Alligators Increase Plant Diversity

Photo by mbarisson licensed under CC BY-ND 2.0.

When you think of gardening, alligators don’t readily jump to mind. Hang out long enough in places like the Everglades and that might change. I was only recently introduced to the concept of a “gator hole” and I must say, I was surprised what a quick search of the literature revealed. It turns out that alligators are important ecosystem engineers and do a wonderful job at increasing plant diversity in the wetlands they inhabit.

Throughout southeastern North America, gators change their behaviors with the seasons. During the rainy season, alligators can be found floating in open water or sunning themselves on land. Except when hunting, they don’t seem to do anything with much urgency. Their activity level changes during the dry season when water is in short supply. Gators don’t sit back and let nature take its course. They spring into action and create their own aquatic refuges.

As the surrounding landscape begins to dry, gators will excavate holes or pits in the soggy ground called gator holes. These holes hold onto water when most of the surrounding landscape isn’t. The process of digging a gator hole may seem destructive but it all must be placed in the context of the surrounding environment. Most gator habitat exists in low lying areas. In places like the Everglades, there isn’t much topography to speak of. When a gator excavates a gator hole, it creates variation in both hydrology and soil conditions.

Photo by Anita Gould licensed under CC BY-ND 2.0.

Soils that have built up over time are lifted out of the hole and piled into mounds. Mounded soils are not only rich in nutrients, they also dry at different rates, creating a gradient in water availability. Plants that normally can’t germinate and grow in saturated soils find suitable spots to live up on the soil mounds while emergent aquatic vegetation fills in along the parameter. Plants that normally prefer to grow in deeper water can also establish within the gator hole itself. In the midst of fairly uniform marsh vegetation, a gator hole quickly becomes a hotbed of plant diversity. The differences in vegetation can be so stark compared to the surrounding landscape that some scientists can actually map gator holes using aerial scans simply by measuring the differences in infrared radiation given off by the leaves of all the different plants that establish around them.

Of course, all of that plant diversity has a huge effect on other organisms as well. Gator holes become important areas for various reptiles, amphibians, birds, and so much more. The paths that alligators take to and from their holes even act like fire breaks, changing the way fire moves through the landscape, which only increases the heterogeneity of the immediate area. Fish, though occasionally eaten, greatly benefit from the stability of water levels within a gator hole. All in all, gator holes are extremely important habitats. Not only do they support a high diversity of plants and animals alike, they make places like the Everglades even more dynamic than they already are.

Photo Credits: [1] [2]

Further Reading: [1] [2] [3] [4]

The Cypress-Knee Sedge

Sedges (Carex spp.) simply do not get the attention they deserve. I am part of this problem because like so many others, I have breezed over them in vegetation surveys as “just another graminoid.” This is truly a shame because not only are sedges absolutely fascinating organisms, they are immensely important ecologically as well. I am working hard to get to know sedges better so that I too can fully appreciate their place in our ecosystems. One of the coolest specialist sedges I just recently learned about is the so-called cypress-knee sedge (Carex decomposita). For all intents and purposes, this sedge is considered something of an epiphyte!

The cypress-knee sedge has a fondness for growing on wood. Most often you will find it rooted to the buttresses and knees of bald cypress (Taxodium distichum) or the swollen trunk of a swamp tupelo (Nyssa aquatica). It can also be found growing out of rotting logs that float on the surface of the water. It is a long lived species, with individuals having records stretching back through decades of wetland plant surveys. When supplied with the conditions it likes, populations can thrive. That is not to say that it does well everywhere. In fact, it has declined quite a bit throughout its range.

Juvenile cypress-knee sedges establishing in moss along the water line of a bald cypress.

One of the key wetland features that the cypress-knee sedge needs to survive and prosper is a stable water level. If water levels change too much, entire populations can be wiped out either by drowning or desiccation. Even before the sedge gets established, its seeds require stable water levels to even get to suitable germination sites. Each achene (fruit) comes complete with a tiny, corky area at its tip that allows the seeds to float. Floating seeds are how this species gets around. With any luck, some seeds will end up at the base of a tree or on a floating log where they can germinate and grow. If water levels fluctuate too much, the seeds simply can’t reach such locations.

Its dependence on high quality wetlands is one of the major reasons why the cypress-knee sedge has declined so much in recent decades. Aside from outright destruction of wetlands, changes in wetland hydrology can have dire consequences for its survival. One of the major issues for the cypress-knee sedge is boat traffic. Boat wakes create a lot of disturbance in the water that can literally scour away entire populations from the base of trees and logs. Another major threat are changes to upstream habitats. Any alteration to the watersheds of wetland habitats can spell disaster for the cypress-knee sedge. Alterations to creeks, streams, and rivers, as well as changes in ground water infiltration rates can severely alter the water levels in the swamps that this sedge depends on for survival.

Closeups of the infructescence showing details of the perigynia (fruit).

Less obvious threats also include changes in plant cover. If the wetlands in which it grows become too dense, the cypress-knee sedge quickly gets out-competed. To thrive, the cypress-knee sedge needs slightly more sunlight than a densely forested wetland can provide. In fact, some have even noted that cypress-knee sedge populations can explode after selective logging of such wetlands. Such explosions have been attributed to not only extra sunlight but also the addition of woody debris, which provides much needed germination sites. That being said, such explosions can only be maintained if woody debris is left in place and further wetland disturbances do not continue.

