What do we know (and need to know) about the role of urban habitats as ecological traps? Systematic review and meta-analysis.

Urban areas represent a spectrum that goes from being safe habitats for biodiversity (i.e., habitats more or equally preferred, without costs to fitness) to being ecological traps (i.e., habitats more or equally preferred, but with costs to fitness). Given the imminent urban expansion, it is valuable to assess how biodiversity is responding to urbanization and thus generate timely conservation strategies. We systematically review the urban ecology literature to analyze how much do we know about the role of urban areas as ecological traps. Using a formal meta-analytical approach, we test whether urban areas are functioning as ecological traps or as safe habitats for different taxonomic groups. We generated a data set of 646 effect sizes of different measures of habitat preferences and fitness from 38 papers published between 1985 and 2020. The data set covered 15 countries and 47 urban areas from four continents, including 29 animal species. Studies from North America and Europe were best represented, and birds were the most studied taxa. Overall, the meta-analysis suggests that urbanized habitats are functioning more as safe sites than as ecological traps, mainly for certain species with characteristics that have allowed them to adapt well to urban areas. That is, many of the studied species prefer more urbanized habitats over other less urbanized sites, and their fitness is not modified, or it is even increased. However, there was high heterogeneity among studies. We also performed meta-regressions to identify variables accounting for this heterogeneity across studies and we demonstrate that outcomes may depend on methodological aspects of studies, such as study design or the approach used to measure habitat preference and fitness. More research is needed for poorly studied regions and on a wider range of species before generalizations can be made on the role of urban areas for biodiversity conservation.

The urban contrast: A nationwide assessment of avian diversity in Mexican cities.

In this study we focused on urban bird diversity across Mexico, a megadiverse country, with a special focus on the relative role of urban greenspaces and heavily-built sites. We considered a country-wide approach, including 24 different sized Mexican cities. Our aims were to describe the urban bird diversity in focal cities and further assess the relationships between it and the biogeographic region where cities are located, their size, elevation, and annual rainfall. Additionally, we evaluated differences in the functional composition of bird communities in both studied urban scenarios (i.e., urban greenspaces, heavily-built sites). Our results confirm that urban greenspaces are home to a large proportion of species when contrasted with heavily-built sites. While total species richness and species richness of greenspaces were related with the cities' biogeographic region -with higher species richness in the Neotropical region and Transition Zone-, the relationship did not hold true in heavily-built sites. We found that annual rainfall was negatively related to bird richness in heavily-built sites, suggesting that species from arid systems can be more tolerant to urbanization. Regarding the bird functional group assessment, results show a clear differentiation between the functional groups of greenspaces and those of heavily-built sites, with granivores and omnivores associated with the latter and a highly diverse array of functional groups associated with urban greenspaces.

Riqueza y abundancia de aves en áreas verdes en la ciudad de San Cristóbal de las Casas, Chiapas, México / Richness and abundance of birds in green areas of San Cristobal de Las Casas city, Chiapas, Mexico

Resumen Las áreas verdes en las ciudades se han considerado elementos importantes para la conservación de la avifauna. En este estudio se presentan estimaciones de riqueza y abundancia de aves, así como su relación con las áreas verdes en la ciudad de San Cristóbal de Las Casas, Chiapas, México. Identificamos un total de 75 especies de aves en diez áreas verdes de diferente tamaño de febrero a julio de 2016 y relacionamos su presencia con variables ambientales a dos escalas geográficas (local y de paisaje). Se utilizaron análisis no paramétricos para comparar la riqueza y abundancia de las aves entre áreas verdes y categorías de tamaño. El porcentaje de superficie total ocupada por vegetación fue la variable más importante al correlacionarse positivamente con la riqueza y abundancia de las aves. Mientras que el porcentaje de cobertura de dosel, de construcciones, de pavimento y de áreas verdes muestreadas se correlacionaron negativamente con la riqueza y abundancia de aves. La heterogeneidad espacial de la estructura vegetal vertical en las áreas verdes fue importante para incrementar la diversidad de la avifauna. El mantenimiento e incremento de áreas verdes en ciudades es significativo para proteger y aumentar la diversidad de aves en áreas urbanas.

Abstract Green areas in urban districts are important elements for bird conservation. In this study, we estimated bird abundance within green areas in the urban district of San Cristobal de Las Casas, state of Chiapas, Mexico. We identified a total of 75 bird species in ten green areas of different sizes from February of 2016 through July of 2016. We related bird presence to environmental variables at two geographical scales (local and landscape). Nonparametric analyses were used to compare richness and abundance of birds between green areas and size categories. The percentage of vegetation area was the most important variable that positively correlated with species richness and abundance. However, negative correlations were found between species richness and abundance and the percentage coverage of canopy, buildings, pavement and sampled green areas. The spatial heterogeneity of vertical vegetation structure in green areas was important to increase the diversity of avifauna. The maintenance and increase of green areas in cities is important to protect and increase the diversity of birds in urban areas.

