Urban Grassland Research Articles

Urban grassland restoration: which plant traits make desired species successful colonizers?

AbstractQuestionsWhich plant traits characterize successful and failed target species in urban grassland restoration? Do traits of successful target species differ from those of resident species? How do plant traits relate to environmental constraints?Location In‐situ experimental sites on wastelands in shrinking residential areas in Berlin, Germany.MethodsWe established grassland restoration treatments and explored plant traits of successful and failed target species (plant height, specific leaf area, seed mass, seed shape, seed longevity index, CSR strategy type, plant life form). To shed light on mechanisms that shape restoration success, we also analysed the same traits of species originating from the soil seed bank and species immigrating from the surroundings. We compared both trait sets to those of resident species. With RLQ analyses we related the trait data to abundance data of species and to variables describing the environmental setting of the sites.ResultsIn the third year after treatment, several plant traits differed between the successful or failed target species and the resident vegetation, e.g. successful target species tended to be as tall as resident species, whereas failed target species were smaller, suggesting insufficient competitive ability of the latter. Species that successfully recruited from the soil seed bank were taller than resident species. Small specific leaf area was important for the establishment success of target species. Trait composition of the species assemblage clearly related to the environmental setting: mean specific leaf area and the proportion of annuals increased and the proportion of C‐strategists decreased with increasing human‐mediated impacts on the restoration sites.ConclusionsOur results reveal clear trait differences between successful and failed target species in grassland restoration on urban wasteland sites, demonstrating that high competitive ability is crucial for success in target species. Grassland species that are successfully integrated into urban wasteland vegetation may thus fill well‐defined vacant niches, while resembling the traits of the resident species in other ways. Our results may allow generalizations and transfer to similar urban settings, as the analysed trait states were assessed as relative values compared to resident species.

Grasshopper response to reductions in habitat area as mediated by subfamily classification and life history traits

Although the loss of species is often attributed to reductions in habitat area, heterogeneity and connectivity, species specific traits and taxonomic relatedness can be important in explaining which species groups may be most impacted by the fragmentation process. In this study, using urban grassland fragments within the Front Range of northern Colorado, USA, we quantified the degree to which grasshopper species numbers declined with reductions in habitat area. We then examined the degree to which subfamily grouping (Gomphocerinae, Melanoplinae, Oedipodinae) and variation in life history characteristics (body size, dispersal ability, feeding preference and juvenile emergence time) explained which grasshopper species groups might be most impacted by reductions in habitat size. Our results showed a strong species-area relationship for grasshoppers across the urban fragments. Grasshoppers in different subfamily groupings were found to respond differentially, with the Melanoplinae being least affected by reductions in habitat area, while the other subfamilies responded similarly and lost species at a higher rate. Species with different feeding strategies were also found to respond differentially, with forbivorous species being least affected by the reductions in habitat area, while the graminivorous and herbivorous (mixed-feeders) responded similarly and lost species at a higher rate. As Melanoplinae tend to be forb feeders, this may partially explain why the subfamily was not as affected by the fragmentation process as the Gomphocerinae and Oedipodinae (who tend to be grass and mixed-feeders, respectively). Species that differed in body size and dispersal ability did not display differential responses to reductions in habitat area.

Determining the effects of habitat fragment area on grasshopper species density and richness: a comparison of proportional and uniform sampling methods

Abstract. 1. The sampling methodology used in diversity studies may at times define perceived biological patterns. In this study we used urban grassland fragments in the Front Range of northern Colorado, USA, to determine whether grasshoppers exhibit a species–area relationship (SAR) and to examine the potential effects of sampling protocol, namely uniform and proportional sampling, and sampling biases on our ability to detect such a relationship.2. A uniform sampling protocol was used to survey grasshoppers across 13 urban fragments and then sample‐based rarefaction curves were used to determine how the SAR would have been modified had we used proportional sampling. We also used individual‐based rarefaction curves to explore the issue of ‘passive’ sampling and used species richness estimators to address the efficacy of our sampling protocols.3. We found that reductions in habitat area due to urban fragmentation lead to a significant SAR between grasshopper species density and richness and fragment size. Using field data we also show, that relative to a uniform sampling protocol, proportional sampling can lead to similar conclusions about the density of species found within an area with the potential benefit of not over sampling smaller relative to larger areas. Finally, the estimators showed that most of the species in each fragment were detected and the individual‐based rarefaction curves showed that the detected SAR was not due to ‘passive’ sampling.

