Effects of winter food supplementation on reproduction, body mass ...

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Effects of winter food supplementation on reproduction, body mass, and numbers of small mammals in montane Australia Peter B. Banks and Chris R. Dickman

Abstract: We used a food-supplementation experiment to test the hypothesis that small-mammal populations are foodlimited during winter in southeastern Australia. We trapped small mammals along 120- to 150-m transects at 12 creek and 12 ridgetop sites (representing high- and low-quality habitats) for 2 months prior to winter and 2 months during winter. High-quality food (peanut butter, honey, oats, and dried cat food) was provided ad libitum for 7 weeks during winter at four sites in each habitat. Eight sites were provided with empty feeding tubes and eight were untreated. Seven weeks of food supplementation caused numerical increases of 4.0- and 5.0-fold for the rodents Rattus fuscipes and Rattus lutreolus, respectively. Increases were due largely to immigration, and were only observed in the highquality creek habitats (R. lutreolus were exclusively captured at creek sites). Food supplementation also led to an increase in body mass and reversed the hiatus in winter breeding for rodents. These results suggest that populations of both species are limited by winter food availability. However, survival rates (indexed from recapture rates) were not affected by food supplementation. Mean body mass of the marsupial Antechinus stuartii also increased with food supplementation, but other demographic parameters showed no response; numbers declined at creek sites after additional food was provided. Trapping-revealed measures of interspecific association showed that A. stuartii avoided areas of high rat numbers after additional food was provided. It is thus likely that interference competition from the much larger and more abundant rodents forced A. stuartii out of the food-supplemented creek sites, hence mediating the direct effects of food supplementation on this species. Résumé : Nous avons procédé à une expérience d’addition de nourriture pour éprouver l’hypothèse selon laquelle les populations de petits mammifères sont limitées de nourriture durant l’hiver dans le sud-est de l’Australie. Nous avons piégé les petits mammifères le long de transects de 120–150 m à 12 sites aux abords de cours d’eau et 12 sites le long de crêtes, (représentant des habitats riches et pauvres) pendant 2 mois avant l’hiver et 2 mois pendant l’hiver. De la nourriture de haute qualité (beurre d’arachides, miel, flocons d’avoine et moulée pour chats) a été donnée ad libitum pendant 7 semaines au cours de l’hiver à quatre sites dans chaque habitat. Des tubes d’alimentation vides ont été installés à huit sites et les huit autres sites ont servi de témoins. Après 7 semaines d’addition de nourriture, le nombre des rongeurs Rattus fuscipes était quatre fois plus élevé et le nombre de R. lutreolus cinq fois plus élevé. Ces augmentations étaient dues en grande partie à l’immigration et n’ont été observées que dans les habitats de haute qualité, près de ruisseaux (R. lutreolus a été capturé exclusivement aux sites placés aux abords de cours d’eau). L’addition de nourriture a également fait augmenter la masse corporelle et annulé l’hiatus dans la reproduction des rongeurs au cours de l’hiver. Ces résultats nous permettent de croire que les populations des deux espèces sont régies par la disponibilité de la nourriture en hiver. Cependant, la survie (déterminée d’après les taux de recapture) n’était pas affectée par l’ajout de nourriture. La masse moyenne a augmenté aussi chez le marsupial Antechinus stuartii, mais d’autres paramètres démographiques n’ont pas été affectés par le supplément alimentaire; les nombres ont même diminué dans les sites aux abords de cours d’eau après l’addition de nourriture. La mesure des associations spécifiques révélées par le piégeage a montré qu’A. stuartii avait tendance à éviter les zones où le nombre de rats avait augmenté après l’addition de nourriture. Il semble donc que la compétition par interférence avec les rats, beaucoup plus gros, beaucoup plus abondants, force les A. stuartii à quitter les abords des cours d’eau où il y a addition de nourriture, modifiant ainsi les effets directs de l’addition de nourriture pour cette espèce. [Traduit par la Rédaction]

