Habacuc Flores-Moreno

High genetic diversity is not essential for successful introduction

Some introduced populations thrive and evolve despite the presumed loss of diversity at introduction. We aimed to quantify the amount of genetic diversity retained at introduction in species that have shown evidence of adaptation to their introduced environments. Samples were taken from native and introduced ranges of Arctotheca populifolia and Petrorhagia nanteuilii. Using microsatellite data, we identified the source for each introduction, estimated genetic diversity in native and introduced populations, and calculated the amount of diversity retained in introduced populations. These values were compared to those from a literature review of diversity in native, confamilial populations and to estimates of genetic diversity retained at introduction. Gene diversity in the native range of both species was significantly lower than for confamilials. We found that, on average, introduced populations showing evidence of adaptation to their new environments retained 81% of the genetic diversity from the native range. Introduced populations of P. nanteuilii had higher genetic diversity than found in the native source populations, whereas introduced populations of A. populifolia retained only 14% of its native diversity in one introduction and 1% in another. Our literature review has shown that most introductions demonstrating adaptive ability have lost diversity upon introduction. The two species studied here had exceptionally low native range genetic diversity. Further, the two introductions of A. populifolia represent the largest percentage loss of genetic diversity in a species showing evidence of substantial morphological change in the introduced range. While high genetic diversity may increase the likelihood of invasion success, the species examined here adapted to their new environments with very little neutral genetic diversity. This finding suggests that even introductions founded by small numbers of individuals have the potential to become invasive.

Climate modifies response of non-native and native species richness to nutrient enrichment

Ecosystem eutrophication often increases domination by non-natives and causes displacement of native taxa. However, variation in environmental conditions may affect the outcome of interactions between native and non-native taxa in environments where nutrient supply is elevated. We examined the interactive effects of eutrophication, climate variability and climate average conditions on the success of native and non-native plant species using experimental nutrient manipulations replicated at 32 grassland sites on four continents. We hypothesized that effects of nutrient addition would be greatest where climate was stable and benign, owing to reduced niche partitioning. We found that the abundance of non-native species increased with nutrient addition independent of climate; however, nutrient addition increased non-native species richness and decreased native species richness, with these effects dampened in warmer or wetter sites. Eutrophication also altered the time scale in which grassland invasion responded to climate, decreasing the importance of long-term climate and increasing that of annual climate. Thus, climatic conditions mediate the responses of native and non-native flora to nutrient enrichment. Our results suggest that the negative effect of nutrient addition on native abundance is decoupled from its effect on richness, and reduces the time scale of the links between climate and compositional change.

Are introduced species better dispersers than native species? A global comparative study of seed dispersal distance.

We provide the first global test of the idea that introduced species have greater seed dispersal distances than do native species, using data for 51 introduced and 360 native species from the global literature. Counter to our expectations, there was no significant difference in mean or maximum dispersal distance between introduced and native species. Next, we asked whether differences in dispersal distance might have been obscured by differences in seed mass, plant height and dispersal syndrome, all traits that affect dispersal distance and which can differ between native and introduced species. When we included all three variables in the model, there was no clear difference in dispersal distance between introduced and native species. These results remained consistent when we performed analyses including a random effect for site. Analyses also showed that the lack of a significant difference in dispersal distance was not due to differences in biome, taxonomic composition, growth form, nitrogen fixation, our inclusion of non-invasive introduced species, or our exclusion of species with human-assisted dispersal. Thus, if introduced species do have higher spread rates, it seems likely that these are driven by differences in post-dispersal processes such as germination, seedling survival, and survival to reproduction.

Mapping local and global variability in plant trait distributions

Significance Currently, Earth system models (ESMs) represent variation in plant life through the presence of a small set of plant functional types (PFTs), each of which accounts for hundreds or thousands of species across thousands of vegetated grid cells on land. By expanding plant traits from a single mean value per PFT to a full distribution per PFT that varies among grid cells, the trait variation present in nature is restored and may be propagated to estimates of ecosystem processes. Indeed, critical ecosystem processes tend to depend on the full trait distribution, which therefore needs to be represented accurately. These maps reintroduce substantial local variation and will allow for a more accurate representation of the land surface in ESMs. Our ability to understand and predict the response of ecosystems to a changing environment depends on quantifying vegetation functional diversity. However, representing this diversity at the global scale is challenging. Typically, in Earth system models, characterization of plant diversity has been limited to grouping related species into plant functional types (PFTs), with all trait variation in a PFT collapsed into a single mean value that is applied globally. Using the largest global plant trait database and state of the art Bayesian modeling, we created fine-grained global maps of plant trait distributions that can be applied to Earth system models. Focusing on a set of plant traits closely coupled to photosynthesis and foliar respiration—specific leaf area (SLA) and dry mass-based concentrations of leaf nitrogen (Nm) and phosphorus (Pm), we characterize how traits vary within and among over 50,000 ∼50×50-km cells across the entire vegetated land surface. We do this in several ways—without defining the PFT of each grid cell and using 4 or 14 PFTs; each model’s predictions are evaluated against out-of-sample data. This endeavor advances prior trait mapping by generating global maps that preserve variability across scales by using modern Bayesian spatial statistical modeling in combination with a database over three times larger than that in previous analyses. Our maps reveal that the most diverse grid cells possess trait variability close to the range of global PFT means.

