Competitive interactions during community assembly: how native species identity, diversity, and functional traits affect invasion.

Recent environmental change associated with human activities has given rise to ecological communities that have no historical counterpart. In particular, introductions of non-native plant species have in many cases altered the structure and functioning of resident plant communities through changes in species composition and the surrounding environment within which species interact. As a result, new combinations of species are forming “novel” communities across an increasingly large portion of the earth’s land surface. Because novel plant communities differ in configuration from original native-dominated communities, they present unique challenges to management, restoration, and conservation efforts. Thus, there is a growing need to understand how novel communities function differently from the original communities they replace. In this thesis, I investigate a variety of interactions in original and novel plant communities. Using a diverse annual plant system that persists within a fragmented agricultural landscape in Western Australia, I focus on the role of local-scale species interactions, an important biotic component of plant community assembly. I explore the complexities of local-scale interactions between native and non-native invasive species in light of coexistence theory, community assembly, and conservation of native floral diversity. This thesis comprises seven chapters. The first chapter serves as a general introduction which places the thesis within the larger context of multispecies coexistence in novel plant communities. The second chapter serves as a description of the York gum (Eucalyptus loxophleba subsp. loxophleba) – jam (Acacia acuminata) woodland annual flora, the study system for the data chapters (3 – 6) which are based on laboratory and field experiments. Chapters 3 and 4 are experimental evaluations of frequency-dependent and density-dependent performance of native and invasive species that co-occur in York gum-jam annual plant communities. Chapter 5 reports on a field experiment, which investigates the performance of common native and non-native invasive annuals experiencing inter- and intraspecific competition in natural York gum-jam annual assemblages. Chapter 5 also assesses changes in community-level functioning due to compositional differences by evaluating diversity effects in novel and original annual communities. The final data chapter, Chapter 6, experimentally investigates how local-scale environmental gradients and a non-native invasive annual grass impacts annual plant community structure in the field. Finally, I conclude with a discussion of my results in Chapter 7, which unites the previous chapters, addresses limitations of the thesis, and presents suggestions for future research. Overall, my results suggest that the local-scale impacts of non-native invasive species on native species may be more variable than those often reported in the literature on plant invasions (i.e. competitive exclusion). Consistent with previous studies, I did indeed observe negative interactions among invasive non-native and native annual plant species in field and laboratory settings. Specifically, I found that some species of invasive annual grass have the potential to negatively impact native populations over very short timescales through direct competition as well as interference from litter. These negative interactions, however, were not representative of interactions in novel communities as a whole. Notably, I found interactions between native annual forbs and an invasive non-native annual grass that ranged from neutral to positive. The direction and magnitude of invader impacts were highly dependent on species identity and the composition of the community, but were generally consistent across community densities and natural environmental gradients. Species interactions play a potentially complex role in the assembly of annual plant communities post-invasion. My results lend empirical support to the notion that species in these novel communities should be considered according to their impacts rather than their origins. These studies serve as some of the first investigations into the processes that stabilize interactions among native and invasive non-native species and contribute to novel community formation and maintenance. When considered alongside large-scale patterns from observational studies, my findings demonstrate that interactions that occur over small spatial and temporal scales have the potential to influence large-scale plant community dynamics. In total, this thesis represents a valuable contribution to the community ecology and biological invasions literature, and has the potential to inform future restoration and conservation efforts in this threatened woodland ecosystem and beyond.

