Owl population restoration in Luxembourg

Central European cultural landscapes used to be mosaics of meadows, orchards, hedgerows, fields, and forests.  Unfortunately, most of these landscapes have been recently homogenized and converted to monocultures.  This process has been a typical result of agricultural intensification.  In the second half of the 20th century, this homogenization has further increased due to land being used to grow plant resources for producing "green energy" such as biofuel and biogas.  The intensification of farming techniques such as the use of inorganic fertilizers and pesticides, and the conversion of diverse landscapes into intensive monocultures, has resulted in widespread severe population declines for a variety of different animal groups.  Local- and country-specific management actions are being implemented to prevent further losses of farmland birds.  For successful conservation, measures need to include actions like hedgrow provision to improve the feeding and breeding habitats of farmland birds.  However, conservation measures vary between species, thus for effective species conservation we must consider species-specific habitat requirements.

The little owl (Athene noctua) is a nocturnal raptor.  It is a small owl, usually 22 cm tall with a wingspan of 56 cm, and weighing about 180 g.  The core of its distribution is located in the temperate steppes and deserts of the Mediterranean region, including north- and northeast Africa; but it inhabits much of the temperate and warmer parts of Europe, and Asia eastward to Korea (figure 1).  This bird uses meadows, grasslands, and fields for hunting, and it nests in trees.  The destruction of forests, and the conversion of these ecosystems to open agricultural land allowed A. noctua to colonize major parts of central Europe.  Today, it often nests in old trees of high-stem orchards, and in buildings and quarries with suitable cavities.  But A. noctua populations have severely declined throughout Europe in recent decades, and the species is now red-listed in several European countries.  Local conservation measures have included the installation of nesting boxes in potentially-suitable habitats, preferrably in high-stem orchards.

Figure 1:  Geographical distribution of the little owl (Athene noctua).

In Luxembourg, A. noctua is near extinction.  85% of Luxembourg's land surface is agriculture and forest, and in recent decades there have been strong increases in agricultural intensification, livestock, and urban expansion.  These landscape changes have resulted in the loss of shrubs and trees; and this loss in landscape- and habitat diversity has led to the loss of many arthropods and small mammals, these being the main food sources for A. noctua.  Inventories of breeding pairs of A. noctua in Luxembourg during recent decades shows a severe population decline:  In 1960, there was an estimated 4200 breeding pairs; but only 15 to 20 breeding pairs existed in 2002.  To prevent further population collapse, 450 nesting boxes were installed since 1999 in major parts of Luxembourg.

63 study sites in high-stem orchards were randomly selected.  The presence- or absence of A. noctua at nesting boxes was recorded for each study site, and the distance to the next settlement was measured.  Presence/absence was assessed during the mating season (march and april) by using audio recordings of male territory calls, invoking responses from other members of the species.

28 of the 63 study sites (27 of the 38 sites with nesting boxes) were occupied by A. noctua in 2012.  The probability of A. noctua presence was much higher at sites with nesting boxes than sites without nesting boxes.  This pattern was consistent across the entire study region, and proximity to other A. noctua breeding pairs had no detectable effect on presence/absence of the species; this further suggests that, in Luxembourg, nesting-site availability is the limiting factor for this species.  The high relevance of nesting boxes for conservation was also seen in studies in Germany, where about 90% of all A. noctua pairs were breeding in artificial nesting boxes.  Originally, old trees in high-stem orchards, as well as old buildings, provided important nesting sites.  However, these structures have largely vanished in today's landscapes.  It should be noted that, for many species, nesting-site availability has been shown to be crucial for maintaining populations.  A combination of fields suitable for hunting and nesting sites for breeding will be the most successful conservation measure for A. noctua.

A. noctua colonized central Europe during the beginning of traditional farming practices in this region.  A. noctua is a Eurasian- and Mediterranean steppe species.  There is the question of whether species whose biogeographical core-distribution is located outside of central Europe should be target-species for nature conservation in central-European countries.  More than 70% (136 species) listed in the European Birds Directive have their core distribution outside of Europe.  Nevertheless, this secures investment in the designation of protected areas for these species.  However, it may be of higher conservation value for these countries to focus on species whose core distribution is in central Europe.

LINK to Habel et al.'s 2015 article in Biodiversity and Conservation.

Butterfly diversity in Prague

Intense increases in urbanization worldwide are causing ecologists and conservation biologists to increase their focus on urban areas.  Responses of butterflies to urbanization have been studied in various regions of the world, and studies agree that densities of specialized species decrease towards city centers.  Species that exploit urban environments have been observed to reach higher densities in cities than elsewhere; and in the case of butterflies, these species may depend on ornamental plants.  Population densities of species that are suburban-adaptable are lower in cities than in rural landscapes, and there should be conservation efforts focusing on enhancing populations of suburban-adaptable species and species that tend to completely avoid urban environments, as populations of these species are more likely to be threatened and declining.

