Research Article |
Corresponding author: Carina Argañaraz ( c.arg.bio@gmail.com ) Academic editor: Ricardo Pinto-da-Rocha
© 2017 Carina Argañaraz, Raquel M. Gleiser.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Argañaraz CI, Gleiser RM (2017) Does urbanization have positive or negative effects on Crab spider (Araneae: Thomisidae) diversity? Zoologia 34: 1-8. https://doi.org/10.3897/zoologia.34.e19987
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Urbanization has a great impact on the diversity of living organisms. Spiders, for example, have been shown to respond negatively (some orb weaving species) and positively (ground dwelling species) to urbanization. The effects of urbanization on crab spiders (Thomisidae) (sit-and-wait predators that generally ambush their prey on flowers and leaves) are not sufficiently known. This paper describes the Thomisidae community that inhabits green patches in a temperate Neotropical city, Cordoba, Argentina, and its surroundings, and ascertains whether there are differences in species richness, abundance and composition between urban and exurban sites. Samples were collected from 30 sites during the summer and spring of 2013 and 2014, using the garden-vacuum method. We compared the abundance, richness and composition of Thomisidae among three habitat categories (urban, suburban and external). Seven species in three genera, Misumenops, Wechselia and Tmarus, were detected. Misumenops was the most abundant genus in Córdoba, and Tmarus elongates Mello-Leitão, 1929 was collected in Argentina for the first time. The abundance, richness and composition of thomisid spiders did not differ between habitat categories, indicating that urbanization does not have a negative effect on the communities of these spiders in open green spaces. Site variability, on the other hand, does, suggesting that local factors may be more relevant than broad-scale factors to explain community patterns in this family.
Arachnid, biodiversity, city, richness, turn-over, urbanization.
Thomisidae Sundevall, 1983, is a species-rich family of spiders comprising 175 genera and 2,153 valid species (
Spiders are highly polyphagous predators that have the ability to store energy and go without food for long periods of time (
Habitat alterations that result from urbanization affect the arthropod fauna (
The effects of urbanization on crab spiders have not been extensively studied. Here we surveyed the Thomisidae community dwelling in green patches in the city of Córdoba, Argentina, and its surroundings, to ascertain whether species composition, richness and abundances differ among urban, suburban and exurban sites. Since predatory species tend to be sensitive to anthropic processes, our working hypothesis was that the community of sit-and-wait spiders would be influenced by the degree of urbanization and would differ. We also expected that species richness and abundance would increase from more intensely urbanized to more natural environments.
Specimens were collected from green patches in the city of Córdoba (31°25’S, 64°11’W, 390 m.a.s.l.) Argentina. The study area is located within the Espinal ecoregion (
Each sampling site was assigned to one of three categories of habitat, based on its relative location and characteristics of the land cover surrounding it within a 400 m radius buffer area (approximately 10 x 10 city block areas). The average number of houses per block and the percentage covered by impervious surface (buildings, road, etc.) were visually estimated from Google Earth images. Based on these variables, the following habitat categories were defined: 1) External patches: located outside the city limits, on natural or agroecosystems (but not within crops), with low impervious cover (≤ 20%). 2) Suburban patches: located close to the city limits, suburban area (approximately 11 houses per block) with moderate proportion (> 20 ≤ 60%) of impervious surfaces. 3) Urban patches: located on the core of the city, with a high proportion (≥ 70%) of impervious surfaces and highly built (22 houses per block on average). Twelve sites were urban, twelve were suburban and six were external (Fig.
Samples were collected using the Garden-Vacuum method to suck spiders from the vegetation, as described by
To evaluate the completeness of the thomisid inventory, the sample coverage (
The spatial turnover or change in the identities of species is a measure of the difference in species composition, either between two or more local assemblages, or between local and regional assemblages (
Differences in species composition between the three habitat categories were assessed with the non-metric multidimensional scaling (NMDS), to ordinate spider diversity composition within different classes. Differences were corroborated with Analysis of Similarity (ANOSIM), a non-parametric test of significant differences between two or more groups (
In all, 298 mature and 1,505 immature individuals were collected. Mature specimens belonged to three genera, Misumenops Pickard-Cambridge, 1900, Wechselia Dahl, 1907 and Tmarus Simon, 1875. A total of five species were identified. One of them, very similar with Tmarus digitatus Mello-Leitão, 1929, was labeled as affinis since it needs further taxonomic confirmation. Additionally, there were three morphospecies. Since adult crab spiders (only immatures, 31 specimens) were not collected from one of the suburban sites, that site was excluded from further analyses. Table
Composition, total number, and incidence (percentage number of sites a species was found) of Thomisidae species in green spaces in Córdoba city, Argentina.
