B. P. Perkins, LSU Biology Undergrad

Introduction

The competitive exclusion principle can be paraphrased in four words: Complete competitors cannot coexist (Hardin, 1960). Although some find this to be an over-simplified maxim that may cause some ecologists to overlook more important underlying evidence, it does raise some interesting questions in the curious mind (Cole, 1960). The principle states that if two distinct populations use the same resources, live sympatrically, and if one population is even slightly better at translating energy to reproductive success than the other; then the fitter population will eventually drive the less fit population to extinction (Hardin, 1960). However, there is another option for the less fit population. Character displacement is an evolutionary process that involves a directional selection toward niche divergence. Sympatric species whose niches originally overlap will have selective pressures on them that cause reduced niche overlap and allow the species to coexist (Molles, 2007).

This experiment looks at the apparent coexistence of three orb-weaver spider species in Louisiana: Gasteracantha cancriformis, Nephila clavipes, and Leucauge sp. Based on the exclusion principle explained above, if the three species inhabit the same ecological niche, the most successful species of spider should drive the others to extinction. Because there is more than one species of spider living sympatrically, it must be assumed that they partition the resources in some way that reduces niche overlap and allows for coexistence. The only question that remains is that of how the spiders actually divide the resources amongst themselves.

Enders (1974) showed that coexisting spider species can partition their niches by building webs at different heights in the forest to catch different types of prey. To test if the orb-weaver spiders that are being studied use this method of niche partitioning, the average web height of each species will be compared. If the spiders being tested use this method of vertical stratification for reducing niche overlap we will expect to see a significant difference in the web heights measured. Craig (1989) lumps understory orb-weaver spiders in Panama into two alternative foraging modes. Large spiders build one large web per feeding period whereas small spiders build a few smaller webs. To test if the orb-weavers in Louisiana use these alternative methods of foraging we will compare the average web area of each species. A significant difference in web area among species would be evidence supporting this method of niche partitioning. Barghusen et al. (1997) showed that the web of the common house spider is more efficient at capturing flies when the strand density is increased. Although orb weaver spiders are being studied, it can still be determined if they use alternative strand densities as a method of reducing niche overlap. To test this we will compare the strand and radii density of the spider webs measured to look for a significant difference among the species. A significant difference supports the hypothesis that they use this method of niche partitioning.

Methods

Field work.—Random samples of orb-weaver spiders were taken at two locations in Baton Rouge, Louisiana (Ben Hur Experimental Forest and Bluebonnet Swamp) over a two year time. One sample was taken during the fall semester of 2006 and the other was taken during the fall semester of 2008. Measurements were taken of the spider size (as measured by length), the height of the center of the web off of the ground, the longest length across a diameter of the web, the length of the diameter perpendicular to the longest diameter, the number of strands per five centimeters of web (strand density), and the number of radii around the web. Data was collected on three species of orb-weaver spiders (Gasteracantha cancriformis, Nephila clavipes, and Leucauge sp.).

Data analyses.—First the data was compiled from the measurements taken in the fall of 2006 and the fall of 2008. The web area was then calculated by multiplying together the two diameter lengths. The radii density was found by dividing the number of radii per web by the area of the web. Bar graphs were made showing the average spider length (to be used as a reference point for comparing spiders), average web height, average web area, strand density, and radii density with 95% confidence intervals. Finally, ANOVA and T-tests were performed on each group of data (spider length, web height, web area, strand density, and radii density) to determine if there are in fact any significant differences among the three species of spider.

Results (Boring and Technical, feel free to skip to the Discussion)

Figure 1 is a representation of the average sizes of the spiders measured. There is a significant difference in size among the three orb-weaver species (F2,318 = 145.11, P = 1.66*10 45). Nephila clavipes is significantly larger than both Gasteracantha cancriformis (t293 = 15.51, tcrit = 1.97, P = 4.86*10-40) and Leucauge sp. (t151 = 7.14, tcrit = 1.98, P = 3.66*10-11). Leucauge sp. is significantly smaller than Gasteracantha cancriformis (t192 = -6.14, tcrit = 1.97, P = 4.73*10-9).

The average web heights for each species of spider measured are presented in Figure 2. There is a significant difference in the web height among the three species of orb-weaver spiders (F2,318 = 40.17, P = 2.79*10-16). Nephila clavipes have significantly higher webs off of the ground than both Gasteracantha cancriformis (t293 = 5.06, tcrit = 1.97, P = 7.33*10-7) and Leucauge sp. (t151 = 7.83, tcrit = 1.98, P = 7.81*10 13). Leucauge sp. have significantly lower webs to the ground than Gasteracantha cancriformis (t192 = -6.40, tcrit = 1.97, P = 1.16*10-9).

