About this Journal Submit a Manuscript Table of Contents
International Journal of Ecology
Volume 2011 (2011), Article ID 704084, 8 pages
http://dx.doi.org/10.1155/2011/704084
Research Article

Trees and the City: Diversity and Composition along a Neotropical Gradient of Urbanization

1Laboratorio de Ecología de Restauración, Centro de Investigaciones en Ecosistemas, Campus Morelia, Universidad Nacional Autónoma de Mexico, Campus Morelia, Antigua Carretera a Pátzcuaro 8701, 58190 Morelia, Michoacán, Mexico
2Laboratorio Genética de la Conservación, Centro de Investigaciones en Ecosistemas, Universidad Nacional Autónoma de Mexico, Campus Morelia, Antigua Carretera a Pátzcuaro 8701, 58190 Morelia, Michoacán, Mexico
3Red de Ambiente y Sustentabilidad, Instituto de Ecología, A.C. Antigua carretera a Coatepec 351, El Haya, 91070 Xalapa, Veracruz, Mexico

Received 30 December 2010; Revised 24 May 2011; Accepted 29 June 2011

Academic Editor: L. Smith

Copyright © 2011 Rubén Ortega-Álvarez et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

In this study we assessed tree species richness, density, and composition patterns along a gradient of urbanization of a megacity. Our results show that total, native, and exotic tree densities were highest in green areas where larger spaces are considered for greening purposes. Conversely, total, native, and exotic tree species richness were highest in land uses with intermediate levels of urban development (residential, residential-commercial areas). Not finding highest tree species richness in less developed urban areas suggests that cultural factors may shape the array of species that are planted within cities. Supporting this, tree composition analyses showed that green areas are comprised of different tree species when compared to the rest of the studied urban land uses. Thus, our results suggest that, to increase the ecological quality of cities, residents and managers should be encouraged to select a greater variety of trees to promote heterogeneous green areas.

1. Introduction

Natural habitats are transformed by urbanization processes to satisfy housing needs and support human activities [14].Such man-made systems include physical, biological, and social processes and result in the development of urban infrastructure such as buildings and streets/roads, often leaving little space for vegetation [5]. Among urban green patches trees commonly comprise the main component, highly represented by exotic species that can become invasive in periurban areas [6]. Thus, urbanization implies not only the alteration of habitat structure, but also the modification of the diversity and composition of its vegetation component [7].

Urban vegetation, comprised of plants from parks, greenways, median strips, playgrounds, cemeteries, gardens, and sidewalks have important ecological and social implications [810]. In particular, trees are critical for the maintenance of some urban ecological processes [11] and have been identified as the most important habitat component known to affect wildlife diversity within urbanized systems [1215]. Additionally, aggregated trees in recreational areas (e.g., parks, playgrounds) allow the interaction of people, although they have also been related to socioeconomic barriers [16, 17].

In this study we examined changes in tree species richness along a neotropical gradient of urbanization in the Metropolitan Area of Mexico City (hereafter Mexico City), assessing patterns of native and exotic species. We also evaluated tree density and composition patterns along this gradient to provide a context for our species richness results. We expected tree species richness to be similar between residential and residential-commercial areas. In contrast, we predicted low species richness and high tree density in urban green areas. Finally, we expected to find a high number of exotic species in all the studied urban land uses (i.e., green areas, commercial areas, residential-commercial areas, commercial areas).

2. Study Area

We performed this study in Mexico City, one of the most populated urban areas in the world [18] (Figure 1). This city covers a current area of >1000 km2, houses a human population that surpasses 20 million inhabitants [19], and has an annual population growth of 0.8% [20]. Although the establishment and continuous growth of this megacity have negatively affected wildlife, it still holds considerable biodiversity values [2123]. Similar to other Latin American cities, Mexico City is represented by four main urban land uses: (1) commercial, (2) residential, (3) industrial, and (4) green areas.

