Nurse Plants: Can They Restore the World One Fragment at a Time?

Did you ever watch the show, "Extreme Couponers" on TLC? I am not sure if it still comes on, but if you have not seen it context clues should tell you that it was great television. Needless to say it is a fascinating display of human resourcefulness, and definitely not exploitation of unsuspecting mothers for mass consumption. Besides that, these people sort for hours and cut coupons that result in them bringing home 7 carts of groceries for less than $18.00 all because their family fell on hard times and couponing became a way to make ends meet. So, for them, couponing provided a cozy cushion to support their family in a time of desperate need. And in this way, nurse plants are kind of like coupons. Nurse plants provide nutrients, shelter, and protection in stressful environments. They can be introduced in extreme habitat conditions to help restore an ecosystem, like those suffering from habitat fragmentation. In this blog post I will discuss the ways in which nurse plants can help restore fragmented habitats as well as potential downfalls of these particular plant-plant interactions.

Plant-plant interactions can be competitive, mutualistic, or even parasitic. Most often when we think of plants interacting in the same space we think competition, especially when that space has been disturbed and destroyed leaving very little behind. However, there is research that suggests that plants can facilitate growth of nearby plants and actually work together under higher levels of abiotic stress (Anthelme, Gómez-Aparicio, & Montúfar 2014; Stachowicz 2001; Aerts, et al. 2006). I dug through 8 articles that talk about these facilitative plant-plant interactions in fragmented habitats. Results from the research tells us that for the most part nurse plants can be an extremely helpful tool in the restoration process, but there are many factors that come into play that can affect whether the outcome is positive or negative.




NURSE EXPECTATIONS 
Nurse plants can have major impacts on biodiversity and community structure through positive plant-plant interactions according to all 8 articles. They serve important functions (as you can see in the chart below) within environments that have experienced lots of stress or have been disturbed by human inflicted habitat fragmentation.

From the research that is currently available, we can read about all of the benefits like: reducing soil erosion, protecting seedlings, reducing herbivory, and providing lots of shade in the face of more extreme weather patterns (Stachowicz 2001; Howe & Miriti 2004). For instance, if the nurse plant is very popular with the nearby pollinators, some of those pollinators just might stop by the little guys and further help the growth of the facilitated species. 1 of the 8 articles reported that there was "high community structure" in the patches where nurse plants were implemented, and the results showed a positive relationship in patches with a focal nurse plant species than in those without (Nuñez et al. 1999). The study also showed a steady increase in diversity over time (Nuñez et al. 1999).

It has been stated in two of the studies, that the shade from the the nurse plant provides lots of shade especially in very dry conditions where seeds have shelter from the sun and can have a greater chance for germination (Valladares & Gianoli 2007; Howe & Miriti 2004). Anthelme, Gómez-Aparicio, & Montúfar report on results from experiments in the Andean tropical forest they saw that seedlings left without a nurse plant or artificial shade were all dead within 10 months. But the results from nurse tussock grass caused reduced soil temperature and composition which made for soil more suitable to grow in; the nurse grass also caused reduced stress from lack of water and reduced stress from herbivory (Anthelme, Gómez-Aparicio, & Montúfar 2014).

Introducing a focal nurse plant changes soil composition, which can lead to richer soil and increase biodiversity over time (Nuñez et al. 1999). Restoring these patches could increase connectivity and promote more spread of plant species and increase biodiversity. However, one study did find that facilitative effects may not be long term, because the positive effects began to fade with time (Anthelme, Gómez-Aparicio, & Montúfar 2014).


 

FACILITATION VS. COMPETITION
So, we can see from these articles that nurse plants can have a major impact on a fragmented ecosystem. Now, we will look at potential downfalls of the nurse plant. While facilitation is a fascinating display of plant-plant interactions, it can sometimes be too much of a good thing. You can be an extreme coupon-er (is that a word?) but at some point you are just spending way too much money on toilet paper, deli-meat, and jello than you actually need, and the cost outweighs the benefits. The 200 packs of chocolate pudding that you purchased for your family of 5 to snack on will most likely expire before it can be consumed, and that is not an example of resourcefulness, that is wasteful.

In fact, 2 of the 8 articles I reviewed found that in some experiments, nurse facilitation was not advantageous at all (Galindo, et al. 2017; Padilla & Pugnaire 2006). Some of these cases reported negative effects after introducing nurse plants. 1 of the 8 articles reported that nurse plants placed can out-compete native species once both plants have grown to a stable size (Galindo, et al. 2017). Padilla and Pugnaire found in their research that nurse plants in fragmented habitats were advantageous for the facilitated plant in "optimal conditions," but when a drought came, all the plants died (Padilla & Pugnaire 2006). They also discovered that in fertile soils, the nurse plants depleted the land of its nutrients and there was less biomass (Padilla & Pugnaire 2006). From studies like this, we can see that there may not be any advantages from nurse plants in restoring habitats.

