Niche Differentiation in Two Sympatric Orb Web Spider Species (Araneus diadematus, Metellina segmentata)
Every animal is required to have its own niche for its very survival in the environment, this is why niche development and security is critical to an animal’s survival and also in diversification of animals due to natural selection (Rundle & Nosil 2005). If two animals operate in the same niche for the whole of their life stages they will be in competition therefore the less adapted species will inevitable go extinct (Berendse 1983). As we can see in the natural world competition is all around us, as linear completion is seen is so many animal and plant species (May & Mac Arthur 1972). Yet many of these competing species have not gone extinct. Co-existence of species is a regularly brought up topic in the field of ecology (Michalko & Pekár 2015). Mechanisms such as Niche filtering and niche partitioning have been suggested to help maintain this co-existence (Hubell 2001; Mayfield & Levine 2010). Co-existence is much more common that one would think with it effecting a great number of animal and plant species. Therefore co-existence is suggested to be a much more significant mechanism in community ecology than previously thought (Silvertown 2004). In plants this can be seem most clearly, as plants are in a constant state of competition be all plants are fighting over the same resource, sunlight (Aarssen & Epp 1990; Goldberg & Barton 1992). They avoid competition and promote co-existence via segregation of niche axes (Slivertown 2004) and have been able to show that in natural community’s the differences between each niche maybe more subtle that once realised (Adler et al 2007). This type of co-existence is also found in many animal species in many difference taxa. A study shown in Glossiphonia complanataandHelobdella stagnalis found that both these species, they were co-existing by temporal differences is feeding causing the adjustments of food partitioning between both species (Wrona et al 1981). Therefore this shows on way of diving resources is feeding at different temperatures. Therefor the same niche is being utilised but in a much more sustainable way providing both species with suitable resources to survive. Therefore in both plants and animals the main mechanism behind how species can survive together is Niche differentiation and division of resources. There are some other theories that explain coexistence just as neutral theory. This is the theory that challenges the niche paragram by suggesting that species similarities can explain the high diversity in natural communities (Bell 2000). These models are based on the assumption that no one species is better than the other in fitness and effects to the other species (Adler et al 2007). This is an interesting theory can could explain some variation in many systems but in this study we are focusing on the differences between two species and how they have differentiated themselves to exploit the same niche while avoiding competition.
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Niche differentiation Therefore n this study we are testing how niche differentiation can occur weather that be by partitioning or any other type of niche division. This can occur in several ways: partitioning of food resources, morphology, behaviour etc. this then leads to a difference in the uptake of resources that a niche provides and can help both species co-exist (Devito et al 2004, Richman et al 1995). This niche differentiation has been noted as one of the most important divers in evolution (MacArthur 1972; Christian & Fenchel 1977) as stated before this is a key driver in both plants and animal species. But how does this process occur and allow for co-existence? In terms of generalists predators and herbivores these species are tend to have niche overlap. Using these terms three possible outcomes can occur coexistence, competitive dominance and competitive bistability. In the later only one species will survive and become the stronger competitor, but in co-existence two species can survive (Schreiber 1996). Thus the need for a differentiation of niches, either by food partitioning (Wrona et al 1981), in their phenology (Gunster 1994) or spatial partitioning (Albrecht & Gotelli 200). Many ecological studies have looking into this and have tried to explain how species of the same functional group can co-exist and how they are effected by niche differentiation. Again this is majorly dominated by plants due to many species competing over the same resource. Another major area that has been looked into in this area is that in the micro-environments set up in cow pats. Cow pats are excellent habitats for small invertebrates to reproduce and feed (Geiger et al 2010). In dung beetles they show that the occur along a seasonal axis and showing that seasonal segregation (in this case) could show that it is a co-evolution in competing species to allow resources partitioning (Holter 1982).
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The study In the UK there are two closely related spider species that seem to be co-existing; the common Garden spider (Araneus diadematus) and the lesser Garden spider (Metellina segmenta). Both these species construct orb webs whose main purpose is to optimise capture rates per construction effort (Zschokke & Vollrath 2000). Both A.diadematus and M.segmenta both feed on similar prey therefore also in theory competing for the same resource (Savolainen & Vepsäläinen 1989, Schneider & Vollrath 1998). Therefore on a face value level would show that both these species are in severe competition. But what are spiders competing for? As explained by schooner (1974) Differences in food types normally explains how resources deviation normally leads to niche differentiation, But as explained beforehand this does not occur in these two orb web spiders. So this leads to the question, how do these closely related sympatric spider species avoid competition? Harston et al. 1960 concluded that most carnivores at the same tropic level is food limited. And therefore spiders being terrestrial carnivores should follow this trend of being food limited (Wise 1993). This can be somewhat explained by carnivores perhaps being explained by the joules of prey available (Hairston et al 1960). This can be shown by spider diets. These do not accurately represent the quantities of insects that ecosystem (Kajak 1965). This could be down to a number of factors. (1) The avoidance of noval prey (Riechert & luckzak 1982). (2) The refusal of harmful or deadly prey items (such as predatory wasps) (Netwig 1983). This then may in fact heighten the degree of resource limitation due to rejection of certain prey items. An early competitions view of spider community structures come from Tretzel (1955). He described that interspecific competition was responsible for the differences in congeneric species community structures. In later studies this point and uncovered patterns in niche parameters and distribution (Wise 1993). So as we can see there is much competition in spider communities, and most of this competition is based around food limiting of the environment. But there must be ways that spiders can differentiate and partition their niches, or many species would be extinct. The key question here is that is there competition for resources? Or can the species or is there mechanisms involved so each species can differentiate the niche? To understand why both of these can be successful and still thrive with niche overlap, we need to understand how they are exploiting each resource and how each web is designed to help with either competition or how each web