Community assembly and coexistence theories predict that both fitness and plant functional traits should influence competitive interactions between native and invasive species. The evolution of the increased competitive ability hypothesis predicts that species will grow larger (a measure of fitness) in their invaded than native range; hence we hypothesized that species might exert greater competitive effects in their invaded range, lessening the importance of functional traits for competitive outcomes. In a greenhouse experiment we compared traits and competitive interactions between Bromus madritensis (an annual grass) and resident species from its native range in Spain, and its invaded range in Southern California. As predicted, B. madritensis collected in California grew larger and had a greater competitive effect on resident species than B. madritensis collected in Spain. However, residents from California also suppressed the growth of B. madritensis more than species from its native range in Spain. Competitive interaction strengths were predicted by different suites of traits in the native versus invaded range of B. madritensis; surprisingly, however, size of the resident species (fitness), did not predict variation in competitive interactions. This study shows that different suites of traits may aid in identifying those native species likely to strongly compete with invaders, versus those that will be competitively suppressed by invaders, with important implications for the design of restoration efforts aimed at promoting native species growth and preventing invasion. More generally, our study shows that fitness differences may not be as important as traits when predicting competitive outcomes in this system.

During community assembly, early arriving exotic species might suppress other species to a greater extent than do native species. Because most exotics were intentionally introduced, we hypothesize there was human selection on regeneration traits during introduction. This could have occurred at the across- or within-species level (e.g. during cultivar development). We tested these predictions by seeding a single species that was either native, exotic 'wild-type' (from their native range), or exotic 'cultivated' using 28 grassland species in a glasshouse experiment. Priority effects were assessed by measuring species' effect on establishment of species from a seed mix added 21 d later. Exotic species had higher germination and earlier emergence dates than native species, and differences were found in both 'wild' and 'cultivated' exotics. Exotic species reduced biomass and species diversity of later arriving species much more than native species, regardless of seed source. Results indicate that in situations in which priority effects are likely to be strong, effects will be greater when an exotic species arrives first than when a native species arrives first; and this difference is not merely a result of exotic species cultivation, but might be a general native-exotic difference that deserves further study.

Trait divergence and the ecosystem impacts of invading species

Summary 1 Two main views have been put forward to explain whether coexisting alien and resident plant species should show converging or diverging functional attributes. According to the ‘try‐harder’ hypothesis, successful aliens should differ from resident species with traits that allow them to deal better with the local conditions than resident species. On the other hand, the ‘join‐the‐locals’ hypothesis stresses the importance of filtering by environmental factors and predicts strong functional trait similarities between alien and native species, especially among the dominants. 2 On the basis of a functional trait comparison between native and alien species of central‐western Argentina across five contrasting ecosystems and four land‐use regimes, we tested these hypotheses over a broad range of habitats. We built a data set with common measurement methods and biogeographical factors but strongly varying environmental conditions, ranging from mesic to extremely dry, and from nearly pristine to heavily disturbed. 3 When considering all species together, the main trend of variation in trait syndromes was between acquisitive (tender, large leaves, with high specific area) and conservative (tough, small leaves, with low specific area and low nutrient content). Although both native and alien species appeared to be well spread across the whole range of trait variation, woody alien species showed a significantly more acquisitive set of attributes (higher specific leaf area, larger and thinner leaves, lower wood density) than native species. No significant difference was detected between herbaceous alien and native species. These general trends were maintained under contrasting climatic and land‐use conditions. 4 Synthesis. The patterns detected for herbaceous species were in line with the ‘join‐the‐locals’ hypothesis. In contrast, those found for woody species, with woody alien species showing more acquisitive attributes than native species in more resource‐rich habitats, provide partial support for the ‘try‐harder’ hypothesis. Overall, our findings reinforce the idea that a universal suit of attributes is unlikely to explain alien plant distribution. They also stress the need for caution when mixing major life‐forms in comparative plant trait analysis.

