Showing papers in "Oikos in 2001"
TL;DR: A review of the evidence for both the natural enemies hypothesis and the biotic resistance hypothesis reveals that: 1) introduced plants can attract a diverse assemblage of native herbivores and that 2) native Herbivores can reduce introduced plant growth, seed set and survival.
Abstract: Two venerable hypotheses, widely cited as explanations for either the success or failure of introduced species in recipient communities, are the natural enemies hypothesis and the biotic resistance hypothesis. The natural enemies hypothesis posits that introduced organisms spread rapidly because they are liberated from their co-evolved predators, pathogens and herbivores. The biotic resistance hypothesis asserts that introduced species often fail to invade communities because strong biotic interactions with native species hinder their establishment and spread. We reviewed the evidence for both of these hypotheses as they relate to the importance of non-domesticated herbivores in affecting the success or failure of plant invasion.
To evaluate the natural enemies hypothesis, one must determine how commonly native herbivores have population-level impacts on native plants. If native herbivores seldom limit native plant abundance, then there is little reason to think that introduced plants benefit from escape from these enemies. Studies of native herbivore-native plant interactions reveal that plant life-history greatly mediates the strength with which specialist herbivores suppress plant abundance. Relatively short-lived plants that rely on current seed production for regeneration are most vulnerable to herbivory that reduces seed production. As such, these plants may gain the greatest advantage from escaping their specialist enemies in recipient communities. In contrast, native plants that are long lived or that possess long-lived seedbanks may not be kept “in check” by native herbivores. For these species, escape from native enemies may have little to do with their success as exotics; they are abundant both where they are native and introduced.
Evidence for native herbivores providing biotic resistance to invasion by exotics is conflicting. Our review reveals that: 1) introduced plants can attract a diverse assemblage of native herbivores and that 2) native herbivores can reduce introduced plant growth, seed set and survival. However, the generality of these impacts is unclear, and evidence that herbivory actually limits or reduces introduced plant spread is scarce. The degree to which native herbivores provide biotic resistance to either exotic plant establishment or spread may be greatly determined by their functional and numerical responses to exotic plants, which we know little about. Generalist herbivores, through their direct effects on seed dispersal and their indirect effects in altering the outcome of native–non-native plant competitive interactions, may have more of a facilitative than negative effect on exotic plant abundance.
821 citations
TL;DR: Hurlbert divides experimental ecologist into ‘those who do not see any need for dispersion, and those who do recognize its importance and take whatever measures are necessary to achieve a good dose of it’.
Abstract: Hurlbert divides experimental ecologist into ‘those who do not see any need for dispersion (of replicated treatments and controls), and those who do recognize its importance and take whatever measures are necessary to achieve a good dose of it’. Experimental ecologists could also be divided into those who do not see any problems with sacrificing spatial and temporal scales in order to obtain replication, and those who understand that appropriate scale must always have priority over replication. If an experiment is conducted in a spatial or temporal scale, where the predictions of contesting hypotheses are convergent or ambiguous, no amount of technical impeccability can make the work instructive. Conversely, replication can always be obtained afterwards, by conducting more experiments with basically similar design in different areas and by using meta-analysis. This approach even reduces the sampling bias obtained if resources are allocated to a small number of well-replicated experiments. For a strict advocate of the hypothetico-deductive method, replication is unnecessary even as a matter of principle, unless the predicted response is so weak that random background noise is a plausible excuse for a discrepancy between predictions and results. By definition, a prediction is an ‘all-statement’, referring to all systems within a well-defined category. What applies to all must apply to any. Hence, choosing two systems and assigning them randomly to a treatment and a control is normally an adequate design for a deductive experiment. The strength of such experiments depends on the firmness of the predictions and their a priori probability of corroboration. Replication is but one of many ways of reducing this probability. Whether the experiment is replicated or not, inferential statistics should always be used, to enable the reader to judge how well the apparent patterns in samples reflect real patterns in statistical populations. The concept ‘pseudoreplication’ amounts to entirely unwarranted stigmatization of a reasonable way to test predictions referring to large-scale systems.
559 citations
TL;DR: Investigation of changes in competition and facilitation between a leguminous shrub and its associate understorey species along an environmental gradient in semi-arid southeast Spain shows a change in the net balance of the interaction, from clearly positive in the water-stressed, infertile environment to neutral or even negative in the more fertile habitat.
Abstract: A combination of competition and facilitation effects operating simultaneously among plant species appears to be the rule in nature, where these effects change along productivity gradients often in a non-proportional manner. We investigated changes in competition and facilitation between a leguminous shrub, Retama sphaerocarpa, and its associate understorey species along an environmental gradient in semi-arid southeast Spain. Our results show a change in the net balance of the interaction between the shrub and several of its associated species, from clearly positive in the water-stressed, infertile environment to neutral or even negative in the more fertile habitat. There was a weakening of facilitation along the fertility gradient as a consequence of improved abiotic conditions. Competition was the most intense for below-ground resources in the less fertile environment while total competition tended to increase towards the more productive end of the gradient. Changes in the balance of the interaction between and among different plant species along the gradient of stress were caused by a decline in facilitation rather than by a change in competition. As both competition intensity and facilitation change along gradients of resource availability, plant interactions are best viewed as dynamic relationships, the outcome of which depends on abiotic conditions.
457 citations
TL;DR: Grasses were generally more abundant than forbs and the relative abundance of grasses and forbs did not change with increasing nitrogen addition or soil disturbance, but high tissue density species became less common as fertility and disturbance increased.
