Structure of the Japanese avian community from city centers to natural habitats exhibits a globally observed pattern
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The number of avian species in urban areas throughout the world, particularly in Europe and the USA is low; however, their total density is higher than that observed in surrounding habitats. Nevertheless, it has not been confirmed whether this is true in Japan. Japanese cities have fewer green areas than European and American cities, and Japanese suburbs are likely to face forests on mountain slopes, whereas cities in most other countries face open grasslands, rural areas, or flatlands. These differences could influence the structure of avian diversity from city to native habitat. We compared the number of species and individuals of all species among city centers, suburbs, and forested areas in Japan. Similar to other countries, the structure of avian communities in Japanese cities was dominated by a handful of species, and total abundance was highest among the other environments. This suggests that the underlying mechanism determining the structure of the avian community is the same between Japan and other previously studied countries. However, species richness was not the highest in the intermediate areas, which is typical in Europe and the USA. This is because suburbs face forested areas and moderately urbanized areas are scarce in the study area. The lack of intermediate area is moderately typical in Japan. This difference is important not only for managing avian diversity but also total diversity from the city to native habitats in Japan.
KeywordsAvian biodiversity Biomass Species abundance Species richness Urban birds
Expanding urban areas are increasing worldwide (Angel et al. 2005; McDonald et al. 2008). This inevitable expansion has a significant negative effect on biodiversity by reducing and fragmenting native landscapes (Wilcox and Murphy 1985; Marzluff et al. 2001; Chace and Walsh 2006). Many studies have tried to measure the degree of fragmentation and identify ways to minimize it (Grimm et al. 2008).
In these studies, bird-community structures (e.g., species richness and number of individuals) along a gradient of urbanization (e.g., from city centers to the native zone) have often been considered an index of the influence of urbanization on biodiversity (Palomino and Carrascal 2006) and, thus, have been well researched. What has become clear is that the number of species in urban areas is low but the total density of birds is higher than that observed in surrounding habitats (Clergeau et al. 1998; Shochat et al. 2006; Luck and Smallbone 2010). This pattern deviates from that observed in less human-influenced habitats, where the total number of individuals usually positively correlates with species richness. This deviation in urban areas implies that the underlying mechanism determining avian community structure differs between urban areas and less-human-influenced areas (Shochat et al. 2006). As a mechanism for creating this urban-specific pattern, Marzluff (2001) suggested that higher resource availability in urban areas supports a higher density of birds. In addition, Shochat et al. (2006) pointed out the importance of the role of species interactions. However, the mechanism remains unclear.
We compared the number of species and individuals of all species among city centers, suburbs, and forested areas in Japan. We also focused on bird biomass among the three environments, because biomass per unit area is often used as a rough proxy for the energy production of an area. Based on the results, we discuss the similarities and differences in the structure of urban avian communities between Japan and other countries.
We conducted randomization tests to explore whether each of the three variables (number of species, number of individuals in all species, biomass) was statistically different among the three environments. As an example, the procedure for the number of species was as follows: three values were randomly chosen from nine values, which included the three environments × the three census routes, and the mean was calculated. Conducting this step 10,000 times created a frequency distribution of the mean values. We used this frequency distribution as a null model that assumed that the mean was not different among the three environments. If each of the means of the observed values (i.e., the mean of the number of species recorded in each environment) was 2.5 % of the smallest value of the frequency distribution, it indicated that the mean of the observed number of species was significantly smaller than the null model. In contrast, if the mean number of species recorded was in the upper 2.5 % of the values, it indicated that the observed number of species was significantly larger than the null model.
We found that the structure of avian communities in a Japanese city was dominated by a handful of species and that total bird abundance was higher than that in the other environments. This is the same pattern observed in European and American cities.
The mean number of species was 6.7 in Japanese city centers, 10.0 in suburbs, and 15.3 in forested areas. Although this is not always the case (Jokimäki et al. 2002), previous studies have shown that the values are likely to be similar among countries (Clergeau et al. 2001). The above values are also similar to those in previous studies. For example, a study conducted by Blair (1996) in USA reported seven species in the business district, 16 in a residential area, and 21 in preserves (see Fig. 4 in that study). In a study conducted by Sandström et al. (2006) in Sweden, 12 species were observed in city centers, 22 in residential areas, and 35 in the periphery (see Table 1 of that study). These values were larger than those in our study, but the proportion among the three environments was similar.
The observed degree of domination by a handful of species in city centers was also similar to that observed in previous studies. In our study, the Eurasian tree sparrow, Passer montanus, occupied 62 % of the total number of individuals in the city centers. In a California city, the rock dove, Columba livia, comprises just 62 % of all individuals (calculated from Table 1 in Blair 1996). In Orebro, Sweden, the house sparrow, P. domesticus (including the Eurasian tree sparrow, due to difficulty in separating them because they create mixed flocks) shares 34 % (Sandström et al. 2006). Beissinger and Osborne (1982) reported that the urban avian community is often dominated by introduced species. The Eurasian tree sparrow is not an introduced species but the rock dove is. Chace and Walsh (2006) stated that urbanization tends to select for omnivores, granivores, and cavity-nesting species. The Eurasian tree sparrow has these characteristics. These consistencies suggest that avian communities in Japanese cities have a similar structure to those in European and American cities, although there are geometric differences among them (Fig. 1).
Although the number of species in our study was smaller than that in the suburbs and forested areas, the biomass in the city centers, which was calculated by body weights of each species, was larger than in the others habitats. A handful of species in the city centers occupied a high proportion of the total biomass. The rock dove, which is 325 g per individual; the large-billed crow (Corvus macrorhynchos), which is 675 g per individual, the carrion crow (C. corone) which is 503 g per individual; and the Eurasian tree sparrow, which is 24 g per individual, occupied 37 %, 33 %, 14 %, and 11 %, respectively, of the total biomass in city centers. This result is also similar to that of the other countries. For example, the rock dove also occupies >90 % of the total biomass in a business district of a city in California (Blair 1996). Similar to other countries, city centers in this study area are able to sustain more birds in terms of biomass than are the surrounding habitats. The following mechanisms for creating this pattern were presented in previous studies: (1) resource availability is higher in urbanized areas (Marzluff 2001); (2) heat generated in highly urbanized areas decreases energy loss for maintaining basal metabolism of birds that defend against cold stress in the temperate zone (Shochat et al. 2006); (3) although it may fit only two crow species and the rock dove in our study area, some species may move from other areas, leading to an overestimate of density (Blair 1996). In addition to the above-mentioned explanations, the following two factors may be essential in our study area: (1) some urban species have smaller territories than those inhabiting natural habitats, which enables them to live at high density. For example, the Eurasian tree sparrows excludes other individuals from only a limited area around their nest, whereas the great tit, Parus major, which has a similar body size to the sparrow, spends a significant amount of time and energy defending its territory by singing, vigilance, and chasing other individuals directly; (2) some individuals of the two crow species do not breed because they do not reach breeding age, which decrease the demand for food for each of them. These five explanations are not mutually exclusive. In future studies, the relative significance of each explanation should be addressed.
In conclusion, we found a consistency in community structure and in characteristics of dominant species among Japanese, European, and USA cities, although there are geographical differences among these countries. This result suggests that the underlying mechanism determining the structure of the avian community is the same between Japan and other previously studied countries. However, the gradient of species richness from city center to the natural habitat differed. The similarities and differences should be a focus of landscape-level planning for species biodiversity.
This research was supported by the Japan Society for the Promotion of Science grant no. 23780028. This study complies with the current laws of Japan.
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