Jörn Pagel

The influence of interspecific interactions on species range expansion rates

Ongoing and predicted global change makes understanding and predicting species’ range shifts an urgent scientific priority. Here, we provide a synthetic perspective on the so far poorly understood effects of interspecific interactions on range expansion rates. We present theoretical foundations for how interspecific interactions may modulate range expansion rates, consider examples from empirical studies of biological invasions and natural range expansions as well as process-based simulations, and discuss how interspecific interactions can be more broadly represented in process-based, spatiotemporally explicit range forecasts. Theory tells us that interspecific interactions affect expansion rates via alteration of local population growth rates and spatial displacement rates, but also via effects on other demographic parameters. The best empirical evidence for interspecific effects on expansion rates comes from studies of biological invasions. Notably, invasion studies indicate that competitive dominance and release from specialized enemies can enhance expansion rates. Studies of natural range expansions especially point to the potential for competition from resident species to reduce expansion rates. Overall, it is clear that interspecific interactions may have important consequences for range dynamics, but also that their effects have received too little attention to robustly generalize on their importance. We then discuss how interspecific interactions effects can be more widely incorporated in dynamic modeling of range expansions. Importantly, models must describe spatiotemporal variation in both local population dynamics and dispersal. Finally, we derive the following guidelines for when it is particularly important to explicitly represent interspecific interactions in dynamic range expansion forecasts: if most interacting species show correlated spatial or temporal trends in their effects on the target species, if the number of interacting species is low, and if the abundance of one or more strongly interacting species is not closely linked to the abundance of the target species.

Quantifying range‐wide variation in population trends from local abundance surveys and widespread opportunistic occurrence records

1. Species’ abundances vary in space and time. Describing these patterns is a cornerstone of macroecology. Moreover, trends in population size are an important criterion for the assessment of a species’ conservation status. Because abundance trends are not homogeneous in space, we need to quantify variation in abundance trends across the geographical range of a species. A basic difficulty exists in that data sets that cover large geographic areas rarely include population abundance data at high temporal resolution. Whilst both broad-scale geographic distribution data and site-specific population trend data are becoming more widely available, approaches are required which integrate these different types of data. 2. We present a hierarchical model that integrates observations from multiple sources to estimate spatio-temporal abundance trends. The model links annual population densities on a spatial grid to both long-term count data and to opportunistic occurrence records from a citizen science programme. Specific observation models for both data types explicitly account for differences in data structure and quality. 3. We test this novel method in a virtual study with simulated data and apply it to the estimation of abundance dynamics across the range of a butterfly species (Pyronia tithonus) in Great Britain between 1985 and 2004. The application to simulated and real data demonstrates how the hierarchical model structure accommodates various sources of uncertainty which occur at different stages of the link between observational data and the modelled abundance, thereby it accounts for these uncertainties in the inference of abundance variations. 4. We show that by using hierarchical observation models that integrate different types of commonly available data sources, we can improve the estimates of variation in species abundances across space and time. This will improve our ability to detect regional trends and can also enhance the empirical basis for understanding range dynamics.

Environmental drivers of demographic variation across the global geographical range of 26 plant species

Summary Understanding how rates of reproduction and survival respond to environmental variation across species’ geographical ranges is a key task for both basic and applied ecology. So far, however, environmental drivers of range-wide demographic variation have only been studied in a few plant species without considering the potentially confounding effects of population density on demographic rates. We present a large-scale demographic study of 26 shrub species (Proteaceae) from the Cape Floristic Region. All study species have a fire-dependent life cycle and are serotinous: they exclusively form a canopy seed bank which contains the seeds produced since the last fire. Fire triggers seed release from the canopy so that recruitment is largely limited to a short period after fire. Across the global geographical ranges of the study species, we collected 3454 population-level records of total fecundity since the last fire (size of individual canopy seed banks), per-capita recruitment (ratio between post-fire recruits and pre-fire adults) and adult fire survival. We used linear regressions to quantify how climate, population density, fire interval and soil nutrients affect demographic variation. A trade-off between survival and reproduction is evident throughout the geographical ranges of our study species: resprouting species with fire-protected buds had much higher fire survival than nonsprouters without fire-protected buds (97% vs. 2%) but they also had substantially lower fecundity and recruitment rates. We found little intraspecific variation in fire survival rates but considerable intraspecific variation in fecundity and recruitment. Range-wide variation in fecundity was dominated by fire interval whereas recruitment was mostly climate-driven. Population density and soil nutrients generally had smaller effects but were important for the fecundity and recruitment of several species. Effects of fire interval on fecundity were consistent across species, but other demography–environment relationships showed substantial interspecific differentiation. Synthesis. This study extends demographic research beyond the population to cover the geographical ranges of multiple species. Such large-scale studies are a necessary first step of a research agenda that aims to understand how range dynamics emerge from first principles of demography, how they are shaped by functional traits and macroevolution and how they will be impacted by global change.

Sugar landscapes and pollinator-mediated interactions in plant communities

Pollinator-mediated interactions between plants can play an important role for the dynamics of plant communities. Pollination services depend on the abundance and the foraging behaviour of pollinators, which in turn respond to the availability and distribution of floral resources (notably nectar sugar). However, it is still insufficiently understood how the ‘sugar landscapes’ provided by flowering plant communities shape pollinator-mediated interactions between multiple plant species and across different spatial scales. A better understanding of pollinator-mediated interactions requires an integrative approach that quantifies different aspects of sugar landscapes and investigates their relative importance for pollinator behaviour and plant reproductive success. In this study, we quantified such sugar landscapes from individual-based maps of Protea shrub communities in the Cape Floristic Region, South Africa. The 27 study sites of 4 ha each jointly comprise 127,993 individuals of 19 species. We analysed how rates of visitation by key bird pollinators and the seed set of plants respond to different aspects of sugar landscapes: the distribution of nectar sugar amounts, as well as their quality, taxonomic purity and phenology. We found that pollinator visitation rates strongly depended on phenological variation of site-scale sugar amounts. The seed set of focal plants increased with nectar sugar amounts of conspecific neighbours and with site-scale sugar amounts. Seed set increased particularly strongly if site-scale sugar amounts were provided by plants that offer less sugar per inflorescence. These combined effects of the amount, quality, purity and phenological variation of nectar sugar show that nectar sugar is a common interaction currency that determines how multiple plant species interact via shared pollinators. The responses of pollinator-mediated interactions to different aspects of this interaction currency alter conditions for species coexistence in Protea communities and may cause community-level Allee effects that promote extinction cascades. This article is protected by copyright. All rights reserved.

Floral resource-landscapes and pollinator-mediated interactions in plant communities

Plant communities provide floral resource-landscapes for pollinators. Yet, it is insufficiently understood how these landscapes shape pollinator-mediated interactions among multiple plant species. Here, we study how pollinators and the seed set of plants respond to the distribution of a floral resource (nectar sugar) in space and across plant species, inflorescences and flowering phenologies. In a global biodiversity hotspot, we quantified floral resource-landscapes on 27 sites of 4 ha comprising 127,993 shrubs of 19 species. Visitation rates of key bird pollinators strongly depended on the phenology of site-scale resource amounts. Seed set of focal plants increased with resources of conspecific neighbours and with site-scale resources, notably with heterospecific resources of lower quality (less sugar per inflorescence). Floral resources are thus a common currency determining how multiple plant species interact via pollinators. These interactions may alter conditions for species coexistence in plant communities and cause community-level Allee effects that promote extinction cascades.