More Like this

1. Temperature and nutrient conditions modify the effects of phenological shifts in predator–prey communities

   https://doi.org/10.1002/ecy.3704  

   Rudolf, V. H. W. ( May 2022 , Ecology)

   Although there is mounting evidence indicating that the relative timing of predator and prey phenologies determines the outcome of trophic interactions, we still lack a comprehensive understanding of how the environmental context (e.g., abiotic conditions) influences this relationship. Environmental conditions not only frequently drive shifts in phenologies, but they can also affect the very same processes that mediate the effects of phenological shifts on species interactions. Therefore, identifying how environmental conditions shape the effects of phenological shifts is key to predicting community dynamics across a heterogeneous landscape and how they will change with ongoing climate change in the future. Here I tested how environmental conditions shape the effects of phenological shifts by experimentally manipulating temperature, nutrient availability, and relative phenologies in two predator–prey freshwater systems (mole salamander–bronze frog vs. dragonfly larvae–leopard frog). This allowed me to (1) isolate the effects of phenological shifts and different environmental conditions; (2) determine how they interact; and (3) evaluate how consistent these patterns are across different species and environments. I found that delaying prey arrival dramatically increased predation rates, but these effects were contingent on environmental conditions and the predator system. Although nutrient addition and warming both significantly enhanced the effect of arrivalmore »time, their effect was qualitatively different across systems: Nutrient addition enhanced the positive effect of early arrival in the dragonfly–leopard frog system, whereas warming enhanced the negative effect of arriving late in the salamander–bronze frog system. Predator responses varied qualitatively across predator–prey systems. Only in the system with a strong gape limitation were predators (salamanders) significantly affected by prey arrival time and this effect varied with environmental context. Correlations between predator and prey demographic rates suggest that this was driven by shifts in initial predator–prey size ratios and a positive feedback between size‐specific predation rates and predator growth rates. These results highlight the importance of accounting for temporal and spatial correlations of local environmental conditions and gape limitation when predicting the effects of phenological shifts and climate change on predator–prey systems.

   « less
2. Reexamining the storage effect: Why temporal variation in abiotic factors seems unlikely to cause coexistence

   https://doi.org/10.1002/ecm.1585  

   Stump, Simon Maccracken ; Vasseur, David A. ( July 2023 , Ecological Monographs)

   The temporal storage effect—that species coexist by partitioning abiotic niches that vary in time—is thought to be an important explanation for how species coexist. However, empirical studies that measure multiple mechanisms often find the storage effect is weak. We believe this mismatch is because of a shortcoming of theoretical models used to study the storage effect: that while the storage effect is described as having just three requirements (partitioning of temporal variation, buffered population growth, and a covariance between environment and density‐dependence), models used to study the storage effect make four assumptions, which are mathematically subtle but biologically important. In this paper, we examine those assumptions. First, models assume that environmental variation leads to a rapid impact on density‐dependence. We find that delays in density‐dependence (including delays caused by competition between cohorts) weaken the storage effect. Second, models assume that intraspecific competition is almost identical to interspecific competition. We find that unless resource or predator partitioning are virtually absent, then variation‐independent mechanisms will overshadow the benefits of the storage effect. Third, models assume even though there is vast variation in the environment, species are equally adapted on average (i.e., zero fitness‐differences). We show that fitness differences are particularly problematicmore »in the storage effect because specializing on temporally rare niches is far less effective than specializing on other types of rare niches. Finally, models assume that stochastic extinctions can be ignored, and invader growth can determine coexistence. We show that storage effects tend to reduce mean persistence times, even if invader growth rates are positive. These results suggest that the assumptions needed for the storage effect are strict: if the first or second assumption is relaxed, it will greatly weaken the stabilizing mechanism; if the third or fourth assumption is relaxed, it will create a diversity‐destroying effect that may undermine coexistence. We examine three real‐world communities—annual plants, tropical forests, and iguanid lizards—and find that empirical studies suggest that all three communities violate multiple assumptions. This suggests that the temporal storage effect is probably not an important explanation for species diversity in most systems.

   « less
3. Quantifying the effects of intraspecific variation on predator feeding rates through nonlinear averaging

   https://doi.org/10.1111/1365-2435.13802  

   Coblentz, Kyle E. ; Merhoff, Stephanie ; Novak, Mark ( June 2021 , Functional Ecology)

   Theory suggests that intraspecific trait variation will alter species interaction strengths through nonlinear averaging when interaction strengths are nonlinear functions of individuals' traits. This effect is expected to be widespread, yet what factors mediate its magnitude in nature and hence its potential effects on ecosystems and communities are unclear.

   We sought to quantify how nonlinear predator functional responses, variation in prey densities and counteracting variation in attack rates and handling times among predator individuals of similar body size alter their population‐level feeding rates through nonlinear averaging in a natural system, and to determine the processes influencing the net magnitude of this effect.

   We used a field caging experiment in the rocky intertidal of Oregon, USA to quantify attack‐rate variation and feeding rates of the whelkNucella ostrinaon its barnacle and mussel prey. We also used empirically parameterized simulations to examine the effects of handling‐time variation among individuals on population‐level feeding rates.