Nitrate Leaching and Nitrous Oxide Flux in Urban Forests and Grasslands

Urban landscapes contain a mix of land-use types with different patterns of nitrogen (N) cycling and export. We measured nitrate (NO(3)(-)) leaching and soil:atmosphere nitrous oxide (N(2)O) flux in four urban grassland and eight forested long-term study plots in the Baltimore, Maryland metropolitan area. We evaluated ancillary controls on these fluxes by measuring soil temperature, moisture, and soil:atmosphere fluxes of carbon dioxide on these plots and by sampling a larger group of forest, grass, and agricultural sites once to evaluate soil organic matter, microbial biomass, and potential net N mineralization and nitrification. Annual NO(3)(-) leaching ranged from 0.05 to 4.1 g N m(-2) yr(-1) and was higher in grass than forest plots, except in a very dry year and when a disturbed forest plot was included in the analysis. Nitrous oxide fluxes ranged from 0.05 to >0.3 g N m(-2) yr(-1), with few differences between grass and forest plots and markedly higher fluxes in wet years. Differences in NO(3)(-) leaching and N(2)O flux between forests and grasslands were not as high as expected given the higher frequency of disturbance and fertilization in the grasslands. Carbon dioxide flux, organic matter, and microbial biomass were as high or higher in urban grasslands than in forests, suggesting that active carbon cycling creates sinks for N in vegetation and soil in these ecosystems. Although urban grasslands export more N to the environment than native forests, they have considerable capacity for N retention that should be considered in evaluations of land-use change.

The importance of fragmentation and habitat quality of urban grasslands for butterfly diversity

Parks, remnants of natural habitats and other green areas are important for preserving biodiversity in urban areas. Here, we investigate the relative importance of habitat type and connectivity for butterfly species richness in the city of Malmö, Sweden. Further, we compare species richness and composition in the urban habitats with that in the surrounding agricultural landscape using previously published data. Both butterfly species richness and density increased with decreasing connectivity, measured as the proportion of urban green areas within 1 km, and were higher in ruderal sites than in traditional and semi-natural parks. Species richness was only slightly lower in the urban habitats than in semi-natural grassland remnants in the agricultural landscape surrounding the city and there was only a small difference in β (between site) diversity between urban and semi-natural landscapes. This study highlights the importance of “townscape” composition for species richness in urban habitats, but also demonstrates clearly that urban habitats, especially those characterized by an early-successional stage, can be of relatively high conservation value in regions dominated by intensive human land use.

Climate Change and Urban Grass Land Soil Moisture Conditions in South-Western Ontario, Canada

Using the past 45 years of climate data in south-western Ontario, Canada and a deterministic continuous simulation model, this study investigates the long-term variability in rain-fed soil moisture in urban areas as influenced by climate change. Statistical analyses of four variables, i.e., soil moisture, precipitation, temperature and evapotranspiration were carried out. As found from other studies for other locations, these analyses confirm in creasing temperatures and average growing season precipitation in south-western Ontario. Results show that both overall soil moisture and evapotranspiration have increased throughout the 45-year period. The probability/frequency distributions of soil moisture were obtained and the analysis shows an increasing average growing season soil moisture availability from the 1960's to the 1990's. The direct influence of precipitation and temperature on soil moisture and evapotranspiration were examined, revealing a stronger relationship of soil moisture and evapotranspiration with precipitation rather than temperature. Overall increasing average growing season soil moistures have very likely resulted from overall increasing rainfall during the growing seasons in south-western Ontario.

