The role of preadaptation, propagule pressure and competition in the colonization of new habitats

To successfully colonize new habitats, organisms not only need to gain access to it, but also need to cope with the selective pressures imposed by the local biotic and abiotic conditions. The number of immigrants, the preadaptation to the local habitat and the presence of competitors are important factors determining the success of colonization. Here, using an experimental set-up, we test the combined effect of propagule pressure, preadaptation and interspecific competition on the colonization success of new habitats using the two-spotted spider mite (Tetranychus urticae) as our model system and the red spider mite (Tetranychus evansi) as a competitor. Our results show that propagule pressure and preadaptation positively affect colonization success. More successful populations reach larger final population sizes either by having higher per capita growth rate (due to preadaptation effect) or by starting a population with a larger number of individuals. Although populations are more successful colonizing non-competitive environments than competitive ones, propagule pressure and preadaptation counteract the negative effects of competition, promoting colonization success. Our results show the importance of propagule pressure and preadaptation to cope both with the exigencies of the new environment and the community context for successful colonization of new habitats.


INTRODUCTION
It is well known that new habitats provide novel ecological opportunities, potentially facilitating speciation and diversification (Simpson 1953, Carson & Templeton 1984, Onstein et al. 2014, Delaux et al. 2015. However, colonization of new habitats very likely relies on the interplay between ecological (e.g. competition, dispersal) and evolutionary (e.g. adaptation) processes that results in complex ecological-evolutionary dynamics. Although For individuals to colonize new habitats, firstly they need to physically access it (via dispersal). Second, the number of individuals arriving in a colonization event (referred to as propagule pressure) affects the probability whether the colonization is successful (Maron 2006). A high propagule pressure increases the chance that some of the immigrants can establish in the new habitat, for instance by having the right genetic makeup.
Also, it reduces the chance of extinction, which is more likely to occur in small population sizes due to Allee effects, founder effects, genetic (inbreeding, drift) and demographic stochasticity (Ellstrand & Ellam 1993 Therefore, preadaptation might positively affect colonization success by avoiding competitive exclusion. Even though these pre-requisites for colonization success are well known, the relative importance of physical access to the habitat, preadaptation and competition to determine success in new habitats, remains unknown. Here, we experimentally tested the importance of propagule pressure (number of immigrants in a single dispersal event), preadaptation and competition on colonization success in novel environments. We used the two-spotted spider mite (Tetranychus urticae) as our study system. T. urticae individuals from different populations were introduced to a novel host plant (tomato), to which some of the populations were pre-adapted and others were not.
Competitive environments were created by including competitive mites (Tetranychus evansi) in the novel environment. Propagule pressure was examined by varying the initial number of immigrants to the new habitat. Population size and per capita growth rate after one generation in the novel environment was used a proxy of colonization success. We specifically tested three hypotheses: (H1) higher propagule pressures will positively affect population sizes but not per capita growth rate, (H2) preadapted individuals will more rapidly increase in population size and attain higher per capita growth rate in new habitats than less adapted individuals; and (H3) competition will reduce population sizes and population sizes irrespectively of propagule pressure or preadaptation. Our results show that large propagule pressures, preadaptation, and low competition are pre-requisites of successful colonization of novel habitats.

Study species
The two-spotted spider mite Tetranychus urticae Koch, 1836 (Acari, Tetranychidae) is a generalist herbivore that feeds on a wide variety of host plants (Gotoh et al. 1993, Bolland et al. 1998). Because of its small body size (female size about 0.4mm length), high fecundity (1-12 eggs/day) and short developmental time (11-28 days; Nacimiento de Vasconcelos et al. was considered as a proxy of adaptation to tomato plants. The population with no preadaptation has never been exposed to tomato (only reared on bean plants), populations of females with medium preadaptation have been exposed to tomato plants for 20 generations (1 generation ~ 13 days), and the population with high preadaptation has been exposed to tomato plants for more than 100 generations.
As a competitor, we used the red spider mite Tetranychus evansi Baker and Pritchard, 1960 (Acari, Tetranychidae), which is a specialist herbivore of (mainly) Solanaceae (incl. tomato).
Adult females are easily distinguishable from T. urticae as they show a characteristic red coloration and are slightly larger (0.5 to 0.6mm length). Fecundity ranges from 10 to 14 eggs per day (Navajas et al. 2013) and development time can vary from 6.3 to 13.5 days, depending on the environmental temperature and host (Bonato 1999).

