Responses of insect herbivores and herbivory to habitat fragmentation: a hierarchical meta‐analysis

Loss and fragmentation of natural habitats can lead to alterations of plant–animal interactions and ecosystems functioning. Insect herbivory, an important antagonistic interaction is expected to be influenced by habitat fragmentation through direct negative effects on herbivore community richness and indirect positive effects due to losses of natural enemies. Plant community changes with habitat fragmentation added to the indirect effects but with little predictable impact. Here, we evaluated habitat fragmentation effects on both herbivory and herbivore diversity, using novel hierarchical meta‐analyses. Across 89 studies, we found a negative effect of habitat fragmentation on abundance and species richness of herbivores, but only a non‐significant trend on herbivory. Reduced area and increased isolation of remaining fragments yielded the strongest effect on abundance and species richness, while specialist herbivores were the most vulnerable to habitat fragmentation. These fragmentation effects were more pronounced in studies with large spatial extent. The strong reduction in herbivore diversity, but not herbivory, indicates how important common generalist species can be in maintaining herbivory as a major ecosystem process.     

Resource complementation and the response of an insect herbivore to habitat area and fragmentation

Few studies have disentangled the effects of the area and fragmentation of a focal habitat type on species that use multiple habitat types within a landscape. We experimentally investigated the effects of habitat area, habitat fragmentation, and matrix composition on the movement and distribution of Melanoplus femurrubrum. Adults of this grasshopper feed preferentially on grasses, but oviposit almost exclusively in soil dominated by forbs. We compared population densities among plots that were made to vary in the area and fragmentation of clover habitat and composition of the matrix (grass or bare ground) within which clover habitat was embedded. In addition, a mark-recapture survey was conducted to examine effects of habitat area, fragmentation, and matrix composition on loss of individuals from a plot’s clover habitat and movement between clover subplots within plots. Overall densities of adult M. femurrubrum (averaged over clover and matrix) were 2.2× higher in plots where the matrix was composed of grass as compared to bare ground, and 1.8× higher in plots with 64 compared to 16 m² of clover habitat. Overall densities of nymphs were also positively influenced by greater clover area, but were unaffected by matrix composition. Within focal clover habitat embedded in grass matrix, adult densities were 2.1× higher in small clover subplots than large clover subplots. We conclude that the grass matrix had a positive effect on adult densities, but not nymph densities, because grass and forb-dominated habitats likely provide complementary resources only for adults. The aggregation of adults on small clover subplots within grass matrix was mainly attributed to a greater rate of emigration loss per unit area. In addition, this study emphasizes that a species’ response to changes in the area of a focal habitat type can depend significantly on the availability of complementary resources in the surrounding landscape. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Effects of habitat fragmentation and soil quality on reproduction in two heathland Genista species

Habitat fragmentation decreases plant population size and increases population isolation, as well as altering patterns of plant–animal interactions, all of which may reduce plant fitness. Here, we studied effects of habitat fragmentation (in terms of population size and isolation) and soil quality on the reproduction of two rare legume species, Genista anglica (13 populations) and Genista pilosa (14 populations), confined to remnants of acidic and nutrient‐poor Calluna heathlands. Single individuals of the Genista plants are impossible to distinguish; population size was therefore estimated according to the area occupied (referred to as population size hereafter). We collected soil samples in all heathland sites to determine content of soil water, C, N, P, Ca, K and Mg. In both species values of soil pH and C/N ratio, as well as content of soil P and base cations, reflected the highly acidic and nutrient‐poor environment of the heathlands. Population sizes were unrelated to soil quality. Although the two Genista species are similar in morphology and ecology, effects of explanatory variables on reproduction were largely inconsistent across species. In G. anglica, population size had a positive impact on all reproductive variables except germination rate, which, in contrast, was the only variable affected positively by population size in G. pilosa. In both species, mean total reproductive output, calculated as the product of total seed mass per shoot and total germination, increased with increasing water content and decreased with increasing P. In G. anglica, we found positive effects of the C/N ratio on all reproductive variables except mean single and total seed mass per shoot. In summary, in both species reproductive success per shoot decreased with increasing soil nutrient availability in the heathland sites. The infestation of two large populations of G. pilosa with the pre‐dispersal, seed‐predating weevil Apion compactum had no significant effect on reproduction of the populations.     

