Part 6

The effect of introduced game species

Add the wild boar to the equation

From 2010 to 2024, roe deer harvest density remained relatively stable at approximately 7 harvested roe deer per 1,000 hectares (Fig. 2.19). During the same period, the wild boar population increased dramatically and has now become the most commonly harvested game species at the national level. At the same time, the numbers of harvested moose and mountain hares have declined. A legitimate question, therefore, is why roe deer do not exhibit a similar trend in harvest numbers.

Roe deer and wild boar do not compete to any significant extent for food resources. However, the rooting activity of wild boar, which exposes patches of bare soil, may create favorable conditions for the growth of new herbaceous vegetation. Such vegetation could improve food availability for roe deer. The recovery of vegetation and the appearance of new plant communities following soil disturbance are well-known ecological phenomena. Whether wild boar rooting actually promotes plant species preferred by roe deer remains unknown. If this is the case, it could help explain why roe deer densities have remained relatively stable over the past 14 years, while the more forest-dependent species, such as moose and mountain hares, have continued to decline. It is also worth noting that the National Forest Inventory (NFI) has reported a positive trend in herbaceous vegetation since 2017.

Roe deer and wild boar appear to share similar environments, where arable and pasture land are mixed with forest habitat. Local wild boar populations are often provided with supplemental forage at baiting sites to facilitate harvesting, reduce crop damage, and keep them in place. Figure 2.18 shows a 3D plot representing the intensity of wild boar rooting. Peak height reflects the amount of rooted patches. These patches coincide with the locations of feeding stations in environments preferred by both species.

Figure 2.18 The 3D plot shows the distribution of rooted ground surface in Björkvik, Södermanland. Peak altitudes indicate the frequency of observed perturbed ground. These peaks coincide with the vicinity of feeding stations. Wild boar are mainly active during dark hours, while roe deer are active during daylight hours. Rooted soil opens for opportunistic herbaceous vegetation, which likely improves roe deer forage conditions.

Concerning climate constraints, wild boar are somewhat like the roe deer—they do not thrive in harsh winter climates with frozen ground—so the northward distribution of boars more or less coincides with southern Dalarna and Gävleborg. The dip in L₅₀ towards its minimum may be due to a rapid response in southern counties, especially along the Baltic coast of southern Sweden, where wild boar populations established early strongholds.

Add fallow deer to the equation

Notice when the golden era of roe deer harvest was over and the species seemed to enter a rather stable period in harvest densities from 2010 and onward. In contrast, moose and mountain hare have been steadily declining during the same time frame. What might be the reason why roe deer do not share this fate?

Figure 2.19 The plot covers the time frame between 2010 and 2024. Within this period, both wild boar and fallow deer are well established in southern counties of Sweden (e.g. where the northern county limit is below 60° N). Both species show significant increasing trends, while moose show a significant decline since 2010. Meanwhile, roe deer appear to have remained stable over the last 15 years. Mountain hare are not included in the plot for readability but show a similar declining trend to moose (~0.6 per 1,000 ha in 2010 and ~0.2 per 1,000 ha in 2024).

Roe deer are picky foragers that need plants rich in nutrients and easy to digest. They mainly choose forbs, delicate grasses, herbs, buds, young leaves, and tender shoots from shrubs and trees. Therefore, a typical roe deer habitat includes both forest and pasture land (see Fig. 2.14).

Fallow deer are much less selective in their feeding. They can consume larger amounts of vegetation with more fiber and lower nutrient concentrations. In many areas they feed heavily on grasses and other abundant herbaceous plants, while also eating browse, mast such as acorns, and various herbaceous plants when available. This broader diet allows fallow deer to exploit grasslands, forest openings such as clear-cuttings, and agricultural fields where vegetation is plentiful but not necessarily of the highest quality. One study shows that about 36% of stomach contents consisted of Avenella flexuosa, a grass that characterizes vegetation in 2–4 year old clear-cuts.

Wild boar are omnivorous and opportunistic foragers. They exploit whatever is available in the environment at a given time. Their diet includes roots, truffles, tubers, bulbs, seeds, crops, invertebrates, and carrion, with mast (e.g. acorns) often being an important seasonal resource. A key feature of wild boar foraging is rooting. By turning over the soil, they actively search for below-ground food such as truffles, roots, and invertebrates. This is also why wild boar struggle in harsh winters—when the ground is frozen, access to food is limited, and mortality can increase unless feeding sites are available. However, remember that Swedish wild boar forage on supplemental food provided at feeding stations.