Introduction

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no regular and predictable fluctuations in density. For speBanks and Dickman

Annual or multi-annual fluctuations in density are characteristic of many small-mammal populations; few species show

cies that exhibit annual fluctuations, populations often have a decline and low phase during late autumn and winter and an increase phase during spring and summer. During the

Received January 3, 2000. Accepted May 30, 2000. P.B. Banks.1 Institute of Wildlife Research, School of Biological Sciences, A08, University of Sydney, N.S.W. 2006, Australia, and School of Biological Sciences, The University of New South Wales, Kensington, NSW 2052, Australia. C.R. Dickman. Institute of Wildlife Research, School of Biological Sciences, A08, University of Sydney, NSW 2006, Australia. 1

Author to whom all correspondence should be sent at the following address: School of Biological Sciences, The University of New South Wales, Kensington, N.S.W. 2052, Australia (e-mail: [email protected]).

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winter low-density phase, reproductive activities typically cease as individuals set up over-winter ranges, lose body mass, and have lower survival rates than in summer months. The postwinter population provides the breeding foundation for the next generation (Lack 1954), and for many species there is a correlation between the winter low density and the subsequent summer peak density (Gilbert and Krebs 1991; Schweiger and Boutin 1995). Consequently, understanding the causes of the low-density phase is often the key to understanding the overall population dynamics of small mammals (Boonstra et al. 1998). A decline in per capita resource availability, especially food, is most commonly hypothesized to be the cause of winter low densities of small mammals (Boutin 1990). Intraspecific competition for food intensifies as resources decline, owing to the progressive onset of unfavourable climatic conditions. The consequent decreases in individual body mass, cessation of breeding, and lower survival rates all contribute to population declines (Boutin 1990). While disease, predation, and intrinsic factors may interact with resource availability to influence demography and density during winter, winter food declines, in particular, are thought to elicit consistent responses in a range of small-mammal species in many environments (see the review by Boutin 1990). However, whereas Boutin (1990) found good experimental support for the general hypothesis that food is a limiting resource for small mammals, specific tests of the winter food limitation hypothesis remain scant (Schweiger and Boutin 1995). Moreover, the general conclusions that could be drawn from many food-supplementation experiments are limited by low replication, the studies typically being restricted to one species in one habitat (Boutin 1990). Also, few studies report information on the behaviour and interaction between individuals and species in response to resource supplementation (Boutin 1990). In this paper we report the results of a replicated foodsupplementation experiment testing the winter food limitation hypothesis for the small-mammal assemblage in two habitats in southeastern Australia. We also investigate the effects of food supplementation on interactions among species within the assemblage. The species that characterize this assemblage are two rodents, the bush rat, Rattus fuscipes, and the swamp rat, Rattus lutreolus, and a marsupial, the brown antechinus, Antechinus stuartii, which are common throughout forested environments in southeastern Australia. All three species exhibit annual fluctuations in density and demography characteristic of winter food limitation. In both species of rats, for example, summer and autumn densities exceed those in winter and early spring 2- to 5fold, the winter low densities being more extreme in dry woodland than in moist heath, rainforest, or riparian habitats (Wood 1971; Press 1986; Happold 1989; Monamy 1995). At high altitudes the winter hiatus in breeding is prolonged in both species and loss of body mass is marked; in R. fuscipes the over-winter survival rate can be as low as 27% compared with usually >50% in coastal localities (Woodside 1983; Happold 1989; Monamy 1995). The demography of A. stuartii is similar to that of the two Rattus species in that numbers and body masses decline over winter and these phenomena are magnified in poor-quality dry habitats (Wood 1970; Statham and Harden 1982; Watt 1997). However, the low survival

