There are many definitions of invasive species, but basically it is any plant, animal, or pathogenic organism that spreads in range without human intervention. Organisms do this naturally—it is a part of being alive.
Some definitions of invasive species include both natives and non-natives, but conservationists have a particular problem with non-native invasive species that have been introduced to areas by humans, because often they grow rapidly and displace the native organisms. The VAST majority of plants and animals introduced to areas are not problems, but a few are, and they can cause lots of damage to the native species.
The Davis area has lots of these species, many being grown in its Town flora. Some are already widely spread and can only be eradicated locally, or perhaps by biocontrols. Other species are still in the early stages of invasion—they are just being grown here and there as landscape or garden plants, and may never escape. On the other hand, these species are invasive in other parts of the world, and invasive species scientists will tell you that the best indicator of invasiveness is if the organism has a bad track record elsewhere.
Invasiveness is very local. As the California Invasive Plant Council puts it: “Invasive plants are by nature a regional or local problem. A plant that jumps out of the garden in one climate and habitat type may behave perfectly in another.” Nandina domestica, the common Heavenly bamboo, is an invasive plant in northern Florida and parts of Georgia, but not here. Lantana x camara, a subtropical with beautiful flowers, has become a serious rangeland invader in New Zealand, but is just an attractive garden plant in California. Scotch broom and gorse have become serious problems throughout northwestern California. Baby’s breath (Gypsophila paniculata) has clusters of delicate, airy-looking flowers used in arrangements, but it may be restricted in some areas (not here) as it invades bunchgrass rangelands. The California Invasive Plant Council (CalIPC) has an inventory of invasive plants by region.
Below are some species of invasive species spotted in Davis, which are damaging to natural areas. The list of species that are strictly agricultural pests is different.
Plants not widely spread
Growing these plants in Davis is just asking for trouble. Even if you don't think they can spread from your garden, birds can carry propagules from your yard to the many wetlands and rivers within an easy bird's flight. Many of these plants are already causing problems in the various wetlands and impoundments that provide value for native animals and migrating waterfowl.
- Barberry (Berberis thunbergii)—a very bad plant in the eastern USA, being grown more frequently in Davis. Sold and planted widely in the Sacramento Valley for over 50 years, it has shown no invasive tendencies.
- Calla Lily (Zantedeschia aethiopiea)—chokes out natives in wet areas in Australia and SoCal. Sold and planted widely in the Sacramento Valley for over 50 years, it has shown no invasive tendencies.
- Carob (Ceratonia siliqua)—extremely weedy in SoCal, impacts yet unknown.
- Chinese tallowtree (Sapium sebiferum)—chokes wetlands, escaping into Davis ponds and streams.
- Cogongrass (Imperata cylindrica)—a pretty red cultivar called 'Red Baron' is invasive, despite what you'll hear from ag reps.
- Empress Tree (Paulownia tomentosa)—makes dense, dark shaded area excluding natives in Eastern USA.
- Jubatagrass (Cortaderia jubata)—excludes wildlife, usually on rocky slopes. More of a coastal problem.
- Kikuyugrass (Pennisetum clandestinum)—excludes plants that provide forage, a Davis turf weed, in some ponds.
- Lantana (including L. camara)—extremely invasive in the tropics, but not bad in California (yet!). Sold and planted widely in the Sacramento Valley for over 50 years, it has shown no invasive tendencies.
- Maiden grass (Miscanthus sinensis)—invasive in the east, currently available cultivars not yet invasive. There are extensive lists of cultivars of Miscanthus in reference books such as the Timber Pocked Guide to Ornamental Grasses, by Rick Darke, and the Encyclopedia of Ornamental Grasses, by John Greenlee. Many are sterile, others blooms so late in the season that they are unlikely to reseed.
- Pampas grass (Cortaderia selloana)—excludes wildlife, usually on rocky slopes. Still mostly a coastal problem.
- Privet (Ligustrum spp.)—very invasive in wetlands in SE USA, not yet a problem here.
- Rattlebox (Sesbania punicea)—chokes wetlands, escaping into Davis ponds.
Plants Already Widely Spread
The plants are already widely spread in the Davis area, and are damaging our native biodiversity. Alas, there's not much we can really do about them in terms of eradicating them. But we can try to remove them when possible. A few of their impacts are listed, all exclude native species by growing very thick or dense.
- Bindweed (Convolvulus arvensis)—really, more of an ag pest.
- Blackberry (Rubus armeniacus)—not to be confused with our native, R. armeniacus at least is very tasty. Yum yum.
- Eucalyptus (Eucalyptus spp.)—in the Davis area, the only real bad boy is E. globulus.
