Orchid bees in riparian and terra-firme forest fragments in an urban matrix in southwestern Brazilian Amazonia

Abelhas das orquídeas em fragmentos de floresta ripária e de terra-firme em uma matriz urbana no sudoeste da Amazônia brasileira

ABSTRACT

Riparian forests are important ecosystems that support an enormous biodiversity in Brazil. Despite being protected under Brazilian legislation, these forests suffer great impact from the fragmentation of habitats. Orchid bees are a key group of pollinators in the Neotropical region, yet few data are available on the assamblage structure of these bees in riparian forests. We evaluated the role of fragments of riparian and terra-firme forest on the conservation and maintenance of orchid bees in an urban landscape in the southwestern Amazon basin. Specifically, we evaluated whether bee assemblages in riparian and terra-firme forests differed significantly in abundance, species richness and composition. We also evaluated whether species richness and abundance of bees vary with the size of the forest fragment. Male bees were attracted using odoriferous baits and collected with entomological nets in 10 forest fragments. There was no significant difference between riparian and terra-firme fragments in species abundance, richness and composition, but there was a positive correlation between fragment size and species richness and abundance. Our results suggest that, in an urban landscape, the remaining riparian and terra-firme forest fragments still could maintain 62.7% of the orchid bee species known to occur in the region, reinforcing the conservation value of these forest remnants. Our findings indicate that these fragments provide a potentially important habitat for the maintenance of local bee populations in the landscape.

KEYWORDS: Euglossini; forest fragmentation; pollination; urban landscape

RESUMO

As florestas ripárias no Brasil são importantes ecossistemas que sustentam uma enorme biodiversidade. Apesar de protegidas pela legislação brasileira, elas têm sofrido grandes impactos decorrentes da fragmentação florestal. As abelhas das orquídeas constituem um grupo chave de polinizadores na região Neotropical, porém, pouco se sabe sobre suas assembleias em florestas ripárias. Nós avaliamos o papel de fragmentos de floresta ripária e de terra-firme na conservação e manutenção da fauna de abelhas das orquídeas em uma paisagem urbana no sudoeste da Amazônia. Especificamente, avaliamos se assembleias de abelhas de fragmentos ripários e de terra-firme diferem significativamente em abundância, riqueza e composição de espécies. Também avaliamos se a abundância e a riqueza de espécies variam em função do tamanho do fragmento. Machos de abelhas foram atraídos por iscas odoríferas e coletados com redes entomológicas em 10 fragmentos florestais. Não houve diferença significativa entre fragmentos ripários e de terra-firme quanto à abundância, riqueza e composição de espécies, mas houve uma correlação positiva entre o tamanho do fragmento e a riqueza e abundância de espécies. Nossos resultados sugerem que, em uma paisagem urbana, os fragmentos de floresta ripária e de terra firme ainda podem manter 62,7% do número de espécies de abelhas das orquídeas conhecido para a região, reforçando o valor da conservação desses remanescentes florestais. Nossos dados indicam que esses fragmentos fornecem um habitat potencialmente importante para a manutenção das populações locais de abelhas na paisagem.

PALAVRAS-CHAVE: Euglossini; fragmentação florestal; polinização; paisagem urbana

INTRODUCTION

Riparian forests provide important ecosystem services, such as the maintenance and protection of water resources, geological stability and biodiversity, the facilitation of gene flow of both fauna and flora, and the protection of soils, while also supporting the wellbeing of human populations (Borges et al. 2011; Monteiro et al. 2013; Martins 2014). In Brazil, despite being protected under forestry legislation, which requires the maintenance of riparian zones on private properties as permanent reserves (BRASIL 2012), riparian forests have been widely impacted by deforestation, habitat fragmentation, and urbanization (Martins 2014; Ferreira et al. 2015).

Concomitantly, the recent worldwide decline of pollinators has reinforced fears of a potential global crisis (Novais et al. 2016). Anthropogenic factors, such as habitat loss, changes in land use, the application of pesticides, and the intensification of farming activities, may all have a major impact on many groups of insect pollinators, including bees (Ferreira et al. 2015; Araújo et al. 2018; Almeida et al. 2020).

