De vos in Meijendel en Berkheide

J.L.Mulder, 2000

Rapport Duinwaterbedrijf Zuid-Holland

Summary: The foxes of the coastal sand dunes in the Dutch province of Zuid-Holland, 1997-2000.

The fox population in the sand dunes along the coast of the province of Zuid-Holland (The Netherlands) was studied from 1997 to 2000. The study area is a nature reserve and at the same time used for producing drinking water from introduced river‑water. There is no game control, except in some private properties at the border of the dune area. The fox's diet (as studied from faeces) consists mainly of rabbits: 84 % by live weight; birds and small mammals make up 5.5 and 4.5 % respectively of the consumed food. In the dunes the amount of human related waste in the diet is negligible; a limited sample of stomachs from the human occupied areas around the dunes (the village of Wassenaar and the town of The Hague) however, contained 13 % digestible refuse (mainly from compost heaps) and 8 % indigestible refuse (plastics, food‑wrappers, etceteras). Through predation and disturbance by foxes the colonies of breeding gulls (four species) all rapidly disappeared after the establishment of the fox population, from 1978 onwards. Several relatively large ground nesting species, i.e. pheasant, curlew, lapwing, woodcock and shelduck, have decreased in numbers since the arrival of the fox. All other breeding bird species however do not seem to have been affected in their numbers (chapter 3).

Fox society was studied by radio‑telemetry (chapter 4). A total of 60 adult and 18 juvenile foxes were radio‑tracked for periods up to almost three years. Throughout the seasons the majority of the foxes live in almost non‑overlapping territories. Average territory size is about 55 hectares. Most territories are occupied by two or three adult foxes, usually one male and one or two females; in two instances two males were present in the same family group. One third to almost half of the number of adult foxes in the area live a non‑territorial life, most of them young individuals in search of a territory, i.e. an opportunity to reproduce. At first they roam over relatively large areas, some of them also exploring the human inhabited surroundings of the dunes. Most of these itinerant foxes restrict their activities to an area of 400 to 600 hectares after several months of unsuccessful wanderings, apparently waiting for an 'opportunity' there. Some individuals are still roaming when three or four years of age, and die early without ever having been territorial. Old territorial foxes, 6 to 10 years of age, often start to wander again, after which they die within weeks or months. The age of live foxes was determined by examining an extracted tooth, the first upper premolar.

Average litter size at an age of 6‑8 weeks is 3.8 cubs (chapter 5). The very skewed sex ratio, in favour of males, suggests a high nest mortality rate among female cubs (chapter 6). Before independence at an age of 7 months, about 50 % of all cubs have died. Yearly mortality rate among roaming subadults is about 35 %, among territorial foxes between 25 and 30 %. Adult foxes die from diseases, affecting lungs, liver and intestines, and from infected bite‑wounds and other injuries; some are killed on the road or shot during short trips to areas outside the dunes and some drown in water with steep concrete or wooden sides. The fox tapeworm Echinococcus multilocularis is absent from this population (chapter 7).

The human inhabited areas around the dunes only slightly influence the dune fox population. Reproduction in the border zone seems to be somewhat higher, possibly due to extra food sources outside the dunes (compost heaps, chickens, and road‑killed animals). Near the border the variation in territory size is higher than more inwards, presumably because on the one hand territories can be smaller (extra food sources) but on the other hand can temporarily be larger when neighbouring foxes have been killed (on roads or shot) outside the dunes. The neighbouring village of Wassenaar and the town of The Hague harbour their own fox populations; cubs are born in many of their estates and parks (chapter 8). 

Population density (just before reproduction) is estimated to be somewhere between 7 and 11 foxes per km²; higher fox densities have only been found in urban areas in Great Britain (chapter 9). The low reproduction rate, the high mortality rate and the high proportion of roaming foxes all suggest that the population size is limited by natural causes. This means that the population will show a high resilience against possible attempts to limit the number of foxes artificially, compensating for the losses by an increased reproduction rate, a decreased (natural) mortality rate and more opportunities for roaming adult foxes. A population model ('Reinaerd') was developed to demonstrate this population resilience (chapter 10). 

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