Substrate Availability May Be More Important than ...

THE JOURNAL OF TROPICAL BIOLOGY AND CONSERVATION

BIOTROPICA *(*): ***–*** ****

10.1111/j.1744-7429.2008.00463.x

Substrate Availability May Be More Important than Aquatic Insect Abundance in the Distribution of Riparian Orb-web Spiders in the Tropics Eric K. W. Chan1 , Yixin Zhang2 , and David Dudgeon1,3 1 Division

of Ecology & Biodiversity, School of Biological Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China

2 Department

of Biology, Texas State University, 601 University Drive, San Marcos, Texas, U.S.A.

ABSTRACT Spiders that are abundant along streams may depend on energy subsidies across land–water ecotones, but the effects of season and habitat structure on this trophic linkage remain poorly understood in the tropics. We carried out surveys and a manipulative experiment to investigate the effects of season and substrate availability on the distribution of riparian orb-web spiders in Hong Kong, southern China. In the surveys, spider abundance, prey, substrate use, and web orientation were recorded. The experiment involved installation of in-stream artificial substrates (ropes and bamboo poles) to increase substrate availability for web attachment. We found no seasonal difference in web abundance, but seasonal differences were observed for the prey on webs: aquatic insects (mostly Ephemeroptera and chironomid midges) contributed 69 percent of total prey collected during the wet season, but only 38 percent during the dry season. Most webs (50–80%) were < 0.5 m above the water and 45–51 percent of them tended to be orientated horizontally to the water surface and supported by overhanging vegetation and boulders. The addition of artificial substrates resulted in a 23–34 percent increase in the number of webs at the four treatment sites compared to controls, indicating that availability of web-building substrates is a critical determinant of the spider distribution. Our results suggest that riparian zones are potential ‘hotspots’ of food availability for spiders, and that the aquatic insect subsidy allows this habitat to support increased densities of spiders when the constraint of substrate availability is relaxed. Key words: BACI; China; ecotone; habitat structure; Hong Kong; riparian vegetation; subsidy.

HABITATS ARE OFTEN LINKED BY THE TRANSFERS OF NUTRIENTS AND ENERGY (hereafter referred to as ‘subsidy’), and stream riparian zones are land–water ecotones where there may be reciprocal exchanges of nutrients and energy of importance to consumers in recipient habitats (Polis et al. 1997, Baxter et al. 2005, Naiman et al. 2005, Ballinger & Lake 2006). For example, the diet of drift-feeding fishes may be subsidized by terrestrial insects falling into streams (Nakano & Murakami 2001). Conversely, terrestrial predators (e.g., lizards, birds, bats, spiders) consume emergent aquatic insects, which show strong seasonal fluctuations in magnitude of emergence and mostly aggregate within 10–20 m of streams (Iwata et al. 2003, Sanzone et al. 2003, Power et al. 2004, Uesugi & Murakami 2007). A recent study in New Zealand has also demonstrated a positive correlation between the biomass of benthic stream insects and riparian arachnids (spiders and harvestmen), while spider web density also declined strongly with increasing distances from stream margins (Burdon & Harding 2008). As a result, stream riparia may be ‘hotspots’ of food availability for terrestrial predators during some parts of the year. Manipulative experiments have shown that the reductions in the subsidy of adult aquatic insects can reduce the densities or growth of riparian insectivores and may indirectly affect the community structure in these habitats (e.g., Sabo & Power 2002a, b; Kato et al. 2003; Marczak & Richardson 2007). Given the potential importance of land–water interactions for streams and their riparian zones, a better understanding of their dynamics is needed, but such data are generally lacking from the seasonal tropics (but see Lynch et al. 2002, Chan et al. 2007). In addition, very few studies (including those carried out in temperate Received 22 March 2008; revision accepted 19 July 2008. author; e-mail: [email protected]

