Regional population viability of grassland songbirds: Effects of agricultural management

Although population declines of grassland songbirds in North America and Europe are well-documented, the effect of local processes on regional population persistence is unclear. To assess population viability of grassland songbirds at a regional scale (∼150,000 ha), we quantified Savannah Sparrow Passerculus sandwichensis and Bobolink Dolichonyx oryzivorus annual productivity, adult apparent survival, habitat selection, and density in the four most (regionally) common grassland treatments. We applied these data to a female-based, stochastic, pre-breeding population model to examine whether current grassland management practices can sustain viable populations of breeding songbirds. Additionally, we evaluated six conservation strategies to determine which would most effectively increase population trends. Given baseline conditions, over 10 years, simulations showed a slightly declining or stable Savannah Sparrow population (mean bootstrap λ = 0.99; 95% CI = 1.00-0.989) and severely declining Bobolink population (mean bootstrap λ = 0.75; 95% CI = 0.753-0.747). Savannah Sparrow populations were sensitive to increases in all demographic parameters, particularly adult survival. However for Bobolinks, increasing adult apparent survival, juvenile apparent survival, or preference by changing habitat selection cues for late-hayed fields (highest quality) only slightly decreased the rate of decline. For both species, increasing the amount of high-quality habitat (late- and middle-hayed) marginally slowed population declines; increasing the amount of low-quality habitat (early-hayed and grazed) marginally increased population declines. Both species were most sensitive to low productivity and survival on early-hayed fields, despite the fact that this habitat comprised only 18% of the landscape. Management plans for all agricultural regions should increase quality on both low- and high-quality fields by balancing habitat needs, nesting phenology, and species’ response to management.     

Post-fledging dispersal, habitat use, and survival of Sprague’s pipits: Are planted grasslands a good substitute for native?

Loss and degradation of native grasslands on the North American landscape are some of the key factors influencing population declines of grassland songbirds. It is unclear what role anthropogenic grasslands play in the conservation of grassland specialists and whether demographic parameters of grassland songbirds differ between anthropogenic and native grasslands. Furthermore, there has been little research examining the overlap (or lack thereof) between nesting and post-fledging habitat. We initiated a radio-telemetry study from 2004 to 2008 on dispersal, microhabitat use, and survival of juvenile Sprague’s pipits (Anthus spragueii), a threatened grassland-obligate songbird, in native pastures and fields planted with exotic species (planted grasslands). Dispersal distance was greater in planted compared to native grasslands, but appeared to be constrained to natal fields up to 26-d post-fledging. Pipits used microhabitats in planted grasslands with vegetation that was on average 11 cm taller than used locations in native pastures, possibly due to the rapid growth of planted vegetation by the time individuals fledged. Individuals that were reared in planted grasslands had consistently lower daily survival rates compared to those in native pastures. Our observation that young birds rarely left their natal field suggests that conserving nesting habitat in native pastures would be an effective management strategy for this species. However, our results suggest that planted grasslands may act as population sinks given the lower survival of juvenile pipits reared in planted fields in our region. Demographic studies during the post-fledging period are essential to determine the conservation value of anthropogenic grasslands for North American songbirds.     

Density and success of bird nests relative to grazing on western Montana grasslands

Grassland birds are declining at a faster rate than any other group of North American bird species. Livestock grazing is the primary economic use of grasslands in the western United States, but the effects of this use on distribution and productivity of grassland birds are unclear. We examined nest density and success of ground-nesting birds on grazed and ungrazed grasslands in western Montana. In comparison to grazed plots, ungrazed plots had reduced forb cover, increased litter cover, increased litter depth, and increased visual obstruction readings (VOR) of vegetation. Nest density among 10 of 11 common bird species was most strongly correlated with VOR of plots, and greatest nest density for each species occured where mean VOR of the plot was similar to mean VOR at nests. Additionally, all bird species were relatively consistent in their choice of VOR at nests despite substantial differences in VOR among plots. We suggest that birds selected plots based in part on availability of suitable nest sites and that variation in nest density relative to grazing reflected the effect of grazing on availability of nest sites. Nest success was similar between grazed plots and ungrazed plots for two species but was lower for nests on grazed plots than on ungrazed plots for two other species because of increased rates of predation, trampling, or parasitism by brown-headed cowbirds (Molothrus ater). Other species nested almost exclusively on ungrazed plots (six species) or grazed plots (one species), precluding evaluation of the effects of grazing on nest success. We demonstrate that each species in a diverse suite of ground-nesting birds preferentially used certain habitats for nesting and that grazing altered availability of preferred nesting habitats through changes in vegetation structure and plant species composition. We also show that grazing directly or indirectly predisposed some bird species to increased nesting mortality. Management alternatives that avoid intensive grazing during the breeding season would be expected to benefit many grassland bird species.     

