Ecological and genetic effects of introduced species on their native competitors

Species introductions to new habitats can cause a decline in the population size of competing native species and consequently also in their genetic diversity. We are interested in why these adverse effects are weak in some cases whereas in others the native species declines to the point of extinction. While the introduction rate and the growth rate of the introduced species in the new environment clearly have a positive relationship with invasion success and impact, the influence of competition is poorly understood. Here, we investigate how the intensity of interspecific competition influences the persistence time of a native species in the face of repeated and ongoing introductions of the nonnative species. We analyze two stochastic models: a model for the population dynamics of both species and a model that additionally includes the population genetics of the native species at a locus involved in its adaptation to a changing environment. Counterintuitively, both models predict that the persistence time of the native species is lowest for an intermediate intensity of competition. This phenomenon results from the opposing effects of competition at different stages of the invasion process: With increasing competition intensity more introduction events are needed until a new species can establish, but increasing competition also speeds up the exclusion of the native species by an established nonnative competitor. By comparing the ecological and the eco-genetic model, we detect and quantify a synergistic feedback between ecological and genetic effects.Comment: version accepted at Theoretical Population Biolog

Handbuch der Raumfahrttechnik

50 Jahre nach dem Start des ersten künstlichen Erdsatelliten - Sputnik 1 - im Oktober 1957 wird dem Leser mit der dritten Auflage dieses Handbuchs der Raumfahrttechnik erneut ein Standardwerk präsentiert. Das komplett vierfarbig gedruckte Handbuch bietet Studierenden, Ingenieuren und Wissenschaftlern sowie ambitionierten Raumfahrtinteressierten detaillierte Einblicke in die faszinierende Welt der Raumfahrt. Ausgehend von den Grundlage, werden in den Hauptkapiteln - Einleitung (Historischer Überblick, Raumfahrtmissionen) - Grundlagen (u.a. Bahnmechanik, Aerothermodynamik/Wiedereintritt, Space Debris) - Trägersysteme (u.a. Stufentechnologien, Antriebssysteme, Startinfrastruktur) - Raumfahrt-Subsysteme (u.a. Struktur, Energieversorgung, Thermalkontrolle, Lageregelung, Kommunikation) - Aspekte bemannter Missionen (u.a. Der Mensch im Weltraum, Lebenserhaltungssysteme, Rendezvous und Dockung) - Missionsbetrieb (u.a. Satellitenbetrieb, Kontrollzentrum, Bodenstationsnetzwerk) - Raumfahrtnutzung (u.a. Erdbeobachtung, Kommunikation, Weltraumastronomie, Materialwissenschaften, Weltraummedizin, Robotik) - Konfiguration/Entwurf eines Raumflugkörpers (u.a. Missionskonzept, Systementwurf, Umweltsimulation, Systemdesign, Galileo-Satelliten) - Management von Raumfahrtprojekten (u.a. Projektmanagement, Kostenmanagement, Raumfahrtrecht) in 42 Unterkapiteln vor allem die Abläufe und Methoden für die Entwicklung, den Bau, den Betrieb und die Nutzung von Raumfahrtsystemen beschrieben

Population Genetic Consequences of the Allee Effect and the Role of Offspring-Number Variation

Wittmann M, Gabriel W, Metzler D. Population Genetic Consequences of the Allee Effect and the Role of Offspring-Number Variation. Genetics. 2014;198(1):311-15S.A strong demographic Allee effect in which the expected population growth rate is negative below a certain critical population size can cause high extinction probabilities in small introduced populations. But many species are repeatedly introduced to the same location and eventually one population may overcome the Allee effect by chance. With the help of stochastic models, we investigate how much genetic diversity such successful populations harbor on average and how this depends on offspring-number variation, an important source of stochastic variability in population size. We find that with increasing variability, the Allee effect increasingly promotes genetic diversity in successful populations. Successful Allee-effect populations with highly variable population dynamics escape rapidly from the region of small population sizes and do not linger around the critical population size. Therefore, they are exposed to relatively little genetic drift. It is also conceivable, however, that an Allee effect itself leads to an increase in offspring-number variation. In this case, successful populations with an Allee effect can exhibit less genetic diversity despite growing faster at small population sizes. Unlike in many classical population genetics models, the role of offspring-number variation for the population genetic consequences of the Allee effect cannot be accounted for by an effective-population-size correction. Thus, our results highlight the importance of detailed biological knowledge, in this case on the probability distribution of family sizes, when predicting the evolutionary potential of newly founded populations or when using genetic data to reconstruct their demographic history

