Stream Proximity and Tetragnathidae Web Inclination Along a Tropical

Stream Proximity and Tetragnathidae Web Inclination Along a Tropical Headwater Stream in Puerto Rico Kirsten Verster1 and Sean Kelly2 1Biology, University of Florida, Gainesville FL 2Department of Biology, University of Puerto Rico, San Juan PR Abstract

Much ecological research has been focused on streams found in temperate regions, while those found in tropical regions remain relatively uninvestigated. Tropical streams, however, can be home to diverse ecosystems which reciprocally supports the adjacent riparian and terrestrial environments. One such specimen that capitalizes on this riverine output are web-building spiders, which have been shown at times to derive significant nutrition from emerging aquatic insects. Certain characteristics of spider webs, such as their angle and location, are optimally built to catch prey. Prior research suggests that horizontal webs may be most efficacious in capturing vertically-moving prey, such as emerging aquatic insects. We hypothesized that spider web angles should change relative to their proximity to this food source. We used spiders from the family Tetragnathidae and placed lateral transects along the headwater tropical stream Quebrada Prieta in El Yunque National Rainforest of northeastern Puerto Rico; we measured spider web angles along these transects to determine the relationship between stream proximity and web inclination. We determined that there was no relationship between web angle and distance from the stream. In addition to learning more about where the two choose to place their webs, we also determined that the abundances of Leucauge and Chrysometa did not change from riparian to upland habitats, thus undermining a prior assumption that all genera within the family Tetragnathidae are riparian specialists.

Introduction

While the fauna of European and North American streams have been studied extensively and are well known, much less organized research has been focused on the diverse ecosystems that inhabit tropical streams. Web-building spiders are particularly good model organisms for ecological study due to their high abundance, immobile habitat, and easily discernible webs which facilitate censusing (Greenstone, 1984). Orb-weaving spider ecology and web-building behaviorin temperate zones are well documented, but the results of these studies cannot, without further information, be justifiably generalized to their counterparts in the tropical regions.

While the allochthonous input from the terrestrial environment has always been an important facet of stream ecology, there is also a reverse flow of carbon from the aquatic to riparian ecosystems surrounding it (Collier, Bury, & Gibbs, 2002). One example of this is when organisms of lotic origins, such as some amphibians and insect larvae, migrate to their terrestrial

1 surroundings. Imagoes, the sexually mature, recently metamorphosed insects emerging from their natatorial homes, provide significant nutrition to riparian spiders (Collier et al., 2002; Greenstone, 1984; Henschel, Mahsberg, & Stumpf, 2001), whose webs are often found directly above the land water interface to capitalize on emerging insects (Uetz, 1992).

A spider’s web, which plays a critical role in its daily nutrition, is effectively its extended phenotype and, as such, should directly influence its fitness (Bishop & Connolly, 1992). Therefore, we can conclude that web architecture that maximizes foraging success is likely to be selected for (Eberhard, 1990). While there are many aspects of web construction that are highly conserved (Vollrath & Selden, 2007), there are certain aspects, such as symmetry, angle, and location, that can be altered with experience (Eberhard, 2000; Heiling & Herberstein, 1999).

There are two key characteristics that determine the efficacy of a spider web: the likelihood of a prey falling into it (interception), and its ability to retain the prey until the spider can consume it (Bishop & Connolly, 1992). One property of a spider’s web that can be crucial in determining its ability to capture prey is web angle (Chacon & Eberhard, 1980). However, there is a discrepancy in the literature as to which web angle relative to the substrate is most efficacious for prey interception. In one study that was conducted in an open field of grass and weeds in Colombia, vertical traps were much more likely to intercept prey species than horizontal or tilted traps (Chacon & Eberhard, 1980). However, in a similar study conducted in the tropical rainforests of Puerto Rico, horizontal and tilted traps caught more than vertical ones. Despite this, the actual web angles tended towards the vertical (Bishop & Connolly, 1992). It is likely that the different sampling environments were responsible for this discrepancy. Prey retention may be greater in vertical webs because, though gravity may pull prey free in horizontal webs, it would only thrust them back into contact with other sticky threads in vertical ones (Eberhard, 1972).

