Autumn tree colours as a handicap signal.

Many species of deciduous trees display striking colour changes in autumn. Here, we present a functional hypothesis: bright autumn coloration serves as an honest signal of defensive commitment against autumn colonizing insect pests. According to this hypothesis, individuals within a signalling species show variation in the expression of autumn coloration, with defensively committed trees producing a more intense display. Insects are expected to be averse to the brightest tree individuals and, hence, preferentially colonize the least defensive hosts. We predicted that tree species suffering greater insect damage would, on average, invest more in autumn-colour signalling than less troubled species. Here, we show that autumn coloration is stronger in species facing a high diversity of damaging specialist aphids. Aphids are likely to be an important group of signal receivers because they are choosy, damaging and use colour cues in host selection. In the light of further aspects of insect and tree biology, these results support the notion that bright autumn colours are expensive handicap signals revealing the defensive commitment of individual trees to autumn colonizing insect pests.     

Opposing deer and caterpillar foraging preferences may prevent reductions in songbird prey biomass in historically overbrowsed forests

Overbrowsing by ungulates decimates plant populations and reduces diversity in a variety of ecosystems, but the mechanisms by which changes to plant community composition influence other trophic levels are poorly understood. In addition to removal of avian nesting habitat, browsing is hypothesized to reduce bird density and diversity through reduction of insect prey on browse‐tolerant hosts left behind by deer. In this study, we excluded birds from branches of six tree species to quantify differences in songbird prey removal across trees that vary in deer browse preference. Early in the breeding season, birds preyed on caterpillars at levels proportional to their abundance on each host. Combining these data with tree species composition data from stands exposed to experimentally controlled deer densities over 30 years ago, we tested whether overbrowsing by white‐tailed deer reduces prey biomass long after deer densities are reduced. Our analysis predicts total prey availability in the canopy of regenerating forests is fairly robust to historic exposure to high deer densities, though distribution of prey available from host species changes dramatically. This predicted compensatory effect was unexpected and is driven by high prey abundance on a single host tree species avoided by browsing deer, Prunus serotina. Thus, while we confirm that prey abundance on host trees can act as a reliable predictor for relative prey availability, this study shows that quantifying prey abundance across host trees is essential to understanding how changes in tree species composition interact with ungulate browse preference to determine prey availability for songbirds.     

Evolutionary relationships, cospeciation, and host switching in avian malaria parasites

We used phylogenetic analyses of cytochrome b sequences of malaria parasites and their avian hosts to assess the coevolutionary relationships between host and parasite lineages. Many lineages of avian malaria parasites have broad host distributions, which tend to obscure cospeciation events. The hosts of a single parasite or of closely related parasites were nonetheless most frequently recovered from members of the same host taxonomic family, more so than expected by chance. However, global assessments of the relationship between parasite and host phylogenetic trees, using Component and ParaFit, failed to detect significant cospeciation. The event-based approach employed by TreeFitter revealed significant cospeciation and duplication with certain cost assignments for these events, but host switching was consistently more prominent in matching the parasite tree to the host tree. The absence of a global cospeciation signal despite conservative host distribution most likely reflects relatively frequent acquisition of new hosts by individual parasite lineages. Understanding these processes will require a more refined species concept for malaria parasites and more extensive sampling of parasite distributions across hosts. If parasites can disperse between allopatric host populations through alternative hosts, cospeciation may not have a strong influence on the architecture of host-parasite relationships. Rather, parasite speciation may happen more often in conjunction with the acquisition of new hosts followed by divergent selection between host lineages in sympatry. Detailed studies of the phylogeographic distributions of hosts and parasites are needed to characterize these events.     

Host specificity in spinturnicid mites: do parasites share a long evolutionary history with their host?

Host specificity in parasites can be explained by spatial isolation from other potential hosts or by specialization and speciation of specific parasite species. The first assertion is based on allopatric speciation, the latter on differential lifetime reproductive success on different available hosts. We investigated the host specificity and cophylogenetic histories of four sympatric European bat species of the genus Myotis and their ectoparasitic wing mites of the genus Spinturnix. We sampled >40 parasite specimens from each bat species and reconstructed their phylogenetic COI trees to assess host specificity. To test for cospeciation, we compared host and parasite trees for congruencies in tree topologies. Corresponding divergence events in host and parasite trees were dated using the molecular clock approach. We found two species of wing mites to be host specific and one species to occur on two unrelated hosts. Host specificity cannot be explained by isolation of host species, because we found individual parasites on other species than their native hosts. Furthermore, we found no evidence for cospeciation, but for one host switch and one sorting event. Host‐specific wing mites were several million years younger than their hosts. Speciation of hosts did not cause speciation in their respective parasites, but we found that diversification of recent host lineages coincided with a lineage split in some parasites.     

