1876; when do we start considering them just a highly successful endemic species?
DISCLAIMER: I am not an ecologist; I just have a fascination with ecology and have developed a recent interest in grey/red squirrels in the UK :o)
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They say that it somehow matters for the overall habitat/natural environment.
Just to tack onto Aravona's excellent answer:
isn't losing a rare species to a very common species a bad thing in its own right?
Un-natural tampering in the diversity in an ecosystem has always been proven to be a bad thing (see examples below from Australia). Greys will effect the ecosystem in many different ways when compared to the smaller Red. Any (unnatural/man made) change in an ecosystem should be resisted.
On a wider context (i.e. not just squirrels). Introduction of foreign species to an ecosystem can be totally devastating, see rabbits or cane toads in Australia. The introduction of grays has had relatively minor consequences when compared to these issues.
So again, history teaches us that introduction of foreign species can have wide ranging and unforseen problems, so all introductions by man should be prevented. whether they have minor consequences or wide ranging and dangerous ones.
BTW this isn't natural selection, man has taken a species with natural checks and measures in it's natural habitat and transplanted it thousands of miles away into an environment without these checks. Likely (eventually) nature will catch up but humans influence these processes far too fast and it takes time for nature to re-balance.
As a side note to this, humans now influence their envrionment so much that it's thought we may now no longer be continuing to evolve! So humans and human consequences are well outside of natural processes.
Why are Grey squirels bad for the red?
The main issues with Grey Squirrels when introduced into a Red Squirrel population are twofold:
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Squirrel pox
The Grey carries a disease (Squirrel pox) that the Greys have a genetic immunity to but the Reds don't. So if a population of Greys comes into contact with a population of Reds the pox is often spread killing the Reds but leaving the Greys unaffected.
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Competition
Basic natural selection, the Greys are bigger, stronger, reproduce faster and eat more than the Reds. So the Reds lose out here, lowering their numbers even further.
The UK is an excellent case study in this. Reds used to be the only squirrel in the UK prior to the introduction of the Grey. They are now critically endangered with a population of only 140,000 (compared to 2,500,000 Greys) left. The future is increasingly bleak for the Reds in the UK with some major holdouts losing large numbers of individuals despite fierce protection and a shoot on sight policy for Greys.
For example the small population on the Sefton coast near me, lost nearly 85% of its population recently due to an outbreak of Squirrel pox. This is starting to bounce back but it's tough to bounce back from losing that number of individuals in short period of time.
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