Seasonal variation in diversity and abundance of understorey birds in Bunduki Forest Reserve,Tanzania: evaluating the conservation value of a plantation forest

Plantation forests generally support lower bird diversity than natural forests. However, in some instances the plantations have been found to provide suitable habitat for a number of bird species. In the Eastern Arc Mountains, there is limited knowledge how understorey birds, some of which make seasonal altitudinal movements, use plantations. Using mist netting we assessed seasonal use of the plantation forest by the understorey bird community in Bunduki Forest Reserve in the Uluguru Mountains. Species diversity and capture rates were significantly higher during the cold season than during the hot season possibly due to seasonal altitudinal migration by some species. The use of plantations by those species that make seasonal altitudinal movements shows that plantation forests can enhance indigenous biodiversity by enabling connectivity between two or more natural forest patches. Our findings suggest that in a situation where there is no natural forest, an exotic plantation with suitable indigenous understorey cover can help in protection of birds, including endemic and near-endemic species.     

Does plantation forestry restore biodiversity or create green deserts? A synthesis of the effects of land-use transitions on plant species richness

Plantations are established for a variety of reasons including wood production, soil and water conservation, and more recently, carbon sequestration. The effect of this growing land-use change on biodiversity, however, is poorly understood and considerable debate exists as to whether plantations are ‘green deserts’ or valuable habitat for indigenous flora and fauna. This paper synthesizes peer-reviewed articles that provide quantitative data on plant species richness in plantations and paired land uses, most often representative of pre-plantation land cover. The results of this synthesis suggest that the value of plantations for biodiversity varies considerably depending on whether the original land cover is grassland, shrubland, primary forest, secondary forest, or degraded or exotic pasture, and whether native or exotic tree species are planted. The results of this study suggest that plantations are most likely to contribute to biodiversity when established on degraded lands rather than replacing natural ecosystems, such as forests, grasslands, and shrublands, and when indigenous tree species are used rather than exotic species. These findings can help guide afforestation and reforestation programs, including those aimed at increasing terrestrial carbon sequestration.     

Modelling land use changes for landscape connectivity: The role of plantation forestry and highways

Landscape connectivity is a key aspect for the maintenance of biodiversity and ecosystem viability. Nowadays, the competition between economic development and nature conservation is intense. In most territories natural vegetation is being replaced by exotic tree plantations, which have a better performance in terms of timber productivity but often, a lower ecological value. We evaluated potential natural forest connectivity improvement in the Cantabria region (Northern Spain) through two main actions: protection of environmentally valuable forest areas, and reforestation with indigenous species of those patches of exotic plantation trees with a particularly important role for the connectivity of the forest network. We established a variety of scenarios to calculate least cost paths, considering the presence or absence of plantation forestry and highways to examine connectivity. Then, we applied two habitat availability indices (integral index of connectivity and probability of connectivity) attending to different dispersal distances. Our analyses show a great potential for improving connectivity using plantation forests in the natural forest network, and a dramatic impact of the highway in the north–south connectivity of the study area. Based on these results, we identified those patches of plantation forest and natural forest that are more important for the maintenance of overall landscape connectivity, and propose their protection or conversion through reforestation. The final proposed network constitutes a larger and better connected natural forested landscape than the existing one.     

The Spatial Ecology of Chacma Baboons (<Emphasis Type="Italic">Papio ursinus</Emphasis>) in a Human-modified Environment