- Exotic grasses (many genera)—nearly all the grasses you see in the wild are exotic and of little forage value. Boo hoo.
- Fennel (Foeniculum vulgare)—not as tasty as its culinary progenitors, this is another habitat excluder.
- Fig (Ficus carica)—when this escapes into a wildland, it is nearly impossible to eradicate!
- Fountaingrass (Pennisetum setaceum)—a pity this is invasive; some sterile clones, others very invasive.
- Giant reed (Arundo donax)—a gigantic grass that makes dense groves.
- Iceplant (Carpobrotus edulis, C. chilensis)—a carpeting plant that covers all. Mostly a coastal menace.
- Johnson Grass (Sorghum halepense)—makes dense banks of growth.
- Milkthistle (Silybum marianum)—a thistle with beautiful leaves, but extremely prickly.
- Perennial pepperweed (Lepidium latifolium)—aka tall whitetop, the makes dense monocultures.
- Periwinkle (Vinca major)—makes native-excluding mats in the understory and along rivers.
- Tree of Heaven (Ailanthus altissima)—a stinky tree that makes dense stands.
- Water hyacinth (Eichhornia crassipes)—a floating plant that can reduce oxygen in the water.
- Yellow iris (Iris pseudacorus)—a pretty wetland iris, but it impedes wildfowl from coming ashore.
- Yellow oxalis (Oxalis pes-caprae)—a relatively new invader, which is very bad in coastal areas.
- Yellow starthistle (Centaurea solstitialis)—dominates our grasslands.
- English sparrow (Passer domesticus)—An omnivore competing for food.
- European starling (Sturnus vulgaris)—An omnivore competing for food.
- Fire ants (Solenopsis invicta)—no, we don't have these monsters yet, but they've been found in Sacramento!
- Humans ("Homo sapiens")—usually, the concept of "invasive species" is discussed within the context of the consequences of human activity, and so humans are kind of outside the definition. But there is no denying that humans are the underlying cause of all artificial invasions!
- New Zealand mud snails (Potamopyrgus antipodarum)—Found in Putah Creek.
- Opossums (Didelphis virginiana)—impact native birds by nest predation. Native to the eastern USA.
- Roof rats (Rattus rattus)—impact native birds by nest predation.
- Eastern fox squirrels (Sciurus niger)—likely impact native western gray squirrels and jays through competition. Garden and tree pest.
- Sudden oak death (not the band of the same name)—killing oaks and tanoaks
- West Nile Virus (Flavivirus sp.)—These could hammer our birds, including crows and the endemic yellow-billed magpie.
What Can You Do?
The number one, most cost effective way to deal with harmful invasive species is to prevent them from getting a foothold in the first place. In Davis, the number one vector for new invasive plants is the horticulture trade. The plants you can get at nonspecialist nurseries like Ace Hardware are not likely to be a problem in this way, since most of their plants are ones already in wide cultivation. Even so, Ace will sometimes have some invasive plants for sale, like the 'Red Baron' cultivar of Cogongrass or Periwinkle.
The UC Davis Arboretum is, unfortunately, a source for some invasive plants. The worst ones they have in this category are invasive grasses. Each year they sell lots of different clones of Maiden grass (Miscanthus sinensis), and it is only a matter of time before they introduce a clone that causes the same amount of trouble to California that they are causing in the eastern USA.
Landscaping invasives control
A few of the species of plants being grown (and even promoted) for landscape work in Davis should be removed from use, and replaced by native species or non-invasive exotics. Lists of ecologically responsible alternatives for the really bad players (such as Chinese tallow tree, Fountaingrass, or Periwinkle) are maintained by the California Invasive Plant Council.
Natural area invasives control
Davis maintains a great set of natural areas that act as sanctuaries for native animals such as waterfowl. Unfortunately, many of these are becoming degraded because non-native plants are invading them. These plants are either growing into clumps that are so thick that native animals are unable to pass through them, or are excluding more desirable plants that could provide food for the native animals.
- North Area Pond: This is one of the best places to observe native waterfowl in Davis. Is is being invaded by Chinese tallow tree and also has at least one Fig on one of its islands that could expand and spread. In years past, with permission from the city of Davis, the Chinese tallow tree was manually removed by volunteers, but some plants were so large they would have to be killed with herbicide. New plants have reinvaded (seeds were washed in from trees growing in nearby park plantings).
- Toad Hollow Dog Park: This dog park has the dubious honor of being the only place in Yolo County where rattlebox has been found. This species is creating lots of problems along the American River Parkway. The rattlebox was removed in 2005, but the plants probably originated by seeds washed in through the storm drains from some local horticulturist's garden. The park also has populations of Chinese tallow tree and a carpet of what might be Kikuyugrass