city size, regional landscape context, and local resource characteristics influence patterns of bee...

8/3/2019 City size, regional landscape context, and local resource characteristics influence patterns of bee occurrence: A cas

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City size, regional landscape context, and local resource characteristics influence patterns of bee occurrence: A case study from northwestern Costa Rica

by Victoria A. Wojcik

Pollinator Partnership, 423 Washington Street, 5th floor, San Francisco, CA 94111

University of California, Berkeley Department of Environmental Science, Policy,& Management, 132 Mulford Hall, Berkeley, CA 94620

E: [email protected]; [email protected]

T: 510-697-3890/415-362-1137

F: 415-362-3070

Abstract

Understanding the ecological patterns of bees in cities can aid in their conservation and management. This is especially important in regions experiencing rapidanthropogenic change that threatens the survival of these important pollinatorsand the ecological services that they provide. To study occurrence trends at the community level, bees visiting a common and abundant floral resource species,Tecoma stans, were sampled across three urban landscapes located in the tropicaldry forest of northwestern Costa Rica. Frequency-of-visitation counts that measured both bee visitor abundance and bee taxon richness were used to assess foraging variation and individual resource constancy, and to evaluate the effects ofplant resource characteristics (floral abundance, resource size, and vertical structure), city size, and regional landscape context on bee occurrence. Of the resource characteristics studied, only the total number of flowers at a food resource had a significant impact on bee occurrence, with resources that possessed mo

re flowers attracting more bees and more bee types. The taxon richness observedat a resource increased significantly as city size increased, but there was no association between city size and bee visitor abundance. Cities located within similar regional landscapes had similar bee community composition, indicating thatthe surrounding landscape influenced species presence and absence patterns. Thepatterns of occurrence recorded in this study can be seen as a positive resultfor bee conservation and management in urban landscapes; the correlation of highbee visitation with abundant flowers suggests that efforts to increase resourceavailability should be successful in producing a corresponding increase in beepresence.

Keywords: bees, hymenoptera, pollinators, foraging, resource characteristics, re

gional landscape, tropical dry forest, Costa Rica, Tecoma stans

Introduction:

Bees are essential ecosystem service providers that are in large part responsible for maintaining terrestrial diversity and productivity. The association of bees with flowering plants structures and sustains ecosystems (Kevan 1999; Kevan 2001; Costanza et al. 1997) from which humans and other species derive direct andindirect benefits (Buchmann and Nabhan 1996; Daily 1997; Kevan and Wojcik 2007).Declines in populations of bees have been noted in recent years and are drivenin part by land-use intensification, such as the conversion of wildlands into agricultural and urban landscapes (NRC 2007; Kremen et al. 2007; Winfree et al. 2007; Winfree et al. 2009). Land-use intensification often diminishes or removes t

he nesting and foraging habitats of native bees (Kremen et al. 2002). The tropical dry forest region of Costa Rica is an example of an area of rapid urbanization where declines in bee numbers have been noted at the interface of urban develo


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pment (Frankie et al. 2009). Declines in the abundance and diversity of bees areparalleled by declines in pollination services (Kevan and Phillips 2001). Continued declines in pollination could have serious ecological and economic impacts,as one third of the food we eat is dependent on pollinators (Buchmann and Nabhan 1996), and the majority of flowering plants require pollinators for reproduction (Kevan 1999).

Early twentieth-century taxonomic surveys verify that bees have been present inurban landscapes for some time (Robertson 1929; Graenicher 1930). More recently,rich communities of bees have been documented in cities in North America (Frankie et al. 2005b; Tommasi et al. 2004; Fetridge et at. 2008; Matteson et al. 2008), Central America (Frankie et al. 2009a), South America (Matteson et al. 2008;Loyola and Martins 2006; Nates-Parra 2006), Europe (Banaszak 1982; Berezin 1995;Saure 1996), and Asia (Sakagami and Fukuda 1973; Hisamatsu and Yamane 2006). Although the presence of bee communities within cities does not suggest that urbanlandscapes can entirely compensate for lost habitat, the colonization of urbangreen space by bees might provide a buffer for some of the effects of wildland degradation on pollination services. This observation has led scientists to investigate the conservation and management potential that human landscapes hold for

ecologically and economically important species.