Genetic Diversity in Introduced Populations with an Allee Effect

Wittmann M, Gabriel W, Metzler D. Genetic Diversity in Introduced Populations with an Allee Effect. Genetics. 2014;198(1):299-310.A phenomenon that strongly influences the demography of small introduced populations and thereby potentially their genetic diversity is the demographic Allee effect, a reduction in population growth rates at small population sizes. We take a stochastic modeling approach to investigate levels of genetic diversity in populations that successfully overcame either a strong Allee effect, in which populations smaller than a certain critical size are expected to decline, or a weak Allee effect, in which the population growth rate is reduced at small sizes but not negative. Our results indicate that compared to successful populations without an Allee effect, successful populations with a strong Allee effect tend to (1) derive from larger founder population sizes and thus have a higher initial amount of genetic variation, (2) spend fewer generations at small population sizes where genetic drift is particularly strong, and (3) spend more time around the critical population size and thus experience more genetic drift there. In the case of multiple introduction events, there is an additional increase in diversity because Allee-effect populations tend to derive from a larger number of introduction events than other populations. Altogether, a strong Allee effect can either increase or decrease genetic diversity, depending on the average founder population size. By contrast, a weak Allee effect tends to decrease genetic diversity across the entire range of founder population sizes. Finally, we show that it is possible in principle to infer critical population sizes from genetic data, although this would require information from many independently introduced populations

Decomposing propagule pressure: the effects of propagule size and propagule frequency on invasion success

Wittmann M, Metzler D, Gabriel W, Jeschke JM. Decomposing propagule pressure: the effects of propagule size and propagule frequency on invasion success. Oikos. 2014;123(4):441-450.Propagule pressure quantifies the inflow of individuals to a location and appears to be a key driver of invasion success. It is often defined as the average number of individuals introduced per time unit, or equivalently as the product of the average number of individuals introduced per introduction event (propagule size) and the frequency of introduction events (propagule frequency). Here we study how the influence of propagule size, frequency, and their product depends on the underlying ecological conditions. While previous studies have focused on introductions under environmental heterogeneity or a strong Allee effect, we examine a range of ecological scenarios that differ in the type of density dependence and in the sign of per capita growth rate. Our results indicate that the relative influence of propagule size and frequency depends mainly on the sign of per capita growth rate. Given a certain average number of individuals introduced per time unit, a high propagule frequency accelerates invasions under ecological scenarios with positive average per capita growth rate throughout the invasion process (‘easy’ scenarios). If per capita growth rate is negative throughout the invasion process (‘difficult’ scenarios) or if there is both an easy and a difficult stage (‘mixed scenarios’), a high propagule size leads to a faster invasion than a high propagule frequency. To explain this finding, we argue that for a fixed value of the product of propagule size and frequency, an increase in propagule size leads to an increase in demographic variance, which promotes invasion success in difficult and mixed but not in easy scenarios. However, we also show that in many of these cases, the product of propagule size and frequency still correlates more strongly with invasion success than either of the single components. Finally, we illustrate our approach with empirical examples from the literature

Decomposing propagule pressure: the effects of propagule size and propagule frequency on invasion success

