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Grey squirrel, Sciurus carolinensis , populations have been subjected to various degrees of control in thewoodlands of Ireland and Britain since their introduction. The populations readily recover, but therecolonisation rates and other ecological effects of the culls have not been fully examined. Monthlylive trapping programmes were used to monitor the grey squirrel populations in two woodlands.
Culls, where as many grey squirrels as possible were removed by trapping, were conducted on fiveoccasions; three in one woodland and two in the other. The squirrel populations recovered withinten weeks in four of the five culls. Post-cull populations tended to consist of unmarked adult squirrelsthat had entered from outside the woodland. One cull attempted in the autumn was not a success, ascapture rates were low and the population had returned to its pre-cull levels within three weeks.
Many young squirrels entered the population following this cull. Some long-term repercussions ofthe culls were evident, even after the population density had recovered. The age profile changed,with the removal of breeding females reducing the numbers of young squirrels for the followingyear. Breeding levels were also reduced as the recolonising populations were slow to start breeding.
Home ranges of surviving squirrels expanded if the population density remained at a reduced level,and core areas often shifted following a cull.
removed are often those that would have been lostto the population through other mortality factors (Mosby 1969). The use of the anti-coagulant Shortly after its introduction in 1911, the American warfarin has been shown to reduce a squirrel grey squirrel (Sciurus carolinensis Gmelin 1788) population and possibly reduce bark-stripping became established and began to spread through Ireland (e.g. Stelfox 1927; Moffat 1938; O’Tean- damage as well (Kenward et al . 1988; Gurnell and gana et al . 2000). Despite efforts to control the Pepper 1998). Warfarin is not recommended where invading species, eradication became virtually red squirrels may also be present (Sheail 1999) (i.e.
impossible. The prolific breeding, expansive feed- in many areas of Ireland), although progress has ing habits and robust nature of the individuals been made in the design of red-squirrel-proof ensured that the total removal of the species from a hoppers (Pepper 1997). The removal of grey woodland was never permanent. A greater emphasis squirrels by trapping is an effective method, as had to be placed on keeping populations reduced to grey squirrels are a very easy animal to trap an acceptable level. Control programmes are now (Seymour 1961). Although it can prove labour focussed on protecting red squirrel (Sciurus vulgaris intensive, trapping is currently considered to be the Linnaeus 1748) populations in specific areas from best tactic available to control grey squirrel grey squirrel incursion and reducing bark stripping numbers, particularly where reds are present, and it can be very efficient when removing grey squirrel numbers are shooting (often employing Genovesi 2003). Attempts have been made in drey-poking to increase the numbers of grey producing an immunocontraceptive control * squirrels removed), poisoning and trapping. These orally introduced antibodies of sperm-specific are accessible and easily implemented by all sperm Á egg interactions and cross-react with the frequently recreational, and the grey squirrel squirrel spermatozoa (Moore et al . 1997). This population is rarely reduced far below the normal could be cheaper than trapping and is considered carrying capacity of the woodland. The numbers more humane by the general public (Barr et al .
BIOLOGY AND ENVIRONMENT: PROCEEDINGS OF THE ROYAL IRISH ACADEMY, VOL. 107B, NO. 1, 19 Á 29 (2007). # ROYAL IRISH ACADEMY 2002), but its potential effectiveness is under north-east of Navan, in County Meath. There are question, as a reduction of births of juveniles may 12ha of woodland within the 40ha estate. The result in an increase in the survival rates of adults, woodland consists of mature mixed hardwoods and compensatory breeding by non-controlled (mainly oak (Quercus sp.), beech (Fagus sylvatica ), individuals may lessen the effects (Rushton et al .
sycamore (Acer pseudoplatanus ) and ash (Fraxinus excelsior )), with an understorey of young trees (10,000 new trees were planted in 1998). A small programme is also important, with the impact on patch of coniferous wood, to the west of the main the damage levels only being significant before and woodland was included in the study site, to during normal damage periods (March to July) (Seymour 1961; Taylor 1966; Rowe 1980). If included in the study. Damage levels were at a exceptionally early breeding is manifest, damage moderate level (14% (910%) damaged (from a periods may begin earlier (January and February) random survey of 100 trees)), although it was (Pentamedes 1983). A general constant attempt to considered to be at risk of escalating alarmingly remove squirrels has no effect, as the bounty once the newly planted saplings reach a more schemes in Britain of the 1950s proved (Shorten vulnerable age. The estate was relatively detached from other woodlands, the nearest being Hayes removal serves only to fill a similar role to that of House woodland, 2km to the east, across the wide predation and hunting in their native North River Boyne (which acts as a barrier against squirrel America, which has little effect on the population density overall (Mosby 1969; Nixon et al . 1975).
