Effects of Patch Area and Habitat on Bird Abundances

CHAPTER 6

Effects of Patch Area and Habitat on Bird Abundances, Species Numbers and Tree Health in Fragmented Victorian Forests Richard H. Loynl

f)ata were analr.sedon birds in 56 forest patches(0.1 to 1171ha) that had been isolatedby, clearing for agriculture or pine plantationsir-rtl'reLatrobeVallel,of southeasternVictoria. Positiverelationships\\'ere found for forest and total bird speciesand abundanceu'ith patch area or its logaritl'rm,and several other variables made additional contributions, e.g. grazing intensitv.Habitat indices R'ereclosel).correlated and explained almc>st as much variation.Forest bird abundancedid not increasewith area above 10-30ha. Nr-rn-rbers of finsectdamage.Protectior-r of understorevhabitatcan haveu,ide benefitsir-rmaintainingthe l-realtl'rof ti-rerural ecosvstem.

INTRODUCTION \7rrnx forests are clearedmany plant and animal species become confined to remnant patches of native vegetation, especially when land is permanently altered for uses such as agriculture. These patchesand their fauna can continue to plav a role in the local ecology, protecting land from erosion, saliniry and other degradation and providing habitat for wildlife including speciesthat benefit from clearing (farmland species) (e.g. Breckwoldt 1983). Hence it is important to know how to design and managesystemsof remnant vegetation,and how to conserve plant and animal species in these fragmented systems. This chapter dealswith a study of birds on 56 fragmented forest patchesin the Latrobe Vallev of southeastern Victoria, following a similar study of mammalson the samepatches(Suckling 1980,7982, 1984). Some preliminary dataon birds have alreadv

been presented (Loyn 7984,7985);the latter paper included a list of species with details of habitats occupied. Here more attention is paid to mathematical relationships and a subsequenr paper is planned to analyse effects of fragmentation on individual specie.s. STUDY REGION A large part of easternVictoria is hilly or mountainous and remains as forested public land. The Latrobe Valley is an exception, as it has become a major centre for agriculture and industry. Much of the study region has heen cleared for agriculture and pine plantations over the last 100 years and in 7978 about 34% remained as fragmented native forests and woodlands, with 52% as agricultural land and 1.4"/"as pine plantations (Suckling 1982) By 1983 an additional 4% had been cleared(mainly for a new coal-firedpower-stationat

ment of Conscn'ation, F'orestsand Lancls). Pages65.17itlNATLjRECoNSERVATIoN:TIIERoLEoFREN{N.ANTSoFNATnE\TGETATIoNedbl'DcnisA.Saunclers.Graha SurreY Beatn' and Sons Pn' Limirecl in association\\'itlt CSIRO and CALM, 1987.

OF NATNE \EGETATION NATURE CONSERVATION:THE ROLE OF REMNANTS Loy Yang), including three of the 59 original fragments. The fragments studied ranged in size from 0.1 to 7777haand were scatteredthrough the region mostly on private propert.v.Thev had been isolated fcrr periods from 7 to 100 vears and three l-radbeer-r partlv cleared for use as shire rubbish tips. The largest area has subsequently been converted tcr pine plantations.No fragment was further than 1.5 km from another patch of native forest. Two were isolated by as little as a main road; details are given in the Appendix. More extensiveforestsstill exist on public land in the hills and coastalsand-plainsouth of the region, and continuous forest occurs on higher ground north of the LatrobeValley. The 56 fragments contained mixecl-speciesdn' sclerophl'll forest including stands of narrow-leaf peppermint Eucalyptus radiata, manna gum E uiminalis and yertchukE. consicieniana. The understorey was dominated either by heatl-rvshrublands, swards of austral bracken fern Pteridium esculentum or in heavil,vgrazed patches b,v introduced grasses. Thickets of burgan Leptospermum pbylicoiries occurred frequentl,v and some gullies contained tall gully shrubs, though generall,vthe region was too low (60 to 360 m above sealevel) ar-rd dry (about 750 mm mean annual rainfall at 180 m) ,for gullv vegetation to be well developed. X,{ore information on individual studv areas is given bt, Suckling (1980) and Loyn (1984). METHODS Nl 56 fragmentswere searchedin each of three spring/summer seasonsfrom November 1980 to March 7983.All birds observedwere recorded, and numbers seen or heard on 20-minute counts were used as measuresof abundance (Lct,vn7986).On tl-re smallestpatchesone or t!v'ocounts were sufficient to cover the whole areaon eachvisit,whereason larger patchesup to 19 countswere made on a visit with the aim of searching all habitats.Areas of about 3 ha could be covered in each count, so birds per count on smaller patchesdo not give a true reflection of bird densit)'. At the end of tl-restudy, counts were continued until few extra species could be found. Ba.sicobservationswere made on the use birds were making of their habitats. ANALYSIS Data were tabulated and statisticscalculatedfor each patch (numbers of bird species, and bird abundances expressed as individual birds per count). Depending on their known use of habitats elsewhere, the 732 birircls(Appenclix). Of tl-re31 patchesof this size cla.ss, forest birclsmaclelesstl-rar-r 20%,ftotal poplrlatictnsin eleven, iutcl tnore than 85% in onh'six, all of n hich haclreceivecla clegreectf protection fr-on'rstock tl-rrotrghf-encing or l)eiltg lxrr:ncle cl lrv pine plantationsmther than pasture.ht tl-rosepatches, clensitiesr>f firrest bircls appearecl