Wittmann M, Metzler D, Gabriel W, Jeschke JM. Decomposing propagule pressure: the effects of propagule size and propagule frequency on invasion success. Oikos. 2014;123(4):441-450.Propagule pressure quantifies the inflow of individuals to a location and appears to be a key driver of invasion success. It is often defined as the average number of individuals introduced per time unit, or equivalently as the product of the average number of individuals introduced per introduction event (propagule size) and the frequency of introduction events (propagule frequency). Here we study how the influence of propagule size, frequency, and their product depends on the underlying ecological conditions. While previous studies have focused on introductions under environmental heterogeneity or a strong Allee effect, we examine a range of ecological scenarios that differ in the type of density dependence and in the sign of per capita growth rate. Our results indicate that the relative influence of propagule size and frequency depends mainly on the sign of per capita growth rate. Given a certain average number of individuals introduced per time unit, a high propagule frequency accelerates invasions under ecological scenarios with positive average per capita growth rate throughout the invasion process (‘easy’ scenarios). If per capita growth rate is negative throughout the invasion process (‘difficult’ scenarios) or if there is both an easy and a difficult stage (‘mixed scenarios’), a high propagule size leads to a faster invasion than a high propagule frequency. To explain this finding, we argue that for a fixed value of the product of propagule size and frequency, an increase in propagule size leads to an increase in demographic variance, which promotes invasion success in difficult and mixed but not in easy scenarios. However, we also show that in many of these cases, the product of propagule size and frequency still correlates more strongly with invasion success than either of the single components. Finally, we illustrate our approach with empirical examples from the literature

Data from: Can Daphnia lumholtzi invade European lakes?

The cladoceran Daphnia lumholtzi is a subtropical and tropical zooplankter, and an invasive species in North America. Thus far, D. lumholtzi has not been detected in Europe. Here we investigated whether a hypothetical introduction to Europe could result in a successful invasion, either now or in the near future when facilitated by climate change. In laboratory experiments, we tested whether different clones of D. lumholtzi can invade a resident community consisting of native Daphnia from lake Klostersee, Germany, and how invasion success depends on temperature and the presence or absence of planktivorous fish. In some treatments, invasion success was consistently high, and D. lumholtzi reached densities similar to the native competitors by the end of the experiment. Surprisingly, the presence of a planktivorous fish inhibited the invasion of D. lumholtzi, and a clone with an inducible defense against fish predation was a more successful invader than a permanently defended clone. Of the three temperatures tested in this study (15, 20, and 24°C), invasion success was highest at 20°C. To understand the competitive interaction between native and introduced Daphnia, we fit a Lotka-Volterra-type competition model to the population dynamics. Our experimental and modeling results suggest that D. lumholtzi can invade European lakes and can cause substantial declines in the population size of native Daphnia, with potential consequences for higher trophic levels

Data from: Can Daphnia lumholtzi invade European lakes?

The cladoceran Daphnia lumholtzi is a subtropical and tropical zooplankter, and an invasive species in North America. Thus far, D. lumholtzi has not been detected in Europe. Here we investigated whether a hypothetical introduction to Europe could result in a successful invasion, either now or in the near future when facilitated by climate change. In laboratory experiments, we tested whether different clones of D. lumholtzi can invade a resident community consisting of native Daphnia from lake Klostersee, Germany, and how invasion success depends on temperature and the presence or absence of planktivorous fish. In some treatments, invasion success was consistently high, and D. lumholtzi reached densities similar to the native competitors by the end of the experiment. Surprisingly, the presence of a planktivorous fish inhibited the invasion of D. lumholtzi, and a clone with an inducible defense against fish predation was a more successful invader than a permanently defended clone. Of the three temperatures tested in this study (15, 20, and 24°C), invasion success was highest at 20°C. To understand the competitive interaction between native and introduced Daphnia, we fit a Lotka-Volterra-type competition model to the population dynamics. Our experimental and modeling results suggest that D. lumholtzi can invade European lakes and can cause substantial declines in the population size of native Daphnia, with potential consequences for higher trophic levels