Aim
Data on species occurrences are far more common than data on species abundances. However, a central goal of large‐scale ecology is to understand the spatial distribution of abundance. It has been ...proposed that species distribution models trained on species occurrence records may capture variation in species abundance. Here, we gauge support for relationships between species abundance and predicted climatic suitability from species distribution models, and relate the slope of this relationship to species traits, evolutionary relationships and sampling completeness.
Location
USA.
Time period
1658–2017.
Major taxa studied
Mammal and tree species.
Methods, Results
To explore the generality of abundance–suitability relationships, we trained species distribution models on species occurrence and species abundance data for 246 mammal species and 158 tree species, and related model‐predicted occurrence probabilities to population abundance predictions. Further, we related the resulting abundance–suitability relationship coefficients to species traits, geographic range sizes, evolutionary relationships and the number of occurrence records to investigate a potential trait or sampling basis for abundance–suitability relationship detectability. We found little evidence for consistent abundance–suitability relationships in mammal (r¯ = .045) or tree (r¯ = −.005) species, finding nearly as many negative and positive relationships. These relationships had little explanatory power, and coefficients were unrelated to species traits, range size or evolutionary relationships.
Main conclusions
Our findings suggest that species climatic suitability based on occurrence data may not be reflected in species abundances, suggesting a need to investigate nonclimatic sources of species abundance variation.
Species with broader niches may have the opportunity to occupy larger geographic areas, assuming no limitations on dispersal and a relatively homogeneous environmental space. Here, we use data on a ...large set of mammal (n = 1225), bird (n = 1829) and tree (n = 341) species to examine the 1) relationship between geographic range size and climatic niche area, 2) influence of species traits on species departures from this relationship and 3) sensitivity of these relationships to how species range size and climatic niche area are estimated. We find positive geographic range size–climatic niche area relationships for all taxa, with residual variation dependent on latitude, and differing from a null model for mammals and birds, but not for trees. Together, we provide support for this general macroecological relationship which is dependent on space, weakly influenced by species traits, and different enough from a null model to suggest that geographic and demographic processes are important.
Species have been commonly hypothesized to have high population densities in geographic areas which correspond to either the centre of the species geographic range or climatic niche (abundant–centre ...hypothesis). However, there is mixed empirical support for this relationship, and little theoretical underpinning. We simulate a species spreading across a set of replicated artificial landscapes to examine the expected level of support for abundant–centre relationships in geographic and niche space. Species niche constraints were modeled as a single axis which was related directly to population growth rates. We found strong evidence for abundant–centre relationships when populations follow deterministic growth, dispersal is high, environmental noise is absent and intraspecific competition is low. However, the incorporation of ecological realism reduced the detectability of abundant–centre relationships considerably. Our results suggest that even in carefully constructed artificial landscapes designed to demonstrate abundant–centre dynamics, the incorporation of small amounts of demographic stochasticity, environmental heterogeneity or landscape structure can strongly influence the relationship between species population density and distance to species geographic range or niche centre. While some simulated relationships were of comparable strength to common empirical support for abundant–centre relationships, our results suggest that these relationships are expected to be fairly variable and weak.
Species interactions may vary considerably across space as a result of spatial and environmental gradients. With respect to host–parasite interactions, this suggests that host and parasite species ...may play different functional roles across the different networks they occur in. Using a global occurrence database of helminth parasites, we examine the conservation of species' roles using data on host–helminth interactions from 299 geopolitical locations. Defining species' roles in a two-dimensional space which captures the tendency of species to be more densely linked within species subgroups than between subgroups, we quantified species' roles in two ways, which captured
if
and
which
species' roles are conserved by treating species' utilization of this two-dimensional space as continuous, while also classifying species into categorical roles. Both approaches failed to detect the conservation of species' roles for a single species out of over 38 000 host and helminth parasite species. Together, our findings suggest that species' roles in host–helminth networks may not be conserved, pointing to the potential role of spatial and environmental gradients, as well as the importance of the context of the local host and helminth parasite community.
This article is part of the theme issue ‘Infectious disease macroecology: parasite diversity and dynamics across the globe’.
Understanding the role of biotic interactions in shaping natural communities is a long-standing challenge in ecology. It is particularly pertinent to parasite communities sharing the same host ...communities and individuals, as the interactions among parasites-both competition and facilitation-may have far-reaching implications for parasite transmission and evolution. Aggregated parasite burdens may suggest that infected host individuals are either more prone to infection, or that infection by a parasite species facilitates another, leading to a positive parasite-parasite interaction. However, parasite species may also compete for host resources, leading to the prediction that parasite-parasite associations would be generally negative, especially when parasite species infect the same host tissue, competing for both resources and space. We examine the presence and strength of parasite associations using hierarchical joint species distribution models fitted to data on resident parasite communities sampled on over 1300 small mammal individuals across 22 species and their resident parasite communities. On average, we detected more positive associations between infecting parasite species than negative, with the most negative associations occurring when two parasite species infected the same host tissue, suggesting that parasite species associations may be quantifiable from observational data. Overall, our findings suggest that parasite community prediction at the level of the individual host is possible, and that parasite species associations may be detectable in complex multi-species communities, generating many hypotheses concerning the effect of host community changes on parasite community composition, parasite competition within infected hosts, and the drivers of parasite community assembly and structure.