Functional features of tropical montane rain forests along a logging intensity gradient

Exotic species are sometimes phenologically distinct from native species in the invaded community, allowing them to be active when there may be reduced competition for resources. In southern California, annual species are particularly problematic invaders, and prior work has shown that these species germinate earlier in the growing season, giving them a competitive advantage over later‐germinating native species. This result begs the question, if being active earlier is advantageous, why have not native species adapted earlier cues for germination? We hypothesized native species would benefit less from earlier germination than exotic species (potentially due to slower growth following germination), thus negating potential selection for early germination. Here we manipulated planting time for common native and exotic species, growing them in all possible species pairs, to evaluate how competitive outcomes were altered by the time of arrival and the origin of competing species. In contrast to our hypotheses, the exotic species often had lower biomass when planted first, potentially due to disturbance when the second species was planted. In contrast, three out of our four native species benefited from earlier planting (a priority effect). Unlike the potential benefit of arriving early, we found no evidence that being planted one week later resulted in a competitive disadvantage, when compared to being planted simultaneously with a competitor. Further, we found that the magnitude and even direction of priority effects varied depending on the identity of the interacting species. Together these results suggest that a lack of directional selection may prevent adaptation towards earlier germination times of native species. Although this experiment was conducted with a limited suite of species, the results show that the role of seasonal priority effects varies among species, and that native species could benefit from seasonal priority effects in restoration efforts even when in competition with fast‐growing exotic annual species.

ABSTRACTQuestionNative and invasive species interact simultaneously with each other and with their species‐specific soil biota, yet the relative importance of native plant–soil feedback (PSF) on the outcome of competition between these species with different origins is poorly understood. Therefore, we studied the influence of native PSF on the performance of two invasive and two native target species in two situations: (1) when the species were grown alone, and (2) when the target species were grown in pairwise competitive setup with the native conditioning species. We also tested the importance of phylogenetic relatedness between target and conditioning species on the simultaneous effect of PSF and competition.LocationCluj‐Napoca, Romania.MethodsWe used native species from a semi‐dry grassland dominated by Brachypodium pinnatum and Festuca rupicola to study how their species‐specific PSFs affect the performance of invasive (Solidago canadensis, Erigeron canadensis) and native (Centaurea jacea, Crepis foetida) Asteraceae species. In the first year, soil was conditioned by six native grassland species (three Asteraceae and three species from other families); while in the second year, we performed a pairwise competition experiment in pots between the four target and six native species in conditioned and control soils.ResultsWe found that although the native species exerted a strong negative PSF on the performance of the target species, this effect mostly disappeared in the real presence of the native competitors. We also showed that the identity of native resident species is more important in determining PSF and competitive outcome than whether it is dominant or subordinate, or whether it is phylogenetically related to the target species.ConclusionsWe showed that PSF of native species may not influence the competitive outcome between invasive and resident native species, thus PSF does not significantly contribute to the invasion resistance of the studied grassland community.

CHAPTER 1: The understory environmental variables guide to a very specific flora to this stratum, and thus a distinct functional structure from the rest of the community. This study evaluated the floristic diversity, leaf phenology and dispersal of tree species in nine understories of semideciduous forests in different stages of disturbances. Was tested the hypothesis that environmental changes of this stratum, with the increase in disturbance intensity, directly affects the floristic diversity and understory functional structure. Phytosociological parameters were evaluated to species and families of each understory and to all nine understory altogether. Floristic similarity was evaluated between the nine understories, and among understories under the same disturbance intensity. The leaf phenology and dispersal syndrome of species were compared between the understory and the upper strata in each area and among the nine understories. Myrtaceae, Rubiaceae, Celastraceae, and Meliaceae Siparunaceae were the five most important families. The five most representative species were Cheiloclinium cognatum, Cordieria sessilis Siparuna guianensis, Siphoneugena densiflora and Trichilia catigua. The floristic similarity revealed a higher floristic similarity between the understory under the same disturbance intensity. Some species and families were indicative of disturbance to the understory. Comparisons about leaf phenology and dispersion syndrome demonstrated a very low proportion of deciduous and anemochoric species in the understory compared to the upper layers. Was observed a significant increase in deciduous species proportion to the more disturbed forests but not to the anemochory. Even with very distinct floristic diversity, was possible to establish functional patterns to leaf phenology and dispersal syndrome to the understory, and then can be use as a parameter in the classification of successional stages of seasonal forests. CHAPTER 2: The understory of forests is a shaded environment, with species which have functional traits that increase carbon net gain in photosynthesis under low light intensity. The light available raise in the understory, related to disturbance intensity, causes changes in functional leaf traits and tree architecture, related to the capture and light use to understory species. It is also expected that distinct disturbance in understory, favors species groups with different functional characteristics. This study evaluated the intensity of disturbance influence on functional leaf traits (leaf area, dry matter content and specific leaf area), the canopy architecture and wood density of understory species to nine forests. It was also evaluated the leaves functional traits variability into inter / intraspecific levels, and species distinction based on functional traits. The understory functional diversity was estimated based in indices of richness, evenness and functional divergence and species specific leaf area. The results showed that the leaf traits variability were concentrated in the interspecific level, and then has great importance in the species functional differentiation. The increase of disturbance intensity in the understory favored species with higher specific leaf area, contrary to many studies that observed a reduction of this trait with disturbance increase. The differences in relative densities of functionally similar species under different disturbance stages of allowed to establish a model of succession to the understory of seasonal forests. The results also showed that understory with low floristic similarity were functionally very similar. The functional diversity indices established relationships between functional diversity and the conservation state of understory too. The wide variation in species functional traits allowed to identify different strategies of light use and absorption to the understory and thus may explain the high coexistence of species in the understory.

