What is an Invasive Species?

If you do anything with natural resources (the land, water, soil, plants, and animals that support our lives) then you’ve probably heard someone bemoan invasive species.  Invaders are generally viewed as bad, but there are levels of badness, from pretty OK to super-villain, which is what I’d like to explain here through examples from the wetlands I work in around Great Salt Lake.  Plants aren’t the only invasive species - there are plenty of invasive swimming, flying, and running things - but I have the most pictures and knowledge of plants. 

Weeds – Badness Level: Pretty OK

A weed is not actually an invasive species, it’s just an undesirable plant for that place, nothing more.  Undesirable to whom?  Whomever is doing the name calling.  Native cattails (species in the genus Typha) are often regarded as weeds because wetland people would rather see something else, like bulrushes or submerged plants, growing in its place.    

Broadleaf cattail (Typha latifolia) – Don’t be hatin’

Duckweed (Lemna minor) is another example of a wetland weed.  Duckweed is native to North America but disliked because it shades out the water and makes boat travel challenging.

Duckweed (Lemna minor) covering open water, probably hated because it signals something off in the water

Introduced Species – Badness Level: Not Great

An introduced species is a species living outside its native range and that has arrived in its new place through human activity.  Many terms are used to describe these species, including alien, exotic, and non-native.  Some introduced species have been introduced to their new habitats deliberately because they were planted in gardens and then escaped.  Watercress (Nasturtium officinale) is not native to North America, but escaped garden fences and can be found in many slow-moving waterways now.

Watercress (Nasturtium officinale) a pretty introduced species (this will become a theme) that might also taste good

European seaheath (Frankenia pulverulenta), another European species, can be found in more and more Utah wetlands, seeming to follow cattle herds around.  However, the way it got to Utah is likely more accidental. Landowners or managers might try to remove introduced species because they are not native, but they’re usually low priority because they aren’t causing much harm.    

European seaheath (Frankenia pulverulenta) another cute, non-native species

Invasive Species – Badness Level: Villainous

Invasive species are those which are both non-native and likely to cause harm to the environment, economy, or human health.  Who decides when a species has passed beyond introduced into invasive?  I don’t know and I think often invasive species are discussed using all of the terms above.  However, since invasives have been called harmful, people are out there combating the invasion.  Reed canarygrass (Phalaris arundinacea) was deliberately introduced around the U.S. as a plant to prevent erosion following road construction (a common method for the introduction of grasses), but has become a plant bully, pushing aside other plants instead of playing nice alongside them.  Reed canarygrass is often sprayed with herbicide to weaken it. 

Reed canarygrass (Phalaris arundinacea) might not be as foreign as originally thought, but is still considered too pushy to put up with

Saltcedar (Tamarix chinensis) is another pushy, invasive species from Eurasia.  Its ability to grow fast, dense, and deep can prevent other wetland species from reaching the sunlight and water they need to grow.  Controlling tamarisk is done through more interesting means: either by ripping up the entire plant (mechanical control) or by bringing its natural enemy – a beetle – into battle. 

Tamarisk (Tamarix chinensis) was planted as an ornamental tree, but was bound for much greater things

Noxious Species – Badness Level: Worst-est

Noxious species reign supreme among invasive species, both in terms of impact and attention.  Agricultural authorities (in Utah, the Department of Agriculture and Food) have legally declared noxious species ‘injurious to public health, crops, livestock, land, or other property’ and require a counties to develop a combat strategy.  Some species are considered so noxious that they are prohibited from even crossing state borders.  Phragmites (Phragmites australis), undoubtedly the worst-est of the worst, was added to Utah’s Noxious SpeciesList just last year.  What makes Phragmites so odious?  Its ability to actually engineer an ecosystem: it displaces native species, obliterates sunlight at the soil surface, changes the course of water flow, and actually elevates the surface of the wetland. 

Phrag (Phragmites australis): beautiful, mean, and injurious

Many thistles, including musk thistle (Carduus nutans), are noxious species, in addition to their status as unnecessarily pokey.  Musk thistle forms such dense stands of solid thistle it is widely regarded on The Internet as ‘aggressive.’  Further, musk thistle might actually release chemicals that stunt the growth of other native species (a phenomenon called allelopathy).  Supervillain stuff, for sure!

 Musk thistle (Carduus nutans) might have been introduced to the US in ship ballast (water stored in boats to balance them) and then expanded across the country. 

Why?  And what can I do about it?

Why do some many rotten plants invade wetlands?  First, invasive plant species tend to have biological super powers like rapid growth and cloning that allow them to be everywhere (and they use this power for evil).  Second, wetlands tend to be located downstream of sources of invader reproductive bits and experience frequent disturbances like scouring floods.  This means there are plenty of plant bits waiting in wetlands to take advantage of bare soil when it is exposed.  Finally, being downstream of everything also means that wetlands often have lots of nitrogen and phosphorus, which are invasive species steroids. 

Purple loosestrife (Lythrum salicaria): each plant can produce more than 2 million seeds every year.

Water milfoil (Myriophyllumsibiricum): small pieces of the plant can hitch a ride on boat propellers and then create a new clone in a new lake.

Fuller’s teasel (Dipsacusfullonum): often sprouts on bare ground and is capable of growing a 2-foot deep taproot.

Oh my.  I need to step back for a minute.  Invasive species are scary…  

Once they’re established we have a whole suite of poisons, digger and cutter machines, fire starters, and natural enemies (often bugs) to combat the invaders.  However, it’s difficult work and generally the purview of professionals (or hapless graduate students).  So here I’m advocating for preventing the spread of new invasions, which is totally something you can handle.  How?  Don’t plant invasive species in your garden.  University Extension programs across the country, local gardening organizations, and a whole variety of herbariums and gardens have guidance on beautiful native species you could use instead.  An added bonus of avoiding non-native species: you’ll be planting something naturally adapted to the environment you live in.  That’s pretty cool. 

Itching for more?  Check out this video I made about my love/hate relationship with Phragmites and then look in on the research the Kettenring Lab at USU is doing on invasions and restorations.