Foodwebs based on unreliable foundations: spatiotemporal masting merged with consumer movement, storage, and diet

Abstract

Mast‐fruiting trees represent a pulsed resource that both supports and destabilizes consumer populations. Whereas a reliable resource is abundant on average and with limited variation in time and space, masting is volatile and localized, and that variability ramifies throughout food‐webs. Theory is developed to evaluate how the space–time structure of masting interacts with consumers who exploit alternative hosts, forage widely in space, and store reserves in time. We derive the space–time–species covariance in resource supply and combine it with the space–time–diet breadth of consumers, or ambit. Direct connection to data is made possible with Mast Inference and Forecasting (MASTIF), a state‐space autoregressive model that fits seed‐trap and canopy observations and predicts resource availability within the canopy and on the forest floor with full uncertainty. A resource score can be assigned to each consumer–habitat combination that integrates the benefits of a high mean supply weighed against the variance cost. As the consumer ambit increases, the volatility of an unreliable resource shifts from a variance cost to a mean benefit. Consumers foraging in the canopy (arboreal arthropods and rodents, song birds) experience space‐time covariance between host trees. Consumers on the forest floor (seed and damping‐off fungi, arthropods, rodents, ground‐nesting birds, mammals) experience instead a redistribution of that covariance by dispersal. For consumers lacking mobility, demographic storage in the form of episodic birth cohorts following mast years is important for population persistence. Consumers additionally compensate volatility with diet breadth. Depending on the dominant masting strategies of host tree species in the diet, habitats differentially limit consumers depending on the misalignment between consumer ambit and spatiotemporal covariance of hosts. The impact of adding or subtracting a diet item can be gauged with the standard error (SE) rule or the benefit of an added diet item balanced against the variance cost, both of which depend on the existing diet, the abundance of the new host, and the consumer's foraging ambit. Results rank habitats by their capacities to support wildlife and other consumers from a resource perspective. Results are connected directly to data, with full uncertainty, by MASTIF.

Continent-wide tree fecundity driven by indirect climate effects

“…Data are non-Gaussian (including zeros for immature trees and failed crops). Trends estimated by meta-analysis may not be comparable across studies due to divergent methods and transformations intended to force non-Gaussian data into traditional time-series models 20 . Efforts to determine whether a species is increasing or decreasing are further challenged by the uneven distribution of publications.…”

“…As with many ecological processes 14 – 16 , noisy, spatially variable fecundity trends are hard to quantify 8 , 17 , but this is only the first problem. Attributing trends to environmental variables is complicated by individual size, growth, and resource access 18 – 20 . Conservation efforts must anticipate not just the direct climate effects on this trajectory but also the indirect effects as growth and changing size structure also affect fecundity.…”

From theory to experiments for testing the proximate mechanisms of mast seeding: an agenda for an experimental ecology

“…Our aim here is to outline a research agenda that takes the biology of masting from a largely observational field of ecology to one rooted in mechanistic understanding. This understanding can be incorporated into global vegetation models to improve their accuracy and realism in terms of seed production but also growth tradeoffs, seed dispersal, establishment, migration, cascading trophic interactions, and ecosystem resilience to disturbances or climate change (Vacchiano et al ; Clark et al ). We outline explicit predictions of prevalent hypotheses explaining intermittent and synchronised reproduction at the population level and describe what experiments would be necessary to test them.…”

Is forest fecundity resistant to drought? Results from an 18‐yr rainfall‐reduction experiment

“…Few studies, however, have compared the impacts of environmental change on the reproductive ecology of trees with other effects such as growth, carbon sequestration, mortality, or phenology (Barbeta et al , 2013; Hacket‐Pain et al , 2016; Zohner et al , 2018; Luo et al , 2020). Ecosystem services, such as mitigating the risk of avalanches, carbon storage, habitat availability and value for the economy and recreation, can suffer if reduced reproduction slows forest expansion or limits the recruitment of merchantable tree species and seed producers that support wildlife (McShea, 2000; Ostfeld & Keesing, 2000; Clark et al , 2007, 2019; Bogdziewicz et al , 2016). The volatility of seed production and our poor understanding of the mechanisms that govern it are challenges for anticipating alternations in forest reproduction (Bogdziewicz et al , 2020a).…”