Bioinvasions: When Alien Species Displace Native Species
Paul Hargraves, Professor
Graduate School of Oceanography
Paul Hargraves became interested in Narragansett Bay in the 1950s when he played
in the mudflats at Gaspee Point in Warwick. More recently (the last 30 years),
he has been studying phytoplankton diversity in Narragansett Bay and in oceans
around the world.
The incredible array of life on our planet consists of plants, animals, microorganisms of many kinds, the genetic structure that defines them, and the ecosystems to which they belong. We call it biodiversity. We are mostly ignorant or unaware of the ways in which this diverse array moves around the planet. Without human intervention, these movements usually occur slowly, with gradual changes in climate or ocean circulation patterns extending or decreasing the natural geographic range of a critter. At its slowest, evolutionary change results in the appearance of new species in places where they never existed.
Occasionally, we become aware of sudden novel appearances, and we recognize the guilty parties as invasive, exotic, or alien species. These are mostly a result of some event humans have caused. World War II was such an event. The huge increase in global shipping brought things to new places around the world at an unprecedented pace. At the time, we didn't care much about it and, in many cases, it's now too late to figure out what went where and when. Transportation is still the primary way in which invasive species are spread. Global climate change is another such event. We're right in the middle of it (the evidence that the planet is warming and receiving more ultraviolet light is overwhelming, despite what Flat-Earth Society members think). In the upcoming decades, there will be obvious changes in the ranges and abundances of plants and animals that inhabit our shores, fields, woods, and waters.
It would be nice to say that we will someday have a handle on all of this, but we won't. Even the definition of exactly what an invasive is varies among disciplines (see Invasive species: What are they?). The subject of invasive or alien species is tied to the knowledge (or lack thereof) of biodiversity. How many species are there? Estimates range from 1 to 100 million. Including micro-organisms, I personally lean towards the higher end. Some steps have been made towards figuring out just what we are living with here on the blue marble. The need to know is driven in part by advances in molecular genetics and by the discovery that lots of species produce chemicals that might cure our diseases or extend our lives. There's an economic incentive to discover them.
Locally, we have been invaded, whether intentionally or not. A number of land plants, insects, and vertebrates have popped up here since the Europeans showed up, and Narragansett Bay has its share of invaders, too. It's hard for us now to imagine what our shorelines would be like without the green crab (Carcinus maenas), the intertidal periwinkle (Littorina littorea), and the green seaweed descriptively named dead man's fingers (Codium fragile). But these are alien, invasive, exotic species that have been here for about 150, 70, and 40 years respectively. More recently, around 1975, the tunicate Stylea clava and two planktonic diatoms Coscinodiscus wailesii and Thalassiosira punctigera appeared. In this decade, the red seaweed Grateloupia doryphora and the Asian shore crab Hemigrapsus sanguineus were discovered. Ironically, Hemigrapsus may be displacing Carcinus. If we knew more about the biodiversity of our area, we would know more about the invasion rate.
Are invasions benevolent, injurious, or something in between? In part, the answer depends on one's point of view. Purple loosestrife (Lythrium salicaria) is eye candy when it blooms along marsh and pond edges, but it is nasty for healthy food webs. Yet in human terms, the question would be better answered if we knew what happens when an invader appears. Many invaders may find the local scene inhospitable and disappear. Others may be assimilated into the ecosystem without a ripple. To the alien invaders that we clearly find nasty, we say "klaatu barada nicto"1 and set about dealing with them, with varying degrees of success, as the authors in this magazine show. Alien invasions clearly present opportunities for the confluence of basic and applied research. A number of people at URI are probing for answers to many questions. In some cases, we're finding them. To learn more about invasive species, start at the website: nas.er.usgs.gov.