Marcel Rejmánek

Plant invasions – the role of mutualisms

Many introduced plant species rely on mutualisms in their new habitats to overcome barriers to establishment and to become naturalized and, in some cases, invasive. Mutualisms involving animalmediated pollination and seed dispersal, and symbioses between plant roots and microbiota often facilitate invasions. The spread of many alien plants, particularly woody ones, depends on pollinator mutualisms. Most alien plants are well served by generalist pollinators (insects and birds), and pollinator limitation does not appear to be a major barrier for the spread of introduced plants (special conditions relating to Ficus and orchids are described). Seeds of many of the most notorious plant invaders are dispersed by animals, mainly birds and mammals. Our review supports the view that tightly coevolved, plant‐vertebrate seed dispersal systems are extremely rare. Vertebrate‐dispersed plants are generally not limited reproductively by the lack of dispersers. Most mycorrhizal plants form associations with arbuscular mycorrhizal fungi which, because of their low specificity, do not seem to play a major role in facilitating or hindering plant invasions (except possibly on remote islands such as the Galapagos which are poor in arbuscular mycorrhizal fungi). The lack of symbionts has, however, been a major barrier for many ectomycorrhizal plants, notably for Pinus spp. in parts of the southern hemisphere. The roles of nitrogen‐fixing associations between legumes and rhizobia and between actinorhizal plants and Frankia spp. in promoting or hindering invasions have been virtually ignored in the invasions literature. Symbionts required to induce nitrogen fixation in many plants are extremely widespread, but intentional introductions of symbionts have altered the invasibility of many, if not most, systems. Some of the worlds worst invasive alien species only invaded after the introduction of symbionts. Mutualisms in the new environment sometimes re‐unite the same species that form partnerships in the native range of the plant. Very often, however, different species are involved, emphasizing the diffuse nature of many (most) mutualisms. Mutualisms in new habitats usually duplicate functions or strategies that exist in the natural range of the plant. Occasionally, mutualisms forge totally novel combinations, with profound implications for the behaviour of the introduced plant in the new environment (examples are seed dispersal mutualisms involving wind‐dispersed pines and cockatoos in Australia; and mycorrhizal associations involving plant roots and fungi). Many ecosystems are becoming more susceptible to invasion by introduced plants because: (a) they contain an increasing array of potential mutualistic partners (e.g. generalist frugivores and pollinators, mycorrhizal fungi with wide host ranges, rhizobia strains with infectivity across genera); and (b) conditions conducive for the establishment of various alienalien synergisms are becoming more abundant. Incorporating perspectives on mutualisms in screening protocols will improve (but not perfect) our ability to predict whether a given plant species could invade a particular habitat.

Alien plants in checklists and floras: towards better communication between taxonomists and ecologists

The number of studies dealing with plant invasions is increasing rapidly, but the accumulating body of knowledge has unfortunately also spawned increasing confusion about terminology. Invasions are a global phenomenon and comparison of geographically distant regions and their introduced biota is a crucially important methodological approach for elucidation of the determinants of invasiveness and invasibility. Comparative studies of alien floras provide substantial new insights to our understanding of general patterns of plant invasions. Such studies, using information in previously published floras and checklists, are fundamentally dependent on the quality of the assessment of particular species with respect to their taxonomic identity, time of immigration and invasion status. Three crucial decisions should be made when defining the status of a plant species in a given region: (1) whether the taxon is native or alien to that region (origin status); (2) what is its position in the invasion process, i.e., when was it introduced (residence status); and (3) what is the degree of its naturalization and possible invasion (invasion status). Standard floras differ hugely in their treatment of non-native species and those with appropriate categorization of alien species according to their status are rather rare. The present paper suggests definitions of terms associated with plant invasions and places these into the context of floras. Recommendations are outlined on how to deal with the issue of plant invasions in standard floras with the aim of contributing to a better understanding between taxonomists and ecologists and allowing more detailed comparative analyses of alien floras of various regions of the world.

Toward a causal explanation of plant invasiveness: Seedling growth and life-history strategies of 29 pine (Pinus) species

We studied 29 pine (Pinus) species to test the hypothesis that invasive species in disturbed habitats have distinct attributes. Seedling relative growth rate (RGR) and measures of invasiveness were positively associated across species as well as within phylogenetically independent contrasts. High RGR, small seed masses, and short generation times characterize pine species that are successful invaders in disturbed habitats. Discriminant analysis and logistic regression revealed that RGR was the most significant factor among these life‐history traits separating invasive and noninvasive species. We also explored the causes of differences in RGR among invasive and noninvasive species. While net assimilation rate, leaf mass ratio, and specific leaf area (SLA) were all found to be contributing positively to RGR, SLA was found to be the main component responsible for differences in RGR between invasive and noninvasive pines. We investigated differences in SLA further by studying leaf anatomy, leaf density, and leaf thickness. We also evaluated relative leaf production rate as an important aspect of SLA. We proposed a hypothetical causal network of all relevant variables.