3 Corresponding

regions) have examined the influence of riparian habitat structure on the distribution and abundance of insectivores that feed on adult aquatic insects. For instance, riparian conditions could limit spider foraging along streams (Laeser et al. 2005). In monsoonal Hong Kong, tetragnathid spiders are a conspicuous component of streamside habitats and orb webs are attached to overhanging vegetation, lianas, logs, and boulders along forest streams. The webs are situated close to the water surface, suggesting that emerging adult aquatic insects may be a major food source for these spiders. We carried out field surveys of webs along ten reaches of four different streams during the wet and dry seasons to record seasonal changes in the abundance of spiders and their prey, and the substrates used for web attachment. We hypothesized that riparian spiders and their catches of aquatic prey would be more abundant during the wet season, when more adult aquatic insect prey are available (Chan et al. 2007). Anecdotal observation indicates that tetragnathid spiders in Hong Kong usually aggregate in stream reaches with dense overhanging vegetation or boulders, suggesting that the availability of suitable substrates is a crucial factor in affecting their abundance in food-rich ‘hotspots’ along streams. The distribution and abundance of riparian spiders may be particularly sensitive to the distribution and availability of robust substrates (e.g., tree branches), which are usually more limiting above streams than in other habitats such as forest and shrubland (cf. Buskirk 1975, Laeser et al. 2005). To investigate this, we conducted a manipulative field experiment to test the hypothesis that spider density is a function of substrate availability for web-building along streams. We expected that increasing the substrate availability for web-building would result in higher spider densities in sites where aquatic insect prey are abundant.

C 2008 The Author(s) C 2008 by The Association for Tropical Biology and Conservation Journal compilation

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2

Chan, Zhang, and Dudgeon

TABLE 1. The number of webs and spiders recorded in field surveys along ten reaches of four forest streams in Hong Kong during the wet and the dry season. No. webs

Stream

Section

Reach

order

length (m)

Wet

Dry

T1

2nd

100

64

10

T2 T3

2nd 3rd

100 80

57 36

11 29

T4 N1 N2

3rd 3rd 3rd

150 100 80

70 34 53

135 26 21

Tai Shing Stream

L1 L2 TS1

2nd 3rd 3rd

100 150 100

52 72 47

113 84 44

Total

TS2 –

4th –

80 –

27 512

33 506

Stream Tai Po Kau Forest Stream

Needle Hill Stream Lead Mine Pass Stream

METHODS STUDY SITES.—Hong Kong (22◦ 30 N, 114◦ 10 E) has a markedly seasonal tropical monsoon climate. Over 70 percent of the annual precipitation (mean: 2214 mm) occurs during the hot, humid wet season (May–Sept, mean monthly temperature: 26–29◦ C), when the southwest monsoon dominates. The northwest monsoon prevails during the dry season (Nov–Mar), in which the weather is cool and dry (mean monthly temperature: 16–21◦ C), and < 20 percent of annual rainfall is recorded. April and October are periods with transitional weather. The ten stream reaches (length: 80–150 m, width: 1–8 m) used as study sites for field surveys were located along four unpolluted forest streams (> 150 m asl) situated within the Hong Kong Country Parks system at Tai Po Kau and Shing Mun. They consisted of four reaches in Tai Po Kau Forest Stream, two reaches each from the Lead Mine Pass Stream, Needle Hill Stream, and Tai Shing Stream, respectively (Table 1). The reaches situated within the same drainage were either located in different tributaries or separated by at least 50 m, and we treated them as independent replicates. All reaches were deeply shaded by riparian vegetation (canopy shading > 60%) and consisted of alternating patterns of riffles and pools with water depth usually < 1 m. Benthic substrates were dominated by cobbles and gravels with scattered boulders, and sand only in pools. Many of the boulders exceeded 50 cm in diameter and protruded from the water surface. Forest streams in Hong Kong have soft, nutrient poor, slightly acidic waters that are well oxygenated, and standing stocks of allochthonous detritus are usually high (Dudgeon 1992, Dudgeon & Corlett 2004). PRELIMINARY STUDY.—A preliminary study conducted to determine species diversity of the orb-web assemblage indicated dominance (> 80% of spiders) by Orsinome diporusa (Tetragnathidae),

