The Concrete Landscape

                Though they may appear only to consist of monotonous hues of gray, the concrete jungles of urban areas in fact provide a multitude of minute habitats open to those plants who are able to survive in the some of the harshest of conditions. These small habitats – unmaintained gardens, vacant lots, and even small cracks in sidewalks – are all open to colonization by the most opportunistic of pioneers – weeds. Populations of plants that do get established within these urban habitats then have to cope with being trodden on by commuters, heat radiating off of nearby surfaces, and higher levels of pollution, in addition to dealing with simply existing in fragmented habitats.

                Environments that consist of many smaller habitat fragments inherently provide limits to plant population growth. Space itself may be a limiting factor, as large trees and woody shrubs take up a large amount of space that is simply not available in individual small fragments. Separation between fragments may lead to less effective pollination, particularly if pollinators have trouble moving from one habitat fragment to another. Even seed dispersal – the process by which these weeds arrived in this environment in the first place – is limited by many seeds falling outside of suitable patches and not becoming established.  

Figure 1. Crepis sancta. From http://flore.la.rochelle.free.fr

                Crepis sancta, commonly known as hawksbeard, is one such plant that has successfully colonized many fragmented habitats within urban environments, and it has been the target of multiple studies investigating plants in these fragmented, human-dominated environments. C. sancta produces two types of seeds – those designed to be dispersed by the wind, and those designed to simply fall from the flower to adjacent patches of soil. Pierre-Olivier Cheptou and colleagues have studied populations in southern France to look for evidence of evolutionary response of C. sancta to fragmented, urban environments. In one study (Cheptou et al. 2008), they hypothesized that in largely concrete environments, dispersal-type seeds are unlikely to land in new, suitable areas, so producing more non-dispersal-type seeds would be an evolutionary adaptive trait in these fragmented habitats.

                Cheptou et al. first counted the overall proportion of C. sancta seeds that are likely to land in a patch of suitable habitat, and found that in fragmented habitats, fewer seeds ended up in viable areas, meaning that plants in fragmented habitats have higher costs of dispersal. In these fragmented, urban habitats, the authors found that populations of C. sancta produce a higher proportion of non-dispersing seeds compared to contiguous, rural populations. By creating mathematical models of the evolutionary processes that would allow for this shift in reproductive strategy, the authors estimated that the observed difference in non-dispersing seed proportion represents around 12 years of evolutionary divergence, a number that is consistent with construction records for concrete areas near their study populations.

Figure 2. Proportion of non-dispersing seeds of the study populations from Cheptou et al. (2008), showing how fragmented populations generally produce higher proportions of non-dispersing seeds than contiguous populations.

This study (Cheptou et al. 2008) was the first to quantify evolutionary changes in dispersal strategies – toward an increased proportion of non-dispersal-type seeds – showing that C. sancta in fragmented habitats have adapted to better persist in these largely urban areas. Interestingly, as the authors point out, this decrease in production of dispersal-type seeds implies that it is now harder for these plants to colonize new areas, which may exacerbate the impact of continued habitat fragmentation and urban development. If plants in urban environments become less able to disperse to new areas, these plants are likely to have increasingly patchy distributions and be less able to react to ongoing changes in climate. 

                In an ongoing research project here on the Grinnell College campus, my lab group is working to characterize the distribution of multiple plant species within a largely concrete patio environment. Our study area consists of a large courtyard enclosed on all sides by the Noyce Science Center, which serves to isolate this courtyard from much of the surrounding college campus. This courtyard is further divided into many small (less than half of a centimeter wide) cracks between bricks, and seeds that colonized this area were likely carried into the courtyard from elsewhere on the Grinnell College campus by the wind. If the plant populations in our study area have been subject to fragmented environments for multiple generations, dispersal should be very limiting to the distribution of these plants within the courtyard, resulting in patchy populations of these weed species. Alternatively, if the influx of new seeds from outside the courtyard is greater than the production of seeds within it, the distribution of plants will reflect small-scale habitat features that promote seedling germination and establishment, such as the size of the gap between paving stones. By measuring the distribution of plant species within this courtyard, my group aims to determine how isolated these plant populations are from those elsewhere on campus, and how the micro-habitats in the courtyard differ in their ability to support plant growth.

