The Impacts of Traffic Intensity on Taxonomic and Functional Diversity in Understory Spiders from the Brazilian Atlantic Forest

Abstract

Although it has its advantages for the development of urban areas, road construction is among the greatest threats to biodiversity, due to fragmentation, habitat loss, and changes in landscape structure. This study investigated the effects of different traffic intensities on the understory spider assemblage in the Brazilian Atlantic Forest. Understory spiders were collected between 09:00 h–16:00 h using beating tray samples on roadside vegetation on roads with and without traffic. In total, 1616 spiders belonging to 24 families and 317 morphospecies were collected. The families Araneidae and Theridiidae were more abundant and showed a higher number of morphospecies on both roads. Understory spiders were classified into seven guilds. However, no significant differences were found in functional and taxonomic richness and abundance between the roads. These results indicate that understory spider assemblages showed no significant response to traffic intensity, suggesting potential resilience to this disturbance in the studied context. Additionally, the proximity between locations may result in the founder effect, with spiders migrating from the preserved site to the impacted site. Overall, this study indicates that traffic presence does not significantly impact the diversity and composition of understory spider assemblages in the studied region.

1. Introduction

The conversion of natural landscapes into urban settings is a global problem for biodiversity conservation. Currently, natural landscapes face great pressure from urban areas [1,2,3]. Growing human activity has caused large-scale invasions into natural areas, leading to the fragmentation and degradation of native vegetation [4,5]. In this way, deforestation, which is closely linked to human activities, serves as the main precursor to negative impacts on natural landscapes, leading to clear cutting and the expansion of agricultural and urban areas [6]. In urban areas, a portion of vegetation suppression is designated for the construction of roads, in order to promote direct connections between new urban environments [7,8]. Road building can be immediately considered a high-impact fragmentation mechanism, particularly as it removes landscape function and structure [9]. As a result, roads are responsible for creating new edge environments, which expose animals to urban limits with greater edge contrasts [10,11,12].

Roadside vegetation provides important habitats and corridors for some species. However, due to their vulnerability to high volumes of vehicle traffic and negative impacts caused by pollution and environmental fragmentation, some species experience increased mortality rates [13]. For example, road edges can promote an increase in floral richness [14,15], which in turn can attract more pollinators. However, this floristic richness can also act as an ecological trap, leading animals to settle in low-quality habitats, which may reduce their survival or reproductive success [16]. Therefore, by investigating road environments, it is possible to observe impacts on biodiversity, such as dispersal barriers, population isolation, increases in mortality, and changes in species abundance, richness, and composition [17,18,19]. High sensitivity was found for ballooning spider species to the presence of heavy road traffic, which resulted in a decrease in their abundance [20]. It is important to note that traffic intensity is a key factor in spider diversity, as increased traffic can lead to higher frequencies of trampling compared to other groups [21].

Studies on the effects of roads on invertebrates are scarce, but the few that are available indicate that different taxa respond differently to road impacts [22,23,24]. Spiders, in particular, have been shown to be sensitive to landscape changes [25,26], especially road creation and traffic intensity [20]. Therefore, studying spider assemblages in each region has great potential, as they are related to local biological control, subject to different levels of disturbance, and their species composition varies along the landscape scale [27,28]. In addition, spiders are known to occupy a wide range of niches, and their functional traits can vary greatly depending on their feeding strategies, hunting behavior, and habitat preferences [29]. For example, some spiders are web builders, while others are active hunters that use camouflage to ambush their prey [30]. Studying functional diversity in spiders is important because it can provide insights into the ecological roles and functions that different spider species play within ecosystems [31]. This knowledge can be used to develop more effective conservation strategies and to better predict the impacts of human activities, such as road construction and traffic, on spider communities and their associated ecosystem services. This study aims to investigate how different traffic intensities affect the assemblages of understory spiders in the Brazilian Atlantic Forest, with a focus on both taxonomic and functional diversity. We predicted that spider assemblages would be negatively impacted by traffic intensity.

