Leaf litter decomposition rates in degraded and fragmented tropical rain forests of Borneo

Previously extensive tracts of primary rain forest have been degraded by human activities, and we examined how the effects of forest disturbance arising from habitat fragmentation and commercial selective logging affected ecosystem functioning in these habitats by studying leaf litter decomposition rates in litter bags placed on the forest floor. The rain forests of Borneo are dominated by trees from the family Dipterocarpaceae, and we compared leaf litter decomposition rates of three dipterocarp species at eight forest fragment sites (area 3–3529 ha) that had different histories of disturbance pre‐fragmentation: four fragments had been selectively logged prior to fragmentation and four had been formed from previously undisturbed forest. We compared these logged and unlogged forest fragments with sites in continuous forest that had been selectively logged (two sites) and fully protected and undisturbed (two sites). After 120 d, undisturbed continuous forest sites had the fastest rates of decomposition (52% mass loss). Forest fragments formed from unlogged forest (32% mass loss) had faster decomposition rates than logged forest fragments (28% mass loss), but slower rates than continuous logged forest (39% mass loss). Leaves of a light‐demanding species (Parashorea malaanonan) decomposed faster than those of a shade‐tolerant species (Hopea nervosa), but decomposition of all three dipterocarp species that we studied responded similarly to logging and fragmentation effects. Reduced decomposition rates in logged and fragmented forest sites may affect nutrient cycling and thus have detrimental consequences for forest regeneration. Conservation management to improve forest quality should be a priority, particularly in logged forest fragments.     

Spatial and temporal variation in the parasitoid assemblage of an exophytic polyphagous caterpillar

Abstract 1. Over 3400 larvae of the polyphagous ground dwelling arctiid Grammia geneura were sampled and reared over seven generations in order to characterise its parasitoid assemblage and examine how and why this assemblage varies over time and space at a variety of scales.
2. The total parasitoid assemblage of 14 species was dominated both in diversity and frequency by relatively polyphagous tachinid flies.
3. Both the composition of the parasitoid assemblage and frequency of parasitism varied strikingly among and within sampling sites, seasons, and years.
4. Overall rates of parasitism increased consistently over the duration of caterpillar development.
5. Within sampling sites, parasitism rates were non-random with respect to habitat structure and caterpillar behaviour for the most abundant parasitoid species.
6. The large variability in parasitoid assemblage structure over space and time in this system may be a function of local host population abundance, habitat-specific parasitism, and indirect interactions between G. geneura and other Macrolepidoptera through shared oligophagous and polyphagous parasitoids.     

Forest fragmentation truncates a food chain based on an old-growth forest bracket fungus

We studied the effect of forest fragmentation on the insect community inhabiting an old-growth forest specialist bracket fungus, Fomitopsis rosea , in eastern Finland. Samples of the fungus from large non-isolated control areas were compared with samples from forest fragments in two isolation time classes; 2–7 yr and 12–32 yr since isolation. Fomitopsis rosea hosted a species-rich community with relatively many specialized old-growth forest insects. The numerically dominant food chain consisted of F. rosea , the tineid moth Agnathosia mendicella and the tachinid fly Elfia cingulata , a specialist parasitoid of A. mendicella . The frequency of F. rosea on suitable fallen spruce logs and the frequency of A. mendicella in fruiting bodies were significantly lower in the forest fragments than in the control areas. The median number of trophic levels decreased from three in the control areas to one in the fragments that had been isolated for the longest period of time. The parasitoid was completely missing from the fragments isolated for 12–32 yr. Our results show that in boreal forests habitat loss and fragmentation truncate food chains of specialized species in the course of time since isolation.     