Abstract: For 76 annual, biennial, and perennial species common in the grasslands of central Minnesota, USA, we determined the patterns of correlations among seven organ-level traits (specific leaf area, leaf thickness, leaf tissue density, leaf angle, specific root length, average fine root diameter, and fine root tissue density) and their relationships with two traits relating to growth form (whether species existed for part of the growing season in basal, non-caulescent form and whether species were rhizomatous or not). The first correlation of traits showed that grasses had thin, dense leaves and thin roots while forbs had thick, low-density leaves and thick roots without any significant differences in growth form or life history. The second correlation of traits showed a gradient of species from those with high-density roots and high-density erect leaves to species with low-density roots and low-density leaves that were held parallel to the ground. High tissue density species were more likely to exist as a basal rosette for part of the season, were less likely to be rhizomatous, and less likely to be annuals. We examined the relationships between the two axes that represent the correlations of traits and previously collected data on the relative abundance of species across gradients of nitrogen addition and disturbance. Grasses were generally more abundant than forbs and the relative abundance of grasses and forbs did not change with increasing nitrogen addition or soil disturbance. High tissue density species became less common as fertility and disturbance increased.
361 citations
TL;DR: It is demonstrated that much variation in trait evolution occurs along a fire-surviving/fire-embracing axis.
Abstract: Fire may act as a selective force on plants both through its direct effects by killing or wounding susceptible individuals and through its effect on the environment: the post-fire environment may select specific physiological traits or life histories. We used phylogenetic independent contrasts to test the hypothesis that fire has selected for correlated evolution among alternative suites of traits in pines: a survival/avoidance suite characterized by thick bark, height, and self-pruning of dead branches; and a fire-embracing strategy in which plants invest little into survival, exhibit traits which enhance flammability, and use fire as a means to cue seedling establishment to the post-fire environment through serotinous cones. We created a set of alternative ‘supertree’ phylogenies for the genus Pinus from published sources. Using these alternative phylogenies, published ecological data for 38 pine species, and newly collected morphological data, we demonstrate that much variation in trait evolution occurs along a fire-surviving/fire-embracing axis. Pines vary in their susceptibility to ignition since a tree that retains dead branches is more likely to carry a fire into the canopy than a tree that self-prunes. The evolution of increased flammability may have altered evolutionary trajectories prompting an evolutionary switch from a fire-surviving to a fire-embracing life history. Alternatively, the fire-embracing strategy may in fact select for increased flammability to ensure canopy ignition and the realization of serotinous seed-release.
323 citations
TL;DR: A set of hypotheses is offered that describe how diversity might influence productivity in plant communities based on three well-known mechanisms: complementarity, facilitation, and the sampling effect, and why productivity per se is a relevant management concern in non-agricultural systems once relationships among diversity, productivity, andThe qualities of the surrounding environment are considered.
Abstract: The effect of species diversity on ecosystem productivity is controversial, in large part because field experiments investigating this relationship have been fraught with difficulties Unfortunately, there are few guidelines to aid researchers who must overcome these difficulties and determine whether global species losses seriously threaten the ecological and economic bases of terrestrial ecosystems In response, I offer a set of hypotheses that describe how diversity might influence productivity in plant communities based on three well-known mechanisms: complementarity, facilitation, and the sampling effect Emphasis on these mechanisms reveals the sensitivity of any diversity-productivity relationship to ecological context (ie, where this relationship should be found); ecological context includes characteristics of the surrounding environment, temporal and spatial scales of observation, and the intensity of human management In particular, the legitimacy of the sampling effect as a mechanism of productivity enhancement is dependent upon the degree to which stochastic events influence immigration and extinction processes in a given ecosystem A mechanistic approach also requires that the three mechanisms be separated and quantified in diversity experiments, and I examine the most appropriate analyses for doing so, focusing on the overyielding technique Finally, I question why productivity per se is a relevant management concern in non-agricultural systems once relationships among diversity, productivity, and the qualities of the surrounding environment are considered
320 citations
TL;DR: In this paper, a discussion and debate about important issues in ecology, e.g. theory or terminology, may be included, and contributions should be as precise as possible and references should be kept to a minimum.
Abstract: P 0 I I Opinion is intended to facilitate communication between reader and author and reader and reader. Comments, viewpoints or suggestions arising from published papers are welcome. N Discussion and debate about important issues in ecology, e.g. theory or terminology, may I I also be included. Contributions should be as precise as possible and references should be P 0 kept to a minimum. A summary is not required. O N
312 citations
TL;DR: Two of the four mechanisms of the seed size/number trade-off models can be considered to hold true generally, while the third mechanism may be valid in particular environments.
Abstract: This paper examines four key mechanisms of the seed size/number trade-off (SSNT) models to assess their relevance to a general understanding of plant community structure. Mechanism 1 is that large seeds have a greater probability of winning in competition against smaller seeds. I provide interspecific experimental evidence that there is a competitive hierarchy among seedlings based on seed size. Mechanism 2 is that a trade-off exists between the number and size of seeds produced for a given reproductive allocation. Negative correlations between seed size and number were found consistently across a range of species from a range of habitats, from published literature. Mechanism 3, that seedling-seedling competition is an important influence on species composition, was found to exist potentially in a range of environments, including annual-dominated, post-fire and gap-dynamic communities. However, there is little quantitative evidence available and this is likely to be a restrictive mechanism. Mechanism 4, that small seeds are superior colonists due to their greater number, was tested in a field experiment in a calcareous grassland community. No supporting evidence was found, suggesting that the SSNT is not an important determinant of structure in this community. Thus two of the four mechanisms can be considered to hold true generally, while the third mechanism may be valid in particular environments. The fourth mechanism did not apply in the community tested, but could be tested in a wider range of communities.