although other tetragnathids such as Tylorida ventralis were present also. Araneid orb-weavers were scarce (< 5%). In the survey and the field experiment, tetragnathids and araneids were not differentiated as we focused on understanding the distribution of the orb-web assemblage. Orsinome spp. usually occurs along streams, where they attach their webs to boulders and vegetation, but other ecological information available on the genus is scant (Murphy & Murphy 2000, Zhu et al. 2003). The preliminary study indicated that spiders attending webs tended to hide in the vicinity making direct counts of individuals impractical; instead we used the number of webs at a site to indicate spider abundance, with the assumption that the abundance of spiders and webs would be positively correlated. Webs were occasionally found connected to each other, but the spiders seemed to defend their own webs and no evidence of cooperation among them was seen. FIELD SURVEYS.—Surveys of webs were carried out once along each of the ten reaches during the 2005 wet season (Aug–Sept) and once during the following dry season (Jan–Mar 2006). During the survey, all orb-webs within 2 m of the water surface at each site were recorded (visual censusing technique; Lubin 1978). Apart from the number of webs, we measured: (1) vertical height of the web from water surface (categorized as 0–50 cm, 51–100 cm or 100– 200 cm); (2) substrates for web attachment: the substrate types to which webs were attached: e.g., ‘boulder’ was recorded if the threads of a web were entirely supported by boulders, while ‘boulder and vegetation’ was recorded when the web was attached to these two substrates; (3) condition of webs: either complete with an intact hub, or incomplete/broken; and (4) orientation of webs with respect to water surface (categorized as horizontal: 0–30◦ to water surface, tilted: 31–60◦ or vertical: 61–90◦ ). As the ‘incomplete/broken’ webs lacked a planar structure, orientation was recorded for complete webs only. However, the other data (height and substrate) were recorded and analyzed in the same manner for both complete and incomplete/broken webs. Prey found on all webs were removed with forceps, preserved in 70 percent alcohol, and later identified and counted in the laboratory. Non-orb webs such as sheet webs were scarce (< 5% of all webs) and they were not included in the present study. We compared seasonal differences in web abundance by paired t-tests with square-root transformed data, using sites as replicates (N = 10). As web abundances in some sites were low (< 50 recorded), data on web height and orientation, substrate for web attachment and prey catches from all ten stream reaches within the same season were pooled during seasonal comparisons. These comparisons included the percentage of webs recorded at each height category, and the relative importance of different substrates for web attachment, which was compared between seasons based on the percentage frequency of attachment to different substrates. Because some webs were attached to more than one type of substrate, the frequency of web attachments to all substrate types in a season could exceed 100 percent. We also tested the null hypothesis that the ratio of web orientations (horizontal: tilted: vertical) was 1:1:1 with Chi-square tests. Rejection of the null hypothesis would indicate a tendency to construct webs of a particular orientation category.

Riparian Spiders Along Tropical Streams 3

MANIPULATIVE EXPERIMENT.—We conducted this experiment in four similar-sized, second-order tributaries within the Tai Po Kau Forest Stream drainage. Mean wet width of each tributary varied from ca 1 m during the dry season to 1.6–1.7 m during the wet season. Two 30-m stream reaches (separated by 20 m) were selected along each tributary, with one stream reach designated as a treatment site and the other as a control site, giving a total of four treatment and four control sites. Treatment sites were located upstream of the control sites in two tributaries, and downstream in the other two tributaries. The design was based on a Beyond BACI (before-aftercontrol-impact) design with repeated sampling before and after manipulation at multiple control and treatment sites (Underwood 1994). Surveys were made along the control and treatment sites on four occasions (each separated by 7–15 days) on randomly selected days during the ‘before’ stage. On each occasion, we finished recording data from both control and treatment sites on a tributary before moving on to other tributaries, but the sequence of visiting the four tributaries was random. Data from the four tributaries were either collected on the same day or on two consecutive days (two tributaries on one day, and the other two on the next day). Only the abundance of webs and substrates used for web attachment were recorded, and prey items were not removed so as to avoid disturbing the spiders. After completion of the ‘before’ stage, artificial substrates were added to each treatment reach to enhance availability of substrates, with each reach receiving eight Hessian ropes (2 m × 2 cm) and ten bamboo poles (1.2 m × 5–8 cm). Three weeks later (during the ‘after’ stage), surveys were repeated four times (separated by 4–12 days) along each reach in the manner described above. The effect of additional substrates on the densities of webs was assessed by dividing the number of webs in each reach by the area of the wetted stream surface, which was obtained by multiplying the mean wetted width by the reach length. The entire experiment was conducted twice: from February to early April during the 2006 dry season, and between May and August in the subsequent wet season. We tested the effects of treatment (control sites vs. treatment sites, fixed factor) and stage (before vs. after, fixed factor) on the web abundance and density separately by season using analysis of variance. Time was a random factor nested within stage, and stream reaches were replicates (N = 4). A significant impact due to the addition of substrates would be indicated by a significant interaction between treatment and stage (Underwood 1994). Data were squareroot transformed to improve the homogeneity of variances before analysis. All the statistical analyses in this study were carried out using SPSS version 15.0 (SPSS Inc., Chicago, Illinois).

RESULTS FIELD SURVEYS.—A total of 1018 webs were recorded from the ten stream reaches within the two seasons; 512 webs from the wet season and 506 from the dry (Table 1). We did not detect any significant difference in web abundance between seasons (t = 0.618, df =

FIGURE 1. Seasonal differences in: (A) substrates used for web attachment; and (B) the height distribution of webs above the water surface; data have been pooled among ten stream reaches. Solid bars: wet season; open bars: dry season.

9, P = 0.552). A total of 156 prey items were recovered from the webs, 64 percent of them during the wet season (Table S1). Adult aquatic insects comprised more than half of prey items, but they were considerably more abundant during the wet (68% of total prey) than the dry season (36%). Adult Ephemeroptera (42% of total prey) was the most numerous taxon collected from webs during the wet season, followed by adult Chironomidae (23%) and terrestrial Diptera (21%). During the dry season, terrestrial Diptera (51%) accounted for more than half of the sample, followed by adult Chironomidae (25%) and adult Ephemeroptera (11%). Other taxa contributed to < 15 percent of all prey collected. During the dry season, most webs were attached to boulders scattered along the streams, but both boulders and overhanging vegetation were equally important for web attachment during the wet season (Fig. 1). Dead wood was used also in both seasons but to a relatively limited extent (Fig. 1). Seasonal differences in web height from the water surface were also observed; webs tended to be closer to streams during the dry season when 85 percent were within 50 cm of the surfaces, compared to 50 percent during the

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FIGURE 2.

Mean number of webs and density for the control (solid bars) and treatment (open bars) sites during the before and after stages of the experiment in

dry (plots A, C) and wet (plots B, D) seasons. Error bars indicate SE.

wet season (Fig. 1). Conversely, > 20 percent of webs were 100– 200 cm above the water surface in the wet season, compared to < 10 percent during the dry season. In both seasons, more webs were orientated horizontally (0–30◦ ) with respect to the water surface (45% in the wet and 52% in the dry season), rather than tilted (31 and 30%, respectively) or perpendicular (24 and 18%, respectively) to it. We found that these proportions (horizontal:tilted:vertical) varied significantly from a ratio of 1:1:1 in both seasons (wet season: χ 2 = 6.86, df = 2, P = 0.032; dry season: χ 2 = 17.8, df = 2, P = 0.001), but the orientations were evidently very similar between seasons. FIELD EXPERIMENT.—The added substrates were used frequently by spiders during both seasons, and a total of 25–26 percent of the webs in the treatment sites were attached to them. These values remained consistent among seasons for bamboo poles (7%) and varied only slightly for ropes (18–19%; Table S2). As in the survey results, boulders were used more frequently than vegetation for web attachment during the dry season (boulders: 56–85%, vegetation: 34–59%). However, vegetation (74–92%) became more important than boulders (34–59%) during the wet season in the field experiment (Table S2). Dead wood was the least important substrate in both seasons (wet: 9–15%; dry: 9–12%). More webs were recorded at treatment than control sites in both before and after stages, but the percentage differences in web abundance were consistently larger during the after stage (Fig. 2A). For example, the mean number of webs was 18 percent higher at treatment sites before the addition of substrates during the dry season, but the difference increased to 41 percent after manipulation (Fig. 2A). The equivalent values during the wet season were

