Browsing by Subject "Density dependence"

Sort by: Order: Results:

Now showing items 1-3 of 3
  • Jakubaviciute, Egle; Candolin, Ulrika (2021)
    The invasion of non-native species into an ecosystem can markedly alter the structure and functioning of the system. Yet, we have limited knowledge of the factors that determine invasion success. Behavioural interactions have been suggested as critical determinants of invasion success in animals, but the exact mechanisms are less well known. We investigated if density-dependent behavioural interactions could have facilitated the invasion of the shrimp Palaemon elegans into the spawning habitat of the threespine stickleback Gasterosteus aculeatus in the Baltic Sea. This was done by manipulating the densities of the two species in mesocosms. We found the stickleback to dominate behaviourally over the shrimp through higher aggression, but that the impact on the shrimp was density-dependent; a high density of sticklebacks increased aggressive interactions, which caused the shrimps to decrease their activity and restrict their habitat use to dense vegetation, while a low density of sticklebacks had no impact on the distribution and activity of the shrimps. The density of the shrimps had no impact on stickleback behaviour. These results suggest that the present density of the stickleback has allowed the invasion of the shrimp into the habitat. However, a current increase in stickleback abundance caused by human-induced ecological disturbances could limit the further expansion of the shrimp. Thus, our results indicate that a behavioural mechanism-density-dependent aggression-can influence invasion success and subsequent population expansion. At a broader level, our results stress the importance of considering density-dependent behavioural interactions when investigating the mechanisms behind invasion success.
  • Hartmann, Marcelo; Hosack, Geoffrey R; Hillary, Richard M; Vanhatalo, Jarno Petteri (2017)
    Density dependent population growth functions are of central importance to population dynamics modelling because they describe the theoretical rate of recruitment of new individuals to a natural population. Traditionally these functions are described with a fixed functional form with temporally constant parameters and without species interactions. The Ricker stock-recruitment model is one such function that is commonly used in fisheries stock assessment. In recent years, there has been increasing interest in semi-parametric and temporally varying population growth models. The former are related to the general statistical approach of using semi-parametric discrepancy functions, such as Gaussian processes (GP), to model deviations of data around the expected parametric function. In the latter, the reproductive rate, which is a key parameter describing the population growth rate, is assumed to vary in time. In this work, we introduce how these existing Ricker population growth models can be formulated under the same statistical approach of hierarchical GP models. We also show how the time invariant semi-parametric approach can be extended and combined with the time varying reproductive rate using a GP model. Then we extend these models to the multispecies setting by incorporating cross-covariances among species with a continuous time covariance structure using the linear model of coregionalization. As a case study, we examine the productivity of three Pacific salmon populations. We compare the alternative Ricker population growth functions using model posterior probabilities and leave-one-out cross validation predictive densities. Our results show substantial temporal variation in maximum reproductive rates and reveal temporal dependence among the species, which have direct management implications. However, our results do not support inclusion of semi-parametric discrepancy function and they suggest that the semi-parametric discrepancy functions may lead to challenges in parameter identifiability more generally.
  • Andreassen, Harry P.; Sundell, Janne; Ecke, Fraucke; Halle, Stefan; Haapakoski, Marko; Henttonen, Heikki; Huitu, Otso; Jacob, Jens; Johnsen, Kaja; Koskela, Esa; Luque-Larena, Juan Jose; Lecomte, Nicolas; Leirs, Herwig; Marien, Joachim; Neby, Magne; Rätti, Osmo; Sievert, Thorbjörn; Singleton, Grant R.; van Cann, Joannes; Vanden Broecke, Bram; Ylönen, Hannu (2020)
    Most small rodent populations in the world have fascinating population dynamics. In the northern hemisphere, voles and lemmings tend to show population cycles with regular fluctuations in numbers. In the southern hemisphere, small rodents tend to have large amplitude outbreaks with less regular intervals. In the light of vast research and debate over almost a century, we here discuss the driving forces of these different rodent population dynamics. We highlight ten questions directly related to the various characteristics of relevant populations and ecosystems that still need to be answered. This overview is not intended as a complete list of questions but rather focuses on the most important issues that are essential for understanding the generality of small rodent population dynamics.