Understanding why a species occurs at a site and what factors are important to this occurrence can inform conservation planning and management. Associations between bees and flowering plants are thought to be distinct and predictableseasonality, host-specificity, and daily flight times are patterned (Linsley 1958; Ginsberg 1983; Wojcik et al. 2008). Resource usage and exploitation are often optimized with the heavy metabolic demands of flight (Heinrich 1979). In tropical systems, floral resources occur at many strata with corresponding vertical stratification, or structural niche partitioning, in different bee species (Roubik 1993).Bees also display site-specific preference patterns in their foraging behaviors,and not all resources within a landscape are visited equally. The study of beeecology in urban landscapes has focused mostly on identifying species presence a

nd absence, describing faunas, or outlining host plant interactions. More detailed studies that outline local occurrence patterns are needed to develop a betterunderstanding of bee foraging strategies and behaviors in cities.

Click image to enlarge

Figure 1: Examples of Tecoma stans plants growing in tropical urban landscapes and displaying a range of structural morphology. Clockwise from top left: (a) shrub, (b) hedge, (c) tree, and (d) a close-up of the inflorescence.

Tecoma stans Kunth (Bignoniaceae) is a common, mass-flowering, woody perennial found across Costa Rica and much of the tropics. In urban environments, it is prominent in home gardens and city squares and along city streets. It has been usedextensively as a decorative ornamental because of its year-round production ofstriking, yellow, trumpet-shaped flowers. Previous observations indicated that T. stans attracts a wide range of bee species from many genera, diverse guilds, and many foraging strata, representing over 10% of the local wildland bee fauna (Wojcik 2009). In the three urban landscapes under study here, T. stans is ubiquitous and presents substantial variability in plant form and structure, particularly in urban landscapes where human management, such as pruning, has resulted inindividuals that range from low-growing shrubs through full hedges to tall trees (Figure 1). Its ubiquitous distribution, structural variability, and observedhigh attractiveness to bees make T. stans a good case-study species for investigations of urban bee foraging preferences in multiple tropical cities.

The goal of this study was to determine what makes a food resource located within a city attractive to local bee species. Are the structural and morphological p


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atterns documented in wildland systems conserved in urban systems? Are all cities similar, and do bees choose the same types of resources in any urban landscape? Furthermore, given the diversity of bee species within urban systems, are allof these bees looking for, or responding to, the same factors? To answer these questions, a forager visitation survey was conducted in the cities of Bagaces, Caas, and Liberia, Costa Rica, focusing on the community of bees that visited T. stans resources. The recorded patterns of bee visitation were examined with respec

t to resource characteristics (floral abundance, height, and plant form), city size, general local landscape characteristics, and regional landscape context todetermine if any other patterns became evident. Gaining a better understanding of how bee foraging strategies and behaviors manifest in cities will add valuableinformation to the new and growing discipline of urban bee ecology.

Materials and Methods

Site description

The cities of Liberia (1037'47.19"N, 8526'17.75"W), Bagaces (1031'34.29"N, 8515'17.73"W), and Caas (1025'35.95"N, 8505'28.02"W) have developed along Central America Hi

ghway 1 in the Guanacaste province of Costa Rica. Each city is about 25 kilometers from its neighbor, separated by remnant tropical dry forest that is fragmented by ranching and agriculture. Urbanization and growth in this region are abovethe national average, with the population doubling in the past decade (INEC 2000). These three cities have experienced corresponding urban and peri-urban growth. The most recent national census puts the population of Liberia at 39,242 (34,469 urban; 4,773 rural), Bagaces at 9,261 (3,645 urban; 5,616 rural), and Caas at18,798 (16,512 urban; 2,286 rural) (INEC 2000). The area currently covered by Liberia is approximately 8 square kilometers; Bagaces, the smallest city, covers approximately 1 square kilometer; and Caas covers approximately 3 square kilometers (urban land coverage estimated in ArcMap 9.3, ESRI Inc. 2009). Demographic projections point to significant growth in this region; by 2015, Liberia is expected to have nearly 45,000 inhabitants, Caas will grow to over 38,000 inhabitants, a

nd Bagaces will double to over 22,000 inhabitants (INEC 2000). Bagaces and Liberia lie in close proximity to the biological reserve network of Lomas Barbudal and Palo Verde and are surrounded by land that is used primarily for cattle grazing. Caas resides within an intensive crop-production region. Building density andthe size of individual lots is smaller in both Liberia and Bagaces than in Caas,but Caas has more public green space and parks. Bagaces and Liberia are bisectedby many riparian zones, but there is only one riparian corridor in Caas.