黄河三角洲景观组分判定与景观破碎化分析

以地理信息系统（ＡＲＣ／ＩＮＦＯ）为手段,利用遥感卫星图片（ＴＭ５）及其他相关图件,以地貌、土壤与植被作为景观分类指标,把黄河三角洲的景观分成８大类型,３０个景观类型．在此基础上,以斑块的周长面积比值、相对面积及与其他景观类型之间的空间相关关系作为识别基质的指标,判定黄河三角洲景观的基质是柽柳芦苇潮盐土斜平地景观;根据斑块的周长面积比值识别廊道,并进行定量化研究．采用斑块密度对黄河三角洲景观的破碎化进行分析,发现生态交错带内斑块密度大;老河道附近景观破碎化严重     

生境破碎化对生物多样性的影响

生境破碎化对生物多样性和生态系统功能的影响是当前国内外生态学家研究的热点问题之一。生境破碎化导致原生境的总面积减小,产生隔离的异质种群,从而影响个体行为特性、种群间基因交换、物种间相互作用及生态过程。生境破碎化的过程引起栖息地内部食物、繁殖场所、局部小气候、边缘效应等生物和非生物条件的变化,从而影响植物种群的大小和灭绝速率、扩散和迁入、遗传和变异以及存活力等,影响动物种群的异质种群动态、适宜生境比例、灭绝阈值、种间关系等。随着景观生态学与农业科学的融合,探索利用景观布局控制害虫发生将是人类利用生境破碎化为人类服务的一条新途径。     

京杭运河邵伯高邮段栖息地破碎化对狗獾种群数量分布的影响

栖息地破碎是生物多样性下降的主要原因之一。栖息地破碎引起的面积效应、隔离效应和边缘效应能影响动物种群的绝灭阈值、分布、多度、种间关系以及生态系统过程, 最终影响动物种群的数量分布。2006年10-11月和2007年10-11月, 利用全球定位系统(GPS)、地理信息系统(GIS)和样方法定量分析京杭运河邵伯至高邮段狗獾栖息地破碎化程度, 确定不同斑块的面积、斑块距离、斑块隔离度以及栖息地质量对斑块中狗獾数量分布的影响。结果表明, 各个斑块内狗獾的洞口数、粪堆数与该斑块的面积显著的正相关(r=0.961, P=0.039; r=0.999, P=0.023), 但与斑块距离、斑块隔离度无显著的相关性(P>0.05)。栖息地的质量也会影响狗獾的数量分布, 多元线性逐步回归分析表明, 人类干扰和与栖息地的郁闭性显著的影响狗獾的数量分布。以上结果说明, 京杭运河邵伯高邮段栖息地的破碎化程度对狗獾的数量分布还没有造成显著的直接影响, 但会间接降低栖息地的质量, 进而影响狗獾的生存。     

The effect of periodic habitat fluctuations on a nonlinear insect population model

 Laboratory data show that populations of flour beetles (Tribolium), when grown in a periodically fluctuating volume of flour, can exhibit significant increases in numbers above those attained when grown in a constant volume (of the same average). To analyze and explain this phenomenon a discrete stage-structured model of Tribolium dynamics with periodic environmental forcing is introduced and studied. This model is an appropriately modified version of an experimentally validated model for flour beetle populations growing in a constant volume of flour, in which cannibalism rates are assumed inversely proportional to flour volume. This modeling assumption has been confirmed by laboratory experiments. Theorems implying the existence and stability of periodic solutions of the periodically forced model are proved. The time averages of periodic solutions of the forced model are compared with the equilibrium levels of the unforced model (with the same average flour volume). Parameter constraints are determined for which the average population numbers in the periodic environment are greater than (or less than) the equilibrium population numbers in the associated constant environment. Sample parameter estimates taken from the literature show that these constraints are fulfilled. These theoretical results provide an explanation for the experimentally observed increase in flour beetle numbers as a result of periodically fluctuating flour volumes. More generally, these integrated theoretical and experimental results provide the first convincing example illustrating the possibility of increased population numbers in a periodically fluctuating environment. Received 23 April 1996; received in revised form 28 March 1997     