Anthropogenic habitat alteration can have a dramatic effect on the spatial distribution and ranging patterns of primates. We characterized the spatial ecology of a free-living troop of chacma baboons (Papio ursinus) in a human-modified environment in the Cape Peninsula, South Africa. We used GPS and behavioral observations collected over 1 yr to quantify the troop’s home range size, habitat selection, choice of sleeping site, and foraging patterns. The troop comprised 115 individuals living in a home range of 9.50 km², giving a density of 12.1 baboons/km². Area use correlates positively with exotic vegetation and negatively with indigenous vegetation and altitude. The troop spent significantly more time in low-lying human-modified environments, i.e., plantations, vineyards, and urban habitat, than in indigenous vegetation that was largely restricted to steeper slopes at higher elevations. The troop slept exclusively in exotic trees, 94% of which were located in the plantation, 3% in urban habitat, and 3% in vineyards. The most consumed food items were exotic grasses, subterranean food items, and exotic pine nuts. The survival and persistence of the focal troop in close proximity to the urban edge while ≥3 neighboring troops were previously extirpated suggests that access to low-lying land in conjunction with a land-use practice that does not preclude baboon presence has been fundamental to both their survival and persistence at such a high density. The almost exclusive use of exotic vegetation both as a food source and as a safe refuge for sleeping highlights the ecological flexibility of baboons, but the systematic loss of low-lying productive land poses the single greatest threat to their continued persistence on the Cape Peninsula.     

Habitat complementarity and butterfly traits are essential considerations when mitigating the effects of exotic plantation forestry

Habitat loss threatens insect diversity globally. However, complementary vegetation types in remaining habitat increases opportunities for species survival. We assess the extent to which indigenous forest patches moderate the impact of exotic commercial afforestation on grassland butterflies. Butterflies were sampled in grassland along uncorrelated gradients of landscape-scale indigenous forest and plantation cover, while controlling for variation in local vegetation composition. We separately assessed responses by butterfly groups differing in habitat preference, larval diet, and mobility. There was no effect of landscape- or local-scale variables on species richness, but there was a strong interactive effect of forest and plantation cover on butterfly assemblage structure. The effect varied according to species traits. When forest cover was high, assemblages did not differ at different levels of plantation cover. However, plantation cover significantly influenced assemblage structure when forest cover was low. Grassland with limited forest cover in the protected area supported unique assemblages with high frequency of less mobile, specialized species with herbaceous larval host plants, whereas grassland with low forest cover near plantations had a prevalence of mobile, generalist species. A positive association between forest cover and butterflies with woody larval host plants suggests that indigenous forest patches improved the suitability of fragmented grassland for a subset of butterflies, emphasising the value of natural heterogeneity in transformed areas. However, certain butterfly traits associated with large, open grassland were under-represented in grassland between plantations, underscoring the importance of open areas in the broader landscape to conserve the full diversity of species.

     

Taxonomic and functional β-diversity of ants along tree plantation chronosequences differ between contrasting biomes

The replacement of natural by anthropogenic habitat changes biological communities in any biome. Variations in environmental conditions along the chronosequence of tree plantations may act as a gradient of environmental filtering where the gain or loss of species occurs. It is expected that environmental filtering increases with the decrease in environmental similarity between the plantations and the natural habitat. Young tree plantations are structurally more similar to grasslands than to mature plantations, which in turn are structurally more similar to subtropical forest than young plantations. This study compares patterns of beta diversity across exotic pine plantation chronosequences in contrasting biomes. We predict that taxonomic and functional beta diversity between plantation and the natural habitat assemblages increase with plantation age in grasslands and decrease in the subtropical forest. We sampled epigean ants and measured environmental variables at 54 plantations of different ages and natural habitats in grassland and forest biomes in Argentina. Taxonomic and functional beta diversity between natural habitat and pine plantations were estimated through dissimilarity indexes of turnover and nestedness. To assess the response of beta diversity estimators to plantation age we performed general linear and non-linear models. Results revealed opposite beta diversity patterns between biomes along the plantation cycle. Turnover increased and nestedness decreased with declining environmental similarity between pine plantations and the natural habitats; changes in the identity of the species were coupled to changes in their functional characteristics. Thus, a given environmental gradient may produce different diversity patterns depending on the regional species pool. Forestry practices that generate environmental conditions similar to natural environments could help to conserve species from the natural habitat.     

