Density-dependent melanism in sub-arctic populations of winter moth larvae (Operophtera brumata)

Abstract.  1. The aim of this 4-year observational study was to test for the presence of direct and delayed density-dependent larval melanism in the geometrid moth species Operophtera brumata (winter moth) in northern Norway.
2. Data from many populations with a wide range of population densities in time and space facilitated statistical analyses that could separate the effects of current and past density. The data also included different phases of the 10-year population cycle of this species so that eventual non-linear density effects due to population phase could be detected.
3. The results showed that the prevalence of melanism had a strong positive, linear relation to population density within years, whereas there was no evidence for a delayed effect from the year before or dependency on the phase of the population cycle.
4. In combination, these results limit the range of possible explanations of larval melanism in this outbreaking species. The possible reasons why winter moth larvae might benefit from crowding-induced melanism are discussed.     

Spatial genetic structure of a small rodent in a heterogeneous landscape

Gene flow in natural populations may be strongly influenced by landscape features. The integration of landscape characteristics in population genetic studies may thus improve our understanding of population functioning. In this study, we investigated the population genetic structure and gene flow pattern for the common vole, Microtus arvalis, in a heterogeneous landscape characterised by strong spatial and temporal variation. The studied area is an intensive agricultural zone of approximately 500 km² crossed by a motorway. We used individual-based Bayesian methods to define the number of population units and their spatial borders without prior delimitation of such units. Unexpectedly, we determined a single genetic unit that covered the entire area studied. In particular, the motorway considered as a likely barrier to dispersal was not associated with any spatial genetic discontinuity. Using computer simulations, we demonstrated that recent anthropogenic barriers to effective dispersal are difficult to detect through analysis of genetic variation for species with large effective population sizes. We observed a slight, but significant, pattern of isolation by distance over the whole study site. Spatial autocorrelation analyses detected genetic structuring on a local scale, most probably due to the social organisation of the study species. Overall, our analysis suggests intense small-scale dispersal associated with a large effective population size. High dispersal rates may be imposed by the strong spatio-temporal heterogeneity of habitat quality, which characterises intensive agroecosystems.     

The Function of Hitchhiking Behavior in the Leaf‐cutting Ant Atta cephalotes1

In some leaf‐cutting ant species, minim workers ride on the fragments of leaves as they are carried back to the nest from the cutting site. There is convincing evidence that these “hitchhikers” can protect the leaf carriers from attack by phorid (Diptera: Phoridae) parasitoids, but we consider the possibility of other functions for the hitchhiking behavior. It has been hypothesized that the hitchhikers (1) feed on leaf sap from the edges of the cut leaves; (2) ride back to the nest to save energy; (3) get caught on the fragments as they are cut, and hitchhike because they cannot (or will not) get off; and (4) begin the process of preparing the leaf to enter the fungal gardens in the nest, perhaps by removing microbial contaminants. We observed hitchhikers of Atta cephalotes in 14 nests at the La Selva Biological Station in Costa Rica. There was no difference in the proportion of leaf carriers with hitchhikers between day and night. Because the nests we observed were largely nocturnal, more than 90 percent of the hitchhiking occurred at night. The phorid parasitoids are usually considered to be diurnal, so the preponderance of nocturnal hitchhiking suggests other functions in addition to parasitoid defense. Hitchhikers spent more time in the defensive head‐up posture during the day, but spent more time in the head‐down posture at night. The head‐down posture may indicate cleaning or other leaf preparation. The hitchhikers were never observed feeding on sap. Hitchhikers frequently got onto and off of the fragments, and so they were not “marooned.” Few hitchhikers rode all the way back to the nest and were often moving on the leaf fragment; these observations make the energy conservation hypothesis less likely, although we cannot reject it. We conclude that parasitoid defense is an important function of hitchhiking but also that there are probably other functions when parasitoids are absent. Based on available data, the most likely possibility is preparation of the leaf fragment before it enters the nest.     

Habitat use of the endangered butterfly Euphydryas maturna and forestry in Central Europe

The knowledge of ecological requirements of declining butterflies of European woodlands remains limited, which hinders conservation management of their localities. This also applies for continentally threatened scarce fritillary Euphydryas maturna . On the basis of the largest data set on its habitat use ever collected in Central Europe, we analyse habitat requirements of its populations in Austria (A), the Czech Republic (Cz) and Germany (D). All studied populations inhabit open-canopy sites within woodlands, but larval survival decreases under full sun and preferred sites are relatively humid and sheltered. Nests of pre-hibernation larvae occur at terminal branches of Fraxinus excelsior , 1.5–3 m above the ground. Pre-hibernation mortality reaches 70% (Cz, D). Another limiting factor is quality of woodland vegetation: post-hibernation larvae consume a wide range of herbs and shrubs, and adult distribution is linked to nectar availability. The butterfly thus depends on highly heterogeneous early successional stages of deciduous woods, historically maintained by coppicing (Cz, D) and forest pasture (A). Restoration of these traditional methods offers the only chance for survival of E. maturna in Central Europe, and the butterfly may become a flagship for other threatened organisms of open-canopy woodlands.     

Edge effects reduce the nesting success of Acadian Flycatchers in a moderately fragmented forest

ABSTRACT.   Forest fragmentation can create negative edge effects that reduce the reproductive success of birds nesting near the forest/nonforest interface, and threaten bird populations deeper in remnant forest habitats. Negative edge effects may be more pronounced in landscapes that are moderately fragmented, particularly where agriculture is the primary land-use fragmenting forests. Information about the extent and strength of edge effects at a site can help guide conservation actions, and determine their effectiveness. We examined edge effects for birds breeding in a nearly contiguous forest fragmented by relatively narrow agricultural corridors in Illinois (USA). We measured rates of nest predation and brood parasitism for Acadian Flycatchers ( Empidonax virescens ) over a continuum of distances from the edge of an agricultural inholding. Nest predation and brood parasitism were highest near the edge and decreased with increasing distance from the edge. Given the cumulative effects of nest predation and brood parasitism on reproductive success, we determined that forest within 600 m of the inholding was sink habitat. We found, however, that deeper forest interior areas currently serve as source habitat, and that conversion of the entire 205 ha agricultural corridor to forest would add 1350 ha of source habitat for Acadian Flycatchers. Such results provide support for a local conservation strategy of forest consolidation and establish baseline measures necessary to determine the relative effectiveness of any subsequent reforestation efforts.