In the Neotropical region, the orchid bees (Apidae: Euglossini) are a key group of insect pollinators (Roubik and Hanson 2004), more abundant and diverse in well-preserved environments (Antonini et al. 2016; Allen et al. 2019), although some, more generalist species are found in degraded areas (Silva and De Marco-Júnior 2014; Aguiar et al. 2015). They are pollinators of native and cultivated plants and the conservation of this group of bees is a fundamental prerequisite for the conservation of many orchids and a wide variety of other plants found in tropical forests (Cavalcante et al. 2012). The fragmentation and loss of forest habitat impact orchid bee communities through the interruption of ecological processes, such as pollination, and a reduction in the availability of flowers that produce pollen, nectar, and aromatic substances (Armbruster 1993; Tonhasca et al. 2002, Livingston et al. 2013, Brito et al. 2018). Larger fragments are expected to contain a higher number of species (MacArthur and Wilson 1967), as they tend to have greater environmental heterogeneity, diversity of food sources and floral fragrances, and nesting sites, which favor greater richness and abundance of bees (Morato and Campos 2000; Tonhasca et al. 2002). Higher visitation rates to odoriferous baits and greater species richness of orchid bees have been reported in larger forest fragments (Powell and Powell 1987), but data on this subject are generally scarce for orchid bees (Powell and Powell 1987; Ramalho et al. 2009; Brosi 2009).

Most studies of orchid bees have focused on fragments of terra-firme forest (Storti et al. 2013; Storck-Tonon et al. 2013; Brito et al. 2018, but see Storck-Tonon and Peres 2017). However, some bees are known to prefer riparian habitats (Moura and Schlindwein 2009; Brito et al. 2017), which could be related to favorable conditions of temperature and humidity, necessary for the development of the brood. Futhermore, the plants found in tropical riparian forests offer floral resources all year round for a wide range of pollinators, including many species of solitary and social bees that nest in riparian forests and their marginal habitats (Moura and Schlindwein 2009; Pires et al. 2012; Montoya-Pfeiffer et al. 2020).

Relatively few data are available on the diversity and ecology of euglossine in riparian forests (Carvalho et al. 2006; Moura and Schlindwein 2009; Faria and Silveira 2011), in particular in the Amazon biome (Brito et al. 2017). Another little known aspect in the Amazon is the adaptability of euglossines to urban environments. Many species of orchid bees occur and nest in urban landscapes (Storti et al. 2013). Relatively small forest fragments, such as public parks, in urban areas can act as dispersion corridors and increase habitat connectivity for bee assemblages (Nemésio and Silveira 2007; Storck-Tonon et al. 2013; Storti et al. 2013; Nemésio 2014).

No previous study in the Amazon biome has compared the orchid-bee fauna in remnants of riparian and terra-firme forest in an urban matrix. In this context, we evaluated the diversity and abundance of orchid bees in urban fragments of riparian and terra-firme forest in the southwestern Brazilian Amazon, to assess their role in the maintenance of the orchid bee fauna of an urban landscape in the region. Our predictions were: (i) riparian and terra-firme forest fragments differ in abundance, species richness and species composition of orchid bees; and (ii) larger fragments, independently of forest type, have greater abundance and richness of bees.

MATERIAL AND METHODS

Study area

We sampled male orchid bees from September 2015 to February 2016 in 10 forest fragments in the urban landscape of the city of Rio Branco, Acre state, in northern Brazil (Table 1; Figure 1). We selected forest fragments with an area between 1.6 and 162.2 ha, which were assigned to one of two forest categories, either riparian or terra-firme (forest that is never flooded), based on their proximity to the nearest water course (Table 1). In general, the terra-firme fragments were larger (average size = 55.5 ha) than the riparian ones (average size = 45.4 ha), with the exception of the Via Parque private area, which was the smallest sampled fragment (Table 1). All the selected fragments were located within a predominantly urban matrix, including some open habitat (built areas, streets, roads, vacant lots and small pastures) in the areas surrounding the fragments. The distance between fragments varied from 2.1 to 13.9 km (average = 7.7 km). Only three fragments were at an average distance above 7.7 km from other fragments. The predominant vegetation of the study region is open tropical terra-firme forest with bamboo and palm trees, and dense forest (Silveira 2005). The local climate is tropical hot and humid, with a mean annual temperature of 32°C (ACRE 2010). During the study period, the mean monthly precipitation was 171 mm, with November being the rainiest month (327.1 mm) and January the driest month (96.3 mm). The mean relative humidity was 83.8 ± 7.7%, with mean daily temperatures of 23.5 ± 1.2 °C (INMET 2016).

