Plant traits are critical for understanding invasion success of introduced species, yet attempts to identify universal traits that explain invasion success and impact have been unsuccessful because environment-trait-fitness relationships are complex, potentially context dependent, and variation in traits is often unaccounted for. As introduced species encounter novel environments, their traits and trait variability can determine their ability to grow and reproduce, yet invasion biologists do not often have an understanding of how novel environments might shape traits. To uncover which combination of traits are most effective for predicting invasion success, we studied three different urban habitat types along the Nile Delta in Egypt invaded by the Pink Morning Glory, Ipomoea carnea Jacq. (Family: Convolvulaceae). Over two years, we measured ten plant traits at monthly intervals along an invasion gradient in each habitat. No single trait sufficiently explained survival probability and that traits linked to invasion success were better predicted by the characteristics of the invaded habitat. While the measured traits did influence survival of I. carnea, the importance of specific traits was contingent on the local environment, meaning that local trait-environment interactions need to be understood in order to predict invasion.

Invasion success, Exotic species, Survival probability, Morning Glory, Disturbed habitat

Biological invasion is a significant threat to biodiversity and often leads to habitat degradation (Elton 1958, Genovesi and Monaco 2013, Qureshi et al. 2014). There is a great need for efficient tools to predict invasion success and impact on native ecosystems (Funk et al. 2008). Yet, the relative importance of particular life history and functional traits for invasion success and impact is strongly context-dependent and likely to change across environments (Dawson et al. 2009). In cases of significant impact by invasive plants, estimating ‘invasiveness’ using plant traits that predict plant survival could be used to determine invasion resistance of native plant communities (Ortega and Pearson 2005, Radosevich et al. 2007, Colautti et al. 2014).This is particularly important to understand in urban habitats where urbanization can favor non-native species (McKinney 2006). Urbanization reduces the competitiveness of established vegetation (Wilson and Tilman 1995) and increases the availability of resources (Johnstone 1986, McConnaughay and Bazzaz 1991), which can be exploited by non-native species, further influencing their invasiveness.

At a more basic level, we often lack a basic understanding of how size and life history traits contribute to the successful growth and reproduction of most species and especially non-native species. The attributes that are associated with successful species is undoubtedly correlated with local environmental conditions (Laughlin et al. 2012, Laughlin 2014). For example, Laughlin et al. (2012) show how models that match trait values to local climate predict species abundances. Such a relationship between traits and the environment are useful to explain presence/absence or abundance patterns, but little work has been done to see if intraspecific trait variation can explain difference in growth, reproduction, and survival. Further, it is unclear how changes in size-based traits through the growing season and in different environments influence success. What is lacking in our current knowledge is how specific traits or suites of traits influence species performance in different habitats.

A number of studies have shown that invasion success can be linked to specific traits and the degree to which they promote survival in novel environments. These include for example, traits linked with reproduction and dispersal, leaf traits that are believed to reflect competitive strategies, overall resource allocation into growth, and seedling growth patterns (Usher 1988, Pattison et al. 1998, Reichard and Hamilton 1997, Kolar and Lodge 2001, Richardson and Rejmánek 2004, Hamilton et al. 2005, Rejmánek et al. 2005). Leaf traits linked to species success are associated with their ability to capture resources, particularly in environments that are resource limited (Funk and Vitousek 2007). For invasive plants, leaf carbon fixation strategies that improve productivity are closely linked to success in introduced habitats (Wright et al. 2004, Westoby and Wright 2006).

While the search for the attributes that influence species performance and especially invasion would undoubtedly lead to advancing general theory, it is often underappreciated just how sensitive trait-performance relationships can be to local environmental conditions. The appreciation of the importance of intraspecific trait variation has greatly increased in community ecology (Albert et al. 2012, Violle et al. 2012), but our understanding of how specific traits, and their variation, influence plant species growing in different habitats is lacking.

This study investigates the aboveground and belowground plant attributes, and especially those that reflect resource allocation, that influence the survivorship of the invasive pink morning glory, Ipomoea carnea Jaq. (Family: Convolvulaceae), in three unique urbanized habitats that it invades in the Nile Delta region in Egypt. Ipomoea carnea is an annual vine that is native to Central and South America, but occurs worldwide in many habitats, including the Nile Delta where it is invasive (Eid 2002, Shaltout et al. 2010). This species reproduces vegetatively and the reproductive vegetative unit is called ramet (an individual stalk from one individual plant) which originates from a semi-underground organ, the caudex. This type of reproduction is very common and considered as a proxy for the successful invasion of I. carnea, additionally it reproduces by seeds and the reproductive unit is called genet which is rarely occurred due to the harsher conditions that required for seed germination. To explore population growth rates of the invasive plant, and how they respond to environmental variation, we used density-independent matrix models (Engelen and Santos 2009, Griffith 2010) to evaluate the survivorship of I. carnea in three introduced habitats (Eid 2002, Jerde and Lewis 2007, Bates et al. 2013). We tested three hypotheses 1) there are specific plant traits that predict the survival probability of I. carnea along an urbanization gradient 2) there is variation in the survival probability in different urban habitat types which is highest in canal banks habitat followed by roadsides and waste lands, representing an urban gradient, and that 3) leaf traits are the best indicators of the successful invasion of I. carnea. Our findings provide valuable information on specific traits that determine invader survival in different novel urban habitat types for a species that is prevalent worldwide.

There was a highly significant effect of habitat on survival probability of I. carnea with lower survival probability in wastelands and roadsides compared to canal banks which showed a remarkably highly survival probability. For adult ramet mortality, wasteland and roadside had the highest, while canal banks had the lowest (Fig. 1a).