Deterministic processes have limited impacts on foliar fungal endophyte communities along a savanna-forest successional gradient

Patterns and drivers of succession provide insight into the mechanisms that govern community assembly, but remain poorly understood for microbial communities. We assess whether successional trends of trees are mirrored by foliar endophyte communities of three tree species across a deterministic woody successional gradient. Additionally, we test the relative contribution of abiotic predictors, biotic factors, and spatial distance between sites in predicting composition and richness of endophyte communities. Unlike the tree community, endophyte communities showed no consistent evidence of deterministic succession. Host identity was the most important factor structuring endophyte community composition; within hosts, spatial distance from the indigenous forest and between samples was important, while environmental predictors had small and inconsistent effects. Much variation in endophyte composition remained unexplained. In contrast, endophyte richness was well-explained by predictor variables. Host identity was most important in predicting endophyte richness, while the effect of other predictors on richness differed between host species. We conclude that deterministic succession in trees did not result in deterministic succession in endophyte communities; instead community assembly was most strongly influenced by host identity; while within hosts, neutral processes may be more important for endophyte assembly than deterministic factors.     

The Quest for a Suitable Host: Size Distributions of Host Trees and Secondary Hemiepiphytes Search Strategy

As climbing plants lack the capacity to hold themselves upright, they need to encounter a suitable host. Vines, lianas, and secondary hemiepiphytes need, therefore, an effective searching strategy. Various hypotheses have been put forward on searching strategies, including ‘skototropism’—growth toward darkness—and random searching. We studied host searching strategies of three secondary hemiepiphyte species belonging to the genus Heteropsis. We recorded information on the diameter distribution of host and evaluated whether these hosts were ‘suitable’, i.e., sufficiently tall for Heteropsis individuals to reach reproductive size. The diameter distribution of host trees bearing Heteropsis seedlings was similar to that of the trees in our study plots. Also, we found that 72–81 percent of the Heteropsis seedlings were present on unsuitable hosts (seedlings, saplings, herbs). These results suggest that Heteropsis seedlings search hosts in a random manner and not by skototropism. We found quite a distinct pattern for adult Heteropsis individuals, which predominantly occur on host trees bigger than 10 cm dbh. Host diameter distribution for Heteropsis adults differed significantly from that of the entire tree community. This difference suggests that Heteropsis individuals may change hosts if they are initially present on nonsuitable hosts. We observed that Heteropsis seedlings and juveniles on unsuitable hosts often produced vegetative shoots that searched for another host. In many cases, such shoots did not find a suitable host. For Heteropsis, our results suggest that host tree searching is a long‐term trial and error process that is governed by a random searching strategy. Abstract in Spanish is available in the online version of this article.     

Abundance trumps quality: bi‐trophic performance and parasitism risk fail to explain host use in the fall webworm

Factors including host abundance, quality, and the degree to which hosts provide enemy‐free space (EFS) may drive host plant choice by phytophagous insects. Herbivores may also experience fitness tradeoffs among hosts, promoting polyphagy. The fall webworm Hyphantria cunea is a dietary generalist that feeds on a broad array of trees across its geographic range. Here, we investigate the drivers of host tree use by the fall webworm in Connecticut (CT) and Maryland (MD). Neither caterpillar performance nor EFS was associated with the frequency with which host trees were used, and no tradeoff between host quality and EFS was identified. Vegetation surveys adjacent to host trees showed that at both localities, host use was non‐random with respect to tree species, and that the main predictor of use among suitable host trees was host tree abundance. This suggests that webworms are under selection to reduce search time for oviposition sites. Although we did not detect a tradeoff between host plant quality and availability in MD, we did identify that tradeoff in CT. This disparity amid otherwise similar patterns of host use between CT and MD may be explained by the relative rarity of high quality hosts in CT compared to MD. Our results illustrate that geographic mosaics in patterns of host use may arise in the absence of local adaptation if host use is based upon availability rather than host plant attributes.     