伞裙追寄蝇和双斑截尾寄蝇对草地螟的寄生特性

为认识寄生蝇对草地螟Loxostege sticticalis L.的寄生特性,调查了河北康保田间两种寄生蝇——伞裙追寄蝇Exorista civilis Rondani和双斑截尾寄蝇Nemorilla maculosa Meigen对草地螟各龄幼虫的寄生率、寄生部位及着卵量,并通过室内饲养对寄生蝇的存活率与寄主龄期、着卵部位及着卵量的关系进行了研究。结果表明： 两种寄生蝇在田间主要寄生草地螟5龄幼虫;寄生蝇可在每头寄主幼虫上着1～8粒卵不等,其中以1粒的居多（45.3%）,2粒的次之（33.6%）,但1头寄主幼虫中仅能羽化出1头寄生蝇;寄生蝇以寄生寄主幼虫头部和胸部较多,腹部较少,并且以寄生侧面居多,背面次之,腹面最少。寄生蝇的存活与寄主幼虫着卵量之间呈相关关系,在着卵量为1粒的寄主幼虫中,寄生蝇的存活率仅为66.7%,在着卵量为2粒及以上的寄主幼虫中,单头幼虫羽化出寄生蝇的比例为100%。     

Detection, identification and geographical distribution of European corn borer larval parasitoids using molecular markers

Biological control requires specific tools for the accurate detection and identification of natural enemies, and to detect unusual variations in their density, which may follow changes in agricultural practices. Here we have developed specific molecular markers to detect Lydella thompsoni (Herting) and Pseudoperichaeta nigrolineata (Walker) (Diptera: Tachinidae) within the European corn borer, Ostrinia nubilalis (Hübner) (Lepidoptera: Crambidae). Primers amplifying fragments of the mitochondrial COI gene were designed following alignment of comparable sequences for a range of parasitoid and host species. Each of the primer pairs proved to be species specific to a tachinid species, amplifying DNA fragments of 191 and 91 bp in length for L. thompsoni and P. nigrolineata, respectively. This DNA-based technique allowed molecular evaluation of parasitism in O. nubilalis natural populations. In order to study the geographical distribution of both species in France, O. nubilalis diapausing larvae in maize stalks were collected from 12 locations over the whole country. The molecular evaluation of parasitism was compared with the traditional method of maintaining O. nubilalis populations in controlled conditions before breaking off the diapause. The percentage parasitism found in both species of tachinids was higher--approximately three times--using the molecular method, suggesting an underestimation by the traditional rearing protocol. Tachinid parasitism on O. nubilalis was not significantly different between geographical areas (south, central and north France) for both species. This study shows that molecular methods are very promising for the correct detection and identification of tachinid parasitoids in natural field populations.     

Gall wasps and their parasitoids in cork oak fragmented forests

Abstract.  1. This paper explores the potential effects of host-plant fragmentation on cork oak gall wasp populations (Cynipidae, Hymenoptera) and on their predators, lethal inquilines, and parasitoids. To address this objective, galls were collected across a gradient of cork oak ( Quercus suber ) forest fragmentation in the East Pyrenees (Albera, Spain), and they were incubated to obtain the parasitism rates.
2. Two hypotheses were tested: (1) Host-plant fragmentation may induce a decline in gall wasp populations because of area and isolation effects on local extinction and dispersal; as a consequence of that, parasitoids may decline even more strongly in fragmented habitats than their prey. (2) Host-plant fragmentation may cause a decline in gall wasp parasitoid populations that, in turn, can lead to an ecological release in their prey populations.
3. Among the eight cork oak gall wasps sampled in the study area of Albera, the gall abundances of three species ( Callirhytis glandium , Callirhytis rufescens , and Andricus hispanicus ) were significantly related to forest fragmentation. The overall abundance of gall wasps was affected by a radius of ≈ 890 m surrounding landscape, presenting constant abundances with forest loss until forest cover is reduced at ≈ 40%; below that value the abundance increased rapidly. Three inquilines and 23 parasitoids species were recorded after gall incubation. In 25 cases, species of inquilines and parasitoids were newly recorded for the corresponding host in the Iberian peninsula.
4. Although the overall parasitism rate was high (1.1), it was uncorrelated with fragmentation and with overall cynipid abundance. These results indicate that host-plant fragmentation was correlated with higher abundance of gall wasps, whereas the parasitism rate could not explain this hyper-abundance in small forest fragments.     

