Thus they made a covenant at Beersheba. So Abimelech rose with Phichol, the commander of his army, and they returned to the land of the Philistines. Then Abraham planted a tamarisk tree in Beersheba, and there called on the name of the LORD, the Everlasting God. And Abraham stayed in the land of the Philistines many days. Genesis 21:31-33
Tamarisk Tree by Robert Wallace on Flickr
A few thousand years after Abraham purportedly planted a tamarisk tree to mark his treaty with Abimelech, the Middle Eastern tree came to America. Gardeners thought it pretty, and drought tolerance gave it a place in the widespread ‘shelterbelts’ built during the Great Depression as giant windbreaks to help control drought and erosion. While these shelterbelts did help some farmers, using tamarisk had unintended consequences.
Tamarisk seeds, ready to be blown to new habitats
See, tamarisk didn’t stay where we planted it. Like dandelions, lightweight tamarisk seeds float on the wind – and an adult tamarisk can make a quarter billion seeds in a year. Since the 1930s, those little seeds have spread over two million acres of riparian habitat.
In its new home in southwestern America, tamarisk forms very dense stands – seedlings have been found at densities of 16 thousand in a square meter. In addition to crowding out native plant species, tamarisk makes the chance that native species can compete with it even lower by changing the very environment it inhabits. It has remarkably high rates of evapotranspiration – so the soil is drier and the streams tamarisk grows along have reduced flows. Its enormous and deep root system does more than suck up all the water: as the roots spread into streams, they slow streamflow, increase buildup of silt, and can eventually rechannel the river or stream. Tamarisk also pulls salt out of the soil and concentrates it in its leaves. Over time, as tamarisk pulls the salt from deep soil and drops its leaves, the top soil becomes too salty for other species to survive. Tamarisk has totally changed the nature of many southwestern streams, and control is difficult and expensive:
When control is needed, saltcedar is difficult or impossible to kill by burning, drought, freezing, hypersalinity, prolonged submersion, or repeated cutting at ground level.
Dense stand of tamarisk along the Colorado River by Rachel Zurer on Flickr
Tamarisk’s story isn’t unique. Fast spreading non-native species that cause economic and ecological problems are, unfortunately, quite common. Many invasives are introduced on purpose before we know what they can do – like tamarisk or starlings. Others just hitch a ride in our boats or food. Regardless of our intentions, we don’t have anyone but ourselves to blame for the sometimes catastrophic consequences of invasive species. (It is important to distinguish invasive from non-invasive introduced species.)
Invasive species are number four on Novacek and Cleland’s list of big things we’re doing to screw up the world. For more on their paper “The current biodiversity extinction event: Scenarios for mitigation and recovery,” check out these posts:
Novacek and Cleland charge scientists with figuring out exactly how invasive species are changing ecosystems and how problematic those changes are. Perhaps most important, they urge us to consider how other human activities worsen the invasive species problem:
The probability and impact of biotic exchange is also closely tracked to other drivers, such as land use policy and introduction of excess nitrogen deposition through use of fertilizers.
We can’t make a case for spending money on removing invasive species or changing human activities when we can’t draw strong connections between the invasive and a problem or the activity and the invasive.
Their policy recommendations are smart and hinge on the idea that a healthy ecosystem and healthy communities are less susceptible to invasion, or at least severe invasions. Their first major suggestion is to keep habitats from getting too fragmented. If communities become ‘patchy’ – split up by cities and golf courses and farms – gene flow exchange and movement of the natives is restricted, preventing them from fighting the onslaught of the invasive species on a somewhat equal playing field.
Their other suggestion is to make sure that normal ‘disturbance regimes’ are maintained. If a location has frequent flooding or fire, for example, the native species are adapted to it, whereas the invasive non-native species probably isn’t. By letting a location flood or burn as it did before we built our dams or started suppressing fire, we may give native species a competitive edge.
Tamarisk and other scrub used to build bee lodge by greenheron47 on Flickr
One promising solution that Novacek and Cleland didn’t mention is human use of invasive species. We cause the invasive species problem because we’re so mobile, and we make it worse because of the way we change the land, but we also have the potential to dramatically reduce both the impact of many invasive species and our own negative impacts while improving our lives and economy.
For example, many invasive species are edible, like crayfish in the west or garlic mustard in the midwest. By eating invasive species (harvested from invaded landscapes, not grown!), we could reduce the impact of invasive species on ecosystems, boost our economy (jobs for harvesting!), save money (groceries from the weed filled lot!), and reduce our use of pesticides and oil (less factory farming!).
Not all invasive species are tasty, but almost all probably have other uses. Tamarisk, for example, has interesting and beautiful bark. I’ve seen young tamarisk used to good effect as a privacy fence. Many invasive species form extensive monocultures that could potentially be harvested for biofuels. Some could be turned into fertilizer or compost.
Invasive species removal is usually pretty expensive and often relies on volunteer efforts. And in many places, as soon as the funding and volunteers dry up, the invasives come back. But humans are incredible consumers and there are an awful lot of us – finding a use and a market for invasive species could keep them under control and give natives more of a chances.
1 Novacek, M. (2001). The current biodiversity extinction event: Scenarios for mitigation and recovery. Proceedings of the National Academy of Sciences, 98 (10), 5466-5470 DOI: 10.1073/pnas.091093698