The Use of Vanilla Plantations by Lemurs: Encouraging Findings for both Lemur Conservation and Sustainable Agroforestry in the Sava Region,Northeast Madagascar

Extensive areas of forest are cleared every year to establish new agricultural land in the tropics, resulting in a catastrophic loss in habitat for the world’s primates. A prominent example of this process is Madagascar, where an increasing demand for arable land has led to the once-forested landscape to be now dominated by agricultural areas used for the cultivation of food and cash crops. Despite the prominence of these plantations throughout Madagascar, their suitability as a habitat to support endemic lemur populations remains unclear. Here, we assessed lemur presence in vanilla plantations, Madagascar’s principal export crop, within the northeastern Sava region with the use of line transects. We confirmed the presence of five lemur species, four of which were nocturnal cheirogaleids, in these vanilla plantations. Intensively farmed vanilla plantations and those in existing stands of vegetation supported at least one species of lemur. Furthermore, lemurs were significantly more likely to be present in plantations grown close or adjacent to natural forest fragments, compared to more intensively farmed, anthropogenic sites. In comparison, we observed eight lemur species in natural forest fragment sites in close proximity to the vanilla plantation sites, four of which we did not observe in any of the plantation sites. Our results provide evidence of lemurs using vanilla plantations and show that vanilla plantations may act as extensions of suitable habitat for lemurs, suggesting that they may also function as matrices between isolated forest fragments through which gene flow can occur. These are important and encouraging findings for both lemur conservation and for sustainable agroforestry undertaken by local farming communities.     

Relevance of exotic pine plantations as a surrogate habitat for ground beetles (Carabidae) where native forest is rare

Plantation forests are of increasing importance worldwide for wood and fibre production, and in some areas they are the only forest cover. Here we investigate the potential role of exotic plantations in supporting native forest-dwelling carabid beetles in regions that have experienced extensive deforestation. On the Canterbury Plains of New Zealand, more than 99% of the previous native forest cover has been lost, and today exotic pine (Pinus radiata) plantations are the only forest habitat of substantial area. Carabids were caught with pitfall traps in native kanuka (Kunzea ericoides) forest remnants and in a neighbouring pine plantation, grassland and gorse (Ulex europaeus) shrubland. A total of 2,700 individuals were caught, with significantly greater abundance in traps in young pine, grassland and gorse habitats than in kanuka and older pine. Rarefied species richness was greatest in kanuka, a habitat that supported two forest specialist species not present in other habitat types. A critically endangered species was found only in the exotic plantation forest, which also acts as a surrogate habitat for most carabids associated with kanuka forest. The few remaining native forest patches are of critical importance to conservation on the Canterbury Plains, but in the absence of larger native forest areas plantation forests are more valuable for carabid conservation than the exotic grassland that dominates the region.     

Reliance on Exotic Plants by Two Groups of Threatened Samango Monkeys, <Emphasis Type="Italic">Cercopithecus albogularis labiatus</Emphasis>, at Their Southern Range Limit

Understanding how threatened species adapt their behavior to landscapes shaped by humans is increasingly important to ensuring they persist in a changing world. Matrix habitats can be shared spaces where human and nonhuman primates coexist. We set out to determine how an endemic, threatened forest specialist, the frugivorous, arboreal samango monkey (Cercopithecus albogularis labiatus), has responded to a matrix habitat made up of residential gardens and commercial plantations in Eastern Cape province, South Africa. We followed two groups from dawn to dusk for a mean of 3 days/mo for 12 mo (February 1, 2011 to January 31, 2012) using scan sampling to collect data on their diet, activity, and ranging patterns. We used resource abundance transects to describe the groups’ home ranges and monitored tree phenology to calculate fruit and seed availability indices. Monkeys from both groups consumed large quantities of exotic plant species, accounting for >50% of their overall annual diet, with seeds of the invasive black wattle (Acacia mearnsii) the most commonly consumed exotic species followed by acorns of two oak species (Quercus robur and Q. palustris.). However, monkeys responded to the availability of indigenous rather than exotic fruits and seeds and increased their consumption of exotics when indigenous fruits were less available. Although monkeys spent less time moving when feeding on exotic species compared to indigenous species, eating exotics did not free up monkeys’ time to rest or socialize, as additional time was required to process exotic foods. To offset the possible negative consequences of the monkeys’ reliance on exotic seeds, including escalating conflict between monkeys and people in gardens, we suggest gradual removal of exotic plant species in the habitat and replacement with indigenous species as one mitigation strategy.     

