PredatorÐprey relationships] arctic foxes and lemmings - PDF

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Ecology 0888\ 7\ 23Ð38 PredatorÐprey relationships] arctic foxes and lemmings ANDERS ANGERBJOÝRN\ MAGNUS TANNERFELDT and SAM ERLINGE Department of Zoology\ Stockholm University\ SÐ09 80 Stockholm\ Sweden^

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Ecology 0888\ 7\ 23Ð38 PredatorÐprey relationships] arctic foxes and lemmings ANDERS ANGERBJOÝRN\ MAGNUS TANNERFELDT and SAM ERLINGE Department of Zoology\ Stockholm University\ SÐ09 80 Stockholm\ Sweden^ and Department of Ecology\ Lund University\ SÐ112 1 Lund\ Sweden Summary 0[ The number of breeding dens and litter sizes of arctic foxes Alopex lagopus were recorded and the diet of the foxes was analysed during a ship!based expedition to 06 sites along the Siberian north coast[ At the same time the cyclic dynamics of co! existing lemming species were examined[ 1[ The diet of arctic foxes was dominated by the Siberian lemming Lemmus sibiricus on one site the Norwegian lemming L[ lemmus#\ followed by the collared lemming Dicrostonyx torquatus[ 2[ The examined Lemmus sibiricus populations were in di}erent phases of the lemming cycle as determined by age pro_les and population densities[ 3[ The numerical response of arctic foxes to varying densities of Lemmus had a time lag of 0 year\ producing a pattern of limit cycles in lemmingðarctic fox interactions[ Arctic fox litter sizes showed no time lag\ but a linear relation to Lemmus densities[ We found no evidence for a numerical response to population density changes in Dicrostonyx[ 4[ The functional or dietary response of arctic foxes followed a type II curve for Lemmus\ but a type III response curve for Dicrostonyx[ [ Arctic foxes act as resident specialist for Lemmus and may increase the amplitude and period of their population cycles[ For Dicrostonyx\ on the other hand\ arctic foxes act as generalists which suggests a capacity to dampen oscillations[ Key!words] Arctic\ cycles\ functional response\ numerical response\ tundra[ Ecology 0888# 7\ 23Ð38 Introduction The Arctic tundra communities may appear simple due to low diversity and relatively uncomplicated food webs[ Nevertheless\ the population dynamics of many tundra species and interactions between their deter! minants are intriguingly complex[ One of the major features of these systems are drastic ~uctuations of some herbivore populations\ which in turn in~uence a majority of the mammalian and avian species in the community[ In boreal forests in North America\ snowshoe hares Lepus americanus Erxleben are the pivot of these ~uctuations\ with a period of roughly 09 years Elton + Nicholson 0831^ Sinclair et al[ 0882^ Boutin et al[ 0884#[ In Eurasia\ the pattern is governed by 2Ð4 years ~uctuations of lemmings Lemmus and Dicrostonyx spp[# and voles Clethrionomys and Mic! Correspondence] A[ Angerbjorn\ Department of Zoology\ Stockholm University\ S!09 80 Stockholm\ Sweden[ Fax] 3!7! [ E!mail] tannerýzoologi[su[se rotus spp[# Collett 0800Ð01^ Hansson + Henttonen 0874^ Stenseth + Ims 0882#[ The ~uctuations are referred to as cycles\ although they may\ in fact\ be chaotic with a strong periodic element Oksanen + Oksanen 0881^ Hanski et al[ 0882#[ The cause of these hare and lemming cycles are not yet fully understood\ but a number of recent studies have suggested that predators play a critical role e[g[ Erlinge et al[ 0872\ 0873^ Erlinge 0876^ Tostel et al[ 0876^ Korpimaki + Norrdahl 0878^ Korpimaki\ Norrdahl + Rinta!Jas! kari 0880^ Hanski et al[ 0882^ Hanski + Henttonen 0883^ Hanski + Korpimaki 0884^ Krebs et al[ 0884#[ Small mustelids are suggested as the most in~uential predators {in the north Hanski et al[ 0882#[ In models of this predatorðprey complex\ the least weasel Mus! tela nivalis L[# and Microtus voles are the presumed key species Korpimaki et al[ 0880^ Hanski + Kor! pimaki 0884#[ Most of these studies have concentrated on the boreal taiga zone[ On the Arctic tundra\ however\ the dominant rodents are lemmings and it