Bottom-up effects may not reach the top: the influence of ant-aphid interactions on the spread of soil disturbances through trophic chains.

Soil disturbances that increase nutrient availability may trigger bottom-up cascading effects along trophic chains. However, the strength and sign of these effects may depend on attributes of the interacting species. Here, we studied the effects of nutrient-rich refuse dumps of the leaf-cutting ant, Acromyrmex lobicornis, on the food chain composed of thistles, aphids, tending ants and aphid natural enemies. Using stable isotopes tracers, we show that the nitrogen accumulated in refuse dumps propagates upward through the studied food chain. Thistles growing on refuse dumps had greater biomass and higher aphid density than those growing in adjacent soil. These modifications did not affect the structure of the tending ant assemblage, but were associated with increased ant activity. In contrast to the expectations under the typical bottom-up cascade effect, the increase in aphid abundance did not positively impact on aphid natural enemies. This pattern may be explained by both an increased activity of tending ants, which defend aphids against their natural enemies, and the low capacity of aphid natural enemies to show numerical or functional responses to increased aphid density. Our results illustrate how biotic interactions and the response capacity of top predators could disrupt bottom-up cascades triggered by disturbances that increase resource availability.

Abundance, reproduction, and seed predation of an alpine plant decrease from the center toward the range limit.

Biogeographic models predict that, because of increasingly unfavorable and stressful conditions, populations become less frequent, smaller, less dense, and less reproductive toward the range edges. These models have greatly influenced the thinking on geographical range limits and have broad implications for ecology, evolution, and conservation. However, empirical tests of the models have rarely investigated comprehensive sets of population properties. We studied population size and density and a broad set of fitness-related traits in 66 populations of the alpine thistle Carduus defloratus along a latitudinal (615 km) and altitudinal (342-2300 m) gradient from the European Alps in the south to the northern range limit in the low mountain ranges of central Germany. Regression analysis indicated that population size and plant density declined with decreasing altitude from the center to the range margin, but plant size increased. In spite of the larger size of plants, the number of seeds produced strongly declined toward the range margin, mainly due to an increase in seed abortion. The number of flowering plants in a population influenced all components of reproduction. Plants in large populations initiated more seeds, aborted fewer seeds, and produced more and larger seeds per plant. The probability that seeds were attacked by insect larvae and the proportion of seeds damaged decreased strongly from the center to the margin of the distribution. However, in spite of the much lower level of parasitization, plants at the range margin produced far fewer viable seeds. Fluctuating asymmetry of leaf width, an indicator of developmental instability, was similar across the range and not related to population size.

Integrating multiple disturbance aspects: management of an invasive thistle, Carduus nutans.

BACKGROUND AND AIMS: Disturbances occur in most ecological systems, and play an important role in biological invasions. We delimit five key disturbance aspects: intensity, frequency, timing, duration and extent. Few studies address more than one of these aspects, yet interactions and interdependence between aspects may lead to complex outcomes. METHODS: In a two-cohort experimental study, we examined how multiple aspects (intensity, frequency and timing) of a mowing disturbance regime affect the survival, phenology, growth and reproduction of an invasive thistle Carduus nutans (musk thistle). KEY RESULTS: Our results show that high intensity and late timing strongly delay flowering phenology and reduce plant survival, capitulum production and plant height. A significant interaction between intensity and timing further magnifies the main effects. Unexpectedly, high frequency alone did not effectively reduce reproduction. However, a study examining only frequency and intensity, and not timing, would have erroneously attributed the importance of timing to frequency. CONCLUSIONS: We used management of an invasive species as an example to demonstrate the importance of a multiple-aspect disturbance framework. Failure to consider possible interactions, and the inherent interdependence of certain aspects, could result in misinterpretation and inappropriate management efforts. This framework can be broadly applied to improve our understanding of disturbance effects on individual responses, population dynamics and community composition.

Pollinator Behavior Mediates Negative Interactions between Two Congeneric Invasive Plant Species.