Patterns of use and damage by exotic deer on native plant communities in northwestern Patagonia

Ungulate overabundance can strongly alter composition and structure of forest communities through selective damage independently of the herbivory history of the recipient system. In the early 1900s, red and fallow deer (Cervus elaphus and Dama dama, respectively) were introduced to northwestern Patagonia. We studied patterns of relative habitat use, damage, and species selection by introduced deer populations in three major plant communities that have the longest history of invasion in the region. We also evaluated community structure and composition characteristics as possible explanatory factors for the observed patterns of browsing, fraying, and bark-stripping. At the species level, exotic deer browsed more than expected on two evergreen species (Austrocedrus chilensis and Schinus patagonicus) and two spiny species (Colletia hystrix and Dasyphyllum diacanthoides), while one dominant shrubland tree (Lomatia hirsuta) was selected for fraying and bark-stripping. Browsing was the more widespread damage in all plant communities, while fraying and bark-stripping occurred at low incidence. Furthermore, species identity was found as the main driver of deer damage over plant community type. Finally, variation in damage and in habitat use was explained by community structure and composition characteristics. Bark-stripping and fraying are best predicted by community composition, whereas browsing and habitat use are best predicted by structural properties including understory cover, cover of fallen logs, and tree density. The patterns of damage and the association with community structure characteristics reported here provide insight for developing management strategies.     

Arthropod assemblages deep in natural forests show different responses to surrounding land use

Many contemporary landscapes have vast areas of production land-uses within landscape mosaics, which may impact species dispersal and occurrence. Here, we determined the extent to which commercial exotic plantation forests affect arthropod diversity associated with natural Afrotemperate forests in the southern Cape Afrotemperate landscape mosaic, South Africa. Natural forests and fynbos vegetation naturally coexist here, with the addition of exotic plantation forests to form a heterogeneous landscape. Epigaeic arthropods were collected by means of pitfall trapping at stations along transects from inside natural Afrotemperate forest, across the edge and into the surrounding land use, which included natural fynbos vegetation, mature forestry plantation blocks (Pinus radiata) and areas where plantations have been clear-felled. Stations were set at 5, 10, 20, 30 and 50 m to both sides of the forest edge with the addition of 100 m stations situated in the natural forest. Arthropod assemblages were distinct in all land-use types. Natural edge effect between forest and fynbos, as measured by arthropod compositional changes, was 20 m into natural forests, yet when bordered by plantations this edge increased up to 30 m into the forest. Once plantations were clear-felled, edge effects increased up to 50 m into natural forests. Responses in terms of assemblage composition and species richness were however taxon specific. Results show that (1) pine plantations are not alternative habitat for native Afrotemperate forest arthropods, (2) there were stark changes in arthropod assemblage composition at edges between these land-use types and (3) that the effects of timber plantation practices (re: clear-felling) also penetrate deep into surrounding natural forests and need to be considered in regional landscape planning. The need for an effective rehabilitation strategy of clear-felled areas is identified as key priority for bordering natural forests. Ongoing monitoring in both the disturbed area and the adjoining natural forest should be undertaken to ensure sufficient recovery.     

