B. P. Perkins, LSU Biology Undergrad


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.


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).


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.


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)


The Attini tribe rely solely on the cultivation of Fungus Gardens for food. When an Attine Daughter Queen leaves her maternal home, she must carry within her mouth a Nucleus of Fungus to serve as the Starting Culture for her new Garden (Schultz and Brady 2008).


In a paper published in PNAS in 2008, Schultz and Brady provide detailed insights into the transition from simple agriculture to complex agriculture of the Attini tribe. The study suggests the Attini first developed agriculture approximately 50 million years ago in the forests of South America, coinciding with the early Eocene climatic optimum (50-55 mya). During this time there was a period of global warming and an extraordinary diversity of tropical plants occurring at middle and high latitudes in South America. The methods used in Attine agriculture have been divided into five distinct systems:

1. Lower Agriculture (practiced by the majority of the Attine)
2. Coral Fungus Agriculture (practiced by the “Pilosum Group”)
3. Yeast Agriculture (practiced by the “Rimosus Group”)
4. Generalized Higher Agriculture (practiced by the “Higher Attine”)
5. Leaf Cutting Agriculture (practiced by the Atta and Acromyrmex)

(Schultz and Brady 2008)

All five systems of agriculture utilize remarkably proficient planting, manuring, weeding, and sheltering techniques (Mueller and Rabeling 2008).


The original Attine agriculturalists collected withered plant bits and other debris on which to cultivate an unspecialized fungus that retained close genetic ties to free-living fungal populations (Mueller and Rabeling 2008). The “parasol mushrooms” grown using this method are, so far as is known, entirely capable of free-living existence without the help of the Attine growers. A paraphyletic grade of Escovopsis is known to infect the the paraphyletic fungal food sources used by the Lower Attine; but, like all Attine agriculturalists, they utilize an antibiotic produced by Actinomycete bacteria to control the parasite.


The “Pilosum Group” of the Attini tribe began to cultivate coral fungi (Pterulaceae) between 10 and 20 million years ago. Recent research indicates that Coral Fungus Agricultural products are infected by a specialized grade of Escovopsis that is derived from an Escovopsis species that infects Lower Agricultural products. This species subsequently gave rise to a clade that switched hosts and began infecting the Higher Attine food sources (Schultz and Brady 2008).


Unlike typical Attine Mycelial Gardens, Yeast Gardens consist of small, irregularly shaped nodules of fungus growing in the yeast phase. Yeast Agriculture is confined to the “Rimosus Group” and originated sometime between 5 and 25 million years ago. The yeast grown are capable of a free-living, feral existence; however, they grow in the mycelial phase rather than the yeast phase. Indeed, these fungi are only known to grow in the yeast phase when attended by the Attine growers (or depending on conditions in artificial culture). The parasite Escovopsis is unknown to Yeast Agriculture (Schultz and Brady 2008).


The transition to higher agriculture and the subsequent origin of leaf cutting are arguably the two most ecologically significant developments in the history of the Attini tribe. The fungi grown by the Higher Attine suggest a significant degree of “domestication”, or modification for life with the Attine. These fungi do not appear capable of free-living existence separable from their growers. And only the fungi grown by the Higher Attine produce “gongylidia”, nutritious swollen hyphal tips that are harvested by the Higher Attine for food(Schultz and Brady 2008).


The development of Leaf Cutting Agriculture (rather than the debris collecting that is used in all the other systems) coincided with marked ecological transitions in South America (5-15 mya). The coincidence of grassland expansion with the development of Leaf Cutting Agriculture supports the hypothesis that early Leaf Cutters may have been Grass Cutting specialists with specializations in Broadleaf Cutting developing later. The most wide ranging Leaf Cutting Agriculturalists originated and expanded within the last 1 to 2 million years. Such a rapid acceleration in diversification and expansion of the Attini tribe underscores the belief that Leaf Cutting Agriculture represents one of the key innovations in Attine history (Mueller and Rabeling 2008).

Mueller, U., & Rabeling, C. (2008). A breakthrough innovation in animal evolution Proceedings of the National Academy of Sciences, 105 (14), 5287-5288 DOI: 10.1073/pnas.0801464105

Schultz, T., & Brady, S. (2008). Major evolutionary transitions in ant agriculture Proceedings of the National Academy of Sciences, 105 (14), 5435-5440 DOI: 10.1073/pnas.0711024105
-Bryan Perkins