Simultaneously flowering plant species may indirectly interact with each other by influencing the quantity of pollinator visitation and/or the quality of pollen that is transferred. These effects on pollination may depend on how pollinators respond to floral resources at multiple levels. In this study, we demonstrate pollinator-mediated negative interactions between two invasive plants, Carduus acanthoides and Carduus nutans. Using constructed arrays of the two species, alone and in mixture, we quantified pollinator visitation at the patch and individual plant levels and measured seed production. We found that co-occurrence of our species led to a shift in pollinator services at both levels. Greater interference occurred when arrays were small and spacings between neighboring plants were large. A spatially explicit movement model suggests that pollinator foraging behavior, which mediates the interactions between plants, was driven by floral display size rather than species identity per se. Pollinator behavior significantly reduced the proportion of seed set for both species relative to that in single-species arrays. Overall, the dependence of pollinator behavior on patch size, spacing between plants, and patch composition can lead to pollinator-mediated plant interactions that range from facilitative to competitive.

Importance of individual and environmental variation for invasive species spread: a spatial integral projection model.

Plant survival, growth, and flowering are size dependent in many plant populations but also vary among individuals of the same size. This individual variation, along with variation in dispersal caused by differences in, e.g., seed release height, seed characteristics, and wind speed, is a key determinant of the spread rate of species through homogeneous landscapes. Here we develop spatial integral projection models (SIPMs) that include both demography and dispersal with continuous state variables. The advantage of this novel approach over discrete-stage spread models is that the effect of variation in plant size and size-dependent vital rates can be studied at much higher resolution. Comparing Neubert-Caswell matrix models to SIPMs allowed us to assess the importance of including individual variation in the models. As a test case we parameterized a SIPM with previously published data on the invasive monocarpic thistle Carduus nutans in New Zealand. Spread rate (c*) estimates were 34% lower than for standard spatial matrix models and stabilized with as few as seven evenly distributed size classes. The SIPM allowed us to calculate spread rate elasticities over the range of plant sizes, showing the size range of seedlings that contributed most to c* through their survival, growth and reproduction. The annual transitions of these seedlings were also the most important ones for local population growth (lambda). However, seedlings that reproduced within a year contributed relatively more to c* than to lambda. In contrast, plants that grow over several years to reach a large size and produce many more seeds, contributed relatively more to lambda than to c*. We show that matrix models pick up some of these details, while other details disappear within wide size classes. Our results show that SIPMs integrate various sources of variation much better than discrete-stage matrix models. Simpler, heuristic models, however, remain very valuable in studies where the main goal is to investigate the general impact of a life history stage on population dynamics. We conclude with a discussion of future extensions of SIPMs, including incorporation of continuous time and environmental drivers.

Mycorrhizal densities decline in association with nonnative plants and contribute to plant invasion.

Belowground interactions between herbaceous native species and nonnative species is a poorly understood but emerging area of interest to invasive-species researchers. Positive feedback dynamics are commonly observed in many invaded systems and have been suspected in California grasslands, where native plants associate strongly with soil mutualists such as arbuscular mycorrhizal fungi. In response to disturbance, invading nonnative plants proliferate, and to the degree these species associate weakly with soil mutualists, we would expect mutualist efficacy to degrade over time. Degraded mutualist efficacy would negatively impact mutualist-dependent native species or their recruitment following a disturbance. We investigated the feedback dynamics of soil conditioned both with native and nonnative herbaceous communities of southern California grasslands to test this degraded mutualist hypothesis. Using a mesocosm approach, we inoculated each community with live soil originating from a remnant native grassland and varied the plant communities (i.e., native or nonnative) along a plant-species-richness gradient. After one year, we then used this conditioned soil for reciprocal feedback tests on a native and nonnative indicator species. We show that a native herbaceous forb (Gnaphalium californicum) grows best in soil conditioned by a diverse mix of other native species that includes G. californicum but is inhibited by soil conditioned by a diverse mix of nonnative species. We also show that an invasive, nonnative herbaceous forb (Carduus pycnocephalus) exhibits strong growth in soil lacking arbuscular mycorrhizal fungi and in soil conditioned by a diverse mix of nonnative species that include C. pycnocephalus, and that it is inhibited by the same soil that best promotes the native, G. californicum. Separate bioassays for mycorrhizal density show a reduction of arbuscular mycorrhizal fungi in the nonnative-conditioned soil relative to the native-conditioned soil, which suggests that nonnative species do not promote the growth of mycorrhizal fungi in the same way that native species do. The growth patterns resulting from the vegetative history of these distinct soil communities provide evidence of a biotic feedback mechanism that may account for the maintenance of persistent communities of nonnative (and often invasive) plants ubiquitous throughout California grasslands.