Tecoma stans (Bignoniaceae) is common along city streets and on private land inall three cities, as well as throughout the wildlands in this region. The nativerange of this species in the Americas extends from the south-central and southeastern United States (Arizona to Florida) to the Antilles and down into Argentina (Hammel 2005; Zuchowski 2007). In Costa Rica, T. stans is most often found onPacific slopes in Guanacaste province and into the central mountainous region, including the greater San Jos area of Puntanenas province (Hammel 2005). Individual plants can reach a height of 10 meters (Zuchowski 2007) but are commonly shorter in urban landscapes (Figure 1).

Resource Classification

The location of each plant in the study was recorded using a hand-held Garmin eTrex Vista C GPS unit. A digital photograph was taken at each site to serve as a reference and to assist with resource height assessment. Height was estimated tothe nearest 25 centimeters using either a meter measure or photographic scalingwith the author (measuring 158 cm) as a reference. The T. stans individuals in t

his study were then grouped into three height classes: small (< 1 meter), medium(13 meters), and large (> 3 meters). Plant form was categorized as shrub, hedge,or tree as follows: shrubs were low-growing (generally shorter than 1 meter) in


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dividuals that lacked a clear dominant axis; hedges ranged in height and were predominantly characterized by having top-to-bottom foliage; trees had a distinctdominant axis (trunk) and a crown of foliage and flowers.

The number of flowers that an individual T. stans possessed was estimated usinga stratified subsample of the inflorescences in the canopy. The canopy was fractioned into a manageable sector based on the overall resource size (either 2, 4,

8, or 10 sectors). The number of fractioned sectors was then used as an expansion factor (k) to calculate the estimated number of inflorescences (i) and the subsequent total flowers per resource (total flowers). The total number of inflorescences in one randomly selected sector was counted (i). A random subsample of the enumerated inflorescences was then selected, and the total number of flowers contained in each inflorescence was counted and averaged over the total sample ().The average number of flowers per inflorescence was then multiplied by the number of inflorescences and by the expansion factor using the following equation: total flowers = x i x k. In the case of some individual plants that possessed very few flowers, the total number of flowers was enumerated using a census. T. stans individuals were then classified into three categories of floral resource abundance: low (< 100 flowers per individual), medium (100300 flowers per individual

), and high (>300 flowers per individual).

Bee visitation assessment

Between July 2007 and March 2009, 2,088 visitation counts documenting the richness and abundance of foraging bees were conducted at 120 unique T. stans individuals across the three landscapes. A standardized 1 x 1-meter square observation frame was visually projected onto an easily observable and unobstructed area of the plant (floral resource). The visits of bees to all of the flowers within thisvisual frame were counted for a period of three minutes. To achieve a representative sample of overall bee visitation, counts were taken approximately every 45minutes during the daylight hours of 6 a.m. to 6 p.m. Efforts were made to randomize the observation frames over repeated visits to each resource. A visitation

was recorded only if a bee entered the corolla of the flower.

Generalized flight patterns and overall gestalt (morphology, coloration, etc.) were used to identify bee visitors "on the wing". Eleven distinguishable taxonomic categories of visitors were observed: African honey bees (Apis mellifera scutellata, abbreviated AHB), stingless bees (Trigona spp., abbreviated STB), Centris(Centris spp. and other resin bees), Eulema (Eulema spp.), Epicharis (Epicharisspp.), Euglossa (Euglossa spp.), Halictid (members of the sweat bee family Halictidae), Mesoplia (Mesoplia spp.), Melitoma (Melitoma spp.), Xylocopa (Xylocopaspp.), and small bees (when a specific identification could not be made). Abundance was recorded as the total number of individual bees that were observed per count. Taxon richness was recorded as the total number of different distinguishable taxonomic categories observed per count.

Representative samples of foraging bees were collected with aerial netting for more refined taxonomic identification. The bee collections were labeled and stored in standard solid wood entomological collection boxes. Bee identification wasassisted by Laurence Packer and Jason Gibbs of York University, Toronto, Canada,and Ricardo Ayala of Universidad Nacional Autnoma de Mxico (UNAM). The majority of the collection resides at the University of California, Berkeley Essig Museumof Entomology, with some parts at the Pollinator Partnership offices in San Francisco, California. The bee-plant interactions described here are also cataloguedwith the Pollinator Thematic Network and can be found by searching the bee species listed in the Appendix.