NATLIRECONSERVATION:THE ROLE OF REI{NANTS OF NATIVE \EGETATION

68

fttble -J Stcpwise contribution of variables to r-ntrltiplercgressi()n eqr-rations

Dependentvariable Furestbird species

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- - P < 0 . 0 5*, * = P < 0 . 0 1 , * * * : P < 0 . 0 0 1 . = a d c l i t i o nvarl r i a b l e m s aclenofirrthersigniflcantcurrtributirxr

only slightlv lower than ir-rlarger parches(Apper-rdix). The apparentreduction in patchessmallertl-ran 3 ha mav be partlv an artefactof the srudv method n'l-ricl'rwas designed to search areasof about 3 l-raon eachcount. Sigr-rsof eucalvpt dieback associatedn'ith insect attack were evident ir-rall patches lacking Llnderstorev,which contained low densitiesof insectivoroLrsfi)restbirds (Appenclix). Forest Bird Abundance - Because popularion cler-rsities of forest birds were iow in tl-resmallest patches, significant regressions were four-rd fbr abundance againstarea for forest birds (Table 2). The relationship v!'asimproved greatly br. considering IcrgA notA, explaining54.7%ofvariance,but u.as clearlv not linear and for patcheslarger tl-rar-r 30 ha there was little difference ir-rdensitv witl-rincreasing patch size (Appendix). The relationship witl-rlog A was better mainlv because large patches received le.ssweightir-rgin the equatior-rs. Addition of the gr^zing, index improved the relationship sigr-rificantlv,in a negative sense, explaining an additior-ral70.60/,' ofvariance(Table 3). No other variable in tl-refirst package(variables2 to 8) maclesignificar-rt additional contriburions,tl-rougl-r several were strongly correlared (Table 2). In the enlarged packagetl-revariable most stronglrrcorrelated n'ith forest bird abundancewas the index of forest habitats,and its positiveeffectsuper.sededthe negativeeffectof the grazing index in the predictive equation. The vears isolatecl also entered this equation ( negativelv), suggestiltg a pr()gressive clegradationof these patches as I'rabitatfirr fctrest

bircl.s,apart from an\r ef-fbctsof area reclllction or changesin other measllreclvariables.The equation explair-red79.9%of varialtce(Table 3). Farmland and Water Bird Abtutdance - Abur-rdances of farrnlanclar-rcls,raterbircls \\'ere negatir.elv correlated n'ith log A (Table 2), but the regression onh.explained16.0%of variance.The grazingindex supercecleclthe effect of log A, ir-ra positive sense, explaining 40.8%of variarrce.Farmlat-rcl bircls \\,ere onll'abundar-rtin heavilv gr^zedpatches,regarclless of size but generallt' these patches\\rere sntall. No other variable in tl-refirst packagentacleaclditir-ralclistributior-r of l-rabitat should be cot'tsiclered. h-rtl'reLatrobe Valler.. clearir-rghas been concelltratecl in tl're more open forest and n'oocllancl habitats, so the more critical need locallv is tct conset"r'especies associated vn'ithtirose l-rabitats.This car-r now onlt, be done bt' consen.ation of fragn-rer-rted patches. srere represented ir-r Most species rvhose l-rabitat.s tl-re .studv patcl-res u,'ere recorclecl on thent, arrcl it seerns tl'ratmost if r-rotallspecie.s hal'e sun'ived there despite the 70% recluctic>rtin area c>fforest l-rabitat. Distribut ion o.l'Species IJetu, eett Patc bes; Species nor on Large Patcltes Generallv the distributior-r of species reflected their particular habitat requirements (Lovn 1985). No sir-rglepatch supported all species, but the largest patcl-r(F11, 7771ha,) supported the greatest numbe r of forest species w'itl-r62 of the 79 recrdecl.Tl-risis a sligirtlv loner proportion than for mantmals witere the same patch supportecl 18 of tl-re 20 species recordecl (Sr-rckling 1982). Eight of tl-re forest bird species missir-rg from that patcl-r \vere mereh' occasional or irregular visitors to other patches, and tltree others werc olrsen etl on tltc ser'ottcl largcst p a t c l - r( N I 1 1, 9 1 3 h a ) . l ' h c r e m a i n i n g s i x a p p a r e n t l v

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I,O\TI: EFFECTSOF PATCH AREA AND FIABITAT oN BIRD ABUNDANCES

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