what is the required minimum landscape size for dispersal studies

7/27/2019 What is the Required Minimum Landscape Size for Dispersal Studies

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Journal of AnimalEcology 200776, 12241230

2007 The Authors.Journal compilation 2007 BritishEcological Society

Blackwell Publishing Ltd

What is the required minimum landscape size for dispersal

studies?

MARKU S FRANZN and SVEN G. NILS SON

Department of Animal Ecology, Ecology Building, Lund University, SE-223 62 Lund, Sweden

Summary1.

Among small animals dispersal parameters are mainly obtained by traditionalmethods using population studies of marked individuals. Dispersal studies mayunderestimate the rate and distance of dispersal, and be biased because of aggregatedhabitat patches and a small study area. The probability of observing long distancedispersal events decreases with distance travelled by the organisms. In this study a newapproach is presented to solve this methodological problem.

2.

An extensive markreleaserecapture programme was performed in an area of81 km

2

in southern Sweden. To estimate the required size of the study area for adequatedispersal measures we examined the effect of study area size on dispersal distance usingempirical data and a repeated subsampling procedure. In 2003 and 2004, two species of

diurnal burnet moths (Zygaenidae) were studied to explore dispersal patterns.

3.

The longest confirmed dispersal distance was 5600 m and in total 100 dispersalevents were found between habitat patches for the two species. The estimated dispersaldistance was strongly affected by the size of the study area and the number of markedindividuals. For areas less than 10 km

2

most of the dispersal events were undetected.Realistic estimates of dispersal distance require a study area of at least 50 km

2

.

4.

To obtain adequate measures of dispersal, the marked population should be large,preferably over 500 recaptured individuals. This result was evident for the mean moveddistance, mean dispersal distance and maximum dispersal distance.

5.

In general, traditional dispersal studies are performed in small study areas and basedon few individuals and should therefore be interpreted with care. Adequate dispersalmeasures for insects obtained by radio-tracking and genetic estimates (gene flow) is still

a challenge for the future.

Key-words

: dispersal distance, Lepidoptera, markreleaserecapture, movement,Zygaenidae.

Journal of Animal Ecology

(2007) 76

, 12241230doi: 10.1111/j.1365-2656.2007.01285.x

Introduction

Accurate estimates of distances travelled by differentspecies is central to metapopulation ecology, gene

flow estimates, population dynamics, species survivalin fragmented landscapes, and species conservationactions, e.g. reserve planning (Hanski & Gaggiotti2004). Yet, for most small animals, dispersal is one ofthe most difficult population parameters to measure(Nathan 2005). Direct traditional estimates, e.g. based

on markreleaserecapture (MRR) techniques, oftenresult in biased dispersal data reflecting the study areasize (Barrowclough 1978; Koenig, van Vuren & Hooge1996). Schneider (2003) examined different MRR

studies performed with butterflies and concludedthat the measured mean dispersal distance increasedwith study area size. Butterflies have become modelorganisms in metapopulation studies (Hanski 1994;Hanski, Kuussaari & Nieminen 1994), but remarkablyfew studies have critically examined the effects ofstudy area size on dispersal parameters (cf. Englund& Hambck 2007). Thus, even if dispersal distancesare substantially underestimated in many studies(Barrowclough 1978; Shreeve 1992; Wilson & Thomas2002), it is not obvious what study area size is requiredfor a realistic measure of dispersal distances.

Correspondence: Markus Franzn, Department of AnimalEcology, Ecology Building, Lund University, SE-223 62Lund, Sweden. Tel.: +46 46 222 38 20. Fax: +46 46 222 47 16.E-mail: [email protected]


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2007 The Authors.Journal compilation 2007 BritishEcological Society,Journal of AnimalEcology

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This study presents a new approach to obtainaccurate estimates of dispersal distances and appliesthe new method to data from two species of burnetmoths (Zygaenidae). Based on our data we giverecommendations on required study area size and thenumber of recaptured individuals required for accurateestimates of dispersal parameters for species withsimilar dispersal patterns.