A theory of seed plant invasiveness: The first sketch

Abstract Although biological invasions are clearly one of the most important impacts humans have had on the Earths ecosystems, we still do not have reliable tools which can help us to predict which species are potential invaders. At present, several limited generalizations are available for seed plants: (1) invasiveness of woody species in disturbed landscapes is significantly associated with small seed mass, short juvenile period, and short mean interval between large seed crops; (2) vertebrate dispersal is responsible for the success of many woody invaders in disturbed as well as ‘undisturbed’ habitats; (3) primary (native) latitudinal range of herbaceous Gramineae, Compositae, and Fabaceae seems to be the best predictor of their invasiveness, at least for species introduced from Eurasia to North America; (4) low nuclear DNA content (genome size) seems to be a result of selection for short minimum generation time and, therefore, may be associated with plant invasiveness in disturbed landscapes; (5) analysis of exotic Gramineae and Compositae introduced from Europe to California supports Darwins suggestion that alien species belonging to exotic genera are more likely to be invasive than alien species from genera represented in the native flora. Fortunately, these seemingly disparate stories can be brought together and provide a foundation for building a general theory of seed plant invasiveness.

High seedling relative growth rate and specific leaf area are traits of invasive species: phylogenetically independent contrasts of woody angiosperms

Understanding causal factors of exotic species invasions is important not only for prevention and prioritizing control efforts, but also for providing valuable insights into the underlying biology of contrasting life-history strategies. In seedling growth analyses, invasive woody species were compared with less-invasive woody species commonly cultivated in California using phylogenetically corrected procedures (12 phylogenetically independent contrasts). Invasive species were hypothesized to have higher seedling relative growth rates (RGRs) and specific leaf areas (SLAs) than did related less-invasive species. In phylogenetically independent contrasts conducted among taxa within families, high seedling RGRs and SLAs have significant positive associations with woody plant invasiveness. For contrasts containing species invasive in mediterranean regions, invasive species had significantly larger root biomass allocation than did less-invasive species. Optimization of fast seedling growth (high RGR) associated with opportunistic resource acquisition (high SLA) and increased root allocation to survive summer drought may be critical for the success of plant invaders in regions with mediterranean climates.

EVOLUTION OF GENOME SIZE IN PINES (PINUS) AND ITS LIFE‐HISTORY CORRELATES: SUPERTREE ANALYSES

Abstract Genome size has been suggested to be a fundamental biological attribute in determining life‐history traits in many groups of organisms. We examined the relationships between pine genome sizes and pine phylogeny, environmental factors (latitude, elevation, annual rainfall), and biological traits (latitudinal and elevational ranges, seed mass, minimum generation time, interval between large seed crops, seed dispersal mode, relative growth rate, measures of potential and actual invasiveness, and level of rarity). Genome sizes were determined for 60 pine taxa and then combined with published values to make a dataset encompassing 85 species, or 70% of species in the genus. Supertrees were constructed using 20 published source phylogenies. Ancestral genome size was estimated as 32 pg. Genome size has apparently remained stable or increased over evolutionary time in subgenus Strobus, while it has decreased in most subsections in subgenus Pinus. We analyzed relationships between genome size and life‐history variables using cross‐species correlations and phylogenetically independent contrasts derived from supertree constructions. The generally assumed positive relation between genome size and minimum generation time could not be confirmed in phylogenetically controlled analyses. We found that the strongest correlation was between genome size and seed mass. Because the growth quantities specific leaf area and leaf area ratio (and to a lesser extent relative growth rate) are strongly negatively related to seed mass, they were also negatively correlated with genome size. Northern latitudinal limit was negatively correlated with genome size. Invasiveness, particularly of wind‐dispersed species, was negatively associated with both genome size and seed mass. Seed mass and its relationships with seed number, dispersal mode, and growth rate contribute greatly to the differences in life‐history strategies of pines. Many life‐history patterns are therefore indirectly, but consistently, associated with genome size.

