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29/08/2016, 17:00

Social behavior in spiders is a way to destroy a whole colony of them

The method by which individuals in a population compete for limited resources such as food, mating opportunities or nesting space is known to have a powerful selective influence on individuals.

Less well studied is how intraspecific competition can affect the stability and long-term persistence of a population, as well as potential links between properties of the resources, individual behaviour and population dynamics. In general, it has been accepted that there are two extreme forms of intraspecific competition – contest and scramble.

These were originally proposed by Nicholson who defined contest competition as occurring when each successful individual gets sufficient access to a limited resource to survive and reproduce, whereas unsuccessful individuals fail to do so.

Scramble competition, in contrast, occurs when resources are shared more or less evenly among all individuals in a population or allocated to those in most immediate need. In the latter case, when resources are limited, no individual may get sufficient resources to reproduce or even survive.

Scramble competition has been demonstrated in species such as the southern pine beetle when attacking trees or in frugivorous primates.

Contest competition, on the other hand, can be seen in species that defend a territory or in those with dominance hierarchies, such as in social primates where individuals at the top of the hierarchy get preferential access to resources. The majority of species are likely to lie somewhere between the extremes of these two forms of competition, with a variety of factors, both biotic and abiotic, potentially affecting how resources are allocated.

Such factors, which may change with season, population size, or geographic location, include the density of conspecific competitors and the prevailing size and spatial distribution of resource patches or prey.

One of the main factors thought to determine intraspecific competition type is the extent to which resources can be monopolized. When resources are clumped either in space or time, single individuals can monopolize them so that contest competition prevails. Dispersed resources, on the other hand, are not defendable, causing scramble competition to predominate.

Vahl, for example, showed that in a wading bird spatially clumped food increased the difference in food intake between dominant and subordinate individuals. Likewise, Weir & Grant showed that in cichlids food that arrives asynchronously is more likely to be monopolized when compared to synchronously arriving food.

Resource size may also determine the extent to which monopolization by individuals is possible. If prey are small, individuals can exclude conspecific competitors, so that contest competition prevails. In contrast, scramble competition is more likely when resources are too large for single individuals to capture and defend solitarily.

In the latter case, group-hunting strategies may evolve, but this does not preclude intraspecific competition and uneven distribution of captured resources.

In any population that shares resources, the method of resource allocation may, in turn, have profound consequences for population stability, especially when resources are limited. Under scramble competition, no individual may get enough of the limited resource to reproduce and the population is thus at greater risk of extinction.

In contrast, under contest competition, the most competitive individuals can get sufficient resources to reproduce, which may result in the population shrinking, but persisting. In an early empirical demonstration of the effect that scramble competition may have on the health of individuals, Bakker fed low food levels to groups of Drosophila melanogaster larvae.

This resulted in the majority of flies pupating but emerging half starved, rather than a few individuals emerging fully fed. In a more recent study, Camphuysen showed that an observed mass mortality of eiders in the Dutch Wadden Sea was due to a reduction in available food resources.

The fact that all individuals measured had reduced body condition suggested that scramble competition predominated and contributed to the population's collapse. In bark beetles, it has been shown that an increase in tree attack density above an optimum level results in a dramatic decline in the number of mature beetles emerging. A deeper understanding of the conditions that lead to the preponderance of one form of competition over the other could therefore be vital in predicting a population's stability and potential persistence in a given environment.

Here, we study the mode of competition in a social spider whose colonies are known to have boom and bust dynamics  and high rates of extinction, which is consistent with scramble competition predominating in this system. In particular, using a simple mathematical model, we show that large prey is not economically defensible and thus more likely to be shared. We then test this prediction experimentally by feeding small and large prey to artificial colonies of the neotropical social spider Anelosimus eximius. Social spiders typically feed on prey items that can range in size from smaller than an individual spider to many times larger. Therefore, prey size is an important factor to consider, particularly as the size of the prey captured has been shown to increase with colony size. Consequently, social spiders, and A. eximius especially, provide an ideal opportunity to investigate the potential triple link between properties of the resources, individual behaviour and population dynamics. There have been a few studies investigating how different species’ behavioural response to differing food patch characteristics can influence the severity of those species’ population declines when resources are scarce, suggesting this is an important factor to consider when investigating a species’ response to resource scarcity and habitat change. However, there have been surprisingly few studies that have considered this triple link despite the several examples of scramble and contest competition in the literature and the underlying conditions that may be responsible for them.

Social spiders are unusual among social organisms in that their colonies represent not only social groups, but also self-sustaining populations. Colony members, typically multiple females and their offspring, remain together throughout their lives and mate with each other to produce successive generations. Through this process of intracolony mating, colonies may grow to contain hundreds to tens of thousands of individuals, depending on the species. Once a colony has reached a large size, dispersal to produce daughter colonies may take place. Colonies, however, may also suffer relatively high rates of extinction, which may involve the sudden crash of large and apparently healthy colonies, often with no individuals dispersing before the colony dies. In our study species, for example, it has been estimated that 21% of well-established colonies in the forest understorey go extinct per generation. Hart & Avilés reconstructed the parameters governing the growth of A. eximiuscolonies in the lowland tropical rain forest and obtained results consistent with their dynamics being intrinsically unstable. Boom and bust dynamics may arise when rates of growth are high, generations discrete, and scramble the predominant form of intraspecific competition A. eximius colonies appear to fulfil these conditions as parents usually die before their offspring reach maturity and large rates of growth may result from the spiders’ cooperative behaviours and highly female-biased sex ratios.

Moreover, Grinsted & Bilde found that increasing competition within artificial colonies of the social spider Stegodyphus dumicola did not increase size asymmetry among the spiders, further suggesting a predominant role of scramble competition in these systems.

We use a simple mathematical model to show that prey above a certain size may not be economically defensible as prey surface area, which needs to be defended, increases at a faster rate than prey volume, which is proportional to the energy a prey item provides. Our model thus suggests that larger prey items would have a higher probability of being evenly shared among colony members. Therefore, we predicted that scramble competition would be more pronounced when prey were large, as individual spiders would not be able to prevent others from joining at either the prey capture or feeding stage. We test this prediction using artificial colonies of A. eximius. We provided prey to these colonies that were either small (approximatively the same size as a single spider) or large (two to three times larger). We also investigated the effect of an individual's body condition and participation in prey capture on the likelihood that it fed on captured prey. In spiders, growth rate and body size are highly correlated with mating success and fecundity, making foraging success an important factor determining an individual's fitness. Consequently, we expected that individuals in poorer condition (i.e. hungrier individuals) would have greater access to larger prey. A finding that prey sharing and scramble competition are more pronounced when prey are large would help explain why large colonies of this social spider, which capture larger prey, tend to be subject to boom and bust dynamics and high rates of colony extinction.

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