Wild boar rooting disturbs field layer patches, opening up space for opportunistic vascular plants to establish. Over time, these pioneer plants are replaced by more permanent vegetation of herbaceous and woody species. The large but delayed rise in roe deer harvest density may reflect this timing in relation to moose and mountain hare. For the same reason, one might argue that wild boar rooting helps create more suitable forage conditions for roe deer. However, this is not confirmed. Other factors may also play a role, such as longer vegetation seasons and reduced competition due to declining populations of moose and mountain hare. The increasing fallow deer population does not seem to affect roe deer in the same way, likely because overlap in forage is limited.

The most reasonable and simplest explanation is a marked change in foraging conditions that promoted roe deer preferences. Through clear-cutting, vegetation succession drastically increased carrying capacity, primarily promoting the expansion of moose. This led to rapidly increasing moose densities and eventually to overbrowsing, which suppressed the field layer vegetation and instead favored mountain hare. The combined effects of moose and mountain hare foraging ultimately shaped a field layer vegetation that elevated roe deer carrying capacity to unprecedented levels. This chain of events reflects Archimedes’ words: “Don’t disturb my circles.” In simple terms, any action—whether random or man-made—will have consequences, often long-lasting.

Roe deer require a broader foraging range compared to moose and mountain hare. They utilize both forest and grassland habitats to meet their demands for energy and nutrients. In contrast to moose and mountain hare, which are still declining, roe deer densities have remained relatively stable since 2010 (Fig. 2.19).

To understand why, we need to consider the population development of wild boar and fallow deer. Red deer are still at generally low harvest densities and are therefore not included. The same applies to mountain hare, which are already at very low densities (see Fig. 2.1). Today, wild boar and fallow deer are among the most common game species. This raises the question—do these introduced species affect roe deer habitat? The answer is likely yes for wild boar, and no for fallow deer.

What the harvesting history of game species may tell us

The findings presented in this communication focus on the development of harvest numbers for native and introduced game species: moose, roe deer, mountain hare, wild boar, and fallow deer. The data are either based on national data between 1939 and 2023 or county-based data between 1966 and 2023. The red deer is still at a low national population density (although locally abundant) and is therefore excluded from this analysis. The reason for this communication is the discussion about the moose population and damage to production forests, unfolded in letters to the editor in Dagens Nyheter (DN) about a year ago and more recently in Svensk Jakt.

These discussions reveal a two-dimensional way of thinking about solutions in complex ecosystems—seeking to find a balance or a status quo between competing desires. This is the core problem. The point is that any change, whether due to random natural events or man-induced actions, will instead start a chain of consequences. There are no everlasting solutions in complex ecological systems other than temporal ones. With that said, any imposed change will have consequences, regardless of what caused the change. That is the problem with management concerning forest habitats. Forestry has one goal, and wildlife management has another. To put it more simply, the consequences of a rationalized forestry, generating even-aged stands of production monocultures, worked out fine economically—for a while.

The rationale was: clear-cuttings—replanting—harvest and repeat. Consequences in the wake of this major change in forestry were likely not considered. The wake-up call came when damaged replanted stands became a serious backlash of a rapidly increasing moose population, promoted by a stark improvement of forage abundance at clear-cut patches. On the other side of the fence, hunters became enthusiastic about improved culling rates. The development of lagged rises and declines for mountain hare and roe deer behind the elevated moose population is an example of an ecological chain reaction.

What emerges from the actions mentioned above, to cite Archimedes, is an ecosystem of disturbed circles governed by time-lagged feedbacks between vegetation and herbivore populations. Primarily induced by changes in forestry practices aimed to optimize forestry rates of return (e.g. economic yield) to produce even-aged, denser stands of predominantly conifer monocultures. The use of herbicides to further increase productivity added another dimension of management actions. By that, the path toward habitat alteration was already set—the remaining question is how?

Changes in forest habitat conditions first alter forage availability, followed by delayed responses in herbivore populations. As populations increase, they in turn modify the vegetation, often with a time lag that depends on growth rates and regeneration capacity of the field layer. This creates a sequence where cause and effect are not synchronized in time. Moose respond first to increased forage following clear-cutting, mountain hare follow as the composition of the field layer shifts, and roe deer respond later as the field layer becomes more herbaceous. The system therefore does not move toward equilibrium, but instead progresses through a series of delayed adjustments, where each phase sets the conditions for the next.