289 citations
TL;DR: The results provide clear evidence that effects of mycorrhizal fungal diversity on productivity are context dependent and may be positive, negative or neutral depending on the situation considered.
Abstract: While there has been much recent interest about the relationships between plant diversity and plant productivity, much remains unknown about how the diversity of mycorrhizal fungi affects plant productivity. We investigated the effects of ectomycorrhizal fungal community composition and diversity on the productivity and growth characteristics of seedlings of two tree species (Pinus sylvetris and Betula pendula) as well as their interactions with each other. This involved setting up a mycorrhizal fungal diversity gradient from one to eight species using a design previously demonstrated to be able to separate diversity effects from compositional effects. We found that the eight mycorrhizal fungal species differed in their effects on seedling productivity and that the nature of effects was determined by the fertility of the substrate. Fungal species richness effects were also important in affecting seedling productivity over and above what could be explained by “sampling effect” but only in some situations. For B. pendula in a low fertility substrate there were clear positive causative effects between fungal species richness and productivity with the eight species treatment having over double the productivity of any of the eight monoculture treatments; no diversity effects were, however, detected in a high fertility substrate. For P. sylvestris in a high fertility substrate there were significant negative effects of fungal diversity on productivity while in a low fertility substrate no effects were apparent. The possible mechanistic bases for these results are discussed. The growth of P. sylvestris relative to that of B. pendula when grown in combination was unaffected by mycorrhizal treatments. Our results provide clear evidence that effects of mycorrhizal fungal diversity on productivity are context dependent and may be positive, negative or neutral depending on the situation considered.
246 citations
TL;DR: This article evaluates each of eight experiments from six recent publications in which the effect of varying plant diversity on the success of invasive species was investigated and proposes that the apparent discrepancy between the results of many observational and experimental studies at least in grasslands is because of the role of competitive dominants.
Abstract: Several recent studies have claimed to present experimental evidence from synthesised plant communities in which diversity was varied as a treatment that diversity reduces community invasibility by other plant species. This type of result contrasts from that of many observational studies which find diversity and invasibility to be positively correlated in nature, but some recent literature has claimed that these observational studies are confounded by extrinsic covarying factors while experimental studies are not. In this article I evaluate each of eight experiments from six recent publications in which the effect of varying plant diversity on the success of invasive species was investigated. In each case that invasibility was identified by the authors as being adversely affected by plant species richness, the result can be explained by factors that covaried with diversity in the experiment, most notably as a consequence of “sampling effect” (in which the most competitive species or species combination in the total species pool has a greater probability of occurring as species richness is increased), or through the incorrect use of statistical techniques. It is proposed that the apparent discrepancy between the results of many observational and experimental studies at least in grasslands is because: (1) in observational studies competitive dominant species are often associated with the most productive plots, and these dominants both reduce diversity through competitive exclusion of subordinates and competitively suppress invasive species; and (2) in recent experimental studies “sampling effect” results in the most competitive species (and therefore those most likely to suppress the invader) occurring with greater frequency as diversity is increased. Both observational and experimental studies therefore point to the role of competitive dominants in reducing invasibility, and in both situations species richness of the plant community need not be invoked as an explanation for the results.
233 citations
TL;DR: Seed mass, whether measured as dry mass or as N or P, appears to be the principal driver of variation in seed output per m 2 occupied, and consequently is among the most important dimensions of ecological variation across coexisting species.
Abstract: In patch-occupancy models for vegetation, propagule output per area occupied is a key species trait, influencing the potential to colonize vacant patches, and hence species dynamics and coexistence. We estimated seed output across a range of species and quantified its relationship to seed dry mass, seed N and P content, and accessory costs in fruiting structures. Fruiting and seed production data were obtained for 47 woody perennial species, spanning an almost 3000-fold range of seed mass, over a period of one year in Ku-ring-gai Chase National Park, New South Wales, Australia. Seed output was measured as numbers per m 2 canopy outline and per m 2 leaf area. Of cross-species variation in seed output per m 2 canopy outline per year. 72% could be predicted from seed mass alone, with a directly inverse relationship (log-log slope not significantly different from -- 1). Seed output per m 2 leaf area could be predicted somewhat more tightly (75%), indicating leaf area per canopy outline area accounted for some cross-species variation. Reproductive production per m 2 occupied per year varied much less than seed mass and accounted for the remaining variation in seed output. Although accessory costs were about equal in magnitude to seed mass as a component of aggregate investment per seed, they were strongly correlated with seed mass, and consequently did not acid substantially to the predictive power. Total mass of N or P per seed were found to be slightly but significantly better predictors of seed output variation than dry seed mass (83% and 78%, respectively). This supports the idea that mineral nutrients are a more fundamental currency for seed production than dry mass. Seed mass, whether measured as dry mass or as N or P, appears to be the principal driver of variation in seed output per m 2 occupied, and consequently is among the most important dimensions of ecological variation across coexisting species.