Table 1
Location, size and classification of the forest fragments sampled for orchid bees in the urban landscape of Rio Branco, Acre, Brazil.

Forest fragment	Forest type	Coordinates	Area (ha)
Batista Stream	Riparian	09°58’11”S, 67°51’31”W	5.2
Tucumã Stream	Riparian	09°57’11”S, 67°51’02”W	12.7
Ouricuri Stream	Riparian	09°56’02”S, 67°48’24”W	5.3
São Francisco Stream	Riparian	09°57’11”S, 67°47’33”W	121.4
Judia Stream	Riparian	10°02’00”S, 67°46’20”W	82.5
Zoobotanical Park	Terra-firme	09°57’23”S, 67°52’21”W	162.3
Municipal Forest Nursery	Terra-firme	09°56’47”S, 67°49’44”W	22.1
Via Parque Private Property	Terra-firme	09°59’11”S, 67°50’31”W	1.6
Amapá Environmental Protection Area	Terra-firme	10°03’15”S, 67°49’30”W	35.5
Chico Mendes Environmental Park	Terra-firme	10°02’20” S, 67°47’38” W	56.1

Figure 1
Location of the study area in the state of Acre in Brazil, and of the sampling sites in the urban matrix of the municipality of Rio Branco, Acre, Brazil (RF = riparian forest, TF= terra-firme forest).

Bee sampling

In each fragment, we collected bees at three sampling points that were at least 50 m from the edge of the fragment, and 100 m from each of the other two points. Each point was sampled twice, on different days within the study period, totalling six samples and approximately 48 hours of sampling time in each fragment. This sampling effort has been successfully used to sample bees from orchids in the region (Storck-Tonon et al 2009; Storck-Tonon et al. 2013; Cândido et al. 2018). The average sampling interval in almost all fragments varied around 30 days, although the interval between sampling occasions withn fragments was quite variable, from three to 86 days (Supplementary Material, Appendix S1). At the end of six months, all fragments were sampled six times.

On each sampling day and at each sampling point, we soaked two cotton swabs in each of the odoriferous substances benzyl acetate, cineol, skatole, eugenol, methyl salicylate, and vanillin, and hung them from a horizontal nylon line at a height of 1.5 m above the ground, with an interval of 2 m between each pair of swabs, following the protocol of Storck-Tonon et al. (2009). We captured bees from 7:00 to 15:00 using an entomological net, as this method is more efficient in the region than trapping with plastic bottles (Nemésio and Morato 2006).

We collected data on the ambient and soil temperatures, luminosity, and relative humidity on an hourly basis during each sampling period using a portable digital thermo-hygro-anemometer-luximeter (THAL-300, Instrutherm, São Paulo, Brasil) and an infrared soil thermometer (± 2°C, MT-350, Minipa, São Paulo, Brasil). We estimated the area (in hectares) of each fragment in ArcGis 10.1, analyzing the polygon of each area derived from LANDSAT8 OLI sensor (Orbit/point: 002/067) satellite images, which have a 30 m spatial resolution in the multispectral band and a 15 m resolution in the panchromatic band.

We identified most of the collected bees to species level by comparing the specimens with the vouchers deposited in the reference collection of Universidade Federal do Acre (UFAC), in Rio Branco, Acre, Brazil. The specimens of the genus Eufriesea were identified by Dr. José Eustáquio dos Santos (Universidade Federal de Minas Gerais, UFMG), and some of the specimens of the genera Euglossa, Eulaema and Exaerete were identified by Dr. Danielle Storck-Tonon (Universidade do Estado de Mato Grosso, UNEMAT) and Dr. Marcio Luiz de Oliveira (Instituto Nacional de Pesquisas da Amazônia, INPA). The specimens were deposited in the Entomological Collection at the Center for Biological and Nature Sciences of UFAC.