Species | Urban | Suburban | External | Total | Incidence (%)* |
---|---|---|---|---|---|
Misumenops maculissparsus | 106 | 65 | 19 | 190 | 89.7 |
Misumenops pallidus | 21 | 16 | 7 | 44 | 67.0 |
Misumenops sp. 1 | 11 | 11 | 2 | 24 | 48.3 |
Misumenops sp. 2 | 5 | 4 | 2 | 11 | 31.0 |
Misumenops sp. 3 | 7 | 2 | 0 | 9 | 10.3 |
Wechselia steinbachi | 1 | 2 | 5 | 8 | 17.2 |
Tmarus elongates | 3 | 1 | 5 | 9 | 17.2 |
Tmarus aff. digitatus | 2 | 1 | 0 | 3 | 6.9 |
Mean (± SD) estimates of Coverage, Observed richness, Chao1-bc and Abundances for each habitat category and total community.
Urban | Suburban | External | Total | |
---|---|---|---|---|
Coverage | 0.99 | 0.98 | 1.00 | 1.00 |
Observed richness | 3.00 ± 1.53 | 3.10 ± 1.37 | 2.33 ± 0.81 | 2.89 ± 0.25 |
Chao1-bc | 3.37 ± 2.23 | 3.50 ± 1.70 | 2.50 ± 1.04 | 2.92 ± 0.33 |
Abundances | 13.00 ± 10.36 | 9.27 ± 9.90 | 6.60 ± 3.90 | 10.28 ± 1.73 |
The average observed and estimated richness were similar, ranging from 2.33 to 3.1 and 2.5 to 3.5, respectively, and did not differ significantly between categories (F2,26 = 0.65, p = 0.53 and F2,26 = 0.62, p = 0.55, respectively for observed and estimated richness). No statistical differences were detected either between patch categories in total average abundance (F2,26 = 1.32, p = 0.29; ranging from 6.6 to 13 specimens) (Table
The NMDS did not show clear discriminations among sites within categories; neither did the Simpson nor Bray-Curtis distances (only NMDS based on Bray-Curtis is shown for reference, Fig.
Within each category, the Simpson index qualitatively indicated a comparably higher average species turnover than between categories (Simpson indexes were equal to zero), which was increasingly more evident from urban to external sites (urban = 0.2 (range 0-0.5); suburban = 0.23 (0-1); external = 0.57 (0-1)).
Diversity profiles from three habitat categories in Córdoba city, Argentina. Alpha values are represented on X axis and diversity values on Y axis. Urban, suburban and external categories are represented by solid cyan, red and orange lines, respectively and their 95% confidence intervals by dashed lines.
Non-Metric Multidimensional Scaling of three habitat categories (urban, suburban and external) in Córdoba city, Argentina. Bray-Curtis similarity index was used (two axes, stress = 0.16). Urban sites are represented by cyan triangles, suburban sites by red squares and external sites by oranges circles.
Species richness estimates and cover analysis indicated that the samples obtained, comprising seven species, provide an adequate representation of the Thomisidae fauna at all habitat categories. To our best knowledge, there are no published studies on urban Thomisidae from Argentina; however comparisons with other environments, using a combination of sampling strategies, show similar or lower richness values in Argentinian sites. From soya crops,
This is the first report of Tmarus elongates Mello-Leitão, 1929 in Argentina. Since the species had already been documented in neighboring Brazil (
Misumenops was the most abundant genus in Córdoba, consistent with other studies on the spider communities of cotton (
The effect of urbanization is generally more evident in the core of a city, which is surrounded by areas of decreasing habitation and development with moderate or low disturbance levels (
The abundance of Thomisidae spiders, on average, was not affected by urbanization at the scale considered, neither was evenness in species abundances as inferred from the diversity profiles. These results differ from some studies that found a negative correlation between abundance and increased human disturbance, for example in Berkeley, USA (
A number of studies on spider assemblages in altered landscapes indicate that urbanization may affect diversity most noticeably in species composition (
Greater differences between sites were found within the same habitat category, suggesting that local factors may be more relevant to explain community patterns. These results are consistent with
In conclusion, our results suggest that in a temperate city like Córdoba, the Thomisidae community of parks and other green spaces that are at least 1 ha are not negatively affected by urbanization. The effect of local factors such as vegetation structure should be assessed to better explain crab spider community patterns in cities and further understand the impact of environmental alterations on them. Also, future studies considering landscape composition and structure will contribute to a better understanding of the effects of urbanization on these spider communities and aid biodiversity conservation policies.
We specially thank Arno Antonio Lise and Renato Augusto Teixeira from Pontifícia Universida de Católica do Rio Grande do Sul, Brazil, for their assessment on the taxonomic identifications and Alejandro Barbeito for help with graphic design. We are grateful to three anonymous reviewers for their careful reading of our manuscript and for their constructive and insightful comments that helped improve our manuscript. The present study was partially supported by grants from SeCyT – UNC, PICT 2012-2014, and PIP CONICET, Argentina. R.M. Gleiser is a Career Researcher of CONICET; C.I. Argañaraz is a doctorate of FCEFyN, UNC and holds a scholarship from CONICET.