Species richness in urban green spaces

Relevant aspects for nature conservation.

authored by

Sarah Matthies

supervised by

Rüdiger Prasse

Abstract

Maintaining species richness became a main subject of nature conservation due to a decline in biodiversity on different scales. In cities high species richness can be found in urban green spaces. As German cities experience population increase in some places and population decrease in other places, green spaces are under threat or, respectively, could be developed. Both phenomena give rise to the need and the opportunity for nature conservation actions. Therefore, the study of the driving factors which lead to high species richness in urban green spaces becomes ever more important. Species richness in urban green spaces is influenced by various driving factors. Factors known to be decisive for promoting species richness include patch size, patch shape, distance to the urban edge and to other urban green spaces, as well as habitat heterogeneity within urban green spaces and in the surrounding area. These factors are derived from species-area effect, distance effects, and effects due to habitat structure. As there are numerous variables which have an impact on species richness in urban green spaces, a current challenge is how to reduce the influencing variables to the smallest relevant set. This set could be adapted for different taxonomic groups, as species richness reacts differently to driving factors in urban green spaces. Reference to the value of certain species for nature conservation, subgrouping within taxonomic groups regarding species’ first occurrence, conservation status or specific traits may all be useful. A lack of research still exists regarding a combination of several response variables and a combination of influencing and response variables. Research gaps include an investigation of different taxonomic groups with different mobility abilities, of endangered species richness, of proportions of different species groups compared to numbers, and of an ‘overall species richness’ containing different taxonomic groups. In addition, not only single factors drive the occurrence of certain species in urban green spaces, but these are also driven by a combination of multiple, interacting factors. Therefore, multivariate analyses are very useful in the identification of driving factors of species richness in urban green spaces. As it is not necessarily the same factors which drive the species richness in urban green spaces located in cities in different climatic regions, the transferability of results and subsequent identification of robust driving factors is an additional aspect relevant for nature conservation. There are various methods available which can be used to define species richness and to verify the capability of driving factors of high species richness. However, each of these survey methods is characterized by specific advantages and disadvantages and the effectiveness and efficiency of the different approaches also differ. Therefore, they each need to be explored for their suitability especially in urban green spaces, as these are usually very diverse habitats. To address in part these research gaps, the aim of my thesis is to name relevant aspects for nature conservation related to high species richness in urban green spaces. The first objective is to detect driving factors of species richness in urban green spaces within and across taxonomic groups. Another objective is to test the transferability of driving factors to cities located in different climatic regions. Finally, a review of the suitability of different field assessments in urban green spaces to survey species richness is performed and the methods evaluated. The thesis addresses the question, of which single and combined factors deriving from species-area effect, distance effects, and effects of habitat structure primarily drive species richness within and across taxonomic groups. After combining the species richness of all the studied taxonomic groups, I then test which combined factors drive overall species richness. The research presented in the thesis is based on an identification of vascular plant, bird and mammal species richness in 32 urban green spaces in Hannover, Germany. The investigated species groups were categorized to be total, native, non-native, endangered, ornamental, and nitrophilous species for vascular plants, as well as total, native and endangered species for birds and mammals. In the study influencing factor patch size was determined from species-area relations. The factors distance to the urban edge, distance to the nearest green space, as well as density, percentage, connectivity, and diversity of the green spaces in a 500 m buffer were checked describing distance effects. The patch shape, number of habitat types, and diversity of habitat types within green spaces described effects of habitat structure and tested to be driving factors as well. Additionally I examine, if the same factors drive species richness of vascular plants in urban green spaces of cities located in different climatic regions. For this, a colleague conducted vascular plant surveys in Haifa, Israel, a city located in a Mediterranean climate. The study results were compared to test transferability of results gained in Hanover to other climatic regions and to identify robust driving factors. Finally, I answer the question, of whether a nested plot method with the possibility to extrapolate species richness is an effective and efficient way compared to a complete field survey that surveys vascular plant species in forested urban green spaces. For this study, results from Modified Whittaker Plots were compared to complete field surveys. In single factor analysis, I detected that the bigger the patch size, the higher is the species richness of vascular plants, birds, and mammals, as well as the proportion of endangered vascular plant species. Additionally, the number of ornamental and proportions of ornamental and non-native vascular plant species were found to higher when urban green spaces were more compact. In urban green spaces with numerous habitat