Manuel A. Morales

MECHANISMS AND DENSITY DEPENDENCE OF BENEFIT IN AN ANT–MEMBRACID MUTUALISM

I examined mechanisms and patterns of benefit for the membracid (treehopper) Publilia concava tended by the ant Formica obscuriventris to test two hypotheses: that treehoppers benefit from ant attendance only by protection from predators, and that density-dependent benefit depends on the presence of predators. I used a factorial design, manipulating ants and predators in 1996, and ants and removal of uncollected honeydew in 1997. Results showed that treehoppers benefit from ant attendance by protection from predators. Additionally, results suggested that treehoppers benefit from ant attendance in ways other than protection from predators; tended treehoppers outperform untended treehoppers even with predators excluded. There was no support for the hypothesis that a proximate benefit of ant-tending includes removal of uncollected honeydew. A possible benefit (untested) of ant-tending is increased feeding rates. Treehoppers in small aggregations benefited more than treehoppers in large aggregations, indicating a density-dependent benefit in this mutualism, independent of predator level. Correspondingly, the number of ants per treehopper was highest for small aggregations. This study suggests that individuals can benefit from mutualisms in complex ways. Additionally, it adds to a growing number of studies that support the hypotheses that mutualisms may be stabilized by density-dependent benefit and that density-dependent benefit may be driven by the recruitment patterns of mutualists.

Effects of Host Plant Quality and Ant Tending for Treehopper Publilia concava

Abstract Few studies have evaluated the relative importance of bottom-up regulation for the population dynamics of mutualism. To address this, we tested the hypothesis that host plant quality affects the strength of the mutualism between Publilia concava (Say) (Hemiptera: Membracidae) treehoppers feeding on Solidago altissima L. and ants. P. concava is a phloem-feeding treehopper that excretes a sugary waste product called honeydew. Ants collect this honeydew as a food resource, and treehoppers benefit both directly (e.g., feeding facilitation) and indirectly (e.g., protection from predators). We evaluated the effect of host plant quality on both the direct and indirect effects of this mutualism by using a factorial design that manipulated N–P–K fertilizer level and ant presence. For the experiment that focused on the direct effects of ant tending, both body size and survivorship were monitored. For the experiment that included the indirect effects of ant tending, only survivorship was monitored. Both host plant quality and ant tending increased the performance (survivorship and size) of Publilia treehoppers. However, we find no support for the hypothesis that host plant quality influences the strength of the mutualism in this system—there was no significant interaction between ant tending and fertilization for any measure of treehopper performance considered. We suggest that this result is explained by the independence of per capita tending levels with both host plant quality and treehopper density in this experiment.

Acoustic alarm signalling facilitates predator protection of treehoppers by mutualist ant bodyguards

Mutualism is a net positive interaction that includes varying degrees of both costs and benefits. Because tension between the costs and benefits of mutualism can lead to evolutionary instability, identifying mechanisms that regulate investment between partners is critical to understanding the evolution and maintenance of mutualism. Recently, studies have highlighted the importance of interspecific signalling as one mechanism for regulating investment between mutualist partners. Here, we provide evidence for interspecific alarm signalling in an insect protection mutualism and we demonstrate a functional link between this acoustic signalling and efficacy of protection. The treehopper Publilia concava Say (Hemiptera: Membracidae) is an insect that provides ants with a carbohydrate-rich excretion called honeydew in return for protection from predators. Adults of this species produce distinct vibrational signals in the context of predator encounters. In laboratory trials, putative alarm signal production significantly increased following initial contact with ladybeetle predators (primarily Harmonia axyridis Pallas, Coleoptera: Coccinellidae), but not following initial contact with ants. In field trials, playback of a recorded treehopper alarm signal resulted in a significant increase in both ant activity and the probability of ladybeetle discovery by ants relative to both silence and treehopper courtship signal controls. Our results show that P. concava treehoppers produce alarm signals in response to predator threat and that this signalling can increase effectiveness of predator protection by ants.

Allee dynamics generated by protection mutualisms can drive oscillations in trophic cascades

Understanding the relative effect of top predators and primary producers on intermediate trophic levels is a key question in ecology. Most previous work, however, has not considered either realistic nonlinearities in feedback between trophic levels or the effect of mutualists on trophic cascades. Here, we develop a realistic model for a protection mutualism that explicitly includes interactions between a protected herbivore and both its food plant and generalist predators. In the absence of protection, herbivores and plant resources approach a stable equilibrium, provided that predation is not so high as to cause herbivore extinction. In contrast, adding protection by mutualists increases the range of dynamical outcomes to include unstable equilibria, stable and unstable limit cycles, and heteroclinic orbits. By reducing the impact of predators, protection by mutualists can allow herbivores to exert strong negative effects on their host plants, which in turn can lead to repeated cycles of overexploitation and recovery. Our results indicate that it may be essential to consider protection mutualisms to understand the dynamics of trophic cascades. Conversely, it may be essential to explicitly include dynamical feedback between plants and herbivores to fully understand the population and community dynamical consequences of protection mutualism.