References:

Cheptou, P.-O., O. Carrue, S. Rouifed, A. Cantarel. 2008. Rapid evolution of seed dispersal in an urban environment in the weed Crepis sancta. PNAS 105(10): 3796-3799. 

Scattering Seeds & Colonized Courtyards

            When Noyce Hall was built in 2007, its courtyard contained only two species of woody foliage: a couple hornbeam trees planted near the edges and some serviceberry bushes. Looking at the same area in 2016, however, I discovered an unexpectedly diverse assortment of trees: scrawny sycamores popping up through the pavement, redbuds arcing up from the bottom of the courtyard’s slope, even a tiny little pine tree wedged between two wobbly stones in a retaining wall. The extent to which the seeds from these species – most of them recognizable from elsewhere on campus – had managed to infiltrate such a small and isolated habitat was impressive and confusing. How far, really, do seeds usually spread from their source, and how well do they really colonize microhabitats like this courtyard? Given the extent to which habitat fragmentation has become a part of the world we live in – especially here in Iowa, where the once-widespread prairie persists mostly as isolated patches – the ability of seeds to travel long distances between habitat could be crucial for species survival.

One paper by Amy B. McEuen and Lisa M. Curran, titled “Seed Dispersal and Recruitment Limitation Across Spatial Scales in Temperate Forest Fragments,” attempts to answer some of the questions about seed dispersal in fragmented habitats. Noticing that most studies of plant seed dispersal focused either on single trees or on dispersal within an individual, continuous area of habitat, McEuen and Curran decided to study how seeds spread among multiple small, separate patches of forest, and how this influenced species composition. In five small forest remnants within an area in Southeastern Michigan, they sampled species present and used traps to collect “seed rain” (the seeds that fall onto a given area) over a two-year period. They found that many species did not disperse well between sites, and those that did generally produced large amounts of small, wind-dispersed seeds.

Image: Unsuccessful vs. successful distribution for two species. This figure shows the seed trap results for two species (Carpinus caroliniana, American hornbeam, above, and Fraxinus pensylvanica, green ash, below), where an X represents a trap where the species’ seeds where found and a dash represents a trap where the species was not found (contour lines represent an approximation of dispersal patterns). On the right – both sites where the species in question was already common – the seeds of the species were abundant, but little dispersal seems to be occurring in the images of Site B on the left. As Site B was a location where neither of these two species was already well-established, this supports the paper’s conclusions that species had difficulty dispersing between sites.

Just because a species had many seeds landing in a given site, however, didn’t necessarily mean that the seeds were growing well – the authors noted that many of the species abundant in their seed rain samples had few to no saplings in the area, a reminder that colonization depends on more than just dispersal. It seems likely, however, that increased habitat fragmentation can still have a large impact on which species spread between areas, to the detriment of species that depend on animals to spread their seeds, or that produce large seeds that do not disperse very far.

This paper seems to suggest an interesting avenue of inquiry that relates to my own research on the tree colonization history of the courtyard in Noyce Hall: of the woody species present in this microhabitat, how many are wind-dispersed as opposed to dispersed via other methods? How large are their seeds? While my research focuses on the colonization history of only a couple of the courtyard’s most common invaders (Sycamores and mulberries, to be specific), it’s important to keep in mind the question of how and why this particular set of species ended up in this unusual microhabitat in the first place. Although many of the species are not native to the area – having likely spread from cultivated individuals elsewhere on campus – the small size and isolation of this habitat makes its array of saplings and small trees surprising. The narrower focus of my own studies doesn’t stop me from wondering why these species are the most abundant courtyard trees in the first place – and what that can tell us about which species, in our increasingly patchwork ecosystems, are most likely to persist.

