Diversity and abundance of birds in relation to forest fragmentation, habitat quality and heterogeneity

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Diversity and abundance of birds in relation to forestfragmentation, habitat quality and heterogeneityÅ. BergVersion of record first published: 29 Mar 2010.

To cite this article: Å. Berg (1997): Diversity and abundance of birds in relation to forest fragmentation, habitat qualityand heterogeneity, Bird Study, 44:3, 355-366

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Habitat fragmentation has several compo-nents including reduction in patch size,

increased isolation of patches and oftenchanges in habitat quality.1,2 Several studieshave found that communities and populationsof different species in fragments are not random samples of larger patches.1,3�6 Thismight result from differences in area, habitatdiversity8 or a sampling artefact, i.e. more individuals in large areas give more speciesthrough passive sampling.9 However, speciesdiffer markedly in the response to habitat fragmentation,10 e.g. some birds have been classified as �forest interior species� due to their

sensitivity to forest fragmentation, while othersare referred to as �edge species� because theyare favoured by forest fragmentation.11,12 Forestfragmentation is assumed to be detrimental tomany forest birds, but some studies suggestthat fragmentation of forest areas in farmlandmight be beneficial to several farmland birds.13

Modern forestry is a major factor influencinglandscape structure.14 Of the 23.5 millionhectares of forest in Sweden, only 2% are virginforest. Clear-cutting regimes have dominatedforest management during recent decades creating a mosaic landscape dominated byclear-cuts and young coniferous forests withfew deciduous trees,15 and this has had a greatimpact on flora and fauna in Sweden. Speciesdependent on old trees (especially deciduous

Bird Study (1997) 44, 355�366

© 1997 British Trust for Ornithology

Diversity and abundance of birds in relation to forestfragmentation, habitat quality and heterogeneity

ÅKE BERG Department of Conservation Biology, The Swedish University ofAgricultural Sciences, Box 7002, S-750 07 Uppsala, Sweden

This study investigated the importance of habitat quality and habitat heterogeneity for the abundance and diversity of breeding birds in continuousforest and in forest fragments surrounded by farmland in central Sweden.Positive correlations were found between species number and area, volume of Aspen Populus tremula and habitat heterogeneity. Spatial segregation of habitats at a relatively fine-grained scale is suggested to allow for the co-occurrence of more species. The abundance of at least 18 of the species in thisstudy was influenced by fragmentation, and nine of these species preferred fragments to forest sites. The total density of birds was higher in fragmentsthan in forest sites, probably because several fragment species forage in farmland surrounding the sites and a few also forage at edges. Nine specieswere more common in forest sites than in fragments, but only one species wasrestricted to continuous forest. However, several fragments were relatively closeto forests (150 m) and forest was common in larger scale contexts. The abundance of most species (25 of 33 species) in this study was correlated with habitat quality variables (i.e. variables measuring the size, volume anddiversity of �tree species�). Among these habitat variables the most importantwas the occurrence of deciduous trees which seemed to be important for 14species. The second most important habitat factor seemed to be the diameter oftrees, which was positively correlated with the abundance of eight species ofwhich five are hole-nesters. Among coniferous trees, six species were positivelycorrelated with the volume of Norway Spruce Picea abies, whereas no speciesseemed to be correlated with the volume of Pine Pinus sylvestris.

Email: [email protected]

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trees) or dead wood are common on theSwedish Red-lists.16,17 However, patch quality(e.g. forest age, volume of dead wood, old anddeciduous trees) is not the only important factor. The structure of the whole landscape isoften important for the composition anddynamics of the community,18 and the type ofsurrounding habitat might strongly influencethe composition of communities in forest fragments.

The aim of this study was to compare theabundance and diversity of birds in continuousforest and forest fragments surrounded byfarmland. The importance of habitat quality(size, volume and diversity of �tree species� anddead wood) and habitat heterogeneity in relation to fragmentation for abundance anddiversity of breeding birds is discussed.

