The Effects of Urbanization on Tree Patch Dynamics

In urban areas, plant communities are greatly affected by how humans decide to develop the surrounding areas.  These plant communities exist in several forms: their original state, a state completely altered in composition by gardening, or even a state altered unintentionally through random side effects, such as changes in the soil pH from nearby roads.  Urban development also affects how successful plants are at dispersing their seeds to other areas.  For example, seeds dispersed in the middle of a parking lot would not be able to grow into trees because they are obstructed by concrete, which lacks the proper resources for growth.  In addition to that, weeding also affects dispersal by reducing the number of ‘nuisance’ plants that grow in yards and other public spaces.  In general, all these changes made by urban development create distinct patches of where plants are able to thrive.

The figure above was taken from the Zipperer et al. (1997) paper.

The paper “Urban tree cover: an ecological perspective” by Wayne Zipperer et al. (1997) explains the significance of evaluating the tree cover in different tree patches that occur within an entire urban landscape rather than only in small portions of it (as researchers have done previously).  Because little was known about ecological processes of urban systems at the time the paper was written, the authors wished for future researchers to approach the problem through analysis of vegetation in different patches of trees.  They proposed that these researchers determine the differences between the patches through their origin (untouched or planted), structure of vegetation (e.g. leaf size or canopy cover), landscape configuration (building or patch locations), and level of management.  One of the vegetation structures that they looked at was species richness which, in one study, was determined to decline due to clearing for development, but had the potential to increase quickly as new species were planted.  With this and future research, the authors would be able to create simulation models that would estimate ecological changes in urban areas.

Link to paper: http://link.springer.com/article/10.1023/A:1018587830636

This picture shows the tree patch that we are studying.  It is located on the Grinnell campus between Noyce and the Forum.

The topics presented in this paper contain components that are prevalent in our current study.  We are attempting to discover how microhabitat affects tree sapling dispersion in a single patch between Noyce and the Forum.  Both we and the authors want to determine the amount of tree cover, as it affects the survival of dispersed seeds.  However, we are only looking at a single patch rather than for the entire urban area.  In our case, we are looking at a patch that has a woody-understory (one of the two types that Zipperer et al. wished to study).  One of the things that they proposed was to determine whether or not the patches had been managed or had been allowed to remain in a more natural state.  Our study focuses on a patch that is partially managed, but has seen lack of recent tending.

In addition to our project, this paper has stimulated research in at least 102 other projects.

References

Zipperer, W.C., S.M. Sisinni, R.V. Pouyat, T.W. Foresman, and T.W. Foresman. 1997. Urban tree cover: an ecological perspective. Urban Ecosystems. 4:229-246

One Man's Trash...

One Man’s Trash…

An all too common issue that we often hear about when discussing urban areas and urban ecology is habitat destruction. As humans move into an area, we often make those areas unsuitable for other species to live in. We convert over 4 million square kilometers of rural land to urban areas every year. One way people try to fix this issue is by preserving small areas of animal habitat in rural areas, but even this leads to the issue of habitat fragmentation, causing changes in what lives in the area and the community structure. Thus, in order for conservation efforts to be more successful, research must be done on how urbanization has affected and is affecting community dynamics and species density. 

Urban raccoon foraging at its finest.

. Retrieved from 

http://www.huntingtonny.gov/images/

EWM/Racoon%20In%20Trash%20Can.jpg

So, how can we look at this issue with some common urban mesopredators? Mesopredators are medium sized predators that benefit from the elimination of larger predators, and I’m sure everyone is familiar with one specific masked menace. Raccoons are often seen rummaging through garbage, or roaming around in alleys looking for garbage to rummage through, especially in urban areas. So, are these fuzzy foragers simply making the best of a bad situation, or do they choose to live in urban communities? A study by Prange and Gehrt in 2004 determined that raccoons, and to some extent opossums and skunks, may actually benefit from urbanization. They looked to see if these three species had a different response to urbanization by live trapping and studying the number of road-kills in each of several study areas around Illinois, which varied in their degree of urbanization. They found that there were a lot more raccoons than anything else, and more raccoons were found at the urbanized sites than other sites. They concluded that raccoon populations reach much higher densities in urban areas. This could be because raccoons are more efficient at using anthropogenic resources than skunks and opossums. According to Prange and Gehrt, it’s all in their grabby little dexterous paws, which allow raccoons to knock stuff over more easily. You can find their study here: http://eds.b.ebscohost.com/ehost/pdfviewer/pdfviewer?sid=b72bc8da-f962-46fc-82ff-b02ca56eb702%40sessionmgr115&vid=2&hid=104 