Can. J. Zool. Vol. 78, 2000

rate in A. stuartii is associated primarily with the short, synchronous period of breeding in late winter, after which all males die. It is suspected that food limitation drives demographic changes over winter in each species (Warneke 1971; Norton 1987; Dickman 1989); in R. fuscipes physiological stress caused by poor nutrition (Stewart and Barnett 1981, 1983; Robinson 1988; White et al. 1996) may also be responsible for the winter hiatus in breeding. We conducted our food-supplementation experiment in protected, dense creek vegetation and exposed, open ridge vegetation. These represent high- and low-quality habitats, respectively, for the small mammals (Statham and Harden 1982; Taylor and Calaby 1988a, 1988b; Bennett 1993), and hence differing degrees of likely winter food limitation (Lima and Jaksic 1999). We also provide a procedural control for the possible effect of augmenting the habitat by using feeding tubes which, once empty, may provide shelter or refuge for these hollow-dependent species (Dickman 1991). If scarcity of winter food causes the demographic symptoms of the annual low-density phase of these small mammals, addition of food during winter should cause local population increases and produce higher body mass, higher winter survival rates, and early recommencement of breeding.

Materials and methods Study sites Twenty-four sites were established in an area of the upper Blue Mountains National Park near Blackheath, 130 km west of Sydney, Australia, at an altitude of 1100 m. In this area, seasonal temperature differences are pronounced, with warm summers (January mean maximum temperature 29°C) and cold winters (July mean minimum temperature 0°C). Twelve sites were chosen in each of two distinct habitat types, moist creek vegetation and dry ridgetop vegetation. Creek sites were situated in shallow valleys along running water courses where the sedge Gymnoschoenus sphaerocephalus and coral fern, Gleichenia dicarpa, formed a very dense ground cover. Established trees were characteristically scarce, but Hakea teretifolia and Grevillea acanthifolia shrubs up to 2.5 m tall were common. The 12 ridgetop sites skirted deeper valleys and were dominated by dry woodland. The major tree species, Eucalyptus sclerophylla, E. mannifera, and E. stricta, formed a sparse canopy layer, while Banksia serrata, B. ericifolia, and H. teretifolia formed a denser shrub layer. Ground cover was typically sparse, consisting of scattered grasses and fallen debris. All sites were >1 km apart and no movements of marked animals between sites were recorded.

Study animals Rattus fuscipes is a small omnivorous rodent (�150 g) common in a variety of forest habitats along the east and west coasts of Australia. Fungi are the major dietary item in winter, while insects and grasses are eaten in the warmer months (Carron et al. 1990). Individuals live for up to 15 months, reaching maturity after only 7 weeks. Breeding occurs in all months except winter (May–July) for populations around Sydney (Press 1976), but in mountain environments the winter hiatus in breeding extends into September and October (Stewart 1979). Litter size varies between three and five (Taylor and Horner 1973) and females can produce up to three litters in 93 days (Taylor and Horner 1971). Movements of both males and females are extensive, and associated with dietary opportunism (Braithwaite and Lee 1979); mating is promiscuous. Rattus lutreolus (�180 g), also common along the east coast of Australia, is restricted to moister habitats such as heathland and swamps, where densities can reach 15 per hectare (Lunney 1983; © 2000 NRC Canada



Banks and Dickman Monamy 1995). Grasses and sedges, which are dependent on rainfall for growth (Robinson 1988), form the staple diet (Watts and Braithwaite 1978; Cheal 1987). Breeding typically occurs from spring to autumn; females may produce several litters per season and the young reach sexual maturity at 3 months of age (Braithwaite 1980). Three to five young are born after 3 weeks’ gestation and leave the nest weighing 25–40 g within 4 weeks of weaning. Mating is less promiscuous than for R. fuscipes, and this may relate to the dietary specialization of the swamp rat, which is restricted to patches of high-density and high-quality food (Braithwaite and Lee 1979). Antechinus stuartii is a small (30–50 g) insectivorous dasyurid marsupial abundant in a variety of habitats along the east coast of Australia. It is sexually dimorphic, with males weighing up to twice as much as females, and has a precisely timed reproductive cycle (Braithwaite and Lee 1979; Dickman 1982). Females are monoestrous and mate during a 2- to 3-week period in late winter. Females may live to breed in a second or third season (Cockburn et al. 1985), but each generation is separated by 1 year. All males, however, are semelparous and die of stress-related disease following their first, frantic mating season (Lee et al. 1977). Litter size is typically high and limited by the number of teats (6–10), which are almost always fully occupied (Cockburn et al. 1983). The young are born in late winter and remain attached to their mother for approximately 35 days (Wood 1970).

