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Review
. 2023 Dec;203(3-4):251-266.
doi: 10.1007/s00442-023-05397-8. Epub 2023 Jun 20.

How ecological and evolutionary theory expanded the 'ideal weed' concept

Jennifer A Lau #  1 , Jennifer L Funk #  2
Affiliations
Review

How ecological and evolutionary theory expanded the 'ideal weed' concept

Jennifer A Lau et al. Oecologia. 2023 Dec.

Abstract

Since Baker's attempt to characterize the 'ideal weed' over 50 years ago, ecologists have sought to identify features of species that predict invasiveness. Several of Baker's 'ideal weed' traits are well studied, and we now understand that many traits can facilitate different components of the invasion process, such as dispersal traits promoting transport or selfing enabling establishment. However, the effects of traits on invasion are context dependent. The traits promoting invasion in one community or at one invasion stage may inhibit invasion of other communities or success at other invasion stages, and the benefits of any given trait may depend on the other traits possessed by the species. Furthermore, variation in traits among populations or species is the result of evolution. Accordingly, evolution both prior to and after invasion may determine invasion outcomes. Here, we review how our understanding of the ecology and evolution of traits in invasive plants has developed since Baker's original efforts, resulting from empirical studies and the emergence of new frameworks and ideas such as community assembly theory, functional ecology, and rapid adaptation. Looking forward, we consider how trait-based approaches might inform our understanding of less-explored aspects of invasion biology ranging from invasive species responses to climate change to coevolution of invaded communities.

Keywords: Adaptation; Community assembly; Functional traits; Invasiveness; Species interactions.

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Conflict of interest statement

The authors declare that they have no conflict of interest.

Figures

Fig. 1
Fig. 1
As noted by van Kleunen et al. (2015) and others (see main text), different traits may be favored at different stages of invasion and their importance may shift from local to regional scales. These traits may be characteristics of the invading species (“Ecological Traits”) or modified by evolution pre- or post-introduction as suggested by a number of hypotheses. Schema modified from van Kleunen et al. (2015). 1Hufbauer et al. (2012), 2Ricciardi and Mottiar (2006), 3Blossey and Notzold (1995), 4Fridley and Sax (2014), 5Darwin (1859)
Fig. 2
Fig. 2
a Species composition within a local community is determined by a series of ecological filters that sort species based on their traits. In this example, both native (blue) and non-native (orange) species are represented in the local community (adapted from Funk 2021). b Analyses that compare trait values with relative abundance or performance in the presence of an ecological filter can determine how invaders succeed; by acting like the natives (shift along common slope) or doing something different (not aligned with slope). In this example, seed predation, a strong biotic filter in some systems, favors species with smaller seeds that are more likely to evade predation from rodents (adapted from Pearson et al. 2018). Some invasive species may under- or over-perform relative to expectations and this information may be used to understand and potentially manipulate the invasion process. c Multivariate trait analyses can identify mechanisms of invasion. For example, two invasive desert annuals have higher growth rates than expected based on water use traits. This pattern suggests that these invaders may not be constrained by growth-conservation trade-offs associated with different rainfall environments (adapted from Huxman et al. and Angert et al. 2009)
Fig. 3
Fig. 3
In this hypothetical example, resistance to specialist herbivores and resistance to generalist herbivores are negatively correlated (here resistance is defined as susceptibility to herbivores and in empirical studies is commonly measured as the inverse of herbivore damage, concentrations of chemical defenses, or level of morphological defenses). In the native range, there is strong selection for increased resistance to both types of herbivores (“native” arrow shows the direction of selection). This selection is perpendicular to the direction of genetic correlation. As a result, there is reduced genetic variation for selection to act upon and the evolutionary response will be slowed. In the introduced range, the invader has escaped specialist enemies leading to selection favoring reduced resistance to specialists and increased resistance to generalists. Here, the direction of selection parallels the direction of the genetic correlation (“introduced” arrow), and the evolutionary response will be greater and more rapid. Each point on the graph depicts the resistance values of an individual or genotype to specialist and generalist herbivores
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