Last September, two invasive aquatic plants, water lettuce (Pistia stratiotes) and water hyacinth (Eichhornia crassipes), were discovered in the Lehigh Canal in Bethlehem PA. Both species are floating plants, like duckweeds but much larger, and they often grow in dense mats in tropical and subtropical regions. Although this was the first confirmed occurrence in natural habitat within Pennsylvania, both species are sensitive to freezing temperatures so they have not not been regarded as major threats in the Northeast. A description of the discovery of these populations and some background on the species, including a discussion of recent work suggesting that the overwintering potential may be greater than previously thought, can be found in my post from last year (New invaders in the Lehigh Valley? Or Just Summer Visitors?).
The discovery last year prompted several questions. In particular, are these populations really persisting from year-to-year and therefore surviving freezing temperatures? I suspected that they were introduced last summer from someone’s pond and that they would not survive the winter. However, the winter was mild and the recent discovery of some overwintering populations in the lower Great Lakes gave me pause. I road my bike along the canal towpath last week to have a look.
I was wrong. Both species have overwintered. A harbinger of things to come? Below are some pictures, and I’ll update this post with more later in the summer.
Back in June an alligator was found in the Lehigh Canal. Apparently it wasn’t the first one found in the broader Lehigh Valley.
But perhaps just as surprising are a couple of potentially new plant arrivals. Or are they just summer visitors? Last week I noticed sizable populations of two aquatic plant species, water lettuce (Pistia stratiotes) and water hyacinth (Eichhornia crassipes), in the canal at Sand Island in Bethlehem. Both of these species float unattached on the water surface, like the more common duckweeds, and they often grow in dense mats that make fishing and boating difficult, crowd out other plant species, and alter water chemistry and light penetration. To my knowledge, neither species is confirmed to occur naturalized in Pennsylvania but it is not uncommon to see them cultivated in backyard ponds (USDA Plants: water hyacinth, water lettuce).
The populations of water lettuce and water hyacinth in the Lehigh Canal consist of scattered colonies extending from about the Hill-to-Hill Bridge (Route 378) east past the New Street Bridge (Fahy Bridge), to about the point where the Sand Island Trail meets the towpath (D&L Trail). The total distance is about a half mile. The water lettuce appears to cover a slightly greater distance than the water hyacinth, and the plants are generally smaller in height as you head east (downstream) from the Main Street Bridge.
Water lettuce and water hyacinth are tropical or subtropical in origin. The two species have dramatically expanded their range in warmer regions in recent years, where they have cause considerable ecological and recreational impacts. However, given that both species are sensitive to freezing temperatures, they have not not been regarded as major threats in the Northeast. However, some uncertainty about this assumption has emerged in the last several years. For example, a few years ago populations were found in the lower Great Lakes (Adebayo et al. 2010), and resurveys found both species in three subsequent years (Maclsaac et al. 2016), raising concerns about the potential for the establishment of persistent populations in more northerly locations. Although freezing typically kills individuals of both species they can produce seeds that survive cold temperatures; in fact, water lettuce seeds can still be viable after a few weeks in solid ice (Pieterse et al. 1981). Maclsaac et al. (2016) suggested that the two species likely persist in the lower Great Lakes due to annual reintroductions by humans (both species are sold for ponds/aquariums), but also noted that at least in the case of water hyacinth, seasonal regeneration from viable seeds may be occurring.
For background, the Lehigh Canal was built in 1827 to transport anthracite coal from the upper Lehigh Valley, and it remained in operation until the early 1940s. Heavy transportation and industrial activity along the canal and river corridor, as well the development of the surrounding Allentown-Bethlehem-Easton region led to numerous environmental problems, including pollution, habitat degradation, the spread of invasive species, and eutrophication of the canal. However, the towpath along the canal is now a natural-area corridor and the old towpath is a great place to bike, run, hike, fish, bird, and observe nature from within the urban and suburban matrix of the Lehigh Valley. Near Sand Island in Bethlehem, the canal itself gets pretty green by mid-summer, as the slow-moving water warms and algae proliferate. Invasive eurasian water milfoil (Myriophyllum spicatum) and curly-leaf pondweed (Potamogeton crispus) are common submerged plants within the canal, and provide a favorable substrate for filamentous algae. The habitat is ideal for water lettuce and water hyacinth, except for the fact that it freezes in the winter.
Have these species been in the canal in previous summers? Are these populations persisting, or did this expansion occur just this year? Perhaps the two species came into the canal with the pet alligator 🙂 Although this was the first time I noticed the two floating species, I don’t frequent this particular area of the towpath often. Will they reemerge next summer? Are they producing viable seed? Lots of questions, and certainly something to watch. The observations have been submitted to iMap Invasives, a database of invasive species.
Of course, floating plants are also very good at moving. Maybe not as fast as an alligator, but fast enough for me to watch a cluster of water lettuce floating down the canal. Perhaps on its way to Easton?