Trapping Line-trapping was used to provide an index of population size at each site; 10 trap stations spaced 12–15 m apart were used in suitable microhabitats (e.g., animal runways, fallen logs; Norton 1987; Read et al. 1989). Line-trapping was more suitable than grids in this area because the moist creek habitat we targeted typically occupied a narrow band, often less than 20–30 m wide. Each trap station was composed of at least one Elliott live trap (30 × 10 × 10 cm) baited with a mix of peanut butter, oats, and honey and provided with cotton wool for bedding and a plastic bag cover for shelter from rain. Multiple traps (up to four) at trap stations were used to minimize competition for traps, and trap success at a site did not exceed 50%. Traps were checked each morning for 4 consecutive days during a trapping session. Trapping was conducted in the 2 months before winter (April and May, prior to manipulations) and 2 months during winter (June and July–August, 3 and 7 weeks, respectively, after manipulations began). Traps were cleaned in dilute soapy water prior to each trapping session at each site. Rattus spp. were marked individually using tiny metal ear tags and A. stuartii were marked by ear-notching. Individuals were sexed and weighed to the nearest 0.5 g. Male rats were scored as having abdominal or scrotal testes (Warneke 1971); female rodents were classified as mated if the vagina was open, bruised, or bloody, as mature (previously parous) if they possessed large, obvious nipples, and as immature if the nipples were small or invisible (Press 1982). Rats were considered juvenile if they weighed 0.25). © 2000 NRC Canada



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Fig. 1. Numbers of adult Rattus fuscipes (mean ± SE) captured at creek (a) and ridgetop sites (b) during autumn–winter 1991. ×, sites with supplemental feeding; �, sites with empty feeding tubes; +, sites with no treatment. Food supplementation began in late May.

Fig. 2. Numbers of unmarked (immigrant) R. fuscipes (mean ± SE) captured at creek (a) and ridgetop sites (b) during autumn– winter 1991. ×, sites with supplemental feeding; �, sites with empty feeding tubes; +, sites with no treatment. Food supplementation began in late May.

Table 1. Mean monthly survival rates for Rattus fuscipes, R. lutreolus, and Antechinus stuartii (sexes combined) at control, tube control, and food-supplementation sites in creek and ridge habitats during autumn–winter 1991. Creek habitats Treatment R. fuscipes Control Tube control Feeding R. lutreolus Tube control Feeding A. stuartii Control Tube control Feeding

Ridgetop habitats

May

June

July–August

May

June

July–August

0.23 (0.15) 0.33 (0.12) 0.62 (0.09)

0.46 (0.07) 0.39 (0.18) 0.65 (0.12)

0.08 (0.14) 0.36 (0.15) 0.65 (0.18)

0.52 (0.23) 0.16 (0.16) 0.56 (0.23)

0.61 (0.20) 0.33 (0.12) 0.37 (0.06)

0.50 (0) 0.57 (0.32) 0.38 (0.05)

0.58 (0.21) 0.67 (0.33)

0.72 (0.32) 0.33 (0.16)

0.61 (0.24) 0.61 (0.05)

0.37 (0.23) 0.59 (0.17) 0.46 (0.22)

0.35 (0.24) 0.31 (0.33) 0.39 (0.21)

0.45 (0.20) 0.25 (0.15) 0.20 (0.14)

0.54 (0.21) 0.50 (0.20) 0.46 (0.08)

0.61 (0.21) 0.64 (0.06) 0.43 (0.15)

0.35 (0.10) 0.25 (0.20) 0.21 (0.21)

Note: Values in parentheses show the standard error.