Patterns of exotic plant invasions in fragmented urban and rural grasslands across continents

Linear native grassland remnants in fragmented landscapes are usually at a great risk of exotic species invasion from their edges. Changes in species distribution near habitat edges are extensively studied in ecology as knowledge about edge responses is important to understand the development of patterns and processes in landscapes. However, elucidating robust general principles for edge effects has been difficult as species responses to habitat edges are highly variable and dependent on a large number of attributes which affect the function and structure of edges and therefore the distance that edge effects penetrate into fragmented natural vegetation. The objective of this study was to investigate the generality of exotic species invasion patterns from edges in native grassland patches surrounded by urban and rural landscapes. This was done by comparing the results of research from Victoria, Australia with a similar study from North-West Province, South Africa. Despite their occurrence on different continents, the grasslands are floristically and structurally similar and are dominated by the same grass species. Invasion patterns were quantified using two spatial statistics methods; block kriging and spatially constrained clustering. Two distinct patterns of exotic species invasion were identified in native grassland remnants in South Africa and Australia, namely exotic species invasion from the edge where the cover of exotic species increased with increasing proximity to the edge and a pattern that suggests that gap phase vegetation dynamics may also drive exotic species invasion at urban grasslands. Although urbanization and weed invasions are complex processes similar patterns of exotic species invasion in urban grasslands were found in two different continents suggesting that general patterns may occur. Implications of this for the conservation of native grasslands in contrasting landscapes are discussed.

Effects of urbanization on ground beetle assemblages (Coleoptera, Carabidae) of grassland habitats in western Canada

We pitfall‐trapped carabid beetles across an urban‐rural gradient in central Alberta, Canada to assess the relative contributions of landscape structure and habitat quality in explaining variation in composition of assemblages. The gradient ranged from the “urban” zone in the city of Edmonton, through a “suburban” zone just outside the city, into a “rural” zone 25–60 km east of the city. We trapped beetles at three replicated sites for each of two habitats (unmanaged grasslands and highly managed graveyards) in each of the three zones. Most of the 24 species (3162 individuals) captured were commonly associated with open habitats.Pterostichus melanarius, an introduced species, made up 80% of the total catch. Carabid assemblages of grasslands and graveyards were distinct, with lower species richness and lower catch rates of native, introduced and brachypterous carabids in graveyards. Graveyard assemblages also had proportionally fewer brachypterous carabid individuals (32 vs 52%) and species (20 vs 36%), suggesting that populations recruit locally from elsewhere after disturbances. Native carabids were least abundant in the urban areas. Brachypterous carabids were similarly abundant across the gradient, but more occurred in the urban zone than elsewhere (32 vs 5–10%). Samples from graveyards were characterized by lower beta diversity (variation among samples) than those from grasslands, but beta diversity was more‐or‐less equally variable across the gradient within habitats. The amount of unmanaged grassland habitat and the degree of urbanization significantly influence carabid assemblages, and therefore are relevant to management and conservation of urban grassland areas.

Plant traits and local extinctions in natural grasslands along an urban–rural gradient

Summary The effect that the surrounding landscape matrix has on the loss of species from fragmented patches remains largely unknown. To determine whether there were differences in the persistence of plants inhabiting remnant patches in contrasting landscape types we examined the local extinction of grassland plants along an urban–rural gradient in western Victoria, Australia. Thirty small grassland remnants that had been comprehensively surveyed between 1979 and 1990 were intensively re‐surveyed. A total of 289 (26%) of the 1104 plant populations present in the 1980s were not relocated and were presumed to be locally extinct. The proportion of populations lost differed along the gradient, with higher local extinction rates at patches in urban (37%) and peri‐urban landscapes (27%) than those in the rural landscape (20%). We calculated the probability of local extinction of species in urban, peri‐urban and rural landscapes using Bayesian logistic regression models. Across all plant functional traits examined, species had a consistently higher probability of local extinction in the urban landscape. Species that were geophytes or hemicryptophytes with a flat rosette and species with seeds dispersed by wind or ants had substantially increased risks of extinction in the urban landscape. Low seed mass, the lack of vegetative reproduction and the presence of a soil‐stored seed bank increased the probability of local extinction in all landscapes. Regionally rare species had a higher probability of local extinction in rural and peri‐urban landscapes but rarity had little influence on extinction risk in urban landscapes. Urbanization has a strong influence on the species composition of urban grasslands and substantially increases the probability of local extinction of plants with particular combinations of functional traits.

