Eduardo T. Mezquida

The geographic selection mosaic for squirrels, crossbills and Aleppo pine

The interactions between many species are structured in a geographic mosaic of populations among which selection is divergent. Here we tested the hypothesis that such a geographic selection mosaic arises for common crossbills (Loxia curvirostra) feeding on seeds in the cones of Aleppo pine (Pinus halepensis) because of geographic variation in the occurrence of European red squirrels (Sciurus vulgaris). On the Iberian Peninsula, Sciurus exerted directional selection favouring larger cones with larger scales, which has caused cones there to be larger than in the Balearic Islands where Sciurus are absent. Moreover, cones on the Iberian Peninsula are so large that they are apparently little used by the relatively small‐billed crossbills on the Peninsula; selection by Sciurus seems to have made the cones so difficult to feed on that crossbills rely mostly on the seeds of other conifers. Where crossbills are present but Sciurus are absent (Mallorca Island), cones were smaller as a result of relaxation of selection by Sciurus. However, cones on Mallorca had proportionally thicker scales in comparison to where both Sciurus and crossbills are absent (Ibiza Island), presumably as an adaptation against crossbill predation. Here crossbills specialize on Aleppo pine, have relatively large bills and have apparently coevolved in an arms race with Aleppo pine. These results suggest that Sciurus has influenced both the geographic selection mosaics for crossbills and conifers and the adaptive radiation of crossbills in Eurasia much like Tamiasciurus has done in the North America.

ARE RESULTS OF ARTIFICIAL NEST EXPERIMENTS A VALID INDICATOR OF SUCCESS OF NATURAL NESTS

Abstract Studies using artificial nests to assess rates or patterns of nest predation have increased during recent decades. However, the critical assumption that temporal or spatial patterns of relative predation rates of artificial nests parallel those of natural nests has begun to be questioned. We compared the daily survival rate for 365 artificial nests with that estimated for 295 open nests of passerines, and analyzed the trends in survival rates for both nest types among three breeding seasons and three species of nest plants in the central Monte Desert, Argentina. Daily survival rate for artificial nests (0.737) was significantly lower than that for natural nests (0.925). Trends in survival rates of artificial nests over years and among species of nest plants only partially reflected those of natural nests. Our results support the well established finding that artificial nests may not accurately estimate actual rates of nest predation. Artificial nests may provide an additional source of data to natural nests when testing ecological hypotheses, but artificial nest experiments should be carefully designed (i.e., realistic) and should attempt to identify predators of artificial and natural nests to validate the experimental results.

SAGE-GROUSE AND INDIRECT INTERACTIONS: POTENTIAL IMPLICATIONS OF COYOTE CONTROL ON SAGE-GROUSE POPULATIONS

Abstract ABSTRACT Coyotes (Canis latrans) are lethally controlled throughout the range of Greater Sage-Grouse (Centrocercus urophasianus) and it has been suggested that such control may benefit sage-grouse. However, the perceived benefits of control are based on the direct effects of coyotes on sage-grouse and largely ignore potential indirect interactions. Here, we summarize some of the evidence for direct effects in a simplified food web including coyotes and sage-grouse. There is very little evidence to suggest that coyotes have much of a direct negative effect on sage-grouse, but there is considerable evidence supporting direct interactions that would lead to positive indirect effects between coyotes and sage-grouse. The three likely forms of positive indirect effects arise because coyotes reduce the potential negative effects resulting from mesopredator release and apparent and exploitative competition. Mesopredator release would adversely affect sage-grouse if a decrease in coyotes allowed an increase in foxes (especially Vulpes vulpes), badgers (Taxidea taxus), and Common Ravens (Corvus corax), mesopredators that prey on sage-grouse eggs and young. A decrease in coyotes is likely to allow jackrabbits (Lepus spp.) to increase, which would cause sage-grouse to suffer from apparent competition if Golden Eagles (Aquila chrysaetos), which are perhaps the most important predator of adult sage-grouse, then increase in response to the increase in jackrabbits. This increase in jackrabbits may also depress the availability of sagebrush (Artemisia spp.) and forbs, leading to an increase in exploitative competition with sage-grouse. For these reasons, we argue that intense and extended lethal coyote control is likely detrimental to sage-grouse conservation.

