Ecological Monographs最新文献

Climate change is altering interspecific interactions globally, yet community-level responses are difficult to predict due to both the direct and indirect effects of changing abiotic and biotic conditions. Snowshoe hares (Lepus americanus) are particularly vulnerable to decreasing snow cover and resultant camouflage mismatch. This species shares a suite of predators with alternative prey species including porcupines (Erethizon dorsatum) and ruffed grouse (Bonasa umbellus), and all three species historically exhibited synchronized population dynamics. Recently, the community has become partially disassembled, notably with the loss of snowshoe hares and associated enemy-mediated indirect interactions resulting from declining snow duration. Specifically, we hypothesized that the extirpation of hares in the early 1990s indirectly increased predation pressure on ruffed grouse and porcupines. To test our hypothesis, we experimentally translocated 96 snowshoe hares to a site within a regional ecotone between northern and southern forests where snowshoe hares were recently extirpated and monitored community members before, during, and after translocation. Ruffed grouse were only loosely associated with the biotic interactions that linked porcupines and snowshoe hares, likely due to predation occurring from avian predators and strong negative direct effects of declining winter snow depths. In contrast, predation of neonate porcupines was virtually non-existent following repatriation, compared with periods without hares. This abrupt attenuation of predation did not increase overall survival due to increased non-predation mortality from cold, early spring weather. Porcupines directly benefited from warming winters: decreased snow cover increased adult survival and warmer temperatures around parturition increased maternal condition and reduced non-predation causes of mortality for neonates. Our experimental manipulation suggests that enemy-mediated indirect interactions were likely to be important features of this community; however, climate change has disrupted these interactions, resulting in extirpation of a central prey species (snowshoe hare) and increased predation of an alternative prey species (porcupine). We show complex effects from climate change with some species directly and negatively affected, while others benefited from direct effects of warming winters, but suffered negative effects from indirect interactions. Due to absent snowshoe hares and associated biotic interactions, continued persistence of this community module is unlikely, potentially resulting in altered no-analog communities along trailing edge distributions.

Plant competition may intensify with climate warming, but whether this will occur equally for conspecific and heterospecific competition remains unknown. Competitive shifts have the potential to instigate community change because the relative strengths of conspecific and heterospecific negative density dependence mediate the stabilizing mechanisms underpinning species coexistence. We examined a mature temperate forest to assess both direct and indirect climate effects at multiple scales: individual species, interspecies relationships, and community stability mechanisms. Our coupled approach (1) quantified tree mortality risk dependence on the interactive effects of competition, climatic water deficit, snowpack, and soil moisture for 28,913 trees over 8 years (3149 mortalities), then (2) used a climate-projection ensemble to forecast changes in conspecific and heterospecific competition from 2020 to 2100. We predict that projected climate warming will destabilize the foundational forest community by increasing the strength of heterospecific competition at a greater rate and to a greater degree than conspecific competition for four of five abundant tree species, particularly on dry microsites. Modeling showed that these findings were most pronounced after the year 2038, at which point snowpacks were projected to be too small to ameliorate the effects of drought on competitive interactions. Our finding that heterospecific competition is more sensitive than conspecific competition to climate warming may indicate the impending loss of ecosystem functioning. We join the growing body of work showing a predominance of indirect drought effects, yet coupled climate models still fail to consider how changing community dynamics may impact forest cover and, in turn, disrupt forest–climate carbon feedbacks. Ecosystems sharing characteristics with our example forest—those with low species richness and therefore a limited biodiversity insurance effect—may be similarly vulnerable to climate-mediated destabilization. In such communities, increased heterospecific competition among even a small number of species can more easily destabilize communities without recourse from redundant species. This study of an overlooked but vital mechanism of community change can be adapted by research in a range of ecosystems to improve the understanding of climate change consequences.

We explored whether parasites are important in kelp forests by examining their effects on a high-quality, high-resolution kelp-forest food web. After controlling for generic effects of network size, parasites affected kelp-forest food web structure in some ways consistent with other systems. Parasites increased the trophic span of the web, increasing top predator vulnerability and the longest chain length. Unique links associated with parasites, such as concomitant predation (consumption of parasites along with their hosts by predators) increased the frequency of network motifs involving mutual consumption and decreased niche contiguity of free-living species. However, parasites also affected kelp-forest food web structure in ways not seen in other systems. Kelp-forest parasites are richer and more specialized than other systems. As a result, parasites reduced diet generality and decreased connectance in the kelp forest. Although mutual consumption motifs increased in frequency, this motif type was still a small fraction of all possible motifs, so their increase in frequency was not enough to compensate for the decrease in connectance caused by adding many specialist parasite species.