In my ecology class, my lab partner and I decided to carry out an experiment on raccoon foraging in a semi-urban area.  We now know that raccoons are typically found in greater densities in urban areas, and can easily exploit resources generated by humans. With this in mind, we hope to find out whether their foraging behavior is affected by the amount of human traffic in the area or the distance from a road. Judging from this research it seems that raccoons don’t really have issues living in habitats modified by humans, but we thought it would be interesting to see if they preferred secluded urban areas to exposed ones, or if they take what they can lay their grubby mitts on.

Too Easy For Crows

Too Easy for Crows

            The urban ecosystem is one that we often forget to consider when we think of ecology. It is much easier for our minds to associate nature with views of vast green fields or lush forests full of wildlife, but, for me, it is an awesome change of pace to see the same complex organization of trophic levels right in the place where I live. Due to humans continuing to diminish natural habitat, more and more animals and plants have to make tough decisions about their survival. Recently, my lab partner and I started an experiment on raccoon foraging in urban areas that showed evidence of these kinds of decisions. The next day we found that crows had eaten all of the eggs we used as raccoon bait! Since we do not live in an area that is by any means overpopulated with crows, this event turned my attention to what caused the crows to forage so heavily in our research area.

Being in an open space forces crows to be more vigilant;
 thus, forage less. However, it is much easier to spot resources
in open spaces, which makes foraging more efficient.
Retrieved from http://texascrowpatrol.com/Regulations.html

            A study by Ward and Low in 1997 examines the impact of different aspects of the urban environment on crow foraging based on crow vigilance (http://www.jstor.org/stable/4163843?seq=1).

They observed and measured the vigilance of the crows in an urban college town, much like ours. Since vigilant behavior and foraging are mutually exclusive, observing the amount of time a crow spends scanning its environment (being vigilant), tells a lot about its foraging patterns. The authors found that the crows became more vigilant as human disturbance increased; thus, foraging decreased. In addition, many factors such as group size, time of day, and duration of the day’s precipitation showed an effect on crow vigilance and foraging. However, the effect most applicable to our situation was that decreases in the distance from cover protective cover caused for greater foraging.  This could explain why the crows would be particularly comfortable foraging in our study site that has plenty of tree cover.

Eggshells are all that remain after the crow's visit. It was just too easy.

In fact, our study site is better than just having good tree cover: it has very few shrubs creating almost no visual obstruction between the eggs on the ground and any bird flying under the tree tops.  Further, the property is located just off of one of the least busy streets in our small town. Though we are attempting to use this street for a treatment of human traffic, we have created an almost perfect foraging environment for the crows: easy targets with low risk required. It is no wonder they got to the eggs before the nocturnal mammals did. So to all of you reading this and trying to decide where to lay your eggs, I would suggest finding a spot where the crows need to be a little more vigilant. I’m sure even the crows would appreciate a challenge.


Reference:

Ward, C. and B.S. Low. 1997. Predictors of vigilance for American Crows foraging in an urban environment. The Wilson Bulletin 109: 481-489.

Why should we care about beetles and their response to the changing urban landscape?