connections to other woodlands existed in the An intensive removal programme is more effective, form of corridors of mature hedgerow and, in taking as many squirrels from the woodland as one instance, a line of trees along the southern bank possible, thus severely reducing their impact on the of the River Boyne. Before this study began, ecosystem, until the population recovers. This recovery rate can be as short as four weeks mainly through shooting and trapping, with about (Rowe 1980), and the pace of the reinvasion will 50 squirrels being removed each year (4.17 squirrels be dictated by the degree of isolation of the ha(1 y(1). This ceased during the course of this woodland and the type and extent of management study. The population was monitored monthly carried out in the neighbouring wood areas (Taylor from December 1996 to May 1999, with culls 1966). The fluctuations evident in grey squirrel carried out following the normal live trapping populations from year to year (Warner 1976; sessions in March 1997, September 1998 and Rowe 1980; Ratcliffe and Pepper 1987; Gurnell 1989) will also influence the effectiveness of the management programme and the speed of recovery, estate, again on the south bank of the River due to variations in the sources of recolonising Boyne, lies Beau Parc Estate (N9472), which contains more than 150ha of woodland stretching Although it is generally accepted that the for 4km along the river. The study site chosen was recolonisation rates of managed woodlands by grey also 12ha, at one end of the estate along the squirrels are very rapid, little attempt has been made entrance drive. As with the Ardmulchan Estate, to measure the rate of recovery or the factors that mature mixed hardwoods (oak, beech, sycamore may influence it. The profile of the recovering population is also important, as this will have a dominated the area, with some coniferous species bearing on the future success of the population.
also. The study area was connected to the rest of This paper aims to investigate the efficiency of the Beau Parc woodlands, albeit through a bottle- an intensive removal programme in clearing grey neck. Damage levels in this site were high (36.42% squirrels from woodlands and to study the rates of (98.45%) damaged (from a random survey of 140 recovery of the squirrel population with reference trees)), with one sycamore stand in particular to the woodland, season and year in which the cull extensively damaged by the squirrels. An average of 100 squirrels were being removed annuallyover the entire woodland, before the study began(0.66 squirrels ha(1 y(1). No removal of squirrels was conducted in the study area during theprogramme, other than the culls comprising the Grey squirrel populations were monitored in two experiments. Squirrel management was, however, maintained in the remainder of the woodland, with known numbers of squirrels being removed from Survey Ireland grid reference N9069) is located the large woodland in the two years of the study on the southern bank of the River Boyne, 3km (300 squirrels in 1997 and 80 in 1998). The GREY SQUIRREL POPULATION RECOVERY FOLLOWING CULLS population in Beau Parc was monitored from animal, using a ‘follow to source’ technique January 1997 to October 1998, with culls carried (Kenward 1982a). The number of fixes were out following the live trapping sessions in May number of fixes vs. home range area showed nofurther increase in home range was being achievedby extra fixes. The surviving squirrels were left for a couple of weeks following the last day of the cull toallow them to settle and to ensure any changes in The squirrels were trapped monthly using pre- home range due to the removal investigation were baited single-catch standard mink traps, which were modified by attaching wooden nest boxes and base Of the numerous methods available to monitor boards and adjusting the treadle sensitivity to capture grey squirrels. Trap density in each site number of squirrels present (MNP) was deemed to was between 1.08 and 1.75 traps ha(1, depending be the most suitable. Once the populations were on availability of traps. Traps were placed to culled and the recolonisations were taking place, it maximise the number of squirrels caught during was effectively a new population that was being each trapping session. Captured squirrels were examined, as marked animals were removed along checked for marks indicating previous capture and with those that had no marks at the time. Capture Á examined for sex and breeding status. Each squirrel mark Á recapture techniques that rely upon the was also weighed to the nearest 10g using a 1kg recapture of previously marked animals from Pesola spring balance and its right shin-bone length earlier months were therefore not suitable (e.g.
was measured to the nearest 0.1mm using callipers.
