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Orb-Weaving Spiders And Niche Partitioning In Louisiana Bottomland Hardwood Forests

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Introduction

An ecological niche is defined as an organism’s habitat and resource use through competition and relationships to other organisms (Slagsvold). The Louisiana bottomland hardwood forest is a unique environment, with year-round moisture and a warm temperate climate creating an abundance of ecological niches for different organisms to fill. Yet despite the number of niches, many species of orb-weaving spiders have evolved to apparently share the same niche. The particular niche an organism occupies is primarily dependent on feeding behavior (Sanders), but it is also dependent on the ability to avoid predation by other species. There is a definite limit to the similarities of species that occupy the same niche (Macarthur), as too many similarities would lead to one species outcompeting other species and being the sole occupant of that niche. Thus, there may be some sort of partitioning mechanism that allows for the multiple species of orb-weaving spiders to coexist. Orb-weaving spiders share the same general strategy for capturing prey, using their webs as a sort of “aerial sein” to capture prey that flies into them (Uetz), but minute differences in other factors can allow for that viable partitioning (Cloyed). The presence of multiple species of orb-weaving spiders in Louisiana bottomland hardwood forests is possible due to varying web structure, design, and location through niche partitioning.

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Methods

Three separate research groups traveled to the Jean Lafitte National Historic Park and Preserve in New Orleans, LA on September 10-12, 2018. Sampling occurred in the afternoon all three days. For the first two days, sampling occurred during and directly after a severe thunderstorm that produced minor flooding in some areas of New Orleans, with residual rain and runoff from the tree canopy maintaining moist conditions in the study area during sampling. The final day was warm, humid, and sunny. The study area is classified as a bottomland hardwood forest, characterized by the high amount of moisture and plant species like Taxodium distichum (bald cypress) and Sabal minor (dwarf palmetto). There was an elevated boardwalk that winded throughout the park area. To randomly determine a sampling plot, after walking ten meters away perpendicular to the boardwalk, we placed a chaining pin. Using an online random number generator, we generated two values between 1-10: the first number was the number of steps walked perpendicular to the first 10-meter transect, and the second number was the number of steps walked parallel to the 10-meter transect. After the second set of steps, we determined a circle with 5-meter radius with a center at our location. A second 5-meter sampling plot was created with the same process as the first, but the steps went the opposite direction of the chaining pin as the first set. Within each sampling plot, we searched for visible and intact orb-weaver webs and recorded data from multiple variables. The webs are always surrounded by contact or anchoring points for the spiders to attach to, but we were thorough and ensured there weren’t any webs hiding in too much foliage. The study organisms included seven species of orb-weavers found in the area: Argiope aurantia, Nephila clavipes, Gasteracantha cancriformis, Micrathena sagittate, Micrathena gracilis, Leucauge venusta, and Verrucosa arenata. We searched for all orb-weavers present, but only obtained sufficient data for three species: Nephila clavipes, Gasteracantha cancriformis, and Leucauge venusta. Data was only recorded for webs with the spider present for verification. For each web, along with plot number (one or two) and spider species, the following statistics were recorded:

Variable Method of Obtainment Units

Spider Length

Length of spider from head to the end of the abdomen mmWeb Area Recorded longest diameter of the web, found a second diameter perpendicular to the first, averaged the two, divided the average in half, and used the resulting radius to find a circular area 〖cm〗

2Strand Density Counted the number of web strands within a 5 cm section of the longest diameter #Orientation Angle of the web with respect to the ground (a horizontal web is 0°, and a vertical web is 90°) Degrees (°)Substrate Proximity Qualitative measurement of the closest vegetative substrate to the center of the web in cm; measurement is recorded as close (C à <10cm), medium (M à 10-50 cm), or far (F à >50cm) C, M, or F.

Discussion

Despite the similar niche that orb-weavers seem to share at Jean Lafitte, there are clear and identifiable differences between the three species of spiders and their webs. Nephila clavipes proved to be substantially larger than the other species, potentially allowing for the ability to capture and consume larger prey than the other species. Additionally, Nephila clavipes built the largest webs by a significant margin, perhaps to allow them to catch more prey to sustain themselves. Leucauge venusta was almost the only spider consistently placing webs near substrate levels, suggesting the possibility of capturing walking prey or other insects that stay near trees and the palmettos. That tendency may also play a role in evading predators since the spiders and their webs are more hidden.

In contrast, Nephila clavipes occupied lots of open spaces far away from substrate, indicating the potential of capturing fast-moving flying prey and no real fear of predatoin. Leucauge venusta was also only species to have a low web angle, whereas Gasteracantha cancriformis and Nephila clavipes had high angle, near-vertical webs, enforcing that they both seek flying prey. Unfortunately, most webs recorded for this study lacked prey, preventing us from definitively determining how the webs differentiated in prey types and capture techniques. One glaring problem of this study is that all the data was recorded by different research groups all working on separate days. Despite the clear instruction and detailed description of how to carry out the experiment, there were undoubtedly slight differences in the ways each group recorded their data. The wet and unideal conditions may have also influenced our results. While it has been suggested in the past that orb-weaver webs will not lose effectiveness when they become wet (Boutry), the conditions may still have influenced out data. Grueling, mosquito-dense conditions made data collection difficult at times, potentially encouraging lazy or low-effort data collection. Alternatively, the weather conditions on the first two days may have affected prey behavior, causing some species to be more active while possibly causing others to become less active. We had no method of measuring webs out of reach, presenting us with our biggest source of bias in the study. There may have been additional webs to collect data from, either from species we studied or the species we lacked enough data for. The inability to measure those higher webs, along with a general lack of observations of prey caught in webs, put a large hindrance on how far we could extend biological interpretations from the data we gathered.

03 December 2019

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