Sudden Oak Death’s Grip on California Forests

Oak trees have been in danger in California since the mid-1990’s, and researchers are now predicting a grim future for California’s emblematic tree. The killer, termed “Sudden Oak Death”, has killed an estimated 3 million trees since its introduction to California.

Quick Facts

  • Sudden Oak Death is caused by a fungus-like pathogen. It is fatal to many California-native oak species.
  • Since its introduction to California, the pathogen has killed an estimated 3 million trees.
  • Scientists have concluded that the pathogen’s spread in California is too great to completely eradicate it. Rather, management of the disease in localized areas is suggested to be our best option.


A Fatal Pathogen and Its Spread

Something weird began happening to California’s oak trees in the mid-1990’s.

Trees were plagued with a sudden onset of dying leaves and trunks that “bled” dark-colored sap, and locals concerned about the sudden tree deaths reported their observations to specialists.

The pathogen causing the symptoms was identified in 2001 as Phytophthora ramorum, a fungus-like species that infects plants. P. ramorum arrived to California by infested rhodendron plants. The fungus spread across the United States through shipments of nursery plants before it could be detected. It is spread through spores, which can be carried by rain, wind, and infected soil.

The symptoms exhibited by infected plants vary by plant species. Some plants can be infected with the pathogen and not have symptoms serious enough to be fatal, allowing the plants to continue living and spread spores from the pathogen as “carriers”. The California bay laurel is a well-known carrier, since its symptoms are limited to spots on some parts of the leaves and diebacks of twigs.

P. ramorum is known to cause death in a number of oak species, including coast live oak, Shreve oak, and California black oak. P. ramorum is also known to infect a relative of oaks, the tanoak. The main symptom of infected trees is a canker on the trunk, with blackish to reddish sap that “bleeds” from the sore. Infected trees take an estimated 2 years to die from the pathogen itself; however, other pathogens and insects readily infect the already-sick trees and speed up the tree’s death.

A Disease With Far Reaches in California

Since its introduction to California in the mid-1990’s, Sudden Oak Death has killed an estimated 3 million trees along the coast from Big Sur to Oregon. The effects of this disease also go beyond just the deaths of trees – trees that have died from Sudden Oak Death are dangerous fuels for California wildfires, a threat that has become more prominent as we face the effects of climate change.

Management practices have focused on the identification, quarantine, and removal of infected plants as a way to stop its spread. Public education initiatives have also focused on educating California residents on how to avoid spreading the disease, through practices such as sterilizing pruning tools and washing off equipment, tires, and shoes that have come in contact with infected soil.

Predicted spread of the sudden oak death epidemic through California, 1990–2030
An image from Meentemeyer et al. 2011 showing the predicted spread of Sudden Oak Death from 1990 – 2030, based on the assumption of similar weather conditions to those in California from 1990 – 2008.

Despite California’s efforts, a recent study (Meentemeyer et al. 2011) conducted by scientists from Cambridge, the University of California at Davis, and the University of North Carolina has concluded that completely eradicating the disease from California is not possible. The sheer number of trees infected makes it impossible to rid the state of the disease or stop its spread.

Instead, the study suggests that management should focus on the restoration and treatment of small, local forests as the most practical and cost-effective option. One such study focusing on removing dead and diseased trees is currently being conducted around Bolinas Ridge, giving us hope for the development of best management practices moving forward.


Migrations: The Monarch Butterfly

Cover photo: © 2017, Mike Bessler, some rights reserved.

Did you know that there is a species of butterfly that migrates south in the winter? Meet the monarch, one of California’s most interesting butterflies.

Cover photo: © 2017, Mike BesslerPhoto Library (Flickr), some rights reserved.

Places to See Monarch Butterflies

Monarch butterflies make an annual migration to warmer temperatures, with some populations traveling as far as 3,000 miles.

Millions of monarch butterflies undergo an annual migration, just as birds migrate south in the winter to escape cold temperatures. Cooling temperatures, shorter days, and the aging of milkweed (the primary source of food for monarch caterpillars) signal to the butterflies that it’s time to begin the journey south. Upon reaching their destinations, the monarchs congregate together and form large clusters on the branches of trees. This allows the insects to retain enough warmth to survive the winter.

You can see this migration in the Bay Area! From mid-October to mid-February, the monarch butterflies overwinter at Lighthouse Field State Beach and Natural Bridges State Beach.