Responses of natural enemies to experimentally increased populations of the forest tent caterpillar, Malacosoma disstria

1. The responses of predators and parasitoids to increased forest tent caterpillar populations were studied by introducing eggs to two trembling aspen, Populus tremuloides Michx., forests where natural populations were at very low density.
2. Of five parasitoid species recovered, only the braconid Aleiodes malacosomatos (Mason) and the tachinid Patelloa pachypyga (Aldrich & Webber) exhibited spatially density-dependent responses, in dry upland forest and mesic lowland forest, respectively. Forest type restrictions on the abundance of these species and the weak density-dependent response by A. malacosomatos suggests that parasitoids may not be capable of regulating low-density tent caterpillar populations.
3. Predation of final-instar larvae and pupae by birds, in particular the northern oriole, Icterus galbula (L.), virtually eliminated the experimental populations. Avian predation was widespread and dominated mortality at all densities.
4. Implications for the initiation of outbreaks of cyclic defoliators are discussed.     

How parasitism affects critical patch-size in a host-parasitoid model: application to the forest tent caterpillar

Habitat structure has broad impacts on many biological systems. In particular, habitat fragmentation can increase the probability of species extinction and on the other hand it can lead to population outbreaks in response to a decline in natural enemies. An extreme consequence of fragmentation is the isolation of small regions of suitable habitat surrounded by a large region of hostile matrix. This scenario can be interpreted as a critical patch-size problem, well studied in a continuous time framework, but relatively new to discrete time models. In this paper we present an integrodifference host-parasitoid model, discrete in time and continuous in space, to study how the critical habitat-size necessary for parasitoid survival changes in response to parasitoid life history traits, such as emergence time. We show that early emerging parasitoids may be able to persist in smaller habitats than late emerging species. The model predicts that these early emerging parasitoids lead to more severe host outbreaks. We hypothesise that promoting efficient late emerging parasitoids may be key in reducing outbreak severity, an approach requiring large continuous regions of suitable habitat. We parameterise the model for the host species of the forest tent caterpillar Malacosoma disstria Hbn., a pest insect for which fragmented landscape increases the severity of outbreaks. This host is known to have several parasitoids, due to paucity of data and as a first step in the modelling we consider a single generic parasitoid. The model findings are related to observations of the forest tent caterpillar offering insight into this host-parasitoid response to habitat structure.     

Long-term effects of forest fragmentation on Amazonian ant communities

Aim To analyse the effects of forest fragmentation on ant communities in an Amazonian landscape that has been fragmented for over a century. Location The region surrounding the village of Alter do Chão in the Brazilian Amazonian state of Pará (2°30′ S, 54°57′ W). Methods Collection of ants and measurements of tree density were performed along transects established in eight sites in continuous forest and in 24 forest fragments surrounded by savanna vegetation. Data on size, perimeter, and degree of isolation (distance to continuous forest and distance to nearest area of forest > 5 ha) of each fragment were obtained from a georeferenced Landsat image of the study area. Results There were significant differences in species richness and composition between fragments and continuous forest, and these differences were not related to intersite variation in vegetation structure (tree density). Fragments supported fewer ant species per plot, and these species tended to represent a nested subset of those found in continuous forests. Fragments had significantly fewer rare species and fewer ant genera. However, fragments and continuous forest had similar numbers of species that also occur in the savanna matrix (i.e. that are not forest specialists). Multiple linear regression analyses indicated that species richness and composition in the fragments are significantly affected by fragment area, but not by fragment shape and degree of isolation. More species were found in larger fragments. Main conclusions Forest fragmentation influences the organization of ant communities in Amazonian savanna/forest landscapes. Forest fragments harboured, on average, 85% of the species found in continuous forest. That these fragments, despite their long history of isolation, support a relatively large complement of the species found in continuous forest is surprising, especially given that in some recently fragmented landscapes the proportion of species surviving in the fragments is lower. Differences in inter‐fragment distance and type of matrix between Alter do Chão and these other landscapes may be involved. The fact that fragments at Alter do Chão are surrounded by a natural (rather than an anthropogenic) habitat, and that most of them are less than 300 m from another forest area, may have helped to ameliorate the adverse effects of forest fragmentation.     