TL;DR: It is argued that the authors must go beyond measures of pollinator effectiveness to investigate pollinator-mediated fitness trade-offs over a range of floral phenotypes, and recognize that the effectiveness of pollinators is not tightly coupled to their importance in selecting for phenotypic novelty.
Abstract: The assumption that flowers readily evolve specializations for pollination by particular animals has been central to a standard view of pollinator-mediated adaptive divergence in angiosperms. Stebbins’ Most Effective Pollinator Principle (MEPP) formalized this assumption in proposing that a plant should always evolve specializations to its most effective pollinator. I argue that the MEPP and its corollaries are unsupported by basic models of phenotypic selection which predict that a plant should evolve greater specialization to a particular pollinator when the marginal fitness gain exceeds the marginal fitness loss from becoming less adapted to all other pollinators. Differences in pollinator effectiveness are neither necessary nor sufficient to predict specialization. Differences in effectiveness certainly can foster floral specialization to the most effective pollinator in some cases, but when adaptation to a relatively ineffective pollinator requires little loss in the fitness contribution of a more effective pollinator, plants may exhibit striking specializations for the less effective pollinator. Recognizing that the effectiveness of pollinators is not tightly coupled to their importance in selecting for phenotypic novelty may resolve the mismatch between floral features that appear to represent clear evolutionary responses to specific pollinators and patterns of flower visitation that often seem generalized. To shed light on agents of selection and the adaptive value of floral traits I argue that we must go beyond measures of pollinator effectiveness to investigate pollinator-mediated fitness trade-offs over a range of floral phenotypes.
TL;DR: This work maintains that PACF is not designed with biological populations in mind and has the wrong null model for detecting the structure of density dependence, and suggests an alternative diagnostic, the partial rate correlation function (PRCF), which is specifically designed for biological populations and has an appropriate nullmodel for detecting their density dependent structures.
Abstract: A central problem in ecology is explaining the causes of population fluctuations, and an important step in the solution is determining the structure of the negative feedback (density dependent) process regulating population dynamics. The conventional way to determine the dimension or order of density dependence in a time series is to calculate the partial autorcorrelation function (PACF). We maintain, however, that PACF is not designed with biological populations in mind and has the wrong null model for detecting the structure of density dependence. We suggest an alternative diagnostic, the partial rate correlation function (PRCF), which is specifically designed for biological populations and has an appropriate null model for detecting their density dependent structures. Tests with simulated data show PRCF to be superior to PACF in detecting the underlying density dependent structure of two simple mathematical models.
TL;DR: In this article, the authors point out that there are several very general law-like propositions that provide the theoretical basis for most population dynamics models that were developed to address specific issues, such as population self-limitation and resource-consumer oscillations.
Abstract: There is a widespread opinion among ecologists that ecology lacks general laws. In this paper I argue that this opinion is mistaken. Taking the case of population dynamics, I point out that there are several very general law-like propositions that provide the theoretical basis for most population dynamics models that were developed to address specific issues. Some of these foundational principles, like the law of exponential growth, are logically very similar to certain laws of physics (Newton's law of inertia, for example, is almost a direct analogue of exponential growth). I discuss two other principles (population self-limitation and resource-consumer oscillations), as well as the more elementary postulates that underlie them. None of the “laws” that I propose for population ecology are new. Collectively ecologists have been using these general principles in guiding development of their models and experiments since the days of Lotka, Volterra, and Gause.
TL;DR: Predicting how female-biased natal dispersal may differentially affect the extinction risk of populations and species with contrasting distributions, migratory behaviours, life histories and mating systems finds vulnerability to extinction should be greater for migratory than for resident species and greater for short-lived than for long-lived species.
Abstract: Small and isolated populations are usually assumed to be at a high risk of extinction due to environmental or demographic stochasticity, genetic problems, or too little immigration. In birds, natal dispersal is usually female-biased, but the consequences of such a pattern on vulnerability to extinction of isolated populations has not received much attention before. In this paper I derive predictions as to how female-biased natal dispersal may differentially affect the extinction risk of populations and species with contrasting distributions, migratory behaviours, life histories and mating systems. Female-biased dispersal will lead to male-biased sex ratios in small, isolated or fragmented populations, in particular because recent research has shown that females often have a limited ability to search for mates and may therefore effectively be lost from the breeding population if they disperse into areas empty of conspecifics. I reviewed published studies on birds and found that a high proportion of unpaired males is common in isolated populations or populations in small habitat fragments. Dispersal of females may therefore increase the vulnerability to extinction of small or isolated populations, or populations at the periphery of a species’ distribution range. I also predict that vulnerability to extinction should be greater for migratory than for resident species and greater for short-lived than for long-lived species because of differences in the time available for females to locate unpaired males. Further, extinction risk may also be greater for birds than for mammals due to differences in which sex disperses and patterns of parental care. Finally, mating system will also affect vulnerability to extinction when natal dispersal leads to biased sex ratios. I review available evidence for these predictions (e.g. songbird declines in North America) and discuss implications for conservation.
TL;DR: It is concluded that aquatic insects had a bottom-up effect on spiders and that this subsidy facilitated a top-down effect that cascaded from spiders to leafhoppers to plants.