METHODS AND STUDY AREA

Study sites

A total of 24 forest sites (mature forest 60�100years old) situated within 40 km of Uppsala(59°52′N, 17°39′E) in central Sweden was censused for breeding birds during the springof 1993. The mean area (± sd) of the sites was7.8 ± 2.9 ha and a total of 186 ha was censused. Of these sites, 12 were forest fragments surrounded by farmland (mean distance from alarger forest area was 450 m). The other 12 siteswere situated within continuous forests largerthan 10 km2 (mean distance to farmland was800 m) and delimited by forest roads or by adjacent forest stands of different type. Thesites differed in the volume of old trees (mainlyAspen Populus tremula), deciduous trees anddead wood (logs and snags); forest sites andfragments were selected in as balanced a designas possible with regard to these factors.However, habitat composition differed slightlybetween fragments and forest sites, i.e. the volume of other deciduous trees (all deciduoustrees except birches Betula pubescens/pendulaand Aspen) was significantly greater in frag-ments (3.4 m3) than in forest sites (0.8 m3).

Habitat mapping

Habitat mapping was carried out at five or sixpoints at each site. The habitat mapping pointswere situated at least 100 m apart, otherwise

they were randomly chosen. At each point thebasal areas of different �tree species� were measured with a relascope (see Karlsson &Westman19) and recalculated (with the help oftree height) to volume (m3/ha) of Scots PinePinus sylvestris, Norway Spruce Picea abies(herewith referred to as pine and spruce,respectively), Aspen and birches. The volumeof other deciduous trees (mainly Oak Quercusrobur, Alder Alnus glutinosa, Rowan Sorbusapricaria and Goat Willow Salix caprea) weregrouped together (m3 other deciduous forest).Height and diameter of lying and standingdead wood was measured and summarized asvolume of dead wood (m3 dead). The meandiameter at breast height (Dbh) of spruce/pineand birches/Aspen was also estimated.Furthermore, the ground vegetation was classified as dominated by Cowberry Vacciniumvitis-idea, Bilberry Vaccinium myrtillus, grasses,herbs, lichens or bare ground.

However, the type of ground vegetation wasrelated to type of tree cover; sites dominated byCowberry or Bilberry had more pine, and sitesdominated by grass vegetation had a large vol-ume of other deciduous trees (t-tests, all P <0.001). Therefore ground vegetation type wasomitted from habitat preference analyses.

The number of holes suitable for hole-nestingspecies within 30 m from each point was counted. However, the abundance of nest-holes was positively correlated with the Dbh of birches/Aspen (Spearman rank correlation,rs = 0.63, n = 24, P < 0.001) and also to the Dbh of spruce/pine (Spearman rank correla-tion, rs = 0.54, n = 24, P < 0.01) and this vari-able was omitted from analyses, because correlations between bird abundance and type of forest are more related to forest management.

Mean number of �tree species� with volumesof more than one cubic metre at the habitatmapping points was used as a measure of taxonomic diversity of the sites, but this indexmight also be expected to reflect structuraldiversity. Deciduous trees other than birch andAspen were grouped together due to theirsparse occurrence, which gave a possible rangeof tree diversity from one to five. Furthermore,a heterogeneity index (HH) for the variation inthe volume of different �tree species� betweendifferent points within sites (i.e. a measurementof the amount of variation between territories)

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© 1997 British Trust for Ornithology, Bird Study, 44, 355�366

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Fragmentation, habitat quality and birds 357

was calculated from the Shannon informationfunction, after Orloci20 and Freemark &Merriam.21

r c

HH = ∑∑ ij ln ( ij / i)

i = 1 j = 1

where r is the total number of classes for vari-ables used to characterize c habitat plots, ij isan entry for the ith class in the jth habitat plot.

i is the mean for class i and ln is the naturallogarithm. Thus, when habitat conditions at a point are identical to the average for the site,the index accumulates no heterogeneity, but itbecomes increasingly greater as habitat variability among points causes accumulateddeviations from i. HH is therefore sensitiveto the number of measurement points, butthese were kept relatively constant (five to sixin each site).