SummaryRecovering biological diversity and ecosystem functioning are primary objectives of ecological restoration, yet these outcomes are often unpredictable. Assessments based on functional traits may help with interpreting variability in both community composition and ecosystem functioning because of their mechanistic and generalizable nature. This promise remains poorly realized, however, because tests linking environmental conditions, functional traits, and ecosystem functioning in restoration are rare.Here, we provide such a test through what is to our knowledge the first empirical application of the ‘response–effect trait framework’ to restoration. This framework provides a trait‐based bridge between community assembly and ecosystem functioning by describing how species respond to environmental conditions based on traits and how the traits of species affect ecosystem functioning.Our study took place across 29 prairies restored from former agricultural fields in southwestern Michigan. We considered how environmental conditions affect ecosystem functioning through and independently of measured functional traits. To do so, we paired field‐collected trait data with data on plant community composition and measures of ecosystem functioning and used structural equation modelling to determine relationships between environmental conditions, community‐weighted means of functional traits and ecosystem functioning.Environmental conditions were predictive of trait composition. Sites restored directly from tillage (as opposed to those allowed to fallow) supported taller species with larger seeds and higher specific leaf area (SLA). Site age and fire frequency were both negatively related to SLA. We also found a positive relationship between soil moisture and SLA.Both trait composition and environmental conditions predicted ecosystem functioning, but these relationships varied among the measured functions. Pollination mode (animal pollination) increased and fire frequency decreased floral resource availability, seed mass had a negative effect on below‐ground biomass production, and vegetative height increased decomposition rate. Soil moisture and fire frequency both increased while site age decreased above‐ground biomass production, and site age and soil moisture both increased decomposition rate.Synthesis and applications. Our results suggest that both trait composition and environmental conditions play a role in shaping ecosystem function during restoration, and the importance of each is dependent on the function of interest. Because of this, environmental heterogeneity will be necessary to promote multiple ecosystem functions across restored landscapes. A trait‐based approach to restoration can aid interpretation of variable outcomes through insights into community assembly and ecosystem functioning.