704084.fig.001
Figure 1: Map of study area. Green: green area, Comm.: commercial area, Res.: residential area, R.-C.: residential-commercial area.

We focused our sampling effort in the central and southwestern section of the city, where industrial areas are practically absent. We established a gradient of urbanization considering the four major urban land uses, from green to commercial areas, including residential and residential-commercial areas. To discriminate among highly developed urban areas (i.e., residential, residential-commercial, commercial areas) and green areas, we followed the classification of urban development proposed by Marzluff et al. [24]. We used the presence of commercial lots to differentiate between commercial and residential urban land uses, and determined areas as residential-commercial where both residential and commercial components were present. As reported in a previous study [23], the four studied urban land uses represent a gradient that affects birds, from less developed (green areas) to highly developed urban sites (commercial areas).

3. Methods

3.1. Tree Surveys

We surveyed trees within 25 m radius circular plots at green, commercial, residential, and residential-commercial areas (modified from [25]) during June-July 2007. We sampled 30 plots within each urban land use, separated by a minimum distance of 250 m to represent independent sampling units, giving a total of 120 plots along the gradient of urbanization. We located survey plots along our study area to meet two main criteria: (1) that the land use was homogeneous in surrounding areas, and (2) that they were heterogeneously distributed along our study area. At each plot, we recorded the number of trees and identified each individual to species level or classified them as morphospecies.

3.2. Data Analysis

To compare tree species richness among the studied urban land uses, we calculated average tree species richness and 95% confidence intervals using EstimateS [26]. This approach allows comparison among treatments using accumulated abundance, as average species richness is calculated by the repeated resampling of all pooled samples [27]. We calculated tree density per hectare by extrapolating the number of trees recorded at each plot (0.19 ha) to one hectare. To compare the calculated species richness and density values, we contrasted their 95% confidence intervals [28], assuming statistical differences with nonoverlapping intervals (α < 0.01; M. E. Payton, pers. comm.). Finally, we used a Bray-Curtis multivariate cluster analysis to compare tree taxonomic composition of the studied land uses [29]. As community diversity patterns could be obscured by the origin of the recorded tree species, we also conducted species richness, density, and composition analyses for each land use distinguishing between native and exotic species. We considered all species that dwell within the southern region of the valley of Mexico, the area in which Mexico City is located, as native sensu [30], while we considered all others as exotic.

4. Results

We recorded a total of 89 tree species, with 48 in green areas, 64 in residential areas, 43 in residential-commercial areas, and 39 in commercial areas (Table 1). Of the total recorded species, 55 were exotic (61.8%), 30 were native (33.7%), and 4 morphospecies remained uncertain (4.5%). Total tree species richness differed among the studied land uses when compared at a constant calculated abundance (524 individuals based on the lowest abundance recorded in residential-commercial areas), with highest values in residential areas (49.0 ± 8.1 computed species), followed by residential-commercial areas (44.0 ± 7.3 species), commercial areas (32.3 ± 6.3 species), and green areas (20.2 ± 4.8 species; Figure 2). This pattern changed when we analyzed native and exotic species separately. On the one hand, native species richness was highest in residential areas (17.42 ± 4.69 species), followed by residential-commercial areas (13.0 ± 3.38 species), and lastly by green and commercial ones (8.8 ± 3.4 and 6.9 ± 3.0 species, resp.; Figure 2). On the other hand, exotic tree species richness was highest in residential, residential-commercial, and commercial areas (30.7 ± 6.0, 31.0 ± 6.4, and 23.9 ± 5.2 species, resp.) and lowest in green areas (11.3 ± 2.8 species; Figure 2).

tab1
Table 1: Tree species recorded along the studied gradient of urbanization. Numbers represent the total number of individuals recorded in this study. Species are displayed alphabetically. (N): native species, (E): exotic species (see Section 3 for details). Green: green areas, Res: residential areas, Res-Com: residential-commercial areas, Com: commercial areas.
fig2
Figure 2: Computed tree species richness for the studied urban land uses. Values represent average values (±95% confidence intervals) computed at 524, 281, and 243 individuals for total, exotic, and native species, respectively. Letters represent statistical differences. Green: green areas, Res: residential areas, ResCom: residential-commercial areas, Com: Commercial areas.