Materials and methods

The burnets Zygaena viciae

(D&S.) and

Z. lonicerae

(Scheven) have similar ecology and a single annualgeneration in Sweden, with a main flight period inJuly. These two species are dependent on flower-richseminatural grasslands and occurs locally in most ofEurope (Naumann, Tarmann & Tremewan 1999). Bothspecies have recently declined in many west Europeancountries (Visnen & Somerma 1993; Naumann et al

.

1999). In the study area mating takes place on nectarrich flowers, mainly on the herb Knautia arvensis

(L.)(Dipsacaceae). When approached the moths remainexposed on the flowers, making them ideal for studiesof dispersal.

The study area, covering 81 km

2

(Fig. 1), was situatedin Stenbrohults parish in southern Sweden (56

37

N,14

11

E). The area borders the large Lake Mckeln inthe west, and forest-dominated land to the south, east

and north. The landscape in this region is mixed,although forest dominates (> 75%).

All open grassland patches, about 5% of the studyarea with seminatural grasslands covering 35%, weremapped in 2003. Grassland patches occupied by thestudy species in 2003 and/or 2004 were defined ashabitat patches and each patch considered separate ifthe distance to the nearest patch was 100 m or more.Patches separated by tall tree stands, an additionalbarrier to dispersal, were also regarded as separatehabitat patches if the distance between them was 75 mor more. The two burnet species were recorded from 68

habitat patches with a mean size of 12 ha (range 00681) (Fig. 1). At 38 patches more than 10 individualsper patch were marked. These 38 patches ranged in sizefrom 006 to 50 ha (mean 139) and were all situatedon abandoned grasslands (previously pastures) or haymeadows with late harvest. The amount of grasslandoccupied by burnet moths was low, only 06% of thestudy area (Fig. 2). The proportion of the area surveyedand the proportion occupied by burnet mothsreached a maximum 15 km from the study area centreand decreased towards the edges of the study area(Fig. 2).

Fig. 1. The study area in Stenbohult, southern Sweden.Habitat patches occupied by burnet moths are filled in blackand other grassland patches unfilled areas.

Fig. 2. (a) The total area (), the area of grasslandhabitats (), and the area occupied by burnet moths(), in increasing distances from the study area centre.The area in log 10 transformed hectares. (b) The proportionof the total study area that was surveyed for burnet moths(i.e. the proportion of grassland habitats) (), theproportion of the study area that was occupied by burnetsmoths (), in increasing distances from the study areacentre.


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An extensive MRR programme for burnet moths wascarried out 26 June31 July 2003 and 23 June9 August2004. The MRR programme started when the firstindividual was observed in the study area and continuedthroughout the flight period of the species. We definedispersal as a movement between habitat patches byone individual. It was impossible to separate natal andbreeding dispersal (cf. Paradis et al

. 1998). At each

marking occasion the date and the species name, sex,activity and habitat patch of capture were recorded. Allpatches were visited every third day during idealweather conditions and all encountered individualswere captured with a butterfly net. Every patch wassearched with equal frequency and intensity. Eachindividual caught was marked with a fine-point per-manent ink pen. Unique marking codes were used ateach patch and occasion of marking. Recapturedindividuals that had moved between patches weresubsequently individually marked according to Kreusel(1999). Using this simplified marking method we could

extend the study area and increase the possibilities offinding long distance movements between patches.With this method it is impossible to track each singleindividual but each marked animal could be traced topatch and date. This information is crucial whenstudying dispersal patterns, as opposed to movementswithin a habitat patch where it is necessary to traceeach animal individually.