Mixing tree species associated with arbuscular or ectotrophic mycorrhizae reveals dual mycorrhization and interactive effects on the fungal partners

1. Recent studies found that the majority of shrub and tree species are associated with both arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) fungi. However, our knowledge on how different mycorrhizal types interact with each other is still limited. We asked whether the combination of hosts with a preferred association with either AM or EM fungi increases the host tree roots’ mycorrhization rate and affects AM and EM fungal richness and community composition.
2. We established a tree diversity experiment, where five tree species of each of the two mycorrhiza types were planted in monocultures, two‐species and four‐species mixtures. We applied morphological assessment to estimate mycorrhization rates and next‐generation molecular sequencing to quantify mycobiont richness.
3. Both the morphological and molecular assessment revealed dual‐mycorrhizal colonization in 79% and 100% of the samples, respectively. OTU community composition strongly differed between AM and EM trees. While host tree species richness did not affect mycorrhization rates, we observed significant effects of mixing AM‐ and EM‐associated hosts in AM mycorrhization rate. Glomeromycota richness was larger in monotypic AM tree combinations than in AM‐EM mixtures, pointing to a dilution or suppression effect of AM by EM trees. We found a strong match between morphological quantification of AM mycorrhization rate and Glomeromycota richness.
4. Synthesis. We provide evidence that the combination of hosts differing in their preferred mycorrhiza association affects the host''s fungal community composition, thus revealing important biotic interactions among trees and their associated fungi.

     

Tree pests and diseases: the threat to biodiversity and the delivery of ecosystem services

The increasing number of invasive pests and pathogens entering North America and Europe indicate that the threat which they pose to forests is increasing concurrently with climate change and globalisation. To date research has mainly focussed on the protection of trees of economic importance. In most countries the major portion of research costs tend to be borne by the state with governments funding tree health research and the implementation of statutory work (risk assessment, border inspection and surveillance), whilst commercial enterprises cover the costs of ongoing management of established pests and pathogens. The costs of responding to new outbreaks tend to be shared with government or regional authorities organising initial response but owners covering subsequent operational costs without state compensation. However in recent years a number of major epidemics have devastated natural ecosystems and landscapes valued both for timber and for their wider benefits to the general public. Against this background it is helpful to consider more explicitly the consequences of pests and pathogens for the full range of ecosystem services. Biodiversity was originally perceived to be an ecosystem service but is now recognised as fundamental in supporting ecological function. Many ecosystem services are uncosted and enjoyed by a range of stakeholders raising important questions about who is responsible for measures to protect tree health. We present data here on the new outbreaks which have occurred in the UK and, as an example of rapid spread of a disease, on the development of Hymenoscyphus fraxineus in the UK over the last 3 years. These data indicate that tree pests and diseases represent a major contemporary problem to which the ecosystem services concept, and its associated implications for cost sharing, can move forward our approach to prevention and outbreak management and may improve the outcomes of international measures to minimise the man-mediated movement and impacts of pests and pathogens.     

Host tree utilization by epiphytic orchids in different land-use intensities in Kathmandu Valley, Nepal

We studied the influence of site conditions on epiphytic orchids under a subtropical climate in the Kathmandu Valley, Nepal. We analysed 96 systematically distributed grid points situated in Kathmandu Valley across a land-use intensity gradient (national park to urbanised city area). Geographical Information System (GIS) and remote sensing were used for classification of land-use types. We identified 23 species of epiphytic orchids, within 13 genera, from 42 different host tree species. Host preference is obvious for some orchid species (e.g., Dendrobium nobile), with certain tree species (e.g., Schima wallichii, Ficus religiosa) hosting more orchid species than others. The orchid Rhynchostylis retusa was the most common species found on many different host tree species across the land-use intensity gradient. Host species and host bark characteristics (e.g., rugosity, pH and exposure to wind) played a vital role for orchid distribution, with lower abundance in areas of higher impact. Under strong human impact (urban city area), F. religiosa was the dominant host tree, with large individual trees (mean diameter in breast height, dbh?=?1.3?m) providing the habitat for considerable populations of R. retusa individuals. In general, epiphytic orchids were found on larger host trees in urban areas than in areas of lower human impact. We found that some hosts are more likely to harbour orchid species, especially native host species. Older larger trees with rougher bark, low pH, exposed to wind and reduced human impact provided better habitats for orchids. We suggest these characteristics should be considered in urban planning to reduce human impact on the associated orchid epiphytic community.     