2. Materials and Methods

2.1. Study Area

This study was conducted in the Campo de Instrução Marechal Newton Cavalcante, a 6280-hectare military area comprising a fragment of semideciduous seasonal Atlantic Forest (07°46′55″ S, 35°09′02″ W) located in Abreu e Lima, Pernambuco, Brazil. The region is characterized by a mean annual temperature of 28 °C and an average annual rainfall of 1634 mm (IBGE). The rainy season extends from March to August (Climate-Data 2023). The remaining Atlantic Forest within this military area features two main roads that traverse its entirety (Figure 1). The active road, which runs east–west, is subject to regular traffic (observed military convoys and private vehicles during sampling periods). In contrast, a decommissioned road, inactive since the 2000s, lies within the current forest core. These roads are separated by a distance of approximately 2 km, providing a clear distinction in their environmental impacts.

2.2. Spider Collection

During the rainy season of 2022 (May, June, and July), spider collections were conducted on three consecutive days of each month, as this period typically corresponds to higher spider activity and abundance in tropical environments [32], making it optimal for effective data collection. The collections took place daily from 09:00 to 16:00 h, totaling 7 h of sampling per day. Thus, over the three months, a total of nine sampling days were conducted, resulting in 63 h of collection. The spiders were collected at forest edges adjacent to roads, comparing areas with heavy traffic to those without traffic (Figure 2). In each locality, six standardized quadrants (30 m × 10 m) were established, positioned 10 m from the roadside toward the forest. Importantly, the quadrants were not repeated across months; each month, 6 new quadrants were established, resulting in 18 quadrants sampled for each road type over the course of the rainy season, totaling 36 quadrants by the end of the study. Within each quadrant, all vegetation taller than 1.5 m was struck 10 times with a wooden stick, and the spiders that fell onto a white sheet (60 cm × 60 cm), known as a beating tray, were collected. The collected specimens were immediately preserved in 70% ethanol and sent to the University of São Paulo (USP) for taxonomic identification. Adult spiders were distinguished from juveniles based on the following criteria: (1) fully developed genitalia (visible under stereomicroscope); (2) the presence of sexual dimorphism characters (e.g., modified pedipalps in males and epigynal plates in females); and (3) sclerotization patterns. Juveniles were distinguished from adults by their underdeveloped or absent genital structures. The categorization of spiders into guilds followed the criteria of previous studies [29,30]. All specimens were duly deposited in the Arachnology Collection of the Butantan Institute, São Paulo, Brazil.

2.3. Data Analysis

The sampling coverage was measured using the collector curve, which compares the number of species collected to the number of individuals collected over the three-month sampling period. The species accumulation curve was generated by performing 999 randomizations using the iNEXT package in the R software version 4.2.0 [33,34]. We assessed the functional diversity of understory spiders using the guild classification [29,30]. The taxonomic and guild diversities of the understory spiders for both roads were assessed by non-metric multidimensional scaling (NMDS) through 1000 randomizations based on a Bray–Curtis similarity matrix. The statistical significance of the NMDS was done through Similarity Analyses (ANOSIM). The contribution of each family and morphospecies (%) among roads with different traffic intensity was performed through Percentage Similarity Analyses (SIMPER) based on Bray–Curtis similarity matrices with 1000 randomizations. All analyses were performed using the PAST 3.18 software [35]. We also calculated the dominance (Palissa index) of each family and morphospecies for each road as follows: D% = (i/t) × 100, where it is the species abundance and are the total abundance. In this way, spider species were classified as eudominant (D > 10%), dominant (5% < D ≤ 10%), subdominant (2% < D ≤ 5%), recessive (1% < D ≤ 2%), or rare (D < 1%) [36].

3. Results

In total, 1616 spiders were collected, distributed across 24 families and 317 morphospecies, with only 18% (n = 317) represented by adult individuals. The species accumulation curve indicated that the sampling effort captured 78% and 88% of the understory spider species on roads with absent and present traffic, respectively (Figure S1). The road with present traffic exhibited a higher diversity (richness = 65 morphospecies, abundance = 931 individuals) of understory spiders compared to the road without traffic (richness = 44 morphospecies, abundance = 685 individuals). According to the SIMPER analysis, the families Theridiidae, Araneidae, and Thomisidae together contributed to 71% of the understory spider assemblage (

Table 1. Summary of the similarity percentage analysis of understory spider families in roadside vegetation with different traffic levels in the Brazilian Atlantic Forest.