For more information on their migration, check out this Bay Area Naturalist article.

The western population of monarch butterflies has been facing declines over the past 20 years.

In 2018, citizen scientists recorded the lowest numbers of monarch butterflies seen in California for 5 years. According to one source, 1.2 million butterflies were recorded on a California survey two decades ago. In 2018, a survey using almost the same number of locations recorded just 300,000 butterflies.

A recent study conducted by a Cornell University scientist posits that loss of overwintering habitat, the loss of habitat for butterflies to stop during their migration, and the declines of sites with nectar-producing flowers for adults during their migration are the main causes of the monarch butterfly declines.

You can help participate in the counts that help scientists know the status of monarch butterfly populations! For more information, check out the Western Monarch Count’s page on volunteering.

Monarch butterflies are vibrant orange.

Monarch butterflies have unmistakable orange wings, with black veins creating beautiful windows in the color. The caterpillars are just as vibrant, with black, yellow, and white stripes across their bodies.

The viceroy butterfly is a species that mimics, or copies, the physical appearance of monarch butterflies. Can you tell the difference in the photo below?

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Monarch versus viceroy butterfly; Source:

Monarch caterpillars only eat milkweed.

Milkweed plants are rich in toxic chemicals called cardenolides. Monarch caterpillars, which feed exclusively on milkweed, are able to store the chemicals in their bodies through adulthood, making adult monarch butterflies bitter-tasting and potentially toxic to predators.

Monarch butterflies don’t specialize on one type of plant like the caterpillars do. They will consume nectar from a variety of flowers.

Monarch butterfly males and females look different from one another.

Male monarch butterflies have a small black spot in the middle of their hindwings (their bottom pair of wings). These “spots” are actually areas that emit chemicals for the courtship displays of some butterflies; however, the spots are not known to be important in the courtship displays of monarchs.

Male and Female Monarch Butterfly
Male versus female monarch butterfly; Source:

Cool facts about monarch butterflies:

  • Only one generation of monarch butterfly migrates out of the 4-5 generations of butterflies that live each year. The generation that migrates is termed the “migratory generation”.
  • Monarch butterflies weigh between 0.27 and 0.75 grams. For some perspective, a U.S. quarter weighs 5.67 grams.

Resources to learn more:

Towering Trees: The Coast Redwood

Coast redwoods are the tallest trees on Earth – and they also only live in the Bay Area.

Places to See Coast Redwoods

Coast redwoods are the tallest trees on Earth.

Coast redwoods grow to heights of over 379 feet – as tall as a 38-floor skyscraper. They can also grow to widths of 26 feet in diameter.

Coast redwoods get their name from the rich color of their bark.

True to their name, these giants have beautiful reddish-colored bark. The rich color is due to high contents of tannins, a chemical which helps them repel damage from insects. Their bark is also especially thick to help protect them from forest fires, which seasonally occur in California.

Coast redwoods live for thousands of years.

Coast redwoods are known to live over 2,000 years. They are an ancient species that dates back to the Jurassic Period over 200 million years ago.

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Hiker Mary Gibbs admires coast redwoods at Big Basin State Park.

Coast redwoods only grow in one place on Earth: the coast of Northern California to Southern Oregon.

Coast redwoods can only be found on the coast of Northern California up into Oregon. In their range, they do not extend more than 50 miles inland. The heavy rains in the winter provide them with plenty of water. In the dryer months, the coastal fog provides much-needed moisture to the redwoods.

Millions of years ago, there used to be many species of redwood tree in the Northern Hemisphere. Today, only three species have survived the millions of years of changes on Earth: the coast redwood, the giant sequoia, and the dawn redwood. 2 of the 3 remaining species (coast redwood and giant sequoia) can be found in California, whereas the dawn redwood can be found in China.

Only 5% of the original old-growth trees survived logging along California’s coast.

The California Gold Rush in 1848 brought about a huge population boom – and with the rise of the human population came a rise in demand for lumber. Settlers looked to California’s coast for sources of wood, decimating stands of coast redwoods in the process. Extensive logging has reduced the population of old-growth trees (trees that have survived a prolonged period of time without disturbance) to 5% of its original size.