Recaptures occurring on the same day as of markingwere not included in the analyses to avoid underesti-mating dispersal (Gall 1984). Each individual washandled with all possible care, as marking and handlingindividuals may affect their mobility. All individualsreceived the same treatment during the marking pro-cedure, on average 9 s. In addition to the MRR pro-gramme all seminatural grasslands were surveyed forbutterflies and burnet moths every 14 days in 2003and 2004 (Nilsson & Franzn 2006). The number ofindividuals, marked individuals and their markingcode was registered.

The effect of the study area size on estimated dispersaldistances was explored by a repeated subsampling

procedure using pre-defined smaller sections of thestudy area. The subsampling procedure was performedto sequentially reduce the area and randomly place thesubsampled area within the original study area. It wasa prerequisite that each subsampled area fit completelyinside the original study area and that there wererecaptures inside each subsampled area. From eachsubsampled area information about recaptures anddispersal events that had occurred in the subsampledarea was extracted/intersected. All recaptures anddispersal events with both patch of marking and patchof recapture inside the subsampled area were selected

and average movement distances, mean and maximumdispersal distances were calculated for the two burnetspecies. Dispersal events were those found in thesubsampled area and based on our empirical findings.This procedure was repeated 100 times for eachsubsampled area, species and year. The subsamplingstarted with a rectangular study area with the side lengths10

8 km. By reducing the side length in 5% intervals,and then examining which dispersal events that occurredwithin the subsampled study area, the effect of the size of

the study area on dispersal distances could be explored.Reducing the side length in percentage intervals doesnot result in identical reduction of the study area size.For example, subsampling using a 5% shorter side-length result in a new study area covering 90% of theoriginal area, and a 90% reduction of side length resultin a new study area covering 1% of the original area.

The mean distance from the centre to the edge ofan average habitat patch was approximately 40 m.Thus, we calculated the mean distance moved for allrecaptured individuals, assuming a movement distanceof 40 m for the individuals that were recaptured within

the same patch as where marked. A within patchmovement of 40 m corresponds well with other studieson burnet moths (Bourn 1995; Kreusel 1999; Menndez,Gutirrez & Thomas 2002). For the three individualsthat dispersed between more than two patches onlythe longest distance was considered in the calculationof mean dispersal distances. In the subsamplingprocedure all dispersal events were included. Thesubsampling of moved mean distance was based onthe number of recaptured individuals at each patch.When the exact number of recaptured individualswas unknown (because of the marking procedure) thelowest possible number of individuals recaptured wasused. This was the case when several individuals fromthe same patch and date were recaptured at severallater marking occasions. The minimum number ofrecaptured individuals did not differ substantiallyfrom the mean and maximum number of recapturedindividuals. For transferring individuals we used theactual dispersal distance, counted as the shortestdistance between the two habitat patches.

We subsampled: (1) the mean movement distance ofall recaptured individuals for respective species in 2003and 2004 (where within patch movement was set to40 m); (2) mean distance travelled for individuals

transferring between different habitat patches (betweenpatch movements called dispersal, i.e. actual dispersaldistances) for respective species and year; (3) maximumdispersal distance for respective species and year.For Z. lonicerae

there was not enough individualsrecaptured in 2004 to obtain a useful result. For therepeated subsampling procedures

65 were used.

Results

In 2003 a total of 4202 Z. viciae

individuals weremarked and 944 individuals were recaptured. Of these,


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80 individuals (85%), had dispersed to another habitatpatch. Mean transfer distance was 1078 m and themaximum dispersal distance 5561 m. Mean movementdistance was 95 m. In 2004 a total of 2361 individual

Z. viciae

were marked. The number of individualsrecaptured was 232, and of these nine individuals(39%), had dispersed to another habitat patch. Meantransfer distance was 1805 m and the maximum dispersaldistance was 4062 m. Mean movement distance was108 m. Approximately 20% of the total number of

individuals in the populations were estimated to havebeen marked (unpublished data).