            With increased urbanization comes increased disturbance of natural habitats for organisms around the globe. As skyscrapers, vast concrete surfaces, small green areas, and landscaped parks and gardens become more and more common, organisms have to find ways to survive in new and fragmented habitats. Many scientists have struggled with the difficult question of how urbanization will affect different and ecologically important species; however, it is difficult to generalize about the effects of urbanization. Therefore, it is important to understand this concept in a localized way since there are a variety of factors that come into play, including the scale of urbanization, the intensity, the geographic location, and the natural ecology of the area. Grinnell College’s campus, which is set in the middle of rural Iowa, is teeming with biodiversity. From butterflies to grasshoppers to vultures to native prairie plants, there is a lot to be studied. Grinnell College is a moderately urbanized area with lots of green space, landscaped gardens, as well as reconstructed prairie patches. Our research team was interested in understanding the local effects of urbanization on Grinnell’s campus.

            In Dr. Michael L. McKinney’s study, “Effects of urbanization on species richness: A review of plants and animals,” he dives into some complex questions about the effects of urbanization on species richness (or the number of different species) in different taxonomic groups (2008). In an attempt to answer these difficult questions, he reviews a total of 105 studies on plants, mammals, reptiles, amphibians, and invertebrates with low, moderate, and high levels of urbanization. While there is significant evidence in the literature for a consistent decrease in species richness with increased urbanization, there are some studies that show a surprising increase in species richness with intermediate levels of disturbance. In his review, McKinney interestingly found that the majority (65%) of the urbanization studies done on plants show an increase in species richness going from low to moderate levels of urbanization. However, this was not the case for vertebrates and invertebrates. Plants were the only taxa studied that showed a peak in species richness at moderate levels. All other taxa had species richness peaks in low urbanization settings. Additionally, there was no evidence for an increase in species richness going from moderate to high urbanization levels across all taxa. McKinney suggests here that human importation of non-native plants for landscaping purposes is the main reason for this trend in species richness this importation will outpace native species extinctions and lead to increased species richness at moderate levels of urbanization. This is not the case for other taxa; however, there are a lot of factors that affect an organism’s response to urbanization and it may differ widely on a species by species basis.

              

(Carabidae Caminara: http://commons.wikimedia.org/wiki/File:Carabidae_Caminara_Starred_ground_beetle_(Calosoma%3F).jpg)

            Carabid beetles (ground beetles), the focus of our research, have been studied as a group of organisms that can be used as environmental bioindicators (Rainio et al. 2003). Bioindicators are species that can be observed in order to determine the effects of environmental disturbance in an area. Bioindicators often mirror the responses of other organisms as well as overall biodiversity. In order to try and understand how Grinnell’s Carabid beetle populations are responding to moderate levels of urbanization, we looked at the difference in species richness and diversity between restored prairie habitats on campus and landscaped garden habitats. While each of these habitats is fragmented, one is a remnant of the native Iowa landscape, and one is the result of human landscaping. It will be interesting to compare how ground beetles respond to these two different types of vegetation in a moderately urbanized setting. In accordance with McKinney’s findings, we predict that species diversity and abundance will probably decline in the landscaped areas, which simulate more urbanization, as opposed to the restored prairie, which simulates rural and native areas.

  

                                               (Reconstructed prairie at Grinnell College)

References:

McKinney, M.L. 2008. Effect of urbanization on species richness: A review of plants and animals. Urban Ecosystems 11 (2): 161-176.

Rainio, J., Niemela, J. 2003. Ground beetles (Coleoptera: Carabidae) as bioindicators. Biodiversity and Conservation 12 (3): 487-506.

On the relationship between Pulliam (2000) and urban ecology

Like many ideas in biology, the ecological niche is often misunderstood, making it difficult to learn and challenging to teach, both in my experience as a biology professor and in the data of biology education researchers.

One reason why the niche concept is difficult is that in ordinary use "niche" often means a place. It's tempting to think of a species' niche as the same thing as its geographic distribution. That view contains two subtle misconceptions. The first has to to with a theoretical perspective. Most contemporary ecological niche concepts, starting with Hutchinson (1957), consider a niche to be a mathematical space (the multivariate distribution of resources and conditions within which a species population can persist), not a geographic space, per se. The fact that real resources and conditions occur in geographic spaces makes this idea hard to get one's mind around. The second misconception has to do with the emerging recognition that populations can occur in places--maybe temporarily, and perhaps only as long as they receive immigrants from elsewhere--where environmental variables lie outside the species' niche. Populations also might not occur in places where they populations could sustain themselves just fine, if they could only get there.