To our knowledge, there have been no studies to date that seek to discern the relationship between web angle and distance from a prominent food source. The studies discussed used random plots but did not identify extraneous variables that could have influenced prey assemblages, such as the nearby presence of streams. The surfaces of tropical streams are often abundant with insects (Buskirk, 1975), which could influence the behavior of nearby predators such as spiders (Kato, Iwata, Nakano, & Kishi, 2003). It has also been demonstrated that distribution and abundance of aquatic insects gradually changes at a distance from the water source, with a significant “drop-off” occurring at around 25 meters (Jackson & Resh, 1989; Sanzone, 2001). These insects emerge nearly perpendicularly to the stream surface (Masteller & Buzby, 1993). A recent paper indicated that Tetragnathidae are specialized to catch aquatic insects—as a result, their webs are believed to be horizontal near fresh bodies of water in order to capitalize on emerging aquatic insects (Laeser, Baxter, & Fausch, 2005).

In this study we sought to examine the difference, if any, between web angles of Tetragnathidae spiders at different distances away from a tropical stream on the island of Puerto

Rico. In doing this we sought to contribute to what is presently a lack of tropical arachnological research, and to help further elucidate the relationship between stream and riparian habitats. We also aimed to correct for the numerous monotemporal experiments by including nocturnal samplings, since sampling time can dramatically influence the composition of spider collections. Additionally, some genera within Tetragnathidae, such as Chrysometa, are considered nocturnal taxa (Green, 1999).

Our objective was to determine the effect of stream proximity on web angle in Tetragnathidae. We hypothesized that if the spiders near the stream are using vertically emerging aquatic insects as a primary food source, the more horizontal webs near the stream will be most efficacious in prey capture. Therefore, we would expect those webs nearest the stream to be more horizontal than those further from the stream.

Methods

Study Site

This study was conducted in the El Yunque National Forest, a tabonuco forest which is located in the Luquillo Mountains in northeastern Puerto Rico. We placed the transects adjacent to Quebrada Prieta (A), a small headwater second-order stream with a slope averaging 20% (Masteller & Buzby, 1993). A 100 meter section of stream was chosen for its relatively level riparian zone, since steep gradients could adversely affect the methods of our study.

Study Organism

Though there are a plethora of tropical spiders in El Yunque National Forest, we examined one family of spiders that has been said to use horizontal webs to catch emerging aquatic insects, Tetragnathidae “long jawed orb weavers” (Henschel et al., 2001). From personal observation the Tetragnathidae, or long-jawed orb weavers, are common throughout El Yunque.

The Tetragnathidae webs, which are broad, planar, and easy to spot in the field (Hormiga, Eberhard, & Coddington, 1995), lend themselves to angular plasticity and determination; coupled with their abundance, this makes them a perfect family to examine. Three genera in the Tetragnathidae family found during the study: Tetragnatha, Leucauge, and Chrysometa. Species identification was not necessary because differences in web type are found at the genus level and thus make this discrimination irrelevant to our study.

Experimental Procedure

A total of 10 lateral transects were established every 10 meters along the Quebrada Prieta for a total sampling space of 100 meters. Each lateral transect was 30 meters in length. We exhaustively searched the area along each lateral transect for webs of Tetragnathidae spiders. Upon finding a spider web (within arm’s reach), we lightly dusted it with rice flour in order to enhance visibility. We measured the angle of the hub from horizontal as determined by a simple

3 level and protractor, as well as web size, height above the ground, and distance to the stream edge. Sampling was conducted under fair weather conditions since web-spinning spider activity is adversely affected by rain (Riechert & Gillespie, 1986). The spiders were collected live in plastic vials to be later identified. In order to control for the potential effects of spider size on web inclination (Herberstein & Heiling, 1999), we recorded the wet weight (mg) and cephalothorax width (mm) of each specimen.

