INTRODUCTION

Natural boreal conifer forests and man-madeconifer plantations are resource-limited systems forHolarctic tree squirrels (Gurnell 1987; Andrén & Lemnell 1992; Larsen & Boutin 1994; Lurz et al. 1997). The costs and benefits associated withspacing behavior and with dispersal vary greatlywith the degree of heterogeneity of the habitatand, in particular, with the spatial and temporal

predictability of critical resources, that is, the sizeof conifer seed crops (Ostfeld 1985; Lurz et al.1997). Moreover, because males and females ofpromiscuous tree squirrel species use differentspacing and behavioral strategies to increase theirreproductive success, they respond differently totemporal and spatial variation in food supplies(Wauters & Dhondt 1992; Lurz et al. 1997). In redsquirrels (Sciurus vulgaris L.) living in conifer plan-tations in northern England, most adult females onspruce-dominated territories shifted home rangeafter breeding (breeding dispersal, Lurz et al. 1997)and female site-fidelity was high only in thosepatches with good and relatively constant tree seedcrops (Lodgepole pine stands). Although homeranges of both sexes were much larger than those

Ecological Research (2000) 15, 271–284

Interspecific effects of grey squirrels (Sciurus carolinensis) on the space use and population demography of red

squirrels (Sciurus vulgaris) in conifer plantationsLuc A. Wauters,1* Peter W. W. Lurz2 and John Gurnell1

1School of Biological Sciences, Queen Mary and Westfield College, London E1 4NS, UK and 2Centre for Land Use and Water Resources Research, Porter Building, University of Newcastle,

Newcastle upon Tyne NE1 7RU, UK

Interspecific competition between red squirrels and grey squirrels was investigated by comparingthe population demography, spacing behavior and habitat use of red squirrels in two large coniferplantations in northern England: one site had only red squirrels (the ‘red-only’ site), in the otherboth red and grey squirrels occurred (the ‘red–grey’ site). Despite more abundant food at thered–grey site, red squirrel densities (0.26 ha-1 at the red–grey site, 0.29 ha-1 at the red-only site),adult survival rates and the breeding rates of females were similar at both study sites. Grey squir-rels at the red–grey site occurred at higher densities (0.92–1.1 ha-1) than did the reds and tendedto have higher breeding rates. In the presence of grey squirrels, the recruitment pattern of red squir-rels changed and there was little recruitment of subadults. The juvenile recruitment rate in thered–grey site (13%) was much lower than in the red-only site (50%). Grey squirrels, in contrast,had higher juvenile recruitment rates at the red–grey site (41%). The core areas of the home rangesof red squirrels in the red–grey site were more strongly overlapped by grey squirrels than by con-specifics. Red squirrels did not select the habitat with the best tree seed crop (Scots pine) but pre-ferred dense Sitka spruce plantations; they appeared to avoid the Scots pine area with its high greysquirrel density. Data on foot length and body condition indicated decreased body growth in youngred squirrels when grey squirrels were present. Our data suggest that adult red squirrels sufferedlittle from interspecific competition with grey squirrels and that the key factor is decreased juvenile recruitment in red squirrels.

Key words: grey squirrel; habitat use; interspecific competition; population demography; red squirrel; spacing behavior.

*Author to whom correspondence should beaddressed. Email: j.gurnell@dial.pipex.com

Received 24 August 1999.Accepted 27 March 2000.

reported in high-quality conifer woodlands andranges often had several non-adjacent activitycenters, breeding females still maintained mutu-ally exclusive core areas (Wauters & Dhondt 1992;Lurz et al., in press). Local survival of adult maleswas very low and the males appeared to track themovements of the females. Thus, there is good evidence that intraspecific competition (especiallywithin each sex) shapes spacing behavior and dispersal and, therefore, affects the populationdynamics of red squirrels. However, the effects ofintensive interspecific competition between closelyrelated tree squirrel species in these low qualityhabitats have not been studied.

In Wales and north and north-west England,grey squirrels (Sciurus carolinensis Gmelin) continueto colonize mixed woodlands and conifer forestswhere, until recently, only red squirrels occurred(Armitage et al. 1997; Cartmel 1997; Skelcher1997). Red squirrels have already been replaced bygrey squirrels in most broad-leaved and mixedforests in England and Wales (Gurnell & Pepper1993). Although several hypotheses have been proposed to explain the replacement of red by grey squirrels, the mechanism of replacement,especially in conifer forests, is not yet understood(Gurnell & Pepper 1993). Kenward et al. (1998)found some correlation between female densityand female reproduction with the size of pine seedcrops that was similar in red and grey squirrels.Moreover, in conifers, grey squirrels had a higheradult mortality rate than did red squirrels andtheir densities were maintained by immigra-tion from nearby deciduous woodlands. Kenwardet al. therefore suggested that the grey squirrelmight be at a disadvantage in conifer woods whenunable to exploit nearby acorn crops (Kenward et al. 1998; also see Gurnell 1996a). They were,however, unable to compare directly the dynamicsand space use of both species within the samewoodland.

To test the effects of interspecific competitionbetween red and grey squirrels, we first describethe dynamics (adult local survival, recruitmentrates, female breeding success) and space andhabitat use of sympatric red and grey squirrels ina large conifer forest, Hamsterley Forest (KielderForest District), in northern England, which has ahigh degree of spatial and temporal heterogeneityin tree seed availability. In the past, red squirrels

272 L. A. Wauters et al.

occurred throughout the forest from the broad-leaved vegetation in the valley bottoms to theconifer plantations on the hilltops. However, sincethe arrival of grey squirrels in the early 1980s, therange of red squirrels has contracted with theincrease of the congener. Now, they are only foundin low numbers in the conifer plantations higherup on the hillsides.