Abstract: Rivers produce an abundance of aquatic insects that traverse land, where they can have bottom-up effects on predators, who, in turn, can have top-down effects on terrestrial herbivores. This effect can cascade down to plants. These trophic relationships were demonstrated in a field of stinging nettles, Urtica dioica, along a river in Germany. At the shore compared to similar microhabitats 30-60 m away the abundance and biomass of: midges were highest, spiders were also highest, while herbivorous leafhoppers were lowest. At the shore, nettle plants were less damaged by herbivores and thus had less regrowth. Spiders regularly captured both aquatic midges as well as terrestrial leafhoppers and they captured more individuals of both groups at the shore than further away. Midges supported high densities of shore spiders. This was inferred from correlation of distribution and diet in the absence of other environmental gradients. Removal of spiders from experimental plots caused leafhoppers to increase at the shore, causing more plant damage. These effects were not evident at spider-removal sites away from the shore. This demonstrated that spiders depressed leafhoppers and decreased herbivory on plants only at the shore. It is concluded that aquatic insects had a bottom-up effect on spiders and that this subsidy facilitated a top-down effect that cascaded from spiders to leafhoppers to plants. Similar effects would explain the distribution of arthropods along many rivers. Allochthony connects river food webs with shore food webs, making both components essential for each other.
TL;DR: The results in the tomato system support the plant vigor hypothesis that predicts positive association between insect performance and plant growth and contradict the plant stress hypothesis that rank stressed plants as better hosts for insects.
Abstract: We conducted a controlled experiment to test the plant vigor and the plant stress hypotheses. The two hypotheses associate plant physiological conditions to insect feeding mode and performance. We exposed tomato, Lycopersicon esculentum ,t o different types of growing conditions: optimal (vigorous plants), resource based stress (water and/or nutrient deficit), and physical stress (punched hole in terminal leaflets). Plant performance, foliar nutritional value for insects and chemical defenses were analyzed after 14 d. These plants were offered to insects belonging to distinct feeding guilds: the silverleaf whitefly, Bemisia argentifolii, a phloem feeder; the leafminer, Liriomyza trifolii; and the corn earworm, Heliothis zea, a leaf chewing caterpillar. The experimental conditions generated a gradient of plant growth in the following order: optimal (vigorous)control =hole punchedno fertilizerno waterno water and no fertilizer. The last two treatments resulted in plants with poor nutritional value (based on %water, C/N, %N) and higher levels of defensive compounds (i.e., peroxidase and total phenolics) compared with control and the vigorous plants. Hole-punching neither affected plant growth nor any of the phytochemicals measured. In a choice experiment adult whitefly ovipositioning was not affected by either vigor or punching but was reduced on the other plants (P0.01). Leafminer feeding and oviposition and corn earworm larval growth rates were higher on the vigorous plants and lower on the punched, no fertilizer, no water, and no water and no fertilizer host plants (P0.01). Regardless of insect species or bioassay method, the results in the tomato system support the plant vigor hypothesis that predicts positive association between insect performance and plant growth. The results contradict the plant stress hypothesis that rank stressed plants as better hosts for insects. The mechanisms involved are a combination of poor nutritional value and chemical defenses. We demonstrate a negative association between plant growth and chemical defense. However, induced response triggered by hole-punching was not cost effective to the plants.
TL;DR: It is argued that much of this discussion of what causes the decline in productivity with stand age has taken place at the wrong scale, that proximate and ultimate causes have been confused, and that an essential perspective has been ignored.
Abstract: The decline in the productivity of forests after a period of growth has been much discussed in recent years, with numerous research papers and several review articles on the topic. Most of these articles have focussed on alternative physiological mechanisms to explain this phenomenon (Gower et al. 1996, Ryan and Yoder 1997, Ryan et al. 1997, Smith and Resh 1999, Magnani et al. 2000). Proposed mechanisms include (a) an altered balance between photosynthetic and respiring tissues, (b) decreasing soil nutrient availability, (c) reduced photosynthetic rates due to increasing limitations to water transport, and (d) changes in allocation. At the recent meeting of the Ecological Society of America (5-10 Aug. 2000, Snowbird, Utah), there was a well-attended discussion entitled "What DOES [emphasis theirs] cause age-related decline in forest productivity?" Behind many of the discussions lies the hope that if we can understand what causes the decline in productivity with stand age, perhaps we can do something about it. We contend that much of this discussion has taken place at the wrong scale, that proximate and ultimate causes have been confused, and that an essential perspective has been ignored. This perspective is simply that trees, like all organisms, exhibit sigmoid growth curves, and that ageand/or size-dependence of growth rates is an expression of this pattern. In this note we present three fundamental and related points which we believe have not been appropriately considered in the discussions to date:
TL;DR: The results suggest that benthic prey populations should be only moderately depressed until predator fish abundance grows to a critical biomass at which benthos collapses due to overexploitation, and that the model may capture the essence of the mechanism causing a discontinuous effect of benthivorous fish on lake ecosystems.
Abstract: Introduction of the benthivorous common carp (Cyprinus carpio) has been identified as one of the main causes of loss of biodiversity and water clarity in numerous shallow lakes and ponds worldwide. Recent observations in experimental fish ponds suggest that the effect of carp on the ecosystem is catastrophic in the sense that a substantial impact occurs only when a critical carp density is exceeded. In search for an explanation, we analyzed a simple model of the interaction between benthivorous fish and their invertebrate benthic prey, and of sediment resuspension resulting from fish feeding behavior. Our results suggest that benthic prey populations should be only moderately depressed until predator fish abundance grows to a critical biomass at which benthos collapses due to overexploitation. This drop in prey density is predicted to result in a sharp increase in water turbidity due to an increase in prey search activity of the fish. For less eutrophic and deeper lakes, where benthos productivity and hence benthivorous fish carrying capacity are lower, water turbidity is predicted to be much less affected. The qualitative patterns are quite robust against assumptions on parameter values and correspond closely to the experimental results and data from lakes suggesting that the model may capture the essence of the mechanism causing a discontinuous effect of benthivorous fish on lake ecosystems.