Changing climatic conditions are shaping how density mediates resource competition. Colonies of the seed-eating red harvester ant, Pogonomyrmex barbatus, live for about 30 years in desert grassland. They compete with conspecific neighbors for foraging area in which to search for seeds. This study draws on a long-term census of a population of about 300 colonies from 1988 to 2019 at a site near Rodeo, New Mexico, USA. Rainfall was high in the first decade of the study, and then declined as a severe drought began in about 2001–2003. We examine the effects on colony survival and recruitment of the spatial configuration of the local neighborhood of conspecific neighbors, using Voronoi polygons as a measure of a colony's foraging area, and consider how changing rainfall influences the effects of local neighborhoods. The results show that a colony's chances of surviving to the next year depend on its age and on the foraging area available in its local neighborhood. Recruitment, measured as a founding colony's chance of surviving to be 1 year old, depends on rainfall. In the earlier years of the study, when rainfall was high, colony numbers increased, and then began to decline after about 1997–1999, apparently due to crowding. As rainfall decreased, beginning in about 2001–2003, recruitment declined, and so did colony survival, leading to a trend toward earlier colony death which was most pronounced in 2016. As rainfall declined, apparently decreasing food availability, more foraging area was needed to sustain a colony: although the number of colonies declined, the impact of crowding by intraspecific neighbors increased. These processes maintain overdispersion on the scale of about 8 m, with transient clustering at larger spatial scales. In addition, other factors besides crowding, such as the colony's regulation of foraging activity to manage water loss, appear to contribute to a colony's survival. The adaptive capacity for selection on the collective behavior that regulates foraging activity may determine how the population responds to ongoing climate change and drought.

A large and growing body of theory has demonstrated how the presence of trait variation in prey or predator populations may affect the amplitude and phase of predator–prey cycles. Less attention has been given to so-called intermittent cycles, in which predator–prey oscillations recurrently disappear and re-appear, despite such dynamics being observed in empirical systems and modeling studies. A comprehensive understanding of the conditions under which trait changes may drive intermittent predator–prey dynamics, as well as their potential ecological implications, is therefore missing. Here we provide a first systematic analysis of the eco-evolutionary conditions that may give rise to intermittent predator–prey cycles, investigating 16 models that incorporate different types of trait variation within prey, predators, or both. Our results show that intermittent dynamics often arise through predator–prey coevolution, but only very rarely when only one trophic level can adapt. Additionally, the frequency of intermittent cycles depends on the source of trait variation (genetic variation or phenotypic plasticity) and the genetic architecture (Mendelian or quantitative traits), with intermittency occurring most commonly through Mendelian evolution, and very rarely through phenotypic plasticity. Further analysis identified three necessary conditions for when trait variation can drive intermittent cycles. First, the intrinsic stability of the predator–prey system must depend on the traits of prey, predators, or both. Second, there must be a mechanism causing the recurrent alternation between stable and unstable states, leading to a “trait” cycle superimposed on the population dynamics. Finally, these trait dynamics must be significantly slower than the predator–prey cycles. We show how these conditions explain all the abovementioned patterns. We further show an important unexpected consequence of these necessary conditions: they are most easily met when intraspecific trait variation is at high risk of being lost. As trait diversity is positively associated with ecosystem functioning, this can have potentially severe negative consequences. This novel result highlights the importance of identifying and understanding intermittent cycles in theoretical studies and natural systems. The new approach for detecting and quantifying intermittency we develop here will be instrumental in enabling future study.

El Niños and marine heatwaves (MHWs) are predicted to increase in frequency under greenhouse warming. The impact of climate oscillations like El Niño-Southern Oscillation on coastal environments in the short term likely mimics those of climate change in the long term; therefore, El Niños may serve as a short-term proxy for possible long-term ecological responses to an increasingly variable climate. Understanding and prediction of ecosystem responses requires elucidating the mechanisms underlying different organizational scales (organism, space, and time). We analyzed spatiotemporal variation in the effect of the 2015–2016 El Niño and the overlapping 2014–2016 East Pacific MHW on three intertidal kelps (Hedophyllum sessile, Egregia menziesii, and Postelsia palmaeformis) at seven sites across 300 km of the Oregon coast and over three years post El Niño. We measured percent cover, density, maximum length, growth, and carbon : nitrogen (C:N) ratios monthly in spring/summer at each site from 2016 through 2018. Results revealed a complex interplay between spatial, temporal, and biological factors that modified the effects of these thermal anomalies on Oregon intertidal kelp populations. Our findings generally agree with prior literature showing detrimental effects of El Niño on kelp. However, El Niño and possibly MHW effects can be mitigated or amplified by environmental processes and kelp life history strategies. In our study, coastal upwelling provided regional relief for the kelp individuals with respect to their growth needs and mitigated the adverse effects of warming. On the other hand, we also found that coastal upwelling amplified, or compounded, detrimental effects of El Niño by increasing phytoplankton-induced shading and mollusk grazing on juvenile and adult kelps, thereby reducing their density. Given the greater uncertainty associated with warming events and climate change in the California Current Upwelling System and its biological implications, our findings reiterate the importance of acquiring better understanding of how context-specific underlying conditions modify ecosystem processes. More specifically, understanding how demographic traits and life history stages of kelp change with biological interactions and environmental forcing over temporal and spatial scales is crucial to anticipating future climate change ramifications.