Total tree density also differed among the studied urban land uses, with highest values recorded in green areas (  trees/ha), followed by residential areas (  trees/ha), commercial areas (  trees/ha), and residential-commercial areas (  trees/ha; Figure 3). The same pattern was observed for exotic tree density, with highest values recorded in green areas (  trees/ha), followed by residential areas (  trees/ha), commercial areas (  trees/ha), and residential-commercial areas (  trees/ha; Figure 3). The density of native species showed highest values in green areas (  trees/ha), while no differences existed among the rest of the studied land uses (residential:  trees/ha, residential-commercial:  trees/ha, commercial:  trees/ha; Figure 3).

fig3
Figure 3: Total, exotic, and native tree species density (ind/ha) average values (±95% confidence intervals) for the studied urban land uses. Letters represent statistical differences. Green: green areas, Res: residential areas, ResCom: residential-commercial areas, Com: commercial areas.

Finally, total tree species composition was more similar among residential, residential-commercial, and commercial areas (>50% similarity), than green areas, which in turn showed an average dissimilarity of 75% with the rest of the studied land uses (Figure 4). Composition among land uses showed a similar pattern for both native and exotic tree species, with residential and residential-commercial areas exhibiting the highest similarity value among all land uses, followed by commercial areas, and finally by green areas (Figure 4).

704084.fig.004
Figure 4: Abundance-based Bray-Curtis multivariate cluster analysis depicting tree composition similarity among the studied urban land uses. GA: green areas, RES: residential areas, RES-COM: residential-commercial areas, COM: commercial areas.

5. Discussion

Along the studied neotropical gradient of urbanization of Mexico City, total, native, and exotic tree species richness were highest in residential and residential-commercial areas, which represent intermediate levels of urban development. Thus, the species richness pattern recorded in this study was consistent with the intermediate disturbance hypothesis [31]. This pattern is different from that observed for other wildlife groups (e.g., birds, mammals, reptiles), which exhibit higher species richness in less developed sites, such as green areas [23, 32]. Our results show that higher species richness of both native and exotic trees in Mexico City is related to the residential component of land uses, suggesting that residents promote tree species richness by choosing diverse tree species for their gardens and homesteads. Thus, tree species richness seems to be molded by cultural forces, as well as by landscaping, horticultural, and recreational practices [33]. Lower tree species richness in land uses with commercial components was not surprising, as the space available for planting trees is often reduced. In the case of green areas, trees are usually used to achieve greening purposes, which could lead to the creation of stands comprised by few tree species. Thus, native and exotic tree species richness were low within such urban land uses. Moreover, some green areas have original vegetation patches (e.g., shrublands, fir forests; [34]) which are characterized by the presence of a limited number of tree species.

Although total tree density also varied among urban land uses, the pattern was contrastingly different to that found for tree species richness. In fact, highest total, native, and exotic tree densities were recorded in green areas. This result was expected, as larger spaces within these areas are used for greening purposes [35] and are generally managed by the city council. Residential areas showed less total, native, and exotic tree densities than green areas, but exhibited higher values than residential-commercial and commercial areas. This could result from socioeconomic considerations, reflecting the preponderance of gardens in high-income residential areas [36, 37], as people who live in these sites tend to appreciate and afford greener neighborhoods. Similar to our tree species richness results, land uses with commercial components (i.e., residential-commercial, commercial areas) had the lowest total, native, and exotic tree density values, mainly because a major proportion of the land is occupied by urban infrastructure components.

