Gonzalo A. Ramírez-Cruz’s research while affiliated with Arizona State University and other places

Hummingbirds play an important role as pollinators and are one of the primary examples of animal–plant coevolution. However, factors such as land‐use change, urbanization, exotic species introductions and the disappearance of native plants can negatively affect the persistence of hummingbird populations. Thus, understanding the ecological factors that may favour their presence is essential for their effective conservation in urban areas. We conducted repeated surveys in a native scrubland reserve within Mexico City from 2015 to 2018, and used conditional two‐species occupancy models to estimate a species interaction factor, and to analyse the effect of five native plants ( Agave salmiana , Echeveria gibbiflora , Opuntia lasiacantha , O. tomentosa and Wigandia urens ) and one exotic plant ( Leonotis nepetifolia ) on the occupancy probability of two hummingbirds: the Broad‐billed Hummingbird ( Cynanthus latirostris ) and the Berylline Hummingbird ( Saucerottia beryllina ). Considering that both of the hummingbird species could be competing for similar resources, we also tested the effect of the presence of the Berylline Hummingbird on the occupancy of the Broad‐billed Hummingbird, assuming its dominance based on larger body‐size. We found evidence of positive spatial interactions between plants and hummingbirds, but we did not find these to be consistent throughout the 3 years of monitoring, which could indicate a modification of the original hummingbird–plant interaction networks resulting from urban disturbance. In addition, there were no negative spatial interactions between the two hummingbirds. Instead, we detected an aggregation of both species during two separate seasons, probably owing to one species using the other as a cue for profitable sites. Even though the populations of these two hummingbird species can thrive in an urban ecosystem, changes in spatial interactions might reflect alterations on the structure and functioning of a community. Therefore, we consider it crucial to continue studying their population dynamics and their implications for the conservation of urban hummingbirds.

Urbanization has forced animals to respond to novel environments. One of the basic questions in urban ecology is if ecological processes that occur in natural conditions also occur in urban environments. In natural ecosystems, key predators can limit prey abundance and have a widespread effect on trophic levels. Here, we examine a predator-prey interaction in Mexico City, an urban ecosystem where prey can be evaluated in the presence or absence of apex predators. Nearly all original ecosystems in Mexico City have been transformed for human purposes. Many of the green spaces in the city contain populations of the Mexican gray squirrel, a species native to this area. These green spaces are also inhabited by different species of raptors, the most frequent being Harris's hawk, Cooper's hawk, and sharp-shinned hawk. Little is known about the interactions between raptors and squirrels, particularly how different environmental factors influence this ecological relationship. We do know, however, that raptors prey on squirrels. We predicted that in parks where raptors were present, there would be fewer squirrels than in parks where there were no raptors. We studied the relationship between raptors and squirrels using occupancy models, which also allowed us to evaluate environmental factors that affect the presence of both squirrels and raptors. We also tested if the presence of raptors influenced the occupancy probability of squirrels, and vice versa. Lastly, we estimated the abundance of squirrels in parks, both where raptors were present and where they were absent. Contrary to our predictions, in our first two sampling periods we found a positive relationship between the presence of raptors and squirrel abundance, but this relationship was absent during our second two survey periods, which was due to a decline in squirrel abundance in parks where raptors were present. These results suggest that predator-prey interactions occur in urban settings much as they do under natural conditions, but that other factors unique to urbanization can also impact the abundance of raptors and squirrels. Our findings also suggest that raptors might be an effective means of controlling squirrel populations under some conditions.

Urban ecological reserves are large green areas immersed within cities where native and exotic species of plants and animals coexist. Here, we examined the environmental features that facilitate the occurrence of nine species of native birds in an urban ecological reserve located within one of the largest cities in the world, Mexico City. We also searched for changes in occupancy rates among the three distinct climatic seasons that occur in central Mexico: warm-dry, rainy, and cold-dry. Using data collected during four years and multi-season occupancy models, we found that most of our study species prefer the urbanized sites that surround the reserve over the core conservation areas. This urban affinity can be explained by the diverse vegetation that prevails in such urban sites, which offers a high habitat heterogeneity that facilitates the presence of bird species with distinct ecological needs. In contrast, the reserve consists of a relatively homogeneous xerophytic scrubland where a few species of shrubs and small trees are dominant. We also detected seasonal changes in five species, with highest occupancy during the warm-dry season of each year, which coincides with both their breeding season and the driest period of the year. This finding indicates that these birds find in the reserve and surrounding urban areas enough food and water during this limiting season as well as safe nesting sites. Our study provides evidence that some native birds can become urban exploiters and that the benefits that they obtain from urban settings are greatest during their breeding season.