Work Cited

McEuen, A.B., & Curran, L.M. (2004). Seed dispersal and recruitment limitation across spatial scales in temperate forest fragments. Ecology, 85(2), 507-518. Retrieved from  http://www.jstor.org/stable/3450214

Between the Cracks: Plant Diversity, Abundance, and Root Health in Paved Areas

"Why doesn't Grinnell have more sidewalks?" students at Grinnell College frequently question while navigating the town. Although paved areas create smooth paths, pavement often creates problems for the surrounding ecological community. While ecological concerns likely did not cause lack of sidewalks in Grinnell, the effect on the surrounding ecological community might be a positive benefit. For instance, pavement decreases the filtration of storm water that would otherwise percolate through the soil. Additionally, paved surfaces reduce gas exchange between the soil and the air. This air exchange may be important for plants to maintain healthy root systems. Some companies marketed paved surfaces with permeable properties because of the negative environmental impacts of paved surfaces. These permeable surfaces allow for greater gas exchange and water filtration. The authors of the paper thought that these properties of permeable pavement might lead to healthier plant root systems for mature Sweetgum trees (Figure 1).

Figure 1. A mature American Sweetgum tree, Liquidambar styraciflua (Arbor Day Foundation).

This paper studies Sweetgum trees to determine the effects of pervious and impervious pavement on their root structure. The scientists created a mini underground laboratory with windows into the soil profile in order to study Sweetgum root structure. They found that pervious pavement did not positively affect any of their measures of Sweetgum performance in comparison to impervious pavement or no pavement at all (Figure 2). 

Figure 2. Mean cumulative (a) new root production, (b) dead root length production and (c) net root length production per cm² tube window until 80 cm depth through time. Open symbols are root production in plots with no pavement (control), black symbols are root production in plots with impervious pavement, and grey symbols are root production in plots with pervious pavement

However, the researchers found that the diameter of the tree trunks were unaffected by pavement installation. The researchers concluded that the negative effects of pavement on Sweetgum root performance likely result from both types of pavements’ prevention of gas exchange between the soil and surrounding air.

The Sweetgum article considers potential factors that may affect my group’s results for our project on the cracks between bricks. Besides crack width and the differences between edge and inner cracks, the differences in soil-air gas exchange could play a role in the performance of plants living in brick crack communities. Additionally, these plants may be nurtured by storm water lacking filtration. In the paper, the authors studied the root systems of ornamental trees. In contrast, my group will only identify the presence and abundance of shoots that arrived on their own (Figure 3). 

 

Figure 3. Different species of plants observed to grow in the cracks of the Noyce Courtyard, my group's experimental site.

My group will not study the root system, which might limit us from observing the negative effects of pavement. The work on pavement and Sweetgum trees shows that the choice to pave and the type of material used tangibly affects surrounding plants. Rather than asking about the effect of different types of concrete on root systems, my group is studying pavement structure by asking how different crack widths affect the diversity and abundance of plant communities.

References

Arbor Day Foundation. 2016. American Sweetgum. Liquidambar styraciflua.

Volder, A., B. Viswanathan, and W.T. Watson. 2014. Pervious and impervious pavement reduce production and decrease lifespan of fine roots of mature Sweetgum trees. Urban Ecology 17: 445-453.

Monarch Butterflies Respond to Habitat Fragmentation

Monarch butterflies choose where they want to live

Monarch butterflies, Danaus plexippus, have sturdy wings which allow them to fly many miles and choose their own habitat. In the spring, monarchs choose to fly north in order to lay their eggs on suitable milkweed plants. In the fall, monarchs sense the cooling temperatures, and respond by flying south to sunnier climates, aggregating in southern Florida, Mexico, and Southern California. In order to fuel these long flights, the butterflies must find flowers with sugary nectar along the way. The monarchs have migrated for thousands of years, but lately their population has been shrinking. Habitat fragmentation is one explanation for their shrinking population; their habitat has become increasingly fragmented as humans build houses, roads, cities, and farm large portions of the United States. A fragmented habitat may be more challenging to navigate than an unfragmented habitat.