To efficiently conserve butterfly populations, knowledge of the distribution of individual species is required, as well as an understanding of how species respond to urbanization.  Local studies from individual cities may show us general patterns that may be applicable between different geographic regions.  Researchers in Prague, Czech Republic, addressed this by comparing local butterfly communities (including burnet moths) observed in 25 urban reserves and parks.

With 88 nature reserves (total area of 2350 hectares), Prague harbors a noteworthy diversity of species.  This is due to Prague's rugged terrain, diverse bedrock, and its location in a warm Vlatava River valley.  These nature reserves consist of a broad diversity of habitats including wetlands, old-growth forest, and dry grasslands on rocky slopes.  Towards the city center, nature reserves are replaced by parks.

85 butterfly species (47% of total butterfly species in Czech Republic) were recorded in the Prague butterfly study, and 22 of the species recorded are threatened in Czech Republic.  The three most species-rich sites (Prokopske udoli, Dalejsky profil, Radotinske udoli) were associated with deep calcareous valleys with rich- and diverse ecosystems, from rocks to grasslands and woodland.  The most species-poor sites were parks near the city center, or small nature reserves with high proportions of forest cover.

Butterfly communities inhabiting Prague's nature reserves and parks were shown to respond positively to a gradient from small sites with homogenous conditions (low altitude range, low plant species richness) to large, diverse sites at the outskirts of Prague.  This is in agreement with other butterfly biodiversity studies.  Sites with suitable conditions for diverse butterfly assemblages are typically far from the city center, as there is more unaltered space in the city's peripheral areas.  Therefore, as in other butterfly studies in urban areas, it was observed that butterfly communities change along a gradient of urbanization.  Habitat availability is the most likely factor shaping a site's butterfly community.  Particularly, sites harboring more plant species will likely contain the larval host-plants of more butterfly species.

From the data in this study, the authors grouped Prague's butterfly species into three categories, regarding each individual species' response to urbanization:  urban-avoiding (increasing outwards from city center), suburban-adaptable (optimum conditions at intermediate urbanization levels), and urban-tolerant (no recognizable response to urbanization).

In Prague, urban-avoiders included some extreme habitat specialists, such as sensitive grassland species (Pseudophilotes vicrama, Pyrgus carthami) and a dry habitat species that requires very large areas of suitable habitat, Hipparchia semele.  Most urban-avoiders were inhabitants of rural woodlands and grasslands; remnants of rural landscape are being increasingly fragmented by urban sprawl, and are practically nonexistent in city centers.

Suburban-adaptable butterflies were the majory of the species observed in this study.  This group contained many dry-habitat species that are supported by large dry grassland reserves in the outskirts of Prague, and also by a dense network of industrial barrens, railways tracks, and abandoned quarries scattered throughout Prague's suburbs.  Some common (Coenonympha pamphilus, Thymelicus lineola) and highly mobile (Issoria lathonia, Inachis io) species were associated with this group, perhaps due to their ability to develop on weedy plants colonizing urban barrens, ot their use of ornamental plants in parks and gardens.  Numerous researchers have indicated that warm- and dry microclimates of industrial- and urban barrens harbor insects.  Approximately one-third of the 85 species (and approximately half of the 60 common species) recorded from this study find their optimum habitat in Prague's suburban belt.  Prague's suburban belt represents an important sanctuary for endangered butterflies.

There were no positive responses to increasing urbanization in this study, indicating that no butterfly species preferred urban environments.  Green areas near the center of Prague provide suitable habitat for arboreal canopy butterflies, but not for grassland butterflies occurring on the ground.  The common grassland butterfly species are absent here most-likely due to inappropriate management of lawns (too frequent- or too clean mowing).  In rural hay meadows, frequent- and uniform mowing has been shown to cause rapid crashes in butterfly populations; and in urban areas this impact on nectar availability for adults, and survival of egg and larva stages, must be even stronger.  Simple, cheap modifications, such as leaving parcels of land unmanaged, would considerably improve to suitability of urban green spaces for butterflies.  Planting selected host plants in parks and gardens would be an additionally-effective measure.  Small measures applied across large urban areas can considerably increase resources for butterflies, increase connectivity between urban nature reserves, and help prevent the loss of species.

LINK to Konvicka and Kadlec's 2011 article in European Journal of Entomology.