Another aspect of man-made action is the introduction or re-introduction of game species. The most frequent species were red deer, roe deer, and wild boar. Most of these escaped from deteriorating game enclosures in the late 1990s. At that time, there were more than 250 registered deer enclosures and an unknown number of enclosures dedicated to wild boar. The important remark is not the number of enclosures. What is important is the ecological consequences these game species posed on the carrying capacity of the forest habitat. One important aspect of this was elevated foraging pressure. The harvest statistics show that mountain hare has suffered the most, and the moose population seems to be in a steady decline. In fact, the trace of its harvest densities suggests changes in the field layer that no longer sufficiently support mountain hare demands.

How to interpret the results

The reason that brought me back to the concept of management of renewable resources was the letters to the editor published by Dagens Nyheter (DN) about the declining moose population in Sweden. These opinions, sometimes slant about the problem, are based on simple reasoning about cause and consequences. By simple, I refer to the dimension of reasoning - like if A is reduced then B will increase. Such deductions only work in finite systems.

What about ecosystems like forest habitats, which have several organisms responding to changes? Obviously, the simple A–B analogy won’t do the work. Pull one string, and you start a chain of reactions in the ecosystem. To succeed with lasting adjustments, managers need to consider the complete forest ecosystem and ask relevant questions about why, followed by a question of how to solve it. However, that is not enough—you must specify a goal: what the expected result of a management action is supposed to be. Before launching actions, a pragmatic manager will specify what possible other consequences decided actions can have. To be blunt but fair, some management decisions apparently skipped the step of scrutinizing what consequences an action might bring in a wider perspective. Apparently, effects in the wake of various management actions still do not seem to have settled population levels of the game species.

Typically, wildlife management has focused on single-species actions—in essence, mainly the moose. The conflicting demands between forestry and the hunter cadres’ desire for decent harvest yields have shaped this approach. In my opinion, this kind of tunnel vision has led to the present situation: a declining moose population with reduced reproductive capacity, recently listed as near threatened (NT). Meanwhile, the mountain hare, obviously at dangerously low densities, is still listed as least concern (LC).

Most of these kinds of actions are practically doomed to fail or even become counterproductive. The same applies to the letters to the editor, which tend to point out isolated causes to blame. To succeed with lasting adjustments, managers need to consider the complete forest ecosystem and ask relevant questions about why. Why is the moose population in poor condition? Why has the mountain hare declined in density following its prolific years in the late 1980s? Why is the fallow deer gaining ground?

To investigate the reasons, begin with the most important factor—forage abundance—followed by interspecific competition, predation, disease outbreaks (note that this category does not include diseases caused by starvation), and accidents.

Can the habitat support the needs of the game species? The answer is yes and no. Obviously, fallow deer and wild boar seem to thrive. However, wild boar are not a good example because of the provisioning of supplemental forage at feeding stations. Thus, wild boar density reflects an artificial carrying capacity. Fallow deer, though, seem to be thriving, showing a steady growth in population numbers. Roe deer also seem to be in parity with foraging conditions, with a 14-year stretch of stability.

The native moose and mountain hare population developments tell a different story, with long-term declines, particularly for the mountain hare. This observation suggests an ongoing alteration in the forest habitat that is negative for moose and mountain hare. What seems to be different, and what caused the alteration in the forest habitat?

The main difference is found in the field and bottom layers and in the early bush and tree layer. The visual difference is striking for those who remember how clear-cuts looked a couple of years after harvest. National forest inventory data support this observation. The richness and density of regenerated natural vegetation at clear-cuts since the mid-1990s are different from those at today’s clear-cuts.

So yes, the forest habitat has changed, leading to the next question—why has it changed? This question has two main answers: modern forestry routines including the use of herbicides, and the effects of overly dense populations. Modern forestry introduced even-aged monocultures, which over time reduced the natural richness of the seed bank. The first generation of clear-cuts provided an enormous boost in natural re-vegetation and forage abundance, promoting population growth of moose, mountain hare, and roe deer. Over time, however, seed bank diversity declined, and the regenerating bush and tree layer became noticeably thinner in comparison.