Bird censuses

Each plot was censused for breeding birds byterritory mapping.22 All sites were visited seventimes during early morning (mainly from sunrise to 10 a.m.), once in each of the periods15�30 April, 1�10 May, 11�20 May, 21�31 May,1�10 June, 11�20 June and 20�30 June in 1993.All bird observations were recorded on maps of1:2000�4000 scale. Birds observed just outsidethe plots were included in order to decidewhether a territory was situated within or outside a plot. Territories across the border of aplot were counted as belonging to the plotwhen more than half of the observations fellinside its borders. Several plots (2�3) were censused during the same morning in randomsequence to avoid effects of time and tempera-ture on the conspicuousness of the birds.Details of methods and criteria used to determine the number of breeding territoriesfor most species are given in Robertson &Skoglund.23

The territory mapping method is less suitable for birds breeding in colonies or semi-colonial aggregations. The number ofbreeding pairs of these species, namely,Jackdaw Corvus monedula, Fieldfare Turduspilaris, Starling Sturnus vulgaris and GreenfinchCarduelis chloris was estimated from the number of birds observed.

X

X

X

XXX

Statistics

Parametric tests were used for the 21 most common species with frequency distributionsapproximately normally distributed. Non-parametric tests were used for 12 relativelyuncommon species, because at many sites theydid not occur and their frequency distributionswere strongly skewed when present. Thesespecies usually occurred with just one territory;presence or absence of the species, and theirhabitat preferences were therefore analysedfurther with logistic regression. Effects of fragmentation, habitat quality and habitat heterogeneity were not analysed for the 17 leastcommon species, i.e. species occurring withfive or fewer territories (for details of type oftest on separate species see Appendix andTable 3). In multivariate models (stepwiseregression and stepwise logistic regression), thesignificance level to enter and to remove variables from the model was set to 0.1.

RESULTS

A total of 1244 territories of 50 species was estimated to occur at the 24 sites (Appendix). A model comparing species-richness in forestsites and fragments, which included site area,suggested a weak but significant difference inspecies number between fragments (mean ± sd= 3.6 ± 1.3 species/ha) and forest sites (mean ±sd = 2.8 ± 1.2 species/ha, ANCOVA, F = 4.2, P < 0.05). However, in a model that also included habitat quality and heterogeneity, site type failed to enter the model; positive correlations were found between number ofspecies and area, volume of Aspen and habitatheterogeneity, measured as variation in the volume of different �tree species� on a scale similar to between-territory differences formost species (Table 1). Thus, the higher species number in fragments seems to be an effect ofhabitat differences between fragments and forest sites. The total number of territories, onthe other hand, was positively correlated with diameter of birch/Aspen, �tree species�diversity (measured as the number of �treespecies� on a within-territory scale) and area.Furthermore total territory number was greaterin fragments (827 territories/100 ha) than inforest sites (607 territories/ 100 ha, see Table 1).

There were, however, large differences in

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358 Å. Berg


densities of different species in forest sites andfragments. Six species (out of 33 whichoccurred in more than five territories) weremore common in fragments than in forest sites,and two species showed similar tendencies,although not significant (Appendix). Twenty-two species showed no significant differencesin densities in forest sites and fragments. Fivespecies were significantly more common in forest sites than in fragments (see Appendix fordetails).

These species might have preferred or avoided fragments due to differences in the

surrounding landscape (open farmland versus forest) or because of differences in habitat composition between forest sites and fragments. Habitat composition differed slightly between fragments and forest sites; the volume of other deciduous trees (i.e. alldeciduous trees except birches and Aspen) wassignificantly greater in fragments than in forestsites (Table 2). A comparison of the number ofterritories of different species when site areaand habitat variables were included into analyses revealed that all these 11 species still had significantly different densities in

Table 1. Correlations with site type, area and habitat variables (see Methods) forspecies-richness and total number of territories in multiple regressions/covarianceanalysis.

Species- Total number ofVariable richness territories

Site type (fragment/forest site) +Area + +Dbh spruce/pineDbh Aspen/birch +Volume of Aspen ++Volume of birches Volume of spruce Volume of pine Volume of other deciduous trees Volume of dead wood Habitat diversity +Habitat heterogeneity +

++P < 0.01, +P < 0.05. Positive sign for site type indicates higher value in fragments thanin forest sites.

Table 2. Mean values for measured habitat variables in forest sites and fragments.