Microhabitat structure and avian nest predation risk in an open Argentinean woodland: an experimental study

Abstract We studied the effect of the general structure of the nest plant, especially the presence of thorns, and the structural homogeneity of the nest patch, on the vulnerability of nests to predation, using natural and artificial nests. Artificial nests placed in non-thorny plants had a significantly lower predation rate and higher daily survival rate than those in thorny plants. The addition of a ‘thorny microhabitat’ around the immediate proximity of nests placed in non-thorny plants did not have any effect on vulnerability of nests to predation. Conversely, natural nests were located in patches of habitat with a higher density of the species of plant that supported the nest compared to patches selected at random. However, daily survival rate was similar for natural nests placed in patches with a higher or lower density of the species of nest plant in the four bird species analysed. Similarly, survival of artificial nests did not increase with the presence of a higher number of plants similar to the nest plant in the nest patch. Thus, the observed patterns of survival for natural and artificial nests did not seem to support the potential prey-site hypotheses. Birds appeared to be the main nest predators in this ecosystem. Behavioural aspects of the identified predators and habitat structure could explain the lack of effect of thorns and nest patch characteristics on nest survival.

Patterns of coevolution in the adaptive radiation of crossbills

Although evidence for coevolution and geographic variation in its apparent strength is increasing, we still have a relatively poor understanding of why coevolution varies among interactions. Here we review how variation in the occurrence of competitors, resource stability, habitat area, and time has affected the extent of trait escalation in coevolutionary arms races between crossbills (Loxia) and conifers. Competitors for conifer seeds, particularly tree squirrels, have limited the extent of crossbill–conifer coevolution; however, seed crop fluctuations reduce the extent to which tree squirrels limit crossbill–conifer coevolution. Crossbill densities increase with forest area, which results in greater escalation of seed defenses apparently as the result of stronger selection exerted by crossbills. The extent of trait escalation appears to increase toward lower latitudes where crossbill–conifer interactions have likely persisted locally for longer periods of time. However, because most crossbills occur at higher latitudes, much of the extant diversification of crossbills has occurred since the last glacial retreat, and the extent of trait escalation is limited. Nevertheless, coevolution has caused considerable trait evolution even in temperate regions. The conditions favoring coevolution between crossbills and conifers are widespread, and coevolution has played at least some role in at least three fourths of the taxa of crossbills.

COEVOLUTION BETWEEN HISPANIOLAN CROSSBILLS AND PINE: DOES MORE TIME ALLOW FOR GREATER PHENOTYPIC ESCALATION AT LOWER LATITUDE?

Abstract Crossbills (Aves: Loxia) and several conifers have coevolved in predator–prey arms races over the last 10,000 years. However, the extent to which coevolutionary arms races have contributed to the adaptive radiation of crossbills or to any other adaptive radiation is largely unknown. Here we extend our previous studies of geographically structured coevolution by considering a crossbill–conifer interaction that has persisted for a much longer time period and involves a conifer with more variable annual seed production. We examined geographic variation in the cone and seed traits of two sister species of pines, Pinus occidentalis and P. cubensis, on the islands of Hispaniola and Cuba, respectively. We also compared the Hispaniolan crossbill (Loxia megaplaga) to its sister taxa the North American white-winged crossbill (Loxia leucoptera leucoptera). The Hispaniolan crossbill is endemic to Hispaniola whereas Cuba lacks crossbills. In addition and in contrast to previous studies, the variation in selection experienced by these pines due to crossbills is not confounded by the occurrence of selection by tree squirrels (Tamiasciurus and Sciurus). As predicted if P. occidentalis has evolved defenses in response to selection exerted by crossbills, cones of P. occidentalis have scales that are 53% thicker than those of P. cubensis. Cones of P. occidentalis, but not P. cubensis, also have well-developed spines, a known defense against vertebrate seed predators. Consistent with patterns of divergence seen in crossbills coevolving locally with other conifers, the Hispaniolan crossbill has evolved a bill that is 25% deeper than the white-winged crossbill. Together with phylogenetic analyses, our results suggest that predator–prey coevolution between Hispaniolan crossbills and P. occidentalis over approximately 600,000 years has caused substantial morphological evolution in both the crossbill and pine. This also indicates that cone crop fluctuations do not prevent crossbills and conifers from coevolving. Furthermore, because the traits at the phenotypic interface of the interaction apparently remain the same over at least several hundred thousand years, divergence as a result of coevolution is greater at lower latitude where crossbill–conifer interactions have been less interrupted by Pleistocene events.