Reduced thermal variability in cities and its impact on honey bee thermal tolerance.

Urbanization is one of the most significant land cover transformations, and while climate alteration is one of its most cited ecological consequences we have very limited knowledge on its effect on species' thermal responses. We investigated whether changes in environmental thermal variability caused by urbanization influence thermal tolerance in honey bees (Apis mellifera) in a semi-arid city in central Mexico. Ambient environmental temperature and honey bee thermal tolerance were compared in urban and rural sites. Ambient temperature variability decreased with urbanization due to significantly higher nighttime temperatures in urban compared to rural sites and not from differences in maximum daily temperatures. Honey bee thermal tolerance breadth [critical thermal maxima (CTmax)-critical thermal minima (CTmin)] was narrower for urban bees as a result of differences in cold tolerance, with urban individuals having significantly higher CTmin than rural individuals, and CTmax not differing among urban and rural individuals. Honey bee body size was not correlated to thermal tolerance, and body size did not differ between urban and rural individuals. We found that honey bees' cold tolerance is modified through acclimation. Our results show that differences in thermal variability along small spatial scales such as urban-rural gradients can influence species' thermal tolerance breadths.

The role of singing style in song adjustments to fluctuating sound conditions: A comparative study on Mexican birds.

Many bird species adjust their songs to noisy urban conditions by which they reduce masking and counteract the detrimental impact on signal efficiency. Different species vary in their response to level fluctuations of ambient noise, but it remains unclear why they vary. Here, we investigated whether noise-dependent flexibility may relate to singing style and signal function of the flexible acoustic trait. Species with highly variable songs may generally be more flexible and strongly repetitive singers may be more limited to stray from their stringent patterns. We exposed males of four passerine species with contrasting singing styles (repertoire size, immediate or eventual variety singing and syllable diversity) to three experimental sound conditions: 1) continuous urban noise; 2) intermittent white noise and 3) conspecific song playback. We found no spectral or temporal changes in response to experimental noise exposure in any of the four species, but significant temporal adjustment to conspecific playback in one of them. We argue that the consistency in song frequency and timing may have signal value, independent of singing style, and therefore be an explanation for the general lack of noise-dependent flexibility in the four species of the current study.

Deforestation Impacts on Bat Functional Diversity in Tropical Landscapes.

Functional diversity is the variability in the functional roles carried out by species within ecosystems. Changes in the environment can affect this component of biodiversity and can, in turn, affect different processes, including some ecosystem services. This study aimed to determine the effect of forest loss on species richness, abundance and functional diversity of Neotropical bats. To this end, we identified six landscapes with increasing loss of forest cover in the Huasteca region of the state of Hidalgo, Mexico. We captured bats in each landscape using mist nets, and calculated functional diversity indices (functional richness and functional evenness) along with species richness and abundance. We analyzed these measures in terms of percent forest cover. We captured 906 bats (Phyllostomidae and Mormoopidae), including 10 genera and 12 species. Species richness, abundance and functional richness per night are positively related with forest cover. Generalized linear models show that species richness, abundance and functional richness per night are significantly related with forest cover, while seasonality had an effect on abundance and functional richness. Neither forest cover nor season had a significant effect on functional evenness. All these findings were consistent across three spatial scales (1, 3 and 5 km radius around sampling sites). The decrease in species, abundance and functional richness of bats with forest loss may have implications for the ecological processes they carry out such as seed dispersal, pollination and insect predation, among others.

Beta Diversity in a Highly Heterogeneous Area: Disentangling Species and Taxonomic Dissimilarity for Terrestrial Vertebrates.

Quantifying differences in species composition among communities provides important information related to the distribution, conservation and management of biodiversity, especially when two components are recognized: dissimilarity due to turnover, and dissimilarity due to richness differences. The ecoregions in central Mexico, within the Mexican Transition Zone, have outstanding environmental heterogeneity and harbor huge biological richness, besides differences in the origin of the biota. Therefore, biodiversity studies in this area require the use of complementary measures to achieve appropriate information that may help in the design of conservation strategies. In this work we analyze the dissimilarity of terrestrial vertebrates, and the components of turnover and richness differences, among six ecoregions in the state of Hidalgo, central Mexico. We follow two approaches: one based on species level dissimilarity, and the second on taxonomic dissimilarity. We used databases from the project "Biodiversity in the state of Hidalgo". Our results indicate that species dissimilarity is higher than taxonomic dissimilarity, and that turnover contributes more than richness differences, both for species and taxonomic total dissimilarity. Moreover, total dissimilarity, turnover dissimilarity and the dissimilarity due to richness differences were positively related in the four vertebrate groups. Reptiles had the highest values of dissimilarity, followed by mammals, amphibians and birds. For reptiles, birds, and mammals, species turnover was the most important component, while richness differences had a higher contribution for amphibians. The highest values of dissimilarity occurred between environmentally contrasting ecoregions (i.e., tropical and temperate forests), which suggests that environmental heterogeneity and differences in the origin of biotas are key factors driving beta diversity of terrestrial vertebrates among ecoregions in this complex area.