Figure 1: Monarch butterfly migration scheme. 

Experimental Habitat Fragmentation in Ohio

In the summer of 1999, Summerville and Crist investigated the impact of habitat fragmentation on butterfly abundance and species diversity near Oxford, Ohio. In June of 1999, they divided the land into 25 separate plots and mowed created habitat fragmentation by mowing various spaces (Figure 2). On six different sampling days, from June to August, they visually monitored each plot for butterflies.  They found that butterflies preferred the patches with the most flowers. Additionally, they found that butterflies preferred the patches with the largest habitat area.

Figure  2: Summerville and Crist used 25 plots of land, measuring 15mx15m for each plot. The mowed area is represented in white, while the intact habitat areas are shaded gray.

Habitat Fragmentation at Grinnell College

In October of 2016, a group of three Ecology students at Grinnell College set out with the question, do monarch butterflies travel between fragmented habitats? To answer their question they observed the flight and landing patterns of monarch butterflies on planted prairie patches  on the Grinnell College campus (Figure 3).  They hope that their experiment will help them better understand where monarch butterflies like to spend their time. Based on Summerville and Crist’s findings, they expect to find more monarchs in the larger patches and patches with the most flowers. However, they should consider that the fall timing of their study may impact their results, as the monarchs have already started migrating south for the winter.

Figure 3: Layout of the Grinnell College project. The prairie patches are divided by a sidewalk and mowed grass. To left of the picture are railroad tracks, and to the right of the picture are athletic fields.

References

Summerville, K. S., & Crist, T. O. (2001). Effects of experimental habitat fragmentation on patch use by butterflies and skippers (Lepidoptera).Ecology, 82(5), 1360-1370.

Concrete Jungles of the Noyce Courtyard and North-Western France

      From song lyrics to film titles, the term concrete jungle is often used ironically to refer to urban areas filled with buildings and sidewalks rather than trees or other plants. However, given the many plants to be found in urban landscapes, it is clear that the two are not mutually exclusive. The relationship between urban environments of concrete and steel and the species of plants that continue to grow in them is the subject of plenty of research by urban ecologists. Many people expect that the diversity of plant species and the well-being of the plants themselves would decrease as environments develop from wild forests to towns and cities, but research suggests that those assumptions don’t capture the complexity of plants’ abilities to occupy urban landscapes. The paper Plant species response to urbanization: comparison of isolated woodland patches in two cities of North-Western France explores the relationship between plants and their urban and rural environments in depth, and also connects to research I’ve done on Grinnell’s campus examining the effect of pavement on which plant species occur in a courtyard in the Noyce Sciencec Building.

      The aforementioned paper by Vallet et al. built off of previous research showing that urban environments tend to be home to more non-indigenous species and plants adapted to high levels of disturbance. The researchers in Vallet worked in 22 sites across two cities in France, all of which were then characterized along an urban to rural gradient. The classification of each site is shown in Fig. 1.  Scientists measured each site for species abundance, as well as measurements of light, moisture, soil pH, and soil nitrogen content. The researchers found a marked difference in plant composition across proportion of sites with impervious surfaces (i.e. concrete); namely three non-native species and seven native species were associated with larger cover of impervious surfaces. Vallet et al. mentioned research by other teams showing that concrete can increase nitrogen content and affect pH, causing some plants to perform quite well when growing near concrete. They concluded that some plant species, both native and non-native, are able to withstand changes to their environment due to urbanization, and many urban plants thrive in the conditions caused by abundant concrete.