We then compare the red squirrel data withthose gathered in a control site, Spadeadam Forest(Kielder Forest District), where only red squirrelsoccur, and investigate possible changes in redsquirrel population dynamics and space use causedby interspecific competition with grey squirrels.

METHODS

Study areas

We studied squirrels in two large conifer forests innorthern England: Spadeadam Forest in KielderForest, Cumbria (55°02¢N, 2°35¢W; red-only site,6500 ha); and Hamsterley Forest, County Durham(54°40¢N, 1°55¢W; red–grey site, 2000 ha). Thestudy site at Spadeadam Forest (red-only site) con-sists of mature conifer trees (> 30 years old), whichare capable of cone production; Sitka spruce Piceasitchensis Carr, Norway spruce Picea abies Karst. andlodgepole pine Pinus contorta Dougl. are the domi-nant species. Here, 38 traps were placed at a heightof 1.5 m against tree trunks in two different grids,covering a total of 41 ha (0.9 traps ha-1).

Hamsterley Forest (red–grey site) is a mixedforest with deciduous trees in the valley bottomsand conifer plantations on the slopes higher up.The study area had a long but relatively narrowtrapping grid of 27 ha, where 22 traps (0.8 ha-1)were placed in two rows. The forest consisted ofsome beech, Fagus sylvatica L., a pure Scots pinestand (Pinus sylvestris L.), a mixed plantation ofSitka spruce with some Scots pine and a pure Sitkaspruce stand. On two sides, the study area was bordered by open fields and newly planted coniferplantations. In both study sites and for both squirrel species, an edge correction of the trap-ping area of 100 m (the study area extended 100 m beyond the area covered by the traps on allsides) was used to estimate densities (see alsoKenward et al. 1998).

Food supplies

Northern conifer forests are characterized bymarked interannual changes in cone-crop size, seasonal variation in food availability due to cone maturation and seed dispersing strategies,and spatial variation in cone abundance and seedquality among the different tree species (Grönwall1982; Andrén & Lemnell 1992; Lurz et al. 1995).Sitka spruce (introduced species), lodgepole pine(introduced), and Norway spruce (native species)were the tree species at Spadeadam Forest (red-onlysite) and Sitka spruce (introduced), Scots pine andbeech (both native species) at Hamsterley Forest(red–grey site; see Table 1). Scots and lodgepolepine seeds are preferred food sources, Sitka spruceseeds are less palatable and very small, whereasNorway spruce seeds are likely to be intermedi-ately attractive as a food source (Wauters &Dhondt 1987; Wauters & Dhondt 1992).

We estimated the cone crops of Sitka spruce andNorway spruce in Spadeadam Forest (red-only site)by counting the fallen cones on cone-transect lines(Lurz 1995). Because average Sitka spruce crops arevery similar throughout the Kielder Forest District(Forestry Commission, unpubl. data, 1997), weused the Sitka spruce cone counts of the red-onlysite as Sitka spruce cone production estimates forthe Hamsterley Forest experimental site (red–greysite). In 1996 and 1997, the lodgepole pine andNorway spruce cone crops at Spadeadam Forestand the Scots pine cone crops and beech seeds cropsat Hamsterley Forest were scored on a scale from1 (very poor crop) to 4 (good crop).

Trapping and handling squirrels

We trapped squirrels in the spring and earlyautumn of 1997 and 1998 to monitor femalebreeding success and the main dispersal periods(Wauters & Dhondt 1993, 1995; Lurz et al. 1997).Additional data were available for the red-only sitefor 1993–1995 (Lurz 1995). Traps were prebaitedfor 5 days and set for 5–7 days. Detailed descrip-tions of the trapping methods are given elsewhere(Wauters & Dhondt 1993; Lurz 1995). Eachtrapped squirrel was individually marked, usingnumbered metal ear-tags (type 1003 S NationalBand and Tag Co., Newport, Kentucky, USA).Squirrels were weighed to the nearest 5 g by usinga Pesola spring balance (British Trust for Ornithol-ogy, Thetford, UK) and the length of the righthind-foot (without the nail) was measured (to thenearest 0.5 mm) with a thin ruler. The sex and ageof the squirrel were determined. Subadult malesare between 4 and 10 months old, with abdomi-nal testes and a small scrotum. Subadult femalesare between 4 and 12 months old, have a very smallvulva and have still-invisible nipples. Adults aremore than 12 months old (Wauters & Dhondt1993; Wauters & Lens 1995).

In red squirrels, foot length and body mass arecorrelated (Wauters & Dhondt 1995). In order toseparate effects of structural size (foot length) fromeffects of body condition, we calculated a linear re-gression of log body mass on foot length using datafrom 30 adult red squirrels from both study sites(log body mass = 1.64(± 0.20) + 0.015(± 0.003)¥ foot length, R2 = 0.39, P = 0.0002). We then

Competition between red and grey squirrels 273

Table 1 Seed energetics as kJ cone-1 (seed-1 for broad-leaved trees), tree species composition (% of total tree-cover)and estimates of cone crops in kJ m-2 (cones m-2 in parentheses) in both study sites

Red-only site (Spadeadam Forest) Red–grey site (Hamsterley Forest)Seed energetics Tree Cone crop Tree Cone crop

Tree species (kJ cone-1) cover (%) 1996 1997 cover (%) 1996 1997

Sitka spruce 4.62 65 2.77 (0.6) 2.77 (0.6) 52 2.77 (0.6) 2.77 (0.6)Norway spruce 17.69 23 4.42 (0.25) 2.48 (0.14) –– –– ––Lodgepole pine 2.48 12 6.20 (2.5) 1.74 (0.7) –– –– ––Scots pine 2.76 –– –– –– 33 55.2 (20) 13.2 (4.8)Beech (3.61) –– –– –– 15 10.8 (3) 10.8 (3)Average kJ ha-1 35 611 25 797 212 764 74 164

Data are derived from: Smith (1968, 1981), Wauters et al. (1992; unpubl. data, 1998) and Lurz (1995).

used this regression model to calculate the resid-uals of body mass for all squirrels and the residualswere then used as a measure of body condition(Wauters & Dhondt 1995).