Adebayo, A., E. Briski, O. Kalaci, M. Hernandez, S. Ghabooli, B. Beric, F. Chan, A. Zhan, E. Fifield, T. Leadley, and H. MacIsaac. 2011. Water hyacinth (Eichhornia crassipes) and water lettuce (Pistia stratiotes) in the Great Lakes: playing with fire? Aquatic Invasions 6: 91-96. DOI 10.3391/ai.2011.6.1.11.
MacIsaac, H.J., A.P. Eyraud, B. Beric, and S. Ghabooli. 2016. Can tropical macrophytes establish in the Laurentian Great Lakes? Hydrobiologia 767: 165-174. doi:10.1007/s10750-015-2491-y
Pieterse, A. H., L. Delange, and L. Verhagen. 1981. A study on certain aspects of seed germination and growth of Pistia stratiotes L., Acta Botanica Neerlandica 30: 47–57. doi:10.1111/j.1438-8677.1981.tb00386.x
After a morning of wetland classification, the Pymatuning wetlanders had a tour of Morgan Swamp led by the Nature Conservancy’s Alex Czayka. We had a fantastic time, and the students were introduced to range of wetland types including marshes, swamps, and vernal pools. They also had the opportunity to learn about the effects of invasive species, and the ongoing management efforts employed by the Nature Conservancy in the preserve.
The Lehigh Canal was built in 1827 to transport coal from the upper Lehigh Valley. The canal was in operation until the 1940s, and it is now primarily used as a recreational trail for hiking, walking, and biking. Within the Bethlehem-Allentown-Easton area, it has all the problems of an isolated natural area surrounded by development (habitat degradation, invasive species, eutrophication of the canal), not to mention the legacies of once being a transportation corridor (e.g., even more invasive species). However, regardless of these problems, the area still provides considerable ecosystem services, including recreational opportunities, a great path for carbon-neutral commuting to work (for me!), and an easily accessible place to observe nature within the greater Bethlehem area.
Below are a few observations, made by students along the towpath this semester…highlighting the educational value of this interesting legacy of Lehigh Valley history.
Restoration at Sand Island
Last weekend, Dennis Scholl of the Delaware and Lehigh National Heritage Corridor led a group of volunteers in an effort to replant native species along the canal just east of Sand Island in Bethlehem, PA. I spent many hours of my youth exploring this area, visiting the playground, walking the bridge, and returning via the towpath adjacent to the canal. The area, and particularly the banks of the canal and river, were thick with vegetation. Although located in the center of the city of Bethlehem, as a child I really believed that we were walking deep in a magical forest. However, a few years ago most understory vegetation was cleared, giving it the appearance of a park. You could see through the trees all the way to the streets of Bethlehem. Of course, the intentions of this work were good – much of the understory vegetation was composed of invasive species, and native species were extensively planted in their place – but a lot bare ground remained. However, the removal of the vegetation didn’t just change the appearance of the area; without the thick vegetation the area became vulnerable to increased erosion, which was made worse by the unusual rain and flooding of last year. I hope the new restoration effort succeeds in reintroducing local plants to all of the areas of Sand Island, turning it back into a magical forest in the middle of the city of Bethlehem.
As I was on a bike ride near the Lehigh River, I came across something that we had just discussed in class – a fish ladder! To me, the ladder looked like a very difficult maze for the fish to navigate. The dam was originally constructed to help boats cross the river and was called the chain dam because there was a link chain across the river to prevent boats from falling over the edge of the dam. The entrance to the ladder was surprising small – only 2 meters or so wide. It is hard to believe that a fish could find it, let alone actually decide to enter it.
Apparently the ladder was added to the dam in 1992. One year fish tallies were made to assess how many fish actually use the ladder. 645 fish made it up the ladder from March 31-June 7. The tallies were achieved by using cameras that recorded the ladder 24 hours a day. The tapes were then looked through by interns, and the fish were each counted. Must have been about as exciting as counting sheep.
I observed a fish or possibly an eel that seemed to be stuck to one of the concrete walls. I wonder how many fish die in the ladder each year, or from injuries sustained while trying to navigate the maze? Perhaps another job for the interns….
Turtles, Turtles, and More Turtles
Today after class, I went for a run on the canal path. Usually I don’t pay much attention, but today I happened to look into the canal and saw a painted turtle basking in the sun! I continued to look for them while I ran. I saw another immediately…and then another right after! So I decided to count and by the end of my run I had seen 36 turtles! I have always assumed that the murky, algae-covered waters of the canal couldn’t possibly contain any life besides some bottom feeders. However, after a little research I discovered that painted turtles are extremely common in Pennsylvania, and they prefer slow-moving water and ditches where algae and insects are abundant. So in contrast to what I had thought, the canal is probably really good habitat for them.
