Search for: All records

Abstract The relationship between infection prevalence and host age is informative because it can reveal processes underlying disease dynamics. Most prior work has assumed that age‐prevalence curves are shaped by infection rates, host immunity and/or infection‐induced mortality. Interactions between parasites within a host have largely been overlooked as a source of variation in age‐prevalence curves.We used field survey data and models to examine the role of interspecific interactions between parasites in shaping age‐prevalence curves. The empirical dataset included quantification of parasite infection prevalence for eight co‐occurring trematodes in over 15,000 snail hosts. We characterized age‐prevalence curves for each taxon, examined how they changed over space in relation to co‐occurring trematodes and tested whether the shape of the curves aligned with expectations for the frequencies of coinfections by two taxa in the same host. The models explored scenarios that included negative interspecific interactions between parasites, variation in the force‐of‐infection (FOI) and infection‐induced mortality that varied with host age, which were mechanisms hypothesized to be important in the empirical dataset.In the empirical dataset, four trematode parasites had monotonic increasing age‐prevalence curves and four had unimodal age‐prevalence curves. Some of the curves remained consistent in shape in relation to the prevalence of other potentially interacting trematodes, while some shifted from unimodal to monotonic increasing, suggesting release from negative interspecific interactions. The most common taxa with monotonic increasing curves had lower co‐infection frequencies than expected, suggesting they were competitively dominant. Taxa with unimodal curves had coinfection frequencies that were closer to those expected by chance.The model showed that negative interspecific interactions between parasites can cause a unimodal age‐prevalence curve in the subordinate taxon. Increases in the FOI and/or infection‐induced mortality of the dominant taxon cause shifts in the peak prevalence of the subordinate taxon to a younger host age. Infection‐induced mortality that increased with host age was the only scenario that caused a unimodal curve in the dominant taxon.Results indicated that negative interspecific interactions between parasites contributed to variation in the shape of age‐prevalence curves across parasite taxa and support the growing importance of incorporating interactions between parasites in explaining population‐level patterns of host infection over space and time. Read the freePlain Language Summaryfor this article on the Journal blog. 

Abstract Many critical drivers of ecological systems exhibit regular scaling relationships, yet the underlying mechanisms explaining these relationships are often unknown. Trophic interaction strengths, which underpin ecosystem stability and dynamics, are no exception, exhibiting statistical scaling relationships with predator and prey traits that lack causal, evolutionary explanations. Here we propose two universal rules to explain the scaling of trophic interaction strengths through the relationship between a predator’s feeding rate and its prey’s density --- the so-called predator functional response. First, functional responses must allow predators to meet their energetic demands when prey are rare. Second, functional responses should approach their maxima near the highest prey densities that predators experience. We show that independently parameterized mathematical equations derived from these two rules predict functional response parameters across over 2,100 functional response experiments. The rules further predict consistent patterns of feeding rate saturation among predators, a slow-fast continuum among functional response parameters, and the allometric scaling of those parameters. The two rules thereby offer a potential ultimate explanation for the determinants of trophic interaction strengths and their scaling, revealing the importance of ecologically realized constraints to the complex, adaptive nature of functional response evolution. 

Abstract Ecologists differ in the degree to which they consider the linear Type I functional response to be an unrealistic versus sufficient representation of predator feeding rates. Empiricists tend to consider it unsuitably non-mechanistic and theoreticians tend to consider it necessarily simple. Holling’s original rectilinear Type I response is dismissed by satisfying neither desire, with most compromising on the smoothly saturating Type II response for which searching and handling are assumed to be mutually exclusive activities. We derive a “multiple-prey-at-a-time” response and a generalization that includes the Type III to reflect predators that can continue to search when handling an arbitrary number of already-captured prey. The multi-prey model clarifies the empirical relevance of the linear and rectilinear models and the conditions under which linearity can be a mechanistically-reasoned description of predator feeding rates, even when handling times are long. We find support for linearity in 35% of 2,591 compiled empirical datasets and support for the hypothesis that larger predator-prey body-mass ratios permit predators to search while handling greater numbers of prey. Incorporating the multi-prey response into the Rosenzweig-MacArthur population-dynamics model reveals that a non-exclusivity of searching and handling can lead to coexistence states and dynamics that are not anticipated by theory built on the Type I, II, or III response models. In particular, it can lead to bistable fixed-point and limit-cycle dynamics with long-term crawl-by transients between them under conditions where abundance ratios reflect top-heavy food webs and the functional response is linear. We conclude that functional response linearity should not be considered empirically unrealistic but also that more cautious inferences should be drawn in theory presuming the linear Type I to be appropriate. 

Abstract Predator feeding rates (described by their functional response) must saturate at high prey densities. Although thousands of manipulative functional response experiments show feeding rate saturation at high densities under controlled conditions, it remains unclearhowsaturated feeding rates are at natural prey densities. The general degree of feeding rate saturation has important implications for the processes determining feeding rates and how they respond to changes in prey density. To address this, we linked two databases—one of functional response parameters and one on mass–abundance scaling—through prey mass to calculate a feeding rate saturation index. We find that: (1) feeding rates may commonly be unsaturated and (2) the degree of saturation varies with predator and prey taxonomic identities and body sizes, habitat, interaction dimension and temperature. These results reshape our conceptualisation of predator–prey interactions in nature and suggest new research on the ecological and evolutionary implications of unsaturated feeding rates. 

While the use of networks to understand how complex systems respond to perturbations is pervasive across scientific disciplines, the uncertainty associated with estimates of pairwise interaction strengths (edge weights) remains rarely considered. Mischaracterizations of interaction strength can lead to qualitatively incorrect predictions regarding system responses as perturbations propagate through often counteracting direct and indirect effects. Here, we introduce PressPurt , a computational package for identifying the interactions whose strengths must be estimated most accurately in order to produce robust predictions of a network's response to press perturbations. The package provides methods for calculating and visualizing these edge-specific sensitivities (tolerances) when uncertainty is associated to one or more edges according to a variety of different error distributions. The software requires the network to be represented as a numerical (quantitative or qualitative) Jacobian matrix evaluated at stable equilibrium. PressPurt is open source under the MIT license and is available as both a Python package and an R package hosted at https://github.com/dkoslicki/PressPurt and on the CRAN repository https://CRAN.R-project.org/package=PressPurt. 

ABSTRACT Identifying species with disproportionate effects on other species under press perturbations is essential, yet how species traits and community context drive their ‘keystone‐ness’ remain unclear. We quantified keystone‐ness as linearly approximated per capita net effect derived from normalised inverse community matrices and as non‐linear per capita community biomass change from simulated perturbations in food webs with varying biomass structure. In bottom‐heavy webs (negative relationship between species' body mass and their biomass within the web), larger species at higher trophic levels tended to be keystone species, whereas in top‐heavy webs (positive body mass to biomass relationship), the opposite was true and the relationships between species' energetic traits and keystone‐ness were weakened or reversed compared to bottom‐heavy webs. Linear approximations aligned well with non‐linear responses in bottom‐heavy webs, but were less consistent in top‐heavy webs. These findings highlight the importance of community context in shaping species' keystone‐ness and informing effective conservation actions.