Our cluster analyses reinforce the point that residents can shape the diversity and density of total, native, and exotic trees in residential areas, even within those that contain commercial components [38, 39]. Higher taxonomic similarity of urban land uses that include residential components could result from a similar incorporation of both exotic and native species by residents. Differences among the latter land uses and commercial areas could emerge as local managers tend to prefer exotic over native species for aesthetic purposes in commercial sites. Finally, green areas were highly different from the rest of the studied land uses, likely due to higher densities of few species (e.g., Gum Trees—Eucalyptus spp., Mexican Cypress—Cupressus lusitanica, Mexican Ash—Fraxinus uhdei).

Although few studies have analyzed the variation of tree species richness along gradients of urbanization, our results agree with them, indicating that tree species richness, native or exotic, does not decrease with urbanization intensity [32]. Particularly, higher values of plant species richness have been reported within moderately urbanized areas, likely enhanced by different plant cultivation choices [32, 40]. Thus, resident preferences and decisions play a fundamental role in determining the vegetation component of urban systems. To increase the potential benefits of human activities on the ecological quality of cities, residents and green area managers should be encouraged to select for a greater variety of tree species to avoid monospecific stands, promote heterogeneous green areas that benefit different wildlife groups, and maintain several ecological processes within urban ecosystems [41]. Moreover, further efforts are needed to enhance higher densities of trees outside green areas (e.g., residential areas, residential-commercial areas), which could derive in benefits for urban wildlife and local residents.

Acknowledgments

The authors thank Susana Valencia and Ramiro Cruz for their valuable assistance on the identification of tree species and Mariela Gómez-Romero, Martha Bonilla-Moheno, Carlos Muñoz-Robles, and two anonymous reviewers for their helpful comments that improved the quality of our paper. As part of the Posgrado en Ciencias Biológicas of the Universidad Nacional Autónoma de México, R. Ortega-Álvarez and H. Hernando A. Rodríguez-Correa received a Master's (327503) and Ph. D. scholarship (329733) from CONACYT, respectively.