Sorry, little ash trees. Just because you germinated in a pavement crack doesn't mean you can sustain a population there.

H. Ronald Pulliam's "On the relationship between niche and distribution" (2000) does a masterful job of explaining the above distinctions. Among other routes, Pulliam does this by showing that demographic processes and dispersal link the niche to distributions. A species' niche is defined as the multivariate environmental set in which populations' finite rates of increase, lambda, are greater than or equal to 1. Dispersal from locations with high growth rates may maintain populations in "sinks" where lambda < 1. Dispersal limitation may keep populations from occupying sites that where lambda (would be) > 1.

The article has three sections. The first sets up the problem and introduces a perspective that makes sense of it. I've reproduced this section's key figure below. A key point is that in Fig. 1, the points are not in geographic space.

Pulliam then analyzes a simulation model (NICHE) in which a species with a two-dimensional niche (in mathematical space) and a defined dispersal ability is introduced into a two-dimensional landscape grid (the geographic space), Grid cells are assigned, at random, values of the two niche dimensions, values that may or may not lie within the species' niche. Pulliam explores the model to identify situations in which distributions reflect niches closely and those in which they don't. (While I love this article, I wish Pulliam also had explored a few more realistic landscape structures with some degree of spatial autocorrelation in the environment: patchiness.)

Finally, Pulliam reviews the limited evidence (as of 2000) on the extent to which natural species distributions correspond to most of, more than, or less than the geographic distribution of suitable habitat, and he identifies data worth collecting. My colleagues and I are trying to collect the right data in long-term studies of distribution, demography, and adaptation in the California annual plant, Clarkia xantiana (my ResearchGate page). But that's another story for another blog.

What does Pulliam's perspective add to studies of organism distribution and abundance, the focus of my Ecology students' independent projects on the Grinnell College campus? I had my students read Pulliam's paper earlier in the semester. I hope they'll think about whether, for their study organisms (e.g., ground beetles; hardwood tree saplings; "ovivores" such as crows and raccoons; ragweed and sweet clover), the campus represents a source environment or a sink environment, and how their dispersal ability (possibly human-assisted) affects their small-scale distributions. In a rapidly urbanizing world, increasing attention to the niches and distributions of urban occupants makes sense.

Urban Ecology

When we are asked where we lived before moving to Grinnell, my wife Liz and I often give the specific--yet teasingly uninformative--reply that we've lived within 3 miles (~ 5 km) from Interstate 80 for the last 30 years.(Deeper history: I was an "I-10" child in the LA area, and Liz is from Oregon; I-84 was her home highway.) Though our past and present I-80 residences roughly share a latitude, Salt Lake City (PhD studies at the University of Utah), Berkeley (postdoctoral research at the University of California), and Grinnell (real jobs) don't seem much alike. Aside from the obvious differences in the dominant view (the Wasatch Front, the Golden Gate Bridge, and a whole lot of corn and beans, respectively), our third I-80 residence is on the small side.

The Joe Rosenfield Center at Grinnell College-
one of Grinnell's tallest buildings and a whole lot of masonry

Hence my students in Grinnell College's BIO 368 ([advanced] Ecology) raised their eyebrows a bit in response to their Urban Ecology lab assignment, in which I ask them to design and complete independent projects on the distribution and/or abundance of organisms on the Grinnell College campus. Grinnell doesn't fit everyone's image of "urban," but as our neighbors hastened to point out when we arrived, it's not the country. The distribution and performance of organisms (and the movements of energy and matter) here clearly depend strongly on variation in conditions and resources created by humans acting as mutualists, enemies, disturbance agents, and “ecosystem engineers” in this built environment.

Most of the next several posts will give brief summaries of published articles that are relevant to my student's projects. First, though, I'd better try the assignment myself.

Committee Meeting

While nearly everyone at Grinnell may know (or at least know of) Joyce Stern, my guess is that rather few of us know her official title. I didn't, before I looked it up: "Dean for Student Success and Academic Advising." It was fitting, then, that Joyce recently told my wife and me another title (a collective noun, anyway) that we didn't know: a "committee" of vultures.

