Indirect Species Interactions Research Articles

Invasive vegetation encroachment modulates dual threats faced by island-endemic scaly crickets

The mechanisms by which non-native species impact island-endemic populations are varied and often convoluted. For effective invertebrate conservation where capacity is limited, it is vital to understand both direct and indirect species interactions. Those interactions define specific threats and therefore highlight possible solutions. We quantified several of the non-native species threats faced by an endemic scaly cricket, Discophallus ascension (family: Mogoplistidae), found on Ascension Island’s naturally barren coastlines. We hypothesized that encroaching non-native Mexican thorn Prosopis juliflora (1) directly reduced microclimate suitability, (2) indirectly increased resource competition with non-native species, and (3) indirectly increased non-native predation. Our aim was to assess how rapidly spreading Mexican thorn might reduce endemic cricket populations. Lab trials first confirmed the temperature and diet preferences of D. ascension. Mexican thorn significantly increased ground-level temperature but not above the estimated optimum for D. ascension within key habitat. Observed interspecific resource competition was significantly greater in habitat degraded by Mexican thorn and was driven by non-native ants, house crickets Gryllodes sigillatus and cockroaches Periplaneta americana. Recorded cricket predation was almost entirely by non-native rats Rattus rattus in barren habitat but primarily by ants in Mexican thorn habitat. Threats to D. ascension were therefore modulated by invasive vegetation, but driven by non-native insects and rats at all sites. We conclude that removal of vegetation alone is likely to reduce resource competition but not predation. Managing multiple invasive species is likely necessary for successful species conservation.

Development of a reduced oxidation mechanism with low-temperature chemistry for real biodiesel methyl esters

Accompanied with the development of advanced engine combustion modes, biodiesel as an important biorenewable fuel has the potential to be a regular engine fuel due to its advantages in carbon neutrality and reducing the emission of major pollutants. In the present study, a compact biodiesel oxidation mechanism for the numerical study of low-temperature biodiesel combustion is developed by reducing a comprehensive mechanism based on five major biodiesel methyl esters. For the mechanism reduction, a method based on direct and indirect species interactions is developed and proven to have improved effectiveness in reducing large complex mechanisms compared to existing methods. The method is preceded by reaction path analysis and followed by isomer lumping and sensitivity analysis, and the overall reduction process finally leads to a compact biodiesel mechanism with 187 species and 689 reactions. The fidelity of the reduced biodiesel mechanism is first demonstrated by simulating the auto-ignition of homogeneous biodiesel-air mixtures from various initial conditions. Predicted ignition delay data show discrepancies in the order of milliseconds, compared to those predicted using the original mechanism and experimental data. Agreement is also found in the instantaneous mole fraction of major species and mixture temperature, indicating that the reduced mechanism preserves the key reaction paths of the original mechanism. A further validation of the reduced mechanism is also accomplished by simulating the low-temperature oxidation of saturated and unsaturated biodiesel methyl esters in a jet-stirred reactor, and reasonable agreement is observed between predicted mole fraction of major product species and the corresponding experimental data at different temperatures.

The Self-Organization of Marine Microbial Networks under Evolutionary and Ecological Processes: Observations and Modeling.

Simple SummaryThe properties and structure of ecological networks in marine microbial communities determine ecosystem functions and stability; however, the principles of microbial network assemblages are poorly understood. In this study, we revealed the influences of species phylogeny and niches on the self-organization of marine microbial co-occurrence networks and provided a mathematical framework to simulate microbial network assemblages. Our results provide deep insights into network stability from the perspective of network assembly principles and not just network properties, such as complexity and modularity.Evolutionary and ecological processes are primary drivers of ecological network constrictions. However, the ways that these processes underpin self-organization and modularity in networks are poorly understood. Here, we performed network analyses to explore the evolutionary and ecological effects on global marine microbial co-occurrence networks across multiple network levels, including those of nodes, motifs, modules and whole networks. We found that both direct and indirect species interactions were evolutionarily and ecologically constrained across at least four network levels. Compared to ecological processes, evolutionary processes generally showed stronger long-lasting effects on indirect interactions and dominated the network assembly of particle-associated communities in spatially homogeneous environments. Regarding the large network path distance, the contributions of either processes to species interactions generally decrease and almost disappear when network path distance is larger than six. Accordingly, we developed a novel mathematical model based on scale-free networks by considering the joint effects of evolutionary and ecological processes. We simulated the self-organization of microbial co-occurrence networks and found that long-lasting effects increased network stability via decreasing link gain or loss. Overall, these results revealed that evolutionary and ecological processes played key roles in the self-organization and modularization of microbial co-occurrence networks.