References

  1. R. M. DeGraaf, A. D. Geis, and P. A. Healy, “Bird population and habitat surveys in urban areas,” Landscape and Urban Planning, vol. 21, no. 3, pp. 181–188, 1991. View at Scopus
  2. P. M. Vitousek, H. A. Mooney, J. Lubchenco, and J. M. Melillo, “Human domination of earth's ecosystems,” Science, vol. 277, no. 5325, pp. 494–499, 1997. View at Publisher · View at Google Scholar · View at Scopus
  3. J. Jökimaki, “Occurrence of breeding bird species in urban parks: effects of park structure and broad-scale variables,” Urban Ecosystems, vol. 3, pp. 21–34, 1999.
  4. L. F. V. Leston and A. D. Rodewald, “Are urban forests ecological traps for understory birds? An examination using Northern cardinals,” Biological Conservation, vol. 131, no. 4, pp. 566–574, 2006. View at Publisher · View at Google Scholar · View at Scopus
  5. A. R. Berkowitz, C. H. Nilon, and K. S. Hollweg, Understanding Urban Ecosystems—a New Frontier for Science and Education, Springer, New York, NY, USA, 2003.
  6. H. R. Grau, M. E. Hernández, J. Gutierrez et al., “A peri-urban neotropical forest transition and its consequences for environmental services,” Ecology and Society, vol. 13, no. 1, article 35, 2008. View at Scopus
  7. J. S. Walker, N. B. Grimm, J. M. Briggs, C. Gries, and L. Dugan, “Effects of urbanization on plant species diversity in central Arizona,” Frontiers in Ecology and the Environment, vol. 7, no. 9, pp. 465–470, 2009. View at Publisher · View at Google Scholar · View at Scopus
  8. R. Costanza, R. D'Arge, R. de Groot et al., “The value of the world's ecosystem services and natural capital,” Nature, vol. 387, no. 6630, pp. 253–260, 1997. View at Publisher · View at Google Scholar · View at Scopus
  9. S. T. A. Pickett, M. L. Cadenasso, and J. M. Grove, “Resilient cities: meaning, models, and metaphor for integrating the ecological, socio-economic, and planning realms,” Landscape and Urban Planning, vol. 69, no. 4, pp. 369–384, 2004. View at Publisher · View at Google Scholar · View at Scopus
  10. A. P. Kinzig, P. Warren, C. Martin, D. Hope, and M. Katti, “The effects of human socioeconomic status and cultural characteristics on urban patterns of biodiversity,” Ecology and Society, vol. 10, no. 1, article 23, 2005. View at Scopus
  11. U. G. Sandström, P. Angelstam, and G. Mikusinski, “Ecological diversity of birds in relation to the structure of urban green space,” Landscape and Urban Planning, vol. 77, no. 1-2, pp. 39–53, 2006. View at Publisher · View at Google Scholar · View at Scopus
  12. K. S. Brown and A. V. L. Freitas, “Butterfly communities of urban forest fragments in campinas, Sao Paulo, Brazil: structure, instability, environmental correlates, and conservation,” Journal of Insect Conservation, vol. 6, no. 4, pp. 217–231, 2002. View at Publisher · View at Google Scholar · View at Scopus
  13. I. MacGregor-Fors, “Relation between habitat attributes and bird richness in a western Mexico suburb,” Landscape and Urban Planning, vol. 84, no. 1, pp. 92–98, 2008. View at Publisher · View at Google Scholar · View at Scopus
  14. G. Brearley, A. Bradley, S. Bell, and C. McAlpine, “Influence of contrasting urban edges on the abundance of arboreal mammals: a study of squirrel gliders (Petaurus norfolcensis) in southeast Queensland, Australia,” Biological Conservation, vol. 143, no. 1, pp. 60–71, 2010. View at Publisher · View at Google Scholar · View at Scopus
  15. A. S. Ackleh, J. Carter, L. Cole, T. Nguyen, J. Monte, and C. Pettit, “Measuring and modeling the seasonal changes of an urban Green Treefrog (Hyla cinerea) population,” Ecological Modelling, vol. 221, no. 2, pp. 281–289, 2010. View at Publisher · View at Google Scholar · View at Scopus
  16. O. J. Furuseth and R. E. Altman, “Who's on the greenway: socioeconomic, demographic, and locational characteristics of greenway users,” Environmental Management, vol. 15, no. 3, pp. 329–336, 1991. View at Scopus
  17. W. D. Solecki and J. M. Welch, “Urban parks: green spaces or green walls?” Landscape and Urban Planning, vol. 32, no. 2, pp. 93–106, 1995. View at Publisher · View at Google Scholar · View at Scopus
  18. United Nations, World Urbanization Prospects: The 2007 Revision United Nations, Department of Economic and Social Affairs, Population Division, New York, NY, USA, 2008.