Female red squirrels are in estrus for approxi-mately 24 h per breeding-cycle, but the postestrusperiod (from estrus to parturition, 38–42 days) caneasily be recognized because the vulva remainsslightly swollen and has a longitudinal scar. Youngsquirrels of both species are weaned when they are 10–12 weeks old (Gurnell 1987; Wauters et al.1993). Thus, we scored a female’s reproductivestatus as: (i) anestrus (vulva small, no longitudinalopening) or estrus (or postestrus; vulva partly orstrongly swollen with longitudinal opening), earlyin the breeding season; (ii) lactating (nipples large,milk excretion can be stimulated) or not lactating(nipples small or invisible) later on (Wauters &Lens 1995; Wauters et al. 1995). For males, we scored testes position (1, abdominal; 2, part-scrotal; 3, scrotal) and testes size (ranging from 1,very small, to 5, very large).

All the resident adults on the site had beenmarked by May 1997 and adults immigrating intothe population after that time could be identified.However, as we were not able to distinguishbetween young animals (subadults, juveniles) thatwere born in the study areas and those that immi-grated from outside the study areas, we have usedthe term recruitment for both unmarked adultsand young animals. Juvenile recruitment rate isthe number of autumn juveniles recaptured thefollowing spring.

Radiotracking

We radiotracked both red and grey squirrels at thered–grey site. We used radio-collars with loopantennae and an adjustable neck size of between9.5 and 11.5 cm, weighing approximately 12 gand with a lifetime varying from 1 to 8 months.Two different types of collars were used, a TW-4transmitter (Biotrack, Wareham, UK) and a TXP-1 transmitter (Televilt, Lindesberg, Sweden). Wemonitored the radio-tagged squirrels by using aMariner M57 receiver (Mariner Radar, Lowestoft,UK) and a portable three-element Yagi antenna(Mariner Radar, Lowestoft, UK). We determinedthe locations of the squirrels by following the radiosignal until the squirrel was seen or pinpointed by

274 L. A. Wauters et al.

signal strength and direction (Wauters & Dhondt1992). Then we plotted each fix to the nearest 10 m ¥ 10 m on detailed maps (scale 1 : 2500) ofthe study site.

Habitat use analysis

There were four types or patches of habitat in thered–grey site: Sitka spruce/Scots pine mixture(SS/SP, 35% of available habitat), pure Sitka spruce(SS, 23%), pure Scots pine (SP, 28%) and beechwoodland (BE, 15%). We used compositionalanalysis to assess habitat preference (Aebischer et al. 1993). This takes the log-ratio differences ofused and available habitat proportions to deter-mine habitat-use preferences for each squirrelspecies using radio locations (Lurz & Garson1998). In the initial step, a Wilks’ lambda statis-tic is calculated and a statistically significant c2

value (P < 0.05, with d.f. = number of habitattypes – 1) indicates a non-random use of the avail-able habitat types (Aebischer et al. 1993; Lurz &Garson 1998). The second step produces a matrixof pairs of compared habitat types in which nega-tive values imply avoidance of the numeratorhabitat type by comparison with the denominatorhabitat type, while positive values imply selection.The statistical significance of these comparisonswas assessed by computing t-values (Aebischer et al. 1993). We also calculated a niche overlapindex for habitat type (Pi) for red and grey squir-rels in the red–grey site by using a measure of pro-portional overlap Pi = SI min Pik, Pil, where i is thehabitat type and k and l are the proportions withwhich the two squirrel species used that habitattype (Hurlbert 1978).

Space use analysis

We collected between 30 and 40 radio-locations(fixes) for each animal in each season for which it wasmonitored. We found that a plot of estimated homerange size against the number of squirrel locationsfor all animals reached an asymptote at approxi-mately 30 fixes, indicating that 30 fixes adequatelydescribed the home range area (see also Kenward1985; Wauters & Dhondt 1992). Drey locationswere used only once when determining ranges toavoid artificial clustering of fixes. We used theranges V program (Kenward & Hodder 1995) tocalculate for each squirrel:

1. The total home range size (100% of the point-fixes) using the minimum convex polygon(MCP) method (e.g. Wauters & Dhondt 1992).

2. Cluster analysis to produce 100% polygons andcore-area polygons for home ranges that weremultinuclear, that is, displayed a distributionof radio locations with two or more distinctareas (e.g. Wauters et al. 1994). The core-areaestimates were defined by determining theinflection point or marked decrease in the slopeof the utilization distribution (Harris et al.1990) and a value of 85% best described thecore area of red squirrels in the red-only site andof both squirrel species in the red–grey site.

3. Core-area overlap, expressed as the percentageof overlap of a squirrel’s core area with the corearea of all other squirrels (e.g. Wauters &Dhondt 1992).

Density estimates

We estimated the prebreeding densities (March) ofboth squirrel species by using the minimumnumber alive (MNA) at the study sites, which wascalculated by combining capture–recapture data,radiotracking and direct sightings. Because suc-cessful trapping allowed us to tag most of the non-juvenile squirrels in both study sites, we feelconfident that these estimates realistically repre-sent squirrel densities. We calculated postbreedingdensities (adults and locally born juveniles) byadding an estimate of the number of weaned juveniles (WJ) on the study sites to the MNAindex for adults. WJ = 1.5 ¥ number of success-fully lactating females, with 1.5 being a conserva-tive estimate of the mean number of weanedjuveniles per litter (Wauters et al. 1993, 1994a,b;Kenward et al. 1998).