A new view on EU agricultural landscapes: Quantifying patchiness to assess farmland heterogeneity

Mapping and assessment of ecosystem services in agricultural landscapes as required by the EU biodiversity policy need a better characterization of the given landscape typology according to its ecological and cultural values. Such need should be accommodated by a better discrimination of the landscape characteristics linked to the capacity of providing ecosystem services and socio-cultural benefits. Often, these key variables depend on the degree of farmland heterogeneity and landscape patterns. We employed segmentation and landscape metrics (edge density and image texture respectively), derived from a pan-European multi-temporal and multi-spectral remote sensing dataset, to generate a consistent European indicator of farmland heterogeneity, the Farmland Heterogeneity Indicator (FHI). We mapped five degrees of FHI on a wall-to-wall basis (250 m spatial resolution) over European agricultural landscapes including natural grasslands. Image texture led to a clear improvement of the indicator compared to the pure application of Edge Density, in particular to a better detection of small patches. In addition to deriving a qualitative indicator we attributed an approximate patch size to each class, allowing an indicative assessment of European field sizes. Based on CORINE land cover, we identified pastures and heterogeneous land cover classes as classes with the highest degree of FHI, while agroforestry and olive groves appeared less heterogeneous on average. We performed a verification based on a continental and regional scale, which resulted in general good agreement with independently derived data.     

武夷山景区主要景观类型斑块大小分布规律及其等级尺度效应分析

以武夷山风景名胜区为研究对象,运用景观分类生态原则和群落生态学原则,将武夷山风景名胜区划分为10类景观类型,并提取各类型斑块的面积、周长、数量等信息。在此基础上,运用正态分布、对数正态分布、Weibull分布、г-分布、Bata-分布及负指数分布等6个常见的概率分布,分别在3种等级划分条件下,对各景观类型斑块大小分布规律进行研究．结果表明,多数斑块大小服从对数正态分布。少数服从r．分布和Weibull分布,而所有类型的斑块大小均不服从正态分布;对于同一景观类型而言,不同划分等级对其斑块大小的分布规律产生一定的影响,即斑块大小分布存在着等级效应,其中以b等级划分相对比较理想;对不同的景观类型而言,斑块大小分布规律依类型的不同而存在差异,而且就整个景区所有斑块大小分布来说,尚未有一种概率分布能理想地加以刻画;由具有相同分布规律的不同景观类型的斑块构成的整体,其斑块大小分布规律基本保持不变,而由不同分布规律的不同景观类型的斑块构成的整体,其斑块大小分布规律发生变化。     

Understanding the effect of landscape fragmentation and vegetation productivity on elephant habitat utilization in Amboseli ecosystem,Kenya

Understanding factors affecting the distribution of the African elephant is important for its conservation in increasingly human‐dominated savannah landscapes. However, understanding how landscape fragmentation and vegetation productivity affect elephant habitat utilization remains poorly understood. In this study, we tested whether landscape fragmentation and vegetation productivity explain elephant habitat utilization in the Amboseli ecosystem in Kenya. We used GPS (Global Positioning System) telemetry data from five elephants to quantify elephant habitat utilization. Habitat utilization was determined by calculating the time elephants spent within a unit area. We then used generalized additive models (GAMs) to model the relationship between time density and landscape fragmentation, as well as vegetation productivity. Results show that landscape fragmentation and vegetation productivity significantly (P < 0.05) explain elephant habitat utilization. A significant (P < 0.05) unimodal relationship between vegetation productivity and habitat utilization was observed. Results suggest that elephants spend much of their time in less fragmented landscapes of intermediate productivity.     