Global climate change will increase the abundance of symbiotic nitrogen‐fixing trees in much of North America

Symbiotic nitrogen (N)‐fixing trees can drive N and carbon cycling and thus are critical components of future climate projections. Despite detailed understanding of how climate influences N‐fixation enzyme activity and physiology, comparatively little is known about how climate influences N‐fixing tree abundance. Here, we used forest inventory data from the USA and Mexico (>125,000 plots) along with climate data to address two questions: (1) How does the abundance distribution of N‐fixing trees (rhizobial, actinorhizal, and both types together) vary with mean annual temperature (MAT) and precipitation (MAP)? (2) How will changing climate shift the abundance distribution of N‐fixing trees? We found that rhizobial N‐fixing trees were nearly absent below 15°C MAT, but above 15°C MAT, they increased in abundance as temperature rose. We found no evidence for a hump‐shaped response to temperature throughout the range of our data. Rhizobial trees were more abundant in dry than in wet ecosystems. By contrast, actinorhizal trees peaked in abundance at 5–10°C MAT and were least abundant in areas with intermediate precipitation. Next, we used a climate‐envelope approach to project how N‐fixing tree relative abundance might change in the future. The climate‐envelope projection showed that rhizobial N‐fixing trees will likely become more abundant in many areas by 2080, particularly in the southern USA and western Mexico, due primarily to rising temperatures. Projections for actinorhizal N‐fixing trees were more nuanced due to their nonmonotonic dependence on temperature and precipitation. Overall, the dominant trend is that warming will increase N‐fixing tree abundance in much of the USA and Mexico, with large increases up to 40° North latitude. The quantitative link we provide between climate and N‐fixing tree abundance can help improve the representation of symbiotic N fixation in Earth System Models.     

Altitudinal patterns in host suitability for forest insects

Conspecific trees growing at high and low-elevations encounter different growing conditions and may vary in their suitability as hosts for herbivorous insects. Mountain tree populations may be more resistant to herbivory if low temperatures constrain growth more than they constrain photosynthesis, resulting in increased secondary metabolism (temperature hypothesis). Alternatively, mountain trees may be fertilized by atmospheric nitrogen deposition and become more palatable to insects (atmospheric deposition hypothesis). We evaluated these two hypotheses by comparing high- and low-elevation trees with insect bioassays and analyses of foliar nitrogen and condensed tannin. Contrary to the temperature hypothesis, high-elevation foliage had higher leaf nitrogen (six of six tree species) and allowed higher growth rates of Lymantria dispar larvae (five of six tree species). The nitrogen deposition hypothesis was broadly supported by measurements from two mountains showing that high-elevation trees tended to have higher leaf nitrogen, lower leaf tannins, and support higher insect growth performance than conspecific trees from lower elevations. The deposition hypothesis was further supported by fertilization studies showing that simulated atmospheric nitrogen deposition changed the foliar chemistry of valley trees to resemble that of high-elevation trees. Predictions that the altitudinal gradient in foliar chemistry and host suitability should be steepest on mountains receiving more deposition were largely not supported, but interpretations are complicated by lack of replication among mountains. In the northeastern United States, increased host suitability of high-elevation trees seems sufficient to influence the population dynamics and community composition of herbivores. Atmospheric nitrogen deposition offers a promising hypothesis to explain and predict some important spatial patterns in herbivory. Received: 21 September 1997 / Accepted: 12 June 1998     

Phylogenetic congruence between subtropical trees and their associated fungi

Recent studies have detected phylogenetic signals in pathogen–host networks for both soil‐borne and leaf‐infecting fungi, suggesting that pathogenic fungi may track or coevolve with their preferred hosts. However, a phylogenetically concordant relationship between multiple hosts and multiple fungi in has rarely been investigated. Using next‐generation high‐throughput DNA sequencing techniques, we analyzed fungal taxa associated with diseased leaves, rotten seeds, and infected seedlings of subtropical trees. We compared the topologies of the phylogenetic trees of the soil and foliar fungi based on the internal transcribed spacer (ITS) region with the phylogeny of host tree species based on matK, rbcL, atpB, and 5.8S genes. We identified 37 foliar and 103 soil pathogenic fungi belonging to the Ascomycota and Basidiomycota phyla and detected significantly nonrandom host–fungus combinations, which clustered on both the fungus phylogeny and the host phylogeny. The explicit evidence of congruent phylogenies between tree hosts and their potential fungal pathogens suggests either diffuse coevolution among the plant–fungal interaction networks or that the distribution of fungal species tracked spatially associated hosts with phylogenetically conserved traits and habitat preferences. Phylogenetic conservatism in plant–fungal interactions within a local community promotes host and parasite specificity, which is integral to the important role of fungi in promoting species coexistence and maintaining biodiversity of forest communities.     