Experiments
We performed two experiments: in one we tested the effects of propagule pressure and competition on colonization success and abundance and in the other we tested for the effects of preadaptation and competition on colonization success and abundance. Before each experiment we removed epigenetic effects (juvenile and maternal effects) by collecting individual females from each population (non-adapted, medium adapted and highly adapted) and rearing them separately in a common garden for 2 generations (

Statistical analyses
Propagule pressure and competition -to test the effect of propagule pressure and competition on per capita growth rate and population size after one generation, we used a general linear mixed model. Propagule pressure (with three levels: 3, 5 and 10 female mites) and competition (with two levels: competition with T. evansi and no-competition) were considered as fixed categorical factors. Because females coming from the same iso-female line (Fig. S1 in Supplementary material) might respond similarly to a treatment than females that are not related, iso-female line was considered as random factor. Per capita growth rate and final population size (number of adult females after one generation) were considered as response variables. To correct for the initial differences in population sizes on final population sizes, we subtracted the initial number of immigrants (3, 5 or 10 female mites) from the final population size. Model selection (for both the random and fixed part) was performed using a stepwise removal of non-significant effects based on log-likelihood ration test until only significant effects remained. Post-hoc tests were performed to test for differences between the least square means of treatments using the difflsmeans function from the package lmerTest

RESULTS
Propagule pressure and competition -Final population size was best explained by the additive effects of propagule pressure and competition (Table 1, see Table S1 in Supplementary material for model selection). While propagule pressure positively affected population size of T. urticae, both with and without competition, competition always exerted a negative effect on population size irrespective of propagule pressure (Fig. 2a). Populations receiving a higher number of propagules (10 females) attained larger population sizes after one generation than populations receiving fewer propagules (3 females).
Per capita growth rate was best explained by both the additive and interaction effects of propagule pressure and competition ( Table 1, Table S1 in Supplementary material for model selection). While competition always affected growth rate negatively, the effect of propagule pressure depended on the competitive environment (Table 1). In a no competitive environment, propagule pressure exerted a negative effect on per capita growth rate, whereas in a competitive environment propagule pressure did not have an effect (Fig. 2b). Preadaptation and competition -Population size was best explained by the additive and interaction effects of preadaptation and competition (Table 1, see Table S1 in Supplementary material for model selection). Populations that co-occurred with T. evansi were significantly smaller than populations without the competitor for populations with medium and high preadaptation (Fig. 3b). Populations with no preadaptation had the lowest population size in both competitive and non-competitive environments, and populations with high preadaptation in non-competitive environments had the largest population sizes (Fig. 3b).
Per capita growth rate was best explained by the additive and interaction effects of preadaptation and competition. Preadaptation affected growth rate positively, whereas competition affected growth rate negatively (Fig. 3a). Populations without any preadaptation showed the lowest growth rate, both in competitive and non-competitive environments, whereas populations with high preadaptation in non-competitive environments showed the highest growth rate (Fig. 3a).   competition counteracted this effect (Fig. 2b). In a competitor-free environment, propagule pressure negatively affected per capita growth rate, whereas it had no effect in a competitive environment. This strongly suggests that intraspecific competition increases with propagule pressure, which in turn hinders growth. In a competitive environment, interspecific competition reduces population sizes to a degree that intraspecific competition may be of less  (Joshi & Thompson 1996). In our experiment, the most adapted populations not only have larger growth rates and larger final population sizes (Fig. 3), they also exerted a higher competitive effect on the competitor species, T. evansi (Fig. S3a in Supplementary material) and displayed a higher overall competitive ability (Fig. S3b in Supplementary material).

CONCLUSION
Understanding the factors that affect species colonization success in novel habitats is of great importance given the biodiversity crisis we are currently facing. Habitat fragmentation and transformation have forced populations into novel habitats. Inability to successful colonize those habitats may lead species to extinction as populations become more isolated attaining smaller population sizes (Fahrig 1997, Wiegand et al. 2005. In this study, we experimentally tested how propagule pressure, preadaptation and competition affect colonization success in novel habitats. Our results confirm the intuitively evident hypothesis that propagule pressure and preadaptation positively affect colonization success. In competitive environments, however, colonization success is reduced, and to successfully colonize habitats that lack 'empty niches' (which is likely the most common scenario), it is important to be either preadapted or to start a population with a large number of colonizing individuals (higher propagule pressure).