Effects of environmental changes on interspecific interactions of three sympatric pheasants – A study based on long-term monitoring data

• Interactions of 3 pheasants were studied using a new model based on long-term data. • And considered habitat filtering and indirect species interactions together. • Both positive and negative interactions existed among 3 pheasants. • Species interactions varied with habitat heterogeneity and stress levels in 11 years. As the lack of a general understanding of how interspecific interactions change along varying environmental gradients exists, predicting the response of ecological communities to habitat and climate change becomes an arduous task. Researchers generally assess interspecific interactions by investigating species co-occurrence patterns. However, habitat filtering and indirect interspecific interactions can interfere with interspecific interaction signals in co-occurrence data, and these two factors have rarely been considered jointly in previous studies. By using the Markov random fields model which can control the indirect interaction, based on the 11-year co-occurrence data, we analyzed the effects of environmental factors on the interaction of three sympatric pheasant species (Chinese Bamboo Partridge Bambusicola thoracicus , Sichuan Hill Partridge Arborophila rufipectu , Temminck's Tragopan Tragopan temminckii ) in Laojunshan National Nature Reserve, China. To eliminate the interference of habitat filtering, we took the environmental change during the 11-year period as the environmental variable instead of the environmental spatial gradient commonly used in previous studies. We found that the positive interaction increased with the increase of temperature, and decreased with the increase of precipitation, and with the increase of EVI heterogeneity, the positive interaction increased and the negative interaction decreased. The results are consistent with the predictions of habitat heterogeneity hypothesis and stress gradient hypothesis. Our study provides an idea for the follow-up studies of how to eliminate the interference of habitat filtering and indirect interaction in interspecific interaction analysis.

A sit-and-wait predator, but not an active-pursuit predator, alters pollinator-mediated selection on floral traits.

Indirect species interactions are ubiquitous in nature, often outnumbering direct species interactions. Yet despite evidence that indirect interactions have strong ecological effects, relatively little is known about whether they can shape adaptive evolution by altering the strength and/or direction of natural selection. We tested whether indirect interactions affect the strength and direction of pollinator-mediated selection on floral traits of the bumble-bee pollinated wildflower Lobelia siphilitica. We estimated the indirect effects of two pollinator predators with contrasting hunting modes: dragonflies (Aeshnidae and Corduliidae) and ambush bugs (Phymata americana, Reduviidae). Because dragonflies are active pursuit predators, we hypothesized that they would strengthen pollinator-mediated selection by weakening plant-pollinator interactions (i.e., a density-mediated indirect effect). In contrast, because ambush bugs are sit-and-wait predators, we hypothesized that they would weaken or reverse the direction of pollinator-mediated selection by altering pollinator foraging behavior (i.e., a trait-mediated indirect effect). Specifically, if ambush bugs hunt from plants with traits that attract pollinators (i.e., prey), then pollinators will spend less time visiting those plants, weakening or reversing the direction of selection on attractive floral traits. We did not find evidence that high dragonfly abundance strengthened selection on floral traits via a density-mediated indirect effect: neither pollen limitation (a proxy for the strength of plant-pollinator interactions) nor directional selection on floral traits of L. siphilitica differed significantly between high- and low-dragonfly abundance treatments. In contrast, we did find evidence that ambush bug presence affected selection on floral traits via a trait-mediated indirect effect: ambush bugs hunted from L. siphilitica plants with larger daily floral displays, reversing the direction of pollinator-mediated selection on daily display size. These results suggest that indirect species interactions have the potential to shape adaptive evolution by altering natural selection.