RESULTS

Food availability estimates

The cone crop of lodgepole pine was poor (indexvalue 2) in 1996 and very poor (index value 1) in1997. These scores were then compared with conecounts on transect lines, which were carried outfrom 1992 to 1997 and extrapolated to cone den-sities of 2.5 cones m-2 in 1996 and 0.7 cones m-2 in

1997 (after Lurz 1995). The Norway spruce conecrop was very poor in 1996 (0.25 cones m-2) and1997 (0.14 cones m-2) and the Sitka spruce cropwas estimated as 0.6 cones m-2 in the red-only siteand taken as the same at the red–grey site. AtHamsterley Forest, seed production by beech wasvery poor in both years, corresponding to a meanseed fall of three seeds per m2 (after Wauters &Lens 1995). Scots pine crops were scored as averagein 1996 (index value 3) and poor (index value 2)in 1997. Based on cone counts under a Scots pinecanopy in Belgium (Wauters & Dhondt 1995) and in northern England (Kenward et al. 1998),these scores were extrapolated to a mean of 4.8 cones m-2 for the poor crop and 20 cones m-2

for the average crop. Overall, Hamsterley Forest(red–grey site) had more available food than didSpadeadam Forest (red-only site), mainly becauseof the presence of Scots pine (Table 1). This differ-ence in food availability will, at the most, make itmore difficult to demonstrate any effects of inter-specific competition. Therefore, at the red–greysite, estimates of parameters on the effects of thepresence of grey squirrels on red squirrels will beconservative and potentially more convincing.

Squirrel densities

The densities of red squirrels were similar in thered-only and red–grey sites throughout the studyperiod (Fig. 1). In contrast, grey squirrels in thered–grey site occurred at much higher densitiesthan did red squirrels.

Competition between red and grey squirrels 275

Fig. 1. Fluctuations in the density of red and greysquirrels in both study sites. (�), Red squirrels in red-only site; (�), red squirrels in the red–grey site;(�), grey squirrels in the red–grey site.

Demography of red squirrels: Comparisonbetween red-only and red–grey study sites

We tested whether the data in the red-only sitecould be combined over the two study periods(1992–1994 and 1997–1998; Table 2) by testingfor a year effect in each parameter by using a Fisherexact test. In the red-only site, there was no sig-nificant year effect on adult survival from springin one year to spring in the following year (malesP = 0.19, females P = 0.44), on the proportion oflactating females (breeding rate in Table 2: springlitters P = 0.52, summer litters P = 0.24) or on the proportions of recruits of different sex and age groups (contingency table G = 0.167, d.f. = 3, P = 0.98). Therefore, the data from the red-onlysite were pooled over all years to increase thesample size.

We tested adult survival for a site and a sexeffect and a site ¥ sex interaction by using a two-way G-test. G-values were adjusted for smallsample sizes using Williams’ correction (Sokal &Rohlf 1981). There was no significant interactionand no significant site effect (Table 2; two-way G-test: site ¥ sex G = 1.02, d.f. = 1, P = 0.31; site

276 L. A. Wauters et al.

G = 0.22, d.f. = 1, P = 0.65), but there was a sig-nificant sex effect (two-way G-test: sex G = 4.70,d.f. = 1, P = 0.028) with adult females having ahigher local annual survival than adult males inthe red-only site (Table 2).

The effects of study site and season on breedingrate were also compared using a two-way G-test.There was no significant site ¥ season interaction(G = 0.41, d.f. = 1, P = 0.53) and no significant siteeffect (Table 2; G = 0.33, d.f. = 1, P = 0.57). Rela-tively more adult red squirrel females were lactat-ing in summer than in spring (mainly for theperiod 1992–1994; Table 2), but the season effect was statistically not significant (G = 1.761, d.f. = 1, P = 0.18). The proportion of lactating redsquirrel females did not decrease strongly in thepresence of grey squirrels.

As described in the methods section, we esti-mate that at least six offspring were weaned in thered-only site between summer 1997 and spring1998 and eight in the red–grey site. In total, threesubadults were trapped between autumn 1997 andautumn 1998 in the red-only site, but only one inthe red–grey site, which extrapolates to juvenilerecruitment rates of 50% in the red-only site and

Table 2 Demographic parameters of red squirrels in Spadeadam (red-only site) and Hamsterley (red–grey site)forests, and of grey squirrels in Hamsterley Forest (red–grey site)

Red-only site Red–grey site Parameter Year Category Red squirrel Red squirrel Grey squirrel

Adult survival 1992–1994 Male 5/23 (22%)Female 8/14 (57%)

1997–1998 Male 3/6 (50%) 3/6 (50%) 5/10 (50%)Female 5/7 (71%) 2/4 (50%) 8/12 (67%)

Breeding rate 1992–1994 Spring 13/29 (45%)Summer 21/31 (68%)

1997–1998 Spring 3/8 (38%) 4/9 (44%) 16/23 (70%)Summer 3/9 (33%) 4/9 (44%) 11/21 (52%)

Recruitment rate 1992–1994 Subadult male 7 (18%)Adult male 17 (45%)Subadult female 6 (15%)Adult female 8 (22%)

1997–1998 Subadult male 2 (22%) 1 (20%) 7 (35%)Adult male 4 (45%) 1 (20%) 6 (30%)Subadult female 1 (11%) 0 4 (20%)Adult female 2 (22%) 3 (60%) 3 (15%)

For the red-only site, the data are presented for the summer 1992–summer 1994 period (Lurz 1995) and for the spring1997–summer 1998 period. The percentage data are calculated as the number of recruits of each category relative to the totalnumber of recruits. Adult recruits are immigrants, subadult recruits include both immigrants and those born on the study sites.

only 13% in the red–grey site. Hence, red squir-rel juvenile recruitment seemed much reduced inthe presence of grey squirrels.