African honey bees (AHB), stingless bees (STB), members of the family Halictidae(Halictids), and bees in the genus Centris (Centris) were the dominant groups observed in this study. Statistical analysis that aimed to describe trends in fun


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ctional groups focused on these four categories.

Statistical Analysis

The observed variability in abundance and taxon richness seen at Tecoma stans resources was verified using repeated measures ANOVA (nresource = 42, nrepeats = 3, = 0.05). Visit tion d t were n lyzed collectively s well s by city to inve

stig

te site-specific v

ri

bility. Multiv

ri

te ANOVA w

s

lso used to est

blishif t xon richness, visitor bund nce, nd individu l bee groups differed between the three cities studied. A Bonferroni djusted of 0.016 (0.05/3) w s used inthis comp rison to djust for Type 1 Error ssoci ted with multiple tests.

The m in nd inter ction effects of resource ch r cteristics (height, life-form, nd flor l bund nce) on bee bund nce, richness, nd the occurrence r tes of the four domin nt t xon groups (Centris, H lictid, AHB, nd STB) were ex mined using multiv ri te ANOVA, with Bonferroni djusted of 0.016 (0.05/3) to djust for Type 1 Error. All of the multiv ri te tests were further n lyzed with Tukey's HSD to determine the direction nd m gnitude of the observed differences. To determine if there were ny ssoci tions between visit tion r tes nd city size o

r the surrounding l

ndsc

pe, the

bund

nce, richness,

nd visit

tion r

tes of the individu l bee groups were comp red using ANOVA, =0.05, followed by Tukey's HSDfor multiple comp risons. All st tistic l n lysis w s run in SPSS 16 (Rel. 16.0.0, 2007 Chic go: SPSS Inc.).

Results

The bees observed nd collected from Tecom st ns represent 27 species in 20 gener nd 2 f milies. The species were diverse in their feeding styles, nesting h

bit ts, nd degrees of soci liz tion (Appendix). Most visitors to T. st ns in the urb n l ndsc pe were solit ry, ground-nesting species; however, c vity-, twig-, nd wood-nesters, s well s eusoci l nd commun l species, were present. Three groups of n tive bees ccounted for 90.36% of the tot l visitors recorded: Cen

tris, STB,

nd H

lictid. Centris w

s the most

bund

nt

t 52.10%, followed by STB t 19.43%, nd H lictid t 18.86%. Nonn tive AHB (Apis mellifer scutell t )

ccounted for 5.68% of the tot l records. The rem ining seven groups ccounted for less then 2% e ch of the tot l visitors in the entire s mple: Eulem , 1.12%; Xylocop , 0.89%; Eugloss , 0.69%; Epich ris, 0.44%; Mesopli , 0.19%; sm ll bee, 0.16%; nd Melitom 0.15%.

V ri bility in individu l Tecom st ns resources

St tistic l tests verified the observed v ri bility in bee ttr ction to individu l T. st ns pl nts to be signific nt. Between-resource bee v ri tion r tes weresignific ntly different for both bund nce (F1,441 = 0.005744, p = 0.000) nd t

xon richness (F1,441 = 0.003596, p = 0.000). Me sures of within-resource bee visitor bund nce nd t xon richness exhibited contr sting trends. Bee bund nce did not v ry signific ntly from me sure to me sure on the s me resource (ANOVA, F2,882 = 0.517, p = 0.596); however, the number of different bee t x recorded per count did (repe ted me sures ANOVA, F2,882 = 2.329, p = 0.098). In ddition, there w s signific nt inter ction effect between me sures of bee t xon richness nd visitor bund nce nd the individu l resource s mpled (p < 0.000), which suggests th t other v ri bles might be structuring p tterns of bee visit tion to T. st ns in urb n l ndsc pes.

Food resource ch r cteristics nd bee for ging

Click im ge to enl rge

Figure 2: The me n bund nce nd richness of bees ttr cted per count to T. st n


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s s function of three different resource ch r cteristics: height (short = < 1meter; medium = 13 meters; t ll = >3 meters), pl nt form (shrub, hedge, tree),

nd flor l bund nce (low = < 100 flowers; medium = 100300 flowers; high = >300 flowers).

