Introduced and Non-native Species
In the Middle Rio Grande Valley, there are many species that have only recently taken up residence. These plants and animals are taking over areas that native species have lived in for hundreds of years. There are many reasons that nonnative species may be successful, but in general they arrive here without the animals or plants adapted to eat or compete with them in their native environment.
There are three introduced trees that are very common in parts of the Middle Rio Grande Valley: tamarisk/saltcedar (Tamarix chinensis), Russian olive (Elaeagnus angustifolia), and Siberian elm (Ulmus sp.). In general they are increasing because human-caused changes in the river valley provide favorable conditions for them to grow.
Tamarisk trees flower and produce seeds throughout the growing season; their reproduction is not restricted to spring/early summer as are native cottonwoods. When bare ground is colonized late in summer by tamarisk, it will not be bare in spring when cottonwoods are sending out seeds. Both Russian olive and Siberian elm can sprout in shaded area, under the canopy of the cottonwoods, and are becoming very common in the bosque.
Fires in the bosque are much more common today than in previous centuries. Human-cause fires from factors such as fireworks destroy many acres of the bosque each year. Cottonwoods can survive low - to moderate-severity fires, and can re-sprout after high-severity fires, but survivorship of the above-ground tree tends to be low after high severity fire. Although the sprouts can grow quickly, it takes some time before they are able to produce seed. Both tamarisk and Russian olive can re-sprout after fires, while above-ground parts of the plants tend to be killed. These species, however, often can reestablish after fires more quickly than cottonwoods with new seeds coming from plants off the burned site, since they produce seeds for a longer period than do cottonwoods. Also, tamarisks that survive a fire can increase flowering and seed production, again giving it an advantage in re-establishment over cottonwoods.
Cavity-nesting birds (such as nuthatches, chickadees, and woodpeckers) are an important part of the bosque ecosystem. They use the large cottonwoods to build their nests, but they have not been seen nesting in tamarisk or Russian olive. These introduced trees do not provide suitable cavity sites. If the number of native trees in the Rio Grande bosque continues to decline while introduced tree species increase, we may see a change in some of the wildlife along our river corridor.
Arthropods
The isopods, commonly called pillbugs (Armadillidium vulgare) and woodlice
(Porcellio laevis), were brought to this continent in the holds of ships.
Ships carried dirt as ballast on their trips to North America, but then dumped
the soil to load cargo bound to Europe. Isopods spread from these deposits.
In the Rio Grande Valley the isopod has become the major detritivore (eater
of dead plant material). Crickets filled this role before, but are now reduced
in numbers. Crickets do well in areas that receive spring flooding, but isopods
tend to be more numerous in drier sites.
Amphibians
Although native to the eastern U.S., it is unknown whether bullfrogs (Rana
catesbeiana) are native to New Mexico. They were introduced throughout the
west to provide a source of frog legs for people to eat, and this is probably
how they got into the Middle Rio Grande. Bullfrogs are large frogs that eat
almost anything they can capture and swallow, even ducklings! They are known
for eating other frogs and have been blamed for the decline of several species.
The northern leopard frog may have declined in part due to predation by bullfrogs.
Fish
Nonnative fishes have been introduced to the Middle Rio Grande both accidentally
and by intentional Game and Fish stocking programs. There has been a corresponding
reduction in the numbers and distribution of native fishes as the new species
compete for food or prey directly on native species. In some cases an introduced
relative is hybridizing with the native species. Species of mosquito fish
eat mosquito larvae. The installation of Cochiti Dam has changed the temperature
of the water and the amount of water released downstream throughout the year;
those conditions change the fish able to survive in the reach below the dam.
Birds
European starlings (Sturnus vulgaris) were introduced into Central
Park, New York City in 1890; by 1952 they were found across the United States.
They primarily eat insects but also eat seeds and scavenge garbage. They nest
in cavities and so compete directly with native cavity-nesting birds. Starlings
nest early in the year and are very aggressive about claiming nest holes.
They even evict the large woodpeckers that excavated the hole! Many species
of birds are now reduced in numbers due in part to competition from starlings.
House sparrows (Passer domesticus) were introduced to St. Louis in 1875 from England, after initial introductions in the 1850s to Brooklyn died. By 1940 they had spread throughout the United States by nesting in boxcars. They live in and around buildings, close to humans. Like starlings, house sparrows start nesting earlier in the year than native birds and so can claim prime nesting habitat (they nest in cavities but can also build a bulky nest in dense vegetation). They may even appropriate nests of other birds, killing eggs and nestling if occupied. House sparrows tend to have several broods a year.
Mammals
House mice (Mus musculus) move along with humans into an area. They
have large numbers of young that can reproduce when only two months old. Although
they are not common in bosque sites away from the city, in the Albuquerque
bosque they are often captured in areas of dense vegetation, especially near
water. They do not tend to be in areas of mature cottonwoods. With more human
development in or near the bosque, and a shift in vegetation, house mice will
likely spread into more areas. The Norway rat (Rattus norvegicus) was also
introduced to the valley and is found in agricultural areas, but is not very
common in the bosque.
Feral cats eat native rodents and birds. They also eat native lizards, and have caused a dramatic decline in the local lizard populations.
Feral dogs roam in packs and eat many native species. They easily destroy ground-nesting birds such as ducks and geese.