Marginal support for a trophic cascade among sympatric canids in peripheral wolf range.

Trophic cascades reportedly structure ecological communities through indirect species interactions. Though the predator-herbivore-autotroph relationship has received much attention, mechanistic evidence supporting intraguild trophic cascades is rare. We established 348 remote camera sites (1 August-5 September 2019) across seven study areas of varying wolf (Canis lupus) density including one study area where wolves were absent in northern Michigan, USA. Using multi-species occupancy modeling at species-relevant spatial scales, we evaluated the hypothesis that increased wolf occurrence suppresses coyote (C. latrans) occurrence with corresponding increased red fox (Vulpes vulpes) occurrence mediated by land cover edge density, human presence, and temporal partitioning. Remote cameras recorded >600,000 images and included 6,370, 10,137, and 4,876 detections of wolves, coyotes, and foxes, respectively. Fox occupancy probability was more than three times as high (0.29) at camera sites where wolves were present, relative to sites wolves were absent (0.09). Pairwise species interactions supported expected size-based dominance patterns among canids and insignificant effects were directionally consistent with reported reduced strength of top-down effects in peripheral wolf range. Increased edge density also increased co-occurrence of coyote and wolves, likely a function of increased prey availability and refugia for coyotes. Though foxes occurred in spatial proximity to wolves, competition was limited by greater temporal partitioning than observed between coyotes and foxes that were spatially segregated. Collectively, our results provide marginal support for the reported trophic cascade among wolves, coyotes, and foxes wherein top-down effects may be reduced near the edge of current wolf distributions. As predators continue to recolonize portions of their historic range, knowledge of the effects on intraguild predators has implications for species management and predicting prey population responses.

Variation cascades: resource pulses and top-down effects across time scales.

Top-down and bottom-up theories of trophic control have been fundamental to our understanding of community dynamics and structure. However, most ecological theories have focused on equilibrium dynamics and do not provide predictions for communities' responses in temporally fluctuating environments. By deriving the frequency response of populations in different trophic communities, we extend the top-down and bottom-up theories of ecology to include how temporal fluctuations in potential primary productivity percolate up the food chain and are re-expressed as population variability. Moreover, by switching from a time-based representation into the frequency domain, we provide a unified method to compare how the time scale of perturbations determines communities' responses. At low frequencies, primary producers and secondary consumers have the highest temporal variability, while the primary consumers are relatively stable. Similar to the Exploitation Ecosystem Hypothesis, top-down effects drive this alternating pattern of variability. We define the top-down effect of consumers on the variability of lower trophic levels as a variation cascade. However, at intermediate frequencies, variation cascades can amplify temporal variation up the food chain. At high frequencies, variation cascades weaken, and fluctuations are attenuated up the food chain. In summary, we provide a novel theory for how communities will respond to fluctuations in productivity, and we show that indirect species interactions play a crucial role in determining community dynamics across the frequency spectrum.

Seasonal food webs with migrations: multi-season models reveal indirect species interactions in the Canadian Arctic tundra.

Models incorporating seasonality are necessary to fully assess the impact of global warming on Arctic communities. Seasonal migrations are a key component of Arctic food webs that still elude current theories predicting a single community equilibrium. We develop a multi-season model of predator-prey dynamics using a hybrid dynamical systems framework applied to a simplified tundra food web (lemming-fox-goose-owl). Hybrid systems models can accommodate multiple equilibria, which is a basic requirement for modelling food webs whose topology changes with season. We demonstrate that our model can generate multi-annual cycling in lemming dynamics, solely from a combined effect of seasonality and state-dependent behaviour. We compare our multi-season model to a static model of the predator-prey community dynamics and study the interactions between species. Interestingly, including seasonality reveals indirect interactions between migrants and residents not captured by the static model. Further, we find that the direction and magnitude of interactions between two species are not necessarily accurate using only summer time-series. Our study demonstrates the need for the development of multi-season models and provides the tools to analyse them. Integrating seasonality in food web modelling is a vital step to improve predictions about the impacts of climate change on ecosystem functioning. This article is part of the theme issue 'The changing Arctic Ocean: consequences for biological communities, biogeochemical processes and ecosystem functioning'.