Figure 3 is a representation of the average web areas of the spiders. There is a significant difference in web area among the three species of orb-weaver spiders (F2,318 = 19.10, P = 1.47*10-8). Nephila clavipes have significantly larger web area than both Gasteracantha cancriformis (t293 = 5.38, tcrit = 1.97, P = 1.54*10-7) and Leucauge sp. (t151 = 3.44, tcrit = 1.98, P = 0.0007). There is not a significant difference between the web areas of Gasteracantha cancriformis and Leucauge sp. (t192 = 1.74, tcrit = 1.97, P = 0.08).

There is no significant difference among the strand densities (Figure 4; F2,318 = 2.49, Fcrit = 3.02, P = 0.08), but there is a significant difference in the radii densities among the three species of orb weaver spiders (Figure 5; F2,318 = 45.04, Fcrit = 3.02, P = 6.00*10-18). Gasteracantha cancriformis and Nephila clavipes do not have a significant difference in radii density (t293 = 0.27, tcrit = 1.97, P = 0.79). Leucauge sp. has a significantly larger radii density than both Gasteracantha cancriformis (t192 = 7.94, tcrit = 1.97, P = 1.66*10-13) and Nephila clavipes (t151 = 6.94, tcrit = 1.98, P = 1.11*10-10).

Discussion

The analysis of web heights shows that each species does build webs at different heights. The largest species (Nephila clavipes) had the highest webs and as spider size decreases (Gasteracantha cancriformis then Leucauge sp.) so does web height (Figure 1 & 2). This evidence supports the hypothesis that orb-weaver spiders use the vertical stratification method of niche partitioning. The spiders likely build their webs at different heights to catch different types of bugs, affectively reducing niche overlap and promoting coexistence. However, the web height could also be a function of web area. Larger webs will naturally need to be higher off of the ground than smaller webs; therefore, we must also take into consideration the analysis of web area to determine the method of niche partitioning for these spiders.

The web area analysis shows that Nephila clavipes have significantly larger webs than both Gasteracantha cancriformis and Leucauge sp., but there is not a difference between the web areas of the latter two species (Figure 3). This supports Craig’s (1989) observation of two alternative foraging modes in orb weaver spiders. The larger Nephila clavipes builds a larger web whereas the other two species build smaller webs. This allows the spiders to divide the resources and not drive one another to extinction. Because we did not count the number of webs each spider built there is more research that must be done to determine if the spiders use these exact two methods, but this is a step in that direction.

Finally, there was no difference between strand densities of the spiders (Figure 4), and only Leucauge sp. had a different radii density (Figure 5). This does not support the hypothesis that orb-weaver spiders use strand density as a method of niche partitioning. If web thread density was a factor used to decrease niche overlap then we would expect to see each species with a different strand or radii density, but that is not the case. The spiders must use some other method of promoting niche divergence in order to coexist in the same environment.

These data suggest a combination of methods that spiders use to minimize niche overlap. While, to the untrained observer, it seems that the spiders all use the same resources and thus must drive one another to extinction, upon closer observation they are not actually complete competitors . A combination of web height and size is seen to be used by the orb-weavers to divide resources, but the strand and radii densities did not seem to play a role. This all suggests a complicated relationship between many different factors that allow multiple species to live in the same environment. Further research into alternative foraging modes in spiders from different places could shed light on the orb weavers. It would also be interesting to compare these data with data from spiders of the same species from different locations. The methods found to be used by the spiders studied in this experiment could have come about solely as a function of the local fauna. The same species compared from a different location could give a different order in the stratification of the webs, or a whole new method of niche partitioning altogether. Whether, like Cole (1960) said, the competitive exclusion principle is a trite maxim or not; it opens the door for some interesting research in the field of Ecology.

Citations

BARGHUSEN, L., CLAUSSEN, D., ANDERSON, M., & BAILER, A. (1997). The effects of temperature on the web-building behaviour of the common house spider, Achaearanea tepidariorum Functional Ecology, 11 (1), 4-10 DOI: 10.1046/j.1365-2435.1997.00040.x

Cole, L. (1960). Competitive Exclusion Science, 132 (3423), 348-349 DOI: 10.1126/science.132.3423.348

Craig CL. (1989). Alternative foraging modes of orb weaving spiders. Biotropica, 21 (3), 257-264

Enders, F. (1974). Vertical Stratification in Orb-Web Spiders (Araneidae, Araneae) and a Consideration of Other Methods of Coexistence Ecology, 55 (2) DOI: 10.2307/1935219

Hardin, G. (1960). The Competitive Exclusion Principle Science, 131 (3409), 1292-1297 DOI: 10.1126/science.131.3409.1292

Molles M. 2007. Ecology: Concepts and Applications. 4th ed. McGraw Hill. NY. 309-317.

(Pictures thanks to Wikipedia)

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