A model for plant invasions: the role of distributed generation times.

An analytical model consisting of adult plants and two types of seeds (unripe and mature) is considered and successfully tested using experimental data available for some invasive weeds (Echium plantagineum, Cytisus scoparius, Carduus nutans andCarduus acanthoides) from their native and exotic ranges. The model accounts for probability distribution functions (pdfs) for times of germination, growth, death and dispersal on two dimensions, so the general life-cycle of individuals is considered with high level of description. Our work provides for the first time, for a model containing all that life-cycle information, explicit relationship conditions for the invasive success and expressions for the speed of invasive fronts, which can be useful tools for invasions assessment. The expressions derived allow us to prove that the different phenotypes showed by the weeds in their native (exotic) ranges can explain their corresponding non-invasive (invasive) behavior.

On net reproductive rate and the timing of reproductive output.

Abstract: Understanding the relationship between life-history patterns and population growth is central to demographic studies. Here we derive a new method for calculating the timing of reproductive output, from which the generation time and its variance can also be calculated. The method is based on the explicit computation of the net reproductive rate (R0) using a new graphical approach. Using nodding thistle, desert tortoise, creeping aven, and cat's ear as examples, we show how R0 and the timing of reproduction is calculated and interpreted, even in cases with complex life cycles. We show that the explicit R0 formula allows us to explore the effect of all reproductive pathways in the life cycle, something that cannot be done with traditional analysis of the population growth rate (lambda). Additionally, we compare a recently published method for determining population persistence conditions with the condition R0 > 1 and show how the latter is simpler and more easily interpreted biologically. Using our calculation of the timing of reproductive output, we illustrate how this demographic measure can be used to understand the effects of life-history traits on population growth and control.

Dispersal patterns, dispersal mechanisms, and invasion wave speeds for invasive thistles.

Understanding and predicting population spread rates is an important problem in basic and applied ecology. In this article, we link estimates of invasion wave speeds to species traits and environmental conditions. We present detailed field studies of wind dispersal and compare nonparametric (i.e., data-based) and mechanistic (fluid dynamics model-based) dispersal kernel and spread rate estimates for two important invasive weeds, Carduus nutans and Carduus acanthoides. A high-effort trapping design revealed highly leptokurtic dispersal distributions, with seeds caught up to 96 m from the source, far further than mean dispersal distances (approx. 2 m). Nonparametric wave speed estimates are highly sensitive to sampling effort. Mechanistic estimates are insensitive to sampling because they are obtained from independent data and more useful because they are based on the dispersal mechanism. Over a wide range of realistic conditions, mechanistic spread rate estimates were most sensitive to high winds and low seed settling velocities. The combination of integrodifference equations and mechanistic dispersal models is a powerful tool for estimating invasion spread rates and for linking these estimates to characteristics of the species and the environment.

Influence of microsite disturbance on the establishment of two congeneric invasive thistles.