The role of plant fidelity and land-use changes on island exotic and indigenous canopy spiders at local and regional scales

Understanding the processes that lead to successful invasions is essential for the management of exotic species. We aimed to assess the comparative relevance of habitat (both at local and at regional scale) and plant features on the species richness of local canopy spiders of both indigenous and exotic species. In an oceanic island, Azores archipelago, we collected spiders in 97 transects belonging to four habitat types according to the degree of habitat disturbance, four types of plants with different colonisation origin (indigenous vs. exotic), and four types of plants according to the complexity of the vegetation structure. Generalised linear mixed models and linear regressions were performed separately for indigenous and exotic species at the local and regional landscape scales. At the local scale, habitat and plant origin explained the variation in the species richness of indigenous spiders, whereas exotic spider richness was poorly correlated to habitat and plant structure. The surrounding landscape matrix substantially affected indigenous spiders, but did not affect exotic spiders, with the exception of the negative effect exerted by native forests on the richness of exotic species. Our results revealed that the local effect of habitat type, plant origin and plant structure explain variations in the species richness observed at a regional scale. These results shed light on the mechanistic processes behind the role of habitat types in invasions, i.e., plant fidelity and plant structure are revealed as key factors, suggesting that native forests may act as physical barriers to the colonisation of exotic spiders.     

Contrasting Cloud Forest Restoration Potential Between Plantations of Different Exotic Tree Species

The role of exotic tree plantations for biodiversity conservation is contested. Such plantations nevertheless offer various ecosystem service benefits, which include carbon storage and facilitation of indigenous tree species regeneration. To assess forest restoration potential in tropical exotic tree plantations, we assessed native cloud forest tree regeneration in 166 plots in ca. 50‐year‐old plantations of five timber species that are widely used in tropical plantations (Pinus patula, Eucalyptus saligna, Cupressus lusitanica, Grevillea robusta and Acacia mearnsii). Differences in species abundance, diversity and composition were compared among plantations, and between plantations and disturbed and undisturbed indigenous Afromontane cloud forest (southeast Kenya) relicts after controlling for environmental variation between plots (i.e. altitude, distance to indigenous forest, soil depth, slope, aspect) and for environmental and stand structural variation (i.e. dominant tree height and basal area). Regenerating trees were mostly early‐successional species. Indigenous tree species regeneration was significantly higher in Grevillea plantations, where the seedling community also included late‐successional tree species. Regeneration under Eucalyptus was particularly poor. Acacia had a strong invasive nature, reducing its potential role and usefulness in indigenous forest restoration. Our study underlined that exotic tree plantations have differential effects on native tree species regeneration, with high potential for Grevillea plantations and low potential for invasive exotic species.     

外来树种对本地林业虫害的诱发作用

白蜡窄吉丁、萧氏松茎象、光肩星天牛、黄斑星天牛、云斑天牛和桑天牛等林业害虫均为我国本地林业生态系统中的昆虫种类,在北美白蜡树、北美松树和北美黑杨派杨树等外来树种引入和大量栽培之前,一直没有形成严重的危害,以至于白蜡窄吉丁和萧氏松茎象都没有引起人们的关注。这些重要林业害虫在我国的发生都是伴随着敏感外来树种的引进和大量不合理的种植而逐渐严重起来的。本文对外来树种的引进与本地林业虫害的发生之间的关系进行了分析,提出了外来树种的不合理引进可能诱发本地林业新虫害的观点,并对其发生机理以及相应的研究和治理对策进行了讨论。     

Native plantations as an important element for biodiversity in vanishing forested landscapes: A study of the near threatened araucaria tit spinetail (Leptasthenura setaria,Furnariidae)

Although forest loss is still a problem worldwide, estimated rates of deforestation have declined in the last decade, primarily because of an increase in the area of tree plantations. This leads to the central question of how suitable plantations are for indigenous species. Native plantations are thought to have higher value for biodiversity than plantations of non‐native trees; however, not all studies support this view. We assessed occupancy and density of the araucaria tit spinetail (Leptasthenura setaria, Furnariidae), a near threatened species, in the highly endangered araucaria forests of north‐eastern Argentina and in araucaria plantations, which comprise 90% of the remaining habitat for this species. All natural forest remnants were occupied by araucaria tit spinetails. Only 85% of the plantations were occupied; however, density was almost threefold higher in plantations compared with natural forests. Our models indicated that stand age was the most important factor in determining occupancy and density of this bird species in plantations. Plantations <10 years old exhibited lower densities than older plantations. This species does not occur in plantations of non‐native trees, but our results indicate that native plantations may provide important habitat for the araucaria tit spinetail, particularly given that most native forest has been removed. Restoration of natural remnants and conservation of old, connected plantations may assure the protection of significant populations of spinetails. The role of native araucaria plantations as habitat for other species merits further examination.     