TL;DR: Methods for estimating statistical fit (magnitude of effect) for individual ANOVA factors based on variance components are reviewed and examples of their application to field data are provided and simple remediation procedures are described.
Abstract: Although analysis of variance (ANOVA) is widely used by ecologists, the full potential of ANOVA as a descriptive tool has not been realized in most ecological studies. As questions addressed by ecologists become more complex, and experimental and sampling designs more complicated, it is necessary for ecologists to estimate both statistical significance and fit when comparing the relative importance of individual factors in an explanatory model, especially when models are multi-factorial. Yet, with few exceptions, ecologists are only presenting significance values with ANOVA results. Here we review methods for estimating statistical fit (magnitude of effect) for individual ANOVA factors based on variance components and provide examples of their application to field data. Furthermore, we detail the potential occurrence of negative variance components when determining magnitude of effects in ANOVA and describe simple remediation procedures. The techniques we advocate are efficient and will greatly enhance analyses of a wide variety of ANOVA models used in ecological studies. Estimation of magnitude of effects will particularly benefit the analysis of complex multi-factorial ANOVAs where emphasis is on interpreting the relative importance of many individual factors.
TL;DR: Although the aspic viper is a typical “capital breeder” in terms of its reliance on stored reserves for maternal “decisions” concerning reproductive frequency, it is to some degree a facultative “income breeder" with respect to the determination of offspring size and litter size.
Abstract: Feeding rates influence reproductive output in many kinds of animals, but we need to understand the timescale of this influence before we can compare reproductive energy allocation to energy intake. A central issue is the extent to which reproduction is fuelled by long-term energy stores (“capital” breeding) versus recently-acquired resources (“income” breeding). Our data on free-living aspic vipers show that there is no simple answer to this question: reproductive frequency is determined by long-term energy stores, offspring size is influenced by maternal food intake immediately before ovulation, and litter size is influenced by both long-term stores and short-term energy acquisition. Thus, offspring size in free-living vipers reflects the mother's energy balance over the preceding year (via a trade-off between litter size and offspring size) as well as her energy balance in the current breeding season. Hence, different components of a given reproductive output (litter) are not only functionally linked, but also respond to different temporal scales of prey availability. A female's body size has little effect on her reproductive output. Attempts to quantify reproductive energy allocation must take into account the fact that different reproductive traits (such as offspring size versus number) may respond to energy availability over different timespans. Thus, although the aspic viper is a typical “capital breeder” in terms of its reliance on stored reserves for maternal “decisions” concerning reproductive frequency, it is to some degree a facultative “income breeder” with respect to the determination of offspring size and litter size.
TL;DR: Acknowledging the dearth of empirical synthesis, two contrasting literature surveys are employed to determine whether the plant–insect herbivore–natural enemy literature is currently adequate to form a conceptual synthesis of the relative roles of top-down and bottom-up forces.
Abstract: Whether resources (bottom-up forces), natural enemies (top-down forces), or both, determine the abundance of insect herbivore populations in plant–insect herbivore–natural enemy systems remains a major issue in population ecology. Unlike recent surveys of the tritrophic literature we do not seek to quantify whether top-down or bottom-up forces predominate in any given set of experimental systems. Acknowledging the dearth of empirical synthesis we employ two contrasting literature surveys to determine whether the plant–insect herbivore–natural enemy literature is currently adequate to form a conceptual synthesis of the relative roles of top-down and bottom-up forces.
The emergence of a synthesis of the relative roles of top-down and bottom-up forces in plant–insect herbivore–natural enemy systems appears to have been largely prevented by (1) the absence of appropriate empirical data; (2) failure to appreciate the merits of existing data; (3) a continued desire to emphasise either top-down or bottom-up forces to the exclusion of the other; and (4) confusion regarding which processes regulate and which influence the abundance of insect herbivores.
TL;DR: It is documented that both treefrogs and Cascades frogs show plasticity in hatching characteristics in response to the threat of egg predation, which may reduce predation in populations that experience high variability in predation pressure between years.
Abstract: The timing of transitions between life history stages should be affected by factors that influence survival and growth of organisms in adjacent life history stages. In a series of laboratory experiments, we examined the influence of predation risk as a cue to trigger a life history switch in amphibians. In the Oregon Cascade Mountains, some populations of Pacific treefrogs (Hyla regilla) and Cascades frogs (Rana cascadae) are under intense egg predation by predatory leeches (families Glossiphonidae and Erpobdellidae). We document that both treefrogs and Cascades frogs show plasticity in hatching characteristics in response to the threat of egg predation. Pacific treefrogs hatch sooner and at an earlier developmental stage when either predatory leeches or non-predatory earthworms are allowed direct contact with the developing egg mass. The same response is elicited even without direct contact. Chemical cues of predatory leeches and chemicals released from injured eggs appear to elicit the same early hatching response in treefrogs. For Cascades frogs, cues of leeches, but not those of injured eggs, elicit an early hatching response. Hatching early in response to egg predators may reduce predation. Plasticity of hatching characteristics has rarely been examined. However, we suspect that it may be common, particularly in populations or species that experience high variability in predation pressure between years.