The successful establishment of invasive species has been shown to depend on aspects of the invaded community, such as gap characteristics. Biotic resistance may be particularly critical for stopping invaders at early life history stages, but new species can often invade following disturbances, which may create microsites with very different characteristics than are usually present. We examine the response of two invasive thistle species, Carduus nutans L. and C. acanthoides L., to three different microsite characteristics: disturbance type, size, and water availability. The two species initially responded differently to the type of disturbance: C. acanthoides had higher emergence and survival in plots with both above- and belowground disturbance, whereas C. nutans had better early performance in large microsites with above-ground disturbance only. Later in their life cycle, C. nutans performed better in plots that had been disturbed both above- and belowground, whereas C. acanthoides was largely unaffected by disturbance type. Increased emergence and survival, larger size and a higher proportion flowering were observed in larger gaps for both species throughout the life cycle. Watering had a negative impact on C. nutans emergence and fall survival and on C. acanthoides survival to the following summer. Overall, these results suggest that disturbance-generated microsite characteristics (disturbance type and size) may have large impacts on establishment of these two Carduus species, which in turn may persist well beyond the initial stages of growth. Studying invader responses to disturbance can help us to understand under what circumstances they are likely to establish and create persistent problems; avoiding or ameliorating such situations will have significant management benefits.

Decreased structural defence of an invasive thistle under warming.

Plant structural defences play a key role in preventing fitness loss due to herbivory. However, how structural defences are affected by potential climate change is rarely examined. We examined how leaf morphological traits that relate to the structural defence of an invasive thistle, Carduus nutans, change in a warmer climate. We manipulated warming using open-top chambers (OTCs) and examined the morphology of leaves at three different positions (the 5th, 10th and 15th leaves, counted from the top of the plant) in two destructive summer censuses. We found that structural defence traits were different under ambient versus warmed conditions. Prickle densities (both the number of prickles per leaf area and the number of prickles per leaf mass) were significantly lower in plants grown in a warmer climate. Our results suggest that plant structural defences may be reduced under warming, and therefore should be considered when examining species' responses to climate change.

Can soil seed banks serve as genetic memory? A study of three species with contrasting life history strategies.

We attempted to confirm that seed banks can be viewed as an important genetic reservoir by testing the hypothesis that standing (aboveground) plants represent a nonrandom sample of the seed bank. We sampled multilocus allozyme genotypes from three species with different life history strategies: Amaranthus retroflexus, Carduus acanthoides, Pastinaca sativa. In four populations of each species we analysed the extent to which allele and genotype frequencies vary in consecutive life history stages including the summer seed bank, which has been overlooked up to now. We compared the winter seed bank (i.e., seeds collected before the spring germination peak), seedlings, rosettes, the summer seed bank (i.e., seeds collected after the spring germination peak) and fruiting plants. We found that: (1) All three species partitioned most of their genetic diversity within life history stages and less among stages within populations and among populations. (2) All genetic diversity parameters, except for allele frequencies, were similar among all life history stages across all populations in different species. (3) There were differences in allele frequencies among life history stages at all localities in Amaranthus retroflexus and at three localities in both Carduus acanthoides and Pastinaca sativa. (4) Allele frequencies did not differ between the winter and summer seed bank in most Carduus acanthoides and Pastinaca sativa populations, but there was a marked difference in Amaranthus retroflexus. In conclusion, we have shown that the summer seed bank is not genetically depleted by spring germination and that a majority of genetic diversity remains in the soil through summer. We suggest that seed banks in the species investigated play an important role by maintaining genetic diversity sufficient for recovery rather than by accumulating new genetic diversity at each locality.

Phenological synchrony affects interaction strength of an exotic weevil with Platte thistle, a native host plant.

Phenological synchrony of a consumer population with its resource populations is expected to affect interaction intensity. We quantified phenological variation and synchrony of populations of an invasive Eurasian flower head weevil, Rhinocyllus conicus, that consumes florets, ovules, and seeds of developing flower heads of a native North American thistle, Cirsium canescens, in Sand Hills prairie in Nebraska, USA. Variation in timing of adult activity among weevil populations was larger than variation in timing of flower head development among C. canescens populations, and it drove the observed variation in the phenological synchrony between weevil and host plant populations. Furthermore, the degree of phenological synchrony between populations was significant in explaining variation in weevil egg load on the newly acquired host plant. Because population growth of C. canescens is limited by predispersal seed losses to floral herbivores, variation in the synchrony of herbivore and plant flowering will affect the density of the plant population. These results provide strong quantitative support for the hypothesis that the synchrony of insect activity with plant resources can determine the magnitude of impact of floral herbivores on their host plant populations.