Microbial Symbionts of Parasitoids.

Parasitoids depend on other insects for the development of their offspring. Their eggs are laid in or on a host insect that is consumed during juvenile development. Parasitoids harbor a diversity of microbial symbionts including viruses, bacteria, and fungi. In contrast to symbionts of herbivorous and hematophagous insects, parasitoid symbionts do not provide nutrients. Instead, they are involved in parasitoid reproduction, suppression of host immune responses, and manipulation of the behavior of herbivorous hosts. Moreover, recent research has shown that parasitoid symbionts such as polydnaviruses may also influence plant-mediated interactions among members of plant-associated communities at different trophic levels, such as herbivores, parasitoids, and hyperparasitoids. This implies that these symbionts have a much more extended phenotype than previously thought. This review focuses on the effects of parasitoid symbionts on direct and indirect species interactions and the consequences for community ecology.

With or without you: Effects of the concurrent range expansion of an herbivore and its natural enemy on native species interactions.

Global climatic changes may lead to the arrival of multiple range-expanding species from different trophic levels into new habitats, either simultaneously or in quick succession, potentially causing the introduction of manifold novel interactions into native food webs. Unraveling the complex biotic interactions between native and range-expanding species is critical to understand the impact of climate change on community ecology, but experimental evidence is lacking. In a series of laboratory experiments that simulated direct and indirect species interactions, we investigated the effects of the concurrent arrival of a range-expanding insect herbivore in Europe, Spodoptera littoralis, and its associated parasitoid Microplitis rufiventris, on the native herbivore Mamestra brassicae, and its associated parasitoid Microplitis mediator, when co-occurring on a native plant, Brassica rapa. Overall, direct interactions between the herbivores were beneficial for the exotic herbivore (higher pupal weight than the native herbivore), and negative for the native herbivore (higher mortality than the exotic herbivore). At the third trophic level, both parasitoids were unable to parasitize the herbivore they did not coexist with, but the presence of the exotic parasitoid still negatively affected the native herbivore (increased mortality) and the native parasitoid (decreased parasitism rate), through failed parasitism attempts and interference effects. Our results suggest different interaction scenarios depending on whether S. littoralis and its parasitoid arrive to the native tritrophic system separately or concurrently, as the negative effects associated with the presence of the parasitoid were dependent on the presence of the exotic herbivore. These findings illustrate the complexity and interconnectedness of multitrophic changes resulting from concurrent species arrival to new environments, and the need for integrating the ecological effects of such arrivals into the general theoretical framework of global invasion patterns driven by climatic change.