Cool facts about coast redwood trees:

  • The tallest known coast redwood is named Hyperion, who stands at 379 feet tall.
  • Coast redwoods have very shallow root systems relative to the heights that they reach. To provide stability in the face of strong winds, they grow their roots outwards and intertwine them with neighboring trees.

Resources to learn more:

The Bay Area’s Fight for the Western Burrowing Owl

Cover photo: © Nathan Rupert, 2010, some rights reserved.

Learn about the burrowing owl’s current conservation status in the Bay Area and the current initiatives to protect them.

Cover photo: © Nathan Rupert, 2010, Flickr Photo Album, some rights reserved.

Quick Facts

  • The western burrowing owl is a small species of diurnal owl, meaning that it’s active primarily during the day.
  • The burrowing owl used to be common in the Bay Area, but habitat loss has caused a steep decline in their populations over the last 30 years.
  • Local conservation groups are working to improve habitats to make them more attractive to owls, and are working on monitoring sites known to have owls.


Nestled on the edge of a corporate development next to the entrance to a local wildlife preserve lies a small, fenced-off lot of short grass. Here, after minutes of scanning small mounds of dirt, Dan and I encountered our first burrowing owls. An adult sat hunkered down, directly next to a young owl that was curiously peering back at me with large, yellow eyes. With this developed backdrop, it is difficult not to question the fate of this charismatic species.

A Bay Area Species in Peril

The western burrowing owl is a diurnal owl, meaning that it’s active during the day. It’s fairly small, standing at about 10 inches high. True to their name, they nest and reside in burrows in the ground – relying on sites that have already been excavated by burrowing mammals such as the California ground squirrel. They can often be found standing vigilant outside of their burrows during the day.

This endearing bird was once a common bird in the South San Francisco Bay Area. In the mid-1980’s their population was estimated to be about 640 individual birds – with three-quarters of the population residing in the South Bay alone. Today, their population has been reduced to isolated breeding and overwintering populations (overwintering populations migrate to the Bay Area during the winter to escape cold temperatures in their other homes). In 2017, the South Bay reported just 64 adults at 5 breeding sites.

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An adult and juvenile western burrowing owl in the South Bay.

Habitat Loss and Evictions

Habitat loss is the main threat that these owls face. The South Bay was once host to native grassland habitats; however, housing and other commercial developments to address a booming human population have outcompeted the owls for space. The East Bay is facing similar problems – areas that overwintering owls need to be successful have rapidly disappeared, putting stress on their populations.

Even at sites where the owls reside, the active removal of burrowing owls takes place. Developers that are initiating projects at known burrowing owl sites will hire contractors to install one-way doors at burrowing owl sites – allowing the owls to leave their burrow, but not to re-enter.

These “evictions” are rendered legal because individuals or nests are not harmed. However, many groups criticize the practice since there is no monitoring of the evicted birds after the traps are installed. Burrowing owls have high nest site fidelitymeaning that they have high rates of returning to the same breeding site, and even specific burrow, year after year. With the evictions taking place without long-term monitoring of the displaced owls, the effects of practices such as these are unknown and the evictors are not held accountable.

Bay Area Organizations Band Together for Owls

Efforts to increase the population of breeding burrowing owls in the South Bay are currently being implemented. A number of South Bay organizations have banded together to increase the population of breeding owls at Warm Springs in Fremont. Based on the outcome of their efforts, they will apply what they’ve learned at this site to the management of other sites in the South Bay. Similarly, the Santa Clara Valley Audubon Society is working in Alviso, a neighborhood in San Jose, to provide suitable habitats for the owls. The group works to install artificial burrows and mow the area to keep vegetation short.

In the East Bay, the Golden Gate Audubon Society, the City of Berkeley Marina and the Shorebird Nature Center have joined forces to create the Golden Gate Audubon Burrowing Owl Docent Program. The program focuses on raising public awareness about keeping dogs on-leash to help protect the overwintering owls that use the areas in the East Bay. Docents also keep track of the burrowing owl numbers in their area – allowing for long-term population data.

Efforts such as these provide hope for burrowing owl populations, yet their fate is still undecided in the face of the rapidly expanding Silicon Valley.

For more information on how to volunteer for the Golden Gate Audubon Society’s Owl Docent Program, please visit this page and scroll to “Train to Become a Docent” towards the bottom of the page. For more information on volunteering with the Santa Clara Valley Audubon Society doing habitat restoration at Alviso, please visit this page.