ForZ. lonicerae

554 individuals were marked of which117 were recaptured in 2003. Of these, 10 individuals(85%), had dispersed to another habitat patch. Meantransfer distance was 540 m and the maximum dispersaldistance 3499 m. Mean movement distance was 65 m.In 2004, 184 individual Z. lonicerae

were marked ofwhich 25 individuals were recaptured. One individual(40%), had dispersed to another habitat patch. Meantransfer distance as well as maximum dispersal distancewas 494 m. Mean movement distance was 54 m.

Dispersal distances were not related to the numberof days between the day of marking and the day of

recapture (linear regression, B =161, P

= 0556,

r

2

= 0004). Mean distance moved, mean transferdistance, and maximum dispersal distance increasedwith size of subsampled study area (Fig. 3). For a studyarea up to 10 km

2

, many dispersal events betweenpatches were undetected. There was an increase inestimated dispersal distance as study area size increasedup to 5060 km

2

. In areas smaller than 30 km

2

, theestimated dispersal measures were uncertain and low(Fig. 3). For unbiased estimates of dispersal, areas of

50 km

2

or more are necessary for these species in thestudy landscape.

Discussion

Most traditional dispersal studies are performed instudy areas that are too small, with species occurringin low abundance resulting in dispersal patterns thatare often hard to interpret (Koenig et al

. 1996; Wilson& Thomas 2002). Among other organisms relatively

unbiased measurements of dispersal are available forsome birds (Newton, Davis & Davis 1989; Serrano

et al

. 2001), mammals (Roper, Ostler & Conradt 2003;Haythornthwaite & Dickman 2006), amphibians (Smith& Green 2006), reptiles (Rivera, Gardenal & Chiarav-iglio 2006) and noctuid moths (William et al

. 1993;Nieminen 1996), where radio-tracking, genetic studies,or large study areas reveal longer dispersal distancesthan previously. Indeed, some species are very mobileand require much larger study areas than our resultsuggests. Some recent studies of birds have used verylarge study areas, e.g. 10 000 km

2

(Serrano et al

. 2001;Hansson, Bensch & Hasselquist 2002). Although someinsects may only move a few hundred meters (Ingvar-sson & Olsson 1997; Ranius 2006) and do not requirelarge study areas for estimates of dispersal distances.

Burnet moths are generally thought to be sedentaryand movements over 300 m are rarely found (Menndez

et al

. 2002; Young & Barbour 2004). However, wefound a strong relation between the size of the sub-sampled study area and estimated dispersal distances.We can put our data in perspective by comparing

with other dispersal studies of burnet moths. Meanmovement distances vary considerably among differentstudies, but are positively correlated with the size ofthe study areas (Table 1). The largest study areademonstrated the longest dispersal distances (Kreusel1999), a pattern that Schneider (2003) also showed forbutterflies. Other studies of burnet moths with similarstudy area sizes as in our subsamples, report similardispersal distance estimates, suggesting that our resultsare not exceptional (Table 1).

There is a possible effect of the spatial configurationof the habitat patches (i.e. connectivity and the degree

Fig. 3. Estimated movement and dispersal distances in relation to the study area size.(ac) Mean distance moved (in metres) using all recaptures within subsampled studyareas for, respectively, size-class, species and year. All within habitat patch movementswas set to 40 m. (a) Zygaena viciaein 2003, (b) Z. viciaein 2004 and (c) Z. loniceraein2003. (df) Mean transfer distance using all recaptures of individuals transferringbetween patches within each subsampled study area for, respectively, size class, speciesand year. (d) Z. viciaein 2003, (e) Z. viciaein 2004 and (f) Z. loniceraein 2003. (gi)Maximum dispersal distance using the longest between patch dispersal event withineach subsampled study area for, respectively, size class, species and year. (g) Z. viciaein2003, (h) Z. viciae in 2004 and (i) Z. lonicerae in 2003. Error bars show the 95%confidence interval of the mean.