Rattus lutreolus were only captured in feeding and tube control sites, and numbers were higher at tube control sites during the initial autumn trapping session (t6 = 3.02, P < 0.05). Seven weeks of food supplementation reversed this pattern: 5 times as many animals were captured at sites with additional food (t6 = 2.54, P < 0.05) (Fig. 3a). In the case of R. fuscipes, since feeding did not influence the rate of survival (recapture) of resident R. lutreolus (Table 1), immigration was the main cause of the observed population response to feeding, 3.5 and 9 times as many new animals being captured at feeding sites as in tube control sites after 3 (t6 = 1.97, P < 0.1) and 7 (t6 = 3.57, P < 0.05) weeks of food supplementation, respectively (Fig. 3b).

In contrast to the rodents, the A. stuartii population showed no response to winter food supplementation. Prior to feeding, within-treatment variation in A. stuartii numbers was high, ranging from 0 to 10 individuals captured per site, and food supplementation did not alter this pattern. After 7 weeks of feeding, numbers of A. stuartii at creek sites declined sharply, while numbers at ridgetop sites remained unchanged (Fig. 4); however, a habitat × treatment interaction was not significant (F[1,20] = 2.14, P = 0.16). Immigration was generally low for A. stuartii except at high-density ridgetop sites (Fig. 5), where after 7 weeks of feeding, only 4% of individuals were recaptures compared with 20–54% elsewhere (a significant habitat × treatment interaction, F[1,20] = 4.82, P = © 2000 NRC Canada



Banks and Dickman Fig. 3. Numbers of adult (a) and unmarked R. lutreolus (b) (mean ± SE) captured during autumn–winter 1991. ×, sites with supplemental feeding; �, sites with empty feeding tubes. Food supplementation began in late May.

0.04). However, the mean monthly survival rate for resident individuals was not affected by winter food supplementation (Table 1). Body mass and reproduction Food supplementation had a large and consistent impact on mean adult body masses of the three species. Body masses of each species did not differ between treatments prior to manipulations. However, male R. fuscipes at feeding sites were 22% heavier than males at nonfeeding sites after 3 weeks of feeding (F[2,20] = 4.15, P = 0.03) and 48% heavier after 7 weeks of feeding (F[2,20] = 8.77, P < 0.01) (Fig. 6). Body masses of mature and immature females showed similar increases after 7 weeks of food supplementation, but treatment effects were not significant. There were no habitat effects. Resident animals also showed significant mass gains at feeding sites, increasing to 140% after 7 weeks of feeding (F[2,20] = 7.55, P < 0.01). Where food was not provided, many resident individuals lost mass; this trend was particularly evident in ridgetop habitat. For R. lutreolus, differences in body mass were not as immediate as for R. fuscipes, but after 7 weeks of feeding, males were 33% heavier (t6 = 2.91, P < 0.05) and parous females were 20% heavier (t6 = 2.85, P < 0.05) than animals at nonfood-supplemented sites. Resident animals gained 23% more mass, on average, than animals at nonfoodsupplemented sites (sexes combined, t6 = 2.55, P < 0.05), but no unfed animals lost mass over the study period. Similarly, A. stuartii showed a rapid and large response to additional food, males being 18% heavier at foodsupplemented sites than at nonfood-supplemented sites after 3 weeks of food supplementation (F[2,20] = 5.04, P = 0.02) (Figs. 7a, 7b). After 7 weeks of food supplementation, the

1779 Fig. 4. Numbers of adult Antechinus stuartii (mean ± SE) captured at creek (a) and ridgetop sites (b) during autumn–winter 1991. ×, sites with supplemental feeding; �, sites with empty feeding tubes; +, sites with no treatment. Food supplementation began in late May.