Statistical analyses

To assess the sampling efficiency, we constructed species accumulation curves for the riparian and terra-firme fragments separately, using the specaccum function in the ‘vegan’ package (Oksanen et al. 2020) in R 3.2.3 (R Development Core Team 2016). This function sequentially samples random individuals from a data set and measures the number of novel species encountered as additional individuals are sampled. We use the ‘random’ method, which finds sites in random order and samples individuals without replacement. Each curve was plotted as the average of 1000 permutations.

We used generalized linear mixed models (GLMMs) (Bolker et al. 2009) and linear mixed-effects models (LMMs) (Crawley 2012) to test whether the abundance and richness of bees and the microclimatic variables (luminosity, relative humidity, and air and soil temperatures) differed between riparian and terra-firme fragments. We considered the abundance and species richness of bees and each of the microclimatic variables as a response variable and habitat (riparian or terra-firme fragment) as explanatory variable. We also included the identity of each area as random effect in the models to account for variance associated with random site effects, using the two samplings in each of the three sampling points as sampling repetitions. We tested model significance by comparing the evaluated model and the null model (intercept and random effect only) by analysis of variance. The adequacy of model prediction was assessed through residual analysis and tests for overdispersion (Crawley 2012). Due to the overdispersion of residues, which occurs when the deviance of the response is greater than that expected by the chosen distribution (Hinde and Demétrio 1998), we analyzed the abundance of bees through a GLMM with negative binomial error distribution. No data overdispersion was detected for species richness of bees, which was thus analyzed with a GLMM using Poisson error distribution. Finally, each of the microclimatic variables was analyzed with a LMM using Gaussian error distribution. GLMM and LMM were created with the ‘lme4’ package (Bates et al. 2020), the analysis of variance between each model and the null model with the ‘vegan’ package (Oksanen et al. 2020), and the residual analysis of GLMM and LMM with the ‘DHARMa’ package (Florian 2020) in R 3.2.3 (R Development Core Team 2016).

To compare the species composition between riparian and terra-firme fragments, we performed a Permutational Multivariate Analysis of Variance (PERMANOVA) (Anderson 2001) based on Jaccard distance. The method computes a pseudo F-ratio, which is the ratio of the composition dissimilarity within a treatment to the ratio between treatments, and the significance is tested by permutation. Subsequently, we carried out a permutational analysis of multivariate dispersions (PERMDISP) (Anderson 2006). PERMDISP is a multivariate analyses analogue to Levene’s test for homogeneity of variances, and the statistic (average distance of group members to the PCoA group centroid) is tested by permutation (Anderson 2006). PERMANOVA and PERMIDISP were performed with the ‘vegan’ package (Oksanen et al. 2020) in R 3.2.3 (R Development Core Team 2016).

Finally, we adjusted models of simple linear regression to predict the abundance and species richness of bees in the riparian and terra-firme fragments as a function of fragment area. For this analysis, we used the log values of the area of each fragment (Sokal and Rohlf 2011).

RESULTS

Overall, we collected 3,166 bees, representing four genera and 38 species (Table 2). We collected 1,805 (57%) bees belonging to 35 species in the terra-firme fragments, and 1,361 (43%) belonging to 32 species in the riparian fragments. Euglossa ignita was the most common species overall, in both riparian and terra-firme fragments, with 703 individuals (22.2%) overall, followed by Eulaema nigrita, with 371 individuals (11.7%) and Eulaema cingulata, with 321 (10.1%). Six species were recorded only in the terra-firme fragments, and three only in the riparian fragments (Table 2). For both riparian and terra-firme fragments, the cumulative number of species collected approached the asymptote by the 25th sample (Figure 2).