Family	Contribution (%)	Cumulative (%)	Traffic Absent	Traffic Present
Theridiidae	41.33	41.33	149	77
Araneidae	16.14	57.47	33	78.30
Thomisidae	13.61	71.08	12.30	49.70
Salticidae	6.25	77.34	15	32.70
Scytodidae	5.11	82.45	0	19.30
Anyphaenidae	2.71	85.17	1.67	10.30
Uloboridae	2.25	87.42	4.33	9.67
Dictynidae	1.84	89.26	3	5.67
Oxyopidae	1.72	90.99	3	7.67
Senoculidae	1.36	92.36	0.66	5
Tetragnathidae	1.34	93.70	2	5
Corinnidae	1.10	94.81	1.33	3.67
Trechaleidae	0.97	95.78	1.33	2.33
Pholcidae	0.95	96.74	1.67	4.67
Sparassidae	0.77	97.52	1.33	2
Mimetidae	0.61	98.14	1	1.67
Linyphiidae	0.47	98.61	1.33	1.33
Theridiosomatidae	0.29	98.90	0.33	1
Philodromidae	0.26	99.16	0.66	0
Selenopidae	0.24	99.40	0	1
Pisauridae	0.19	99.59	0	0.33
Anapidae	0.18	99.78	0.33	0
Phrurolithidae	0.13	99.92	0	0.33
Caponiidae	0.08	100	0	0.33

). The most abundant spiders were from the families Araneidae and Theridiidae on both roads (Figure 3A). However, the dominance pattern differed between the road with present traffic and the road with absent traffic. For the road with present traffic, the families Araneidae, Theridiidae, and Thomisidae were classified as eudominant, while Salticidae and Anyphaenidae were considered dominant. On the other hand, the absence of traffic allowed for the eudominance of the families Araneidae and Theridiidae (

Table 2. Dominance of understory spider families found in roadside vegetation on roads with different traffic intensities in the Brazilian Atlantic Forest.

Family	Traffic Absent		Traffic Present
	Palissa Index	Dominance	Palissa Index	Dominance
Anapidae	0.14	Rare	-	-
Anyphaenidae	0.43	Rare	3.32	Subdominant
Araneidae	14.45	Eudominant	25.24	Eudominant
Caponiiidae	-	-	0.10	Rare
Corinnidae	0.58	Rare	1.18	Recessive
Dictynidae	1.31	Recessive	1.82	Recessive
Linyphiidae	0.58	Rare	0.42	Rare
Mimetidae	0.43	Rare	0.53	Rare
Oxyopidae	1.31	Recessive	2.47	Subdominant
Philodromidae	0.14	Rare	-	-
Pholcidae	0.72	Rare	1.07	Recessive
Phrurolithidae	-	-	0.10	Rare
Pisauridae	-	-	0.10	Rare
Salticidae	5.54	Dominant	9.23	Dominant
Scytodidae	-	-	5.69	Dominant
Selenopidae	-	-	0.32	Rare
Senoculidae	0.29	Rare	1.50	Recessive
Sparassidae	0.58	Rare	0.42	Rare
Tetragnathidae	0.87	Rare	1.07	Recessive
Theridiidae	65.40	Eudominant	25.24	Eudominant
Theridiosomatidae	0.14	Rare	0.32	Rare
Thomisidae	4.96	Dominant	15.89	Eudominant
Trechaleidae	0.58	Rare	0.75	Rare
Uloboridae	1.45	Recessive	3.11	Subdominant

).