Ecologists working with stable isotopes have to deal with complex datasets including temporal and spatial replication, which makes the analysis and the representation of patterns of change challenging, especially at high resolution. Due to the lack of a commonly accepted conceptual framework in stable isotope ecology, the analysis and the graphical representation of stable isotope spatial and temporal dynamics of stable isotope value at the organism or community scale remained in the past often descriptive and qualitative, impeding the quantitative detection of relevant functional patterns. The recent community trajectory analysis (CTA) framework provides more explicit perspectives for the analysis and the visualization of ecological trajectories. Building on CTA, we developed the Stable Isotope Trajectory Analysis (SITA) framework, to analyze the geometric properties of stable isotope trajectories on n-dimensional (n ≥ 2) spaces of analysis defined analogously to the traditional multivariate spaces (Ω) used in community ecology. This approach provides new perspectives into the quantitative analysis of spatio-temporal trajectories in stable isotope spaces (Ω_δ) and derived structural and functional dynamics (Ω_γ space). SITA allows the calculation of a set of trajectory metrics, based on either trajectory distances or directions, and new graphical representation solutions, both easily performable in an R environment. Here, we illustrate the use of our approach by reanalyzing previously published datasets from marine, terrestrial, and freshwater ecosystems. We highlight the insights provided by this new analytic framework at the individual, population, community, and ecosystems levels, and discuss applications, limitations, and development potential.

The study of ecological networks has progressively evolved from a mostly descriptive science to one that attempts to elucidate the processes governing the emerging structure of multitrophic communities. To move forward, we propose a conceptual framework using trait-based inference of ecological processes to improve our understanding of network assembly and our ability to predict network reassembly amid global change. The framework formalizes the view that network assembly is governed by processes shaping the composition of resource and consumer communities within trophic levels and those dictating species’ interactions between trophic levels. To illustrate the framework and show its applicability, we (1) use simulations to explore network structures emerging from the interactions of these assembly processes, (2) develop a null model approach to infer the processes underlying network assembly from observational data, and (3) use the null model approach to quantify the relative influence of bottom-up (resource-driven) and top-down (consumer-driven) assembly modes on plant–frugivore networks along an elevational gradient. Simulations suggest that assembly processes governing the formation of pairwise interactions have a greater influence on network structure than those governing the composition of communities within trophic levels. Our case study further shows that the mode of network assembly along the gradient is mainly bottom-up controlled, suggesting that the filtering of plant traits has a larger effect on network structure relative to the filtering of frugivore traits. Combined with increasingly available trait and interaction data, the framework provides a timely toolbox to infer assembly processes operating within and between trophic levels and to test competing hypotheses about the assembly mode of resource–consumer networks along environmental gradients and among biogeographic regions. It is a step toward a more process-based network ecology and complete integration of multitrophic interactions in the prediction of future biodiversity.

The terminal investment hypothesis (TIH) predicts that individuals with favorable prospects for future reproduction (i.e., high residual reproductive value, RRV) should moderate current reproductive investment in favor of growth, survival, and future reproduction, whereas those with low RRV should “terminally invest” by diverting somatic resources towards current reproduction at the expense of future reproduction. However, support for the TIH in wild animal populations is fragmentary, and the ecological contexts of terminal investment remain poorly known. We report a remarkable case of simultaneous terminal investment involving five sympatric species of the electric knifefish genus Brachyhypopomus, from Amazonian floodplain and terra firme stream habitats. We found that terminal investment is synchronized by seasonal breeding, in response to circannual environmental variation in mortality risk. Four species exhibit a uniseasonal iteroparous (annual) life history with complete post-reproductive mortality after a single breeding season. One species (Brachyhypopomus beebei) exhibits a 2-year multiseasonal iteroparous life history with breeding in two seasons and post-reproductive mortality after the second. In mature females and (most) males of the annual species, as well as in both mature female and male second-year (but not first-year) B. beebei, we documented an increase in two metrics of reproductive effort (size-adjusted gonad mass and electric signal amplitude) and a concomitant reduction in somatic condition (size-adjusted somatic mass), all in response to proximity to the end of the common breeding season, when RRV approximates zero. In mature first-year B. beebei, we documented neither an increase in reproductive effort nor a decline in somatic condition, implying an alternative strategy of reproductive restraint. Our findings support Kirkwood's disposable soma theory, which posits that death by reproductive exhaustion can be delayed if terminal investment is replaced by reproductive restraint, allowing individuals to survive and breed in a subsequent season. Deferral of the terminal investment response in annual species, and the origin of a gonadal regression-regeneration sequence, may open pathways for rapid evolutionary transitions to multiseasonal iteroparity. Excepting the age (year-group) dependency of terminal investment in B. beebei, we were unable to identify intrinsic cues or extrinsic environmental cues for the terminal investment response in Brachyhypopomus.