The effects of biological invasions are most evident in isolated oceanic islands such as the Hawaiian Archipelago, where invasive plant species are rapidly changing the composition and function of plant communities. In this study, we compared the specific leaf area (SLA), leaf tissue construction cost (CC), leaf nutrient concentration, and net CO2 assimilation (A) of 83 populations of 34 native and 30 invasive species spanning elevation and substrate age gradients on Mauna Loa volcano in the island of Hawaii. In this complex environmental matrix, where annual precipitation is higher than 1500 mm, we predicted that invasive species, as a group, will have leaf traits, such as higher SLA and A and lower leaf CC, which may result in more efficient capture of limiting resources (use more resources at a lower carbon cost) than native species. Overall, invasive species had higher SLA and A, and lower CC than native species, consistent with our prediction. SLA and foliar N and P were 22.5%, 30.5%, and 37.5% higher, respectively, in invasive species compared to native ones. Light-saturated photosynthesis was higher for invasive species (9.59 μmol m-2 s-1) than for native species (7.31 μmol m-2 s-1), and the difference was larger when A was expressed on a mass basis. Leaf construction costs, on the other hand, were lower for the invasive species (1.33 equivalents of glucose g-1) than for native species (1.37). This difference was larger when CC was expressed on an area basis. The trends in the above traits were maintained when groups of ecologically equivalent native and invasive species (i.e., sharing similar life history traits and growing in the same habitat) were compared. Foliar N and P were significantly higher in invasive species across all growth forms. Higher N may partially explain the higher A of invasive species. Despite relatively high N, the photosynthetic nitrogen use efficiency of invasive species was 15% higher than that of native species. These results suggest that invasive species may not only use resources more efficiently than native species, but may potentially demonstrate higher growth rates, consistent with their rapid spread in isolated oceanic islands.

QuestionsCalifornia's grasslands are heavily invaded by exotic species. Counteracting these invasions depends on understanding the functional differences between native and exotic species and how these shift along environmental gradients. Focusing on grasses, we ask: (a) how native and exotic trait means and intraspecific trait variation (ITV) differ; (b) how the functional composition of grass assemblages shifts with precipitation; and (c) how species shift their functional strategies as precipitation changes.LocationMarin County, California (USA).MethodsIn spring 2017, we censused grassland vegetation plots across eight reserves in Marin County that are arrayed along a precipitation gradient. At each plot, we measured traits of grasses, including height, leaf area and specific leaf area (SLA).ResultsWe found modest differences in functional traits between native and exotic species. Exotic species had larger seeds and higher SLAs, indicating higher reproductive investments and a leaf strategy focused on obtaining rapid returns on investments with relatively short leaf life spans. Native and exotic species did not differ in the ITV for any of the measured traits. Variation in precipitation among sites drove strong turnover in the functional trait composition of grasses, as well as shifts within species. Wetter sites had shorter species with smaller leaf areas, smaller seeds and higher leaf N concentrations. There was also strong intraspecific SLA variation along the precipitation gradient; all species displayed lower SLA values as precipitation increased.ConclusionsConsistent with previous results, native species had more conservative leaf strategies. Grasses that display these conservative strategies tend to be more abundant in dry climates, leading to increasing exotic abundance in wetter climates. However, each species shifted towards more conservative strategies as precipitation increased, suggesting that the factors that drive species turnover and ITV differ.

ABSTRACTAimPhenological shifts are regarded to be the most pronounced indicators of global climate change. Worldwide, native species pools are being increasingly colonised by non‐native species, thus shaping novel communities. Here, we investigate whether, and how, phenology varies between native and non‐native species. We also examine whether functional traits and/or phylogeny can explain these phenological variations.LocationEurope and Asia.Time Period2022.Major Taxa StudiedPerennial herbs.MethodsWe studied multiple phenophases and functional traits of 427 plant species across 13 botanical gardens using the PhenObs monitoring protocol. We used linear mixed models to test for phenology and functional trait differences between native and non‐native species. Boosted regression trees were employed to identify the functional trait predictors of phenology variance between native and non‐native species. To test the effect of phylogeny on these phenological variations, we estimated phylogenetic signal using Pagel's λ.ResultsNative and non‐native species exhibited distinct phenological patterns and functional traits. Native species started vegetative phenophases earlier than non‐native species and senesced later. Similarly, reproductive phenophases varied, with native species flowering earlier and having longer flowering and fruiting durations. Native species also had higher specific leaf area, while non‐native species had higher seed mass and larger leaf area. The variations in phenology were explained by garden location, leaf area, plant height, and leaf nitrogen. Most phenophases and functional traits showed low phylogenetic conservatism, suggesting a more important role of local environmental factors in driving these variations.Main ConclusionsOur findings suggest that the phenological differences between native and non‐native species, both in timing and magnitude, may change the community composition and structure under global change. Non‐native species likely occupy a subset of native phenological niches, and this overlap may alter biotic interactions and ecosystem functioning. Future research needs to substantiate our findings under natural field conditions.