The scaling relationship observed between species richness and the geographical area sampled (i.e. the species-area relationship (SAR)) is a widely recognized macroecological relationship. Recently, ...this theory has been extended to trophic interactions, suggesting that geographical area may influence the structure of species interaction networks (i.e. network-area relationships (NARs)). Here, we use a global dataset of host-helminth parasite interactions to test existing predictions from macroecological theory. Scaling between single locations to the global host-helminth network by sequentially adding networks together, we find support that geographical area influences species richness and the number of species interactions in host-helminth networks. However, species-area slopes were larger for host species relative to their helminth parasites, counter to theoretical predictions. Lastly, host-helminth network modularity-capturing the tendency of the network to form into separate subcommunities-decreased with increasing area, also counter to theoretical predictions. Reconciling this disconnect between existing theory and observed SAR and NAR will provide insight into the spatial structuring of ecological networks, and help to refine theory to highlight the effects of network type, species distributional overlap, and the specificity of trophic interactions on NARs.
Aim
Interactor richness in host–parasite networks, corresponding to either parasite species richness for host species or host range for parasite species, can be a function of taxonomic or trait ...constraints. Species appearing in multiple networks can have similar interactor richness in each network owing to these taxonomic and trait constraints, resulting in a spatially conserved mean interactor richness and lower variation in interaction richness relative to a null expectation. Here, we used a global database of host–helminth interactions to examine the variability in interactor richness across a spatially explicit collection of 299 host–helminth networks.
Location
Global.
Time period
1800–2003.
Major taxa studied
Helminth parasite species and their host species.
Methods
We used randomization tests to examine spatial conservation of species interactions for both host and helminth species.
Results
We failed to detect a signal of interactor richness conservation for > 95% of host and helminth parasite species relative to a set of three null models, where both the mean number of interactions per species and the variation in the number of interactions per species did not differ from a random draw. Furthermore, we detected a significant taxonomic signal in divergence in parasite species richness from a null model for host species, indicating that slight departures from null expectations are related to host phylogenetic relationships.
Main conclusions
Overall, this indicates that interactor richness can vary widely for the same species and that host and helminth parasite species can play very different functional roles in interaction networks across spatial or environmental gradients.
How many parasites are there on Earth? Here, we use helminth parasites to highlight how little is known about parasite diversity, and how insufficient our current approach will be to describe the ...full scope of life on Earth. Using the largest database of host-parasite associations and one of the world's largest parasite collections, we estimate a global total of roughly 100 000-350 000 species of helminth endoparasites of vertebrates, of which 85-95% are unknown to science. The parasites of amphibians and reptiles remain the most poorly described, but the majority of undescribed species are probably parasites of birds and bony fish. Missing species are disproportionately likely to be smaller parasites of smaller hosts in undersampled countries. At current rates, it would take centuries to comprehensively sample, collect and name vertebrate helminths. While some have suggested that macroecology can work around existing data limitations, we argue that patterns described from a small, biased sample of diversity aren't necessarily reliable, especially as host-parasite networks are increasingly altered by global change. In the spirit of moonshots like the Human Genome Project and the Global Virome Project, we consider the idea of a Global Parasite Project: a global effort to transform parasitology and inventory parasite diversity at an unprecedented pace.
Populations and communities fluctuate in their overall numbers through time, and the magnitude of fluctuations in individual species may scale to communities. However, the composite variability at ...the community scale is expected to be tempered by opposing fluctuations in individual populations, a phenomenon often called the portfolio effect. Understanding population variability, how it scales to community variability, and the spatial scaling in this variability are pressing needs given shifting environmental conditions and community composition. We explore evidence for portfolio effects using null community simulations and a large collection of empirical community time series from the BioTIME database. Additionally, we explore the relative roles of habitat type and geographic location on population and community temporal variability. We find strong portfolio effects in our theoretical community model, but weak effects in empirical data, suggesting a role for shared environmental responses, interspecific competition, or a litany of other factors. Furthermore, we observe a clear latitudinal signal – and differences among habitat types – in population and community variability. Together, this highlights the need to develop realistic models of community dynamics, and hints at spatial, and underlying environmental, gradients in variability in both population and community dynamics.
Metapopulation dynamics – patch occupancy, colonization and extinction – are the result of complex processes at both local (e.g. environmental conditions) and regional (e.g. spatial arrangement of ...habitat patches) scales. A large body of work has focused on habitat patch area and connectivity (area‐isolation paradigm). However, these approaches often do not incorporate local environmental conditions or fully address how the spatial arrangement of habitat patches (and resulting connectivity) can influence metapopulation dynamics.
Here, we utilize long‐term data on a classic metapopulation system – the Glanville fritillary butterfly occupying a set of dry meadows and pastures in the Åland islands – to investigate the relative roles of local environmental conditions, geographic space and connectivity in capturing patch occupancy, colonization and extinction. We defined connectivity using traditional measures as well as graph‐theoretic measures of centrality. Using boosted regression tree models, we find roughly comparable model performance among models trained on environmental conditions, geographic space or patch centrality.
In models containing all of the covariates, we find strong and consistent evidence for the roles of resource abundance, longitude and centrality (i.e. connectivity) in predicting habitat patch occupancy and colonization, while patch centrality (connectivity) was relatively unimportant for predicting extinction. Relative variable importance did not change when geographic coordinates were not considered and models underwent spatially stratified cross‐validation.
Together, this suggests that the combination of regional‐scale connectivity measures and local‐scale environmental conditions is important for predicting metapopulation dynamics and that a stronger integration of ideas from network theory may provide insight into metapopulation processes.
The relative influence of local scale environmental conditions and regional scale connectivity on metapopulation dynamics is largely unclear. Here, the authors present a framework to incorporate both scales and estimate their relative importance in a foundational metapopulation system, the Glanville fritillary butterfly in the Åland islands. Photo credit: Ana Salgado.
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