Fisher-Ford model, Jolly’s stochastic (or Jolly Á If the squirrel had not been previously marked, a Seber) method (Begon 1979)). The low numbers colour-coded, numbered brass collar was attached of captures in some months, particularly those to its neck and a numbered Minitag ear tag immediately after a cull, meant that other methods that may be applied within a month’s trapping Five grey squirrel removal exercises (or culls), session (e.g. Petersen/Lincoln Index (Begon 1979)) were carried out; three in Ardmulchan and two in were also unsuitable. Squirrels that were found in Beau Parc. The culls took place between trapping the woodland in a certain month, were absent in the sessions, and the subsequent trapping session following month or months and were then caught resumed two weeks after the start of the cull.
subsequently were not considered to be present for the entire period, as there was some evidence from monitored on weeks two, six, ten, etc. following the radio-collared squirrels that individuals left the the cull. It was decided that trapping was the most study sites for extended periods. Squirrels were suitable removal method available. The trapping therefore only considered to be present on the actual periods were approached in the same way as the standard monthly trapping sessions, with a pre- Examination of the incremental lines laid baiting session preceding the trapping.
down in the teeth cementum (Harris 1977; 1978; Fogl 1978) showed that 97% of animals weighing 500g or less removed in the cull were subadults euthanised by a 2.5ml injection of the barbiturate (squirrels less than one year old) (Bresnihan 1998).
Squirrels weighing under 500g throughout the abdominal cavity. The deceased animal was then study were therefore classified as subadult.
removed to the laboratory for further analysis.
Trapped males were classed in each month as Trapping continued for five days, with the success having abdominal or scrotal testes, and the of each cull determined from the decrease in daily percentage of males having scrotal testes was used capture success rate and the number of marked to estimate the extent of sexual activity and squirrels captured in post-cull trapping sessions.
duration of the breeding season. It was more During the fourth and fifth culls, individual difficult to estimate when females were in a squirrels were allowed to survive the culls so that breeding condition, but females were examined changes to the ecology of these animals could be for evidence of lactation, and these data were used to estimate the abundance of litters in the individuals were fitted with ‘Biotrack’ radio transmitters (supplied by Biotrack Ltd, Wareham,Dorset) and their home ranges determined in theweeks before the cull. Data were analysed by minimum convex polygons using ‘Wildtrak’ v.
1.15 (Todd 1993). Twenty to thirty five fixes The MNP in each month for both woodlands are were collected during each session for each shown in Fig. 1 (December 1996 to November Fig. 1 * Recovery of grey squirrel populations at Ardmulchan (Cull 1; A MNP) and Beau Parc (Cull 2; B MNP) following the first culls in spring 1997. (MNP-Minimum Number of Squirrels present per hectare).
1997) and Fig. 2 (October 1997 to September 1998). Fig. 3 shows the MNP in Ardmulchan only respectively), only one previously marked animal from June 1997 to June 1999. The rates of was found in each post-cull sample * on each recovery of the grey squirrel populations are given occasion a squirrel that had escaped following capture during the cull. One further pre-cull In the four springtime culls (Culls 1, 2, 3 squirrel was recorded ten weeks after Cull 2 in and 5), carried out in March 1997, May 1997, Beau Parc. No resident squirrels appeared to March 1998 and March 1999, respectively, greater survive Cull 3 in Beau Parc. In the three months numbers of squirrels were removed than had previ- following Cull 5 in Ardmulchan, besides the six ously been recorded as present in the woodland radio-collared squirrels that had been allowed to (based on the MNP recorded immediately before survive the cull, eight marked animals not removed the culls). For example, the MNP in Ardmulchan during the cull were captured. Only one of these in March 1997, two weeks before the first cull, previously marked animals was found in the stood at 22 squirrels (1.83 squirrels ha(1); during immediate pre-cull sample (February 1999). The the cull 40 squirrels (3.33 squirrels ha(1) were remainder were absent from the trapping sessions for up to four months over the winter, indicating In three out of the four springtime culls, the they may have been forced into marginal habitats populations recovered to pre-cull levels within ten until space was available in the woodland once weeks. However, in Beau Parc in March 1998 more. The numbers of subadults captured in the (Cull 3) the effects of the cull appeared more long ten