Fig. 5. Numbers of unmarked (immigrant) A. stuartii (mean ± SE) captured at creek (a) and ridgetop sites (b) during autumn– winter 1991. ×, sites with supplemental feeding; �, sites with empty feeding tubes; +, sites with no treatment. Food supplementation began in late May.

difference between males disappeared, whereas females were 28% heavier when provided with additional food (F[2,20] = 8.30, P < 0.001) (Fig. 7c). Notably, the mean body masses © 2000 NRC Canada



1780 Fig. 6. Body masses (mean ± SE) of adult male R. fuscipes at creek (a) and ridgetop sites (b) during autumn–winter 1991. Solid bars denote supplemental feeding, shaded bars denote empty feeding tubes, and open bars denote no treatment. Food supplementation began in late May.

and body masses of resident A. stuartii in both supplemented and non-supplemented sites increased over winter. For all species, two-factor ANOVA on the coefficients of variation in body mass during winter showed no association with treatment or habitat (P > 0.1). This suggests that individuals within a species were not monopolizing and benefiting from the additional food more than others. Food supplementation also promoted reproductive activity. After 7 weeks of additional food, 97% of male R. fuscipes at all feeding sites had descended testes compared with 23% at the same time at non-supplemented sites. Most females also showed signs of recent mating where additional food had been provided and freshly weaned juveniles ( 0.1; A. stuartii / R. lutreolus, CA = –0.10, P > 0.1). Similarly, where additional food was provided, A. stuartii avoided traps in which rodents had previously been captured and were more likely to be captured in traps in which conspecifics had previously been captured. Antechinus stuartii showed no such avoidance where winter food was not supplemented (Table 2). The two rodent species © 2000 NRC Canada



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1781 Table 2. Association of first trap capture with second trap capture for each small-mammal species combination during experimental winter food supplementation. Second capture Treatment

First capture

A. stuartii

R. fuscipes

R. lutreolus

Tube control (χ2 = 5.6, P > 0.5)

A. R. R. A. R. R.

6.5 –12.5 +16.6 198.9 –44.7 –68.2

–2.8 +20.1 –100.0 –31.1 +57.5 –18.2

40.6 –9.3 –4.4 –100 +1.4 +68.3

Feeding (χ2 = 29.3, P < 0.01)

stuartii fuscipes lutreolus stuartii fuscipes lutreolus

Note: Values are the percentage and direction of deviation of observed from expected values.

showed no strong or consistent spatial associations (R. fuscipes/ R. lutreolus, CA = 0.09, P > 0.1). Compared with unmanipulated control sites, the experimental addition of empty feeding tubes had no effect on any population parameters for any species.

Discussion Winter food supplementation had clear effects on the smallmammal assemblage studied here, indicating that all three species were under food stress during winter. The three species generally showed large increases in mean body mass after 7 weeks of food supplementation, with recaptured animals also showing higher growth rates during winter than animals at non-supplemented sites. The rodents also showed a strong aggregative response to the rich resource patches, with high rates of immigration into feeding sites in creek habitats. As a result, both R. fuscipes and R. lutreolus populations were dramatically larger after 7 weeks of winter food supplementation at creek sites. Food supplementation also reversed the usual winter hiatus in rodent breeding (Irby et al. 1984; White et al. 1996), with 97% of males of both rodent species becoming reproductively mature in July after 7 weeks of food supplementation, and newly weaned R. fuscipes being captured in four out of eight feeding sites. Food supplementation was terminated before recruitment of juveniles could contribute significantly to populations, but the presence of newly weaned R. fuscipes at four supplemented sites confirmed that reproduction had occurred. Winter reproduction is known in both R. fuscipes and R. lutreolus in lowland and subtropical environments (Wood 1971; Taylor and Calaby 1988b), where declines in food abundance may not be marked, but has not been recorded previously in montane or alpine situations, where food is seasonally more limited (Stewart 1979; Happold 1989). For A. stuartii there was little evidence that adding food caused numerical increases, although food supplementation has been shown to increase population size in the closely related Antechinus agilis (Dickman 1989). Although the higher rate of immigration to the supplemented ridgetop sites is indicative of a demographic effect of food, it is not clear why immigrants rather than residents benefit from the food supplements. In contrast to the effects of food on numbers, body mass, and reproduction, the mean monthly survival rate (an index of winter survival) of all three species was unaffected by winter food supplementation, despite large increases in body mass that often correlate well with higher survival rates (Wood 1970, 1971). This lack of response of recapture or survival