The families Theridiidae, Araneidae, and Salticidae exhibited a higher number of morphospecies associated with the road with traffic presence. Conversely, on the road with no traffic, Araneidae and Theridiidae displayed the highest number of morphospecies (Figure 3B). However, no significant differences were found in terms of abundance (NMDS: stress = 0, ANOSIM: F = 0.1111; p = 0.2061) and morphospecies richness (NMDS: stress = 0.0284, ANOSIM: F = −0.0740; p = 0.3897) between the roads. Furthermore, the families Caponiidae, Pisauridae, Phrurolothidae, Scytodidae, Selenopidae, and Tetragnathidae were exclusively found on the road with traffic, while the families Anapidae and Philodromidae were present only on the road with no traffic. Interestingly, the environments shared 78% of the morphospecies, with the theridiids Episinus sp. 1 and Dipoena sp. 3, and the araneids Alpaida truncata (Keyserling, 1865) and A. delicata (Keyserling, 1892), accounting for 23% of the understory spider assemblage (Table S1). Moreover, the dominance among the morphospecies varied with traffic intensity. Alpaida delicata, A. truncata (Araneidae), Dipoena sp. 1, and Episinus sp. 1 (Theriididae) were dominant on the road with intense traffic, while A. delicata, A. truncata, Micrathena evansi (Chickering, 1960) (Araneidae), Dipoena sp. 1, Dipoena sp. 2, and Dipoena sp. 3 (Theridiidae) were dominant on the road without traffic (Table S1).

The understory spiders were classified into seven guilds: orb-weavers, sheet-weavers, tangle-web weavers, ambush hunters, ground hunters, specialists, and other hunters. According to the SIMPER analysis, tangle-web weavers, orb-weavers, ambush hunters, and other hunters represented 96% of the total sampled individuals (

Table 3. Summary of similarity percentage analysis on spider guilds in roadside vegetation with different traffic levels in the Brazilian Atlantic Forest.

Spider Guild	Contribution (%)	Cumulative (%)	Mean Traffic Absent	Mean Traffic Present
Space web weavers	44.46	44.46	154	87.30
Orb web weavers	19.95	64.41	40	94
Others hunters	17.16	81.57	22.30	77
Ambush hunters	14.94	96.51	12.30	50.70
Specialists	1.74	98.25	2.33	4.33
Ground hunters	1.09	99.35	1.33	4
Sheet web weavers	0.65	100	1.33	1.67

). In terms of dominance, orb-weavers and tangle-web weavers were the eudominant guilds on the road with no traffic, while orb-weavers, tangle-web weavers, ambush hunters, and other hunters were the eudominant guilds on the road with traffic (

Table 4. Dominance of understory spider guilds found in roadside vegetation on roads with different traffic intensities in the Brazilian Atlantic Forest.

Spider Guild	Traffic Absent		Traffic present
	Palissa Index	Dominance	Palissa Index	Dominance
Orb web weavers	17.11	Eudominant	29.46	Eudominant
Others hunters	9.55	Dominant	24.13	Eudominant
Ambush hunters	5.27	Dominant	15.88	Eudominant
Ground hunters	0.57	Rare	1.25	Recessive
Sheet web weavers	0.57	Rare	0.52	Rare
Space web weavers	65.90	Eudominant	27.37	Eudominant
Specialists	0.99	Rare	1.35	Recessive

). However, no significant differences in guild abundance (NMDS: stress = 0, ANOSIM: F = 0.1111; p = 0.4020) and richness (NMDS: stress = 0.0284, ANOSIM: F = 0.1111; p = 0.4019) were found between the roads.

4. Discussion

In this study, we investigated the effects of different traffic intensities on the understory spider assemblage in the northeastern Atlantic Forest in Brazil. Surprisingly, we found a higher level of diversity in understory spiders on the road with intense traffic compared to the road without traffic. Pollution released by vehicles, particularly nitrogen oxides, promotes the growth of vascular plants, especially shrubs and grasses [37,38]. Additionally, the eutrophication caused by pollutants can act as an attractant for insects [39]. Thus, this promotes an increase in food resources and environmental complexity, favoring the presence of spiders in the vegetation.