Red squirrel recruitment rates (Table 2) werecompared in the two sites by using a hierarchicallog-linear model (G-values with Williams’ correc-tion) testing for the effects of site, sex, age, all two-way interactions and the three-way interaction.The three-way site ¥ sex ¥ age interaction was closeto significance (G = 3.64, d.f. = 1, P = 0.053), withproportionally more males than females amongrecruits in the red-only site, but no differencebetween the sexes in the red–grey site (Table 2).Therefore, we calculated two-way G-tests for malesand females separately, investigating site and ageeffects. In males, there was no significant site ¥ ageinteraction (G = 0.65, d.f. = 1, P = 0.58), but a sig-nificant age effect (G = 9.09, d.f. = 1, P = 0.0027):the majority of male recruits were immigratingadults (Table 2). In females, a significant site ¥ ageinteraction (G = 5.45, d.f. = 1, P = 0.019) indi-cated a difference in age-composition between thetwo sites: in the red-only site nearly equal numbersof subadult and adult females moved in, while in the red–grey site only adult female recruits(immigrants) were caught (Table 2). These resultssuggest that recruitment patterns of red squirrelschanged in the presence of grey squirrels.

Demographic parameters of sympatric redand grey squirrels

We also compared the demographic parameters ofthe two squirrel species in the red–grey site (Table2). Adult survival rates from spring one year tospring in the next year were similar for bothspecies, irrespective of sex (two-way G-test: species¥ sex interaction G = 0.20, d.f. = 1, P = 0.66; sexeffect G1 = 0.43, P = 0.52; species effect G = 0.23,d.f. = 1, P = 0.64). Slightly larger proportions ofgrey squirrel females than red squirrels were foundlactating, especially in spring, but these differ-ences were not significant (two-way G-test: species¥ season interaction, G = 0.41, d.f. = 1, P = 0.53;season effect, G = 0.96, d.f. = 1, P = 0.37; specieseffect, G = 1.48, d.f. = 1, P = 0.22). However, therewere no female red squirrels with two litters peryear (one female with no litter, eight with onelitter), but eight of 20 (40%) grey squirrel femalesproduced a second litter within the same year

(Fisher Exact test: P = 0.029), a group that mainlycomprised those occupying the Scots pine stand.Among the remaining grey squirrel females, threehad no litters and nine produced one litter per year.Moreover, an estimated juvenile recruitment rateof 41% between autumn 1997 and spring 1998(11 subadult grey squirrels recruited of a total of 27 weaned juveniles) for grey squirrels washigher than the 13% estimated for red squirrels.However, the higher rate for greys is probably due to the immigration of young born outside the study area in the broad-leaved woodlands inthe valley.

Habitat use

A total of eight red squirrels (three males, fivefemales) and 11 grey squirrels (five males, sixfemales) were radiotracked at the red–grey siteduring a 40-day period in May–June 1998 or inSeptember 1998. At the red–grey site, there weremarked differences in the habitat use of both squir-rel species. Of all red squirrel radio locations, 18%were in mixed SS/SP stands and 82% in pure SSstands. The pure SP stand and the beech woodlandwere not used at all. Radiotracked grey squirrelswere rarely found in the beech woodland (3% ofall radio locations), but intensively foraged in thepure SP stand (21%). They also regularly occurredin the mixed SS/SP (16%) and in pure SS (58%).Based on these proportions of habitat used by thetwo squirrel species, the niche overlap was high (Pi = 0.75).

The habitat preferences of both species wereinvestigated using compositional analysis. For greysquirrels, this index did not reflect the true habitatpreference, because radio-tagged grey squirrelswere not randomly marked over the whole studysite. We selected animals whose home ranges couldpotentially overlap with the home ranges of the redsquirrels. For red squirrels, the index was morereliable because all adult red squirrels that sur-vived at least 6 months on the study site had beenmonitored. For both species combined, habitat usewas not random and the same was true for greysquirrels and red squirrels taken individually(Wilk’s lambda test both species: c2 = 67.4, d.f. =3, P < 0.001; red squirrels only, c2 = 33.3, d.f. = 3,P < 0.001; grey squirrels only, c2 = 36.6, d.f. = 3, P < 0.001).

Competition between red and grey squirrels 277

Red squirrels showed a significant preferencefor pure SS, and the SS/SP mixed stands were alsoused more than expected (Table 3). The pure SPand the beech woodland were avoided (order ofpreference: Sitka spruce > Sitka/Scots pine > Scotspine = beech; where > corresponds to P < 0.001;Table 3). Also, grey squirrels were observed morethan expected in the pure SS stands. The pure SPstand was preferred over the mixed SS/SP and overthe beech woodland, but these differences were sta-tistically not significant (order of preference: Sitkaspruce >> Scots pine = Sitka/Scots pine > beech;>> corresponds to P < 0.01, > to 0.1 > P > 0.05,Table 3).

Space use of red and grey squirrels at thered–grey site

At the red–grey site, individual variations in thehome range size of both red and grey squirrels werevery large and differences in the mean home rangesize were not significant (Fig. 2; two-way anova:species effect, F = 0.23, d.f. = 1,15, P = 0.64; sexeffect, F = 3.39, d.f. = 1,15, P = 0.086, species ¥sex interaction, F = 1.75, d.f. = 1,15, P = 0.21).Male grey squirrels tended to use larger homeranges than male red squirrels, while for femalesthe opposite was true and males of both speciestended to have larger home ranges than did females

278 L. A. Wauters et al.

(Fig. 2). Grey squirrel males used larger core areasthan did grey squirrel females, but in red squirrelsboth sexes had core areas of similar sizes (two-wayanova: species ¥ sex interaction, F = 2.15, d.f. =1,15, P = 0.16; sex effect, F = 2.17, d.f. = 1,15, P = 0.16). There was no significant difference