rates to winter food supplementation is inconsistent with the hypothesis that food stress is a major direct source of mortality, leading to the observed winter population declines. In many studies it has also been reported that food supplementation strongly influenced body mass, immigration rates, and population sizes of small mammals, but did not increase recapture rates or site-persistence times (Gilbert and Krebs 1981; Boutin 1990; Hubbs and Boonstra 1997; Galindo-Leal and Krebs 1998). Although mortality of R. fuscipes has been reported to be highest during June and July (Wood 1971), it is possible that food supplementation at this time benefited only those individuals likely to survive the winter. Alternatively, it is possible that the additional food did not contain appropriate nutrients or the balance of nutrients needed to enhance survival. However, the indices of survival used here cannot distinguish between emigration and death, and it is possible that some animals at feeding sites made only temporary use of the food and spent most time in adjacent areas where they were unlikely to be trapped (Boutin 1984). Also, index line trapping, while often more efficient than grid trapping for gauging relative abundance (Read et al. 1989), would be more susceptible to bias arising from differences in regional pools of individuals in adjacent areas. The role of limited food as a proximal cause of mortality of Rattus spp. and A. stuartii during winter thus remains largely unresolved, and may remain so until direct evidence of the causes of winter mortality is obtained, such as by radio tracking. Despite the failure of additional food to enhance winter survival, winter food supplementation changed the structure of the small-mammal assemblage. Rapid immigration of the two rodents species into supplemented creek sites coincided with a marked decline in A. stuartii numbers in these areas. The measures of spatial association at food-supplemented sites also suggest that A. stuartii avoided areas with high densities of rodents. The frequency of interspecific encounters is generally related to population density (Dickman and Woodside 1983; Dickman 1991) and it is likely that A. stuartii is subordinate to R. lutreolus and R. fuscipes because of its relatively small size. If interference competition intensified under food supplementation, stronger responses by R. lutreolus and R. fuscipes than by A. stuartii would be expected, and the declines in A. stuartii numbers at creek sites might have been due to increased interference competition from the rats. Similarly, the high numbers of A. stuartii immigrating into food-supplemented ridgetop sites may reflect a breakdown in the usual pattern of space use (Lazenby-Cohen and Cockburn 1991). © 2000 NRC Canada



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For R. fuscipes, habitat effects were contrary to predictions. If ridgetops are of low quality because of a lack of food in winter, these drier habitats should have allowed a stronger population response than that which occurred in creek habitat. Given that increases occurred only in creek habitat, food may not limit population size. However, the smaller populations in ridge habitat would have limited the pool of potential immigrants available in these areas to cause rapid population growth after supplemental feeding. We conclude that experimental addition of food in winter elicited large immigration responses by R. fuscipes and R. lutreolus populations, causing significant increases in population within 7 weeks, although only in creek habitats. Together with rapid increases in mean body mass and early breeding, the experiment demonstrated that these rodents are under considerable food stress during winter that is likely to limit population size. A similar response has been reported for several northern hemisphere rodents, though under more sever winter conditions than in this study (e.g., Andrzejewski 1975; Schweiger and Boutin 1995). However, winter survival, as measured here, was not affected by food supplementation, and future work must focus on the causes of mortality during resource bottlenecks such as stressful winters. The smaller A. stuartii also showed increases in body mass after food supplementation, but no increases in population numbers. The latter result was most likely due to interference competition from the increased numbers of rodents at supplemented sites. Hence, the importance of winter food to A. stuartii remains partly unresolved.

Acknowledgements We thank J.M. Tipping, A.N. Banks, R.K. Banks, T. Steeves, and G. Humphries for assistance in the field. Thanks are also extended to Jemby-Rinjah Lodge for accommodation and to B.M. Banks for logistical support. This research was funded by the Institute of Wildlife Research.

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