Bee bund nce nd richness were signific ntly influenced only by the number of flowers th t resource h d (F2,16 = 4.014, p = 0.000; F2,16 = 4.836, p = 0.000,

respectively),

nd not by its height or pl

nt form (T

ble 1). To

lesser extent, the inter ction between tot l flowers nd pl nt form influenced the bund nceof bees visiting resource (F4,16 = 2.063; p < 0.091), but not the tot l numberof bee types. Figure 2 displ ys the gener l trends seen in the bund nce nd richness of bees observed per count s function of resource ch r cteristics. Thethree flor l cl sses re signific ntly different from e ch other; in order of incre sing flor l bund nce, e ch c tegory ttr cted signific ntly more bees ndmore bee types (Tukey's HDS; p >0.05) th n the one before. There w s no observedv ri bility in visitor bund nce nd ttr cted t xon richness b sed on the pl nt form. Although medium nd t ll resources did ttr ct more bees nd more bee types th n shorter resources, this difference w s not signific nt.

When the four domin

nt bee groups were ex

mined for their individu

l responses to resource ch r cteristics, more specific p tterns were seen (T ble 2). The for

ging r tes of Centris nd STB were not signific ntly imp cted by ny of the resource ch r cteristics or their inter ctions. The for ging r tes of H lictids wereimp cted by the pl nt form nd the tot l number of flowers possessed by resource (F2,16 = 5.477; p < 0.005 nd F2,16 = 4.764; p < 0.010, respectively), s well s by the inter ction between the two (F2,16 = 5.094; p = 0.001). AHB were signific ntly influenced by the tot l number of flowers resource h d (F2,16 = 8.540; p = 0.000), nd by the inter ction between pl nt form, height, nd tot l flowers (F3,16 = 7.472; p = 0.007). The community-level for ging p tterns re likely driven by the responses of H lictids nd AHB to individu l resource v ri bility.

City size

nd ch

r

cteristics of the surrounding region

l l

ndsc

pe

E ch city h d unique bee f un nd specific p tterns of for ger occurrence. The me n occurrences of 9 of the 11 bee groups s mpled v ried signific ntly between the 3 cities (T ble 3). Only Euglossine bees nd sm ll bees did not show ny signific nt v ri tion (F2,26 = 1.825; p = 0.161 nd F2,26 = 2.044; p = 0.130, respectively). The size of the city h d signific nt imp ct on the number of bee t

x th t were observed per count. Me n t xon richness incre sed signific ntly inthe order B g ces < C s < Liberi (Tukey's HSD, B g ces: me n = 1.150, 95% CI [1.070, 1.230]; C s: me n = 1.368, 95% CI [1.294, 1.442]; Liberi : me n = 1.479,95% CI [1.413, 1.545]); which is consistent with n incre se in size from the sm

llest to the l rgest urb n l ndsc pe (Figure 3). City size did not h ve line

r correl tion with bund nce p tterns.

Click im ges to enl rge

Figure 3: The me n number of different bee t x observed per three-minute count cross ll counts t ken in the three study cities, B g ces, C s, nd Liberi . The cities re rr nged on the x- xis in order of incre sing size. In e ch c se, there re signific ntly more bee types per count s city size incre ses (Tukey'sHSD, B g ces: me n = 1.150, 95% CI [1.070, 1.230]; C s: me n = 1.368, 95% CI [1.294, 1.442]; Liberi : me n = 1.479, 95% CI [1.413, 1.545]). Error b rs representthe 95% confidence interv ls for e ch me sure.

Figure 4: The represent tive proportions of the domin nt bee groups (AHB, Centris, STB, nd H lictids) recorded on T. st ns resources in the cities of B g ces,


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C s, nd Liberi . "Minor Groups" represents those bees th t ccounted for < 2% of the tot l bund nce found t e ch site, nd in e ch c se included: Xylocop , Epich ris, Eugloss , Eulem , Mesopli , Melict , nd sm ll bees. The domin nt region l l nd use is indic ted in br ckets for e ch city.

