Irvine Ranch Conservancy

The Elusive Trapdoor Spider
Outlives Other Species…Yet Now Struggles to Survive

The California trapdoor spider (Bothriocyrtum californicum) belongs to one of the oldest and most primitive spider groups that includes tarantulas and dates back 200 million years. Among invertebrates it is a high-ranking and feared predator. Living in colonies, trapdoor spiders are found throughout Southern California, including on the Irvine Ranch Natural Landmarks. In fact, the region is considered a global “hotspot” for such spider species. However, the ability of trapdoor spiders to move around the landscape is very limited – it must walk everywhere it goes. As a result, their populations have been in decline, signaling trouble for the survival of the species.

An Insect Ambush
A big part of what makes trapdoor spiders so fascinating is their unique hunting strategy. The spider digs a burrow into clay soil in open grassland and scrub habitats. At the top of the burrow is a D-shaped door lined with web material and even hinged with webbing from the inside. The door, which has a handle that the spider can grab onto with its fangs, is closed most of the time, except when the spider is hunting. When an unsuspecting insect wanders by, the spider pounces out of its door, grabs the prey, and pulls it into its burrow where it is devoured.

Threats to Survival
Trapdoor spiders can live a very long time – up to 20 years. Unfortunately, these fascinating creatures face a variety of dangers. Fire is a major threat, because the spiders’ burrows are shallow and can become a death trap if the fire burns very hot. Land disturbance is another threat in some areas. A major portion of the habitats that were once home to large colonies of trapdoor spiders in Southern California have been converted to agricultural fields and urban centers. And invasive weeds can also cause problems by forming a thick layer of thatch - dead plant debris – that covers burrows, increases fire risk, and blocks the spiders wandering to find a mate.

Natural predators of the trapdoor spider include rodents and parasitic insects. Parasitic insects lay their eggs on a living host after paralyzing it. When the eggs hatch, the larva grows by slowly feeding on its victim. Spider wasps and small-headed flies hunt for trapdoor spiders as their primary source of food for their larvae.

Learning More
IRC’s Science and Stewardship team and volunteers have conducted surveys of trapdoor spiders to evaluate their status and needs on the Irvine Ranch Natural Landmarks. Don Millar has been the lead Citizen Science volunteer, gathering information on:

Spider colonies before and after the fire of 2007
Colony size change since 2007
Death rates of spiders in relation to their size and age
Habitat characteristics of spider colonies.

The information has been very important in helping decide how to manage this important species. “I am convinced that trapdoor spider numbers have declined in Southern California,” says Dr. Jutta Burger, IRC Senior Field Ecologist. “Loss of native grassland and coastal scrub has probably contributed and fires and habitat conversion by weeds have further impacted this unique and important species. Our work shows us that many spiders did not survive the Santiago fire in 2007 and that young spiders in small, shallow burrows were especially hard hit.”

Conservation Activities
Having determined that the trapdoor spider population on the Irvine Ranch is likely in decline, IRC is now evaluating ways to prevent the species’ loss from the area. “No doubt the best approach to conserving trapdoor spiders – and many other species for that matter - is by reducing fire frequency. Also, identifying spider colonies and protecting them from disturbance, and reducing the invasive weed cover in areas where they occur is likely important,” says Dr. Burger. “We know too little about these creatures to be able to breed them in captivity at this point.”

Though many people may dislike spiders, trapdoor spiders are harmless to humans and are an integral and unique part of the ecology of the Irvine Ranch Natural Landmarks. As one of the top predators in the arthropod world, they help control numbers of other insects, creating a balance in species populations and in competition for food. The California Trapdoor spider is a remarkable species, and IRC is doing everything it can to ensure its survival.

To learn more about Resource Management and Stewardship projects on the Irvine Ranch Natural Landmarks, visit http://www.irconservancy.org/stewardship/science.aspx. To get involved or inquire about volunteer opportunities, visit http://www.irvineranchwildlands.org/involved/opMain.asp. To find out more about the trapdoor spider visit, http://creationwiki.org/Trapdoor_spider.