I never imagined that tumbleweed, the iconic extra in any western movie, is actually an ‘obnoxious weed’, to quote from a sign at Cathedral Gorge State Park in Nevada.
Tumbleweeds are the seed dispersing stage of ‘Russian thistle’ a plant that is bright green until after it has flowered. In autumn it dries out, breaks away from its roots and ‘tumbles’ through the landscape spreading more than 250,000 seeds per plant. It probably arrived from the Ural Mountains of Russia via flaxseed imported to South Dakota by Ukrainian farmers in the early 1870s and by the early 1900s had spread to most of the western states. Russian thistle thrives in disturbed soil, requires very little moisture and has proliferated amazingly well.
Kiwis know all about issues with introduced pests in New Zealand and the country spends millions of dollars a year trying to save our vulnerable species but I hadn’t expected to find similar problems in the USA.
And Russian thistle wasn’t the only unwelcome intruder that we saw. Almost as ubiquitous was Tamarisk (or salt cedar), a tree native to Asia which was introduced as an ornamental and for erosion control particularly along stream banks where it has been spectacularly successful. It can spread along riverbanks at an estimated speed of 20 kilometres per year! It also very effectively suppresses the growth of other plants by excreting salt which makes the ground beneath it unbearable for seedlings of other species. And if that wasn’t enough, it drinks a lot of water! Mature trees can have roots at least 30 metres below the soil surface and these suck up water which native plants and animals need.
It is one of the world’s super-plants and is tough to get rid of. If it is cut down it can regrow shoots 40 centimetres long in two weeks and can be more than two metres tall in two years. If you burn it, it grows back. In order to try and control it the tamarisk beetle has been imported to the US. So far they have only eaten tamarisk and over several years can kill a tamarisk tree by continually eating its foliage and stopping the tree photosynthesizing.
The beetles seem to be working in some areas but meddling with eco-systems is never straight forward. Stands of dead tamarisk are a fire hazard and have to be manually cleared and it takes a while for other plants (hopefully native but often not) to recolonize the areas. During this time other plants and animals suffer from the effects of things such as stream-side erosion and lack of shelter and nesting spots.
We spent quite a bit of time getting acquainted with the amazing Colorado River and couldn’t help but be impressed with this mighty river and fascinated (and somewhat horrified) by the many different ways that its water is being used.
We learned that the native fish of the Colorado River are ‘one of the most bizarre and unusual faunas found anywhere in the world’ and that many of them are endemic to that particular river system. Dams and introduced fish (such as Asian carp, introduced as a food source) are having a huge effect on the 14 native fish species which include native carp, minnows and suckers and the astonishing Colorado pikeminnow which can grow up to 2 metres long – or used to.
Of special interest (and slight embarrassment) to us was the problems being caused by New Zealand mudsnails. Yes, I know! Who would think that a native animal that most of us have never heard of would be causing problems overseas. They were first discovered in Idaho in 1987 and have spread widely, probably being dispersed by birds as well as fishermen. It was sobering to realise the extent of problems caused by both the deliberate and accidental introduction of non-native species in a robust country like the USA and that even something like the New Zealand mudsnail which is innocuous in an ecologicaly vulnerable country like New Zealand can be a pest overseas.