      These findings also have applications to a much smaller concrete jungle: the courtyard near the elbow of the Noyce Science Center in Grinnell (Fig. 2). The courtyard serves as a study space in the science building, and many plants were introduced to populate the garden spaces, but some species colonized the pavement cracks on their own. We surveyed the cracks to see if the presence and abundance of certain plant species are affected by the width of the sidewalk cracks (a measure of both permeability and soil space). The research by Vallet et al. shows that we shouldn’t be looking at the concrete in the courtyard purely as an obstacle to plant growth; some species thrive better near concrete than in rural environments. Although the analysis of native and introduced species is not as applicable to our research, since many of the plants in the cracks of the sidewalk are descendants of the ornamental plants in the courtyard, and although our research space is of a much smaller scale, their findings on the effects of concrete still have implications for us. Concrete jungles exist all around us, and the plants that live there are often doing just fine.

Figure 2. We took to the Noyce Courtyard to measure the species abundance and width of sidewalk cracks                        

Vallet, J., H. Daniel, V. Beaujouan, F. Roze. 2008. Plant species response to urbanization: comparison of isolated woodland patches in two cities of North-Western France. Landscape Ecology 23:1205-1217.

Blocking the way: The effect of physical barriers on seed dispersal and colonization

How do new species appear in isolated habitat patches, especially in urban settings where growing space is sparse and plant sources are often few and far between? High levels of disturbance, a common feature of urban settings, often result in more accessible space and greater light availability, and provide prime habitat for invaders to colonize. The composition of these urban flora is determined by several factors, including the endemic species that were present in the original environment, and the addition of new species to an area by human interventions (Williams et al., 2009). Therefore, the floristic composition surrounding a patch will likely have a relatively strong influence on what is able to colonize it. But, in order to successfully colonize a habitat patch, seeds must first be able to make it through to the patch itself.

            A recent study by Cutway & Ehrenfeld (2010) tackles the idea of patch boundaries, by examining the effect of the surrounding urban area on the invasibility of a patch. They compared seed dispersal, through measures of seed rain and seed banks, between wetlands bordered by residential areas, and wetlands bordered by industrial areas. Overall, higher quantities of dispersal through seed rain were found in the residential adjacent wetlands, though there were no differences in total numbers of species found between the two sites. This difference between the residential and industrial wetlands is most likely due to the differing edge structures between these two areas, as seen in Figure 1.  

The wetlands adjacent to residential areas have a much more open edge structure than those adjacent to industrial areas, which are much more dense and overgrown, making it more difficult for seeds to colonize these wetlands compared to the more open ones.

            The Noyce Courtyard is a small, isolated habitat patch that was established in 2007 within the Noyce Science Center at Grinnell College. The Courtyard is completely enclosed, with the walls of Noyce surrounding it for at least two stories on all sides. Although the initial composition of this patch was determined by humans, species that were not original planted have begun to colonize the area. A recent study has worked to investigate these newcomers, with the aim of determining when colonization occurred, as well as how and from where the seeds traveled. Based on the findings of Cutway & Ehrenfeld (2010), the walls of Noyce should have formed an even more effective barrier to seed dispersal into this habitat patch than the dense edges of the industrial wetlands. However, the seedlings of many woody plants, including sycamore (Platanus occidentalis) and white mulberry (Morus alba) can now be found within the Courtyard. The seeds of these two species are dispersed through the air, by wind and by bird. Therefore, although the walls of Noyce may act as a barrier to certain types of seed dispersal, the Courtyard itself is relatively open, providing a potentially favorable habitat for those few seeds that are able to make it through.

References

Cutway, H.B., & Ehrenfeld, J.G. (2010). The influence of urban land use on seed dispersal and wetland invasibility. Plant Ecology, 210(1), 153-167.

Williams, N.S.G., Schwartz, M.W., Vesk, P.A., McCarthy, M.A., Hahs, A.K., Clemants, S.E., . . . McDonnell, M.J. (2009). A Conceptual Framework for Predicting the Effects of Urban Environments on Floras. Journal of Ecology, 97(1), 4-9.

