Threats from Habitat Loss
Habitat loss affects certain plants and animals more strongly than others.
Generalist species like racoons, cockroaches, and Norway rats can live in a range of conditions and consume a wide variety of foods. Generalists are less likely than specialists to face extinction.
Specialist species like the western monarch can survive only on certain food and in certain places. They need milkweed host plants, nectar plants, and protected spaces for overwintering. They are thus at risk of extinction when these necessary elements are unavailable.
Unlike monarchs, raccoons are omnivorous.
Loss of Habitat at Overwintering Sites
Western monarchs migrate to the coast between October and March. During that time, they gather in large colonies to roost, primarily on eucalyptus trees, Monterey pines, and Monterey cypresses and occasionally on oaks, sycamores, and redwoods.
These trees provide wind protection and dappled sunlight, acting as a “thermal blanket and a rain umbrella.” Access to trees of varied height is also important so that monarchs can move around when microclimate conditions such as wind and temperature fluctuate.
Overwintering sites must also provide a source of fresh water as well as nectar plants. One advantage of eucalyptus trees is that they bloom during the overwintering season, offering a convenient source of nectar during a period when relatively few flowers are available.
Loss of Eucalyptus and other Roosting Trees
Protecting and restoring overwintering sites is crucial to the monarchs’ survival. However, many roost trees have been cut down to make room for residential development.
In particular, eucalyptus trees are susceptible to removal due to their flammability. Many were planted a century ago or more; their height and relative weakness also make them vulnerable to falling during the rainy months.
While acknowledging these risks, monarch site management specialists emphasize the important role currently played by the eucalyptus as a roost tree, and recommend that it be strategically protected while replacement options are being developed and implemented.
What is so great about eucalyptus trees, from a monarch’s point of view?
The monarch’s long legs are a perfect length for holding onto the sickle-shaped eucalyptus leaf. Also, eucalyptus typically grow in a north-facing, bowl-shaped area so they offer good wind protection as well as moderate sun exposure.
How did eucalyptus become so widespread?
These trees were introduced from Australia during the Gold Rush in the mid-1880s. The newcomers needed firewood as well as lumber for construction of all kinds. With a growth rate of 4 to 6 feet a year and a mature height of up to 100 feet, the trees were also planted to serve as windbreaks on agricultural land.
A second planting boom occurred in the early 1900s when fears of a timber famine in the eastern US led to the cultivation of millions of acres of eucalyptus in California.
When the timber famine failed to occur, and as it became apparent that the eucalyptus was not suitable for construction, thousands of acres of eucalyptus were simply abandoned. Much of what we see today is what remains of this crop.
Learn more about the rise and fall of the eucalyptus in the Bay Area here.
Loss of Native Milkweed in Inland Habitats
Milkweed is absolutely essential to the survival of the monarch.
It is the only plant on which monarchs lay their eggs because monarch caterpillars eat nothing else. In addition to providing nutrition to caterpillars, it also contains toxic chemical compounds (cardiac glycosides) that are sequestered within the caterpillar and adult butterfly. This toxicity, signaled by the monarch’s distinctive coloration and markings, making them toxic and distasteful to predators.
The best option for monarchs in Marin is narrow-leaf milkweed (Asclepias fascicularis). The only true Marin native, it begins growing in the spring, reaching 2 to 4 feet by summer. It blooms with nectar-rich pink and white flowers beloved by the monarch as well as bees and other pollinators.
The second best option is showy milkweed (Asclepius species). Native to inland California, it was introduced to Marin many years ago. Many local experts deem it an acceptable choice for private gardens in Marin, but not for wild areas.
Check out this great video by Kim Smith of munching monarchs on milkweed. If you have ever wondered how four monarch caterpillars manage to consume all the leaves on a medium size milkweed plant in five days, here’s your answer.
Milkweed is rarely seen in urban areas unless a concerted effort has been made by local government or private gardeners to introduce it for the purpose of sustaining monarchs. Many residents do not know how important milkweed is for monarchs.
Though essential for monarchs, milkweed is not wildly popular among home gardeners. One issue is that it dies back in winter, which is undesirable to home gardeners who prefer plants that remain green year round. A second issue is that it contains toxic sap, creating some concern that pets or children may be harmed by exposure to it.
So what can be done? The problem of dying back can be addressed by interspersing milkweed plants with perennials, so that when the milkweed dies back, there is still foliage in the area.
Concerns about harm from the milky sap tend to be largely unfounded. Medical experts underscore the fact that milkweed sap, while toxic to monarch predators, is unlikely to harm larger animals like pets or children.
Monarch caterpillar on narrowleaf milkweed
Narrowleaf milkweed
Showy milkweed