Variable Forest sites (n = 12) Fragments (n = 12) P

Dbh spruce/pine 30.0 30.7 >0.8Dbh Aspen/birch 29.9 29.6 >0.9Volume of Aspen (m3) 21.6 34.6 >0.3Volume of birches (m3) 9.8 12.8 >0.6Volume of spruce (m3 100.1 84.6 >0.4Volume of pine (m3) 79.8 67.6 >0.5Volume of other deciduous trees (m3) 0.8 3.4 <0.05Volume of dead wood (m3) 7.5 3.6 >0.1Habitat diversity 3.0 2.8 >0.5Habitat heterogeneity 26.6 25.9 >0.9

Dbh is diameter at breast height in cm; habitat diversity is measured as mean number of �tree species� at habitatmapping points; heterogeneity in the tree strata is measured with a habitat heterogeneity index (amount of varation within a site, see Methods). P values are from t-tests.

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fragments and forest sites (Table 3).Additionally (i.e. species for which no differences were found with univariate tests),the abundances of Fieldfare, Stock DoveColumba oenas and Whitethroat Sylvia communis

seemed to be greater in fragments than in forestsites (although these species occurred at only afew sites) and the abundances of Crested TitParus cristatus, Robin Erithacus rubecula,Dunnock Prunella modularis and Blackcap



Table 3. Associations between abundance of different species and different variables: area, fragmentation (forestsite/fragment), habitat variables (amount and size of different tree species), habitat diversity (mean number oftree species at habitat mapping points) and habitat heterogeneity (within-site variation in amount of differenttree species, see Methods).

Habitat variable

Dbh Volume (m3)Test Tree Tree

Species type Area Frag. SP BA Aspen Birch Spruce Pine Other Dead spp. het.

Chaffinch REG +++ +++ +++Robin REG � �Goldcrest REG ++ ++Great Tit REG � +++ +++ � �Willow Warbler REG + +Coal tit REG � �Yellowhammer REG +++ +Blackbird REGGreenfinch REG + +++ +Tree Pipit REG +Song Thrush REG (�) +++Blue Tit REG ++ +++ (+) ++Treecreeper REG � (�)Nuthatch REG (+) + ++Great Sp.

Woodpecker REG ++Woodpigeon REG +Dunnock REG � +Pied Flycatcher REG ++Crested Tit REG � �Siskin REG � � � +++ + (+)Starling LOG +++ (+) +++Redwing REG � � ++Wood Warbler LOG (+) +++ � �Blackcap LOG � +Marsh Tit LOG ++Garden Warbler LOGFieldfare LOG +++ +++ � � ++Jay LOG + � �Willow Tit LOG � � +Wren LOG � �Stock Dove LOG ++ ++Green

Woodpecker LOG � � ++Whitethroat LOG +++ ++

REG, stepwise regression; LOG, stepwise logistic regression; Frag, + = positive association to fragments, � = preference for forest sites; Dbh, mean diameter at breast height; SP, spruce and pine; BA, birch and Aspen;Other, deciduous trees other than birch and Aspen; Dead, dead wood; Tree spp., tree species diversity; Tree het.,habitat heterogeneity. +++/� � � = P < 0.001, ++/� � = P < 0.01, +/� = P < 0.05, (+)/(�) = P < 0.1.

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Sylvia atricapilla were greater in forest sites thanin fragments (Table 3), which was not the casein univariate analyses. This suggests that fragmentation of forest in itself influences the occurrence of 18 species, when habitat differences are included in the analyses.

A factor expected to be important for theoccurrence of most species is habitat qualitywithin sites. Habitat quality was measured asthe volume (m3) and size (Dbh) of different �treespecies� and dead wood. Most bird species (25of 33 species analysed) were significantlyaffected by these habitat variables, and also byhabitat diversity (Table 3, Fig. 1). The size of thetrees (Dbh of spruce/pine and birch/Aspen)was positively correlated with the abundanceof eight species (Fig. 1). The volume of deciduous trees (m3 birch, Aspen or otherdeciduous trees) was significantly correlatedwith the abundance of eight species, of which five were positively correlated withthese variables. The volume of spruce was significantly correlated with the abundance of

eight species (of which six were positively correlated), whereas the volume of pine anddead wood was not included in the models forany species (Table 3). The occurrence of 13 ofthese species seemed to be affected both byhabitat quality and by fragmentation itself(Table 3). Of these species, Chaffinch Fringillacoelebs, Greenfinch, Dunnock, Siskin Carduelisspinus and Willow Tit Parus montanus were allpositively associated with the volume ofspruce, Yellowhammer Emberiza citrinella waspositively associated with the volume of deciduous forest and Blue Tit Parus caeruleus,Fieldfare and Whitethroat were positively associated with tree diversity. Crested Tit wasnegatively associated with Aspen volume,Starling and Blackcap were both positively correlated with Aspen volume and Stock Dovewas positively associated to the Dbh of Aspenand birch.

