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.
Loss of Habitat in Overwintering Sites
Loss of habitat in overwintering sites is a very serious threat to the species.
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.
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 Milkweed in Monarch 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 (Asclepias speciosa). Native to inland California, it was introduced to Marin many years ago. Many local experts advise that it is an acceptable choice for private gardens in Marin but should not be planted in wild areas.
In contrast to narrow leaf and showy milkweed, tropical milkweed (Asclepias curassavica) is not a good choice for Marin. This species is not native to California. Unlike narrow-leaf and showy milkweed, it does not die back in winter. Because it is evergreen, it can confuse monarchs into remaining inland rather than migrating to overwintering sites. It is more likely to host the parasite OE, which persists on its leaves and infects monarchs as they migrate through the county. The CA Department of Food and Agriculture has designated it as a B-rated noxious weed. It can no longer be sold in nurseries in Marin.
