Most of the nine identified species of invasive orchids in Puerto Rico arrived as ornamental or agricultural imports, says University of Puerto Rico, Río Piedras Campus Biologist James D. Ackerman, orchid expert, co-author of The Orchids of Puerto Rico and the Virgin Islands, and author of An Orchid Flora of Puerto Rico and the Virgin Islands and the forthcoming Flora of the Greater Antilles: Orchidaceae. However, the seeds of one invasive orchid, originally from tropical Africa, Oeceoclades maculata, may have been carried here by the wind, country- and island-hopping its way from Brazil. Its seeds germinated and propogated rapidly.
This invasive orchid was first noticed by Roy Woodbury, former professor at UPR, RP in the mid 1960s. “Now you can find it from Guánica to El Yunque. It’s most common in the mogotes [rounded tower-like hills of limestone]. There are places that you can’t walk through the forest without stepping on it,” says Ackerman. When Oeceoclades was found in the sugarcane fields of the Dominican Republic, he says, planters chopped them up. But like the pieces of the brooms in The Sorceror’s Apprentice that grew into new brooms, the workers only succeeded in vegetatively propagating them, as many of the pieces sprouted.
The CATEC–funded project that Ackerman is working on, along with Professors of Biology Paul Bayman and Raymond Tremblay, investigates three invasive orchids, the African Oeceoclades, and two others, from Asia, Arundina graminifolia and Spathoglottis plicata. Oeceoclades is the most pervasive. The scientists want to identify the population growth strategies that these invasive species exploit to proliferate, the relationship of proliferation and land-use history, and whether the invasives are competing with native orchid species for fungi, which are necessary for the seeds to germinate. In other words, what makes some orchid species invasive and others not?
Ackerman imagines a possible answer to this question. “Is it because they are very liberal in what they parasitize? All the other orchids that are not invasive may be highly specific in the fungi that they parasitize. So invasive orchid species may be very catholic in their choices of fungi, or they might be specific, but specific on a fungus that is everywhere. We won’t know this until we get the mycorrhizal information and identify the fungi that the different species use.”
That’s where UPR, RP mycologist, or fungus expert, Paul Bayman’s students come in. Doctoral candidate Naida Viera and undergraduate biology majors Nilbeth Hurtado and Rocío Garriga sequence parts of the DNA to identify a fungus. “We either isolate the fungus or we just take the root that has the fungus in it and amplify a fungal gene, something that is in the fungus but not in the plant, and we get a sequence. Then we compare it with GenBank, which is an enormous database that has millions of DNA sequences, to see what it’s most similar to,” explains Bayman. After identifying the fungus, the researchers test whether it has the capacity to help the orchid seed germinate.
Researcher José Carlos Rodrígues of the Agricultural Experimental Station in Río Piedras is also studying the orchids. “His interest is in invasive mites and the plant viruses that they carry. So he wants to see whether or not invasive orchids are harboring bad things that could jump to horticultural or agricultural crops, but he’s also interested in whether or not they will jump to native vegetation as well,” says Ackerman.
Raymond Tremblay, wildlife researcher and associate professor of biology at UPR, Humacao, provides statistical analysis of the invasive orchids’ ecological data to estimate the effect of the species, how invasive it is, and how quickly it spreads. Scientists call this population viability analysis, and when they talk about invasive species, they discuss its invasive speed, or how fast the species distributes itself, how fast the population grows. “For example, there’s a nice little comment by Darwin in one of his books. He starts with one orchid, and that orchid has a number of fruits and each fruit has hundreds of thousands of seeds. And if all those seeds were to germinate and become a plant equal in size of their mother and produce all the fruit and all the seeds it would only take three generations for the whole earth to become completely covered by this orchid,” says Tremblay.
Many factors limit population growth, such as predators, but an invasive species has an advantage over endemic populations. Oftentimes when an exotic species settles in a new place, it has no natural predators, so it can spread quickly, making the exotic species invasive. “One of the best examples is probably the rabbit that was introduced in Australia, or the toad Bufo marinus that was introduced not too long ago in northern Australia, and it is expanding its range at about 70 kilometers a year, basically because it has no natural predators and the few predators that are present in that area and bite the toad are killed by the toad’s poison glands. So those predators are not very effective, and that often happens, and often on islands,” explains Tremblay.
"Many factors limit population growth, such as predators, but an invasive species has an advantage over endemic populations. Oftentimes when an exotic species settles in a new place, it has no natural predators, so it can spread quickly, making the exotic species invasive."
Sometimes invasives have unchecked population growth followed by a decline in population. This seems to have happened to the population of the invasive Oeceoclades orchid, says Tremblay. “At some point in the last 10-15 years, it used to cover the whole ground of Cambalache Forest, for example. I don’t think it is at the density that it was 10-15 years ago.”
Each scientist has his own hypothesis about the population growth strategies of the invasive orchids. “Ackerman does plant reproductive biology, so his explanation is that Oeceoclades is successful because it can self pollinate. José Carlos Rodrigues is a virologist and plant pathologist. So his explanation is that maybe these orchids are successful because they are resistant to diseases or maybe they carry diseases and they can carry it to native orchids. I work with fungi, so my explanation is that maybe it has special relationships with fungi. And Tremblay is an expert in population biology, so his explanation is that it is successful because it produces so many seeds, thousands in each seed pod,” says Bayman. Each seed is tiny and dust-like. It is only under a compound microscope that their details become visible. The darker spot inside a spindle-shaped seed is the embryo.
“We’ll be doing experiments using plastic slide mounts. We put orchid seeds between some plankton netting and then snap the slide mounts around it, creating a packet of seeds within the slide mount frame. We then put the frames out in the field to try to catch the fungi. The fungi will go through the phytoplankton netting to get to the seeds and the seeds will start to germinate. We pull the slides out of the soil and give them to Bayman to sequence the fungi,” says Ackerman.
“Invasive species carry bad connotations and some of them can severely harm natural and agricultural ecosystems. The Hawaiian Islands are a prime example of an island system that has suffered devastating effects of invasive species,” says Ackerman. “However, the islanders do show tolerance to their invasive orchids. After all, they are orchids and if we have any invasive species, let them be orchids!” Ackerman, Bayman, Rodrigues, and Tremblay hope their studies of invasive orchids determine whether they are benign or harmful to natural or human ecosystems.
jdackerman@uprrp.edu
pbayman@.uprrp.edu
raymond@hpcf.upr.edu
|
|
 |