In early April, after the snow melted, a large committee of turkey vultures regularly roosted overnight in the bare sycamores south of the Noyce Science Center. My subjective impression--not backed up by data, but by an unreliable, middle-aged memory--is that the campus has become an increasingly important gathering place for "TVs" on their southward fall migration and northward spring return trip. The large spring gatherings seem to have dispersed, though solitary soarers remain commonplace.

Insert academic metaphor here.

The Iowa Mangrove Forest?

Destination Iowa (320 – 300 million years ago) – enjoy a lush marine environment where you are not yet threatened by dinosaurs and need not cope with allergies induced by flowering plants. Be sure to visit the thriving mangrove forests that straddle the border between land and sea.

The image of Pennsylvanian Iowa does not resonate with the state’s flat, corn-dominated landscape. However, evidence from petrified plants suggests that mangroves temporarily dominated the flora of central Iowa. These anatomically preserved plant fossils are present in coal balls, spherically shaped coal concretions. They formed through the process of permineralization, when water rich in the mineral calcium carbonate (CaCO₃) trickled into plant cells, resulting in structural preservation. Subsequent debris accumulated on top of the plants, freezing them in time.

Anne Raymond, a paleoecologist at Texas A&M, has spent her academic career documenting prehistoric plant life. With the knowledge that many plants disappeared abruptly in the late Pennsylvanian, she seeks to relate paleoclimate changes to the fossil record. In 1988, she published a study in which she convincingly argued the existence of fossilized mangroves in a collection of 78 coal balls retained from Iowa’s Urbandale mine. Known as cordaitaleans, these ancient mangroves composed 56 percent of identifiable coal ball debris. Conversely, freshwater tree ferns, including the seed plants Medullosa and Psaronius, represented 31 percent of debris, and lycopods composed five percent of debris (Raymond 1988). Raymond not only proposed the mangrove lifestyle of cordaitaleans but also developed a hypothesis for early plant succession in the Urbandale swamp.

Urbandale Cordaitaleans: Saltwater Inhabitants

To persist in their saltwater environments, mangroves have evolved root structures that limit salt intake and enable subsistence at tide’s edge. In many species, roots are coated with suberin, a rubbery material that prevents water from penetrating tissue (Werner and Stelzer 1990). Abundant prop roots systems keep mangrove trunks upright in soft marine sediments. Some mangroves even have pneumatophores, roots that extend out of the mud to take in air (Warne 2007). Likewise, cordaitalean fossils indicate evidence of massive root systems that served as a salt filtration system. Raymond found two indicators of swamp salinity in Urbandale coal ball cordaitalean fossils – the distribution of pyrite crystals and the ratio of shoot to root debris.

Present in sea water, sulfur-reducing bacteria allow for formation of iron-sulfide complexes, including pyrite (FeS₂). Raymond uncovered an abundance of this mineral in the Urbandale coal balls. Disseminated pyrite that occured both inside and outside roots indicated freshwater species that were unable to exclude saltwater. Pyrite that rimmed fossilized roots evidenced saltwater exclusion. Among the Urbandale coal balls, Raymond found that cordaitalean roots lacked internal pyrite. Thus, they remained alive throughout marine inundations. Conversely, pyrite saturated tree fern and lycopod roots, indicating their vulnerability to marine infiltration (Figure 1).

                                                   (a)                               (b)


Figure 1. (after Raymond 1988). (a) Salt water kills the roots of fresh water plants. Sulfate-reducing bacteria inside and outside the root produce sulfide, which combines with iron to form pyrite. (b) Salt-resistant plant roots in salt water exclude the salt. No pyrite forms inside these living roots.