双斑截尾寄蝇对寄主种类及草地螟幼虫龄期和寄生部位的选择性

双斑截尾寄蝇Nemorilla maculosa Meigen是草地螟和其他鳞翅类昆虫的重要寄生天敌,但国内外对其寄生行为知之甚少。为了明确双斑截尾寄蝇的寄生行为特征及其对草地螟种群动态的影响,在22℃,光照16 h的条件下就其对不同寄主种类及草地螟幼虫龄期、寄主部位的选择性进行了研究。结果显示：在草地螟、甜菜夜蛾和粘虫3种幼虫同时存在的条件下,双斑截尾寄蝇对草地螟的寄生率显著高于甜菜夜蛾和粘虫,而对甜菜夜蛾的寄生率又显著高于粘虫。另外,幼虫被寄生的平均次数也呈现相同的趋势,表明双斑截尾寄蝇喜选择草地螟。在3, 4, 5龄草地螟幼虫同时存在时,双斑截尾寄蝇对5龄幼虫的寄生率和平均寄生次数均显著高于4龄和3龄幼虫的,表明该寄蝇主要选择5龄幼虫为其寄主。在被寄生的草地螟幼虫中,胸部被寄生的比率（66.4%）和次数（4.2）显著高于头部（23.4%,1.0）和腹部（10.9%,1.1）,而腹部及头部被寄生的比例及卵粒数没有显著差异,表明双斑截尾寄蝇主要选择幼虫的胸部进行寄生。最后,就双斑截尾寄蝇对产卵寄主选择的原因和机理进行了讨论。     

Are galling insects better protected against parasitoids than exposed feeders?: a test using tenthredinid sawflies

ABSTRACT.

• 1 Data mostly from the published literature were used to assess the effect of galling on the number of parasitoid species per host species in the phylogeny of nematine sawflies from free external feeders (colonial and solitary) to leaf gallers and shoot gallers.
• 2 The strongest effects of galling were the total elimination of the species-rich cocoon-attacking guild of parasitoids, and eonymphal parasitoids, from the parasitoid community on shoot gallers, all of which are in the genus Euura.
• 3 All tachinid larval parasitoids were also eliminated by the galling habit.
• 4 The cumulative effects of these exclusions resulted in a decline in mean number of parasitoid species per host species from almost sixteen species on external colonial feeders to 4.0 species on shoot gallers.
• 5 General patterns in per cent parasitism by non-tachinid and tachinid larval parasitoids, eonymphal and cocoon parasitoids, on exposed feeders to shoot gallers, showed declines in non-tachinid attack and elimination of tachinid, eonymphal and cocoon parasitoids. But leaf gallers tended to be attacked more than exposed feeders by non-tachinid larval parasitoids.
• 6 The galling habit had a long-term impact by reducing the number of parasitoid species attacking nematine sawfly gallers and per cent mortality inflicted, so that natural enemies may have been important as a selective factor in the evolution of galling nematine sawflies.

     

草地螟的寄生蝇种类、寄生方式及其对寄主种群的调控作用

寄生蝇是草地螟的重要寄生天敌之一,其寄生率多数情况下高于寄生蜂,表现出很大的控害潜力。本文通过对文献资料的分析,结合多年来对草地螟寄生蝇的研究结果,对草地螟的寄生蝇种类、寄生方式、优势种及其控制作用,保护利用进行了总结和评述。草地螟寄生蝇种类丰富,目前已知有22个种,在草地螟的主要发生为害区都有分布,其寄生方式多样,寄主范围广,很多种类为多主寄生。优势种有4种：伞裙追寄蝇Exorista civilis Rondani,双斑截尾寄蝇Nemorilla maculosa Meigen,黑袍卷须寄蝇Clemelis pullata Meigen和草地追寄蝇Exorista pratensis Robincau Uesvoidy。这些优势种对控制当代草地螟的作用有限,而对控制下一代草地螟的发生为害起着重要的作用。本文最后讨论了草地螟寄生蝇的保护和利用前景。     