It was a damp afternoon; it had been raining for most of the day and humidity was still in the air when I returned from my run and took some time to recover by stretching my legs. Seating myself on the driveway as I stretched, I noticed a six-inch-long earthworm winding its way slowly across the asphalt. I have often seen dead, desiccated earthworms on asphalt and concrete surfaces –my understanding of the phenomenon is that earthworms burrow up out of the ground during heavy rain to avoid being drowned as the soil becomes saturated. Usually, a few individuals seem to have the misfortune of ending up on impermeable asphalt or concrete surfaces and dying before they are able to find their way back to moist soil after the rain ends. I watched this particular earthworm as it progressed slowly across the porous surface of the asphalt, sticking its anterior end into each chink and crevice it came upon, apparently trying to burrow back down into the ground. Of course, each attempt at burrowing was thwarted by the impermeability of the asphalt. It was sad to watch the futility of the earthworm, guided by instinct, trying to burrow into this strange environment.
A quick Wikipedia search of earthworm physiology and behavior revealed the mechanism by which earthworms move through soil. These mechanisms appeared useless on asphalt—an environment the earthworm is not evolutionarily equipped to deal with. The earthworm’s slow pace brought to mind the issue of scale. The short distance of a driveway, traversable quickly in human terms, is for the tiny earthworm a much more formidable expanse. Watching the confused route the earthworm was taking, I couldn’t help but think it would probably live out the rest of its short life on that driveway, like so many of the poor, desiccated carcasses I had seen before. We often think of the effects that roadways and other manmade barriers have on larger animals like deer, but habitat destruction and fragmentation clearly operate at all scales, including the very small. By placing artificial barriers like roads or driveways in the middle of an earthworm habitat, could humans change the environmental conditions governing the evolution of earthworm populations? Islands of earthworm populations subject to the sorts of processes that take place on oceanic islands? Insular populations separated by concrete barriers?
A few days after my earthworm observations, we discussed the ecological role earthworms and other “soil biota” play in helping regulate soil health in my Science of Environmental Issues course. The soil is a habitat like any other, home to a diverse and extensive ecosystem of organisms – plants, fungi, arthropods, and microbes – that are linked to the aboveground ecosystem through the food web. By recycling nutrients like nitrogen and phosphorus and making them available to plants, soil biota help maintain soil quality and productivity. According to many sources, earthworms in particular help to increase water infiltration and water retention by soil through their tunneling and burrowing activities; they also mix and aerate the soil. These functions are thought to help to make soil more arable so that plant species to take root and develop more easily. But is this true? Apparently not all scientists agree.
Listening to the above NPR Science Friday podcast suggests a more nuanced perspective on the ecological significance of earthworms. It turns out that earthworms, like the one I saw exploring the topography of my driveway, may not be as innocuous or even as beneficial as I originally thought. Apparently, most earthworm species in the forests of the northeastern United States are non-native transplants from Europe and Asia and may be deteriorating soil quality in these ecosystems. It seems hard to believe, but the emblematic nightcrawler species, whose uses as bait and perceived importance in gardening make it such a visible part of American cultural life, is actually an invasive European species. Native North American earthworm species are not found in higher-latitude states because they were killed around 15,000 years ago when the land was glaciated. Considering that I live north of the glacial boundary, it is very likely that the earthworm I saw on my driveway was one of the interloper species.
Conventional wisdom among farmers and gardeners has long held that earthworms improve soil quality—exactly as I was led to believe from my previous class discussions. However, some scientists refute this claim, arguing that while the movement of earthworms has been observed to aerate compacted soil, leading to increased productivity, soil in most agricultural and forest systems is not compacted and receives adequate aeration without earthworm activity. Also, many studies that have attempted to show that earthworms enhance soil productivity in agricultural systems have been anecdotal. Although earthworm activity likely does help to accelerate the movement of water through soil, this may not always be a good thing for soil productivity, as too much earthworm activity can result in water leeching from the soil too quickly.
The main problem with non-native earthworm species in the forest ecosystems of the Northeast is that they consume the rich layer of organic matter that coats the forest floor. This organic “carpet” is essential for soil productivity and the health of the forest ecosystem, as it protects soil from erosion; provides proper environmental conditions for certain plant species, like spring beauty, trillium and trout lily, to take root; provides a habitat for certain animal species, like salamanders and beetles; and stores carbon and nitrogen, preventing the release of these elements into the atmosphere or nearby water systems. However, earthworm populations are extremely hard to remove once they’ve been established. Human efforts should probably be aimed at halting their spread rather than trying to eliminate existing populations. Education and outreach to people who use earthworms in different aspects of their daily lives, he said, is the most effective way to control the problem. Accidental transfer by humans is, of course, the way these organisms likely arrived in the non-native environment of North America to begin with–as is the case with so many invasive species.
All from watching a little earthworm trying to make its way through our world….