Local and landscape-scale environmental filters drive the functional diversity and taxonomic composition of spiders across urban greenspaces
Delgado de la flor, Yvan et al. (2020), Local and landscape-scale environmental filters drive the functional diversity and taxonomic composition of spiders across urban greenspaces, Dryad, Dataset, https://doi.org/10.5061/dryad.3tx95x6cf
1. Urban patch colonization and species establishment within cities are restricted by the behavioral, life history, and physiological attributes of colonizing species, in conjunction with environmental filtering processes at small and large spatial scales. To enhance local biodiversity in urban greenspaces, these filtering processes need to be assessed so that greenspace design and management can guide establishment of local species pools. 2. We investigated the relative importance of local and landscape-scale features on spider community assembly using a functional and taxonomic approach. Within the shrinking city of Cleveland Ohio, we established a field experiment wherein control vacant lots, urban meadows, and low- & high-diversity pocket prairies were established across eight neighborhoods (N = 32). Spiders were sampled in June-August of 2015 and 2016 using pitfall traps and vacuums. Spider functional diversity was assessed using null models, while local and landscape drivers were analyzed via canonical partial least squares and clustered image maps. 3. Increased mowing strongly influenced spider communities leading to lower-than-expected spider functional alpha and beta diversity in 2015. Patch isolation and percentage impervious surface increased the functional dissimilarity and taxonomic diversity of spiders in 2016, resulting in higher-than-expected overall functional alpha diversity. We also found that increasing plant height and biomass favored spiders with large body-size and decreased the abundance of small web-weavers. 4. Synthesis and applications. Our findings suggest that impervious surface is a strong environmental filter that influences the colonization and establishment of spider communities in cities. Additionally, while periodic mowing in vacant lots benefits some spider taxa, it has a negative impact on the establishment of several species, mainly larger spiders and those most sensitive to disturbance. To conserve spiders and the biota depended upon them, investment in managed greenspaces such as pocket prairies that require infrequent mowing is warranted. In so doing, cities can enhance urban biodiversity and beautify local neighborhoods.31-Mar-2020
Our study was conducted in the city of Cleveland, Ohio, USA. In 2014, we established the Cleveland Pocket Prairie Project across eight inner-city neighborhoods and selected 32 vacant lots (15 x 30 m on average) wherein four experimental treatments were established: Control Vacant Lots, Urban Meadows, Low-Diversity Pocket Prairies and High-Diversity Pocket Prairies. Pocket prairies were mown to a height of 20 cm monthly in 2015 to reduce weed competition and facilitate the establishment of flowering species. All data collection occurred within a 7 x 15 m grid of 105 quadrats (1 m2 each), placed within the center of each site.
To account for both active and less-mobile spiders, specimens were collected in each site using four pitfall traps and four vacuum samples three times per year in 2015 (12-22 June, 8-20 July, 11-18 August), and in 2016 (1-9 June, 6-14 July, 3-11 August). Within each site, four quadrats were randomly selected, and pitfall traps were set up for seven consecutive days. Pitfall traps consisted of 1 L plastic cups (12 cm diameter x 14 cm depth) filled halfway with water containing a small amount of dish soap (Dawn® Ultra, original scent). While pitfall traps were active, we vacuumed an area of 0.25 m2 (30-50 cm away from each trap in any direction) for 45 seconds using a modified leaf vacuum (12 cm diameter). Specimens were stored in 80% ethanol and transported to the laboratory for sorting and identification. Due to weather and issues with landscaping contractors, vacuum sampling did not occur in July 2015.
Lycosidae and Linyphiidae adult spiders were identified to species and other adult and sub-adult spiders were identified to genus. Spider functional traits and groups were classified following Cardoso et al. (2011). Functional traits comprised foraging activity (web type or hunting method), prey range (stenophagous or euryphagous), vertical stratification (ground or vegetation), circadian activity (diurnal or nocturnal), and mean body size measured as the community-weighted mean. Voucher specimens were deposited in the Museum of Biological Diversity at The Ohio State University.
Local habitat variables were measured within 20 randomly selected quadrats. Using a 0.5 m2 sub-quadrat, vegetation was sampled twice in 2015 (16 June - 3 July and 22 July - 13 August) and three times in 2016 (13-24 June, 11-22 July, and 4-16 August). We recorded the three most dominant plant species per quadrat and diversity was calculated per site using the Shannon-Wiener index. Plant biomass was estimated with a comparative-yield method and the dry-weight-rank method from the 20 selected quadrats. First, five quadrats were ranked (1 = lowest biomass density, 5 = highest biomass density, and 2-4 in between) and established as the ‘standard yields’ reflecting the range of biomass within each site. In each of our 20 quadrats, we estimated the biomass yield, on a scale of 1 to 5, in comparison to our five standard yields. Only the five standard yields were harvested, oven-dried at 75 °C for 48 hours and weighted. Finally, we plotted our standard yields, obtained an equation from the trendline, and inserted our 20 ranked comparative yields from each site into this equation to estimate biomass in each quadrat. Average site-wide biomass was then calculated as the mean of these 20 comparative yield estimates.
Mean bloom abundance, bloom area, and plant height were also calculated at each site from an additional six randomly selected quadrats. Average plant height was derived from three height measurements (cm) taken in each quadrat. Likewise, bloom abundances were counted per each flowering species present in the six quadrats. For each flowering species present, we recorded five bloom area measurements (mm2) and then multiplied the average bloom size by the number of blooms present at a site to derive an average bloom area. Additionally, twenty soil cores were randomly sampled and pooled per site in April 2014 to measure the concentration of heavy metals. The Contamination Factor of aluminum, antimony, arsenic, barium, cadmium, chromium, cobalt, copper, iron, lead, manganese, nickel, vanadium, and zinc was calculated using regional background levels from eastern United States, and from these values Pollution Load Indices were calculated per site.
Landscape information was obtained from the Cuyahoga County Planning Commission using remotely sensed images at 1-2 m resolution, captured in 2011. Following previous studies that reported spider communities patterns across the landscape we selected buffer zones at 200 m and 1500 m radii from each site. Landscape cover was classified into percentage: grass/shrubs, bare soil, water, buildings, roads/railroads, other paved surfaces, tree canopy (TC) over vegetation, TC over buildings, TC over roads/railroads, and TC over other paved surfaces. We limited the landscape covers to those directly affecting ground-dwelling spiders. To assess landscape composition, percentage grass/shrubs, percentage buildings, percentage flat impervious surface (roads/railroads & other paved), and Shannon landscape diversity were included in the analysis. For landscape configuration, we re-classified our categories into either ‘greenspace’ (grass/shrubs & TC over vegetation) or ‘other’ based on the importance of patch connectivity on our spider functional groups, and calculated the class-metrics patch size (m2) and patch isolation (m). Shannon landscape diversity, patch size, and patch isolation were computed at 200 m and 1500 m radii using Fragstats v4.2.