weeks following the springtime culls were low: term. The pattern of recovery was much slower three after Cull 1; two after Cull 2; none after Cull than seen following the other spring culls, as no squirrels were found two weeks later, and only two Six radio-collared squirrels were allowed to individuals were found four weeks after that, six survive Cull 4, carried out in Ardmulchan in weeks on from the cull. After ten weeks, the September 1998 * the only cull attempted in the population appeared to be recovering, with ten autumn. On this occasion the populations were individuals captured in June 1998. Following this examined three weeks, eight weeks and fifteen already slow recovery, however, the population weeks after the cull. The estimate from before Cull appeared to collapse once more, with very few 4 indicated a population with MNP of fifteen squirrels remaining in the wood. Population squirrels. Only twelve animals were removed throughout the stuttering recovery period in Beau estimated three weeks after the cull, 22 individuals were captured. Twelve of these were new to the Following the springtime culls the squirrels woodland: a recolonisation rate of 0.33 squirrels captured in the post-cull samples were generally unmarked adults, presumably immigrants to the population was different to that found in the woodland. Following Culls 1 and 2 (Ardmulchan, other culls conducted in spring. The animals GREY SQUIRREL POPULATION RECOVERY FOLLOWING CULLS Fig. 2 * Recovery of grey squirrel population at Beau Parc (B MNP) following Cull 3 in spring 1998. The population densities in Ardmulchan for the corresponding times are also shown. (MNP-Minimum Number of Squirrels present perhectare).
known to be within the site after three weeks were: majority of males remaining in a breeding state five animals with radio-collars (one having left the until the cull in September in Ardmulchan (Cull 4).
woodland); ten subadults (new squirrels weighing None of the animals that replaced these individuals under 500g); five previously marked animals missed were in a breeding condition, which is not by the cull, or who had left the woodland and surprising as the majority of them were subadults.
The number of females that were lactating during each trapping session indicates the success of the breeding season. In 1997, lactating females were captured between April and October, although population throughout the study. A relatively numbers were quite low. Breeding was much high proportion of subadults were present at the more successful in 1998, when there was no cull start of the study, but this level quickly dropped, in Ardmulchan, with a much higher percentage of with low numbers of subadults captured in Cull 1.
females lactating following breeding. In total Subadult squirrels were found in the study, however, in nearly every month of 1997. During Ardmulchan in 1998, and six of these were the following winter, the few subadults that were lactating for three months or more, implying that present disappeared or became adults, and none they had two litters during the year. This suggested were detected in the woodlands until the second that up to 22 litters were weaned in the woodland half of 1998. Subadults were found in high numbers over the course of five months. The sample size in following the September 1998 cull in Ardmulchan Beau Parc throughout parts of the study was too (Cull 4; see above), and the survival of these young low to accurately observe trends in the breeding appeared quite good through the following winter.
The sample size in Beau Parc throughout parts of the study was too low to accurately observe trends throughout the study (Fig. 6) and were affected by low sample number, particularly immediately after a cull. However, after three of the culls (Culls breeding condition and females lactating during 1, 2 and 4), the recovered populations had a larger the course of the study in Ardmulchan. The main number of males than the pre-cull populations, and breeding season in 1997 began in March and this situation persisted for a number of months after continued until July. Following Cull 1, the the population had returned to its original density.
recolonising males were mostly in a breeding In the other two culls (Culls 3 and 5) the condition when they arrived at the woodlands.
populations at the time of the cull were still male- The majority of male squirrels had returned to the non-breeding state by November. In 1998 a very The home range and core areas of each animal similar breeding season was apparent, with the studied during Culls 4 and 5 are given in Table 2.
Fig. 3 * Recovery of grey squirrel populations at Ardmulchan following Culls 4 (autumn 1998) and 5 (spring 1998). The population densities in Ardmulchan during the same months in the previous year (when no culls took place) are alsoshown. (MNP-Minimum Number of Squirrels present per hectare).