Species Richness and Resistance to Invasions

Traditionally, tropical forests, and especially tropical rain forests, have been contrasted with extratropical communities in terms of their species diversity and stability (Elton 1958). Unfortunately, ecologists have used the word’ stability’ to mean several different things (Orians 1975; Harrison 1979; Pimm 1984): Resilience can be defined as a rate of return of population densities, community composition, or collective properties like biomass production, to conditions preceding a perturbation. Persistence usually means how long presence of individual populations or community composition last. Resistance means the degree to which a variable of interest remains unaltered following perturbation. Constancy usually means a lack of change (low variability) of variables of interest over time. Finally, systems are defined as stable in a narrow sense if, and only if, variables of interest return to their initial (equilibrium) values, following perturbation. Elton (1958) himself switched between different meanings of stability when he talked about absence of insect outbreaks in tropical forests (high population constancy) and about higher frequency of extinctions and invasions in simple communities (low persistence). Elton suggested that species rich communities like tropical rain forests possess “complex systems of checks and buffers” responsible for their stability. Causal positive connections between biotic diversity and low variability or high persistence of tropical comunities have been questioned many times since Elton’s influential book was published (Futuyma 1973; Farnworth and Golley 1974; Leigh 1975; Wolda 1978, 1983; Maury-Lechon et al. 1984). Elton, however, should be prized for drawing the long-lasting attention of ecologists to relationships between diversity and stability in ecological systems.

Small rodents as significant dispersers of tree seeds in a Neotropical forest

Through seed dispersal and predation, terrestrial mammals should be an important component of the mecha- nisms that determine patterns of tree recruitment in tropical forests. Despite their great abundance and ubiquity in Neo- tropical forests, small rodents as seed predators and dispersers remain largely forgotten. To investigate the fates of seeds in a hunted primary forest in Belize, we tagged seeds ofAstrocaryum mexicanum (Palmae), Ampelocera hottlei (Ulmaceae), and Pouteria sapota (Sapotaceae) and placed them into open plots, exclosures accessible only to small mammals, and exclosures accessible to medium-sized and small mammals. The exclosure experiments and fates of the seeds show that the spiny pocket mouse, Heteromys desmarestianus (Heteromyidae), was the dominant handler of seeds of the first two species and also removed a significant proportion of the very large-seeded Pouteria. Most of the seeds were killed immediately upon removal, but many of the seeds (3 - 18 %) of the first two species were scatterhoarded (dispersed and buried in the soil) by Heteromys. Some of the scatterhoarded seeds (29 %) remain buried and therefore protected from predation by other animals. Agoutis (Dasyprocta punctata), a caviomorph rodent, buried 13 % of the seeds of Pouteria, and Heteromys consumed and dispersed but did not bury Pouteria seeds. Results of this study support predictions by some researchers that small rodents are dominant terrestrial granivores in Neotropical forests. The role of small rodents as seed dispersers, however, has never been fully appreciated.

Contrasting ectomycorrhizal fungal communities on the roots of co-occurring oaks (Quercus spp.) in a California woodland.

Plant host species is considered an important factor influencing ectomycorrhizal (EM) communities. To gain insights into the role of host species in structuring EM communities, EM communities on sympatric oak (Quercus) species were compared in the Sierra Nevada foothills of California. Using molecular methods (polymerase chain reaction, cloning, restriction fragment length polymorphism and DNA sequencing), EM fungi on roots of deciduous Quercus douglasii and evergreen Quercus wislizeni trees were identified from 64 soil cores. The total EM species richness was 140, of which 40 taxa were detected on both oak hosts. Greater diversity and frequency of EM fungi with epigeous fruiting habit were found on Q. wislizeni, while taxa in the Ascomycota were more frequent and diverse on Q. douglasii. Using ordination, it was determined that both soil extractable phosphorus and oak host species explained a significant proportion of the variation in EM species distribution. These results indicate that plant host species can be an important factor influencing EM fungal community composition, even within congeneric trees.

Vegetation dynamics in the emerging Atchafalaya Delta, Louisiana, USA.

1. The wetlands of the Atchafalaya Delta, Louisiana are characterized by a warm climate, highly fertile sediments, and an absence of moisture limitations. Consequently, vegetation succession (on islands that emerged in 1973) was expected to occur relatively rapidly. 2. From 1980 until 1986, annual surveys of vegetation cover and substrate elevation were conducted on 110 permanent plots along six transect lines crossing four islands. In addition, to evaluate the effect of herbivory (predominantly by Myocastor coypus, nutria or coypu), four 50-m × 40-m exclosures and four control plots were constructed on two of the islands