Table 2
Number of bees collected per species in five fragments of riparian forest and five fragments of terra-firme forest in the urban landscape of Rio Branco, Acre, Brazil.

Species	Riparian	Terra-firme	Total
Eufriesea convexa (Friese, 1899)	-	1	1
Eufriesea flaviventris(Friese, 1899)	2	2	4
Eufriesea ornata(Mocsáry, 1896)	-	1	1
Eufriesea pulchra (Smith, 1854)	3	1	4
Eufriesea superba (Hoffmannsegg, 1817)	1	8	9
Euglossa allosticta Moure, 1969	41	39	80
Euglossa amazonica Dressler, 1982	104	100	204
Euglossa augaspis Dressler, 1982	33	35	68
Euglossa avicula Dressler, 1982	1	7	8
Euglossa bidentata Dressler, 1982	3	7	10
Euglossa chalybeata Friese, 1925	-	2	2
Euglossa chlorina Dressler, 1982	1	1	2
Euglossa cognata Moure, 1970	15	15	30
Euglossa despecta Moure, 1968	-	4	4
Euglossa ignita Smith, 1874	221	482	703
Euglossa imperialis Cockerell, 1922	11	37	48
Euglossa intersecta Latreille, 1817	3	6	9
Euglossa gaianii Dressler, 1982	1	-	1
Euglossa laevicincta Dressler, 1982	-	1	1
Euglossa aff. mixta Friese, 1899	13	26	39
Euglossa modestior Dressler, 1982	143	175	318
Euglossa mourei Dressler, 1982	80	67	147
Euglossa orellana Roubik, 2004	1	54	55
Euglossa prasina Dressler, 1982	11	5	16
Euglossa pleosticta Dressler, 1982	-	1	1
Euglossa rugilabris Moure, 1967	1	-	1
Euglossa securigera Dressler, 1982	1	2	3
Euglossa townsendi Cockerell, 1904	1	10	11
Eulaema bombiformis (Packard, 1869)	7	13	20
Eulaema cingulata (Fabricius, 1804)	156	165	321
Eulaema meriana (Olivier, 1789)	125	154	279
Eulaema mocsaryi (Friese, 1899)	26	26	52
Eulaema nigrita Lepeletier, 1841	182	189	371
Eulaema pseudocingulata Oliveira, 2006	92	98	190
Exaerete frontalis (Guérin, 1844)	3	5	8
Exaerete aff. frontalis	1	-	1
Exaerete lepeletieri Oliveira & Nemésio, 2003	24	18	42
Exaerete smaragdina (Guérin, 1844)	54	48	102
Total abundance	1,361	1,805	3,166
Total species richness	32	35	38

Figure 2
Accumulation curve of orchid-bee species (continuous line) and 95% confidence intervals (dashed lines) for riparian-forest fragments (A) and terra-firme forest fragments (B) located within the urban matrix of Rio Branco, Acre, Brazil. N samples = number of samples.

The abundance of bees (mean ± standard deviation) did not differ between riparian and terra-firme fragments (riparian, 48.3 ± 24.3 individuals; terra-firme, 57.2 ± 31.5 individuals; ANOVA: χ² = 1.49, df = 1, p = 0.2220). The species richness of bees also did not differ between riparian and terra-firme fragments (riparian, 12.2 ± 5.7 species; terra-firme, 13.3 ± 3.6 species; ANOVA: χ² = 0.61, df = 1, p = 0.4340).

Regarding climatic variables, the riparian fragments differed from the terra-firme fragments by registering significantly higher values of air temperature (mean ± standard deviation) (riparian, 30.3 ± 1.8 °C; terra-firme, 28.6 ± 1.2 °C; ANOVA: χ² = 10.05, df = 1, p = 0.0015) and luminosity (riparian, 785.3 ± 285.9 Lux; terra-firme, 529.3 ± 216.0 Lux; ANOVA: c² = 13.05, df = 1, p < 0.0003), and significantly lower values of relative humidity (riparian, 66.7 ± 6.1%; terra-firme, 76.4 ± 3.5%; ANOVA: c² = 14.47, df = 1, p < 0.0001) (Figure 3). Soil temperature did not differ significantly between fragment type (riparian, 23.8 ± 5.2 °C; terra-firme, 26.0 ± 5.0 °C; ANOVA: χ² = 0.94, df = 1, p = 0.3329) (Figure 3).