Table 3 Log-ratio differences (mean ± SE) between utilized (radio locations for each squirrel) and available (totalstudy site) habitat compositions for red and grey squirrels at Hamsterley Forest (red–grey site)

Habitat types (denominator)Sitka spruce/

Habitat types (numerator) Scots pine Sitka spruce Scots pine Beech Rank

Red squirrels (n = 8)Sitka/Scots pine –– - 2.15 ± 0.33*** 4.76 ± 0.28*** 4.18 ± 0.28*** 2Sitka spruce 2.15 ± 0.33*** — 6.90 ± 0.06*** 6.32 ± 0.06*** 3Scots pine - 4.76 ± 0.28*** - 6.90 ± 0.06*** — Not computed 1Beech - 4.18 ± 0.28*** - 6.32 ± 0.06*** Not computed — 1

Grey squirrels (n = 11)Sitka/Scots pine –– - 3.68 ± 0.85** - 0.34 ± 1.77 1.42 ± 1.30 1Sitka spruce 3.68 ± 0.85** — 3.33 ± 0.99** 5.09 ± 0.65*** 3Scots pine 0.34 ± 1.77 - 3.33 ± 0.99** — 1.76 ± 0.85* 2Beech - 1.42 ± 1.30 - 5.09 ± 0.65*** 1.76 ± 0.85* — 0

*0.1 > P > 0.05; **P < 0.01; ***P < 0.001. Negative values imply avoidance of the numerator habitat type by comparisonwith the denominator habitat type; positive values imply selection. Habitat types are ranked in order of preference (rank 1, leastpreferred; rank 3, most preferred).

Fig. 2. (a) Individual home range size; (b) individual85% core-area size of adult red and grey squirrelsaccording to sex in the red–grey site. (�), Individual;(�), mean.

between the two species (species effect, F = 0.91,d.f. = 1,15, P = 0.36).

The core area overlap of red squirrels of one sexwith red squirrels of the opposite sex (thus malesoverlapped by females, females overlapped bymales) was much higher (26.4 ± 20.0%, n = 8) thanwith conspecifics of the same sex (1.4 ± 1.5%, n = 8; Wilcoxon matched-pairs signed-ranks test:P = 0.018, two-tailed) and the degree of intra-sexual exclusive core-area use was very stronglypronounced in adult female red squirrels. Com-paring intraspecific overlap with squirrels of thesame sex (thus male red squirrels with other males,female red squirrels with other females) againstinterspecific core-area overlap between red andgrey squirrels of the same sex, we found that redsquirrels were avoiding core-area overlap with con-specifics (mean ± SD, core-area overlap 1.4 ± 0.5%,n = 8) much more than core-area overlap with greysquirrels (mean ± SD, core-area overlap 38.0 ±31.1%, n = 8, Wilcoxon test, P = 0.025, two-tail).Hence, intraspecific (and intrasexual) competitionbetween adult red squirrels was much more im-portant in determining red squirrel spacing be-havior at the red–grey site than was interspecificcompetition with adult grey squirrels.

Space use of red squirrels: Comparisonbetween red-only and red–grey study sites

We compared, for each sex separately, the meanhome range size and the mean 85% core-area size of red squirrels in the red-only and red–greysites (mean home range ± SD; red-only site males,n = 7, 18.0 ± 13.9 ha; red-only site females, n = 13,8.7 ± 5.8 ha; red–grey site males 22.6 ± 10.3 ha;red–grey site females 17.7 ± 14.5 ha; core-area size:red-only site males, 3.40 ± 1.59 ha; red-only sitefemales 2.84 ± 2.16 ha; red–grey site males 3.63 ±0.73 ha; red–grey site females 3.62 ± 2.08 ha). Thesize of both the total home range and the core area of adult males was very similar in both studysites (home range t-test t = 0.509 (two-tail), d.f. =8, P = 0.62; core area t = 0.238, d.f. = 8, P = 0.82).Adult females tended to have larger home rangesin the mixed-species site (red–grey site, t = 1.94,d.f. = 16, P = 0.07), but this difference was mainlydue to one adult female (AF 228, MCP home rangesize 43.2 ha, 85% core estimate 13.4 ha) that had immigrated recently and was still showing

exploratory behavior as indicated by the large dif-ference in home range and 85% core estimates.The home ranges of the other females were com-parable in size to those found in the control site.There was no site difference in the mean core-areasize of adult red squirrel females (t = 0.693, d.f. =16, P = 0.50). Hence, red squirrels did not show amarked increase in home range (or core-area) sizein the presence of grey squirrels.

Body size and body mass of red squirrels

There was no sexual dimorphism in body size or body mass of adult red squirrels in either studysite and no interaction between sex and site (foot length : sex effect in a two-way anovaF = 0.05, d.f. = 1,26, P = 0.83, sex ¥ site inter-action, F = 0.68, d.f. = 1,26, P = 0.42; body mass :sex effect, F = 0.01, d.f. = 1,26, P = 0.94, sex ¥ siteinteraction, F = 0.04, d.f. = 1,26, P = 0.85). Adultred squirrels, however, were significantly larger inthe red-only site than in the red–grey site (meanfoot length ± SD, red-only site, n = 18, 57.2 ± 0.7mm; red–grey site, n = 12, 56.1 ± 1.2 mm, two-way anova site effect, F = 9.08, d.f. = 1,26, P = 0.0057). Moreover, body mass was higher at the red-only site than at the red–grey site,although this was not significant at the P = 0.05level (mean body mass ± SD, red-only site, n = 18,304 ± 15 g; red–grey site, n = 12, 294 ± 18 g, two-way anova site effect, F = 2.50, d.f. = 1,26, P = 0.13). However, when we tested for body con-dition, the site effect disappeared completely andthere was no difference in mean body conditionbetween study sites (mean condition index ± SD,red-only site: n = 18, – 0.001 ± 0.019; red–greysite, n = 12, 0.002 ± 0.021; t = 0.355, d.f. = 28, P = 0.73, two-tail). These results suggest that redsquirrels had a similar mass for their body size inboth sites, with red squirrels being physicallysmaller (shorter foot length) in sympatry with greysquirrels (red–grey site) than when they were theonly squirrel species present (red-only site).