The Multiple Impacts of Tropical Forest Fragmentation on Arthropod Biodiversity and on their Patterns of Interactions with Host Plants

Tropical rain forest fragmentation affects biotic interactions in distinct ways. Little is known, however, about how fragmentation affects animal trophic guilds and their patterns of interactions with host plants. In this study, we analyzed changes in biotic interactions in forest fragments by using a multitrophic approach. For this, we classified arthropods associated with Heliconia aurantiaca herbs into broad trophic guilds (omnivores, herbivores and predators) and assessed the topological structure of intrapopulation plant-arthropod networks in fragments and continuous forests. Habitat type influenced arthropod species abundance, diversity and composition with greater abundance in fragments but greater diversity in continuous forest. According to trophic guilds, coleopteran herbivores were more abundant in continuous forest and overall omnivores in fragments. Continuous forest showed a greater diversity of interactions than fragments. Only in fragments, however, did the arthropod community associated with H aurantiaca show a nested structure, suggesting novel and/or opportunistic host-arthropod associations. Plants, omnivores and predators contributed more to nestedness than herbivores. Therefore, Heliconia-arthropod network properties do not appear to be maintained in fragments mainly caused by the decrease of herbivores. Our study contributes to the understanding of the impact of fragmentation on the structure and dynamics of multitrophic arthropod communities associated with a particular plant species of the highly biodiverse tropical forests. Nevertheless, further replication of study sites is needed to strengthen the conclusion that forest fragmentation negatively affects arthropod assemblages.     

Population structure of a large blue butterfly and its specialist parasitoid in a fragmented landscape

Habitat fragmentation may interrupt trophic interactions if herbivores and their specific parasitoids respond differently to decreasing connectivity of populations. Theoretical models predict that species at higher trophic levels are more negatively affected by isolation than lower trophic level species. By combining ecological data with genetic information from microsatellite markers we tested this hypothesis on the butterfly Maculinea nausithous and its specialist hymenopteran parasitoid Neotypus melanocephalus. We assessed the susceptibility of both species to habitat fragmentation by measuring population density, rate of parasitism, overall genetic differentiation (theta(ST)) and allelic richness in a large metapopulation. We also simulated the dynamics of genetic differentiation among local populations to asses the relative effects of migration rate, population size, and haplodiploid (parasitoid) and diploid (host) inheritance on metapopulation persistence. We show that parasitism by N. melanocephalus is less frequent at larger distances to the nearest neighbouring population of M. nausithous hosts, but that host density itself is not affected by isolation. Allelic richness was independent of isolation, but the mean genetic differentiation among local parasitoid populations increased with the distance between these populations. Overall, genetic differentiation in the parasitoid wasp was much greater than in the butterfly host and our simulations indicate that this difference is due to a combination of haplodiploidy and small local population sizes. Our results thus support the hypothesis that Neotypus parasitoid wasps are more sensitive to habitat fragmentation than their Maculinea butterfly hosts.     

Larval parasitism of a forest pest, the common pine sawfly Diprion pini (L.) (Hym., Diprionidae), during an endemic density phase

Abstract: Species composition and impact of larval parasitoids during an endemic density phase of a pine forest pest, the common pine sawfly Diprion pini (L.), were investigated by exposure of laboratory-reared larvae of the sawfly to parasitoid attack in natural habitats. Colony-associated host larvae of different instars were exposed continuously from early June until end of September of the years 1994 and 1995 in two pine stands with a different history of pine sawfly outbreaks. Host larvae were exchanged after a defined time interval and those collected up were examined in the laboratory for parasitism. Parasitoid assemblages were shown to be very species poor at both pine stands. The dominant species were the ichneumonid Olesicampe macellator and the tachinid Drino gilva , which reached attack rates on the collected host larvae of 60 and 94%, respectively, during peak activity. The patterns of temporal attack, of multiparasitism, and of superparasitism of these dominant species were studied. In an additional experiment, the response of these species to different host-patch sizes (number of host larvae per colony) were investigated. No aggregative response to higher host-patch densities could be detected. The role and function of these larval parasitoids in the population dynamics of D. pini are discussed.     

