The movement of species around the globe raises complex questions about what is native or non-native, if new species are harmful or benign in their new habitat, and even whether or not species originated from places they're now commonly found.
Categorizing species is a helpful way to clarify exactly what role an animal has in an ecosystem or in its distribution in one location or around the globe. Here are six categorizations that are most used in explaining species and their presence in a habitat.
A native species is one that is found in a certain ecosystem due to natural processes, such as natural distribution and evolution. The koala above, for example, is native to Australia. No human intervention brought a native species to the area or influenced its spread to that area. Native species are also called indigenous species.
While a native species can be helped by new species introduced to an area — such as flowers native to North American gaining the help of European honeybees in the last several centuries — the native species itself developed of its own accord in the area and is particularly adapted to its habitat.
The key aspect of a species being native is that it occurs in an area without human influence. In fact, it's that human influence that has helped to create several other species categorizations.
The Galapagos mockingbird is one of four mockingbird species endemic to the Galapagos Islands. (Photo: putneymark/flickr)
A native species can be indigenous, as discussed above, or endemic. When a species is indigenous, it's found in a particular location and surrounding areas. For instance, an indigenous species might be found throughout the Rocky Mountain range as well as the surrounding areas west of the Rockies.
An endemic species, however, is a native species found only in a particular area, large or small. A species can be endemic to an entire continent, or to only a relatively minuscule area. For instance, an endemic species might be found only in a particular mountain range at a certain elevation zone and nowhere else, or only in a particular lake, a single river or a small island.
Often, endemic species are confined to a certain area because they are highly adapted to a particular niche. They may eat only a certain type of plant that is found nowhere else, or a plant might be perfectly adapted to thrive in a very particular climate and soil type.
Because of this specialization and inability to move into new habitats, some endemic species are at particular risk of extinction when a new disease hits, when the quality of its habitat is threatened, or if an invasive species enters its region and becomes a predator or competitor.
Introduced or non-native species
Introduced species are those that occur in an area where they are not native, but were brought there through human influence — either purposefully or accidentally.
A common misconception is that introduced and invasive species are interchangeable terms, but these are actually distinct. Introduced species don't necessarily have a negative impact on their new ecosystem, and could even be beneficial.
The European honeybee is a great example of a beneficial introduced species, as the honeybee is critical to North America's crops and doesn't necessarily have a negative impact on other pollinators.
However, an introduced species has the potential of becoming an invasive species.
An invasive species is one that is introduced into an ecosystem and thrives so well that it negatively affects native species.
The USDA defines invasive species as:
1) non-native (or alien) to the ecosystem under consideration and
2) whose introduction causes or is likely to cause economic or environmental harm or harm to human health.
Invasive species can be plants, animals, and other organisms (e.g., microbes). Human actions are the primary means of invasive species introductions.
The negative impacts can include out-competing native species in the same ecological niche, reducing biodiversity in their new habitat, or altering their new habitat in ways that make it difficult for native species to survive.
Thanks to human travel, thousands of species have been introduced into new habitats and become invasive. Once the species becomes established, and their impact is clear, it can be difficult to figure out how to remove the species and restore the ecosystem. There can be other difficult questions to address, in addition.
As CBC Radio science columnist Torah Kachur points out, "What if an invasive species is wreaking havoc on an ecosystem in one area, but is actually critically endangered in its native ecosystem? This is what is happening to the wattle-necked softshell turtles – they're native to China, and they are endangered. But on the Hawaiian island of Kauai, they are considered invasive. So do we try and eradicate them from Kauai — and potentially doom the entire species?"
The issue of invasive species is never an easy or straightforward one.
While an endemic species is limited to a particular range, a species that's found in a globally broad range, in a particular type of habitat all over the world, or which rapidly extends its range in opportunistic ways, is called cosmopolitan.
The categorization of cosmopolitan is complex. While it typically describes a species with global distribution, it is assumed the polar regions, deserts, high altitudes and other extremes are automatically excluded. The label may also be used to describe species that might be found on most continents but not all, or many ocean habitats but not all. The term is mostly used to describe species that are generally widespread, but doesn't necessarily mean that the species is found absolutely everywhere.
Orcas are one such species. They're found throughout the world's oceans, from the icy waters off North America and Antarctica to the more temperate waters of the Mediterranean and the Seychelles. They do not necessarily appear everywhere in the oceans, but they have a broad distribution.
Houseflies, rats, domestic cats, humans and many other species also fit under the label of cosmopolitan as they're found globally.
Native to the coasts of northern China, Korea, Russia and Japan, it's unknown how the Northern Pacific seastar arrived in Alaska and Canada. (Photo: Lycoo/Wikimedia Commons)
While a native or introduced species is usually easy to categorize, that's not always the case. Sometimes it is nearly impossible to tell whether a species originated in an area or was brought in long ago. A cryptogenic species is one whose origins are unknown, or cannot be definitively determined. So, a cryptogenic species could either be native or introduced, but has settled into its habitat so thoroughly that no one knows for certain.
In a 1996 paper titled "Biological Invasions and Cryptogenic Species," James T Carlton notes, "It is exceedingly difficult to generate quantitative estimates of the frequency of cryptogenic species, because they have been widely neglected as a conceptual category. On occasion workers will puzzle over the native status of a particular species, but the great majority of taxa with widespread if not cosmopolitan distributions are simply stated as being cosmopolitan, with no further discussion."
It isn't necessarily known whether the species were introduced relatively recently by ancient humans, appeared there naturally in relatively recent natural history, or have been there for eons.
When reviewing native or alien species in Ireland's waters, a paper published by REABIC notes, "The sixty-three cryptogenic species arise out of the uncertainty of their origin or as to how they will have arrived. Ireland being a recently deglaciated island and separated from the continental land mass will have acquired the majority of its biota since the last glacial retreat making distinction between native and alien species more difficult."
The difficulty in knowing with certainty where a particular species originated from may mean that the mystery is never solved.