Matrix effects on bird assemblages in fragmented Afromontane forests in South Africa

We examine the effects of matrix type on forest bird species richness in historically fragmented forests of the KwaZulu-Natal midlands, South Africa. Bird species-area relationships for forest fragments within natural grassland were compared with those surrounded by plantation forestry (Pinus spp.). While fragments in grasslands displayed a species-area effect, no such effect (slope ∼0) was detected for fragments in plantations. The critical fragment size to avoid an island effect on species richness was 302 ha for fragments in grasslands, and contained 51 of the 61 forest bird species encountered. Small forest fragments (<50 ha) within grasslands were less species-rich than those surrounded by plantation, while the reverse was true for forests larger than 50 ha. Bird density was significantly lower in larger forest patches within plantation when compared to those in grassland. However, a slight decline in species density with increasing species richness in a patch (i.e. weak density compensation) characterised only those bird assemblages surrounded by a grassland matrix, suggesting these are approaching species saturation. Commercial plantations may increase the likelihood of colonisation of, and immigration from, small forest fragments by birds, reducing the incidence of area-dependent extinction in small fragments, but may also result in lower species richness in larger fragments. The latter is likely a consequence of the promotion by plantations of a wider distribution of generalist species in, and the loss of some specialised or rare species from, large patches. Management options are to avoid planting plantations near large forest patches and to increase the size of small patches where possible. Where afforestation is unavoidable, placing plantations in the vicinity of small forest patches rather than large forest patches may be preferable.     

Brood ranging behaviour and breeding success of the threatened little bustard in an intensified cereal farmland area

Cereal pseudo-steppes hold the largest proportion of declining bird species in Europe. Understanding how this process of decline is driven is essential for designing adequate management practices. The little bustard (Tetrax tetrax) is one of such threatened steppe-land bird species, which has suffered recent dramatic range constrictions and population declines. Although low breeding success has been claimed to be the main threat for the viability of future little bustard populations, the surprising lack of studies dealing with ecological requirements of broods hinders the implementation of efficient conservation practices. We studied the ranging behaviour and the reproductive success of a little bustard population by means of female radio-tracking conducted during 2006–2008 breeding seasons in the Ebro basin (NE Spain). We recorded an average breeding success of 0.27 chicks/hen, which is not enough to sustain a viable population according to the demographic models for the species. Early-laid clutches showed more probabilities to hatch and to fledge. This was associated to two different ranging patterns (i.e. sedentary versus wandering respectively) which depended on the ability of the hen and its chicks to settle in suitable areas harbouring enough arthropods and shelter for chicks. Such resources decrease drastically as the breeding season progresses. Overall, data suggest that agricultural practices associated to agricultural intensification, particularly early harvest, reduce breeding success both directly by nest destruction and indirectly through a decrease on resource availability for broods. Based on our findings on the characteristics and structure of the home range of little bustard broods, we propose the creation of a network of permanent vegetation plots interconnected by high-quality field margins as a management practice to raise the breeding success of little bustards and other ground-nesting birds on cereal farmland.     