has not been shown that small mustelids here play the 23 24 A[ Angerbjorn\ M[ Tannerfeld + S[ Erlinge suggested key role[ In some areas\ as on the Wrangel Island\ where lemming numbers ~uctuate in a pro! nounced cyclic pattern Chernyavskii + Tkachev 0871^ Ovsyanikov 0882#\ small mustelids are absent Dorogoi 0876#[ If the suggested predator!generated ~uctuations are valid also for lemming cycles\ there may be other predators on the tundra that assume a role similar to that of weasels[ Nomadic avian lemming predators can be abundant on the tundra\ especially during summers with rodent peaks Potapov 0886^ Wiklund\ Kjellen + Isaksson 0886#[ The most important of these are long!tailed\ pomarine and arctic skuas Stercorarius longicaudus Vieillot\ S[ pomarinus Temminck\ S[ parasiticus L[#\ snowy owls Nyctea scandiaca L[# and rough!legged buzzards Buteo lagopus Pontoppidan[ However\ these species lack a number of traits which have been assumed for the dominant predator in the models mentioned above[ First\ avian predators are not pre! sent during the winter season\ which means that rod! ents have a complete refuge from these predators for three!quarters of the year[ Secondly\ they usually give up breeding and move elsewhere during rodent lows\ and hence do not deepen and prolong rodent popu! lation crashes in the way mustelids are suggested to do Hanski + Korpimaki 0884^ Potapov 0886#[ Thir! dly\ the numerical response of avian lemming pred! ators shows no time lag Potapov 0886^ Wiklund et al[ 0886#[ This is because many have a generalist diet or migrate when food abundance decreases[ Instead\ the arctic fox Alopex lagopus L[# is a strong candidate for being a most in~uential lemming pred! ator[ Due to its habit of food caching and a slightly less specialized diet\ adult mortality is not so strongly in~uenced by rodent crashes as in mustelids Hiruki + Stirling 0878^ Tannerfeldt + Angerbjorn 088#[ Also\ arctic foxes have the capacity to migrate over vast distances[ Arctic fox breeding success and population dynamics are nonetheless strongly in~uenced by lem! ming populations in areas where the species co!exist Macpherson 088^ Ovsyanikov 0882^ Angerbjorn et al[ 0884^ Kaikusalo + Angerbjorn 0884^ Tan! nerfeldt + Angerbjorn 0887#[ Furthermore\ arctic foxes are present on the tundra also in winter and they often stay in an area once they have established a territory Tannerfeldt + Angerbjorn 088#[ All these features are characteristic of the modelled predators[ Further investigations of the role of arctic foxes in lemming dynamics are thus warranted[ The inter! action has so far only been examined from the view! point that lemmings govern fox populations[ The role of predation in intraguild relationships between prey species is little known\ but has gained recent attention Boutin 0884^ Schmitz 0884^ Abrams + Matsuda 088^ Hanski + Henttonen 088#[ In most of the Arctic\ lemmings of the genus Lemmus co!exist with Dicrostonyx[ These di}er in habitat preference and in diet[ Lemmus occur preferably in wet grasslands and feed mainly on sedges\ grasses and moss Batzli 0882#[ Dicrostonyx prefer dry sandy areas and feed primarily on dicotelydones\ such as Salix spp[ and Dryas spp[ Batzli 0882#[ Co!existing microtines are exposed to similar variations in predation pressure and their dynamics seem to be linked Henttonen et al[ 0876^ but see Pitelka + Batzli 0882#[ Arctic fox predation patterns are also interesting in themselves[ The foxes show a large intraspeci_c variation in diet and with this follow striking di}er! ences in life history traits and population dynamics Hersteinsson 0889^ Tannerfeldt + Angerbjorn 0887#[ Furthermore\ the arctic fox is a species of signi_cant economic value to the human inhabitants of the Arctic[ If we are to evaluate the role of the arctic fox in the tundra community\ we must understand its predation patterns[ In this study\ we examine the predatory relationship\ in terms of functional and numerical response\ of arctic foxes in relation to chan! ges in lemming densities on the Siberian