�Tree species� diversity (measured as themean number of �tree species� at habitat mapping points) was positively correlated with

360 Å. Berg


Num

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of s

peci

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Fragm. Treespeciesdiversity

Area ofsite

Dbh Volumeof

deciduous

Volumeof

spruce

Treeheterogeneity

Volumeof

pine

Volumeof deadwood

0

5

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15

Positive correlation20

Negative correlation

Figure 1. Number of species positively and negatively related to the different variables: fragmentation (positivecorrelation represents a preference for fragments and negative correlation a preference for forest sites), �treespecies� diversity (i.e. mean number of �tree species� at habitat mapping point) area of site, Dbh (diameter at boleheight of trees), volume of single deciduous �tree species�, volume of spruce, tree heterogeneity (i.e. differencesin the volume of different �tree species� between habitat mapping points within sites), volume of pine and volume of dead wood.

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nine species (Table 3, Fig. 1). Four of thesespecies (Song Thrush Turdus philomelos, BlueTit, Great Spotted Woodpecker Dendrocopusmajor and Wood Warbler Phylloscopus sibilatrix)were not significantly correlated with any otherhabitat quality variables (see Table 3).

Habitat heterogeneity (measured as hetero-geneity in volume of different �tree species�within sites) was not positively correlated with the abundance of any species, but theabundance of two species (Great Tit Parus majorand Wood warbler) was negatively correlatedwith this factor (Table 3, Fig. 1).

DISCUSSION

In this study, species-richness was positivelycorrelated with both area and habitat hetero-geneity, but not with measures of habitatdiversity (Table 1). Habitat heterogeneity was measured as variation in the volume of dif-ferent �tree species� on a scale similar tobetween-territory differences for most species,whereas habitat diversity was measured as thenumber of tree �species� on a within-territoryscale. A more heterogeneous habitat is suggested to allow the co-occurrence of more species,24 probably because habitatrequirements of more species are met, but itmight be partly due to spatial segregation thatreduces competition.25 Habitat requirements ofmore species might also be expected to be meton a small scale in diverse habitats, resulting inhigher species-richness in diverse habitats thanin uniform ones,26 but this was not supported inthis study. Species-richness was positively correlated with site area in this study as hasusually been found.5,10,21 However, the relativeimportance of area and habitat diversity and heterogeneity is probably affected by the magnitude of variation in area and diversity/heterogeneity and the correlationbetween these variables which often exists (e.g. Reed,28 Ford29), usually making it difficultto separate them. Some descriptive studieshave not found effects of habitat quality, andarea was then argued to be much more important (e.g. Blake & Karr,30 Nilsson et al.31).However, it is difficult to measure all relevanthabitat variables and therefore experimentalstudies, such as that of Simberloff,32 are preferred for separating effects of area andhabitat variables.

The group of factors associated with theabundance of most species in this study consisted of habitat quality variables (variablesmeasuring the size, volume and diversity of�tree species�), i.e. the abundances of 25 of 33species were correlated with different habitatvariables (see Table 3). Other studies havefound this factor somewhat less important inrelation to landscape variables and isolationfactors, e.g. Hinsley et al.,33 possibly because all censused sites in that study were surrounded by farmland where many speciesforage. Among habitat variables, the mostimportant group seemed to be the occurrenceof deciduous trees, although the volume of different deciduous �tree species� was positively correlated with the abundance ofonly five species (Starling, Blackcap, Siskin,Willow Warbler Phylloscopus trochilus andYellowhammer). However, habitat diversitywas positively correlated with the abundanceof nine species. As this measurement was madeat five or six points in each site, a high meandiversity indicates that there were small areas(territory or parts of territory for most species)with several species of deciduous trees occurring, as pine and spruce occurred at mostsites (90% and 94% of 136 points, respectively).This suggests that the occurrence of deciduoustrees was important for 14 species altogether.Earlier studies have also found that mostspecies had higher densities in mixed naturalforests compared with managed coniferous forest.34,35 Mixed forest might even have higherbird densities than deciduous forests and thehigh densities in mixed forests are suggested to be caused by high insect availability in combination with good predator refuges.7