There w s lso rel tionship between t xon domin nce nd the gre ter l ndsc pecontext. Figure 4 displ ys the community composition of bees observed in e ch ci

ty over the three-ye

r study period. B

g

ces

nd Liberi

re situ

ted within c

ttle r nching l nds nd ne rer to conserv tion l ndsc pes; they h ve simil r proportions of Centris, H lictids, nd STB. C s is situ ted within more gricultur

l l ndsc pe nd h d higher numbers of Centris nd STB, but lower occurrences ofH lictids.

Discussion

The tropic l dry forest of Cost Ric is known for its bee diversity. More th n250 n tive bee species re found in the wildl nds of this region (Fr nkie et l.1983). M ss-flowering ngiosperms common to the s v nn nd mesic h bit ts provide ide l for ge th t supports the bee community (Fr nkie et l. 2004), but r pi

d urb

n growth is tr

nsforming this ecosystem. Tecom

st

ns resources loc

ted inthe three cities studied here ttr cted subs mple of the wildl nd bee community27 species with diverse life histories. The species visiting T. st ns r nge inbody sizes, flor l us ge p tterns, nd nesting h bits. The b l nce of guilds ttr cted, proportion of the n tive bee community (10%) ttr cted, high tempor l ttr ctiveness, nd nnu l blooming suggest th t T. st ns might be n ppropri tet rget resource th t c n highlight trends in the loc l bee community. V ri bility in the over ll ttr ctiveness of individu l trees h s been est blished, nd individu l cities h ve been shown to h ve different communities of bees. The implic tions for s mpling, monitoring, nd conserv tion in rel tion to these trends

re discussed in further det il.

P tterns in resource v ri bility

The popul tion of Tecom st ns ex mined showed v ri bility in bee ttr ction. This to some extent c n be expl ined by the gre ter l ndsc pe content within whichsubs mples were found, suggesting th t the surrounding l ndsc pe imp cts the structure of urb n bee communities. At finer resolution, individu l resources within the three cities were found to v ry signific ntly in both the number of bees nd the number of bee groups ttr cted. Between-resource v ri bility in bee visit tion h s lso been documented in other loc l species in the f mily Bignoni ce e, n mely, T bebui rose (Fr nkie et l. 1982) nd T. ochr ce . The s me is seen in some members of the f mily F b ce e within this region, including D lbergi retus , Gliricid sepium, nd Andir inermis (Fr nkie et l. 1997; Fr nkie et l. 2005 ). Suggested c uses include: the qu ntity nd qu lity of bloom; v ri tion in both the volume nd sug r content of nect r (Kev n 2001); inconsistenciesin se son l phenology, especi lly dv nced or del yed blooming (Fr nkie et l.2005 ); v ri bility in second ry compounds found in nect r (Roubik et l. 1995);competition for pollin tors in the surrounding flor l community; nd the ch r cteristics of the surrounding urb n m trix (C ne et l. 2006).

The for ging str tegies of bees re optimized, b l ncing resource cquisition with competition nd energy consumption. This optimiz tion results in the development of p tterns nd techniques to f cilit te efficient resource cquisition, including: tempor l flight p tterns, feeding speci liz tion, tr p-lining beh vior,

nd resource p rtitioning by l ndsc pe loc tion nd vertic l str tum. While vertic l str tific tion h s been suggested (Roubik et l. 1982; Roubik 1993; Gumbert nd Kunze 1999), in this p rticul r system there were no signific nt p tterns w

ith respect to resource height or pl

nt form. Inste

d, the number of flowers th

t resource possessed, me sure of resource bund nce, w s signific nt driver of bee visit tion. Resources with more flowers ttr cted more bees, irrespecti


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ve of their height nd pl nt form. This trend w s driven, in p rticul r, by thevisit tion of Afric n honey bees (AHB) nd H lictids. The visit tion r tes of Centris nd stingless bees (STB) were not influenced by resource ch r cteristics

nd structure, which is consistent with wildl nd studies of for ging in these species groups (Roubik et l. 1982). Contr ry to the results presented here, Roubiket l. (1982) did not est blish ny rel tionship between flor l bund nce nd bee for ging. The size of the resource p tch w s, however, shown to correl te wit

h bee visit

tion, with l

rger p

tches receiving more visits, in studies of cropfields in Engl nd (Cresswell nd Osborne 2004).