Effects of grazing practices and fossorial rodents on a winter avian community in Chihuahua, Mexico

Chihuahuan Desert grasslands are important wintering grounds for grassland and shrub-adapted birds. Many species belonging to these assemblages are currently exhibiting population declines. One area recognized for its importance to biological diversity, including grassland birds, is the Janos-Nuevo Casas Grandes black-tailed prairie dog (Cynomys ludovicianus) complex in northwestern Chihuahua, Mexico, an area containing 58 colonies with 30,000 ha of prairie dogs. This is one of the largest remaining prairie dog complexes and the only intact complex in the Chihuahuan Desert. In its current condition, a large percentage of this complex is of reduced value to wildlife. Overgrazing on communal (ejido) lands has resulted in areas being comprised of annual grasses and forbs. The density of active prairie dog burrows and banner-tailed kangaroo rat (Dipodomys spectabilis) mounds as well as avian diversity and abundance were lower on ejido lands than an adjacent private ranchland with and without prairie dogs. Few avian species used overgrazed portions of the prairie dog colony. Community similarity among plot types was low due to different management practices and differences on and off colony. To retain, and in many instances restore the biological diversity of this important region it is essential to work with local ejidos on grazing management.     

Predicting juniper encroachment and CRP effects on avian community dynamics in southern mixed-grass prairie, USA

The probability of occurrence of 30 bird species was modeled as a function of landscape covertype in northwestern Oklahoma, USA. This grassland region has been extensively fragmented by agricultural activity, and remnant grassland patches are undergoing severe degradation from encroaching juniper (Juniperus virginiana L.). In addition, many marginal or highly erodable croplands have been placed into perennial pasture dominated by exotic grasses under the Conservation Reserve Program (CRP). Based on temporal patterns of landscape change observed between 1965 and 1995, we estimated the covertype composition of the landscapes in the year 2015 under various CRP administrative and juniper expansion/control scenarios. We then used logistic regression to predict bird responses to these landscape composition estimates. Our estimates suggest that at the current rate of expansion, juniper will overtake substantial areas of remnant grassland even with extensive control measures. As a result, some obligate and facultative grassland birds are projected to decline, while numerous species tolerant of or partially reliant on woody vegetation will increase. Landscape dynamics due to changes in the CRP might be significant and could be designed to benefit declining grassland birds, but these benefits thus far are relatively minor compared to the effects encroaching juniper woodlands will have on the landscape and the avian community.     

The conservation effects of meadow bird agreements on farmland in Zeeland, The Netherlands, in the period 1989-1995

Meadow bird agreements are the most important Dutch agri-environment schemes, both in terms of uptake and of aim. Meadow bird agreements postpone the first agricultural activities on grassland thus reducing egg and chick mortality due to mowing or grazing. We investigated the conservation effects of meadow bird agreements by analysing settlement densities of meadow birds on 34 fields in 1989, 1992 and 1995 in the province of Zeeland, The Netherlands. We compared territory numbers on fields with meadow bird agreements with paired nearby control fields that were conventionally managed. In 1995, the number of territories of black-tailed godwit (Limosa limosa), lapwing (Vanellus vanellus) and the total number of meadow birds were significantly higher on fields with conservation management. These differences were partly caused by the higher quality (i.e. higher groundwater level) of fields with meadow bird agreements. Population trends were similar on fields with and without meadow bird agreements and the observed difference in settlement density in 1995 was already present in 1989. Furthermore the effectiveness of the scheme did not increase with time. Thus we found no conclusive evidence that the conservation measures themselves did result in higher territory numbers. Currently, we do not have sufficient ecological and behavioural knowledge of meadow birds to explain why the higher reproductive success does not result in higher settlement densities.     

The buffer effect of non-breeding birds and the timing of farmland bird declines

Breeding territoriality in birds means that a certain proportion of the adult population may breed in suboptimal habitats or not breed at all. Non-breeding birds, or floaters, can have a `buffer effect' on breeding population size. Farmland bird declines in Britain are measured and analysed in terms of numbers of territory-holding birds. We use a population dynamics model to illustrate how, because of the buffer effect of floaters, there can be a time lag between the start of total population decline and detectable breeding population decline. Differences in the timing of declines of closely related farmland birds could therefore be due to slight differences in their population dynamics, rather than differences in their response to agricultural change.     