Impact of climate change on direct and indirect species interactions

Impact of climate change on direct and indirect species interactions

生态系统基于性状调节的物种间接作用:特征、成因及后果

PDF HTML阅读 XML下载 导出引用 引用提醒 生态系统基于性状调节的物种间接作用:特征、成因及后果 DOI: 10.5846/stxb201610011986 作者: 作者单位: 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金项目（31230012）；国家科技基础性工作专项（2012FY111900）；教育部创新团队发展计划（IRT-16R11）；中国博士后面上基金（2016M590246） Characteristics, causes, and consequences of trait-mediated indirect interactions in ecosystems Author: Affiliation: Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:物种之间的间接作用关系是维持生物群落结构以及生态系统功能的关键因素。目前的理论模型和实验性研究均认为，物种间接作用的传递主要是由物种的密度变化所引起。但大量的实验证据表明，生物个体在形态、生理、行为和生活史性状的适应性变化，是物种间形成间接互作关系的另一个重要机制。生态学家把这种基于物种性状调控的种间关系称为性状调节的间接作用。深入了解性状调节的间接作用类型，发生机制和作用途径，有助于阐释自然界中多物种种间关系的复杂性和多样性形成机制。对性状调节的概念和性状的分类进行系统的总结，指出性状可塑性和物种特异性是物种间性状调节关系形成的两个重要机制。与其他类型的种间互作一样，性状调节的种间作用是驱动物种进化的重要力量。此外，分析了性状调节在自然群落中传递的几个重要途径，并强调这些间接作用在影响和调节生态系统功能和过程中的重要作用。最后，就性状调节种间关系的研究对象、研究尺度、以及研究方法等问题提出若干建议，为今后对这一问题的研究提供相关参考。 Abstract:Indirect species interactions play a critical role in shaping community structure and the functioning of ecosystems. Previous empirical and theoretical studies commonly assumed that indirect interactions were caused in general by changes in population density of interacting species, which is referred to as ‘density-mediated indirect interactions.’ However, there is growing evidence that changes in traits of the individual intermediate species can also be an important mechanism that regulates indirect interactions among species, and ecologists term these species interactions ‘trait-mediated indirect interactions’ (TMⅡs). TMⅡs are prevalent in both aquatic and terrestrial systems, and their effects are often as strong as, or stronger than, those of density-mediated indirect effects. Studies on the characteristics, causes, and consequences of TMⅡs are essential to understand the complexity and diversity of multispecies interactions in natural ecosystems fully. In this paper, we summarize the concept and classification of TMⅡs, and suggest that trait plasticity and species specificity are the two intrinsic mechanisms for the occurrence of TMⅡs. We also review the direct evidence for TMⅡs in various permutations of interaction webs that can have both trophic and non-trophic links, and then highlight the profound effects of TMⅡs on regulation of ecological functions and processes in a wide range of ecological systems. Finally, we propose future developments regarding the effects of TMⅡs to provide a reference framework for related research. 参考文献 相似文献 引证文献

Animal water balance drives top-down effects in a riparian forest-implications for terrestrial trophic cascades.

Despite the clear importance of water balance to the evolution of terrestrial life, much remains unknown about the effects of animal water balance on food webs. Based on recent research suggesting animal water imbalance can increase trophic interaction strengths in cages, we hypothesized that water availability could drive top-down effects in open environments, influencing the occurrence of trophic cascades. We manipulated large spider abundance and water availability in 20 × 20 m open-air plots in a streamside forest in Arizona, USA, and measured changes in cricket and small spider abundance and leaf damage. As expected, large spiders reduced both cricket abundance and herbivory under ambient, dry conditions, but not where free water was added. When water was added (free or within moist leaves), cricket abundance was unaffected by large spiders, but spiders still altered herbivory, suggesting behavioural effects. Moreover, we found threshold-type increases in herbivory at moderately low soil moisture (between 5.5% and 7% by volume), suggesting the possibility that water balance may commonly influence top-down effects. Overall, our results point towards animal water balance as an important driver of direct and indirect species interactions and food web dynamics in terrestrial ecosystems.

Contrasting patterns of short-term indirect seed-seed interactions mediated by scatter-hoarding rodents.

It is well known that direct effects of seed predators or dispersers can have strong effects on seedling establishment. However, we have limited knowledge about the indirect species interactions between seeds of different species that are mediated by shared seed predators and/or dispersers and their consequences for plant demography and diversity. Because scatter-hoarding rodents as seed dispersers may leave some hoarded seeds uneaten, scatter hoarding may serve to increase seed survival and dispersal. Consequently, the presence of heterospecific seeds could alter whether the indirect interactions mediated by scatter-hoarding rodents have a net positive effect, creating apparent mutualism between seed species, or a net negative effect, creating apparent competition between seed species. We present a testable framework to measure short-term indirect effects between co-occurring plant species mediated by seed scatter-hoarding rodents. We tested this framework in a subtropical forest in south-west China using a replacement design and tracked the fate of individually tagged seeds in experimental patches. We manipulated the benefits to rodents by using low-tannin dormant chestnuts as palatable food and high-tannin non-dormant acorns as unpalatable food. We found that seed palatability changed the amount of scatter hoarding that occurred when seeds co-occurred either among or within patches. Consistent with our predictions, scatter-hoarding rodents created apparent mutualism through increasing seed removal and seed caching, and enhancing survival, of both plant species in mixed patches compared with monospecific patches. However, if we ignore scatter hoarding and treat all seed harvest as seed predation (and not dispersal), then apparent competition between palatable chestnuts and unpalatable acorns was also observed. This study is the first to demonstrate that foraging decisions by scatter-hoarding animals to scatter hoard seeds for later consumption (or loss) or consume them can influence indirect effects among co-occurring seeds, and rodent-mediated indirect effects vary depending on whether the harvested seeds are hoarded or eaten.