Maintenance of ladybeetle diversity across a heterogeneous African agricultural/savanna land mosaic

The importance of agricultural land for maintaining indigenous biodiversity is an important contemporary issue. A large agricultural estate in Swaziland, which is a mosaic of citrus orchards, exotic tree windbreaks, indigenous riparian vegetation and savanna was investigated. The distribution of larval and adult coccinellids within the habitat types, and the relative influences of temperature, relative humidity, ground insolation and prey presence was assessed over a 2-year period. The highest coccinellid densities and species richness occurred within the orchards. Riparian habitats on the boundaries of the estate were closer to the orchard habitat in species composition than was the natural savanna. Not surprisingly, the presence/absence of prey was the most important variable which determined coccinellid distribution and density in the agricultural landscape. Relative humidity, temperature and ground insolation had no significant effect on coccinellid densities. Some species, such as the exotic Cheilomenes lunata, and the indigenous Scynmus sp. and Nephus sp. were widely dispersed and occurred in all habitats, while the economically important but exotic species, Rodolia cardinalis was only within the citrus orchards. Overall, the coccinellid assemblage was not a good indicator of habitat type. Nevertheless, a variety of habitat types (i.e. a heterogenous landscape) appear to be essential for the survival of most coccinellids. Indeed, the heterogeneous agricultural and natural land mosaic provides maximum plant diversity and hence coccinellid diversity, thus increasing the potential for natural control of pest prey species in the orchards, while at the same time maximizing survival of indigenous coccinellids.     

Apes in space: saving an imperilled orangutan population in Sumatra

Deforestation rates in Sumatra are amongst the highest in the tropics. Lowland forests, which support the highest densities of orangutans, are particularly vulnerable to clearance and fragmentation because they are highly accessible. Consequently, many orangutans will, in the future, live in strictly or partially isolated populations. Whilst orangutans have been extensively studied in primary forests, their response to living in human-dominated landscapes remains poorly known, despite it being essential for their future management. Here, we focus on an isolated group of critically endangered Sumatran orangutans (Pongo abelii) that co-exist with farmers in a mixed agroforest system consisting of degraded natural forest, smallholder (predominantly rubber) farms and oil palm plantations. Over 24 months we conducted the first ever spatial assessment of orangutan habitat use in the human-transformed landscape of Batang Serangan, North Sumatra. From 1,204 independent crop-raiding incidents recorded, orangutans showed strong foraging preference for mixed farmland/degraded forest habitat over oil palm patches. The core home range areas of the eight adult orangutans encompassed only 14% of the available study area. Monthly home range sizes averaged 423 ha (±139, SD) for males, and 131 ± 46 ha for females, and were positively influenced by wild and cultivated fruit presence, and by crop consumption. The average daily distance travelled was similar for both adult males (868 m ± 350, SD) and females (866 m ± 195), but increased when orangutans raided crops. These findings show that orangutans can survive, demographically, in certain types of degraded landscapes, foraging on a mixture of crops and wild fruits. However, the poor quality habitat offered to orangutans by oil palm plantations, in terms of low food availability and as a barrier to female movements, is cause for concern since this is the land use type that is most rapidly replacing the preferred forest habitat across both Sumatran and Bornean orangutan ranges.     