Diurnal and nocturnal samplings were performed on different days so as to minimize disturbance to spider webs within the study area. These samplings occurred biweekly (one diurnal and one nocturnal) for six weeks from June 17th to July 24th. In sum, there were a total of 60 transects sampled during the study, half being diurnal and the other half nocturnal. Diurnal samplings occurred from approximately 08:00—12:00; nocturnal sampling from 20:00—24:00.

Statistics

We used the program PAST to determine if the data fit the normal distribution; for those that did we performed two-way ANOVAs with three stream distance groups as the main factors: 0—10 m (riparian), 10—20 m (intermediate), and 20—30 m (upland). We ran these against variables such as web angle, height, and area. For those data that did not fit the normal distribution we performed Kruskal-Wallis tests similarly. To graphically represent the relationship between web angle and stream proximity we performed a regression analysis. We also ran a regression analysis between spider size and web angle. We used the programs R (R Development Team, 2013) and PAST (Hammer, Harper, & Ryan, 2001) to create the necessary graphs and organize the data.

Results

Taxa Abundances — The most abundant genera were by far Chrysometa (n=72) and Leucauge (n=158) webs; Tetragnatha (n=5) were not abundant enough to warrant their inclusion in the following analyses. We categorized the webs we found as being within three different distances from the stream source. We also divided these into day and night samplings (Table 1). There was no difference in genera abundance for different distances, with one exception – there were significantly less Leucauge webs closest to the stream at night (ANOVA, p<0.05).

Table 1. Shows the different genera abundances at different times and distances from the stream source.

Time Species Total 0 - 10 m 10 - 20 m 20 - 30 m Day Leu 80 26 27 27 Day Chr 7 1 3 3 Night Leu 78 13 33 32 Night Chr 65 16 27 22

We also examined what substrate the different genera choose to build their webs on (Table 2). Leucauge were significantly more likely to choose vegetation over other substrate (chi-squared: χ2=160.215, DF=3, p<0.001), whereas Chrysometa had a greater inclination for deadwood (chi-squared: χ2=14.667, DF=3, p=0.0021).

Table 2. Shows the proportions of each genera found in each substrate type.

Genus Deadwood Fence Vegetation Mixed Leucauge (n=158) 0.08 0.08 0.68 0.16 Chrysometa 0.42 0.11 0.19 0.28 (n=72)

Web Inclination – Contrary to our hypothesis, we found no significant relationship between web angle and stream proximity in either Chrysometa or Leucauge webs (Figure 1). This was still the case even when standardizing for variables such as web height, size, or substrate.

Figure 1. Relationship between web angle and distance from the stream—blue diamond points represent Chrysometa webs while red squares represent Leucauge webs. There is no relationship between the two in either Chrysometa (r²=0.0108, p<0.05) or Leucauge (r²=0.0043, p<0.05) webs.

Although there was no relationship in web angle and distance from the stream, we found other distinct patterns distinguishing the two most abundant genera. We found that Chrysometa webs were significantly more vertical (tending towards a 90° angle) than Leucauge webs and had a lower standard deviation, while Leucauge webs were more horizontal and displayed a greater spread (Figure 2).

Figure 2. Box-and-whisker plot of web angles for both Leucauge and Chrysometa. Chrysometa webs are significantly more vertical than Leucauge webs (t-test: p<0.001).

Additionally, we also discovered that Chrysometa webs were significantly lower to the ground than Leucauge webs, again with less spread in height (Figure 3).

Figure 3. Box-and-whisker plot of web heights for both Leucauge and Chrysometa. Chrysometa webs are significantly higher than Leucauge webs (t-test: p<0.001).