Spatial and Dietary Overlap Creates Potential for Competition between Red Snapper (Lutjanus campechanus) and Vermilion snapper (Rhomboplites aurorubens)

Understanding the complex nature of direct and indirect species interactions is a critical precursor to successful resource management. In the northern Gulf of Mexico fisheries ecosystem, red snapper (Lutjanus campechanus) and vermilion snapper (Rhomboplites aurorubens) are two commercially harvested species within a larger reef fish complex. These two species share similar habitats and diets; however, little is known about how these species partition habitat and dietary resources. In this study we examined the extent of spatial and dietary overlap between red snapper and vermilion snapper, and experimentally compared their feeding behavior. Field data from multiple gear types demonstrates that red snapper and vermilion snapper frequently cohabited reefs in the northern Gulf of Mexico, and Pianka’s niche overlap indices suggest significantly overlapping diets. Experimental manipulations show that red snapper are the dominant forager of the two species, as red snapper foraging alone ate more shrimp per fish than vermilion snapper in both the single species (p = 0.003) and mixed species (p = 0.02) treatments. In addition, red snapper ate significantly more shrimp per fish in the mixed species treatment than in the single species treatment (p = 0.04). Vermilion snapper shrimp consumption per fish did not differ significantly between mixed and single species treatments. Cumulatively, our results suggest that spatial and dietary overlap could lead to competition between red and vermilion snapper in the study area; however, conclusively determining the existence of such competition would require further research.

The hidden complexity of multi‐species interactions

AbstractIn this issue of the Journal of Vegetation Science, Michalet et al. used species removal experiments to detect direct and indirect species interactions in a sub‐alpine grassland. They found evidence for competition, facilitation and a range of indirect interactions at the species level, but no measurable effects when aggregated at the community level. Their results raise fundamental questions regarding the role and importance of indirect interactions in structuring ecological communities.

Genetic variation in invasive species response to direct and indirect species interactions

Biotic resistance to invasion arises from strong species interactions that decrease the fitness and population growth rates of potential invaders. Strong, direct interactions such as predation and competition are typically thought to drive biotic resistance, but in diverse communities, indirect interactions among species may also affect biotic resistance. Further, genetic variation in traits of the invading species that affect species interactions may allow some genotypes to overcome biotic resistance. We investigated the direct and indirect effects of a native legume (Acmispon wrangelianus) and insect herbivores on the fitness of different genotypes of an invasive legume (Medicago polymorpha) in a California grassland. Insect herbivores decreased Medicago fitness, but only in the presence of Acmispon, suggesting that indirect interactions mediated through insects and Acmispon are important for deterring Medicago invasion. Some Medicago genotypes were less affected by interactions with other species, however. This genetic variance suggests that while biotic resistance reduced the reproductive success of most genotypes, a few genotypes were able to overcome these complex interactions. However, Medicago invasion was unsuccessful in all treatments at several of our sites, suggesting that factors beyond those manipulated here also play a key role at many sites. At sites where biotic resistance is important, spatial and temporal variation in community composition and the genetic composition of the invasion pool may explain the invasion success of Medicago into this community.

