Spiders And Pollution: A Web Of Toxicity

Spiders are an important part of the ecosystem, preying on insects like flies and mosquitoes, and serving as prey for birds and lizards. However, human activities such as habitat fragmentation, fires, sheep grazing, conventional crops, and the use of insecticides have negatively impacted spider populations. A recent study has also found that light pollution, one of the fastest-growing types of pollution, is shrinking the brains of city-dwelling spiders, which could affect their ability to hunt and navigate.

Light pollution can reduce spiders' brain size

Light pollution is having a detrimental impact on spiders, with research finding that it is shrinking their brains. This is particularly evident in the Australian garden orb-weaving spider, a nocturnal species that is commonly found in urban environments, including under streetlights.

The Impact of Light Pollution on Spider Brains

A study conducted by researchers from the University of Melbourne, including Professor Therésa Jones and PhD researcher Nikolas Willmott, examined the effects of light pollution on the development of Australian garden orb-weaving spiders. The researchers found that exposure to artificial light at night led to a reduction in the overall brain volume of these spiders, with the most significant effects observed in the areas of the brain linked to vision. This research adds to the growing body of evidence that light pollution, one of the fastest-growing forms of pollution, has far-reaching consequences for a range of species, including birds and mammals, as well as invertebrates.

Potential Causes and Consequences

The reasons behind the decrease in brain size are not yet fully understood. One possible explanation is that the presence of light at night creates a stressful environment, disrupting the hormonal processes related to growth and development. Alternatively, it may be that spiders alter their "investment" in different parts of the brain when forced to develop under artificial light. This could be a result of limited resources being directed to the more critical parts of the brain necessary for survival.

The Importance of Spiders

Spiders play a crucial role in ecosystems, preying on other invertebrates such as flies and mosquitoes, while also serving as a food source for birds and lizards. Therefore, any changes to their cognitive function and behaviour could have a ripple effect on the ecosystem. The reduction in brain size may impact their ability to navigate, hunt, and perform other vital tasks.

Addressing Light Pollution

The findings of this research highlight the need to address light pollution, especially in urban areas. Some local councils in Australia are taking steps in the right direction by implementing measures such as using warm low-level lighting, reducing lighting hours, and designating dark areas and parks. Individuals can also play a part by minimising outdoor lighting and drawing curtains to prevent indoor lights from shining outdoors.

Light pollution can affect spiders' hunting and climbing abilities

The study, conducted by Therésa Jones and Nikolas Willmott, revealed that the spiders exposed to artificial light had smaller brains overall, with the most significant reduction observed in the visual region. This could potentially impede the spiders' ability to hunt and climb, as these functions are closely linked to their sense of vision.

Spiders rely on their vision to navigate their complex environments and forage for food. By disrupting their brain development, light pollution may hinder their ability to perform these essential tasks. As spiders play a crucial role in ecosystems by preying on pests and serving as prey for other predators, any impairment to their cognitive function could have far-reaching consequences.

Furthermore, light pollution interferes with the production of melatonin, a vital chemical that regulates day-night rhythms and affects reproduction, survival, and immune function. The disruption of these natural processes can have adverse effects on the health and behaviour of spiders.

The study by Jones and Willmott highlights the potential consequences of light pollution on spider behaviour and ecology, indicating that further research is needed to fully understand the impact of artificial light on these arachnids.

Insecticides negatively impact spider diversity

Insecticides have a negative impact on spider diversity. Research has shown that the use of pesticides (insecticides, herbicides, and fungicides) decreases spider biodiversity, affecting their ambulatory speed and causing mortality. The population of spiders decreases after the application of pesticides, leading to a combination of direct and indirect lethal effects, such as changes in physiological and constructive behavior, migration, and sublethal consequences like reduced reproductive capacities and impaired food quality and supply.

The effects of insecticides on spiders are observable in the structures of spider silk. While sublethal effects have received less attention than lethal impacts, they are ecologically significant as they can reduce the performance of populations and create risks for the conservation of spider biodiversity.

In agricultural and pasture ecosystems, insecticides negatively impact spider diversity. Organic farming, which involves more controlled use of insecticides, is more beneficial to arachnid abundance than conventional agriculture. However, the effects of farming practices on spider populations depend on the complexity of the landscape.

To conserve spiders, it is recommended to reduce mechanical alterations to the land, such as harvesting, ploughing, and grazing, and to control the use of insecticides, similar to the practices employed in organic farming. Avoiding habitat fragmentation is also crucial for maintaining spider diversity.

Habitat fragmentation affects spider abundance

Habitat fragmentation is a process by which large and contiguous habitats are divided into smaller, isolated patches of habitats. It is considered an invasive threat to biodiversity, affecting a large number of species. Habitat fragmentation can be caused by endogenous and exogenous processes. Endogenous processes are those that develop as a part of species biology, such as changes in biology, behaviour, and interactions within or between species. Exogenous processes are independent of species biology and can include habitat degradation, habitat subdivision, or habitat isolation.

Habitat fragmentation can reduce the amount of suitable habitat available for organisms, decrease interior habitat, and isolate habitats from one another. These factors can negatively impact gene flow and migration, reducing genetic diversity and increasing inbreeding. This can lead to inbreeding depression, which is associated with conservation risks such as local extinction.

Habitat fragmentation can also alter the resources available and the structure of habitats, affecting the behaviours of species and the dynamics between different species. For example, it can alter predator-prey relationships, forcing species to adapt new behaviours. It can also affect communication behaviours, as seen in Dupont's Lark, where isolation of territories due to fragmentation led to a decrease in territorial calls.

Furthermore, habitat fragmentation can have significant effects on the microbiota of an environment, reducing populations and diversity of fungi responsible for decomposition and altering microenvironments, which are often not conducive to overall forest health.

Overall, habitat fragmentation has far-reaching consequences for species abundance and biodiversity as a whole.

Fire, sheep-grazing, and conventional crops harm arachnid fauna

Fire, sheep-grazing, and conventional crops all have the potential to harm arachnid fauna. While fire and sheep-grazing can destroy spider burrows, conventional farming practices such as tilling, crop residue removal, and monoculture can harm or drastically reduce spider populations.

Fire

Using fire for pest control or to clear spider webs can have disastrous consequences, as seen in several incidents where people accidentally set their houses or buildings on fire. While flaming for pest control is sometimes used in agriculture, it is not sustainable due to the release of greenhouse gases.

Sheep-Grazing

Sheep-grazing can influence ecosystems positively or negatively, depending on the grazing regime and intensity. A study by Clayton and Bull (2017) found that sheep-grazing can destroy shallow spider burrows through trampling and the production of burrow-filling debris. However, moderate levels of grazing may have a low impact on deeper burrows, as seen in the case of the pygmy bluetongue lizard, which relies on co-existing spiders for refuge.

Conventional Crops

Conventional farming practices, such as tilling, crop residue removal, and monoculture, can harm or drastically reduce spider populations. Tilling, for example, increases topsoil erosion, accelerates soil carbon decomposition, reduces soil water infiltration, and disrupts microorganisms in the soil, including spiders. Crop residue removal eliminates the food source that attracts spiders' prey, while bare ground exposes their nesting sites and makes them more vulnerable to predators.

Conservation agriculture, on the other hand, has been shown to support spider populations and diversity, which can help mitigate pest damage and potentially increase yields for farmers.

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