It is estimated that there are 8.7 million species in the world. In most areas the local species have evolved together for thousands of years. They have their own ecological role in the community and all species keep each other in check-that means that they control each other’s populations so that no species grows so fast that it overwhelms the others. The species live in balance with one another. Species that have lived in the same environment for thousands of years are considered native to that environment and they are called native species. Species that arrive in new environments are called non-native species and if the non-native species starts to grow, reproduce, and take over it is called an invasive species.
We are going to investigate invasive species together through learning about the invasion of the green crab in Atlantic Canada.
If you would like to do some interactive activities involving characteristics of invasive species please check out this desmos activity: https://student.desmos.com/activitybuilder/student-greeting/5ee3bb4cd012d450fb51b166
If you would like a lesson plan for teaching about invasive species with an interactive movement based activity check out this PDF;
Vectors of introduction
Before modern humans there was never much chance of a species moving from another country or continent arriving in a new habitat. But when humans started travelling from place to place and shipping goods around the world, invasive species hitched along for the ride. We call these movements of species vectors of introduction. Anything that connects two environments with a vessel in which species can survive is a vector of introduction. A vector is a special word that just means that a quantity has a direction. It’s used in this case because when a species is introduced, where it came from and where it is going is important to the species survival.
Green crabs were introduced from Europe (where they are a native species) to Massachusetts (where they had never existed) in the early 1800’s. It is believed that the green crabs were present on the ships as adults and survived the long voyage over to North America where they began to invade. In the image below you can see the native range of the Green Crab and the areas where it was introduced.
Check out the powerpoint below for some examples of vectors of introduction. Think about what has to happen for a species to establish successfully. Do you think all species carried to a new environment establish themselves? Why or why not? Who is to blame for invasive species arriving in a new place? Is it the species fault?
Once a species has arrived in a new environment in order for the species to become invasive it must have a high growth rate. A growth rate is how much you grow in a certain amount of time. For example, you have a growth rate! If you grew two inches in one year, your growth rate is 2 inches/year.
Or if you grew 6 inches in two years your growth rate would be
6inches/2 years = 6/2 inches/year = 3 inches per year!
If you have measured how much you grew in the past years or months, try to calculate your growth rate in inches or centimetres per months, years, or days.
Humans grow pretty slow compared to animals and animals grow at different rates relative to each other.
An invasive species usually has a growth rate much higher than the surrounding native species. On a graph this looks like a steeper line. For example, growth rate comparisons of a native and invasive species might look like this:
This becomes a problem because once the species has grown enough it can have babies.
Invasive species can usually have lots and lots of babies. So now not only do individuals grow fast so does their population. We can see the same steeper line for invasive species in this population graph:
Now that the invasive species is growing and growing the animals that eat the same food as the invasive species start to suffer because the invasive species is eating all their food.
Animals that the invasive species eats or attacks are also in danger because they cannot keep up with the appetite or destruction of the invasive species.
One green crab can produce 175,000 eggs per year. It can also reach sexual maturity in one year. That means every year every female green crab produces 175000 larvae.
That’s a lot of crabs! This increase in green crabs has reduced the population of Jonah crabs by eating all their food while Jonah crabs are simultaneously fished by humans. They have reduced the population of mussels, clams, and oysters found in eelgrass. They destroy eelgrass beds while they dig for prey. Eelgrass is an important habitat species and so green crabs destroy the home of many marine organisms by digging up eelgrass (click to enlarge the photo to visualize the impact).
Hearty to disturbances
Invasive species are also often very hearty to disturbances. This means they can survive unforeseen events like drought, storm, fire, habitat destruction, and pollution.
This gives them an even bigger advantage over native species because disturbances that really damage native species don’t hurt invasive species and so they continue to take up all the resources.
Green crabs can survive out of water for up to 5 days! They can survive in a wide variety of temperatures, oxygen levels, and salinity levels (that is why you can find them in the ocean and in estuaries). They can survive severe food shortages because they are the only crab that can eat through their gills!
Vector of dispersal
One more thing that makes invasive species a very big problem is once they have invaded a new environment they also have methods of dispersing to new environments.
Invasive species can either move themselves or hitch rides with humans to new environments and then take over that environment as well.
Green crabs have a very long larval stage. That means they stay suspended in water for a long time and can drift to new environments or be taken up in ships ballast water.
This makes them very effective at dispersing and so they have dispersed all over Atlantic Canada in a little over 30 years.
Global Invasive Species
Green crabs are one of the world’s most widespread invasive species. To read about the top ten most impactful invasive species globally check out this link: http://content.time.com/time/specials/packages/article/0,28804,1958657_1958656_1958637,00.html
Check out the powerpoint for attempted solutions to invasive species. Most just involve trying to prevent further invasions, does that seem effective to you? Why or why not?
How do you feel about large scale killing of invasive species? Do the species intend to do lots of harm or are they just animals doing what animals are supposed to do-eat, grow, and reproduce?
Could we consider humans an invasive species?
This is a really interesting article about the science of invasive species and human attitudes towards them.