Impact of treatment in long-term survival patients with follicular lymphoma: A Spanish Lymphoma Oncology Group registry

Background Follicular lymphoma is the second most common non-Hodgkin lymphoma in the United States and Europe. However, most of the prospective randomized studies have very little follow-up compared to the long natural history of the disease. The primary aim of this study was to investigate the long-term survival of our series of patients with follicular lymphoma. Patients and methods A total of 1074 patients with newly diagnosed FL were enrolled. Patients diagnosed were prospectively enrolled from 1980 to 2013. Results Median follow-up was 54.9 months and median overall survival is over 20 years in our series. We analyzed the patients who are still alive beyond 10 years from diagnosis in order to fully assess the prognostic factors that condition this group. Out of 166 patients who are still alive after more than 10 years of follow-up, 118 of them (73%) are free of evident clinical disease. Variables significantly associated with survival at 10 years were stage < II (p <0.03), age < 60 years (p <0.0001), low FLIPI (p <0.002), normal β2 microglobulin (p <0.005), no B symptoms upon diagnosis (p <0.02), Performance Status 0–1 (p <0.03) and treatment with anthracyclines and rituximab (p <0.001), or rituximab (p <0.0001). Conclusions A longer follow-up and a large series demonstrated a substantial population of patients with follicular lymphoma free of disease for more than 10 years.

Model selection analysis of temporal variation in benefit for an ant-tended treehopper

Recent studies of mutualism have emphasized both that the net benefit to participants depends on the ecological context and that the density-dependent pattern of benefit is key to understanding the population dynamics of mutualism. Indeed, changes in the ecological context are likely to drive changes in both the magnitude of benefit and the density-dependent pattern of benefit. Despite the close linkage between these two areas of research, however, few studies have addressed the factors underlying variation in the density-dependent pattern of benefit. Here I use model selection to evaluate how variation in the benefits of a mutualism drives temporal variation in the density-dependent pattern of net benefit for the ant-tended treehopper Publilia concava. In the interaction between ants and treehoppers in the genus Publilia, ants collect the sugary excretions of treehoppers as a food resource, and treehoppers benefit both directly (e.g., by feeding facilitation) and indirectly (e.g., by predator protection). Results presented here show that temporal changes in the relative magnitude of direct and indirect benefit components of ant tending, especially the effectiveness of predator protection by ants, qualitatively change the overall pattern of density-dependent benefit between years with maximum benefit shifting from treehoppers in small to large aggregations. These results emphasize the need for empirical studies that evaluate the long-term dynamics of mutualism and theoretical studies that consider the population dynamics consequences of variation in the density-dependent pattern of benefit.

Prognostic value of event‐free survival at 12 and 24 months and long‐term mortality for non‐Hodgkin follicular lymphoma patients: A study report from the Spanish Lymphoma Oncology Group

Relatively few studies have analyzed the mortality of follicular lymphoma (FL) patients in comparison with a sex‐ and age‐matched general population. This study analyzed the overall survival (OS) of patients with FL and compared their survival with the expected survival of a general population.

Considering Interactions: Incorporating Biotic Interactions into Viability Assessment

Most analyses of population viability focus on changes in numbers of the focal species independent of other members of their community or any other biotic interactions. It is difficult to incorporate all relevant factors into a viability analysis, but leaving out biotic interactions may be a critical flaw in some analyses. While single-species population viability analyses (PVAs) implicitly incorporate the effect of species interactions on population growth rate parameters (i.e., vital rates), models that explicitly consider changes in species dynamics as ecological conditions change may be needed. Unfortunately, these models will significantly increase data requirements. Because robust data sets are notoriously difficult to acquire even for single-species PVAs, it is important to evaluate the relative importance of species interactions before considering development of a PVA model that incorporates them explicitly. In this chapter, we discuss the various kinds of interactions that plants are involved in, evaluate when species interactions are likely to matter, consider strategies for deciding when to incorporate these interactions into PVA models, and discuss relevant modeling approaches.

Mechanisms of aggregation in an ant-tended treehopper: Attraction to mutualists is balanced by conspecific competition

Understanding the spatial structure of populations and communities has been a dominant focus of ecological research, and spatial structure is increasingly seen as critical for understanding population dynamics. Habitat (or host) preference is a proximate mechanism that can generate aggregation or overdispersion, lending insight into the ultimate consequences of observed spatial distributions. Publilia concava is a univoltine phloem-feeding insect that forms mutualistic associations with ants, which consume honeydew and protect treehoppers from predation. Treehopper adults and nymphs are aggregated at the scale of goldenrod plant stems, and previous studies have suggested that this aggregation is an adaptive response that increases feeding performance or maximizes benefits of ant-tending. Previous studies have also shown experimentally that individual treehoppers preferentially oviposit on plants with ants present, but a complimentary hypothesis that treehoppers prefer to oviposit near conspecifics (e.g., to take advantage of density-dependent ant attraction) remains untested. We show that, as expected, the probability of treehopper oviposition increases with ant-presence and relative ant abundance. However, we also find that treehopper oviposition decreases with increasing treehopper density. Thus our results are inconsistent with the hypothesis that treehopper aggregation is a socially cooperative strategy to attract ants; we suggest that aggregation is a form of conflict and an unavoidable by-product of individual responses to ant-tending levels.

Isolation and characterization of nine microsatellite loci in an ant-tended treehopper Publilia concava.

Publilia concava is an eastern North American membracid commonly occurring in large but spatially patchy aggregations, primarily on the host plant Solidago altissima. Like other myrmecophiles, P. concava provides sugary excretions to ants in return for the various protective, competitive or even sanitary benefits that ants provide. We developed nine microsatellite loci from P. concava. Mean per locus allele number was 6.78, and observed heterozygosities ranged from 0.03 to 0.850. One locus exhibited significant heterozygote deficit, possibly due to the presence of null alleles. These markers provide important tools for future spatial ecological studies in this model system for the study of mutualism.