types, high species numbers of all tested taxonomic groups were found. Similarly, the diversity of habitat types positively influenced the numbers of vascular plants and birds, but not mammals. Regarding distance effects, the distance to the nearest green space positively affected all tested species groups. A higher distance to the urban edge only decreased the number of ornamental vascular plant species, whereas a high connectivity of green spaces increased the number of native vascular plants and total mammals. The analysis of multiple influencing variables revealed, that patch area, distance to the urban edge, patch shape, and habitat heterogeneity within an urban green space, measured by the number and the diversity of habitat types, best explained the total number of vascular plants. The number of native and endangered vascular plants, as well as total and native birds was best explained by the same selection of variables, however distance to the urban edge and patch shape were not significant. Patch area, distance to the urban edge as well as compactness defined the number of non-native and ornamental, as well as proportions of native, non-native, and ornamental vascular plant species. The results also revealed that the bigger the patch area and the smaller the distance to the urban edge, the higher was the number of nitrophilous vascular plant species. An increasing patch area was the only factor driving the number of endangered birds, total and native mammals, and also driving the proportion of endangered vascular plant species. None of the tested influencing variables affected the proportion of nitrophilous vascular plant species significantly. The overall species richness was best explained by patch area in combination with habitat heterogeneity. In the different climatic regions of Hannover, Germany and Haifa, Israel we found that in both of these areas the total and native vascular plant species numbers increased with increasing patch size. However, no significant influence of the patch size on the proportion of natives was detected either for Hannover or for Haifa. Regarding the influence of distance to the urban edge we did not find any significant results for the total or native species number, or the proportion of native species for these two cities located in different climatic regions. Finally, I compared the results derived from the Modified Whittaker Plot method to those from a complete field survey in different urban forest habitats in Hannover. The Modified Whittaker Plot method showed some limitations in the estimation of total and endangered species richness of vascular plants and as a result in the habitat evaluation, especially in large habitats. At the same time it was more time efficient to use the plot method instead of the complete field survey in larger habitats. Therefore one advantage is that the Modified Whittaker Plot method can be the source of basic information that can be used for further survey efforts. My work shows that there are diverse relevant aspects for nature conservation that need to be considered regarding species richness in urban green spaces. The driving factors behind species richness of different taxonomic groups are - to some extent - the same. This observation is independent of whether the investigation is based on the analysis of single or multiple variables. Patch size and habitat heterogeneity are both driving factors that have an overriding importance for different species groups as well as for the aggregated measure of overall species richness. A relevance of distance effects was only confirmed for the distance to the next urban green space in the single variable analysis of my work. In the multivariable approach the relevance of this variable was probably too weak to be significant after accounting for the effect of patch size and habitat heterogeneity. The importance of patch size was also underlined in the analysis in the different climatic region of Haifa. The assignment of species numbers and a habitat evaluation based on the Modified Whittaker Plot method revealed important information on the potential value of an urban green space. It does not replace complete field surveys, as it cannot detect specific species that can be assigned to be for instance native or endangered. From my results I draw the following implications for sustainable urban development for the tested taxonomic groups: green spaces should not vanish or be reduced in size; green spaces should be maintained, developed, and planned to include heterogeneous habitats; driving factors of overriding importance can be transferred to cities located in different climatic regions; an effective species monitoring and evaluating system is needed for urban green spaces, whereas the Modified Whittaker Plot method can be used for providing basic information; and finally urban development should place special importance on site specific traits. The results from my studies contribute to closing some of the existing research gaps on relevant aspects of nature conservation. The research demonstrates that sustainable conservation as well as development of species rich urban green spaces is possible. Nevertheless, there is still a clear need for further investigations of effects on and mechanisms behind high species richness. Further studies could include additional response variables and influencing variables, a greater number of studies in similar climatic regions, further research on statistical analysis, and an assignment of a habitat value to urban green spaces. As species loss lasts differently over time in different environments, the maintenance and development of species richness in urban areas with sustainable conservations actions and the implementation of state of the art research in urban planning are both vitally important for promoting a sustainable future.

Organisation(s)

Institute of Environmental Planning

Type

Doctoral thesis

No. of pages

53

Publication date

2018

Publication status

Published

Sustainable Development Goals

SDG 11 - Sustainable Cities and Communities

Electronic version(s)

https://doi.org/10.15488/3093 (Access: Open)