             

DARE TO ENTER: Interpatch movement of the migratory monarch butterfly, Danaus plexippus

Habitat fragmented by road construction. Photo credit: Notice Nature

The roads you travel on may help get you home, but it is likely that they prevented some animals from reaching theirs. As a scientist eloquently put it: “Roads scare the hell out of ecologists” (quoted in Nijhuis, 2015) and there is reason for such fear. Human modifications to the environment through roads, development, and agriculture have been major drivers of habitat fragmentation. This form of habitat loss is one of the primary threats to wildlife in the United States. Fragmentation not only results in the loss of the total amount of suitable habitat, but also compromises the remaining habitat for animals. This process makes it difficult for migratory species to find resting sites and places to feed along their migration routes. Movement between these patches is critical for their survival and can help “rescue” these populations from local extinction and the negative effects of inbreeding depression (i.e., reduced fitness from breeding with relatives). In spite of the importance of mobility and emigration, few studies address how easily animals can move through varied environments—i.e., the rate of interpatch movement. To gain a better understanding of local extinction risks and patch occupancy, it is important to examine the willingness of an organism to leave a patch and their behavioral response to encountering a patch’s edge. Whether they choose to approach it, avoid it, or cross it, affects their dispersal success (and thus, their ability to survive) across varying landscapes. 

Such decisions are crucial for animals like the migratory monarch butterfly (Danaus plexippus). As individuals journey to overwintering sites in Mexico, they encounter miles of landscape heterogeneity. How do monarchs respond to fragmented environments? Previous research by Ries and Debinski (2001) helped to address this question. Their findings piqued our interest in studying edge effects on our campus monarchs.

Specifically, Ries and Debinski (2001) wanted to compare the responses of two butterfly species to edges in fragmented prairies and whether their responses were affected by variables like wind, flower abundance, and time of year. In this study, the researchers focused on two species, Speyeria idalia and D. plexippus: a non-migratory habitat specialist and a habitat generalist, respectively. Using plots in eleven prairies with different boundary types (crop, treeline, roads, fields), Ries and Debinski tracked the behavior of the individuals to quantify their edge response (i.e., proportion that exited patches and proportion that returned to the plot interior; turning frequency to avoid crossing edges). They found that the frequency that D. plexippus would cross edges was strongly affected by wind direction and time of year; individuals were more likely to leave prairie plots later in the summer and when the wind was blowing towards the boundaries of the study sites. Both species were less likely to cross over treeline edges- but the non-migratory S. idalia was a lot more sensitive to subtle changes in vegetation and avoided crossing edges adjacent to row crops. These findings demonstrate that habitat edges can be barriers to movement. The interspecies variability in leaving or remaining in patches shows that environmental factors can influence and modify edge response, regardless of edge type—further emphasizing the importance of local conditions in analyzing species movement.

What does this mean for our local population of Danaus plexippus at Grinnell College? Those individuals that visit the reconstructed prairie patches near the athletic fields encounter various types of edges (concrete paths, turf, railroads). 

Campus study site: Bowers Prairie patches near Springer Field. The patches border a concrete walkway, turf-grass lawns, and a railroad. Scale: Soccer field measures 120 x 75 yards.  Aerial image from Mapquest.

In our campus project, we are attempting to investigate the variation in interpatch movement among monarchs and how local variables like flower abundance, area size, and distance between neighboring patches might affect their flight patterns and visitation frequency in certain plots. We hope to use the results discussed in Ries and Debinski (2001) to inform our project and create a specific framework of how the landscape (near the Bear Recreational Center, at least) will affect small-scale movements in these monarchs. Our study may lend support for a plan to make campus landscaping more butterfly-friendly. But we are also optimistic about informing larger-scale applications such as roadside plantings. Is it possible to combat the harmful effect of roads by providing better feeding and resting sites for pollinators like D. plexippus? We invite you to think about that on your next trip down I-80.

REFERENCES:

Nijhuis, M. (2015, March 20). What Roads Have Wrought. The New Yorker, Retrieved from http://www.newyorker.com/tech/elements/roads-habitat-fragmentation. 

Ries, L., & Debinski, D. M. (2001). Butterfly responses to habitat edges in the highly fragmented prairies of Central Iowa. Journal of Animal Ecology, 70(5): 840-852.