外来红树植物无瓣海桑和拉关木在海南东寨港的人工种植与自然扩散

海南东寨港国家级自然保护区拥有我国主要的外来红树植物引种驯化基地,于1985年和1999年分别从孟加拉国和墨西哥引入外来红树植物无瓣海桑(Sonneratia apetala)和拉关木(Laguncularia racemosa)。此后这两个种又从此基地向我国其他沿海湿地再引种,目前已在该岛及华南多处湿地成林,并引发了可能产生对乡土红树植物群落不利影响的担忧。为此,本研究于2019年7月对我国最早引入和种植无瓣海桑和拉关木的东寨港进行这两个种的人工种植与自然扩散的调查,分析它们的扩散路径和扩散趋势,以期为红树林湿地外来植物的管控提供参考。调查结果显示:无瓣海桑人工种植面积为40.56 hm2,主要在试验区和核心区;自然扩散面积40.12 hm2,大部分零散分布于核心区和缓冲区;由于种植次数多、种植地点分散,扩散路径难以判断。拉关木人工种植面积为6.23 hm2,主要在保护水域和试验区;扩散面积0.53 hm2,主要在缓冲区;拉关木已形成的3个主要扩散区在相对集中的种植区外围,可推断其个体呈辐射状向外扩散,扩散的直线距离达3～5 km。扩散区有众多的幼苗个体,显示数量还存在爆发的趋势。两...     

Invasion of exotic earthworms into ecosystems inhabited by native earthworms

The most conspicuous biological invasions in terrestrial ecosystems have been by exotic plants, insects and vertebrates. Invasions by exotic earthworms, although not as well studied, may be increasing with global commerce in agriculture, waste management and bioremediation. A number of cases has documented where invasive earthworms have caused significant changes in soil profiles, nutrient and organic matter dynamics, other soil organisms or plant communities. Most of these cases are in areas that have been disturbed (e.g., agricultural systems) or were previously devoid of earthworms (e.g., north of Pleistocene glacial margins). It is not clear that such effects are common in ecosystems inhabited by native earthworms, especially where soils are undisturbed. We explore the idea that indigenous earthworm fauna and/or characteristics of their native habitats may resist invasion by exotic earthworms and thereby reduce the impact of exotic species on soil processes. We review data and case studies from temperate and tropical regions to test this idea. Specifically, we address the following questions: Is disturbance a prerequisite to invasion by exotic earthworms? What are the mechanisms by which exotic earthworms may succeed or fail to invade habitats occupied by native earthworms? Potential mechanisms could include (1) intensity of propagule pressure (how frequently and at what densities have exotic species been introduced and has there been adequate time for proliferation?); (2) degree of habitat matching (once introduced, are exotic species faced with unsuitable habitat conditions, unavailable resources, or unsuited feeding strategies?); and (3) degree of biotic resistance (after introduction into an otherwise suitable habitat, are exotic species exposed to biological barriers such as predation or parasitism, “unfamiliar” microflora, or competition by resident native species?). Once established, do exotic species co-exist with native species, or are the natives eventually excluded? Do exotic species impact soil processes differently in the presence or absence of native species? We conclude that (1) exotic earthworms do invade ecosystems inhabited by indigenous earthworms, even in the absence of obvious disturbance; (2) competitive exclusion of native earthworms by exotic earthworms is not easily demonstrated and, in fact, co-existence of native and exotic species appears to be common, even if transient; and (3) resistance to exotic earthworm invasions, if it occurs, may be more a function of physical and chemical characteristics of a habitat than of biological interactions between native and exotic earthworms.     

引种桉树对本地生物多样性的影响

桉树以速生和适应多种环境的特性成为世界著名的造林树种.但引种桉树对环境可能产生负面影响,如导致土地退化、地下水水位下降和多样性降低等,特别是对林下本地生物物种多样性的影响及其原因还存有争议.本文对此进行了综述,认为大部分桉树人工林本地物种数量低于天然林,一般不高于乡土树种人工林,但总体上好于其他外来树种人工林.导致桉树人工林生物多样性低的原因主要是桉树的生理生态特性、人类不合理的规划和砍伐等,其中人为因素起主导作用.若根据引种地的群落性质,通过严格的设计和科学管理,可以将这种负面影响最小化.应按照有利于群落正常发育和植被更新的方式栽种桉树人工林,保留天然植被,减少人为干扰,从而减轻引种桉树的负面影响.