A comparison of the recruitment success of introduced and native species under natural conditions.

It is commonly accepted that introduced species have recruitment advantages over native species. However, this idea has not been widely tested, and those studies that have compared survival of introduced and native species have produced mixed results. We compiled data from the literature on survival through germination (seed to seedling survival), early seedling survival (survival through one week from seedling emergence) and survival to adulthood (survival from germination to first reproduction) under natural conditions for 285 native and 63 introduced species. Contrary to expectations, we found that introduced and native species do not significantly differ in survival through germination, early seedling survival, or survival from germination to first reproduction. These comparisons remained non-significant after accounting for seed mass, longevity and when including a random effect for site. Results remained consistent after excluding naturalized species from the introduced species data set, after performing phylogenetic independent contrasts, and after accounting for the effect of life form (woody/non-woody). Although introduced species sometimes do have advantages over native species (for example, through enemy release, or greater phenotypic plasticity), our findings suggest that the overall advantage conferred by these factors is either counterbalanced by advantages of native species (such as superior adaptation to local conditions) or is simply too small to be detected at a broad scale.

Peeking beneath the hood of the leaf economics spectrum

It has long been known that leaf trait trade-offs exist (Reich et al., 1992) and can be viewed collectively as a leaf economic spectrum (LES; Wright et al., 2004) that identifies a manifold of strategies successful (and thus selected for) within, and among, communities (Falster et al., 2012). In short, a rapid return on investment (‘fast’ economic strategy; Reich, 2014) is typically associated with leaves with shorter longevity, higher nutrient concentrations and gas exchange rates, and thinner and/or less dense leaves (lower leaf mass per area, or LMA);with a slower return on investment associatedwith the opposite set of traits (Reich et al., 1992, 1997; Wright et al., 2004). Taxawith the ‘fast’ strategy are oftenmore successful in higher resource micro-environments in time and space, and slow strategy taxa the reverse (Reich, 2014; Kunstler et al., 2016). However, the anatomical, morphological, and structural mechanisms underpinning the LES have received far more conceptual than empirical consideration. In a paper in this issue ofNew Phytologist, Onoda et al. (pp. 1447–1463) empirically tackle several questions about those underlying mechanisms and, in so doing, help advance our understanding of the anatomical and physiological relationships underpinning the LES, while perhaps involuntarily, also raising important questions about what drives these trade-off relationships. What is most novel about their study is the bringing together of considerable data on rarely measured leaf traits, assessing both chemical (e.g. nitrogen (N) allocation) and diffusive (mesophyll conductance) constraints at the same time, and identifying a key role for cell-wall thickness in both of these. Fig. 1 outlines LES relationships already well demonstrated (black lines), newer relationships (red lines) shownfirst or bolstered byOnoda et al., and how sets of relationships might influence other sets (green arrows). Arguments have beenmade that theLES is strongbecause certain leaf economic ‘design choices’ involving LMA have unavoidable consequences for carbon (C) gain (Reich et al., 1998). For example, slow return on investment strategies are associated with low returns per unit time on investment in leaf nutrients (e.g. low instantaneous photosynthetic N-use efficiency, PNUE; Fig. 1e), but spread over a long period of time (Westoby et al., 2000; Falster et al., 2012; Fig. 1c). This low PNUE in ‘slow’ strategy plants may be due to a greater proportional allocation of C and N to structural than metabolic components of the leaf in high LMA leaves (Poorter et al., 2009), and to the associated higher mesophyll diffusive limitations (Niinemets et al., 2009;Terashima et al., 2011).Onoda et al. provide support for these assertions by showing that greater proportional allocation of C and N to cell walls (i.e. structure), rather than to photosynthetic machinery, is positively related to LMA and increases diffusive limitations (low mesophyll conductance). Their results provide general support for related ideas and evidence in the literature (Flexas et al., 2012; Funk et al., 2013; Tomas et al., 2013; Tosens et al., 2015; John et al., 2017).