Figure 3
Values ​​of the climatic variables air temperature (A), luminosity (B), relative humidity (C) and soil temperature (D) in riparian and terra-firme forest fragments sampled for orchid bees in the urban matrix of Rio Branco, Acre, Brazil. The box represents the 25-75% quartiles, the line the median and the bars the standard error. Circles represent outlier values. Different letters above the boxes represent statistically significant differences between the categories.

There was no significant difference in species composition (PERMANOVA: F = 1.110, r² = 0.121, p = 0.3826), and species composition heterogeneity (PERMDISP: F = 0.001, p = 0.9676) between riparian and terra-firme fragments.

No significant correlation was found between fragment area and species richness in riparian (r = 0.74, p = 0.1513, df = 3) and terra-firme fragments (r = 0.48, p = 0.4151, df = 3). However, when the joint dataset for all fragments was considered, a significant and positive regression was observed for species richness (y = 4.7log (x) + 14.4, r² = 0.47, p = 0.0283, df = 8) and abundance (y = 141.2log (x) + 124.3, r² = 0.44, p = 0.0346, df = 8) on fragment size (Figure 4a,b). Overall, larger fragments tended to have a larger number of both bee species and individuals than smaller fragments.

Figure 4
Relationship between the log area of riparian and terra-firme forest fragments with species richness (A) and abundance (B) of orchid bees in the urban matrix of Rio Branco, Acre, Brazil. In both plots, the white circles represent the riparian fragments and the black circles, the terra-firme fragments.

DISCUSSION

The fact that the cumulative number of species approached the asymptote for both categories of fragments, indicates that the sampling effort was adequate for a reliable estimate of the species richness of orchid bees in the study area, including more exclusive and less abundant species.

Our results point to that there is no significant difference between riparian and terra-firme fragments in the abundance and species richness of orchid bees, independently of forest type. While riparian forests are considered to be relatively humid environments (Silvaet al. 2015), the temperature and luminosity were higher and relative humidity significantly lower in riparian fragments, contrary to our expectations. This indicates that, despite the proximity of the sampling points to streams, the riparian fragments were more open than the primary terra-firme forests. These variables influence the activity of insects in general (Marques 2012), and of orchid bees in particular (Nemésio and Vasconcelos 2013; Vilhenaet al. 2017). However, the environmental differences between the forest types were not enough to significantly influence the structure of the respective bee assemblages.

We found no significant difference in species composition between riparian and terra-firme forest fragments, in contrast with other studies that found low similarity in species composition between riparian forests and other types of habitat in the surrounding matrix for neotropical savanna (Faria and Silveira 2011) and environments in the northern hemisphere (Williams 2011). One factor contributing to the overall similarity among the bee faunas in our study may be the proximity among the fragments (2.1 to 13.9 km), as euglossine bees can fly and forage over a number of kilometers each day in search of widely-dispersed food sources and aromatic substances (Pokornyet al. 2015). Many generalist species are also able to forage over long distances, far from their nests, and in anthropogenic environments in the urban matrix (Raw 1989; Nemésio and Silveira 2007). In the Atlantic Forest, males of these bees were observed to disperse recurrently both within and among forest fragments in search of resources (Tonhascaet al. 2003). Another factor that may have contributed to this result is that the fragments are inserted in an urban matrix and their vegetation structure is more uniform and dominated by open ombrophilous forest (Daly and Silveira 2009; ACRE 2010).