DISCUSSION

Squirrel densities

Grey squirrels first moved into Hamsterley in theearly 1980s, but there have been no data about the

Competition between red and grey squirrels 279

changes in population densities of either speciesuntil the present study. Nevertheless, it is notablethat red squirrels have survived in the presence ofgreys for at least 15 years (see Gurnell & Pepper1993). Parapoxvirus infection in red squirrels mayplay a part in the replacement of red by grey squir-rels in Britain (Sainsbury et al. 1997), but therewere no signs of the virus in the red or grey squir-rels at this site (P. Nettleton, unpubl. data, 1999).

Overall, red squirrel densities were similar inHamsterley (prebreeding density 0.26 ha-1, post-breeding density 0.33–0.37 ha-1) and Spadeadam(prebreeding density 0.29 ha-1, postbreedingdensity 0.34–0.46 ha-1) forests, despite seed foodbeing more abundant in the former. However, red squirrels were not present in the Scots pineplantation in Hamsterley Forest or in the beechwoodland and their density within the rest of thestudy site was correspondingly higher (prebreed-ing density 0.46 ha-1, postbreeding density 0.52–0.65 ha-1), although this was still lower than thedensity of grey squirrels (prebreeding density0.92–1.11 ha-1, postbreeding density 1.04–1.75ha-1). Red squirrel abundance in these coniferplantations has been positively linked to the presence of pine and the availability of conifer cone crops (Lurz et al. 1995) and, thus, our resultssuggest that the presence of grey squirrels ishaving a negative effect on red squirrel density inHamsterley Forest. An obvious test of this will beto remove grey squirrels at some time in thefuture.

Grey squirrel densities in Hamsterley Forestwere broadly comparable to estimates from otherconifer forests in which grey squirrels also hadaccess to oak and other broad-leaved species (Lurzet al. 1995; Cartmel 1997; Smith & Gurnell 1997;Kenward et al. 1998). They were, however, con-siderably lower than densities observed in pre-dominantly broad-leaved woodlands (Gurnell1987; 1996b; Kenward et al. 1998).

Survival, recruitment and body size

The replacement of red by grey squirrels is oftenconsidered a consequence of grey squirrels havinga competitive advantage over red squirrels anddepleting the availability of important tree seeds(resource competition, Gurnell & Pepper 1993;Skelcher 1997). If this is the case, we predict that

280 L. A. Wauters et al.

red squirrels will have reduced survival and/orbreeding success and a lower body mass in thepresence of grey squirrels, because of their in-ability to sufficiently exploit high-quality foodresources. If interspecific competition acts primar-ily on juvenile red squirrels, we expect reducedrecruitment rates and possible changes in the agecomposition of immigrants; increased juvenilemortality will result in fewer subadult recruits inthe presence of grey squirrels compared to the red-only study sites.

We found, however, no effect of study site onthe survival rate of adult red squirrels of either sex,indicating that red squirrel survival did notdecrease in the presence of grey squirrels. More-over, the breeding rate of female red squirrels inHamsterley Forest (44% of females lactating perbreeding season) was not significantly reduced bythe presence of grey squirrels when compared tobreeding rates in Spadeadam Forest (ranging from30 to 68% of females lactating). Thus, there wasno evidence that grey squirrel males interruptedred squirrel mating chases or that grey squirrelskilled red squirrel neonates in the nest (i.e. oneform of interference competition, see Wauters &Gurnell, 1999).

The observed changes in the pattern of recruit-ment of red squirrels at the red–grey site comparedto the red-only site revealed that there was littlerecruitment of subadult red squirrels when greysquirrels were present. Taking into account theproportion of adult females weaning offspring(44%; Table 2), this indicated low juvenile redsquirrel recruitment rates. The two adult redsquirrel females that immigrated to the red–greysite in autumn (see Table 2) had bred before andthe spring immigrant produced a litter on thestudy site. Thus, it is possible that most femaledispersal was breeding dispersal (also Lurz et al.1997). In the red-only site, we know that breed-ing dispersal occurred regularly (Lurz et al. 1997),but there was still a large number of subadultimmigrants each year, suggesting much higherjuvenile recruitment rates than in the red–grey site.

Red squirrels were smaller (according to thelength of the right hind foot) in the red–grey studysite than in the red-only study site, but site dif-ferences in the body mass of adults were not significant. Moreover, the observed variation in

log-transformed body mass among individualsquirrels was mainly explained by variation in footlength (body size) and there was no site effect onbody condition. Hence, our results on recruitmentpatterns and body size differences suggest thatinterspecific competition acts primarily during thegrowth phase of the red squirrels (hence juvenileand subadult stages), causing reduced growth and hence smaller adult size for red squirrels insympatry with greys. The results also suggest thatresource competition between resident adults ofboth species has little additional effect on the condition of adult red squirrels.

Habitat preference and use of space

Interspecific competition between closely relatedspecies often results in niche separation (reducedniche overlap). If grey squirrels are enjoying a com-petitive advantage over red squirrels, we predictthat:

1. The latter will be forced to select habitatpatches of poorer resource availability orquality when in the presence of grey squirrelsand, therefore, patch choice will differ fromthat in the control site.

2. Red squirrels need to increase the size of theirhome range (and/or core area) to find sufficientfood resources in the presence of greys.