TAKEAWAYS
To find a one size fits all answer to the problem surrounding habitat fragmentation would be wonderful, but sadly it is not realistic. There are far too many plant responses in addition to the varying conditions of each ecosystem to take into account; so, it is best to combine different techniques for the most productive outcome. It is so imperative that "caveats regarding species and site characteristics" are taken into account because they could have a major impact on the productivity and restoration of a fragmented habitat (Padilla & Pugnaire 2006).




WORKS CITED

Aerts, Raf, Wouter Maes, Eva November, Aklilu Negussie, Martin Hermy, and Bart Muys. "Restoring dry Afromontane forest using bird and nurse plant effects: Direct sowing of Olea europaea ssp. cuspidata seeds." Forest Ecology and Management 230.1-3 (2006): 23-31.

Anthelme, Fabien, Lorena Gómez-Aparicio, and Rommel Montúfar. "Nurse-based restoration of degraded tropical forests with tussock grasses: experimental support from the Andean cloud forest." Journal of Applied Ecology 51.6 (2014): 1534-543.

Galindo, Víctor, Zoraida Calle, Julian Chará, and Inge Armbrecht. "Facilitation by pioneer shrubs for the ecological restoration of riparian forests in the Central Andes of Colombia." Restoration Ecology (2017).

Howe, Henry F., and Maria N. Miriti. "When Seed Dispersal Matters." BioScience 54.7 (2004): 651.

Nuñez, Cecilia I., Marcelo A. Aizen, and Cecilia Ezcurra. "Species associations and nurse plant effects in patches of high-Andean vegetation." Journal of Vegetation Science 10.3 (1999): 357-64.

Padilla, Francisco M., and Francisco I. Pugnaire. "The role of nurse plants in the restoration of degraded environments." Frontiers in Ecology and the Environment 4.4 (2006): 196-202.

Stachowicz, John J. "Mutualism, Facilitation, and the Structure of Ecological Communities." BioScience 51.3 (2001): 235.

Valladares, Fernando, and Ernesto Gianoli. "How Much Ecology Do We Need to Know to Restore Mediterranean Ecosystems?" Restoration Ecology 15.3 (2007): 363-68.


  
Picture 1: http://rainforests.mongabay.com/facts/rainforest-facts.html

Picture 2: https://www.researchgate.net/figure/265053296_fig1_Figure-1-A-conceptual-framework-for-the-abiotic-and-biotic-mechanisms-of-nurse-plant

Pollination Disruption in Fragmented Habitats

The year is 2042, Oprah Winfrey is president of the United States, the balance between man and Earth has been rightfully restored, and it has been a long two years since you graduated from university. Only one of these statements seems unrealistic, and I can tell you it is NOT the one about Oprah. Your mother is recently very proud of you because you aren't sleeping on your twin-sized Buzz Lightyear sheets at home anymore. You have secured a respectable job that allows you to pay back those hefty student loans in addition to putting some cash towards a proper vacation once the summer rolls around. A lot of hard work and sleepless nights have gone into the past few years of trying to get to where you are now, so you want to take a vacation to relax and unwind. A national park camping trip in California sounds nice, but so does a tropical beach trip in Cambodia. Getting away from the never-ending stress of everyday life to the serene outdoors sounds like the sweetest- and most rewarding- dream. However, the sad truth about the year 2042 is that many vacation destinations have fallen victim to steady habitat fragmentation and destruction due to growing popularity of Earth's natural wonders. So, while you wanted to zip-line through a tropical forest in the middle of July, you could potentially just be zip-lining over: log cabins, winding roads, themed restaurants, and the tiny remnants of what is left of that tropical forest.

Anthropogenic habitat fragmentation, as I discussed in my first post, is happening at a more rapid rate than effected populations can adapt to (Lienert, J. 2004) and has major impacts on plant-pollinator interactions. Fragmentation can put a major damper on the health and biodiversity most ecosystems are trying to maintain. This has been shown especially true when reviewing the important interactions between pollinators and their plants. So, throughout this paper I will discuss how fragmented habitats impact pollination patterns.

LONG DISTANCE RELATIONSHIPS
Long distance relationships are hard. Whether your (human) partner lives halfway across the country or (your plant partner) is just across I-40 and Turkey Creek. When I think of pollinators and land disruption, the first thing that pops into my mind is the increase in distance that the pollinator will have to travel to get these plants. Three of the studies relayed that bees, butterflies, and moths can fly over long distances to food sources. However, there was a study that noted male bees could not make it across a fragmented landscaped of 100 meters, so the data really varies from study to study (Didham et al. 1996). Below the graph on the left depicts pollinator visits in two different areas of a forest (Valdivia 2011). We can see that in this study there were far less visits at the edges of the fragmented forest than in the connected mainland. On the right we can see lack of fruit and seed growth in plants depending on their location in a fragmented forest (Valdivia 2011).
 
Likewise, three of the papers showed a decrease in "the most common pollinator" and as you got closer to the edges of these fragmented landscapes, the plants there were visited less often and in one study these plants exhibited a smaller fruit and seed set (Steffan-Dewenter, Ingolf and Teja, Tscharntke. 2002). Carlos Valdivia noted from his study that plants at the edges of the forest patch have larger petals and foliar displays than flowers on the mainlands, and even with a larger display, they still receive less traffic. The effects of fragmentation and lack of pollination over patches of habitat do not bode well for plant-pollinator interactions.