Although no significant difference in species composition was found, some species (Euglossa gaianii,Eg.rugilabris, andExaereteaff.frontalis) occurred only in riparian fragments, and others (Eufriesea convexa,Ef. ornata,Eg. chalybeata,Eg. despecta,Eg. laevicinctaandEg.pleosticta) only in terra-firme fragments, although in low abundance (1-4 individuals). Some of these species had already been collected in the region (Nemésio and Morato 2006;Storck-Tononet al. 2011; Storck-Tononet al. 2013). Of the three species collected in the riparian fragments, only Eg. gaianiihad been recorded in an urban fragment in Rio Branco. Regarding the species collected in the terra-firme fragments,Ef. convexaandEg. laevicinctahad not yet been recorded in the region.Eufriesea ornatawas previously recorded in two fragments in a rural area in the vicinity of Rio Branco.Euglossa despectawas recorded in eight rural and urban fragments.Euglossa pleostictawas recorded in a rural fragment and in a forest relatively distant from the urban matrix of Rio Branco. Euglossa chalybeatawas recorded in the region of Rio Branco in high abundance but only in two fragments in rural areas. Thefore, except for Eg. chalybeata, all other exclusive species recorded in this study were collected previously in low abundance in the region and could be considered of low density. Euglossa chalybeata, Eg. pleostictaandEf. ornatawere also recorded in forests of the Juruá River basin, Acre (Nemésio and Morato 2005), which suggests that they are widely distributed in the southwestern Amazon.

The number of species recorded in our study represents 62.7% of the orchid-bee species known to occur in the southwestern Brazilian Amazon (Nemésio and Morato 2006, Storck-Tonon et al. 2009; Cândido et al. 2018), indicating that the sampled fragments are occupied by a considerable proportion of the known total regional diversity of orchid bees. Considering that the abundance and diversity of arthropods is declining in tropical forests due to climate change (Novaiset al. 2016), and the important role of orchid bees in trophic webs (Lister and Garcia 2018), urban forest fragments may play a fundamental role in the conservation of ecosystems in the landscape as a whole. Riparian and terra-firme fragments may provide orchid bees with nesting sites, refuge from solar radiation (Moura and Schlindwein 2009; Faria and Silveira 2011), and feeding resources (Neves and Viana 1999). Additionally, riparian forests are considered to be areas of permanent preservation under Brazilian legislation (BRASIL 2012). Both riparian and terra-firme habitats are important in the provision of ecosystem services, such as pollination, production of fruit and seeds, carbone storage, regulation of the hydrological cycle and prevention of erosion and silting (Martins 2014; Mello et al. 2016; Lense et al. 2020). Both types of forest fragments may also provide important ecological corridors for the dispersal of bee populations, contributing to the maintenance of gene flow and the structure of assemblages, and reducing the potential for local extinction (Souzaet al. 2010; Storti et al. 2013; Grayet al. 2016; Lense et al. 2020).

The species richness and abundance of orchid bees may also be affected by the quality and heterogeneity of the matrix surrounding forest fragments (Livingstonet al. 2013; Aguiaret al. 2015; Britoet al. 2017). Given the ongoing urbanization of the study area in the municipality of Rio Branco, the size of local forest fragments (both riparian and terra-firme) is decreasing progressively. The modification of the urban landscape, with the reduction in forest cover and the ongoing expansion of the urban matrix, may result in a reduction in the availability of natural resources (Santoset al. 2019), such as nesting sites and materials, and sources of pollen and nectar, which combine to limit the species richness and abundance of bees (Zanetteet al. 2005; Storck-Tononet al. 2013). This process affects solitary bees in particular, such as the majority of the Euglossine bees, including the forms that nest on the ground and in tree holes (Zanetteet al. 2005; Cardoso and Gonçalves 2018). In this context, areas of natural vegetation located within the urban matrix, no matter how small or disturbed, can be important refuges for bees and other organisms associated with the urban landscape (Aronsonet al. 2017).