 

Low cordaitalean shoot to root ratios further supported Raymond’s theory. Like modern mangroves, cordaitaleans living in a marine environment would have benefitted from extensive root tissue. To determine shoot to root ratios, Raymond superimposed a grid over peels from fifty Urbandale coal balls (a total sample surface area of 4299 cm²). After identifying the organ type (i.e. shoot or root) and plant form for the largest piece of debris in each square, she calculated the percentage of shoot debris for each coal ball. Interestingly, nearly twenty percent of Urbandale coal balls featured debris composed of only zero to ten percent shoot debris. As expected, these low-shoot coal balls contained primarily cordaitalean fossils. Conversely, coal balls with higher shoot percentages contained evidence of Medullosa, Psaronius, and lycopods – the freshwater plants. Thus, the taxonomic composition of low-shoot coal balls aligns with Raymond’s pyrite inferences, implying that Pennsylvanian cordaitaleans were mangroves.

The Urbandale Swamp Flora: A Freshwater-Saltwater Gradient?

If cordaitaleans were indeed ancient mangroves, did they grow in a different subenvironment from their freshwater counterparts? Did freshwater and saltwater plants live at the same time? In 1983, Raymond T.L. Phillips preliminarily approached this question when they analyzed the paleoecology of Iowa coal balls from three mines, including the Urbandale Mine. They discovered a gradient between cordaitalean debris and tree fern debris – within each coal ball, the freshwater plants’ roots were oriented closer to the cordaitalean debris than were their shoots. Based on this relationship, Raymond and Phillips suggested that tree ferns succeeded cordaitaleans in early Iowa swamp forests. However, their gradient analysis was largely quantitative and excluded lycopod fossils (Raymond and Phillips 1983).

In her 1988 study, Raymond completed a root-penetration analysis of the Urbandale coal balls that confirmed her earlier gradient theory. She tabulated the number of instances in which roots belonging to the four major taxa (cordaitaleans, Medullosa, Psaronius, and lycopods) penetrated debris of each of the other taxa. Consistent with the gradient analysis, root penetration data suggested that cordaitaleans colonized the swamp first and were replaced by Medullosa and Psaronius – in terms of root contact, the freshwater tree ferns grew through the cordaitalean debris ninety percent of the time (Figure 2a). Raymond further postulated that the two tree fern species coexisted; the percentages of Medullosa – Psaronius penetrations and Psaronius – Medullosa penetrations were both approximately fifty percent. Although lycopod fossils composed five percent of the Urbandale coal ball debris, Raymond found only root debris. This suggested that lycopods colonized Iowan landscapes following accumulation of cordaitalean and tree fern peat (Figure 2b).
    
 (a)                                                                     (b)


Figure 2. (after Raymond 1988). Peat accumulation and plant colonization in the Urbandale swamp. The amount of peat under each community corresponds to the percentage of debris uncovered in the coal balls. (a) Based on root penetration analysis, cordaitalean trees colonized first, followed by the freshwater tree ferns. (b) Reconstruction of the swamp following the lycopod colonization. Lycopod roots penetrate both cordaitalean and freshwater peat.

 

Iowa Mangroves: A Figment of the Pennsylvanian

After analyzing the pyrite patterns and shoot-to-root ratios of Urbandale coal balls, Raymond asserted that Pennsylvanian cordaitaleans lived mangrove lifestyles. Further, she completed fossil gradient and root penetration analyses to indicate that they preceded the colonization of freshwater tree ferns and lycopods. In light of major paleogeographical events, Iowa’s present lack of marine forests comes as no surprise. The Urbandale coal balls immortalize tropical flora that existed when North American, Europe, and Asia composed a single continent.

References

Raymond, A. 1983. Peat taphonomy of recent mangrove peats and Upper Carboniferous coal-ball peats. Ph.D. Dissertation. University of Chicago, pp 293.

Raymond, A. 1988. The paleoecology of a coal-ball deposit from the Middle Pennsylvanian of Iowa dominated by cordaitalean gymnosperms. Review of Paleobotany and Palynology. 53: 233-250.

Warne, K. 2007. Mangroves: forests of the tide. National Geographic Magazine. ncm.com.

Werner, A. and R. Stelzer. 1990. Physiological responses of the mangrove Rhizophora mangle grown in the absence and presence of NaCl. Plant, Cell, and Environment. 13: 243-255.