Spiders belonging to the families Araneidae, Theridiidae, and Thomisidae accounted for most of the abundance in the samples. These families are typical components of the understory fauna in tropical forests [40,41]. However, regardless of the difference in traffic intensity, we observed a higher abundance of specimens from the families Araneidae and Theridiidae in the understory spider assemblage of both locations. These individuals are web-building spiders that are closely related to the available vegetation structure, using it to support their webs [42,43]. These two families exhibit high ecological plasticity, and are capable of persisting in both natural and disturbed environments, maintaining high richness and abundance [44]. On the other hand, representatives of the family Thomisidae were abundant in the areas with traffic on the road. Unlike the families mentioned earlier, thomisids have distinct predation strategies, acting as ambush predators and using sit-and-wait tactics to capture their prey [45]. Thus, these arachnids can be found in both forests and highly disturbed environments [46], where they explore different parts of plants, such as flowers and tree barks, for foraging [47].

Our analysis of family and morphospecies dominance revealed only minor differences between the traffic conditions. While Araneidae and Theridiidae were eudominant at both sites, the sole distinction was the additional eudominance of Thomisidae at the traffic-present location. This limited variation suggests that the presence of traffic had minimal impact on overall family-level dominance patterns. Members of these families are dominant in impacted areas, such as those with a high human presence and selective pesticide application [48]. Similar results have been described for urban parks, where the eudominance of these families has been observed [49]. The predominance of these spiders can be attributed to the adaptation abilities that these families possess, allowing them to occupy different plant strata and microhabitats, resulting in a broad exploratory capacity in these regions [44,50]. Notably, web-building families like Araneidae and Theridiidae may face hidden costs in traffic-exposed areas, as their webs can accumulate traffic-derived pollutants [51,52], though this requires verification in tropical systems.

While roads with traffic exhibited numerically higher richness and abundance, these differences were not supported by community-level analyses (ANOSIM p > 0.2), suggesting limited biological significance. The unpaved condition of both roads may have preserved habitat connectivity more effectively than paved surfaces, which typically induce stronger edge effects and microclimatic changes [53]. This contrasts with studies showing behavioral impacts on web-building spiders near roads [54], but aligns with our observation of ecological plasticity in the dominant families. Our results indicated that the abundance and richness of understory spider assemblages were not affected by traffic intensity. This finding suggests a high level of plasticity within the assemblages, allowing them to adapt and survive in contrasting environments. It is possible that species turnover is occurring, where species better suited to each habitat are replacing others, thus maintaining overall species richness and abundance. Additionally, the differing vegetation structures along the roadsides may be providing suitable microhabitats for species adapted to these conditions. Our results also indicated that different guilds responded differently under different disturbance regimes. In areas with no traffic, orb-weavers and tangle-weavers were the dominant species. These results are consistent with previous studies that observed a significant presence of these spiders in regions with lower exposure to stress, such as the interior of mature forests [11,55]. On the other hand, on roads with traffic, orb-weavers and tangle-weavers, along with ambush hunters and other hunters, were eudominant. The occurrence and dominance of these guilds on traffic roads could be attributed to the increased availability of food resources in the road’s vegetation, mainly pollinators [56]. Future studies should incorporate standardized traffic monitoring and pollutant assays (e.g., web metal content [52]) to disentangle resource benefits from potential physiological costs and precisely correlate vehicle density with ecological impacts. However, our key conclusion—that spider assemblages showed no significant differences between roads—remains supported by the observed contrast in human activity levels (no traffic vs. frequent traffic), which was the central variable of interest.

5. Conclusions

In summary, our results indicated that the taxonomic and functional diversity of understory spiders were unresponsive to the effects of traffic. This lack of observed impact may be due to the traffic on the active road not being sufficiently intense to significantly affect spider diversity. The military-controlled, unpaved nature of the active road likely generated milder ecological impacts than high-intensity urban or paved road systems. Importantly, without quantitative traffic measurements, we cannot rule out that higher traffic intensities might yield detectable impacts. Additionally, while spiders are known to be sensitive to human pressures, the absence of clear differences could also be related to limitations in our study design, such as insufficient sample sizes or the lack of replicated road conditions. These factors might have obscured any potential effects, rather than solely reflecting the resilience or adaptability of spider assemblages to environmental stressors.