As in our study, larger fragments generally tend to harbor higher species richness (Santos et al. 2013). In the case of bees, larger areas imply a greater availability of sources of odoriferous substances, feeding resources, resin, and potential nesting sites. While some studies have found evidence that the size of the fragment influences the structure of orchid-bee assemblages (Powell and Powell 1987; Aguiaret al. 2015; Botschet al.2017), the empirical data are inconclusive (Nemésio and Silveira 2007). Powell and Powell (1987) recorded a decrease in the visitation rates of bees at odoriferous baits with decreasing fragment size in central Amazonia. In a previous study in the southwestern Amazon, which focused on fragments located within both the urban matrix and the rural zone, Storck-Tononet al. (2013) found no significant relationship between the area of the fragment and the abundance and species richness of bees. The divergence between these findings and the results of the present study may be related primarily to the configuration of the respective study areas and the sampling procedures adopted in each study. The habitat complexity, in terms of vegetation structure, also may be an important determinant of the distribution of orchid bees (Aguiaret al. 2014). In the Brazilian Atlantic Forest, Antoniniet al. (2016) showed that relatively complex habitats tend to have a greater species richness and abundance of bees than less complex environments, even though the species composition was not affected systematically. Further research is needed to evaluate the long-term impacts of the urbanization process on these environments and the biodiversity they support in a more conclusive way.

CONCLUSIONS

Our prediction that species abundance, richness and composition of orchid bees differs between riparian and terra-firme forest fragments in an urban landscape in the southwestern Amazon has not been corroborated. However, the size of the fragment was a predictor of richness and abundance. Therefore, our findings highlight the importance of maintaining large areas of forest within the urban matrix for the conservation of orchid-bee species. Furthermore, riparian and terra-firme forest fragments widely dispersed in the urbanized landscape may provide important habitat for the maintenance of bee populations and their foraging patterns. The fact that a major proportion of the regional species complement of orchid-bees was found in these fragments emphasizes the functional importance of this type of habitat in the urban matrix.

ACKNOWLEDGMENTS

We thank Dr. Márcio Luiz de Oliveira and Dr. José Eustáquio dos Santos Júnior for help with the identification of some critical taxa, Dr. Felipe Martello Ribeiro of Universidade Federal do Acre (UFAC) for making the location map of the sampling sites, and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for partial financial support (407631/2013-5). The first author thanks Programa de Pós-Graduação em Ecologia e Manejo de Recursos Naturais (UFAC) for the support to this project. We would like to thank Secretaria de Estado de Meio Ambiente (SEMA-Acre) and Secretaria Municipal de Meio Ambiente (SEMEIA) for the logistic access to the some urban forest fragments.

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• CITE AS:
  Cândido, M.E.M.B.; Miranda, P.N.; Morato, E.F. 2021. Orchid bees in riparian and terra-firme forest fragments in an urban matrix in southwestern Brazilian Amazonia. Acta Amazonica 51: 214-223.

SUPPLEMENTARY MATERIAL

(only available in the electronic version)

Cândido et al. Orchid bees in riparian and terra-firme forest fragments in an urban matrix in southwestern Brazilian Amazonia

Appendix S1
Sampling dates of orchid bees in five riparian (RF1-RF5) and five terra-firme (TF1-TF5) forest fragments (FRAG) from September 2015 to February 2016 in Rio Branco, Acre state, Brazil. Tabulated values are the numbers of orchid-bee individuals captured on each day. The black bars mark periods over 10 days in which there was no field work.

FRAG	September																					October																								November
	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29	30		1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23		16	17	18	19	20	21	22	23	24	25	26	27	28
RF1				20																		47
RF2																		18					39																		31														14			59
RF3	6																																					17																32
RF4										27																							37																	65
RF5					55																													86										71
TF1		40																	68																							105
TF2									27																						31
TF3			35																	28															39
TF4																	23																				77										58						83
TF5								40																						70																67
FRAG	December							January																								February
	10	11	12	13	14			7	8	9	10	11	12	13	14	15	17	18	19	20	21	22	23	24	25	26	27	28	29	30		1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27
RF1								21																													31							32
RF2		54
RF3																								16															25												33
RF4																										20							44					72
RF5																	86																									136																96
TF1											50																							80													68
TF2																														45											34								38
TF3	37																																		45																			49
TF4					59																							103
TF5																									120																					106							80

Edited by

• ASSOCIATE EDITOR:
  Juliana Hipólito

Publication Dates

• Publication in this collection
  20 Sept 2021
• Date of issue
  Jul-Sep 2021

History

• Received
  29 Aug 2020
• Accepted
  20 Apr 2021