SummaryWe used seed additions to test experimentally whether long‐term community assembly unfolds consistently (deterministic model) or whether different outcomes are possible depending on variations in climate and the presence of exotic dominants (stochastic model). The experiment was conducted in homogeneous semi‐arid grassland on the northern Great Plains of North America. Native grass seed additions in each of 3 years (1994–96) were combined factorially with repeated selective herbicide applications to reduce the cover of the extant dominant, a Eurasian C3grass (Agropyron cristatum). We assessed whether composition converged or diverged among treatments between 1996 and 2004.The outcome of long‐term community assembly varied by establishment year, in association with significant variations in monthly growing‐season precipitation from 68‐year averages during the years when seed was added. In 2004, the added native C4grassBouteloua gracilisdominated plots sprayed and seeded in 1994 and 1995, resisting re‐invasion and having significantly higher plot diversity. In contrast, plots sown in 1996 – one of the driest years on record – reverted to the exotic C3grass. Seeded but unsprayed plots maintained their pre‐treatment dominance by exotic grass. Rainfall variability was also associated with the extent of exotic grass cover and bare soil in unsprayed plots, and natural recruitment by native species from nearby prairie.Establishment success varied significantly among the added species. All seeded grasses, other thanB. gracilis, were mostly absent by 2004. The failure of these native C3species to establish appears consistent with deterministic assembly models where local abiotic conditions filter out unsuitable species.Synthesis. Our results indicate that stochastic and deterministic processes operate simultaneously to influence community assembly, depending on interactions among climate, seed availability, species identity and disturbance during the initial stages of establishment. Multiple assembly trajectories developed, but the assembled communities did not include all the added species. Conversion of degraded grassland back to native‐dominated grassland was possible, but only with the removal of the exotic dominant and seed additions during years that were suitably wet for establishment. Once formed, however, the assembled native community resisted re‐invasion. This suggests that native grassland restoration will depend on establishment measures as intense as those used during initial cultivation almost a century earlier.

Despite decades of study, the relative importance of niche‐based versus neutral processes in community assembly remains largely ambiguous. Recent work suggests niche‐based processes are more easily detectable at coarser spatial scales, while neutrality dominates at finer scales. Analyses of functional traits with multi‐year multi‐site biodiversity inventories may provide deeper insights into assembly processes and the effects of spatial scale. We examined associations between community composition, species functional traits, and environmental conditions for plant communities in the Kouga‐Baviaanskloof region, an area within South Africa's Cape Floristic Region (CFR) containing high α and β diversity. This region contains strong climatic gradients and topographic heterogeneity, and is comprised of distinct vegetation classes with varying fire histories, making it an ideal location to assess the role of niche‐based environmental filtering on community composition by examining how traits vary with environment. We combined functional trait measurements for over 300 species with observations from vegetation surveys carried out in 1991/1992 and repeated in 2011/2012. We applied redundancy analysis, quantile regression, and null model tests to examine trends in species turnover and functional traits along environmental gradients in space and through time. Functional trait values were weakly associated with most spatial environmental gradients and only showed trends with respect to vegetation class and time since fire. However, survey plots showed greater compositional and functional stability through time than expected based on null models. Taken together, we found clear evidence for functional distinctions between vegetation classes, suggesting strong environmental filtering at this scale, most likely driven by fire dynamics. In contrast, there was little evidence of filtering effects along environmental gradients within vegetation classes, suggesting that assembly processes are largely neutral at this scale, likely the result of very high functional redundancy among species in the regional species pool.