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of fragmentation) on the dispersal pattern (Conradt

et al

. 2000; Baguette et al

. 2003). Increased fragmenta-tion increases the size of the study area required foradequate dispersal measures as individuals may move

until they encounter suitable habitat or die (cf. Paradis

et al

. 1998). The required study area size that wasfound is larger than study areas used in most previousdispersal studies of insects, and it seems that a studyarea of at least 50 km

2

is required for adequate dispersalmeasures in some small terrestrial insects.

In favourable situations (e.g. suitable, warm andsunny weather, large population sizes, and a large studyarea) dispersal distances longer than normal could beexpected (Walls, Kenward & Holloway 2005). Thelongest distance between occupied burnet habitatpatches in our study area was 9300 m. The longestdetected dispersal event was 5561 m and this distanceprobably does not reflect the absolute maximumdispersal distance for these species. Our subsamplingprocedure indicates, however, that the actual maximumdispersal distance for these two species is close to thedistance found in this study (Fig. 3). However, thedistance of the longest dispersers will probably never becaptured in any traditional MRR study. The metricsused to measure the longest dispersal events shouldpreferably be called the maximum dispersal eventmeasured.

To detect long distance dispersal events both a largestudy area and large populations (i.e. large number ofrecaptured individuals) are necessary. This is obviousfor Z. viciae

in 2004, for which results from the sub-sampling procedure is more variable, compared with2003 (Fig. 3). If the two species studied had shown anequivalent number of individuals, a low variation indispersal distance would have been expected. Mostspecies of conservation interest occur in small popula-tions and small habitat patches, which constitute amajor problem for traditional dispersal studies. MRR

studies usually handle less than 1000 individuals(Schtickzelle & Baguette 2003). For species with pop-ulations limited to only a few localities MRR studiesshould primary aim to estimate the local population

sizes and within patch movement behaviour.How should we measure dispersal of rare species

when large areas and large sample sizes are required?Indirect measures such as detailed mapping of thedistribution of a species over a longer period of timehas been suggested by Wilson & Thomas (2002), butsuch studies are also to the outmost time consumingand for many species the required information is notavailable. Genetic methods are more frequently appliedto estimate dispersal and this field might offer suitablesolutions of dispersal measures in the future (Nathan

et al

. 2003; Rivera et al

. 2006). Radio telemetry can beused on larger species (Hedin & Ranius 2002). How-ever, direct traditional dispersal studies gain a lot ofinformation about local population structure that isimportant. Our results emphasize that dispersalpatterns gained from small areas and small populationsshould be interpreted with care.

Acknowledgements

Mikael Johannesson gave valuable help with thesubsampling procedure in

. Charlotte Jonsson,Sandra Rihm and Anneli hrstrm helped in the field.Thomas Gosden improved the English. Jens Roland

and one anonymous referee gave valuable commentson an earlier version of the manuscript. Financialsupport for this study was given by The SwedishResearch Council for Environment, AgriculturalSciences and Spatial Planning (FORMAS).

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Table 1. Dispersal data obtained in markreleaserecapture studies of different burnet moths (Zygaena spp.)

SpeciesMean dispersaldistance (m)

Maximumdispersaldistance (m)

Study areasize (km2) Reference

Z. spp. 3300 13 Kreusel (1999)Z. spp. 250400 1700 48 Grnwald (1988)Z. spp. 1500 4 Naumann, Hille & Mller-Tappe (1987)Z. spp. 100125 2000 < 1 Bhmer (1995)Z. spp. < 200 980 < 1 Smolis & Gerken (1987)Z. spp. 700 < 1 Bourn (1995)Z. spp. 250 < 1 Lttmann (1987)Z. filipendulae (L.) 33 533 < 1 Menndez et al.(2002)Z. filipendulae 77 932 < 1 Schmidt-Loske (1993)Z. viciae < 250 001 Young & Barbour (2004)Z. viciae 95 5561 81 This studyZ. lonicerae 65 3499 81 This study


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Received 20 February 2007; accepted 31 May 2007Handling Editor: Stan Boutin


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