气候变化对中国东北主要森林类型的影响

应用林窗模型-FAREAST,模拟未来气候变化对中国东北主要类型森林演替动态的影响.根据大气环流模型ECHAM5-OM和HadCM3预测的气候变化资料,模拟选择了目前气候情景、增暖情景、增暖且降水变化情景3种气候情景.结果表明:维持目前气候不变,东北森林树种组成和森林生物量基本维持动态平衡.气候增暖不利于东北主要森林类型生长,主要针叶树种比例下降,阔叶树比例增加;温带针阔混交林垂直分布带有上移的趋势;增暖幅度越大,变化越明显.气候增暖基础上考虑降水变化,东北森林水平分布带有北移的趋势,降水对低海拔温带针阔混交林影响不大.     

Pest management through tropical tree conservation

Tropical trees can provide various ecological services to adjacent agricultural environments, including maintaining and amplifying the numbers of beneficial insects. In Mexico, certain tree species harbor a diverse guild of hymenopteran parasitoids that attack pest fruit flies (Diptera: Tephritidae) and are at the same time sources of valuable hardwood timber. Indigenous trees and their associated fauna are slowly disappearing due to forest clearance and the expansion of crop monocultures. Here we explore the relationship among pest and non-pest fruit flies, their fruit-hosts and parasitoids in the context of mango orchards and surrounding patches of uncultivated vegetation and propose a novel mechanism to use these associations in favor of conservation purposes and pest management. Trees of conservation biological control interest are classified as: (1) parasitoid multiplier plants, species that serve as alternate hosts for key fruit fly pests when their commercial hosts are not available, but in which they are unusually vulnerable to parasitism; (2) parasitoid reservoir plants, native or introduced trees in whose fruits non-pest fruit flies serve as hosts to generalist parasitoids that are able to attack pest tephritids in other species of commercially grown fruit; and (3) pest-based parasitoid reservoir plants, native or introduced species that are not economically important locally, but which harbor fruit flies that would be pests in other circumstances and that serve as hosts for parasitoids of the important pests in the vicinity. Protection, multiplication and dissemination of such tree species has the potential to increase the number of naturally produced fruit fly parasitoids and could assist in the management of tephritid pests in areas where destruction of forests has impoverished the historical sources of fruit fly natural enemies. Tropical forest conservation may help resource-poor farmers reduce crop losses, increase biodiversity within fruit-growing regions and conserve native forests for both conservation purposes and commercial use of native hardwoods.     

Influence of insecticide treatments on ant-hemiptera associations in tropical plantations

In this survey conducted in southern Cameroon, we compared ant-Hemiptera associations on plantations treated with insecticides, on plantations 2 years after insecticide treatments ceased, and on control lots that never received insecticide treatments. By eliminating arboreal-nesting ants, insecticides favored the presence of "ecologically dominant" ground-nesting, arboreal-foraging species that occupied the tree crowns. The reinstallation of arboreal ants was slow as 2 yr after insecticide treatment ceased differences with the control lots were significant. This intermediary period also illustrated that arboreal ants can found and develop colonies on trees occupied by ground-nesting species. Certain arboreal species were more frequent during this intermediary period than on the control lots, showing that the period of installation in the trees was followed by competition between arboreal ants. We confirm that ground-nesting ants tend a wide range of hemipteran families, including well known agricultural pests, whereas arboreal ants, particularly dominant species, were mostly associated with Coccidae and Stictococcidae that do not pose problems to the supporting trees. A tree effect was also noted for both ant and hemipteran distribution. We concluded that because of insecticide use, ground-nesting ants pose problems through their associated Hemiptera. On the contrary, dominant arboreal ants, strong predators, benefit their supporting trees by excluding ground-nesting species and tending mostly nonpest Hemiptera. Nevertheless, certain of them, carpenter species or species likely to tend Pseudococcidae, have to be eliminated through integrated management.     