The second most important habitat factorseemed to be the size of trees (measured as Dbh of pine/spruce and of birch/Aspen),which was significantly correlated with theabundance of eight bird species (see Table 3).Five of these species (Great Tit, Nuthatch Sittaeuropaea, Pied Flycatcher Ficedula hypoleuca,Stock Dove and Green Woodpecker Picusviridis) are hole-nesters, suggesting that theavailability of trees suitable for nesting isimportant to these species. The abundance ofnest-holes was also positively correlated withthe Dbh of birch/Aspen and spruce/pine.Aspen has earlier been shown to be the mostimportant nest-tree for several species in the



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region of the investigated sites.36,37 Among theconiferous trees, six bird species were positively correlated with the volume ofspruce, while no species seemed to be correlated with the volumes of pine or deadwood (Table 3). Earlier studies have found thatthe volume of pine is negatively correlatedwith species-richness.12 A possible explanationof the preference for spruce over pine might bedifferences in forest structure, i.e. there aremore protected nest sites in spruce forests.However, productivity and possibly also theamount of food might differ between pine-dominated and spruce-dominated forests.

A factor that influenced the abundance of atleast 18 of the species in this study was fragmentation, i.e. nine species were moreabundant in fragments and nine species weremore abundant in forest sites (Table 3).However, the total density of birds was higherin fragments than in forest sites. A possibleexplanation is that several species with higherdensities in fragments than in forest sites foragein farmland areas surrounding the sites, so theareas actually used by these birds are largerthan those of the forest fragments. Whenspecies mainly foraging in farmland (BuzzardButeo buteo, Fieldfare, Stock Dove, WoodpigeonColumba palumbus, Starling, Greenfinch,Yellowhammer, Ortolan Bunting Emberiza hortulana, Linnet Carduelis cannabina, CarrionCrow Corvus corone and Jackdaw) are excluded,the total number of territories in fragments (6.6territories/ha) and forests (5.9 territories/ha)no longer differs significantly, when includingarea in models (ANCOVA, F = 0.4, P > 0.5).Similarly, Loman & von Schantz38 found higher densities of birds in small fragmentsthan in large fragments, probably due to use ofadjacent habitats by several species. Robertsonand Berg13 found that the number of farmlandbird species increased with fragmentation offorests, i.e. edge species were more common insites with fragmented forest. Furthermore, theedge between farmland and forest is a distincthabitat in itself and many species have theirnests there and a few also have their main foraging areas at edges. A more detailed analysis of the distribution of territories withinsites for the species preferring fragmentsshowed that three of the species that do not forage in fields (Whitethroat, Marsh Tit Paruspalustris and possibly Blue Tit: 100%, 80% and

59% of territories at edges, respectively) preferred edge habitats (i.e. territories situatedwithin 50 m of edges). Additionally,Greenfinch, Yellowhammer, Fieldfare and possibly Chaffinch (89%, 100% , 73% and 61%of territories at edges, respectively), showed a preference for edges, but these species also forage in fields. Thus, seven of the species preferring fragments seem to prefer edge habitats, although four of them also forage inother habitats. However, Starling, Jackdaw andStock Dove showed no preference for edgehabitats (i.e. territories at edges were not morecommon than territories inside fragments), sotheir preference for fragments can probably be explained either by their preference for foraging in fields or by other habitat differencesbetween fragments and continuous forest.These explanations are not mutually exclusive.