Vertebrate dissimilarity due to turnover and richness differences in a highly beta-diverse region: the role of spatial grain size, dispersal ability and distance.

We explore the influence of spatial grain size, dispersal ability, and geographic distance on the patterns of species dissimilarity of terrestrial vertebrates, separating the dissimilarity explained by species replacement (turnover) from that resulting from richness differences. With data for 905 species of terrestrial vertebrates distributed in the Isthmus of Tehuantepec, classified into five groups according to their taxonomy and dispersal ability, we calculated total dissimilarity and its additive partitioning as two components: dissimilarity derived from turnover and dissimilarity derived from richness differences. These indices were compared using fine (10 x 10 km), intermediate (20 x 20 km) and coarse (40 x 40 km) grain grids, and were tested for any correlations with geographic distance. The results showed that total dissimilarity is high for the terrestrial vertebrates in this region. Total dissimilarity, and dissimilarity due to turnover are correlated with geographic distance, and the patterns are clearer when the grain is fine, which is consistent with the distance-decay pattern of similarity. For all terrestrial vertebrates tested on the Isthmus of Tehuantepec both the dissimilarity derived from turnover and the dissimilarity resulting from richness differences make important contributions to total dissimilarity, and dispersal ability does not seem to influence the dissimilarity patterns. These findings support the idea that conservation efforts in this region require a system of interconnected protected areas that embrace the environmental, climatic and biogeographic heterogeneity of the area.

Depredación de nidos artificiales en cercas vivas de un sistema agro-urbano en Hidalgo, México / Artificial nest predation in hedgerows of an agro-urban system in hidalgo, Mexico

En muchos agroecosistemas de América Latina las cercas vivas representan un hábitat importante para muchas especies de aves; sin embargo, aún no se conocen cuáles son las características de las cercas vivas y del paisaje circundante que influyen sobre la depredación de nidos en estas estructuras. Utilizando nidos artificiales, se estudió la depredación en 20 cercas vivas de un sistema agro-urbano del centro de México. Los datos de depredación de nidos se relacionaron con variables del hábitat a dos escalas espaciales, local y del paisaje. De los 200 nidos colocados 80 fueron depredados, y no se encontró ninguna relación significativa entre la depredación total y las variables a escala local. A escala del paisaje se observó que la depredación total aumentó en cercas vivas localizadas cerca de áreas cubiertas con vegetación nativa. Se encontró también que los principales depredadores fueron aves, seguidas de carnívoros y roedores.

In many agro-ecosystems of Latin America, hedgerows are an important habitat for numerous bird species. However, the hedgerow characteristics and those of the surrounding landscape affecting nest predation in these structures are not known. Using artificial nests, predation was studied in 20 hedgerows located in an agro-urban system in central Mexico. Predation data was related with habitat variables at two spatial scales, local and landscape. Out of a total of 200 artificial nests put in place, 80 nests were predated and no significant relationship was found between total predation and local scale variables. At the landscape scale, total predation increased in hedgerows located near remnants of native vegetation. It was also found that birds were the main predators in this landscape, followed by carnivores and rodents.

Trends in the measurement of alpha diversity in the last two decades

El interés reciente en biología de la conservación ha fomentado el estudio de la diversidad de especies y el surgimiento de nuevos métodos para medirla. En este trabajo se evalúa la forma en que la diversidad alfa ha sido medida en artículos publicados en dos revistas de ecología de 1982 a 2002. Se encontraron 244 artículos que miden la diversidad alfa, y el número de artículos por año aumenta a través del tiempo con una tasa de incremento mayor después de 1991. La medida más popular de la diversidad es la riqueza de especies, pero desde 1994 se ha incrementado el uso de estimadores de riqueza. La mayoría de los artículos son sobre estudios con animales, pero cuando los dividimos en vertebrados e invertebrados, estos dos grupos incluyen menos artículos que los dedicados a plantas. Para estos tres grupos el número de artículos incrementa en el tiempo. Se observó también un incremento tanto en el número de artículos escritos por autores que laboran en Norte América como por autores que laboran en otras regiones; sin embargo, los autores norteamericanos publican la mayoría de los trabajos. De manera similar, el número de estudios realizados en Norte América y el número de estudios realizados en otras regiones se incrementa a través del tiempo, pero se realizan más estudios en Norte América y estos son publicados a una tasa más elevada