Species invasions offer a perfect setting to look at these mechanism of behavioural change and explore their evolutionary implications: We have information about the mechanism behind the behavioural changes in many non-native species. In a next step, the environmental variability in the home range should be quantified using different indicators (seasonal climatic variability, stochastic climatic variability, biotic variability, …). Quantitatively comparing the distribution of variability indices between mechanisms of behavioural change can answer the question if these different mechanisms evolve in the respective species shaped by their environment. Theory predicts that environments with low variability will foster genetic adaptation, while individual learning particularly evolves in very variable environments. In environments with intermediate variability, social learning is most favourable (Brown 2012). We conducted such a study with data from the study in Chapter 1 and found preliminary evidence for this pattern with stochastic temporal temperature variability as a predictor (Ruland, Wiedenroth et al., in prep.).
This dataset can be expanded for a clearer picture by gathering more instances of behavioural change for a subset of species. That means ideally more than one instance of behavioural change in one species in its native and its invaded range, with evidence for the mechanisms involved. Then the environmental variability of the invaded range – better: ranges – will be quantified in the same way. Now all cross-comparisons are possible: the species might, for example, have evolved through genetic adaptation to a variable home range and use learning as a mechanism of behavioural change in its invaded ranges – evidence for the innovation- precedes-invasion hypothesis. Or it originates from a less variable home range where it shows no clear pattern in mechanisms of behavioural change while it then genetically adapts to very variable invaded ranges – evidence for the selection-for-innovation hypothesis.
The types of behaviour under change are valuable information about potential speciation in the native or non-native species. Drawing from the framework of Duckworth (2009), there are explicit predictions about what kind of evolutionary change is expected for which change in behaviour, covering some, but not all types of behaviour classified in Chapter 1: what we defined as locomotion is called "migratory patterns or habitat selection that causes an organism to move to a novel environment" in Duckworth’s framework and predicted to affect diversification rates (Phillimore et al. 2006). Mating as well as resource use ("feeding") are predicted to result in sympatric speciation (Dieckmann and Doebeli 1999). The behavioural change to
cope with abiotic stress like different temperature and salinity in the environment, defined as "climate- related" in Chapter 1, is predicted to inhibit evolutionary change (Huey et al. 2003). What is missing in these predictions are changes in competitive or anti-predator behaviour, which are common behavioural changes and not included in the framework by Duckworth (2009). I predict behavioural changes to cope with competition to inhibit speciation in a similar way as behavioural adaptation to thermoregulation limits selection. The same will be true for successful behavioural change to cope with predation which will directly reduce selective pressure. Chapter 2 on behavioural change in marbled crayfish showed that these crayfish can be successful in competitive interactions with invasive spiny-cheek crayfish without any genetic variation. However, most schemes on invasion success do not consider evolutionary change in the invader and the recipient community (Whitney and Gabler 2008). The growing database that I created for the studies compiled in this thesis will aid to answer these questions in the future.
The behavioural changes observed in the studies compiled in this thesis support the hypothesis that species thriving in association with humans in their native range are more likely successful invaders (Strubbe et al. 2015), as supported by the evidence that urban ecosystems serve as hotspots and hubs for non-native species (Von Der Lippe and Kowarik 2008). Marbled crayfish that came from laboratories showed a different behavioural response to the human hand as a threat. While life in captivity is more monotonous (Mason et al. 2013), it is also safer and possibly the marbled crayfish show the appropriate "freezing" response to human approach, which corresponds to ignoring. This could reduce stress level in crayfish and decrease non-lethal predation effects - missed opportunity costs in foraging - similar to the decreased flushing distance in birds populations in touristic places (Jiménez et al. 2013). If the human hand is seen as a novel predation threat, the observation is inverse to the prediction by Sih et al. (2010), whereby the non-consumptive effects of predation are smaller the more novel the predator is. In this case, marbled crayfish may have learned to ignore the hand in contrast to the spiny-cheek crayfish, which show a generalized fight or flight response towards it. In the end, it is not clear, however, if the freezing response will be adaptive or maladaptive in frequently visited lakes like Krumme Lanke. While few humans will intentionally seek to predate on marbled crayfish there, freezing may increase the chance of getting accidentally stepped on.
We found evidence for a behavioural syndrome in marbled crayfish between aggression and activity (Chapter 2). This could limit the potential for behavioural change, as the change in one trait will always be associated with a - potentially maladaptive - change. The aggression that is rendering individuals more successful at obtaining resources from hetero- and conspecific competitors is predicted to be a positive trait at low densities, but not at high densities (Hudina et al. 2014). Behavioural syndromes may be less pronounced in the wild, though, with increased predation pressure (Niemelä et al. 2012). However, the high observed rate of cannibalism of own offspring in marbled crayfish in the lab (Stefan Linzmaier, pers. comm.) may limit densities in invaded lakes.