Nine species were negatively affected byfragmentation, i.e. they were more common inforest sites than in fragments, but only onespecies (Wren Troglodytes troglodytes) wasrestricted to continuous forest (see Appendix).However, some fragments were relatively close to forests (150 m) and forest was the dominating habitat in larger scale contexts.Thus, more species might be expected to benegatively affected by fragmentation if isolation is great.5,12,39 Some species not affectedby fragmentation in this study have beenshown to be affected if isolation is greater (i.e.kilometres instead of hundreds of metres); e.g.Nuthatch.40 Furthermore, species requiringlarge areas might be excluded from small fragments, although some species, such as theBlack Woodpecker Dryocopus martius, mightuse several fragments and can therefore befound in the same densities in continuous andfragmented landscapes.41 This might have beenthe case for Green Woodpecker and Jay in thepresent study, because both have relativelylarge territories.42 The habitat surrounding thefragments might be expected to influence thecomposition of bird communities in fragments,and studies from edges at clear-cuts43,44

and farmland45 suggest that there are also differences in the bird fauna when similar forest edges are compared. Thus, the composi-tion of the whole landscape seems to affect the bird community and relatively widespreadspecies might be affected by landscape parameters.33

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Conservation and management implications

The results from this study stress the importance of habitat quality for the abundance and diversity of birds in mixedconiferous forests. First, the volume of deciduous trees and a high �tree species� diversity (i.e. several deciduous �tree species�occurring in the same forest stand) seem to beimportant for many species. Therefore, anincrease in the volume of deciduous trees inconiferous forests (now about 4%) would havepositive effects on many species. Second, thenumber of large trees suitable for hole- nesters (mainly Aspen) affects species-richnessand abundance of several species (mainly hole-nesting ones) and this is suggested to be alimiting factor for many species in managedforests,46-49 but probably not in natural forests.37

However, a relatively large proportion of thesespecies can remain in managed forests if oldtrees are retained at cutting. Another importantresult is that several species are positively correlated with the volume of spruce.However, planting of pine has increased in certain parts of Sweden, sometimes in earlierspruce-dominated areas,50 which will probablyreduce bird diversity in managed forests in thelong term. Dead wood was not found to be afactor of importance for the species in thisstudy. However, dead wood is thought to beimportant for many forest species (includingbirds, cryptogams and insects) on SwedishRed-lists and several rare bird species notfound in the studied sites might depend on theoccurrence of dead wood.16

The degree of isolation in this study was atlevels of hundreds of metres. Only one of 33species was completely restricted to forest sites,although several (eight species) were found inlower densities in fragments than in forest sites.Thus, a grain size of a few hectare(s) might be aproper size for forest management (clear-cutting, thinning operations etc.) if diversityand abundance of birds are to be retained inmanaged forest. In addition, birds such as theCapercaillie Tetrao urogallus requiring largerareas, might interpret these fine-grained landscapes as being continuous.51 However,there might be differences between old forestfragments surrounded by young forest andfragments surrounded by farmland. Old forestsites surrounded by young forests are probably

less affected by isolation than farmland fragments at the same distance from largerareas of old forest, because farmland fields are less suitable for most forest birds than clear-cuts and young forests. However, birdspreferring deciduous trees might be affected by isolation of deciduous patches within coniferous forest.52 Furthermore, habitat heterogeneity, at the scale of hectare(s), waspositively correlated with species-richness.Thus, spatial separation of habitats on a relatively fine-grained scale seems to enhancespecies-richness.

Finally, it appears that forest fragments areimportant for the abundance of many farmlandbirds,13 mainly those correlated with theamount and quality of forest edges45, becausearable fields by themselves are of low value tomost species.53,54

ACKNOWLEDGEMENTS

I am grateful to Mats Edholm, MagnusJohansson and Martin Amcoff for help withfieldwork; also to Lennart Hansson and AllanCarlsson for comments on earlier versions ofthe manuscript. This study was funded by theForest Faculty (research program �Productionand Environment�) at the Swedish Universityof Agricultural Sciences.

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APPENDIX

Total number of territories for different species and their mean densities (territories/100 ha) in 12 forest fragments and 12 forest sites.