Figure 1.

a Comparison between the mean of observed survival probability at habitat level b–d the relationship between fitted survival probabilities from the three top models and survival probabilities from the actual observed data in canal banks, roadsides and wastelands respectively.

Odds ratio (OR)

The odds ratio and confidence interval for modelled variables were interpreted as the ratio of the probability of success (survival) over the probability of failure (mortality). For each top model, we used odds ratios as a measure of statistical significance of the association between each modelled traits and survival probability. Accordingly, all odds ratios were (>1 and <1) (Table 1), therefore the association between each modelled trait and survival probability was statistically significant (Figs 2–4). If we take leaf biomass as an example, we see that one unit increase in this predictor, led to a 9.7% and 9.9% increase in the odds of I. carnea survival in canal bank and wasteland respectively, assuming that other variables are fixed (see Table 2).

Figure 2.

The relationship between survival probability and predictors from the top model for Canal bank habitat.

Figure 3.

The relationship between survival probability and predictors from the top model for wasteland habitat.

Figure 4.

The relationship between survival probability and predictors from the top model for Roadside habitat.

In our study we showed that plant attributes associated with the survival of the invasive species I. carnea differed by urban habitat type. Confirming recent calls for the inclusion of intraspecific variation in ecological studies, we showed that growth and biomass allocation traits were indeed important for predicting species performance, but that the important traits differed among the habitats analyzed. Our results showed that using morphological plant traits provides a simple approach to understand invasive species survival in novel habitats. The critical conclusion is that while the measured traits did influence survival of I. carnea, the importance of specific traits was contingent on the local environment, meaning that local trait-environment interactions need to be understood in order to predict and plan for invasive species.

Our study showed a clear selection of traits in different habitats (Table 2, Figs 2–4). In canal banks habitat, the interaction among leaf biomass, root biomass, number of flowering ramets and non-flowering ramets has a significant effect on the survival rate of I. carnea. In waste lands, leaf biomass, root biomass, number of leaves and stem diameter, but in roadsides stem diameter, number of flowering ramets and non-flowering ramets. Consequently, growth and survival of this species was a function of measured traits, especially those reflecting size.

However, it should be noted that size-based traits were the best predictors for invader survival, which is in some ways not surprising. It is well known that larger plants have higher survival probabilities and greater reproduction (Horvitz and Schemske 2002). Larger plants are those individuals that have already experience successful growth and might reflect the fact that these individuals are in locations with optimal micro environmental conditions. Yet, what was unexpected was that different size traits predict survival in different habitats.

Traits linked to invader colonization in new environments are those most likely to predict invasion success. Trait-environment relationships were also consistent with general patterns observed along large ecological gradients (Ozenda 1985, Pornon et al. 2007). In a meta-analysis of trait comparisons related to invasiveness, shoot allocation, leaf-area allocation, physiology, size, growth rate, and fitness were found to be most important for plant invader success (Van Kleunen et al. 2010).

From the top models, our study showed a significant relationship between leaf traits and different habitats. For example, there was a positive significant effect of the number of leaves on the survival probability of I. carnea in wastelands, whereas survival probability decreased significantly with increasing leaf biomass in canal banks and wastelands habitat. Moodley et al. (2013) stated that for some plant traits there are clear mechanisms for the association of selected traits and invasion success, also some traits show differing responses at the different stages of invasion. Leaf traits in particular have been linked to invader success, including a large meta-analysis of local and global leaf traits predicting invasion (Leishman et al. 2007). Leaf growth and production is dependent upon the rate of supply of limiting resources (Shinozaki and Kira 1956). Both leaf area and leaf consistency are related to the moisture conditions prevailing in the habitat occupied by the plant. The moisture conditions are reflected by climatic and soil factors and it may be difficult to distinguish between the effects of either (Werger and Ellenbroek 1978).

It should be noted that the three habitat types were spatially segregated and thus the analyses would be pseudo replicated if included in single statistical models (Hurlbert 1984). We analyzed the three habitats separately and our expectation was that if trait-performance relationships were robust enough, we should see similar patterns in the different habitats. Yet trait-performance relationships were quite different at the different sites. Future studies should find more habitat replicates or design experiments to delve deeper into the trait-environment relationships.

This study concluded that trait-environment interactions are critical predictors of invader species survival and subsequent success in novel urban habitats. As invasive plant species continue to pose significant threat to natural areas, understanding how they interact in novel, urban habitats is often a first step to understand the dynamics of invasive species in more pristine and protected habitats. Our approach was able to predict the local abundance of I. carnea across a large ecological gradient. Also it can help to assess monitoring of invasive species in native Egyptian ecosystems. We gain a better insight on the rapid growth and adaptability of I. carnea from dry to aquatic habitats which may indicate that this plant is capable of rooting within a few days (Cook 1987, Chaudhuri et al. 1994, Shaltout et al. 2010). Due to the invasion of I. carnea, diversity of native Egyptian wetland flora and fauna is being reduced (Eid 2002). By understanding which traits are correlated to invasion success of I. carnea, what the mechanisms behind such correlations are, and under which conditions invasions are favoured can provide accurate predictive tool to reduce the impact of invasive species.

The authors sincerely thank Sohee Kang (Professor of biostatistics, Department of Math and computer science, Toronto University, Scarborough) for her help in stats analysis that greatly improved our manuscript. MWC was supported by the TD Professor of Urban Forest Conservation and Biology chair, Canada Foundation for Innovation, the Ontario Research Fund, and Natural Sciences and Engineering Research Council of Canada (#386151).