TL;DR: There is a high potential for developing an individual-based, size-dependent community theory using Physiologically structured population models, which offer a much higher degree of realism, precision and testing ability than lumped stage-based or non-structured models.
Abstract: A ubiquitous feature of natural communities is the variation in size that can be observed between organisms, a variation that to a substantial degree is intraspecific. Size variation within species by necessity implies that ecological interactions vary both in intensity and type over the life cycle of an individual. Physiologically structured population models (PSPMs) constitute a modelling approach especially designed to analyse these size-dependent interactions as they explicitly link individual level processes such as consumption and growth to population dynamics. We discuss two cases where PSPMs have been used to analyse the dynamics of size-structured populations. In the first case, a model of a size-structured consumer population feeding on a non-structured prey was successful in predicting both qualitative (mechanisms) and quantitative (individual growth, survival, cycle amplitude) aspects of the population dynamics of a planktivorous fish population. We conclude that single generation cycles as a result of intercohort competition is a general outcome of size-structured consumer–resource interactions. In the second case, involving both cohort competition and cannibalism, we show that PSPMs may predict double asymptotic growth trajectories with individuals ending up as giants. These growth trajectories, which have also been observed in field data, could not be predicted from individual level information, but are emergent properties of the population feedback on individual processes. In contrast to the size-structured consumer–resource model, the dynamics in this case cannot be reduced to simpler lumped stage-based models, but can only be analysed within the domain of PSPMs. Parameter values used in PSPMs adhere to the individual level and are derived independently from the system at focus, whereas model predictions involve both population level processes and individual level processes under conditions of population feedback. This leads to an increased ability to test model predictions but also to a larger set of variables that is predicted at both the individual and population level. The results turn out to be relatively robust to specific model assumptions and thus render a higher degree of generality than purely individual-based models. At the same time, PSPMs offer a much higher degree of realism, precision and testing ability than lumped stage-based or non-structured models. The results of our analyses so far suggest that also in more complex species configurations only a limited set of mechanisms determines the dynamics of PSPMs. We therefore conclude that there is a high potential for developing an individual-based, size-dependent community theory using PSPMs.
TL;DR: Peres-Neto et al. as discussed by the authors proposed an analytical protocol for incorporating habitat suitability as an occupancy criterion in null models, and validated this approach by showing that type I error is not affected by the use of probabilities as a site occupancy criterion.
Abstract: Peres-Neto, P. R., Olden, J. D. and Jackson, D. A. 2001. Environmentally con-strained null models: site suitability as occupancy criterion. – Oikos 93: 110–120.Null models have proven to be an important quantitative tool in the search forecological processes driving local diversity and species distribution. However, thereremains an important concern that different processes, such as environmental condi-tions and biotic interactions may produce similar patterns in species distributions. Inthis paper we present an analytical protocol for incorporating habitat suitability as anoccupancy criterion in null models. Our approach involves modeling species presenceor absence as a function of environmental conditions, and using the estimatedsite-specific probabilities of occurrence as the likelihood of species occupancy of a siteduring the generation of ‘‘null communities’’. We validated this approach by showingthat type I error is not affected by the use of probabilities as a site occupancycriterion and is robust against a variety of predictive performances of the species-en-vironmental models. We describe the expected differences when contrasting classicaland the environmentally constrained null models, and illustrate our approach with adata set of Dutch dune hunting spider assemblages. An environmentally constrainedapproach to null models will provide a more robust evaluation of species associationsby facilitating the distinction between mutually exclusive processes that may shapespecies distributions and community assembly.
TL;DR: This paper measured the albedo of the sand on different beaches, i.e. the percentage of the incident solar radiation that was reflected from the sand surface, and also measured the temperature of sand at nest depths.
Abstract: At Ascension Island and Cyprus, major nesting areas for green turtles (Chelonia mydas) in the Atlantic and Mediterranean, respectively, visual inspection shows some beaches are light in colour while others are darker. We objectively measured the albedo of the sand on different beaches, i.e. the percentage of the incident solar radiation that was reflected from the sand surface. At sites where albedo was recorded, we also measured the temperature of the sand at nest depths. At both rookeries, the sand temperature was markedly higher on darker beaches due to greater absorption of the incident solar radiation over the diurnal cycle. Temperature loggers buried at nest depths revealed seasonal changes in temperature on both islands, but showed that the lowest temperatures found on the darker beaches rarely dropped below the highest temperatures on the lighter beaches. Sea turtles exhibit temperature-dependent sex determination. Since sand albedo is a major avenue for the production of a range of incubation temperatures on both islands, it will also have profound implications for hatchling sex ratios. In comparison with both Ascension Island and Cyprus, for samples collected from sea turtle rookeries around the world there was an even greater range in sand albedo values. This suggests that sand albedo, a factor that has previously received little consideration, will have profound implications for nest temperatures, and hence hatchling sex ratios, for other populations and species.
TL;DR: It is hypothesized that the preferential predation of large A. mexicanum nuts by H. desmarestianus is a response by these rodents to insect predation, and nuts attacked by insects develop to be significantly smaller, with a low proportion of undamaged endosperm, than uninfested nuts.