Squirrel 169A disappeared between the autumn area shifted for two of the squirrels being pre-cull and post-cull sessions, and one of the radio-collared squirrels in the spring sessions left Overall core-area use (i.e. time spent within the woodland before either home range could be the core area) decreased significantly for the ten squirrels (over both sessions), from an average of In the autumn sessions (Cull 4) the population 73% pre-cull, to 60.5% post-cull (paired t -test: t 0 increased to its original size very quickly (three weeks) and home range sizes and core areas did notchange to a significant degree. Three of thesquirrels shifted their core areas from the original location to a new location within the original homerange (i.e. following the cull they started utilising a A cull of five days using the intensive trapping different region in their original home range as their programme was sufficient to remove most, if not core area). The other two squirrels that remained in their previous core areas were lactating at the time concerned. This was evident from the very low numbers of previously marked animals in the post- During the spring session (Cull 5), when the cull samples. It was not viable to continue trapping population remained depleted for a relatively long for further days, given the low numbers of squirrels period (up to ten weeks), the home ranges of all the captured. During Culls 4 and 5, all of the radio- surviving squirrels increased and the core areas collared animals were trapped during the cull increased in size on all but one occasion. The core sessions, indicating that all the present squirrels Table 1 * Details of each cull, including the recolonisation rates based on the minimum number of squirrels to enter the woodland during the recover period.
*Culls took place approximately two weeks after the trapping session in month shown.
GREY SQUIRREL POPULATION RECOVERY FOLLOWING CULLS Fig. 4 * Age profile of grey squirrels in Ardmulchan throughout the study. Squirrels weighing under 500 g were classified as subadults; those that weighed 500 grams or more were considered to be adults.
had been captured (assuming there was an equal to another woodland. The mature hedgerows chance of capturing every squirrel). Other methods surrounding Ardmulchan offered sufficient cover of removing squirrels were not attempted in this and resources through which the squirrels moved study. Poisoning has been shown to work as well (Kenward et al . 1988), but with 94 animals from non-target species captured in traps during the 15 time of year when the populations were naturally days of the five removal experiments, the potential increasing following a post-winter low, with the need for grey-squirrel-specific hoppers is under- spring dispersal bringing new individuals into the lined. However, red squirrels were not present in woodland. It is possible that these animals may have the region concerned, and these non-target cap- been able to enter the woodland even if there had tures were mainly rats (another pest species) or bird not been a cull. In Ardmulchan, the population was species (which are not as susceptible as mammals to higher in January and February of 1999 than that found during the same period of 1998, but was considerably lower in May, despite apparently having recovered following Cull 5 (Fig. 3). The previously been recorded in the woods. This population, although recovering to pre-cull levels, indicates MNP was an underestimate of squirrel numbers. It is also possible that a number of population, had the cull not taken place.
squirrels entered the woodlands in the three The study site in Beau Parc is 12ha of a much weeks between the final pre-cull trapping session larger woodland (150ha), which is the obvious and the last day of the cull in question. MNP did source of recolonising squirrels following the culls appear to be a fairly reliable measure of changes in in this study. Control was conducted in the remaining 138ha of Beau Parc as well, with 300 squirrels removed in 1997 and 80 removed in 1998.
robustly on all but one occasion, with population This control in the surrounding woodland is likely densities recovering within two months on most to have contributed to the faltering recovery after occasions. The number of squirrels removed during Cull 3 in the study site, as movement of squirrels a cull had no bearing on the recovery rate of the from the larger portion may have been reduced.
woodland, but was simply a feature of the carrying Breeding levels and other population dynamics in capacity of the woodland at the time of removal.
the larger portion would also have influenced the Following the springtime culls, the animals making up the recovered population tended to be recovered in ten weeks following Cull 2 in Beau previously undetected adult squirrels, presumably Parc, despite the large number of squirrels removed entering the woodlands from surrounding habitat, a from the remainder of Beau Parc in that year. In common source of recovering populations (Gurnell general the grey squirrel population density in the 1989). The population in Ardmulchan recovered Beau Parc study site was lower than that in quickly even though it was not directly connected Fig. 5 * Breeding status of male grey squirrels (shown as% of captured male squirrels with scrotal, rather than abdominal testes) and lactation in females throughout the study in Ardmulchan.
indicating it was a poorer habitat for grey squirrels, recoveries of the population densities following and this may have further hampered the recovery of most of the removal experiments, the repercussions of the loss of the established individuals of the Cull 4 was conducted in the autumn and was not successful. Few squirrels were captured, a removal of the breeding females in Ardmulchan consequence of the difficulty of trapping squirrels in the spring of 1997 reduced the number of young during the autumn, a time when more of the squirrels within the woodland until the emergence natural food source is available to the population in of the following year’s young. The replacement the woodlands (Seymour 1961; Shorten 1962; population did not breed in high numbers.