   Within cages, individual attack‐rate variation reduced population‐level whelk feeding rates. However, the magnitude of this reduction differed among prey species and cages depending on cage‐specific magnitudes of attack‐rate variation and functional‐response nonlinearity. The inferred effects of handling‐time variation among individuals were of smaller magnitude than those of attack‐rate variation, yet counteracted themmore »to cause a net weakening of the effect of individual attack‐rate variation on population‐level feeding rates. Across cages, attack‐rate and prey‐density variation had non‐additive effects that produced greater feeding‐rate reductions at the experiment scale relative to the cage scale.

   Our results indicate that the effects of trait variation via nonlinear averaging depend critically on the features of systems that determine the magnitudes of nonlinearities and trait variation. Because of counteracting trait variation, nonlinear‐averaging effects may be quite complex, involving both the variances and covariances of all traits and environmental variables influencing the ecological process of interest.

   A freePlain Language Summarycan be found within the Supporting Information of this article.

   « less
4. Long‐term survey data reveal large predator and temperature effects on population growth of multiple zooplankton species

   https://doi.org/10.1002/lno.11340  

   Marino, Jr, John A. ; Vanderploeg, Henry A. ; Pothoven, Steven A. ; Elgin, Ashley K. ; Peacor, Scott D. ( September 2019 , Limnology and Oceanography)

   Predators can strongly affect prey communities, but their influence may be difficult to distinguish from bottom‐up and other environmental effects. The problem of assessing predator impact is especially difficult in large systems that do not allow for comparisons across multiple units (e.g., small lakes) that have varying predator density. For instance, the invasion of the predatory zooplankter,Bythotrephes longimanus, into the Laurentian Great Lakes contributed to the nearly complete disappearance of several zooplankton species, but current effects on extant zooplankton are not well understood. We used generalized additive models (GAMs) applied to long‐term data time series (1994–2012) to examineB. longimanuseffects on zooplankton species in Lake Michigan. BecauseB. longimanusabundance varied over time, our approach allowed assessment of predator effects from field data while accounting for other factors, including food resources, temperature, and seasonality. Results suggest thatB. longimanussubstantially reduces some zooplankton population growth rates, with the largest effects on species thatB. longimanusaffected more strongly in experiments. For example, at maximumB. longimanusabundance,Daphnia mendotae,Bosmina longirostris, andDiacyclops thomasipopulation growth rates were estimated to be reduced by 17%, 30%, and 21%, respectively, compared to no effect on calanoid copepods. Results further indicated positive temperature effects on population growth that differed by species. Our study thus providesmore »field‐based evidence for ongoing impacts of invasive species and temperature on zooplankton production and composition, with potential consequences for planktivorous fish, and exemplifies how GAMs can be used to determine predator effects from time series data.

   « less
5. Fluctuations in age structure and their variable influence on population growth

   https://doi.org/10.1111/1365-2435.13431  

   Hoy, Sarah R. ; MacNulty, Daniel R. ; Smith, Douglas W. ; Stahler, Daniel R. ; Lambin, Xavier ; Peterson, Rolf O. ; Ruprecht, Joel S. ; Vucetich, John A. ; Sandercock, ed., Brett ( August 2019 , Functional Ecology)

   Temporal fluctuations in growth rates can arise from both variation in age‐specific vital rates and temporal fluctuations in age structure (i.e. the relative abundance of individuals in each age‐class). However, empirical assessments of temporal fluctuations in age structure and their effects on population growth rate are relatively rare. Most research has focused on understanding the contribution of changing vital rates to population growth rates and these analyses routinely assume that: (a) populations have stable age distributions, (b) environmental influences on vital rates and age structure are stationary (i.e. the mean and/or variance of these processes does not change over time), and (c) dynamics are independent of density.

   Here we quantified fluctuations in age structure and assessed whether they were stationary for four populations of free‐ranging vertebrates: moose (observed for 48 years), elk (15 years), tawny owls (15 years) and grey wolves (17 years). We also assessed the extent that fluctuations in age structure were useful for predicting annual population growth rates using models which account for density dependence.

   Fluctuations in age structure were of a similar magnitude to fluctuations in abundance. For three populations (moose, elk, owls), the mean and the skew of the age distribution fluctuated without stabilizing over the observed time periods. More precisely,more »the sample variance (interannual variance) of age structure indices increased with the length of the study period, which suggests that fluctuations in age structure were non‐stationary for these populations – at least over the 15‐ to 48‐year periods analysed.

   Fluctuations in age structure were associated with population growth rate for two populations. In particular, population growth varied from positive to negative for moose and from near zero to negative for elk as the average age of adults increased over its observed range.

   Non‐stationarity in age structure may represent an important mechanism by which abundance becomes non‐stationary – and therefore difficult to forecast – over time‐scales of concern to wildlife managers. Overall, our results emphasize the need for vertebrate populations to be modelled using approaches that consider transient dynamics and density dependence and that do not rely on the assumption that environmental processes are stationary.

   A freePlain Language Summarycan be found within the Supporting Information of this article.

   « less