  19. N. B. Grimm, S. H. Faeth, N. E. Golubiewski et al., “Global change and the ecology of cities,” Science, vol. 319, no. 5864, pp. 756–760, 2008. View at Publisher · View at Google Scholar · View at Scopus
  20. Instituto Nacional de Estadística Geografía e Informática, “Cuaderno estadistico de la Zona Metropolitana del Valle de México,” 2006, http://www.inegi.gob.mx/.
  21. O. Flores and P. Geréz, Biodiversidad y Conservación en México: Vertebrados, Vegetación y Uso de Suelo, Ediciones Técnico Científicas, Distrito Federal, México, 1994.
  22. A. T. Peterson and A. G. Navarro-Sigüenza, “Hundred-year changes in the avifauna of the valley of Mexico, Distrito Federal, Mexico,” Huitzil, vol. 7, pp. 4–14, 2006.
  23. R. Ortega-Álvarez and I. MacGregor-Fors, “Living in the big city: effects of urban land-use on bird community structure, diversity, and composition,” Landscape and Urban Planning, vol. 90, no. 3-4, pp. 189–195, 2009. View at Publisher · View at Google Scholar · View at Scopus
  24. J. M. Marzluff, R. Bownam, and R. Donnely, “A historical perspective on urban bird research: trends, terms, and approaches,” in Avian Conservation and Ecology in an Urbanizing World, J. M. Marzluff, R. Bownam, and R. Donnely, Eds., pp. 1–17, Kluwer Academic, Boston, Mass, USA, 2001.
  25. I. F. Spellerberg, Monitoring Ecological Change, Cambridge University Press, Cambridge, UK, 1991.
  26. R. K. Colwell, “EstimateS: Statistical estimation of species richness and shared species from samples. Version 8.2,” 2009, http://purl.oclc.org/estimates.
  27. N. J. Gotelli and R. K. Colwell, “Quantifying biodiversity: procedures and pitfalls in the measurement and comparison of species richness,” Ecology Letters, vol. 4, no. 4, pp. 379–391, 2001. View at Publisher · View at Google Scholar · View at Scopus
  28. M. E. Payton, M. H. Greenstone, and N. Schenker, “Overlapping confidence intervals or standard error intervals: what do they mean in terms of statistical significance?” Journal of Insect Science, vol. 3, 2003. View at Scopus
  29. J. R. Bray and J. T. Curtis, “An ordination of the upland forest communities of Southern Wisconsin,” Ecological Monographs, vol. 4, pp. 325–349, 1957.
  30. G. Rzedowski and J. Rzedowski, Flora Fanerogámica del Valle de México, Instituto de Ecología, A.C., Comisión Nacional para el Conocimiento y Uso de la Biodiversidad, Distrito Federal, México, 2005.
  31. J. H. Connell, “Diversity in tropical rain forests and coral reefs,” Science, vol. 199, no. 4335, pp. 1302–1310, 1978. View at Scopus
  32. M. L. McKinney, “Effects of urbanization on species richness: a review of plants and animals,” Urban Ecosystems, vol. 11, no. 2, pp. 161–176, 2008. View at Publisher · View at Google Scholar · View at Scopus
  33. J. F. Dwyer, H. W. Schroeder, and P. H. Gobster, “The significance of urban trees and forests: toward a deeper understanding of values,” Journal of Arboriculture, vol. 17, no. 10, pp. 276–284, 1991. View at Scopus
  34. M. C. Molles, Ecología: Conceptos y Aplicaciones, McGraw-Hill, Barcelona, Spain, 2006.
  35. M. Anaya-Corona, “Los parques urbanos y su panorama en la zona metropolitana de Guadalajara,” Revista de Vinculación y Ciencia, vol. 9, pp. 4–16, 2002.
  36. K. Dow, “Social dimensions of gradients in urban ecosystems,” Urban Ecosystems, vol. 4, pp. 255–275, 2000.
  37. D. Hope, C. Gries, W. Zhu et al., “Socioeconomics drive urban plant diversity,” Proceedings of the National Academy of Sciences of the United States of America, vol. 100, no. 15, pp. 8788–8792, 2003. View at Publisher · View at Google Scholar · View at Scopus
  38. I. Kowarik, “Some responses of flora and vegetation to urbanization in central Europe,” in Urban Ecology, H. Sukopp, Ed., pp. 45–74, SPB Academic, Amsterdam, The Netherlands, 1990.
  39. M. L. McKinney, “Urbanization, biodiversity, and conservation,” BioScience, vol. 52, no. 10, pp. 883–890, 2002. View at Scopus
  40. C. J. Tait, C. B. Daniels, and R. S. Hill, “Changes in species assemblages within the Adelaide Metropolitan area, Australia,” Ecological Applications, vol. 15, no. 1, pp. 346–359, 2005. View at Scopus
  41. I. MacGregor-Fors, R. Ortega-Álvarez, and J. E. Schondube, “On the ecological quality of urban systems: an ornithological perspective,” in Urban Planning in the 21st Century, D. S. Graber and K. A. Birmingham, Eds., pp. 51–66, Nova Science, Huntington, NY, USA, 2009.