Grassland decline in Kinangop Plateau, Kenya: implications for conservation of Sharpe's longclaw (Macronyx sharpei)

The Kinangop highland grasslands, the world stronghold of the endangered Sharpe's longclaw (Macronyx sharpei), consist almost entirely of privately owned land. As human population in the grasslands increases, the mean acreage of land holdings decreases and more grassland is being converted to other uses. Land subdivision information, existing human population data, and current land use data from a subset of 162 farms were used to extrapolate the past and future extent of Sharpe's longclaw habitat throughout the landscape on Kinangop Plateau. It was estimated that grasslands covered 50% of the plateau in 2000, but only 58% of the grassland area was likely to be tussock grasslands, the preferred habitat of Sharpe's longclaw. Large farms are being subdivided rapidly, and more than half of the remaining tussock grasslands are likely to be found on farms that are not large enough to support large longclaw territories. We predict that by 2010 tussock grasslands will cover only one-fifth of Kinangop, and all farms that are large enough to act as longclaw reserves are likely to have been subdivided. We propose urgent prioritisation of a network of large farms (>30 ha) for conservation action.     

Declines in common, widespread native birds in a mature temperate forest

Common, widespread species are important for ecosystem structure and function. Although such species have declined in some parts of the world, for most ecosystems there is a lack of information about changes in the population status of common species. We studied the abundance of common, widespread forest birds in Nelson Lakes National Park, New Zealand using standardised 5-min bird counts, carried out over a 30-year time span. There was a significant change in the bird community structure during this period. Five native species (bellbird, rifleman, grey warbler, New Zealand tomtit and tui) declined in abundance during the 30 years. All of these declined in abundance at low but not high altitudes, and the decline was substantial for all but New Zealand tomtit and tui. Three other native species increased in abundance (silvereye, yellow-crowned parakeet and New Zealand robin). There was no change in the abundance of introduced blackbirds. We suggest that invasive alien species are the most likely cause of the ongoing declines in common native species. A peak in brushtail possum abundance and the arrival of a new species of Vespula wasp were two large changes in Nelson Lakes forests that occurred during this study. Both are likely to have added to the ongoing impacts of predation by introduced rats and stoats. We suggest that it is necessary to actively manage introduced species in order to maintain populations of widespread, common native bird species in New Zealand.     

The ecology of extinction: population fluctuation and decline in amphibians

Even among widespread species with high reproductive potentials and significant dispersal abilities, the probability of extinctions should be correlated both with population size variance and with the extent of population isolation. To address how variation in demographic characteristics and habitat requirements may reflect on the comparative risk of species decline, I examined 617 time series of population census data derived from 89 amphibian species using the normalized estimate of the realized rate of increase, ΔN, and its variance. Amphibians are demonstrably in general decline and exhibit a great range of dispersal abilities, demographic characteristics, and population sizes. I compared species according to life-history characteristics and habitat use. Among the populations examined, census declines outnumbered increases yet the average magnitudes for both declines and increases were not demonstrably different, substantiating findings of amphibian decline. This gives no support for the idea that amphibian population sizes are dictated by regimes featuring relatively rare years of high recruitment offset by intervening years of gradual decline such that declines may outnumber increases without negative effect. For any given population size, those populations living in large streams or in ponds had significantly higher variance than did populations of completely terrestrial or other stream-dwelling amphibians. This could not be related to life-history complexity as all the stream-breeding species examined have larvae and all of the wholly terrestrial species have direct development without a larval stage. Variance in ΔN was highest amongst the smallest populations in each comparison group. Estimated local extinction rates averaged 3.1% among pond-breeding frogs, 2.2% for pond-breeding salamanders, and negligible for both stream-breeding and terrestrial direct-developing species. Recoveries slightly exceeded extinctions among European pond-breeding frogs but not among North American pond-breeding frogs. Less common species had greater negative disparities between extinctions and recoveries. Species with highly fluctuating populations and high frequencies of local extinctions living in changeable environments, such pond- and torrent-breeding amphibians, may be especially susceptible to curtailment of dispersal and restriction of habitat.     

Land use and habitat gradients determine bird community diversity and abundance in suburban, rural and reserve landscapes of Minnesota, USA

Bird species’ community responses to land use in the suburbanizing Twin Cities, Minnesota, USA, were contrasted among reserves, rural lands, and suburbs. For each land use type, bird composition, diversity, and abundance were recorded for 2 years in ≈99 plots in three sampling units (each ≈4500 ha). A habitat gradient defined by canopy structure (grasslands to savannas to forests) was influenced by land use, so ≈300 plots were used to characterize simultaneous variation in bird communities along land use and habitat gradients. At broad scales (aggregate of 33 plots covering ≈4500 ha) suburbs supported the lowest bird richness and diversity and rural landscapes the most, with reserves slightly below rural. Although reserves were like rural lands in diversity of bird communities, they supported more species of conservation concern, particularly of grasslands and savannas. Differences among land use types varied with habitat structure. Suburbs, rural lands, and reserves had similar forest bird communities, but differed in grassland and savanna bird communities. The extensive rural forests are important for the region’s forest birds. Suburban grasslands and savannas had low shrub abundance, low native bird richness and high non-native bird richness and abundance. However, total bird richness and diversity were as high in suburban as in rural and reserve plots because high native richness in suburban forests and high non-native species richness in suburban grasslands and savannas compensated for lower native richness in suburban grasslands and savannas. Bird conservation here and in the Midwest USA should protect rural forests, expand grasslands and savannas in reserves, and improve habitat quality overall.     

Extension of landscape-based population viability models to ecoregional scales for conservation planning

Landscape-based population models are potentially valuable tools in facilitating conservation planning and actions at large scales. However, such models have rarely been applied at ecoregional scales. We extended landscape-based population models to ecoregional scales for three species of concern in the Central Hardwoods Bird Conservation Region and compared model projections against long-term trend data from the North American Breeding Bird Survey. We used a spatially-explicit demographic model and structured the regional population into ecological subsections on the basis of habitat, landscape patterns, and demographic rates to assess species viability. Our model projections were within 2% of the Breeding Bird Survey trends over the last 40 years for each species. Wood thrush (Hylocichla mustelina) populations remained relatively stable over the simulation and worm-eating warbler (Helmitheros vermivorus) abundance increased throughout most of the time period until reaching carrying capacity. In contrast, the prairie warbler (Dendroica discolor) population steadily declined by 0.59% annually. The combination of habitat and demographic modeling allowed us to create models that address processes driving these populations at all scales, which is critical to understanding how regional populations respond to landscape processes such as habitat loss and fragmentation. Therefore, because it is spatially explicit and directly addresses population growth and viability, this approach provides a valuable foundation to planning conservation strategies, offering the ability to identify the most salient risks to viability and explore ways to address them.     

Could soil degradation contribute to farmland bird declines? Links between soil penetrability and the abundance of yellow wagtails Motacilla flava in arable fields

Major changes to the extent and quality of farmland habitats, brought by the intensification of agricultural practice, are thought to be the main factors driving declines in a suite of farmland bird species in Europe. Recent changes in agricultural techniques have also contributed to widespread soil degradation, arising from increased soil exposure to erosion forces, declining soil organic content and increasing soil compaction. Although soils have a fundamental influence on ecosystem properties, the implications of soil degradation for farmland biodiversity have received little attention. In this study, we measure the influence of soil conditions on the distribution of a declining insectivorous farmland bird, the yellow wagtail Motacilla flava, relative to other habitat features in arable fields. Soil penetrability was found to have a significant influence on the abundance of territorial yellow wagtails at the field scale, together with crop type. Other measured habitat features had little effect on territory abundance, including soil organic content, crop height (within preferred crop types), field boundary habitats and availability of bare ground. Monitoring of invertebrate abundance across 20 cereal fields revealed a significant influence of both soil penetrability and soil organic content on aerial invertebrate capture rates. This relationship was strongest during the latter part of the breeding season, implying that settling yellow wagtails could use soil penetrability as a predictive indicator of prey abundance during the chick-rearing period. The strong relationship between yellow wagtails and soil penetrability suggests a potential causative link between soil degradation and population decline. The role of soils in determining abundance patterns and population declines of other farmland species may have been overlooked in previous studies.     

Distance to edges, edge contrast and landscape fragmentation: Interactions affecting farmland birds around forest plantations

Afforestation often causes direct habitat losses for farmland birds of conservation concern, but it is uncertain whether negative effects also extend significantly into adjacent open land. Information is thus required on how these species react to wooded edges, and how their responses are affected by edge and landscape characteristics. These issues were examined in Mediterranean arable farmland, using bird counts at 0, 100, 200, 300 and >300 m from oak, pine and eucalyptus edges, embedded in landscapes with variable amounts and spatial configurations of forest plantations. Bird diversity declined away from edges, including that of woodland, farmland and ground-nesting birds. Positive edge responses were also found for overall and woodland bird abundances, and for five of the nine most widespread and abundant species (Galerida larks, stonechat, linnet, goldfinch and corn bunting). Strong negative edge effects were only recorded for steppe birds, with reduced abundances near edges of calandra larks and short-toed larks, but not of little bustards and tawny pipits. Edge contrast affected the magnitude of edge effects, with a tendency for stronger responses to old and tall eucalyptus plantations (hard edges) than to young and short oak plantations (soft edges). There were also species-specific interactions between edge and fragmentation effects, with positive edge responses tending to be strongest in less fragmented landscapes, whereas steppe birds tended to increase faster away from edges and to reach the highest species richness and abundances in large arable patches. Results suggest that forest plantations may increase overall bird diversity and abundance in adjacent farmland, at the expenses of steppe birds of conservation concern. Clustering forest plantations in a few large patches and thus reducing the density of wooded edges at the landscape-scale might reduce such negative impacts.     

Nestedness of bird assemblages in fragmented Afromontane forest: the effect of plantation forestry in the matrix

Bird species composition (61 species) of Afromontane forest fragments (n = 19) embedded in a natural grassland matrix or a plantation forestry matrix were compared. Forest bird assemblages in a natural grassland matrix displayed a nested non-random species distribution. Species loss was significantly area-dependent and predictable. Assemblages in the plantation-dominated matrix were also nested but were neither area- nor isolation-dependent and these potential processes had equal influence over bird species composition. Forests patches <50 ha in plantation were more species rich than those in a grassland matrix and visa versa for patches >50 ha. The effect of the matrix is revealed here with arboreal plantation cover promoting dispersal across the landscape, favouring abundant generalist bird species in smaller patches than expected in the plantation matrix, and the loss of some specialists and species living at low densities from large (>50 ha) patches. Because of a significant species-area relationship and possible area-dependent extinction, the largest forest patch in the nested series in a grassland matrix protects most bird species (89%). However, in a plantation matrix the absence of a species-area relationship requires the combination of many larger forest patches to protect most bird species. Plantation forests in the matrix significantly alter forest bird assemblage structure and composition. To avoid the effects of insularisation and to maintain the viability of bird communities in Afromontane forests requires conservation of the larger forests in a natural matrix. However, for forests in a plantation matrix, the management of dispersal processes by manipulation of the matrix may be as important to conserving species and communities as is minimising extinctions. Avoiding placing plantations near large forests, increasing the size of small patches, and where afforestation is unavoidable, placing plantations in the vicinity of small rather than large forest patches, are preferable management practices.     

Avian response to warm-season grass use in pasture and hayfield management

Populations of most avian species associated with grasslands have declined in North America over the last few decades. These declines may be related, in part, to changes in species composition and management of pastures and hayfields. The incorporation of native, warm-season grasses into pasture and hayfield management has been suggested as a means of providing suitable habitat for birds in agricultural areas. To examine this, we compared avian abundance, richness, and reproductive success in warm- [i.e. switchgrass (Panicum virgatum) and big bluestem (Andropogon gerardii)] and cool-season grass [i.e. orchardgrass (Dactylis glomerata)] fields on private farms in southwest Pennsylvania. Point counts, nest monitoring, and vegetation sampling were conducted on nine pairs (warm- and cool-season grass) of fields in 1996, and 12 pairs of fields in 1997. Compared with cool-season grass fields, warm-season grass fields supported a greater abundance and richness of birds, including several declining species [e.g. song sparrows (Melospiza melodia), field sparrows (Spizella pusilla), chipping sparrows (Spizella passerina), grasshopper sparrows (Ammodramus savannarum), and vesper sparrows (Pooccetes gramineus)]. Additionally, due to lower nest destruction and depredation rates, birds in warm-season grass fields had greater nest success and fledge rates. The positive response of birds to the use of warm-season grasses in pastures and hayfields appears to be due to the increased availability of undisturbed cover. Thus, the establishment of warm-season grasses in pastures and hayfields should be an avian conservation and management priority. Convincing farmers to use warm-season grasses in their fields should not be difficult, as several state and federal programs fund their establishment, and the use of these grasses increases forage production and farm income.     

Bird assemblages as bio-indicators of water regime management and hunting disturbance in natural wet grasslands

Responses of the waterbird community to the management regime of a wet grassland system (primarily small changes in the water regime but also hunting disturbance) in a large floodplain lake, Grand-Lieu, were studied during three consecutive years (winter and spring in 1998-2000). These temporarily flooding grasslands (1100 ha) constituted a very important feeding area by supporting large flocks of waterbird species (52 regular spp., totalling up to 14?250-22?850 birds). A global similar seasonal pattern in the presence of waterbird species was recorded whatever the year. The March-May period was a key phase with the maximum species richness (42-48 spp.) and the maximum abundance (30-40% of the total according to year, essentially Anseriformes). Late in spring Ciconiiformes and to a lesser extent some waders exploit this feeding area. Nevertheless some changes in bird assemblages were exhibited according to the management regime (a decreasing spring water level from 1998 to 2000, and an exceptional hunting ban in January 2000, instead of an usual stop in late February). The hunting disturbance clearly limited diurnal accessibility of ducks to wet grasslands in January and February 1998 and 1999. Also, the duck population in flooding grasslands increased by 55-65% in January 2000 when hunting was prohibited and in the same time the richness species grew from 14-19 spp. up to 23 spp. in 2000. The overall impact of a high spring water level on waterbirds as in 1998 and to a lesser extent in 1999 (with respectively 54 and 35 cm of mean water level against 25 cm in 2000) was an increase in diving fish-eating birds and larger flocks of resting gulls. Conversely these conditions, notably a reduced period with a spring water level under 25 cm in spring (20 days in 1998-1999 instead of 40 days in 2000) were adverse to numerous ground-feeding waterbirds (Ciconiiformes and waders) that require shallow waters. This study showed that limitation of human disturbance and moderate flooding of wet grasslands can increase their attractiveness for numerous species of high conservation status.     

Landscape elements as potential barriers and corridors for bees, wasps and parasitoids

Habitat loss and fragmentation in agricultural landscapes lead to severe declines of abundance and richness of many insect species in the remaining isolated semi-natural habitats. We analysed possible barrier effects of large hedges and corridor effects of narrow grass strips that were hypothesized to affect foraging and dispersal of hymenopterans. We selected calcareous grasslands in the vicinity of Göttingen (Germany), which harbour high Hymenoptera diversity and are starting points for foraging and dispersal in the landscape. We installed pan traps to sample bees (i) on the grasslands; (ii) on grassland edges behind adjacent hedges (potential barriers) and without hedges; (iii) on grass strips in 100 m distance to the grassland, which were connected or unconnected to the grassland; and (iv) unconnected (isolated) grass strips in 300 m and 750 m distance to test for corridor and isolation effects on abundance and species richness of foraging wild bees. Additionally we provided trap nests for bees, wasps and their parasitoids on the grasslands and the strips. Species abundance and richness declined with increasing isolation from grasslands for foraging solitary bees, trap-nesting bees, wasps and parasitoids, but not for foraging bumblebees. Hedges did not confine movement of foraging bees. We found no mitigating effects of (100 m) corridor strips on any of the observed groups. We conclude that conservation of semi-natural habitats as sources of bee and wasp diversity is important and that grass strips act as sinks rather than corridors when high quality patches are nearby.