The main reason for the impoverishment of seed banks relates to the increasing extent of density-optimized, even-aged monocultures and actively reducing deciduous woody vegetation. The densities of birch, rowan, aspen, sallow, ash, and oak are lower compared to earlier decades. In fact, aspen and ash are now hard to find on current clearings.

Another aspect of forage impoverishment is browsing pressure. When populations of browsers exceed the carrying capacity of forest habitats, the browsable vegetation becomes overexploited. Ideally, browser density should be in parity with yearly forage biomass production (equilibrium density). However, there is a nuance to this statement. For such a balance to be sustainable, vegetation must fully recover each year, including sufficient regrowth to support populations during the non-vegetative season.

The mountain hare seemed to follow variation in forage abundance—until the effects of modern forestry routines put the forest habitat in disarray. This initiated a nearly 60-year-long ecological chain reaction in the ecosystem. With the addition of introduced game species, the disarray has become even more complex to understand. It may take several more decades to fully comprehend the consequences of these man-made actions.

As of now, moose and mountain hare are getting the dirty end of the stick, while fallow deer, wild boar, roe deer, and likely red deer have got the better end of that stick. If these conditions persist, moose sightings may become rare events, while the mountain hare may disappear from southern Sweden.

Who is to blame for this roller coaster of alterations in the forest system? One thing is certain: there was a poor understanding of biological processes in the 1950s. What about the supervisory role of Naturvårdsverket (Swedish EPA) in habitat protection? Much of the damage had already been done when the EU Habitat Directive was implemented.

What to consider before any management actions

Let’s make an example of what the results above suggest. Clearly, the mountain hare seems to be suffering as an effect of a deteriorating habitat. This means that the foraging conditions suitable for the mountain hare are dwindling. Specifically, this becomes obvious from 1996 onward. How can we see that? Because of the weak response to random changes in the quality of vegetation seasons. It no longer matters, as viable forage has been depleted or almost so.

The moose follows the same track, but the general decline is blurred by management actions, also indicating forage shortage. That shortage is visible in declining reproductive capacity and falling dressed body mass among calves. Notice that we would not see a drop in body mass among calves if the reduction in reproductive capacity was caused by broken population structures. These trends have been ongoing since 2000.

Now - the most important question is where to go concerning the actual population development. What would the aim be? If the answer to that question is to restore the forest habitat suitable for thriving populations of moose and mountain hare populations, this will demand certain actions.

The first priority must be to prevent further depression of field layers and the bush- and tree layers in the forest habitat. We need to give the vegetation a chance to recover completely and abundantly enough to support winter browsing each year. How can we do that? First of all we need to focus on habitat management rather than pointed actions targeting management on moose and mountain hare populations.

The solution is to reduce all forest game ungulates radically. However, the mountain hare which is already at dangerously low levels, does not need further reduction. That action is the only way to recover a balance between forage availability and game species density. In ecological terms we need to keep game species well below actual carrying capacity. Such an action will remove effects of overbrowsing. This is the most crucial action. Therefore, managers need to monitor the progress in vegetation recovery annually until it is restored to provide consumption over a full year.

Remember the ecological hierarchy of importance - vegetation (abundance and diversity) sets the limits - that limit shapes the diversity and density of browsing and grazing game species - which in turn configures the diversity and abundance of predators. When the balance between suitable vegetation and browsers and mixed foragers is back - then reproductive capacity of populations and conditions among offspring will improve. Notice, to reinstate a broken habitat is not a quick fix! It will take a good number of years to reset foraging conditions. This action would be the main task for wildlife managers.

However, the wildlife management will not suffice! The forest habitat has radically changed as a result of forestry routines and pointed actions to delete deciduous tree vegetation. Some methods used were really dubious, leading to a loss of important species such as aspen, ash, and sallow. The deciduous diversity and abundance has to be restored if traditional games species like moose and mountain hare are to thrive. Unfortunately, by doing stomach content analysis under actual conditions in forest habitats will not monitor prior plant diversity. Valid samples have to be collected from studies done during the 1980s.

The seed bank in the forest habitat has become impoverished over time by forestry actions and game population kept at far too high densities. A possible solution is to ensure variety in browse by planting protected smaller stands of rowan, aspen, sallow, ash, and oak, keeping them protected until the stems have grown above the reach of moose browsing. This action would improve the seed bank by time. The responsibility for such actions would lie with landowners.

These actions taken together are an example of management measures aimed at habitat function rather than single-species management.