The Brown Recluse: How Dangerous Is It?

Cover Photo: © Andrew Hoffman, 2012, some rights reserved

Recluse spiders are some of the most feared spiders in America. Rumors of infestations, highly dangerous bites, and even deaths have spread throughout the country. Yet despite the rampant arachnophobia, brown recluses are actually not so dangerous when we take a closer look at the spider behind the hype.

Cover Photo: © Andrew Hoffman, 2012, Photo Library, some rights reserved.

By Dave Frank, Contributor

Recluse spiders are some of the most feared spiders in America. Rumors of infestations, highly dangerous bites, and even deaths have spread throughout the country. Yet despite the rampant arachnophobia, brown recluses are actually not so dangerous when we take a closer look at the spider behind the hype. Let’s explore some common myths and misconceptions about the brown recluse!

Myth #1: Recluse bites are extremely dangerous!

First, we need to address the rates of spider bites in general. There are many myths about spider bites. One common myth is that you can tell a spider bite from another bug bite, such as a mosquito bite, because a spider bite will have two puncture wounds while a mosquito bite will only have one. While it is true that spiders bite with two fangs, in contrast to the bite of a mosquito’s single mouthpart, spider fangs are usually too small and close together for us to actually see both bite marks. A spider bite would appear as a single wound, much like a mosquito bite. In fact, mosquitos are responsible for far more deaths worldwide than spiders.

According to Rainer Foelix’s Biology of Spiders, “only four genera are known to cause potentially deadly bites.” Loxosceles reclusa, the American brown recluse, is one of these spiders, but most recluse bites are harmless. The venom of spiders is used to paralyze their prey, so bites on humans would only be defensive. And this is a rare case, as recluses (as with most spiders) are not aggressive towards humans. They are actually much more scared of us than we are of them!

Their bites are necrotic, which means that their venom causes tissue degeneration at the site of the bite. This is different from spiders such as the black widow, Latrodectus mactans, which have neurotoxic venom. Neurotoxic venom will cause nerve damage and paralysis to prey. For humans, the bite of a recluse is very unlikely to lead to significant injury or death. We are much larger than their usual prey, so their bites do not contain enough venom to hurt us much. As long as one seeks medical attention for wounds causing pain or that won’t heal, recluse bites are survivable.

Myth #2: Brown recluses are common in California!

In America, the range of the brown recluse is contained mainly in the southern states. While there have been sightings in California, these are extremely rare. In fact, an entomologist at University of California, Riverside argues that there are no populations of brown recluses in California. Rick Vetter states that hundreds of spiders have been brought to the entomology department at UC Riverside for identification, and not a single one was a brown recluse.

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Recluse Range Map, taken from UC Riverside Entomology Department,

The map above shows the common ranges of recluse spiders in the U.S. Loxosceles reclusa is the species of brown recluse with bites that are the most medically significant. As you can see, their range does not extend west into California. While other recluse species can be found in California, these species do not have bites that are medically significant. Loxosceles deserta, the desert recluse, can be found in California, but they live far from urban areas and so are not considered dangerous due to their extremely limited interaction with humans.

Myth #3: Recluses are aggressive spiders!

As stated previously, recluses are not aggressive. There’s a reason they’re called “recluses” after all! Recluses like finding hidden spaces to build their nests. Older homes can be perfect habitats for them, but they often coexist peacefully with humans. As far as infestations go, there are many reports of recluses being found in large numbers. However, most of these events are blown out of proportion. In Brentwood, TN, a woman claimed to have been bitten several times due to an infestation in her apartment. However, the articles do not always have scientifically accurate information and may be misleading. For example, the main picture of “brown recluses” in this article does not show a brown recluse, as the spider shown is not even brown! While recluses can be found in high numbers in suitable locations, it is extremely rare for them to interact negatively with humans.

Myth #4: Brown recluses are difficult to tell apart from other spiders!

Brown recluses are quite easy to identify. The most telling feature of a recluse is the pattern of the eyes. Most spiders have eight eyes arranged in two rows. Recluses have only six eyes, and these are arranged in three pairs across the cephalothorax (head and torso region). They also have a violin shaped pattern along their backs, which is why they are sometimes called “fiddleback spiders”. Recluses are around ⅜ of an inch in length, a little smaller than a penny, and are have uniform brown coloration. Other spiders may have some of these aspects, but if a spider you are trying to identify does not have these characteristics, then you have not found a brown recluse.