24 percent (before) and 58 percent (after) (Fig. 2B). A similar trend was found for the data on web densities (Fig. 2C, D). We confirmed with ANOVA that the web densities and web abundances were significantly increased by the addition of substrates during both seasons (treatment × stage, P < 0.05; Table 2).

DISCUSSION Spider abundance in tropical riparian zones may vary substantially with season and in response to habitat structure or complexity (Buskirk 1975). Marked seasonal changes in spider abundance along Hong Kong streams were expected, because more adult aquatic insect prey items are available in riparian zones during the wet season (Chan et al. 2007). Surprisingly, however, a similar number of webs was recorded in surveys during both seasons. We have to interpret this result with caution since the seasonal difference in spider abundance was assessed by comparing the number of webs, and environmental factors may influence the web-building behavior of spiders (Power et al. 2004): e.g., food limitation during the dry season may force spiders to maintain webs for longer time. Another possible explanation is that spiders close to the water were frequently disturbed by spates that may have limited their abundance during the wet season (Dudgeon & Corlett 2004). The more frequent use of overhanging vegetation for web attachment during the wet season can be explained by the fact that it is further from the water surface than boulders. If spiders are limited by spate-induced mortality during the wet season, webs closest to the water surface would be most vulnerable and those suspended from overhanging vegetation would persist longer. Conversely, building webs close

Riparian Spiders Along Tropical Streams 5

TABLE 2. Summary of ANOVA results for the effects of increased substrate availability on the abundance and density of orb-webs along streams during the dry and wet season. Dry season

Wet season

MS

F

Treatment 1 Stage 1 Treatment × stage 1

6.37 36.3 2.09

24.8 2.88 8.12

Time (stage) Treatment × time (stage)