Nest site selection and nesting success of five species of passerines in a South American open Prosopis woodland

I analyzed nest site preferences and whether these preferences were linked to nest success of five bird species breeding in an arid area of southern South America. Most nests (90%) were located in three plant species (Geoffroea decorticans, Capparis atamisquea, and Atriplex lampa). Serpophaga griseiceps, Poospiza ornata, and P. torquata nested in plants with size significantly different from randomly selected plants. At the mesohabitat scale (i.e., habitat patch surrounding the nest), four species showed clear differences from random patches within the general habitat, and the remaining species (Saltatricula multicolor) showed a non-significant trend. The daily survival rate of S. griseiceps nests placed in the preferred plant species (i.e., G. decorticans) was lower than those in the other nest-plants. In the other four bird species, nest survival did not differ between preferred and other plants used for nesting. At the scale of microhabitat (i.e., plant size and location of the nest inside the plant), no significant differences were detected between unsuccessful and successful nests. Similarly, unsuccessful nest patches did not differ significantly in habitat features from those that were successful. High nest predation rates found in this habitat (around 85% for the total assemblage) and a relatively high diversity of predators (and of predation tactics) appear to impede the presence of “safe” sites for nesting (i.e., there are trade-offs between avoidance strategies).

Evaluation of the potential index model to predict habitat suitability of forest species: the potential distribution of mountain pine ( Pinus uncinata ) in the Iberian peninsula

Characterization of the suitability or potentiality of a territory for forest tree species is an important source of information for forest planning and managing. In this study, we compared a relatively simple methodology to generate potential habitat distribution areas that has been traditionally used in Spain (the potential index model) with a statistical modelling approach (generalized linear model). We modelled the potential distribution of mountain pine (Pinus uncinata) in the Iberian peninsula as a working example. The potential index model generated a map of habitat suitability according to the values of an index of potentiality, whose distribution has usually divided into four categories based on quartiles (from optimum to low suitability). Considering all values of the index of potentiality as presences of mountain pine resulted in a low to moderate degree of agreement between the potential index model and the generalized linear model according to the kappa coefficient. Using the cut-off value of the index of potentiality that maximized the degree of agreement between both modelling approaches resulted in a substantial similarity between the maps of the predicted distribution of mountain pine. This cut-off value did lie in the upper-third quartile of the potential index distribution (high suitability category), and roughly coincided with the upper 30th percentile. The use of statistical techniques, which have proved to be powerful and versatile for species distribution modelling, is recommended. However, the potential index model, together with the adjustments proposed here, could be a reasonably simple methodology to predict the potential distribution of forest tree species that forest managers should take into account when evaluating forestation and afforestation projects.

Habitat area and structure affect the impact of seed predators and the potential for coevolutionary arms races

Both habitat patch size and structure affect the abundance and occurrence of species and thereby can affect the ecology and evolution of species interactions. Here we contrast the level of seed predation and selection exerted by Common Crossbills (Loxia curvirostra complex) and red squirrels (Sciurus vulgaris) in the extensive mountain pine (Pinus uncinata) forests in the Pyrenees with their level of seed predation in two small, isolated forests. Crossbills consumed 5.1 times more seeds in the Pyrenees than in the isolated forests, and six of seven cone traits under selection by crossbills were enhanced in the Pyrenees. In contrast, red squirrels tend to be uncommon in the open mountain pine forests, consuming relatively few seeds in both regions and having limited impact on both mountain pine and the interaction between crossbills and mountain pine. Resident crossbills in mountain pine forests in the Pyrenees have larger bills than in nearby forests, consistent with local adaptation by crossbills and a coevolutionary arms race between crossbills and mountain pine. The mechanisms leading to variation in the interaction between crossbills and mountain pine should be general to many systems because habitat patch size and structure often vary across the range of a species.

Efficiency of pollination and satiation of predators determine reproductive output in Iberian Juniperus thurifera woodlands

Fruit production in animal-dispersed plants has a strong influence on fitness because large crops increase the number of seeds dispersed by frugivores. Large crops are costly, and environmental control of plant resources is likely play a role in shaping temporal and spatial variations in seed production, particularly in fluctuating environments such as the Mediterranean. The number of fruits that start to develop and the proportion of viable seeds produced are also linked to the number of flowers formed and the efficiency of pollination in wind-pollinated plants. Finally, large fruit displays also attract seed predators, having a negative effect on seed output. We assessed the relative impact of environmental conditions on fruit production, and their combined effect on seed production, abortion and seed loss through three predispersal predators in Juniperus thurifera L., sampling 14 populations across the Iberian Peninsula. Wetter than average conditions during flowering and early fruit development led to larger crop sizes; this effect was amplified at tree level, with the most productive trees during more favourable years yielding fruits with more viable seeds and less empty and aborted seeds. In addition, large crops satiated the less mobile seed predator. The other two predispersal predators responded to plant traits, the presence of other seed predators and environmental conditions, but did not show a satiation response to the current-year crop. Our large-scale study on a dioecious, wind-pollinated Mediterranean juniper indicates that pollination efficiency and satiation of seed predators, mediated by environmental conditions, are important determinants of reproductive output in this juniper species.