Density (territories/ha)No. of No. of

Species sites terr. Forest Fragment Test value P

Whinchat Saxicola rubetra 1 1 0 2.1 not testedLinnet Carduelis cannabina 1 1 0 0.6 not testedOrtolan Bunting Emberiza hortulana 1 1 0.8 0 not testedPygmy Owl Glaucidium passerinum 1 1 0.8 0 not testedLong-tailed Tit Aegithalos caudatus 1 1 0.9 0 not testedGoldfinch Carduelis carduelis 1 1 0 0.6 not testedCapercaillie Tetrao urogallus 1 1 0.6 0 not testedCuckoo Cuculus canorus 2 2 2.0 0 not testedCrossbill Loxia curvirostra 2 2 1.6 0 not testedNutcracker Nucifraga caryocatactes 2 2 3.3 0 not testedBuzzard Buteo buteo 3 3 0 5.4 not testedGreen Sandpiper Tringa ochropus 3 3 2.9 0 not testedBlack Woodpecker Dryocopus martius 3 3 2.4 1.5 not testedSpotted Flycatcher Muscicapa striata 5 5 0.6 5.6 not testedCarrion Crow Corvus corone 5 5 0 6.7 not testedLesser Whitethroat Sylvia curruca 3 5 0.9 0.3 not testedWhitethroat Sylvia communis 4 7 0 8.5 U = 48 >0.1Green Woodpecker Picus viridis 8 8 5.0 6.2 U = 63 >0.6Stock Dove Columba oenas 5 8 0 9.8 U = 42 >0.05Wren Troglodytes troglodytes 6 9 8.6 0 U = 36 <0.05Willow Tit Parus montanus 8 9 9.1 0.9 U = 36 <0.05

continued

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366 Å. Berg


APPENDIX continued

Density (territories/ha)No of No. of

Species sites terr. Forest Fragment Test value P

Jay Garrulus glandarius 9 10 6.4 3.8 U = 53 >0.2Fieldfare Turdus pilaris 5 11 0 11.2 U = 42 >0.05Jackdaw Corvus monedula 2 11 0 9.2 not testedGarden Warbler Sylvia borin 11 12 6.2 9.2 U = 66 >0.6Marsh Tit Parus palustris 12 13 1.8 14.1 U = 18 <0.001Blackcap Sylvia atricapilla 14 16 12.1 7.2 U = 50 >0.2Wood Warbler Phylloscopus sibilatrix 10 17 10.3 7.2 U = 62 >0.5Redwing Turdus iliacus 11 17 11.0 7.3 t = 0.8 >0.4Starling Sturnus vulgaris 6 18 0 19.3 U = 36 <0.05Siskin Carduelis spinus 14 20 16.8 6.2 t = 2.1 <0.05Crested Tit Parus cristatus 15 20 14.3 9.2 t = 1.0 >0.3Pied Flycatcher Ficedula hypoleuca 14 22 7.4 17.8 t = �1.4 >0.1Dunnock Prunella modularis 16 23 15.8 8.5 t = 1.5 >0.1Woodpigeon Columba palumbus 16 23 10.0 14.9 t = �0.9 >0.4G. Sp. Woodpecker Dendrocopos major 21 23 13.4 14.0 t = �0.2 >0.8Nuthatch Sitta europaea 19 26 14.4 18.6 t = �0.7 >0.4Treecreeper Certhia familiaris 18 26 12.0 9.2 t = 2.6 <0.05Blue Tit Parus caeruleus 14 33 8.0 26.2 t = �2.6 <0.05Song Thrush Turdus philomelos 22 36 18.4 23.0 t = �0.8 >0.4Tree Pipit Anthus trivialis 18 36 17.4 28.5 t = �1.1 >0.3Greenfinch Carduelis chloris 14 39 1.5 42.6 t = �4.0 <0.001Blackbird Turdus merula 23 50 24.2 35.0 t = �1.6 >0.1Yellowhammer Emberiza citrinella 16 53 8.1 59.2 t = �5.6 <0.001Coal Tit Parus ater 22 54 41.2 21.0 t = �2.2 <0.05Willow Warbler Phylloscopus trochilus 21 69 31.4 49.0 t = �1.2 >0.2Great Tit Parus major 24 69 32.2 49.1 t = �1.6 >0.1Goldcrest Regulus regulus 24 85 49.4 46.4 t = 0.3 >0.7Robin Erithacus rubecula 24 100 68.3 48.2 t = 1.5 >0.1Chaffinch Fringilla coelebs 24 234 107.2 160.0 t = �2.5 <0.05

All species � 1244 607.2 827.4 t = �2.5 <0.05

U-values and t-values are from Mann�Whitney U-tests and t-tests comparing densities of species in forest sitesand fragments.

(MS received 1 November 1996; revised MS accepted 26 March 1997)Dow

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Documents

Diversity and abundance of birds in relation to forest fragmentation, habitat quality and heterogeneity