Abstract: Seed size may vary greatly among individuals within plant species. What effects the extremes of this variation have for seeds taken by small mammals are poorly understood. Not all seeds removed by small mammals are necessarily eaten. Small rodents are common seed predators, but they may disperse a significant proportion of seeds by scatter hoarding them via burial. Size-dependent predation and dispersal of seeds has not been directly tested within a plant species for tropical rodents. This study tested whether or not large and small nuts of Astrocaryum mexicanum (Palmae) differed in their fates due to handling by the spiny pocket mouse Heteromys desmarestianus (Heteromyidae). Exclosures were used to give small rodents exclusive access to A. mexicanum nuts. H. desmarestianus preferentially consumed large over small A. mexicanum nuts, but cached (in burrows and by scatter hoarding) similar proportions of these nuts by size. Small nuts tended to be buried farther away from exclosures than large nuts. Although sample sizes of buried nuts were small, the rodents retrieved all buried large nuts, but 30% of the small nuts remained buried long enough to germinate. I also examined predispersal predation by insects and found that insects appear to have no size preference for A. mexicanum nuts, but insect predation appears to hinder nut development. Thus, nuts attacked by insects develop to be significantly smaller, with a low proportion of undamaged endosperm, than uninfested nuts. It is hypothesized that the preferential predation of large A. mexicanum nuts by H. desmarestianus is a response by these rodents to insect predation.
TL;DR: The results suggest that APH is the main mechanism linking lemming cycles and goose nesting success and that nesting associations during peak le mming years (NAH) can enhance this positive link at the local level.
Abstract: The suggested link between lemming cycles and reproductive success of arctic birds is caused by potential effects of varying predation pressure (the Alternative Prey Hypothesis, APH) and protective association with birds of prey (the Nesting Association Hypothesis, NAH). We used data collected over two complete lemming cycles to investigate how fluctuations in lemming density were associated with nesting success of greater snow geese (Anser caerulescens atlanticus) in the Canadian High Arctic. We tested predictions of the APH and NAH for geese breeding at low and high densities. Goose nesting success varied from 22% to 91% between years and the main egg predator was the arctic fox (Alopex lagopus). Nesting associations with snowy owls (Nyctea scandiaca) were observed but only during peak lemming years for geese nesting at low density. Goose nesting success declined as distance from owls increased and reached a plateau at 550 m. Artificial nest experiments indicated that owls can exclude predators from the vicinity of their nests and thus reduce goose egg predation rate. Annual nest failure rate was negatively associated with rodent abundance and was generally highest in low lemming years. This relationship was present even after excluding goose nests under the protective influence of owls. However, nest failure was inversely density-dependent at high breeding density. Thus, annual variations in nest density influenced the synchrony between lemming cycles and oscillations in nesting success. Our results suggest that APH is the main mechanism linking lemming cycles and goose nesting success and that nesting associations during peak lemming years (NAH) can enhance this positive link at the local level. The study also shows that breeding strategies used by birds (the alternative prey) could affect the synchrony between oscillations in avian reproductive success and rodent cycles.
TL;DR: The effects of recruitment limitation on ecosystem functioning illustrate possible effects of diversity loss not captured by synthetic experiments in which diversity gradients are created by adding high densities of seeds to bare soil.
Abstract: A five-year removal experiment in which plant functional group diversity was manipulated found strong limitation of ecosystem functioning caused by the differing abilities of remaining functional groups to recruit into space left unoccupied by the plants removed. We manipulated functional group diversity and composition by removing all possible combinations of zero, one, or two plant functional groups (forbs C 3 graminoids, and C 4 graminoids), as well as randomly chosen biomass at levels corresponding to the functional group removals, from a prairie grassland community. Although random biomass removal treatments showed no significant effect of removing biomass in general on ecosystem functions measured (P > 0.05), the loss of particular functional groups led to significant differences in above-(P < 0.001) and belowground (P < 0.001) biomass, rooting-zone (P = 0.001) and leached (P = 0.01) nitrogen, nitrogen mineralization (P < 0.001), and community drought resistance (P = 0.002). Many of these differences stemmed from the marked difference in the ways remaining functional groups responded to the experimental removals Strong recruitment limitation of C 4 graminoids resulted in large areas of open ground, high nutrient leaching, and high community drought resistance in plots containing just this functional group. In contrast, rhizomatous C, graminoids quickly colonized space and used soil resources made available by the removal of other groups leading to lower soil nitrate in plots containing C 3 graminoids These effects of recruitment limitation on ecosystem functioning illustrate possible effects of diversity loss not captured by synthetic experiments in which diversity gradients are created by adding high densities of seeds to bare soil.
TL;DR: There is a competition for the mutualistic services of ants between aphid species exploiting the same resource and the outcome of this competition depends on the aphid's selective attractiveness to ants, which corresponded with already reported differences in honeydew quality and quantity.
Abstract: Workers of the ant species Lasius niger showed marked preferences when collecting honeydew from three aphid species living on tansy, Tanacetum vulgare. Colonies of Metopeurum fuscoviride were visited most intensively, Brachycaudus cardui had an intermediate position, and colonies of Aphis fabae were visited least intensively. L. niger abandoned colonies of the less preferred species as soon as colonies of a higher-ranked species were available on neighbouring tansy plants. As a consequence, the abandoned colonies suffered a higher predation by aphid predators and persisted for a shorter time than attended colonies of the same species. If two aphid species were present in a mixed colony on the same shoot, the preferred species increased in numbers, while the less preferred species decreased due to predation by L. niger. The results show that there is a competition for the mutualistic services of ants between aphid species exploiting the same resource. The outcome of this competition depends on the aphid's selective attractiveness to ants, which corresponded with already reported differences in honeydew quality and quantity.