Taylor 1966; Gurnell 1983; 1989; 1996). The Breeding was most successful in Ardmulchan in population had recovered to pre-cull levels by the 1998, when there was no cull. The recovered next trapping session, three weeks later. Much of population was male-biased for an extended period the recovered population consisted of young following three of the culls, a possible result of squirrels that were old enough to join the adult greater range expansion by sexually active male population, but only when the space was available.
squirrels, particularly in the spring (Kenward 1985).
Removal of most of the squirrels from a woodland It is the production of juveniles that correlates leaves improved resources for the survivors as well strongly with damage (Kenward and Parish 1986), as possibly encouraging immigration (Kenward and so the reduction in overall breeding may have 1982a). High levels of breeding preceding Cull 4 reduced potential damage to the woodlands, despite meant there were a large number of young squirrels the apparent recovery in squirrel numbers.
available to recolonise the woodland. The cull The home ranges of the surviving squirrels simply made room for these animals to move in.
increased following Cull 5 in spring 1999, but Mortality in squirrels is highest during their first were unchanged following Cull 4 in autumn 1998.
year, but this is partly due to there being no Home range size is dictated by the population available habitat into which the young can move.
density (Don 1983; Kenward 1985), and the These young squirrels were in a better position to population numbers were reduced for a longer survive winter than usual. The high proportion of period after Cull 5. The increased home range size young animals in the following cull (Cull 5) (45% may, however, simply reflect a seasonal change in of the total squirrels removed, compared to 20% home range size. Further examination of the for the first cull, carried out in the same woodland influence of culls on home range size of survivors in 1997) indicates there was better survival through is required. Other changes were apparent following the winter by the young animals following Cull 4.
both culls, with a number of squirrels shifting The culls carried out in the two woodlands core areas following the culls. This may be a influenced the age ratios, sex ratios and breeding of hierarchy within the woodland, as recolonising GREY SQUIRREL POPULATION RECOVERY FOLLOWING CULLS Fig. 6 * Percentage of grey squirrels captured that were male in Ardmulchan (% Males A) and Beau Parc (% Males B) during the study. The timing of the culls are also shown (Ardmulchan-Culls 1, 4 and 5; Beau Parc-Culls 2 and 3).
squirrels are likely to be less dominant than the individuals they replaced. Squirrels spent less time in populations following an intensive cull is quite their core areas following the culls, using other areas rapid, the population can be reduced for up to ten of their home ranges to a greater extent.
weeks, which may offer enough time to protect a Table 2 *The home range and core areas of squirrels allowed to survive culls 4 (Autumn 1998 in Ardmulchan) and 5 (Spring 1999 in Ardmulchan) both before (pre-cull) and after (post-cull) the removal experiments.
Home range (ha) Core area usage (%) Core area size (ha) Home range (ha) Core area usage (%) Core area size (ha) *After adjustment for extreme outlier.
**Signal lost halfway through post-cull sample, before a complete set of fixes had been taken.
%Arbitrary codes given to all captured squirrels.
woodland from damage during the most severe annuli in razor-sectioned teeth. Journal of Wildlife recovering squirrels need to be identified and the squirrels therein controlled if the population is to Gurnell, J. 1983 Squirrel numbers and the abundance of tree seeds. Mammal Review 13 (2 Á 4), 132 Á 48.
be reduced for a longer period of time. Other population dynamics, such as breeding, sex ratio control of grey squirrels. In C. Helm (ed.), Mammals and age profile, are affected by control programmes as pests , 131 Á 43. London. Christopher Helm.
and this could have more long-term implications for the population. Reduced breeding may lead to a to broadleaf woodland in the new forest: a study on the effects of control. Quarterly Journal of Forestry 92, important for removing animals that have learned the habit, as these animals are more likely to Gurnell, J. 1996 The effects of food availability and damage trees in the future (Kenward et al . 1988).
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