Ecological relationships between landscape change and plant guilds in depressional wetlands

Plant guilds used to measure the relationships between wetlandplant community characteristics and landscape change around 31 depressionalwetlands in central Ohio, USA. Characteristics of certain plant guilds withineach wetland site are correlated with changes in: (a) area of urban land cover,forest, grassland, agriculture, and open-water in the local vicinity ofthe wetland; (b) inter-wetland distance; and (c) wetland size (area).Taxa richness is negatively correlated with inter-wetland distance forall plant guilds, except submersed herbaceous plants. Taxa richness of thesubmersed herbaceous plant guild (usually less than 20% of the totalnumber of plant species at a wetland) is positively correlated with the area ofopen-water in the local landscape and with the areaofthe wetland site itself. Significant positive correlationsalso exist between the area of open-water in the vicinity of the wetlandand the proportion of submersed herbaceous plant taxa at the site, the numberofnative submersed herbaceous plant species, the submersed herbaceous plantperennial-to-annual ratio, and the number ofavian-dispersed submersed herbaceous plant species at a site. Theresults suggest that (a) the dominance of submersed herbaceousplantspecies at a site is related to dispersal constraints between wetlands, and (b)the relatively slower physiological response of woody plants to local landscapechange may result in their contribution to greater ‘ecologicalinertia’ in the plant community as a whole. For these reasons,relationships between the plant community and land cover change may not alwaysbe observed unless analyzed at the level of plant-guild.

Diversity and Organization of the Ground Foraging Ant Faunas of Forest, Grassland and Tree Crops in Papua New Guinea.

Thirty samples of ants were taken in each of seven habitats: primary forest, rubber plantation, coffee plantation, oilpalm plantation, kunai grassland, eucalypt savannah and urban grassland. Sixty samples were taken in cocoa plantations. A total of 156 species was taken, and the frequency of occurrence of each in each habitat is given. Eight stenoecious species are suggested as habitat indicators. Habitats fell into a series according to the similarity of their ant faunas: forest, rubber and coffee, cocoa and oilpalm, kunai and savannah, urban. This series represents an artificial, discontinuous succession from a complex stable ecosystem to a simple unstable one. Availability of species suitably preadapted to occupy habitats did not appear to limit species richness. Habitat heterogeneity and stability as affected by human interference did seem to account for inter-habitat variability in species richness. Species diversity was compared between habitats using four indices: Fisher et al.; Margalef; Shannon; Brillouin. Correlation of diversity index with habitat heterogeneity plus stability was good for the first two, moderate for Shannon, and poor for Brillouin. Greatest diversity was found in rubber, the penultimate in the series of habitats according to heterogeneity plus stability ('maturity'). Equitability exceeded the presumed maximum in rubber, and was close to the maximum in all habitats. The mosaic dispersion pattern found among ants elsewhere also appeared to be present in each habitat. The mean maximum number of territories possible to be overlapping is about 4.6; the number in a particular case probably being a function of the specialization of the dominant ant present. Ecological isolating mechanisms among ants are probably similar to those among birds; size and structure of mouthparts in ant communities warrant further study.

Contamination of soil with zinc, copper, lead, and cadmium in the Wollongong city area

From a grid sampling procedure over 56 km2 of the Wollongong city area, levels of acetic acid-extractable soil zinc, lead, and cadmium, and EDTA-extractable soil copper, have been investigated. Mean levels of these metals in four groups of urban grassland, namely, farm land, recreational land, industrial land, and roadsides, are up to 10 times greater than those in a rural control area. Highest levels of all four metals are found in the Port Kembla industrial area, where 21 samples within 1 km of the main heavy metals smelting complex have mean values of 82 p.p.m. zinc, 343 p.p.m. copper, 21 p.p.m. lead (15 samples), and 2.8 p.p.m. cadmium (11 samples); mean levels in the rural area are 2.7 p.p.m. zinc, 5.3 p.p.m. copper, <1 p.p.m. lead, < 1 p.p.m, cadmium. Profile samples show that all four contaminants were very largely concentrated in the top 15 cm soil, confirming an airborne origin.