SPECIALIZATION IN PLANT + POLLINATOR INTERACTION
 

Plants are pollinated in many different ways. They can cross-pollinate by wind, water, or use animals for fertilization. Wind and water pollination make up a much smaller percentage by which plants reproduce, but they are worth mentioning. The reason that animal or insect pollination is particularly cardinal is because plants and their pollinators have co-evolved as long as plants and animals have interacted, and in this fact, many plants have intricate structures made especially for a particular type of pollinator. Plants are often visited by multiple species of pollinator, "but a critical level of pollinator species richness is needed for successful pollination and maintenance of diverse plant communities" (Steffan-Dewenter, Ingolf and Teja, Tscharntke. 2002). And according to one of the studies, if a plant is thriving from the pollination of a small set of species, it would be more difficult for that plant and that pollinator to survive in a fragmented environment (Skogen et al. 2016).

If a species of plant is pollinated by a specialized insect or animal, then the problem of separation and decline in plant number can elicit a significant change in pollinator population number. These pollinators require a certain amount of pollen, nectar, and nesting sites to be provided by their environment in order to survive, and if their environment is being disturbed or destroyed, some of their previously supplied resources may be in limited amounts (Steffan-Dewenter, Ingolf and Teja, Tscharntke. 2002).  Oftentimes, a generalist can swoop in and take over for the previous pollinator, but new pollinators may not be as effective (Kwak et al. 1998). If the insect or animal is not a generalist species, then it will have two choices: the pollinator can look for a new species of plant to feed from, or it can die off.

LOSS OF PLANT DIVERSITY
Pollinators are important drivers of diversity in the ecosystem by providing a mutualistic relationship to crops and wild plants all around the globe (Potts et al. 2010). Two out of eight papers noted, roughly 80% of all flowering plants reproduce with the help of an animal pollinator. So naturally, in a fragmented habitat, the loss of plants and plant diversity can lead to a major change in pollinator abundance and diversity. Lots of insects are tied to specific plants, for instance the monarch butterfly and milkweed plants. Monarchs use these plants to feed on in order to grow. If there is no milkweed, there is no monarch. These interactions are important to take note of because the effects can impact all humans on many levels. We depend on plants as a source of oxygen, for nutrients, to feed animals that we eventually eat,  and we also depend on plants for recreational purposes. I mean, what would the back porch of a cabin be without a full forest to drink your morning coffee to?

Four of the eight sources explicitly note that more research needs to be done on the topic of plant-pollinator interactions specifically in disturbed habitats. Much of the literature shows that both variables are affected, but a lot of the results are varied and it is also possible that some of the results are due to abiotic factors inhibiting plant growth.

TAKEAWAYS
The major takeaways that I got from my research is that: 1. we should do our best to preserve the natural land that we still have and come up with plans to help communities impacted by fragmentation, 2. more experiments need to be conducted on plant-pollinator interactions in these disturbed environments, and 3. we should take a pledge to elect Oprah as president in 2040.



WORKS CITED

Didham, Raphael K., Jaboury Ghazoul, Nigel E. Stork, and Andrew J. Davis. "Insects in fragmented forests: a functional approach." Trends in Ecology & Evolution 11.6 (1996): 255-60.

Ferreira, Patrícia Alves, Danilo Boscolo, and Blandina Felipe Viana. "What do we know about the effects of landscape changes on plant–pollinator interaction networks?" Ecological Indicators 31 (2013): 35-40.

Kwak, Manja M., Odilia Velterop, and Jelte Van Andel. "Pollen and gene flow in fragmented habitats." Applied Vegetation Science 1.1 (1998): 37-54. JSTOR.

Lienert, J. 2004. Habitat fragmentation effects on fitness of plant populations – a review. Journal for Nature Conservation 12:53–72.
 
Potts, Simon G., Jacobus C. Biesmeijer, Claire Kremen, Peter Neumann, Oliver Schweiger, and William E. Kunin. "Global pollinator declines: trends, impacts and drivers." Trends in Ecology & Evolution 25.6 (2010): 345-53.  

Skogen, K. A., T. Jogesh, E. T. Hilpman, S. L. Todd, M. K. Rhodes, S. M. Still, and J. B. Fant. "Land-use change has no detectable effect on reproduction of a disturbance-adapted, hawkmoth-pollinated plant species." American Journal of Botany 103.11 (2016): 1950-963. 

Steffan-Dewenter, Ingolf, and Teja Tscharntke. "Insect communities and biotic interactions on fragmented calcareous grasslands—a mini review." Biological Conservation 104.3 (2002): 275-84.

Valdivia, Carlos. "Negative effects of forest fragmentation and proximity to edges on pollination and herbivory of Bomarea salsilla (Alstroemeriaceae)." Plant Ecology and Evolution 144.3 (2011): 281-87. 


Picture: https://abinashpadhi.wordpress.com/2011/06/30/butterflies-in-hobbs-state-park-arkansas/