Lynxes are known to kill medium sized predators, but the degree of the impact this has on the populations of these medium sized predators might strongly differ from one region to another, depending on many factors.
By radio tagging foxes and raccoon dogs in Paazierrie Forest and Naliboki Forest, the scale in which killing of these medium sized predators by lynxes occurs became apparent: more than half of over 60 foxes and raccoon dogs were killed by lynxes and wolves, almost an equal share each.
From snow tracking and telemetry data on lynx in Belarus (Paazierre Forest and Naliboki Forest) we calculated that an adult lynx kills about 40-50 foxes per year. However, in summer and autumn this may happen more frequently, because young foxes are easier victims. So, this figure is almost surely an underestimation.
So, what impact does this have on the populations of these medium sized predators? Foxes are known to respond to high mortality rates by producing more offspring (larger litter size, more litters). Are the losses large enough to cause a decline in their densities? Which factors are at play?
In the Naliboki Forest lynx, wolf, fox and pine marten densities have been monitored since 2000. Wolf and lynx via terrain wide track inspection, foxes and pine marten via transect snow tracking. Lynx numbers have increased from 5-15 in the whole forest in the early 2000’s to at least 60 nowadays. Over the years 1999-2014, the number of wolf packs varied between 6 and 14, and the number of wolves between 27 and 70. However, since lynxes reached high densities, wolf numbers have become very variable within the year, as a result of the killing of wolves by lynxes and the subsequent immigration and emigration of wolves (more details in ‘Interference competition between lynx and wolf’). Fox numbers have decreased 10-20 fold in 2014-2017 compared to the 2000’s, when there were 50-70 foxes per 100 km².
Snow tracking foxes in the winter 2016-2017 revealed pronounced lynx avoidance behaviour in foxes. Of the remaining foxes of the Naliboki Forest, 80 % live almost exclusively in grassy openings. Trails left by individual foxes during several days showed they hardly ever go inside the surrounding forest habitats. This behaviour is unprecedented in the Naliboki Forest. In the 2000’s foxes were common in all forest habitats and foxes that went hunting in grassy openings used the neighbouring forest habitats a lot as well.
The few foxes that still survive within densely forested habitats are mostly large adult males. Interestingly, the disappearance of foxes from the forest habitats correlates with a four fold increase in pine marten density.
In the Naliboki Forest lynxes clearly have a strong impact on fox populations, but how about other area’s? Let’s have a look at long term data on fox and lynx densities (given as multiannual mean) in several (predominantly forested) study areas in Belarus with different habitat carrying capacity:
- Krasny Bor terrain, western Paazierre Forest: mostly forests on sand and peat, mixed with richer soils, medium habitat carrying capacity (similar to Naliboki Forest)
- 1990’s: 1 lynx/100 km², 56 foxes/100 km²
- 2005-2014: 3-6 lynxes/100 km², 10 foxes/100 km²
- Lovat terrain, eastern Paazierre Forest: forests on rich soil (clay), interspersed with a lot of rough grassland, high habitat carrying capacity
- 1990’s: 3.1 lynxes/100 km², 158 foxes/100 km²
- Palata terrain, central Paazierre Forest: forests on very poor sandy soils, very low habitat carrying capacity
- 1990’s: almost no lynxes, 26 foxes/100 km²
The above examples suggest that the impact of lynxes on fox densities heavily depends on habitat carrying capacity. In the ecologically rich forests interspersed with a lot of rough grasslands, both lynxes and foxes reached high densities in the 1990’s. But if we look at the Krasny Bor terrain with medium habitat carrying capacity, we notice the same pattern we observed in the Naliboki forest: fox numbers in drastic decline when lynxes reach high densities, resulting in unsaturated fox populations. Even in the Palata terrain with very low habitat carrying capacity but with very few lynxes, fox densities are still higher.
So, lynxes seem to be able to reduce the fox population significantly only if habitat carrying capacity for foxes is medium: around 50-70 individuals/100 km² (Naliboki Forest and Krasny Bor). If habitat carrying capacity is very low (both for foxes and lynxes), lynx densities are too low to have an impact on fox numbers, allowing foxes to have a saturated population. If habitat carrying capacity is very high, lynxes are not able to keep the fox population at a low level.
Impact on raccoon dogs and shared prey?
The decline in Tetraonids (mostly black grouse and capercaillie) in Belarus can be attributed largely to predation by the non-native and invasive raccoon dog. Raccoon dogs can reach very high densities and they are more effective than foxes to hunt hens with chicks because they hunt in pairs (Sidorovich 2011).
Raccoon dogs are medium-sized predators with a very high fertility rate. When raccoon dogs are numerous, killing and/or predation by large predators has very little to no effect on their populations. Moreover, raccoon dogs are less voulnerable than foxes to killing and predation in winter, because they spend a lot of time sleeping in their burrows.
However, there is another factor at play. Raccoon dogs in Belarus are characterized by a population cycle of about 40-years due to infestations of the trematode Alaria alata and mange (see Rotenko & Sidorovich 2017 for more information on the raccoon dog population cycle).
When raccoon dogs are in decline and reach their cyclical low, lynxes (when occurring in moderate to high densities) seem to be able to exacerbate their decline and keep their numbers at a low level for a longer period. This in turn impacts shared prey such a Black Grouse and Capercaillie.
In the Naliboki Forest Raccoon dogs reached a cyclical low in the early 2010’s. In 2013 an event took place which has probably exacerbated the impact of lynx and wolf on raccoon dogs even more. Due to a crash in roe deer and wild boar numbers, both lynxes and wolves had to switch to alternative prey. Shortly after the crash, lynxes and wolves started to increase predation on raccoon dogs. An initial period of adaption to lower numbers in roe deer caused wolves to predate on a lot of unusual prey such a foxes and raccoon dogs, but after a few months they adapted to the new situation by starting to predate larger ungulates such as red deer and elk (Sidorovich, Schnitzler et al. 2017).
Lynx numbers increased even after the crash in roe deer and for the moment they are seemingly able to keep the raccoon dog population at a low level.
Even though lynxes predate Tetraonids themselves, recently we notice a 2-4 fold lower mortality of Tetraonids attributed to predators.
Lynx and badger
In ‘Hunting behaviour’ we describe cases of lynxes hunting for badgers at their setts. Moreover, badgers have been found as an item in lynx diets in several study area’s in Belarus.
In Belarus, badgers have declined during the past decades, due to a snow ball effect initiated by raccoon dogs. Blocking tunnels of badger setts, leading to suffocation of sleeping badgers in winter, exploitative competition over carrion in early spring, aggressive interference (particularly killing of cubs) and high parasite infestations at the sett are all direct impacts of raccoon dogs on badgers. Wolves and lynxes played a secondary role in the badger decline (as one of the cumulative effects) by more frequent killing of badgers that are more voulnerable when they occur in low densities (details in Rotenko & Sidorovich 2017).
In the Naliboki Forest, we recently noticed a marked recovery of the badger population after several years of very low raccoon dog densities and despite the increase in lynx numbers.