The effects of energy input,immigration and habitat size on food web structure: a microcosm experiment

It has been hypothesised that larger habitats should support more complex food webs. We consider three mechanisms which could lead to this pattern. These are increased immigration rates, increased total productivity and spatial effects on the persistence of unstable interactions. Experiments designed to discriminate between these mechanisms were carried out in laboratory aquatic microcosm communities of protista and bacteria, by independently manipulating habitat size, total productivity and immigration rate. Larger habitats supported more complex food webs, with more species, more links per species and longer maximum and mean food chains, even in the absence of differences in total energy input. Increased immigration rate resulted in more complex food webs, but habitats with higher energy input per unit area supported less complex food webs. We conclude that spatial effects on the persistence of unstable interactions, and variation in immigration rates, are plausible mechanisms by which habitat size could affect food web structure. Variation in total productivity with habitat area seems a less likely explanation for variation in food web structure.     

Effects of habitat loss, habitat fragmentation, and isolation on the density, species richness, and distribution of ladybeetles in manipulated alfalfa landscapes

Abstract.  1. Habitat loss and fragmentation are the main causes of changes in the distribution and abundance of organisms, and are usually considered to negatively affect the abundance and species richness of organisms in a landscape. Nevertheless, habitat loss and fragmentation have often been confused, and the reported negative effects may only be the result of habitat loss alone, with habitat fragmentation having nil or even positive effects on abundance and species richness.
2. Manipulated alfalfa micro-landscapes and coccinellids (Coleoptera: Coccinellidae) are used to test the effects habitat loss (0% or 84%), fragmentation (4 or 16 fragments), and isolation (2 or 6 m between fragments) on the density, species richness, and distribution of native and exotic species of coccinellids.
3. Generally, when considering only the individuals in the remaining fragments, habitat loss had variable effects while habitat fragmentation had a positive effect on the density of two species of coccinellids and on species richness, but did not affect two other species. Isolation usually had no effect. When individuals in the whole landscape were considered, negative effects of habitat loss became apparent for most species, but the positive effects of fragmentation remained only for one species.
4. Native and exotic species of coccinellids did not segregate in the different landscapes, and strong positive associations were found most often in landscapes with higher fragmentation and isolation.
5. The opposing effects of habitat loss and fragmentation may result in a nil global effect; therefore it is important to separate their effects when studying populations in fragmented landscapes.     

Contrasting effects of habitat fragmentation,population density,and prey availability on body condition of two orb‐weaving spiders

1. Habitat fragmentation is a major threat to biodiversity because it disrupts movement between habitat patches. In addition, arthropod fitness may be reduced in fragmented habitats, e.g. due to reduced prey availability. 2. We studied the relationship of spider body condition with habitat fragmentation, population density, and prey availability. We expected that prey availability and population density of spiders would be affected by landscape composition and patch isolation. Body condition should be enhanced by high prey availability, but negatively affected by population density due to competition. 3. We sampled spiders on 30 groups of cherry trees that varied independently in the level of isolation from other woody habitats and in the percentage of woody habitat within 500 m radius. As a measure of body condition, we used residuals of the relationship between individual body mass/opisthosoma width and prosoma width of the two most common orb‐weaving spider species, Nuctenea umbratica Clerck and Araniella opisthographa Kulczynski. 4. Body condition of A. opisthographa was positively correlated with the abundance of flies, which increased with the percentage of forest in the landscape. In contrast, body condition of N. umbratica was reduced at high population densities, presumably due to intraspecific competition. In addition, body condition and population density of A. opisthographa was lower at isolated sites. 5. Our study suggests that effects of landscape fragmentation on body condition vary strongly between spider species, depending on the relative role of food limitation and intraspecific competition.     