12.6 49.0 < 0.001 0.257 0.440 0.848

df

P

MS

F

P

Number of webs

Error

6 6 48

0.003 6.72 0.141 20.1 0.029 1.85

28.1

42.8 7.94 11.8

2.53 16.1 0.157 0.158

0.001 0.030 0.014 0.002 0.986

47.6

Density of webs 1 1

0.246 52.0 < 0.001 1.08 2.34 0.177

0.088 22.2 0.331 5.94

0.003 0.051

Treatment × stage 1 Time (stage) 6 Treatment × 6

0.090 18.9 0.005 0.460 97.2 < 0.001 0.005 0.232 0.964

0.046 11.6 0.056 14.0 0.004 0.197

0.014 0.003 0.976

time (stage) Error

0.020

0.020

Treatment Stage

48

to water during the dry season may be more effective for capturing emerging aquatic insects, which are more limited at this time (Chan et al. 2007). Building webs close to water during the dry season may also help the tetragnathids avoid desiccation as they are vulnerable to water stress (Power et al. 2004). A number of studies have shown that, as observed in our study, some tetragnathids (e.g., Tetragnatha and Metabus species) tend to spin horizontal webs (Eberhard 1990, Kato et al. 2003). In riparian zones, webs that are horizontal to the water surface may improve both the efficiency of intercepting emerging aquatic insects and the ‘rain’ of terrestrial insects from overhanging vegetation. Construction of horizontal webs could also be a consequence of limitations in vertical structures above the stream surface (Eberhard 1990) and may explain the occurrence of webs close (< 50 cm) to the water surface. Adult aquatic insects, particularly small individuals with weak flying ability (e.g., Chironomidae and certain Ephemeroptera), contributed over half of the prey on webs indicating that aquatic insects are an important energy subsidy for the spiders. Caddisflies (Trichoptera), which are common in Hong Kong streams, were rare (one individual among 156 prey), but this may reflect the ability of larger insects to escape from webs (Nentwig 1987). Seasonal differences in the proportions of aquatic prey on webs reflected changes in the abundance of flying aquatic insects in Hong Kong (Chan et al. 2007). However, even during the wet season, the proportion of aquatic prey (68%) was somewhat lower than that reported for riparian orb-webs in Japan, where aquatic insects made up > 80 percent of prey during May and June (Kato et al. 2003). This minor

difference may result from the more seasonal Palaearctic climate of Japan, where aquatic insect emergence occurs over a shorter period than in tropical Hong Kong (Nakano & Murakami 2001, Dudgeon & Corlett 2004). The results from the experiment were consistent with the hypothesis that enhancing substrate availability would lead to a numerical response of spiders, resulting in an increase in abundance and density of webs in sites along streams where aquatic insect prey are abundant. This suggests that these riparian spiders may be substrate limited, rather than food limited. A similar effect has been reported along a Canadian stream where the abundance of orb-web spiders was higher along a stream bank with dense overhanging vegetation than along the opposite bank with less vegetation (Williams et al. 1995). In California, higher densities of spiders were likewise associated with increased habitat structural complexity (Power et al. 2004). While complexity offers more attachment sites for spider webs, and hence allows them to make use of riparian sites with high food availability, such complexity may provide other benefits to spiders such as protection from predation and extreme weather conditions (Rypstra et al. 1999, Halaj et al. 2000, McNett & Rypstra 2000). We have no information on the frequency with which O. diporusa changes web location, but studies of other tetragnathids show that they may change sites of web attachment daily if they cannot get enough prey at a site (Power et al. 2004). This makes it likely that the 3-wk period we allowed for spiders to respond to extra substrates in the present study was sufficient to produce a realistic response in spider abundance. In conclusion, this study demonstrates the use of aquatic insects as prey by riparian orb-web spiders along tropical streams, but the importance of aquatic prey, as well as the height of webs from water and the use of substrates for web attachment, varied with season. No seasonal difference in web abundance was found in the surveys; however, the experiment showed that abundance and density of webs increased with increasing availability of substrates during both seasons, suggesting that distribution of substrates is a critical determinant of the spider (web) distribution. We suggest that the abundance of aquatic insect prey along streams makes these habitats potential ‘hotspots’ for spider abundance, and that enhancing substrate availability removes a constraint on spider numbers allowing them to become more numerous. In turn, this will likely affect the proportion of emerging aquatic insects that are captured by spiders, and thus the availability or distribution of substrates could have indirect effects on the magnitude of aquatic insect subsidies to terrestrial food webs.

ACKNOWLEDGMENTS We thank Shu-Qiang Li from the Chinese Academy of Sciences for the identifications of spider samples, and two anonymous reviewers for helpful comments on the manuscript. EKWC was supported by a postgraduate studentship from The University of Hong Kong when this study was taken. The work described in this paper was partially supported by a grant from the Research Grants Council of Hong Kong Special Administrative Region, China (Project No.

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[HKU] 7619/05M). We are grateful to the Agriculture, Fisheries and Conservation Department of the Hong Kong Government for permission to conduct this study within protected areas.

SUPPORTING INFORMATION Additional Supporting Information may be found in the online version of this article: Table S1. Number (percentage) of prey items collected from orb-web spider webs during wet and dry seasons. Table S2. Frequencies of orb-web spider web attachments (%) of different substrate types in the experiment during the dry season and the wet season. Please note: Wiley-Blackwell are not responsible for the content or functionality of any supporting materials supplied by the authors. Any queries (other than missing material) should be directed to the corresponding author for the article.

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