Brown Recluse Spider
© Andrew Hoffman, 2012, Photo Library, some rights reserved

For those who may still be arachnophobic, please take a look at this video! This person is holding a brown recluse and a black widow, the most commonly feared spiders, in one hand. As you can see, neither spiders attack the cameraperson. These spiders are actually rather mild-mannered. We do not recommend trying this at home, but hopefully it shows that humans have very little to fear from our eight-legged friends.



Daytime Owls: Western Burrowing Owls

Cover photo: © Julio Mulero, 2017, some rights reserved.

Did you know that not all owls are active at night? Meet the burrowing owl – a small, charismatic species of owl that is out during the day.

Cover photo: © Julio Mulero, 2017, Flickr Photo Album, some rights reserved.

Places to See Western Burrowing Owls

Shoreline Park Circle

Shoreline Regional Park

Western burrowing owls are small, standing at about 10 inches.

Western burrowing owls have a mottled pattern of brown and white across their body. They have bright, yellow eyes and long legs. They are small owls, averaging 8 to 10 inches in height.

Burrowing owls are facing steep population declines in the Bay Area.

Habitat loss and fragmentation have led to steep declines in the Bay Area’s burrowing owl population. In the mid-1980’s, it was estimated that there were 640 individuals, with three-quarters in the South Bay alone. In 2017, a survey in the South Bay reported just 64 individuals during the breeding season.

Projects for owl habitat restoration and public education programs are being implemented now. For more information on how you can get involved, visit the Santa Clara Audubon Society’s volunteer page and the Golden Gate Audubon Society’s docent page.

Burrowing owls are found in two areas in the United States: the west and Florida.

Burrowing Owls are separated into two subspecies: one species found in the western United States (western burrowing owl) and one species found in Florida (Florida burrowing owl). In the San Francisco Bay Area, they are found in isolated areas in the East Bay and South Bay.

In the northern part of their range, western burrowing owls will migrate south for the winter, arriving at their wintering site in October and departing from it in March.

Western burrowing owls live in flat grasslands, occupying abandoned burrows dug by mammals.

Burrowing owls are found in grassland areas, often in close association with California ground squirrels. They prefer areas with very short vegetation so that they are able to easily detect predators.

Burrowing owls are known to reside in close proximity to humans. One example of this is a population of burrowing owls that lives at the Mountain View Shoreline Park golf course. Here, the small population is adored by the regulars – some golfers have even named individual birds.

Western burrowing owls are opportunistic eaters.

Western burrowing owls are raptors, meaning that they capture and kill their prey with their feet. To forage, burrowing owls will hover, fly, and run to chase their prey. Burrowing owls are opportunistic eaters, meaning that they will feed on a variety of sources depending on its availability. They are known to eat insects, small mammals, reptiles, amphibians, and small birds.

Burrowing owls will imitate rattlesnakes to scare off predators.

Burrowing owl calls sound like a two-part coo-coo. Just like parrots are known to mimic humans, the burrowing owl is known to mimic the rattling sound of a rattlesnake. This vocalization is thought to be a means to deter predators, since an angry rattlesnake is a threatening sound to many animals.

For more burrowing owl sounds, check out their sound page by the Cornell Lab of Ornithology. For more information on the hissing mimicry, check out this page by the Audubon Society.

Burrowing owls nest in burrows, using the poop of other animals to deter predators.

Breeding burrowing owls nest inside of their burrows. They will line the inside and entrance of their burrows with other animals’ dung, which is hypothesized to mask the smell of their young from predators.

They will form loose groups when nesting, perhaps to help owls better detect predators since more individuals are present to be vigilant. Young burrowing owls rely on their parents for food until they are about 4 weeks of age. By 6 weeks of age, young burrowing owls are capable of flight.

Cool facts about western burrowing owls:

  • Western burrowing owls are one of two subspecies of burrowing owl – the western burrowing owl, which lives on the west coast of the U.S., and the Florida burrowing owl, which only lives in Florida.
  • Burrowing owls don’t actually excavate their own burrows! They rely on mammals like the California Ground Squirrel to dig the burrows for them.
  • A young adult novel, Hoot, was written about Florida burrowing owls. In the novel, a group of young kids bands together to help save local owls from losing their habitat to development.

Resources to learn more: