The value of paddock trees for regional conservation in an agricultural landscape

Summary Isolated trees and small patches of trees – paddock trees – are a prominent feature of agricultural landscapes in Australia, but are declining in many areas due to natural senescence, clearing, dieback and the general absence of recruitment. We assessed the importance of paddock trees for woodland conservation in a 30 000 ha sample of the New South Wales (NSW) South‐west Slopes using Satellite Pour l’Observation de la Terre (SPOT) panchromatic satellite imagery combined with models predicting the original distribution of vegetation communities. Tree‐cover occurred over 12% of the study area. The patch‐size distribution of vegetation in the study area varied between woodland types. For woodland communities that were confined to hills and ridges, most tree‐cover occurred as few, large remnants. For woodland communities of the foothills and plains (Blakely's Red Gum, Eucalyptus blakelyi and Yellow Box, Eucalyptus melliodora, or White Box, Eucalyptus albens and Red Stringybark), 54% of remnant tree‐cover occurred as patches < 1 ha. The loss of paddock trees will cause substantial reductions to some woodland communities. For example, the loss of patches < 1 ha in woodlands dominated by Blakely's Red Gum and Yellow Box would reduce this association from 7.4% to 3.4% of its predicted pre‐1750 distribution. Mean distance to tree‐cover across the study area increased almost fourfold if patches < 1 ha were removed from the landscape, which may have consequences for movements of some flora and fauna. Failure to protect and perpetuate paddock trees will diminish the likelihood of achieving the conservation objectives of comprehensiveness, adequacy and representativeness in agricultural landscapes.     

农田景观格局变化对昆虫的生态学效应

景观格局变化是全球变化的一个重要内容。农田是由人类赖以生存所种植的人工栽培作物组成的生态系统。在该景观系统中,多种植物-害虫-天敌相互作用、相互制约,形成有机整体。研究农田景观格局对害虫和天敌种群动态影响,不仅在害虫生物防治的实践中有重要意义,而且对于揭示人类活动对生物多样性结构与功能的影响,阐明农田景观中生物多样性整合、维持机理有重大的理论意义。本文从农田景观格局的"质、量、形、度"4个方面,系统地论述了农田景观格局变化对害虫和天敌的作用,分析了农田景观的生物控制服务功能,指出了未来研究发展的方向。     

Conservation value for birds of traditionally managed isolated trees in an agricultural landscape of Madagascar

Isolated trees have distinctive economic, social and cultural value for the Betsileo people living on the edge of the protected forest corridor between Ranomafana and Andringitra national parks, in South-East Madagascar. Many of these trees are Ficus spp., traditionally protected and respected. At the landscape level, they are isolated features in a heterogeneous mosaic, providing fruit, shade and aesthetic services in open cultivated areas. Within the current management system, isolated trees may also contribute significantly to the provision of ecological services by enhancing bird diversity in open areas outside the forest. We identified practices and values linked to isolated tree uses and management through ethnographic data collection. Bird presence and abundance were sampled by 338 point counts in isolated trees and open areas of the agricultural mosaic. Isolated trees were occupied by 18 out of 32 (56%) bird species in the agricultural mosaic, including 8 (25%) endemic forest species. Endemic forest birds were significantly more numerous in isolated trees than in open habitats, both in species richness and abundance (mean P value < 0.001). Overall bird species richness was significantly higher in open areas containing isolated trees, than in areas without isolated trees. Bird species richness in Ficus spp. was significantly higher than in other isolated tree species, although no differences were detected in abundance or within guilds. Community-based management of isolated trees may thus represent an opportunity for convergence between bird conservation goals outside protected areas and local management values and practices.     

Seed dispersal, seedling establishment and gap partitioning among tropical pioneer trees

1 We examined the abundance and distribution patterns of pioneer seeds in the soil seed bank, and of pioneer seedlings in 53 recently formed gaps, in a 50‐ha forest dynamics plot on Barro Colorado Island (BCI), Panama. The aim was to assess the importance of dispersal limitation (failure of seeds to arrive at all sites suitable for their germination) and establishment limitation (failure of seeds having reached a site to germinate successfully and establish as seedlings) in determining patterns of gap occupancy.
2 The abundance of seeds in the soil seed bank was strongly negatively correlated with seed size, but was not correlated with the abundance of reproductive‐sized adult trees in the plot. In contrast, the abundance of pioneer seedlings > 10 cm height in natural gaps was strongly correlated with adult abundance, but was not correlated with seed size.
3 Seedlings were non‐randomly distributed among gaps, but seedling abundance was not directly related to gap size, and there was no evidence of partitioning of the light environment of gaps by small seedlings. Large differences in growth and mortality rates among species were observed after 1 year, and this may result in the gap size partitioning previously found in saplings of the same species.
4 Seedlings of most species, particularly those with large seeds, were relatively more abundant than expected in gaps close to their conspecific adults. Proximity to reproductives, and by inference dispersal limitation, therefore exerts some effect on seedling distribution. None the less, large differences between seed and seedling abundances for some species, and low seedling occupancy rates in some gaps close to adult conspecifics, suggest that seedling emergence probabilities and species‐specific establishment requirements may also be important determinants of local abundance.     

Bird movement patterns in an agricultural landscape are mediated by both habitat conditions and traits

Alterations of the landscape following agricultural expansion and intensification affect animal movement patterns in the resulting mosaic of fragments and surrounding matrix. Here, we analyze the observed movement patterns of 34 individuals from nine tropical bird species from a rapidly changing agricultural landscape in Kenya. We deconstructed the movement patterns into their three components: step length, turn angles, and displacement and categorized them into two states: area restrictive and expansive movement. Using hidden Markov models, mixed models, and species traits, we showed that movement of birds in the fragments comprised of short step lengths and small displacements, characteristic of area restrictive movement to exploit high-quality habitats. On the contrary, movement in the matrix comprised of long step lengths and large displacements, characteristic of area expansive movement to explore or pass-through poorer habitats. The responses of movement components to fragments and the surrounding matrix were mediated by species traits. Habitat specialists showed stronger boundary response, shorter step lengths, and smaller displacements than habitat generalists in both the fragments and the matrix. Their strong preferences for the fragments, coupled with low flight capabilities can make movement in the matrix particularly difficult. Whereas, at the landscape scale, habitat generalist omnivores and habitat specialist frugivores had larger step lengths than the other guilds, as they use the matrix for resources or as a conduit to movement. Therefore, the habitat fragments are intensely utilized and of conservation importance. The matrix quality and permeability can promote animal space use and movement.     

Ditch network sustains functional connectivity and influences patterns of gene flow in an intensive agricultural landscape

In intensive agricultural landscapes, plant species previously relying on semi-natural habitats may persist as metapopulations within landscape linear elements. Maintenance of populations'' connectivity through pollen and seed dispersal is a key factor in species persistence in the face of substantial habitat loss. The goals of this study were to investigate the potential corridor role of ditches and to identify the landscape components that significantly impact patterns of gene flow among remnant populations. Using microsatellite loci, we explored the spatial genetic structure of two hydrochorous wetland plants exhibiting contrasting local abundance and different habitat requirements: the rare and regionally protected Oenanthe aquatica and the more commonly distributed Lycopus europaeus, in an 83 km² agricultural lowland located in northern France. Both species exhibited a significant spatial genetic structure, along with substantial levels of genetic differentiation, especially for L. europaeus, which also expressed high levels of inbreeding. Isolation-by-distance analysis revealed enhanced gene flow along ditches, indicating their key role in effective seed and pollen dispersal. Our data also suggested that the configuration of the ditch network and the landscape elements significantly affected population genetic structure, with (i) species-specific scale effects on the genetic neighborhood and (ii) detrimental impact of human ditch management on genetic diversity, especially for O. aquatica. Altogether, these findings highlighted the key role of ditches in the maintenance of plant biodiversity in intensive agricultural landscapes with few remnant wetland habitats.     

Amphibian diversity increases in an heterogeneous agricultural landscape

As a group amphibians are the vertebrates most affected by anthropic activity, particularly by agriculture. The rapid advance of the agricultural frontier makes it important to identify the role of agroecosystems as habitat supply for amphibians. We analyzed the differences in amphibian assemblages and populations between habitats with different plant covers and different degrees of human intervention in northwestern Argentina. For three years we conducted 114 high frequency trap samplings to quantify abundance, specific composition and species richness of amphibian assemblages in three habitat types (lemon and sugarcane crops and secondary forest) of a piedmont agroecosystem of Tucumán province. Crops hosted more species and individuals than secondary forests, but the specific composition of forest was different from that of crops suggesting that they could be complementary. Although the assemblage abundance of every observation responded to climate, the strong effect of sampling year was not related to climatic factors suggesting that there might be long term fluctuations that were not analyzed. We also found that responses to agricultural practices were species specific, so no generalizations about these practices should be done. Our study shows that cultivated areas are not hostile environments for amphibians since they can lodge huge amphibian populations and that the occurrence of disturbances associated to agricultural practices is not necessarily reflected in a decrease in the abundance and richness of amphibians in the short term. However, our results also show that forested lands are necessary to lodge some specialist species which are very rare in the croplands. This suggests that environmental heterogeneity generated by the combination of natural and cultivated patches can increase biodiversity at landscape scale because this allows the coexistence of species related to either kind of environment.     

Establishment of native perennial shrubs in an agricultural landscape

Abstract Native vegetation has been destroyed or dramatically modified throughout agricultural regions of southern Australia. Extensive restoration of native perennial vegetation is likely to be crucial in these areas for the persistence of native plant and animal species, to ameliorate dryland salinity and soil degradation, and to maintain long‐term agricultural production. The long‐term resilience of these systems will be dependent on the ability of key functional taxa, such as perennial shrubs, to recruit and persist. In this study, we examine the factors limiting establishment of two perennial shrubs in formerly cropped land, the rare Maireana rohrlachii and the common Maireana decalvans. Field and laboratory observations suggest that establishment of both species is not limited by life‐history traits following cultivation. Both species established and persisted under varying levels of plant competition. Similarities existed between species in their initial germination rates. Weak differences were found between species in the growth and survival rates under different levels of competition. The main difference between the two species was in the decline of germinability of fruits with increasing fruit age. From the data, it is difficult to determine what factors limit the establishment of perennial shrubs in these landscapes. The main hypothesis that can be advanced is that establishment of shrub species appears to be limited by propagule availability and this is likely to be a function of past and present grazing management rather than cultivation per se. Further investigation of these land‐use practices may give greater insight into the factors affecting the establishment of this life form across these landscapes.     

区域农田景观格局对麦蚜种群数量的影响

明确农田景观格局对麦田蚜虫种群的影响,是开展区域性害虫生态调控的重要理论依据之一。以区域性小麦种植区为研究对象,基于遥感影像与土地覆盖分类数据以及田间调查的蚜虫种群数据,计算景观格局指数,使用负二项分布的广义线性模型从农田景观、非作物生境景观和区域景观3个方面分析了区域农田景观格局对麦田蚜虫种群的影响。结果表明,蚜虫种群的数量与草地的平均斑块面积和最大斑块指数显著正相关,与县域的平均几何最邻近距离和面积加权平均斑块面积显著负相关,与耕地的面积加权平均斑块面积显著负相关,与耕地的斑块密度显著正相关。草地斑块面积的增大、区域景观与耕地的破碎化、区域景观的聚集会促进蚜虫种群数量的增加。使用草地的斑块面积和最大斑块指数、区域景观的平均几何最邻近距离可以预测蚜虫种群的发生量。非作物生境草地的斑块面积、耕地的破碎化、区域景观的空间分布及破碎化是影响麦田蚜虫种群发生的重要景观因素。     

不同农业景观背景下传粉昆虫群落的分布差异

农业景观中的自然、半自然生境等非农生境可为传粉昆虫提供丰富的食物来源、栖息地、繁殖地、避难所等,对维持生物多样性的稳定起着不可替代的作用。以巩义典型的山地-丘陵-河川混杂的复杂景观和民权的平原农业简单景观为研究区,分析不同景观背景下传粉昆虫群落的物种组成及其在不同生境中的分布特征。结果表明:(1)巩义研究区内累计捕获传粉昆虫18582头,民权研究区内累计捕获传粉昆虫18518头,优势传粉昆虫功能群为双翅目、膜翅目、鞘翅目和鳞翅目等;(2)景观复杂度更高的巩义研究区的传粉昆虫多样性、丰富度以及均匀度均显著高于民权研究区;存在大面积农田斑块的平原景观中则有更多的优势传粉昆虫个体;(3)农田斑块中具有更高的物种丰富度,但林地物种的多样性和均匀度则相对稍高些。农田斑块在作物花期能有效的提高传粉者种群密度,但林地等自然、半自然生境对于维持传粉者多样性和食物缺乏期种群的稳定具有重要作用。因此在未来为确保农业景观中传粉者的多样性与传粉服务的稳定,一方面要关注自然、半自然生境的作用,同时也要考虑不同景观背景下异质性特征对不同生物类群的影响差异。     

Dispersal characteristics of seven odonate species in an agricultural landscape

Dispersal is an ecological phenomenon which is of fundamental importance to population biology. While dispersal behaviour of many orders of winged insects has received a great deal of attention, the dispersal characteristics of odonates have been poorly documented. We used capture-mark-recapture techniques to study dispersal behaviour of seven species of odonates breeding on a network of 11 small ponds in Cheshire, U.K. The ponds ranged in size from 615 to 1300 m²- and varied from 30 to 860 m apart. We found surprisingly high rates of dispersal between ponds, with 10–47% per species of recaptured individuals moving from their natal pond. The mean probability of dispersal differed significantly among species but the relationship between the probability of dispersal and distance moved consistently followed a simple negative exponential curve for all species. Most individuals stayed at their natal pond, but a few moved long distances. Neither the age at which an individual was marked (teneral vs sexually mature) nor its sex significantly affected its tendency to disperse. The negative exponential relationship suggests that dispersal should be relatively easy to incorporate in more complex models of odonate spatio-temporal dynamics. To our knowledge, this is the first large-scale, multi-species study to assess dispersal behaviour of odonates by direct observation.     

Effects of an invading coccinellid on native coccinellids in an agricultural landscape

Seven native coccinellid species inhabited alfalfa, corn, and small grain fields in eastern South Dakota prior to invasion and establishment of Coccinella septempunctata L. Six species occurred in all crops, however, Adalia bipunctata (L.) occurred only in corn. The structure of native coccinellid communities differed significantly for years prior to compared with years after establishment of C. septempunctata in fields of the three agricultural crops. Differences in community structure were accounted for mainly by reduced abundance of two species, C. transversoguttata richardsoni Brown and Adalia bipunctata (L.). Annual abundance of C. transversoguttata richardsoni averaged 20–32 times lower during post-invasion years than in years prior to invasion, depending on crop; while annual abundance of A. bipunctata averaged 20 times lower in corn after invasion. Addition of C. septempunctata to the community did not result in a significant increase in total abundance of coccinellids in the crops. Coccinellid abundance in agricultural crops may be limited by the total abundance of prey or by the availability of other requisites in the landscape as a whole. Therefore, introduction of a new species, while resulting in reductions in native species populations, may not increase total coccinellid abundance, and may therefore have no net effect on biological control of aphid pests.     

Ecosystem wicks: Woodland trees enhance water infiltration in a fragmented agricultural landscape in eastern Australia

Abstract Since European settlement, Eucalyptus box woodlands have been substantially modified by agricultural practices, and in many areas in southern Australia are now restricted to scattered or clumped trees. We report here on a study to examine the impact of trees on water flow (infiltration) in an agricultural landscape with substantial areas of extant native vegetation. We examined infiltration through coarse‐ and fine‐textured soils within four landscape strata, the zones below Eucalyptus melliodora and Callitris glaucophylla canopies, the intertree zone dominated by perennial grasses and a landscape homogenized by cultivation and dominated by annual crops. We measured sorptivity, the early phase of water flow, and steady‐state infiltration with disc permeameters at two supply potentials. These different potentials enabled us to separate infiltration into (i) flow through large (biopores) and small pores and (ii) flow through small pores only where biopores are prevented from conducting water. On the fine‐textured soils, both sorptivity and steady‐state infiltration were significantly greater (approximately fivefold) under the timbered strata compared with the grassy slopes or cultivation. Differences were attributable to the greater proportion of macropores below the tree canopies compared with the nontimbered strata. The lack of a significant difference on the coarse‐textured soils, despite their macropore status, was attributed to differences in surface litter and plant cover, which would maintain continuous macropores at the surface and thus conduct large amounts of water. The tendency of slopes covered by cryptogamic crusts and grasses to shed run‐off and for the trees to absorb substantial quantities of water reinforced the important ecological service provided by trees, which moderates large run‐off events and captures small amounts of water leaking from the grassy patches. In the absence of these ‘ecosystem wicks’, run‐off would find its way into regional groundwater and contribute to rising salinity.     

Roots of pioneer trees in the Amazonian rain forest

. On low fluvial terraces of the Caquetá River, Araracuara region, Colombia, tree root systems were examined with regard to their structure, underground stratification and biomass distribution. Excavations of skeleton roots and microscopic observation of terminal fine roots included ten species belonging to the Cecropia, Vismia, Miconia, Goupia, Clathrotropis and Brosimum genera. Roots of particular species varied in the external features of their periderm and structure of freshly cut slash. Coarse skeleton roots differed in the shape and position of the plagiotropic branches and positively geotropic taproots or sinkers. None of the observed species could be identified as a shallow-rooted tree, in spite of the general fine root concentration in the upper soil horizon. In the two seasons of study, the terminal roots displayed prevailingly secondary anatomical structure and did not maintain much primary anatomical tissue in their apices. Neither ectomycorrhizas nor endomycorrhizas were detected in the samples. In a set of regeneration stages the amount of tree roots linearly increased with the age of growth. However, fine roots below 2 mm in diameter shared 80-90% of all roots in the upper 20 cm layer of all sample plots. In an old-growth forest, the total tree root biomass amounted to 39 ton/ha, thus being comparable to the underground biomass observed in similar tropical forests.     

Metacommunities of spiders in grassland habitat fragments of an agricultural landscape

Arthropod communities in fragmented agricultural landscapes depend on local processes and the interactions between communities in the habitat islands. We aimed to study metacommunity structure of spiders, a group that is known for high dispersal power, local niche partitioning and for engaging in species interactions. While living in fragmented habitats could lead to nestedness, other ecological traits of spiders might equally lead to patterns dominated either by species interactions or habitat filtering. We asked, which community pattern will prevail in a typical agricultural landscape with isolated patches of semi-natural habitats. Such a situation was studied by sampling spiders in 28 grassland locations in a Hungarian agricultural landscape. We used the elements of metacommunity structure (EMS) framework to distinguish between alternative patterns that reveal community organization. The EMS analysis indicated coherent species ranges, high turnover and boundary clumping, suggesting Clementsian community organization. The greatest variation in species composition was explained by local habitat characteristics, indicating habitat filtering. The influence of dispersal could be detected by the significant effect of landscape composition, which was strongest at 500 m. We conclude that dispersal allows spiders to respond coherently to the environment, creating similar communities in similar habitats. Consistent habitat differences, such as species rich versus species poor vegetation, lead to recognisably different, recurrent communities. These characteristics make spiders a predictable and diverse source of natural enemies in agricultural landscapes. Sensitivity to habitat composition at medium distances warns us that landscape homogenization may alter these metacommunity processes.     

Dynamics of Thuja occidentalis in an agricultural landscape of southern Quebec

Abstract. Human impact influences patterns of plant distribution and dominates disturbance dynamics in the agricultural landscape of Haut-Saint-Laurent, Québec. We investigated the influence of such impact on the dynamics of Thuja occidentalis in the hardwood forest of southern Québec. A bimodal species of either dry or wet habitats, T. occidentalis has successfully invaded mesic sites of the study area where, although generally considered a poor competitor, it persists as dense populations that seem to resist invasion by more tolerant species. As Acer saccharum is expected to be a dominant latesuccessional species under mesic conditions, we first evaluated the relative importance of abiotic and historical factors in the distribution of 34 T. occidentalis stands and 21 A. saccharum stands. We then classified 55 stands that include T. occidentalis and investigated the dynamic relationships between the species and its associates in 13 mixed mesic sites. Canonical correspondence analysis (CCA) of T. occidentalis and A. saccharum stands suggested that land use and drainage are the most important variables correlated with plant community composition. A partial CCA with drainage as the constraining variable showed that T. occidentalis stands were significantly associated to present or past use as pasture. Correspondence analysis of 55 stands where T. occidentalis is present produced a clear segregation between nearly monospecific stands and mixed communities, but the chosen environmental variables were not significant in explaining the vegetation gradient at this scale. In all mixed stands, Thuja occidentalis was most often associated with Ulmus americana, Acer rubrum, Betula populifolia, Fraxinus americana, A. saccharum, and Tsuga canadensis. Finally, size-class ordination of 13 mixed mesic stands did not show convergence toward A. saccharum. Our results suggest that grazing has played a significant part in the appearance of T. occidentalis stands on mesic sites, whereas competition processes and landscape patterns possibly contribute to the maintenance of relatively persistent T. occidentalis stands in the landscape.     

Patterns of species richness in dry grassland patches in an agricultural landscape

Eighty-five patches of semi-natural grassland of varying size scattered in a agricultural landscape were investigated for their flora of vascular plants. Relationships between species richness and patch area, spatial isolation and local habitat conditions including heterogeneity were examined. Differences between single species and among groups of species defined by life-history traits were also investigated.
Area was shown to be an important determinant of species richness irrespective of habitat heterogeneity. Isolation in space and habitat heterogeneity also play significant roles. These results are consistent with results from a multitude of studies on fragments of ancient deciduous woodland in northern Europe, They are, however, contradictory to results from previous studies in grasslands within the same region. Seed mass and dispersal syndrome were poor predictors of the degree to which the species were affected by isolation of grassland patches. Seed mass deviation from community median could explain a small percentage of the variation in regional abundance. Logistic regression on species occurrences showed that few species are associated with large patches, and less than half seem to avoid isolated patches.     

Spatial dynamics of plant species in an agricultural landscape in the Netherlands

This study examined the changes in distribution patterns of 13 herbaceous plant species from 1998 to 2000 in ditch banks along the edges of arable fields in the Netherlands. The objective was to test if spatial dynamics could be related to spatial isolation and disturbance of habitat and to the dispersal and seed bank characteristics of the species. Knowledge of these relations should be used to increase the effectivity of agri-environmental schemes aiming at an increase of botanical diversity. All species frequently colonized empty patches and populations in occupied patches frequently went extinct. Most colonization events occurred within 50 m of conspecific source patches in the preceding year, but colonization events in patches at distances more than 200 m from conspecific source patches were also observed. The colonization probabilities decreased with isolation distance. For nine species this relation was statistically significant, after correction for year and habitat. The extinction probabilities increased with isolation. For only four species this relation was statistically significant. Both colonization and extinction probabilities were more often statistically significant related to isolation for species with transient seed banks than species with persistent seed banks. Implications for management options aiming at survival of plant species in fragmented landscapes are discussed.