L ndsc pe context nd loc l bee communities

Ch r cteristics of the surrounding urb n m trix m y be structuring bee visit tion t different sp ti l sc les. B g ces nd Liberi h ve the most simil r p tterns of bee visit tion, nd both re situ ted within simil r region l l nd use: c ttle gr zing in modified gr ssl nds punctu ted by n tive dry forest. Ne rby re the biologic l reserve systems of Lom s B rbud l nd P lo Verde. C s is different, sitting in the center of gricultur l intensific tion. On finer sc le, B g ces nd Liberi re interspersed with more stre ms nd rip ri n re s th n C s. A

more refined species-specific

n

lysis of the bee communities would reve

l if there re more subst nti l trends in t xon presence nd bsence in urb n re s loc ted within different l ndsc pes. An initi l study of urb n l ndsc pes cross C

liforni conducted by Fr nkie et l. (2009b) indic ted th t the species ttr cted to set of t rgeted orn ment l pl nts differed between individu l cities, even between those loc ted within simil r ecoregions.

The community of bees t loc lity is determined by history, context, nd current m n gement pr ctices. Historic l presence/ bsence records re not lw ys v il ble for site, nd this is the c se for the cities of B g ces, C s, nd Liberi . The bee species th t occur in urb n l ndsc pes c n be mix of loc l n tivesmigr ting in from the wildl nd, gener lists th t re successful in novel l ndsc

pes, or introduced exotics ( common species group in urb n re s due to the in

fluence of

ctive

nd p

ssive hum

n introductions). P

tterns in richness

re likely driven by the spor dic occurrence of r re species. Attempts to more dequ tely c t logue species richness will require more extensive s mpling over four-to five-ye r period, due to both n tur l v ri bility in pollin tor popul tions (see E rdley et l. 2006 nd the 2007 NRC report St tus of Pollin tors in North Americ ) nd l rge-sc le cycles of se son l bee v ri bility, which re common tothe tropics (Fr nkie et l. 2005 nd 2009 ).

Finer-sc le investig tions of urb n systems

Urb n l ndsc pes h ve been shown to be ccept ble nd ppropri te h bit ts for some species of bees. The diversity nd import nce of bee communities found within cities present opportunities for conserv tion nd m n gement. The m n gement of ecologic l systems, however, requires n intim te knowledge of ecosystem-levelfunction nd species-specific biology. Further comp risons between different urb n l ndsc pes nd within the heterogeneous urb n m trix re lso necess ry. Studies th t ex mine more th n one l nd-use type re limited in the liter ture. Only four studies of urb n bees h ve considered multiple cities (Dreist dt et l. 1990; Porrini et l. 2003; Fr nkie et l. 2005 ; Fr nkie et l. 2009 ), nd onlyone w s met - n lysis th t ttempted to integr te inform tion cross multiplel ndsc pes (C ne 2005).

Urb n l ndsc pes do sh re m ny simil r ecologic l ch r cteristics in terms of l

nd-use p tterns nd disturb nce regimes, but they re lso situ ted within different ecologic l regions. It is uncle r if there re l rger congruent trends in b

ee ecology between different urb

n l

ndsc

pes,

nd if these trends

re glob

llyconsistent. The v ri bility in the p tchwork of l nd-use types th t m kes up theurb n l ndsc pe must lso be ex mined in more det il to determine if sm ll-sc l


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e, loc l f ctors influence the success of bees in modified l ndsc pes. Sm ller bodied species such s bees respond to sc le-depend nt f ctors, nd n ppropri te underst nding of bee ecology within ny l ndsc pe requires loc l-sc le investig tions. The next step in urb n bee for ging ecology will be to outline microh bit t elements nd sm ll-sc le v ri bility within the urb n m trix nd to determine their imp ct on bee occurrence nd community structure.

Acknowledgments

Funding w s provided by the M rg ret C. W lker Fund in p rtnership with the Essig Museum of Entomology t the University of C liforni -Berkeley. I m very gr teful to L urence P cker nd J son Gibbs of York University, Toronto, C n d , ndRic rdo Ay l of UNAM, for their ssist nce with specimen identific tion. I would like to th nk L ur Fine nd Me gh n J strebski, who provided field ssist nce nd good comp ny throughout the dry se son. Addition lly, the hospit lity of the S ndov l-Argon f mily m de the rese rch experience wonderful.

city size, regional landscape context, and local resource characteristics influence patterns of bee...

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