tundra[ Materials and methods The study was performed during a ship!based expedition along the north coast of Siberia in the summer of 0883 where we visited 06 sites\ from the Kola Peninsula in the west to Wrangel Island in the east Fig[ 0#[ The sites were not situated in coastal habitat[ At each site\ Erlinge and co!workers censused lemming populations\ focusing on the Siberian lem! ming Lemmus sibiricus Kerr# Erlinge et al[ 0884#\ whereas Angerbjorn and Tannerfeldt surveyed arctic fox dens and collected scats for diet analysis Anger! bjorn + Tannerfeldt 0884#[ Some of the western sites were visited twice sites 0Ð4 and 7Ð09#[ For arctic foxes\ data collected during the second visit have been pooled with data from the _rst visit Table 0#[ During the 2!month expedition\ we covered 033 km 1 and inspected 031 arctic fox dens[ Normally\ the predatory response to prey population ~uctuations are discussed for one population along a time scale[ We instead use each population as a data point and construct response curves along a gradient of prey densities[ CENSUSING ARCTIC FOXES Arctic fox dens are usually situated in characteristic landforms and have lush vegetation\ making them relatively easy to locate e[g[ Smits et al[ 0878^ Prestrud 0881a^ Smith et al[ 0881#[ A single visit at a den was su.cient to detect if it was occupied with a litter or not[ We are convinced that we found a similar proportion of dens in all inventoried areas and that this was a majority of all breeding dens in the area[ A longer stay was needed at each den to observe the number of adult foxes and to estimate litter size[ Litter size estimates were made between June 14 and August 1\ i[e[ when the cubs were between 2 and 01 weeks old[ These estimates must be regarded as minimum numbers Garrott\ Eberhardt + Hanson 0873^ Fig[ 0[ Map of northern Siberia with study site numbers as in Table 0[ Table 0[ Summary of arctic fox inventories[ Site numbers and names follow Hedberg 0884#[ Asterisk # denotes data that were excluded from calculations^ at site 2 due to absence of rodents\ at other sites due to small sample sizes[ The summation {Total is only of scats included in the analyses Site Site name Inv[ area Fox dens Breeding foxes Fox litter size No[ fresh No[ old no[ km 1 # per 099 km 1 per 099 km 1 means 2 SD fox scats fox scats 0 Kola Peninsula 098 1=49 2= Kanin Peninsula 68 0=6 1= Kolguyev Island 69 3=33 4= Pechora Bay 73 2=22 6=03 1=49 2 9= W Yamal Peninsula 82 2=2 09=64 1=49 2 1= N Yamal Peninsula 28 1=4 4=02 0=99 2 9= NW Taymyr Peninsula 68 =6 09=02 1=49 2 9= Chelyuskin Peninsula 4 3=99 = NE Taymyr Peninsula =99 8= Olenekskiy Bay 89 0=00 1= Yana Delta 6 02=32 1=76 3=69 2 1= a N[S[I[ Faadeyevskiy =99 17=99 2=01 2 0= b N[S[I[ Kotel nyy 24 03=18 17= Indigirka:Lopatka =20 13=1 2=08 2 0= Kolyma Delta 009 1=62 4= Ayon Island 49 3=99 7=99 1=99 2 9= Wrangel Island 09 =14 01=49 3=99 2 1= Total 033 4=99 00=48 2=43 2 1= N[S[I[ New Siberian Islands[ Tannerfeldt + Angerbjorn 0887#[ It should also be noted that the long time span make litter size com! parisons between populations uncertain[ We have assumed that each breeding den was occupied by two adult foxes[ In a total of 74 breeding dens\ there was only one observation of three adults at the same den[ 26 A[ Angerbjorn\ M[ Tannerfeld + S[ Erlinge The number of occupied dens multiplied by two was used as an index of density of breeding arctic foxes Angerbjorn et al[ 0884#[ The area inventoried at each site varied from 24 to 09 km 1 Table 0#\ mostly depen! dent on the number of hours spent at each site[ EXAMINING LEMMING POPULATIONS We follow Jarrell + Fredga 0882# and regard collared lemmings from all visited sites as one species\ Dicro! stonyx torquatus Pallas#[ The Siberian lemming Lemmus sibiricus is the only Lemmus at all sites except no[ 0\ the Kola Peninsula\ where it is replaced by the Norwegian lemming L[ lemmus L[#[ The brown lemming L[ trimucronatus Davis has been reported from site 0\ Ayon Island\ but is now considered a synonym to L[ sibiricus Corbett + Hill 0880^ Wilson + Reeder 0882#[ When discussing the genera separ! ately\ we use the terms {Lemmus and {Dicrostonyx \ respectively\ while the term {lemmings refers to both genera combined[ The population densities of lemmings were esti! mated by trapping[ We concentrated our trapping e}ort on the Siberian lemming[ Relative density esti! mates of Siberian lemmings were obtained according to a standardized grid snap!trapping program\ the {small quadrate method Myllimaki et al[ 0860#[ On each locality generally 19 quadrates 04 04 m# were set out\ each with 01 traps three in each corner of the quadrate#[ The site of a trap was carefully chosen and if possible the traps were set at the entrance of a nest or across a lemming runway[ We placed the quadrates about 49 m apart on representative and suitable habi! tat for the Siberian lemming wet grasslands#[ The traps were checked every 7 h and trapping was carried out for 13 h on each locality[ Site 0 was an exception with only 49 trap!nights Table 1#[ In this trapping programme\ the number of captured Lemmus per 099 trap!nights was used as an index of their population density[ To obtain further information on the demography of Siberian lemming populations we placed additional traps at selected places where there were signs of recent lemming activity[ Body weight and sex of captured Siberian lemmings were determined[ We removed eye!lenses to be used for age determination according to Hagen et al[ 0879#[ The weight of eye!lenses made it possible to separate _ve cohorts] juveniles and sub!adults less than 1 and 3 months old\ respectively#\ and three categories of adults\ adult 0 3Ð7 months and born in preceding winter#\ adult 1 8Ð03 months and born in previous summer#\ and adult 2 more than 03 months old#[ The detailed data on age determination will be published separately Sam Erlinge et al[ unpublished data#[ The data on Siberian lemmings permitted us to determine in which phase the examined population was[ In doing so\ we used information on present and previous densities together with information on the age pro_le of the population[ Estimates on previous densities were based on the amount and frequency of old lemming faeces and earlier used runways in typical Siberian lemming habitats[ A population in the increase phase is expected to have medium present density and indications of low past density^ typically\ the age pro_le should be dominated by younger age categories[ A population in the peak phase\ on the other hand\ is expected to have a high density\ both in the preceding and present season[ Furthermore\ the population should have a relatively high frequency of older individuals[ A population in the decline phase should have a moderate present density and high past density\ and an age pro_le dominated by older cohorts[ The low phase is characterized by very low present density and indications of higher previous density[ As discussed earlier\ Lemmus and Dicrostonyx have di}erent habitat preferences Rodgers + Lewis 087^ Batzli 0882#[ The grids were set to trap Lemmus\ but Dicrostonyx were also trapped to some extent[ However\ we do not consider this trapping e.cient for estimates on Dicrostonyx density[ Other scientists on the expedition trapped lemmings\ especially Dicro! stonyx\ for genetic and taxonomic analyses Fredga et al[ 0884#[ They used a constant number of 199 Sher! man live traps and 49 snap!traps\ set selectively at active Dicrostonyx holes at each site Vadim Fedorov\ personal communication#[ We have used the number of Dicrostonyx trapped by Fredga and co!workers divided by the time the 149 traps were active\ i[e[ number of trapped animals per 13 h per 149 traps#[ We call this estimate {Dicrostonyx index Table 1#[ It is important to note that this index has a di}erent scale than the number of Lemmus per 099 trap!nights[ For an estimate on both species together\ we have therefore calculated a {total lemming index [ It is derived in the same way as the Dicrostonyx index\ but also includes total captures of Lemmus by selective and grid trapping\ again per 13 