The plight of the cypress-knee sedge stands as a reminder of just how poorly we treat wetlands around the globe. Aside from providing valuable ecosystem services for the human environment (flood control, water filtration, etc.), wetlands are home to countless unique species. Only by treating wetlands betters and attempting to restore some of what has been lost will we ever do better by wetland species like the cypress-knee sedge. Hopefully by showcasing species like this, people will begin to feel a little more compassion towards the ecosystems on which they depend. Please consider supporting a wetland conservation and restoration initiative in your region!

Photo Credits : LDWF Natural Heritage Program [1] & Paul Marcum (Midwest Graminoides) [2] [3] [4]

Further Reading: [1] [2]

Botanical Buoys

Photo by Doug McGrady licensed under CC BY 2.0

American featherfoil (Hottonia inflata) is a fascinating aquatic plant. It can be found in wetlands ranging from the coastal plains of Texas all the way up into Maine. Though widespread, American featherfoil is by no means common. Today I would like to introduce you to this gorgeous member of the primrose family (Primulaceae).

American featherfoil may look like a floating plant but it is not. It roots itself firmly into the soil and spends much of its early days as a vegetative stem covered in wonderful feathery leaves. It may be hard to find during this period as no part of it sticks above the water. To find it, one must look in shallow waters of ponds, ditches, and swamps that have not experienced too much disturbance. More on this in a bit.

Photo by Doug McGrady licensed under CC BY 2.0

American featherfoil lives life in the fast lane. It is what we call a winter annual. Seeds germinate in the fall and by late October, juveniles can be seen sporting a few leaves. There it will remains throughout the winter months until early spring when warming waters signal the growth phase. Such growth is rapid. So rapid, in fact, that by mid to late April, plants are beginning to flower. To successfully reproduce, however, American featherfoil must get its flowers above water.

The need to flower out of water is exactly why this plant looks like it is free floating. The flower stalks certainly do float and they do so via specialized stems, hence the specific epithet “inflata.” Each plant grows a series of large, spongy flowering stalks that are filled with air. This helps buoy the stems up above the water line. It does not float about very much as its stem and roots still anchor it firmly into place. Each inflorescence consists of a series of whorled umbels that vary in color from white to yellow, and even violet. Following pollination, seeds are released into the water where they settle into the mud and await the coming fall.

Photo by Doug McGrady licensed under CC BY 2.0

As I mentioned above, American featherfoil appreciates wetland habitats that haven’t experienced too much disturbance. Thanks to our wanton disregard for wetlands over the last century or so, American featherfoil (along with countless other species) has seen a decline in numbers. One of the biggest hits to this species came from the trapping of beavers. It turns out, beaver ponds offer some of the most ideal conditions for American featherfoil growth. Beaver ponds are relatively shallow and the water level does not change drastically from month to month.

Historically unsustainable levels of beaver trapping coupled with dam destruction, wetland draining, and agricultural runoff has removed so much suitable habitat and with it American featherfoil as well as numerous wetland constituents. Without habitat, species cannot persist. Because of this, American featherfoil has been placed on state threatened and endangered lists throughout the entirety of its range. With the return of the beaver to much of its former range, there is hope that at least some of the habitat will again be ready for American featherfoil. Still, our relationship with wetlands remains tenuous at best and until we do more to protect and restore such important ecosystems, species like American featherfoil will continue to suffer. This is why you must support wetland protection and restoration in your region!

Photo Credits: [1] [2] [3]

Further Reading: [1] [2]

In Search of a Parasitic Orchid

In this episode, In Defense of Plants goes looking for a tiny parasitic orchid called the autumn coralroot (Corallorhiza odontorhiza - http://bit.ly/2xQhzbc). It has no leaves and does not photosynthesize. Instead, it makes its living completely off of mycorrhizal fungi, digesting its hyphae within the cells of its highly derived roots. Along the way we meet plants such as:

 Music by: Artist: Ampacity

Track: Asimov's Sideburns

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Thanks, Ducks!

Photo by loren chipman licensed under CC BY-NC 2.0

Recent research suggests that certain duck species are crucial for maintaining wetland plant diversity in highly fragmented landscapes. Functioning wetlands are becoming more and more isolated each year. As more land is gobbled up for farming and development, the ability for plants to get their seeds into new habitats is made even more difficult. Luckily, many plants utilize animals for this job. Seeds can become stuck in fur or feathers, and some can even pass through the gut unharmed. What's more, animals can move great distances in a short amount of time. For wetland plants, the daily movements of ducks seems to be paramount. 

By tracking the daily movements of mallards, a team of researchers from Utretch University were able to quantify how crucial these water fowl are for moving seeds around. What they found was quite remarkable. In autumn and winter, the diet of mallards switches over to seeds. Not all seeds that a mallard eats get digested. Many pass through the gut unharmed. Additionally, mallards are strong flyers. On any given day they can travel great distances in search of winter foraging grounds. In the evenings, they return to roosting sites with a high degree of fidelity. 

[SOURCE]

The research team was able to demonstrate that their movements cover even greater distances in highly fragmented landscapes. It's these daily migrations that are playing a major role in maintaining plant diversity between distant wetlands. This is especially important for wetlands that function as roost sites. Whereas mallards distribute around 7% of the surviving seeds they eat among foraging sites, that number jumps to 34% for surviving seeds at roost sites. Given the sheer number of mallards on the landscape, these estimates can really add up. 

It is likely that without mallards, North American wetlands would be much less diverse given their increasingly isolated nature. However, not all seeds are dispersed equally. Small seeds are far more likely to pass through the gut of a duck unharmed, meaning only a portion of the plant species that grow in these habitats are getting a helping hand (wing?). Still, the importance of these birds cannot be overlooked. The next time you see a mallard, thank it for maintaining wetland plant diversity. 

Photo Credits: [1] [2]

Further Reading: [1]