Indirect effects and facilitation among native and non‐native species promote invasion success along an environmental stress gradient

Summary The spatial distribution of species is mediated by a combination of biotic interactions and environmental conditions. Understanding the relative importance of these factors and how they interact is particularly important for predicting the spread of non‐native species and their impact on resident communities. We used a 3‐species Lotka–Volterra model parameterized with field and experimental data to understand the potential for continued spread by an introduced, non‐native, dune‐building beach grass (Ammophila breviligulata) and whether this invasion will result in coexistence or displacement in the resident beach grass communities (native Elymus mollis and introduced, non‐native Ammophila arenaria) of the US Pacific Northwest. We also used the model to investigate to what extent different rates of ocean‐driven sand supply mediate the outcomes of beach grass species interactions. Our model suggests that A. breviligulata could invade and dominate dunes across the range of sand supply rates observed in the region. We found that sand supply influenced intra‐ vs. interspecific interactions, the strength of positive and indirect effects among beach grasses and the long‐term abundances of the beach grass species themselves. Of the two non‐natives, A. breviligulata is the inferior dune‐building species. Thus, our results suggest that further invasions by A. breviligulata could reduce the coastal protection services afforded by tall dunes currently dominated by A. arenaria. Synthesis: In systems with strong environmental forcing and stressful conditions such as coastal dunes, environmentally mediated positive and indirect species interactions can govern invasion success and long‐term native–non‐native coexistence. In doing so, these interactions ultimately shape community structure and ecosystem function. Understanding the joint effects of environmental forcing and species interactions on community assembly is particularly important in cases where species introductions can alter ecosystem services, such as coastal protection, which are vulnerable to the effects of climate change.

Long-term exclosure of large terrestrial vertebrates: Implications of defaunation for seedling demographics in the Amazon rainforest

“Empty forests” in which humans have driven large vertebrate species to extinction lack myriad direct and indirect species interactions. This may alter key ecosystem processes, including trophic cascades, disturbance regimes, and nutrient cycling. Past research concerning the effects of tropical forest defaunation mainly compared sites with an intact fauna with sites having degraded faunal communities. However, experimental studies on seedling demographics at individual sites, with the same abiotic and biotic conditions, over a long period of time are still scarce. In this research we conducted a 7-year experiment within a completely intact and pristine forest in Manu National Park, Peru to elucidate the effects of defaunation on tree seedling demographics and densities. We constructed semipermeable exclosures that prevented access of large terrestrial vertebrates, but allowed free passage of small- and medium-sized species. New tree seedlings taller than 5cm were tagged and followed. Jolly–Seber mark-recapture models were used to estimate density, survival, and recruitment of seedlings. Seedling density in the exclosures continued to increase significantly after the first year of the study. Only during the first 2years was seedling survival higher in the exclosures. The numbers of new recruits were consistently higher in exclosures when compared to open control plots throughout the study. In accord with increased density, survival, and recruitment in exclosure plots, we also observed increased understory vegetation cover. Our results suggest that defaunation can have long-term effects on seedling demographics and density, which could ultimately influence the composition and diversity of the canopy tree community.

Missing lynx and trophic cascades in food webs: A reply to Ripple et al.

AbstractRipple et al. (2011) proposed a hypothesis that the recovery of gray wolves (Canis lupus) may positively affect the viability of threatened Canada lynx (Lynx canadensis) populations in the contiguous United States through indirect species interactions. Ripple et al. (2011) proposed 2 key trophic linkages connecting wolf restoration with lynx recovery. First, recovering wolf populations may benefit lynx through reduced interference and exploitative competition with coyotes (C. latrans). Second, recovering wolf populations may benefit lynx through reduced exploitative competition among ungulates and snowshoe hares (Lepus americanus), the primary prey of lynx. Both proposed linkages have weak or contradictory empirical support in the available literature on lynx–hare ecology, casting doubt on the utility of Ripple et al.'s (2011) hypothesis. Debate over Ripple et al.'s (2011) hypothesis demonstrates the importance of experimental or comparative documentation when proposing trophic cascades in complex food webs. In this case, publishing unsupported opinions as hypotheses that concern complex trophic interactions is a potential disservice to lynx conservation through misallocated research, conservation funding, and misplaced public perception. © 2012 The Wildlife Society.