h Table 1#[ We have used the most reliable index type for each category of lemmings and the indices cannot be compared directly[ However\ testing for Lemmus\ the two types of trapping indices were highly correlated r 9=74\ P 9=9990\ n 04#[ ANALYSIS OF ARCTIC FOX SCATS We collected arctic fox scats at occupied dens[ Fresh scats\ from the summer of 0883\ were separated from older scats by appearance[ Older scats are dry and weathered or overgrown by recent vegetation[ Fresh and old scats contained similar proportion of migrat! ing birds\ indicating that scats on the dens were from summers only\ making age separation easier[ We ignored scats that were 1 years or older\ as determined by extensive weathering\ generally being white and brittle\ or overgrowth of vegetation from previous seasons[ Scats were dried at 89 C and prey remains 27 Arctic foxes and lemmings Table 1[ Relative density of Lemmus spp[ and Dicrostonyx torquatus populations\ with age pro_le and phase of examined Lemmus sibiricus populations[ Density of Lemmus is measured as number of animals captured per 099 trap!nights with the small quadrate method\ whereas for Dicrostonyx and total lemming density it is number of animals captured per 13 h with selectively set traps see text for details#[ n is the number of examined L[ sibiricus for age and phase determination[ Age pro_le is based on weight of eye!lenses^ the _gures are the average of the lens!weights of each population[ Note that the lens!weights are not comparable for mainland and island populations lemmings on islands were about 39) heavier#[ Phase determination is based on information on age pro_le of the population\ captures and estimated density during previous season indicated by the frequency of old lemming faeces\ winter nests and lemming runways#[ During the second visit to some sites 1 nd #\ additional captures were made of L[ sibiricus for population phase analyses[ Lemming trapping at site 3 was insu.cient for analysis[ Asterisk # denotes Lemmus indices not used in comparisons with arctic fox data Site Lemmus captures Dicrostonyx captures Total lemming Time at site No[ trap!nights Lemmus sibiricus number per 099 trap!nights# per 13 h# captures per 13 h# h# for Lemmus Sampling date n Lens weight g# : SE Population phase 0 9 9[ June 9 Low June 9 Low 4 09[9 0[36 18[ June 16 23[81 2 2[39 Increase:peak 9 7[62 7[ Aug 7 09[9 3[9 39[ June 71 20[80 2 0[24 Increase:peak 7 1nd# 6[ Aug [6 37[81 67[ June 14 26[0 2 1[69 Peak:decline 8 1nd# Aug [ 1[39 14[ June 0 22[78 2 0[48 Increase:peak 09 1nd# 09[ Aug [40 1[ July 9 Low 01 01[8 8[01 13[ Aug 20 Increase 02a 17[7 40[ July [47 2 0[02 Peak 02b 10[6 9 39[ July 1 41[ [41 Peak 03 [2 9[46 03[ July 14 30[44 2 1[40 Decline 04 7[2 00[99 36[ July 28 22[62 2 0[83 Increase 0 1[9 9 0[ July Low 06 16[4 13[99 46[ July [38 2 0[07 Peak 28 A[ Angerbjorn\ M[ Tannerfeld + S[ Erlinge were identi_ed using reference material[ In the analysis of scats we identi_ed rodent species\ reindeer Rangifer tarandus L[#\ mountain hare Lepus timidus L[\ bird groups ducks and geese\ ptarmigan and grouse\ waders\ passerines#\ insects and plant material\ as far as possible[ At site 2\ Kolguyev Island\ we found rod! ent remains in one of the arctic fox scats n 30#[ This is the _rst report of rodents from the island\ but the remains were only 19) by volume in the single scat and we were unable to determine the species[ Since the amount was negligible\ we excluded data from this site from all analyses of predatory response to rodents[ The remains of Lemmus in the scats fol! lowed known distributions\ with the Norwegian lem! ming Lemmus lemmus only on site 0\ the Kola Penin! sula\ and the Siberian lemming L[ sibiricus on all other sites[ The collared lemming Dicrostonyx torquatus is not known for sites 0 and 02a Faadeyevskiy Island#[ We found remains of Dicrostonyx in _ve out of 39 arctic fox scats from site 02b Kotel nyy Island#\ where the species previously was unknown[ We used a modi_ed frequency of occurrence mea!
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