Urban ecological reserves are relatively large green areas immersed within cities that serve the purpose of preserving the native species of plants and animals that still persist in urban settings. In this study, we examined temporal changes (among seasons and years) in site occupancy rates of nine species of native birds that inhabit an urban ecological reserve located within one of the largest cities in the world, Mexico City. Using data collected during 11 field seasons that spanned a four-year period and implementing multi-season occupancy models, we estimated occupancy, colonization, local extinction, and detection probabilities of all nine species. For more than half of our study species, the highest occupancy and colonization rates occurred at or near the urban areas that surround the reserve. Additionally, in three species local extinction probabilities were lowest also at these urban sites. This urban affinity can be explained by the diverse vegetation that prevails in the surrounding urban sites, which offers a high habitat heterogeneity that facilitates the presence of bird species with distinct ecological needs. In contrast, the ecological reserve consists of a relatively homogeneous xerophytic scrubland where a few species of shrubs and small trees are dominant. We did not detect long-term decreasing trends in site occupancy of any species. Instead, in three species we observed positive trends in occupancy during particular periods. Overall, our findings indicate that the combination of this urban ecological reserve with the green spaces of the surrounding urban areas harbors stable populations of native birds.

Invasive species hinder the conservation objectives of natural protected areas, particularly of those found within or nearby urban settlements. Identifying the habitat and landscape traits that determine the establishment and persistence of populations is essential for implementing effective management plans to control invasive species. We employed multi-season occupancy models to identify the habitat and landscape traits that determined the local colonization and extinction probabilities of an invasive bird (House Sparrow—Passer domesticus), in order to provide recommendations for controlling its population within a natural protected area immersed in Mexico City. We selected traits that exhibited management potential to provide feasible recommendations for controlling the species. We observed that increasing values of shrub cover, tree cover, and distance to developed areas discouraged the sparrow from invading new sites of the reserve. Simultaneously, greater distances to developed areas promoted the extinction of the species across invaded sites. These effects might be related to resource availability, foraging preferences of the species, predatory exposure, and competition. Preserving tree and shrub cover as a natural barrier for dissuading species arrival represents a key management objective for its control in the reserve. Greater management efforts must be focused at those sites of the reserve that are closer to developed areas, given that the latter may function as source habitats for the House Sparrow. Our approach for identifying management actions that impact the population dynamics of an invasive species might provide crucial results to control this type of populations. Such a strategy could be replicated for other species and natural protected areas to enhance the conservation value of reserves and provide alternatives when dealing with invasive species.

The evolution of matrotrophy (post-fertilization maternal provisioning to developing embryos) has been explained through several hypotheses. Trexler and DeAngelis proposed in 2003 a theoretical model that defines the ecological conditions under which matrotrophy would be favored over lecithotrophy (pre-fertilization maternal provisioning). According to this model, matrotrophy offers a selective advantage in environments with abundant and constantly available food, whereas environments with limited and fluctuating food resources should instead promote a lecithotrophic mode of maternal provisioning. This model also proposes that matrotrophy entails the consequence of leaner reproductive females and in turn shorter lifespans. In this study, we examined the Trexler-DeAngelis model using data from 45 populations of five viviparous species from the fish genus Poeciliopsis (family Poeciliidae). We used the matrotrophy index (MI) as a measure of post-fertilization maternal provisioning, and the index of stomach fullness and individual body condition (BC) as proxies for food availability. We also estimated the magnitude of fluctuations in food availability by calculating the temporal variances of these two proxies. Neither abundant nor constantly available food were associated with greater degrees of matrotrophy, which fails to support the predictions of the Trexler-DeAngelis model with respect to the ecological drivers of increased post-fertilization provisioning to embryos. Nonetheless, in all five species we observed that females with greater degrees of matrotrophy had poorer BC compared to females that provided less nutrients to embryos after fertilization. This finding is consistent with one of the expected consequences of advanced matrotrophy according to the Trexler-DeAngelis model, namely, a detriment to the nutritional status of females. Our study provides compelling evidence that gestating females experience a trade-off between post-fertilization provisioning to embryos and self-maintenance, revealing in turn that matrotrophy is a costly reproductive strategy.

Despite being highly modified environments, cities are important reservoirs of biodiversity. The ecological reserve situated within southern Mexico City houses several species of mammals, including the ringtail Bassariscus astutus. However, the urban areas surrounding the reserve are subject to various types of uses that could affect the presence of wild species inside and out of the reserve areas. Here, I tested the hypothesis that recreational activities, especially the walking of domestic dogs, can cause the ringtails, smaller sized carnivores, to avoid certain areas. To achieve this, I carried out a survey throughout the reserve and urban landscape, and registered the presence of domestic dogs and ringtails by direct observation or via the detection of feces and latrines during two sampling seasons. I used single species occupancy models to determine the environmental covariates associated with the presence of these carnivores within the reserve and the urban areas around it. In addition, I used two-species occupancy models to estimate the species interaction factor between these species to assess if the presence of domestic dogs has a negative effect over the presence of ringtails. My results reveal that shrub cover in the area affects the occurrence of these species, and that there is no clear evidence that domestic dogs negatively affect the occupancy of ringtails during the studied seasons. These results highlight the capacity of the ringtails to adapt to rapid urbanization, but do not guarantee that recreational activities, especially those involving dogs, have no negative effects on the local distribution of other smaller mammals within this urban reserve.