3. Red squirrels will shift their home ranges toavoid, or at least limit, interspecific core areaoverlap.

Both species showed a preference for pure Sitkaspruce stands. Consequently, the estimated niche-overlap was high (75%). In grey squirrels thisresult may be partly explained by a non-randomchoice of radiotracked animals (only grey squirrelsthat potentially overlapped with red squirrels weremonitored), but this was not so for red squirrels(all red squirrels that survived for 6 months weremonitored). Sitka spruce was avoided by red squir-rels in the control site when other food resourceswere available, and pine habitat was preferred(Lurz & Garson 1998; Lurz et al., in press). Bothtrapping and radiotracking indicated that greysquirrels at Hamsterley Forest used the Scots pinewoodland intensively but that radio-tagged redsquirrels were never observed in the Scots pine, norwere any animals trapped there. Hence, despite the

fact that Scots pine seeds were the highest qualityfood resource available to squirrels at HamsterleyForest, the red squirrels did not exploit them. Thisresult agrees with our first prediction. The lack ofred squirrels in the pure Scots pine could be causedby red squirrel avoidance of the areas intensivelyused by the grey squirrels. The greater preferenceof both squirrel species for pure Sitka spruce standssuggests a relaxation of competition in these habi-tats in relation to Sitka spruce–Scots pine mix-tures, although this might be due to the timing of the radiotracking, which mainly occurred inspring and early autumn. In spring, spruce buds,flower buds and young spruce shoots are importantred squirrel food in many conifer habitats (e.g.Grönwall 1982; Moller 1983; Wauters, unpubl.data, 1998). In early autumn, the cones of Sitkaspruce are still closed and constitute an energy-richfood supply. Red squirrels also preferred Sitkaspruce stands with a good cone crop in the controlsite in autumn–winter (Lurz & Garson 1998; Lurzet al., in press).

At Hamsterley Forest, individual variation inhome range size of both red and grey squirrels wasvery large and differences in mean home range size between the two species were not significant.In agreement with other studies, males of bothspecies tended to have larger home ranges than didfemales (Kenward 1985; Wauters & Dhondt 1992;Wauters et al. 1994a; Cartmel 1997; Magris et al.1997). Male grey squirrels tended to use largerhome ranges than did male red squirrels, but theopposite was true for females (see also Cartmel1997). In contrast to most other studies in bothconifer and broad-leaved woods, where male redsquirrels used larger core areas than females(Wauters & Dhondt 1992; Wauters et al. 1994a;Magris et al. 1997), there were no significant dif-ferences in the size of the core area between thesexes at Hamsterley Forest. Typically, in the habi-tats where the two species co-occurred, intersexualcore-area overlap (males overlapped by females,females overlapped by males) was much higherthan the core-area overlap with conspecifics of the same sex and the degree of intrasexual exclu-sive core-area use was very strongly pronounced in adult female red squirrels (see also Wauters &Dhondt 1992; Wauters 1997; Lurz et al., in press).Interspecific overlap of grey squirrel core areaswith those of red squirrels was underestimated

Competition between red and grey squirrels 281

because only 11 of 25 (44%) grey squirrels presentin the 1998 prebreeding population were radio-tracked. Nevertheless, the interspecific compari-sons showed that red squirrels were avoidingcore-area overlap with conspecifics much morethan core-area overlap with grey squirrels. There-fore, we conclude that it is possible that interspe-cific competition may have caused red squirrels toavoid the Scots pine woodland at the red–grey site,but throughout the rest of the site, intraspecific(and intrasexual) competition among adult redsquirrels was much more important in determin-ing red squirrel spacing behavior than was inter-specific competition with adult grey squirrels.

Moreover, the mean home range size and themean 85% core-area size of adult male and femalered squirrels (with the exception of one immigrantfemale at the red–grey site) were similar in the red-only and red–grey sites and within the rangetypically found for upland conifer forests in theUK (Lurz 1995; Cartmel 1997; Lurz et al., in press) Hence, red squirrels did not show a markedincrease in home range or core area size in the pres-ence of grey squirrels. Thus, although our data donot support our second prediction, they are con-founded by the differences in food availabilitybetween sites.

Conclusion

Our results regarding red squirrel survival, breed-ing success, body size and spacing behavior do notsupport predictions related to interspecific compe-tition between adult grey and red squirrels, withthe exception of the absence of red squirrels in thehigh-quality Scots pine stands at HamsterleyForest. However, our findings lend support tothose of Wauters and Gurnell (1999) who pro-posed that the key factor in the replacement of redby grey squirrels is reduced juvenile recruitmentin red squirrels in the presence of grey squirrels.Future research should concentrate on the juvenileand subadult survival of red squirrels and aim tocompare these parameters in mixed-species (redand grey squirrels present) and single-species (onlyred squirrels) study sites. Furthermore, we believethat it is important to investigate grey squirrelsurvival, reproduction and recruitment in verylarge conifer forests where grey squirrels do nothave access to large seeded broad-leaved species.

282 L. A. Wauters et al.

ACKNOWLEDGEMENTS

Forest Enterprise, Kielder Forest District and RAF Spadeadam kindly allowed us to work ontheir estates. We thank Bob Bradley, CorrieBruemmer, Bill Burlton, Emma Harding, MarkHerbertson, Malcolm McEwan, Helen Thompson,Tim Venning and Louise Young for assistance in the field. Robert Kenward (Institute of Terrestrial Ecology, Wareham, UK) provideduseful comments on the analysis of space-use patterns. This study was financed by a TMR grant (No. ERBFMBICT 960662) from the Commissionof the European Union to L. Wauters with Queen Mary and Westfield College as host insti-tution. This study received additional supportfrom Dr Steve Rushton, Centre for Land Use and Water Resources Research, University ofNewcastle.

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