Discussion As shown, there was no relationship between the web angle and stream proximity in either spider genus. Web construction is a conserved behavior influenced by a complex array of factors (Riechert & Gillespie, 1986); while stream proximity may play some role in angular determination, ultimately it is not significant enough to lead to a simple relationship between the two. It is likely that other factors, such as temperature, substrate, and sunlight play a more important role in what type of web a spider will build on a given day. An important consideration is that the movements of aquatic insects may be too complex to warrant this simple a relationship in regards to optimal prey capture. A horizontal web may only be optimal immediately above the stream surface, when the emerging insect’s movements are more or less vertical in order to escape the surface of the water (I. J. Davies, 1908). Once the insect is away from the immediate proximity of the stream, its movements (and, indeed, the terrain over which it flies) may be sufficiently irregular and stochastic so as to confound any distinct angular advantage.

Since there was no significant decline in spider abundance at up to 30 meters from the stream source, this suggests that prey availability did not change within these bounds, a fact supported by prior research (Jackson & Resh, 1989; Sanzone, 2001). Prior literature posits that Tetragnathidae are specialists in catching emerging aquatic insects (Akamatsu, Toda, & Okino, 2004; Kato et al., 2003; Laeser et al., 2005) – however, given that these spiders are equally abundant in riparian and upland regions, where the aquatic insect abundance drops exponentially (Jackson & Resh, 1989), this does not seem to be the case. Prior generalizations about Tetragnathidae also suggested that their webs were mainly horizontal (Laeser et al., 2005), which was definitely not the case (see Figure 1), especially with Chrysometa. Much of the prior research on Tetragnathidae occurred in forested regions where the predominant genus is Tetragnatha. It appears as though the researchers overgeneralized their results of a few genera to include all Tetragnathidae, a grievous error that undermines the diversity of the family as well as the wide range of ecosystems they inhabit. We discovered many other interesting facts about the web-building behaviors of Chrysometa and Leucauge. There is a startling lack of published literature about most Neotropical spider genera, particularly Chrysometa. While attempts to classify the various different Chrysometa species have been made, little is known of their life histories and factors responsible for their web formation (Levi & Eberhard, 1986). We found that Chrysometa are primarily nocturnal and have a preference for deadwood as a web-building substrate. erhaps because most deadwood is near to the ground, the average height was relatively low (n 2, 0.79, S.D.=0.44), which is close to the previous estimate of one meter (Levi & Eberhard, 1986). Their webs also tended to be more vertical (Figure 1). All Chrysometa webs observed had a tangle web structure towards the top in which they would hide until prey hit the web, at which point they would drop onto their prey (personal observation). Leucauge, on the other hand, had no such structures (personal observation). They were equally prevalent during the day and night, and overwhelmingly favored vegetation as their substrate of choice (Table 2). Their webs tended to be higher than Chrysometa (n=158, 1.16, S.D. =0.42) and more horizontal ( 34o, S.D. =19o). The variation in their web angles was much higher than that of Chrysometa, which may be because their preferred substrate, vegetation, is more variable in 3D space and so offers greater flexibility (or possibly constraints) in how they construct their webs. It is possible that the difference in preferred angles is the result of having evolved different hunting strategies. The Chrysometa “drop attack” described earlier is facilitated by the high verticality of their webs; gravity facilitates their dropping a long distance relatively quickly. Leucauge, on the other hand, may rely more on using the momentum provided by the slight incline to propel their run across the web (Gregoric et al., 2013). The two spider genera seem to occupy different types of areas in their respective microhabitats; Leucauge tend to be higher and located in vegetation while Chrysometa are lower and mainly reside in deadwood (Table 2). This difference in habitat choice may help reduce competition by enabling them to catch different prey (Hénaut, García-ballinas, & Alauzet, 2006). More research remains to be done on these and other Neotropical spider genera in order to fully understand how they have evolved to fit their specific habitats. In addition, further avenues for investigation lie in examining the differences between webs built directly over water and those far from a lotic source.

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