Selective logging in tropical forests decreases the robustness of liana–tree interaction networks to the loss of host tree species

Selective logging is one of the major drivers of tropical forest degradation, causing important shifts in species composition. Whether such changes modify interactions between species and the networks in which they are embedded remain fundamental questions to assess the ‘health’ and ecosystem functionality of logged forests. We focus on interactions between lianas and their tree hosts within primary and selectively logged forests in the biodiversity hotspot of Malaysian Borneo. We found that lianas were more abundant, had higher species richness, and different species compositions in logged than in primary forests. Logged forests showed heavier liana loads disparately affecting slow-growing tree species, which could exacerbate the loss of timber value and carbon storage already associated with logging. Moreover, simulation scenarios of host tree local species loss indicated that logging might decrease the robustness of liana–tree interaction networks if heavily infested trees (i.e. the most connected ones) were more likely to disappear. This effect is partially mitigated in the short term by the colonization of host trees by a greater diversity of liana species within logged forests, yet this might not compensate for the loss of preferred tree hosts in the long term. As a consequence, species interaction networks may show a lagged response to disturbance, which may trigger sudden collapses in species richness and ecosystem function in response to additional disturbances, representing a new type of ‘extinction debt’.     

Species loss on spatial patterns and composition of zoonotic parasites

Species loss can result in the subsequent loss of affiliate species. Though largely ignored to date, these coextinctions can pose threats to human health by altering the composition, quantity and distribution of zoonotic parasites. We simulated host extinctions from more than 1300 host–parasite associations for 29 North American carnivores to investigate changes in parasite composition and species richness. We also explored the geography of zoonotic parasite richness under three carnivore composition scenarios and examined corresponding levels of human exposure. We found that changes in parasite assemblages differed among parasite groups. Because viruses tend to be generalists, the proportion of parasites that are viruses increased as more carnivores went extinct. Coextinction of carnivore parasites is unlikely to be common, given that few specialist parasites exploit hosts of conservation concern. However, local extirpations of widespread carnivore hosts can reduce overall zoonotic richness and shift distributions of parasite-rich areas. How biodiversity influences disease risks remains the subject of debate. Our results make clear that hosts vary in their contribution to human health risks. As a consequence, so too does the loss (or gain) of particular hosts. Anticipating changes in host composition in future environments may help inform parasite conservation and disease mitigation efforts.     

Spatial structuring of bacterial communities within individual Ginkgo biloba trees

Plant‐associated microorganisms affect the health of their hosts in diverse ways, yet the distribution of these organisms within individual plants remains poorly understood. To address this knowledge gap, we assessed the spatial variability in bacterial community diversity and composition found on and in aboveground tissues of individual Ginkgo biloba trees. We sampled bacterial communities from > 100 locations per tree, including leaf, branch and trunk samples and used high‐throughput sequencing of the 16S rRNA gene to determine the diversity and composition of these communities. Bacterial community structure differed strongly between bark and leaf samples, with bark samples harbouring much greater bacterial diversity and a community composition distinct from leaves. Within sample types, we observed clear spatial patterns in bacterial diversity and community composition that corresponded to the samples' proximity to the exterior of the tree. The composition of the bacterial communities found on trees is highly variable, but this variability is predictable and dependent on sampling location. Moreover, this work highlights the importance of carefully considering plant spatial structure when characterizing the microbial communities associated with plants and their impacts on plant hosts.     

The dynamics of preferential host switching: Host phylogeny as a key predictor of parasite distribution*

Parasites often jump to and become established in a new host species. There is much evidence that the probability of such host shifts decreases with increasing phylogenetic distance between donor and recipient hosts, but the consequences of such preferential host switching remain little explored. We develop a computational model to investigate the dynamics of parasite host shifts in the presence of this phylogenetic distance effect. In this model, a clade of parasites evolves on an evolving clade of host species where parasites can cospeciate with their hosts, switch to new hosts, speciate within hosts or become extinct. Our model predicts that host phylogenies are major determinants of parasite distributions across trees. In particular, we predict that trees consisting of few large clades of host species and those with fast species turnover should harbor more parasites than trees with many small clades and those that diversify more slowly. Within trees, large clades are predicted to exhibit a higher fraction of infected species than small clades. We discuss our results in the light of recent cophylogenetic studies in a wide range of host–parasite systems.