is subtly changed to that direct competition is avoided. As we can see there I much evidence provided that a relative lack of prey can affect the growth, reproduction and the web construction of many spider species (Wise 1993). There has been many ecological studies into spider resource division and niche differentiation. One such experiment was undertaken by Harton & wise (1983) and in this experiment it was found that some negative intrasepsificy density effects do occur such as: an increase of the desist of the conspecifics actually increases the mean height of the web in one study species A.aurantia (74%) but this result was only significant for the survey immediately following density manipulations and was not significant in any other. It was also found that survival rate was significant in A.triasciata when calculated against the last survey undertaken. Yet with the absence of interspecific competition, evidence would point towards niche differentiation as seen in the effects of web placement, growth and survival. But the studies evidence was not strong enough to support this theory. Another study into web structure found that spider species can adapt its niche by adapting its web structure. This is possible in a number of ways; by changing/increasing the web size, web shape, radial spacing etc. This can form over many attempts in a “fine tuning stage” as described by Vollrath & Samu (1997). Another factor that could help relieve competition in spiders, is abiotic factors, these can be for example moisture, Exposure to wind, and temperature. All of these have been found to influence habitat selection in spider species (Ebehard 1971; Cherett 1964; Norgaard 1951). Exposure to sunlight has also been shown to effect the orientation of the web (Biere & Uetz 1981). These factors can cause behavioural responses in the spiders which can drive spiders to much greater densities than normal and could increase the chances of competition. This theory suggests that the abiotic factors do not allow for spiders to reach high enough densities that competition for food becomes a limiting factor. In this study we will be looking at how 3 factors can show niche differentiation in Araneus diadematus, Metellina segmentata. These factors include Habitat variation i.e do these spiders occur in different abundancies in different habitats and show niche differentiation in that sense. This will be taken in a range of habitats showing variation in human management. To see if this has an outside effect on spider abundance. We will also be looking at the phenology of the spiders. This could shed light on this, the theory is that if one spider emerges earlier in the season it avoid competition with the other due to its feeding on the same resource earlier, then as the second emergence the spider feeds on a different food source (bigger invertebrates, different taxa). These species were chosen due to their similarity (same family, same capture method of prey). Hopefully this study can expand on the theory of niche differentiation and find strong evidence to support this theory.
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14 sites were surveyed around the south wales areas of Abergavenny and Usk from the 31st July to the 14th of September. Each site was categorised into 6 categories based on Land use, vegetation make up and the surrounding areas, this was to give a varied sample group and could then be used to show differences between habitats in the tests later. The site categories were as follows: Garden, park, managed woodland, unmanaged woodland, field and field with forest. Replicates of these sites were made three replicate sites of Garden habitats were surveyed, three replicate sites of field were surveyed, two replicate sites were surveyed for park, two replicate sites of managed woodland were surveyed, two replicated of unmanaged woodland were surveyed and two replicate sites were surveyed for field with forest, this was done to increase the significance of my results and also the viability of the results. Each site was given transects, this typically occurred around the edges of each site due to the higher abundance of suitable vegetation present there. For the woodland sites whoever transect were made of these sites following pre made paths, due to the ease of access and to minimise disturbance to these areas. Each transect was measures using Satellite GPS on Google earth, this also gave the altitude for each site. Both of these were measures to see if these variables had any effect or could distort any results given. The date of each survey was also noted to see if season and time had an effect on the results.
In the field every spider spotted of the relevant speciate was measured along the transect, this measuring in the field as opposed to capture and measure was to avoid confusing when gaining the spider and the web which correspond to that spiders data. The measurements of Body size, Abdomen size and radial distances were taken with a digital calliper with and accuracy of 0.01mm. The body length was taken from the tip of the carapace of the spider to the tip of the abdomen. The abdomen length was taken from the end of the pedicel to the tip of the abdomen. A ratio was then taken from this Abdomen length over spider length; this was given to indicate the heath of the spider. With an increase in number signified and increase in health of the spider i.e. the lager the abdomen compared to body size the healthier the spider. The distances of the radial were taken from the top of one radial to the bottom of the other. These were taken along a single transect of the web as best as possible. In some instances the web was damaged, the transect was moved to another part of the web on the same radial line and continued to complete the transect. Measurement from this were all added and dived by the number to give a mean radial distances across the web. The web height and web width were also measured using a tape measure with has an accuracy of 0.1cm. The web width was measured by taking the distance between the web frames of opposite sides of the web. The web height was measured from the ground up to the centre of the hub of the web, were the spider is situated. All the data was taken down via a Dictaphone in the field. Then brought back and input into Microsoft excel spread sheet ready to be statistically analysed on Minitab 17 statistical software.
The initial testing was compiled on Minitab 17 statistical software. A linear regression was used to find the underlying relationships within just the spider species. This was to find how the spider uninfluenced had a relationship with its web. This was achieved by plotting each of our spider measured data points against its own web data points i.e. Spider length x Web height, abdomen length x radial distances etc. Habitat and species were then plotted against the continuous variables to see if there was partitioning between species and habitat. This was done using an ANOVA (habitat) and 2-sample t (species) plotting the results on bar charts to see comparisons. Taking into account the R2, F and P values to see how strong the relationship were. Tests were then carried out to test the Phenology of the Spider to see if this factor could contribute to the partitioning of the niche. Each habitat was giving a day number using the Julian’s day count; these days were then plotted against species size to see if there was any difference in emergence and spices growth. This would then find if this was the main driver of niche differentiation in these species of spiders if the other showed signs of competition.
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