66 research outputs found
Scaling of the risk landscape drives optimal life history strategies and the evolution of grazing
Consumers face numerous risks that can be minimized by incorporating
different life-history strategies. How much and when a consumer adds to its
energetic reserves or invests in reproduction are key behavioral and
physiological adaptations that structure much of how organisms interact. Here
we develop a theoretical framework that explicitly accounts for stochastic
fluctuations of an individual consumer's energetic reserves while foraging and
reproducing on a landscape with resources that range from uniformly distributed
to highly clustered. First, we show that optimal life-history strategies vary
in response to changes in the mean productivity of the resource landscape,
where depleted environments promote reproduction at lower energetic states,
greater investment in each reproduction event, and smaller litter sizes. We
then show that if resource variance scales with body size due to landscape
clustering, consumers that forage for clustered foods are susceptible to strong
Allee effects, increasing extinction risk. Finally, we show that the proposed
relationship between consumer body size, resource clustering, and Allee
effect-induced population instability offers key ecological insights into the
evolution of large-bodied grazing herbivores from small-bodied browsing
ancestors.Comment: 9 pages, 5 figures, 3 Supplementary Appendices, 2 Supplementary
Figure
On the dynamics of mortality and the ephemeral nature of mammalian megafauna
Energy flow through consumer-resource interactions is largely determined by
body size. Allometric relationships govern the dynamics of populations by
impacting rates of reproduction, as well as alternative sources of mortality,
which have differential impacts on smaller to larger organisms. Here we derive
and investigate the timescales associated with four alternative sources of
mortality for terrestrial mammals: mortality from starvation, mortality
associated with aging, mortality from consumption by predators, and mortality
introduced by anthropogenic subsidized harvest. The incorporation of these
allometric relationships into a minimal consumer-resource model illuminates
central constraints that may contribute to the structure of mammalian
communities. Our framework reveals that while starvation largely impacts
smaller-bodied species, the allometry of senescence is expected to be more
difficult to observe. In contrast, external predation and subsidized harvest
have greater impacts on the populations of larger-bodied species. Moreover, the
inclusion of predation mortality reveals mass thresholds for mammalian
herbivores, where dynamic instabilities may limit the feasibility of megafaunal
populations. We show how these thresholds vary with alternative predator-prey
mass relationships, which are not well understood within terrestrial systems.
Finally, we use our framework to predict the harvest pressure required to
induce mass-specific extinctions, which closely align with previous estimates
of anthropogenic megafaunal exploitation in both paleontological and historical
contexts. Together our results underscore the tenuous nature of megafaunal
populations, and how different sources of mortality may contribute to their
ephemeral nature over evolutionary time.Comment: 10 pages, 5 figures, 1 table, 4 appendices, 8 supplementary figure
Collapse of an ecological network in Ancient Egypt
The dynamics of ecosystem collapse are fundamental to determining how and why
biological communities change through time, as well as the potential effects of
extinctions on ecosystems. Here we integrate depictions of mammals from
Egyptian antiquity with direct lines of paleontological and archeological
evidence to infer local extinctions and community dynamics over a 6000-year
span. The unprecedented temporal resolution of this data set enables
examination of how the tandem effects of human population growth and climate
change can disrupt mammalian communities. We show that the extinctions of
mammals in Egypt were nonrandom, and that destabilizing changes in community
composition coincided with abrupt aridification events and the attendant
collapses of some complex societies. We also show that the roles of species in
a community can change over time, and that persistence is predicted by measures
of species sensitivity, a function of local dynamic stability. Our study is the
first high-resolution analysis of the ecological impacts of environmental
change on predator-prey networks over millennial timescales, and sheds light on
the historical events that have shaped modern animal communities
Eco-Evolutionary Origins of Diverse Abundance, Biomass, and Trophic Structures in Food Webs
Organismal traits and their evolution can strongly influence food web structure and dynamics. To what extent the evolution of such traits impacts food web structure, however, is poorly understood. Here, we investigate a simple three-species omnivory food web module where the attack rates of all predators evolve as ecological dynamics unfold, such that predator trophic levels are themselves dynamic. We assume a timescale where other vital rates that govern population dynamics are constant and incorporate a well-known tradeoff between attack rates and the conversion of prey into predator biomass. We show that this eco-evolutionary model yields a surprisingly rich array of dynamics. Moreover, even small amounts of selection lead to important differences in the abundance, trophic, and biomass structure of the food web. Systems in which intermediate predators are strongly constrained by tradeoffs lead to hourglass-shaped food webs, where basal resources and top predators have large abundances, but intermediate predators are rare, like those observed in some marine ecosystems. Such food webs are also characterized by a relatively low maximum trophic level. Systems in which intermediate predators have weaker tradeoffs lead to pyramid-shaped food webs, where basal resources are more abundant than intermediate and top predators, such as those observed in some terrestrial system. These food webs also supported a relatively higher maximum trophic level. Overall, our results suggest that eco-evolutionary dynamics can strongly influence the abundance-, trophic-, and biomass-structure of food webs, even in the presence of small levels of selection, thus stressing the importance of taking traits and trait evolution into account to further understand community-level patterns and processes
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