Temporal variation in the effects of habitat fragmentation on reproduction of the Mediterranean shrub <Emphasis Type="Italic">Colutea hispanica</Emphasis>

Habitat fragmentation poses a major threat to the viability of plant populations. However, the intensity of fragmentation effects may vary among years. We studied two possible effects of habitat fragmentation (patch size and isolation) on the reproduction and proportion of damaged fruits in 24 patches of the self-compatible shrub Colutea hispanica for three consecutive years with different climate conditions. We also studied the effect of fragmentation on the incidence of two main pre-dispersal seed predators, the butterflies Iolana iolas and Lampides boeticus. High between-year variability was found in number of viable seeds per fruit, number of fruits per plant, total number of viable seeds per plant and proportion of damaged fruits. In 2003, small, isolated patches had a higher fruit set and number of fruits per plant. The proportion of damaged fruits was significantly lower in isolated populations in 2003, while it was very high in all patches in 2004 and 2005. High between-year variability was also found in the proportion of fruits per plant with I. iolas eggs. In 2003 isolated patches had a lower proportion of fruits with I. iolas eggs, but no significant effect of patch size and isolation was found in 2004 or 2005. The proportion of fruits with L. boeticus eggs was similar in the three years of study, although it was slightly higher in large, non-isolated patches in 2003. Thus, the effects of fragmentation on plant reproduction cannot be generalized from one single-year survey. In contrast to the generally accepted idea that fragmentation reduces plant reproduction, plant fitness may increase in isolated patches in years with high fruit production and low seed predation.     

Habitat quality predicts the distribution of a lizard in fragmented woodlands better than habitat fragmentation

Organisms often face a higher risk of local extinction in fragmented than in continuous habitat. However, whether populations are affected by reduced size and connectivity of the habitat or by changes in habitat quality in fragmented landscapes remains poorly investigated. We studied the regional distribution and microhabitat selection of the lacertid lizard Psammodromus algirus in a fragmented landscape where the existence of deciduous and evergreen woodlands brought about variation in habitat quality. Lizards never occupied any fragment smaller than 0.5 ha. However, above that limit fragment size no longer predicted lizard occurrence, which was explained by woodland type instead, with lizards being more frequently found in deciduous than in evergreen woodlands. Lizards selected microhabitats that had structural features favouring thermoregulation, foraging and predator avoidance, and we identified better conditions for thermoregulation and food acquisition in deciduous than in evergreen woodlands. Our results support the idea that variation in habitat quality can sometimes override the effect of habitat fragmentation on animal populations. We consider the implications of our study for the conservation of Mediterranean lizards, discussing our results in a broader context framed by previous studies conducted in nearby areas.     

The influence of habitat heterogeneity on host-pathogen population dynamics

The influence of spatial heterogeneity on the population dynamics of a naturally occurring invertebrate host-pathogen system was experimentally investigated. At ten week intervals over a two year period, I quantified the spatial distribution of natural populations of the terrestrial isopod crustacean Porcellio scaber infected with the isopod iridescent virus (IIV). During the seasonally dry periods of summer and early fall in central California, isopod populations were highly aggregated and the degree of patchiness and distance between inhabited patches was greatest. Coincident with increased patchiness and patch spacing was an increase in isopod density within patches. During the wet seasons of winter and spring, isopod population patchiness, inter-patch spacing, and within-patch density was low. Seasonal changes in virus prevalence were negatively correlated with within-patch density, patchiness, and inter-patch spacing. The influence of the spatial distribution of isopods on virus prevalence was also tested in field experiments. The virus was introduced into arrays of artificial habitat patches colonized by isopods in which interpatch distance was varied. The prevalence of resulting infections was monitored at weekly intervals. In addition, dispersal rates between artificial patches and natural patches were quantified and compared. The results showed that isopods in treatments with the smallest inter-patch spacing had the highest virus prevalence, with generally lower prevalence among isopods in more widely spaced patches. The spacing of experimental patches significantly affected virus prevalence, although the experiments did not resolve a clear relationship between patch spacing and virus prevalence. Rates of dispersal between patches decreased with increased patch spacing, and these rates did not differ significantly from dispersal between natural patches. The results suggest that rates of dispersal between isopod subpopulations may be an important component of the infection dynamics in this system. I discuss the consequences of these findings for host-pathogen dynamics in fragmented habitats, and for other ecological interactions in spatially heterogeneous habitats.