Herbivory and seedling performance in a fragmented temperate forest of Chile

Forest fragmentation alters plant-animal interactions, including herbivory. Relying manipulative experiments, we test if the reduction in insect herbivory associated with forest fragmentation translates into increased seedling growth and survival of three tree common species (Aristotelia chilensis, Cryptocarya alba and Persea lingue) in forest fragments and continuous forests in coastal Maulino forest, central Chile. Furthermore, we test if after protecting seedlings from herbivorous insects, plant performance is increased regardless of forest fragmentation. Nursery grown seedlings were transplanted into four forest fragments and a continuous forest during 2002. Insects, important herbivores in this forest, were excluded from half the seedlings by repeated applications of insecticides. Compared to continuous forests, in forest fragments, herbivory was reduced in all three species, seedling growth was greater in A. chilensis and C. alba but not in P. lingue, and survivorship was unaffected by herbivory or fragmentation in all three species. Protecting seedlings from insects reduced herbivory in the continuous forest to similar levels attained in the forest fragments. No change in herbivory results from by protecting seedlings in forest fragments. These results confirm that insects are important herbivores in the Maulino forest and also support the hypothesis that fragmentation can have strong indirect effects on plant communities as mediated through trophic interactions.     

The effects of plant dispersion and prey density on parasitism rates in a naturally patchy habitat

Despite extensive research on parasitoid-prey interactions and especially the effects of heterogeneity in parasitism on stability, sources of heterogeneity other than prey density have been little investigated. This research examines parasitism rates by three parasitoid species in relationship to prey density and habitat spatial pattern. The herbivore Itame andersoni (Geometridae) inhabits a subdivided habitat created by patches of its host plant, Dryas drummondii, in the Wrangell Mountains of Alaska. Dryas colonizes glacial moraines and spreads clonally to form distinct patches. Habitat subdivision occurs both on the patch scale and on the larger spatial scale of sites due to patchy successional patterns. Itame is attacked by three parasitoids: an ichneumonid wasp (Campoletis sp.), a braconid wasp (Aleiodes n. sp.), and the tachinid fly (Phyrxe pecosensis). I performed a large survey study at five distinct sites and censused Itame density and parasitism rates in 206 plant patches for 1–3 years. Parasitism rates varied with both plant patch size and isolation and also between sites, and the highest rates of overall parasitism were in the smallest patches. However, the effects of both small- and large-scale heterogeneity on parasitism differed for the three parasitoid species. There was weak evidence that Itame density was positively correlated with parasitism for the braconid and tachinid at the patch scale, but density effects differed for different patch sizes, patch isolations, and sites. At the site scale, there was no evidence of positive, but some indication of negative density-dependent parasitism. These patterns do not appear to be driven by negative interactions between the three parasitoid species, but reflect, rather, individual differences in habitat use and response to prey density. Finally, there was no evidence that parasitism strongly impacted the population dynamics of Itame. These results demonstrate the importance of considering habitat pattern when examining spatial heterogeneity of parasitism and the impacts of parasitoids. Received: 3 June 1999 / Accepted: 4 October 1999     

Dispersal of Amazonian birds in continuous and fragmented forest

Many ecologists believe birds disappear from tropical forest fragments because they are poor dispersers. We test this idea using a spatially explicit capture data base from the Biological Dynamics of Forest Fragments Project near Manaus, Brazil. We measure bird movements directly, over relatively large scales of space and time, both before and after landscape fragmentation. We found that species which disappear from fragments move extensively between plots before isolation, but not after, and often disperse to longer distances in continuous forest than in fragmented forest. Such species also preferentially emigrate from smaller to larger fragments, showing no preference in continuous forest. In contrast, species that persist in fragments are generally less mobile, do not cross gaps as often, yet disperse further after fragmentation than before. 'Heavy tailed' probability models usually explain dispersal kernels better than exponential or Gaussian models, suggesting tropical forest birds may be better dispersers than assumed with some individuals moving very long distances.