Preface to the Princeton Landmarks in Biology Edition vii Preface xi Symbols Used xiii 1. The Importance of Islands 3 2. Area and Number of Speicies 8 3. Further Explanations of the Area-Diversity Pattern 19 4. The Strategy of Colonization 68 5. Invasibility and the Variable Niche 94 6. Stepping Stones and Biotic Exchange 123 7. Evolutionary Changes Following Colonization 145 8. Prospect 181 Glossary 185 References 193 Index 201

The Theory of Island Biogeography

There is now ample evidence of the ecological impacts of recent climate change, from polar terrestrial to tropical marine environments. The responses of both flora and fauna span an array of ecosystems and organizational hierarchies, from the species to the community levels. Despite continued uncertainty as to community and ecosystem trajectories under global change, our review exposes a coherent pattern of ecological change across systems. Although we are only at an early stage in the projected trends of global warming, ecological responses to recent climate change are already clearly visible.

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Ecological responses to recent climate change.

Human alteration of Earth is substantial and growing. Between one-third and one-half of the land surface has been transformed by human action; the carbon dioxide concentration in the atmosphere has increased by nearly 30 percent since the beginning of the Industrial Revolution; more atmospheric nitrogen is fixed by humanity than by all natural terrestrial sources combined; more than half of all accessible surface fresh water is put to use by humanity; and about one-quarter of the bird species on Earth have been driven to extinction. By these and other standards, it is clear that we live on a human-dominated planet.

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Human Domination of Earth's Ecosystems

Scenarios of changes in biodiversity for the year 2100 can now be developed based on scenarios of changes in atmospheric carbon dioxide, climate, vegetation, and land use and the known sensitivity of biodiversity to these changes. This study identified a ranking of the importance of drivers of change, a ranking of the biomes with respect to expected changes, and the major sources of uncertainties. For terrestrial ecosystems, land-use change probably will have the largest effect, followed by climate change, nitrogen deposition, biotic exchange, and elevated carbon dioxide concentration. For freshwater ecosystems, biotic exchange is much more important. Mediterranean climate and grassland ecosystems likely will experience the greatest proportional change in biodiversity because of the substantial influence of all drivers of biodiversity change. Northern temperate ecosystems are estimated to experience the least biodiversity change because major land-use change has already occurred. Plausible changes in biodiversity in other biomes depend on interactions among the causes of biodiversity change. These interactions represent one of the largest uncertainties in projections of future biodiversity change.

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Global biodiversity scenarios for the year 2100.

Biotic invaders are species that establish a new range in which they proliferate, spread, and persist to the detriment of the environment. They are the most important ecological outcomes from the unprecedented alterations in the distribution of the earth's biota brought about largely through human transport and commerce. In a world without borders, few if any areas remain sheltered from these im- migrations. The fate of immigrants is decidedly mixed. Few survive the hazards of chronic and stochastic forces, and only a small fraction become naturalized. In turn, some naturalized species do become invasive. There are several potential reasons why some immigrant species prosper: some escape from the constraints of their native predators or parasites; others are aided by human-caused disturbance that disrupts native communities. Ironically, many biotic invasions are apparently facilitated by cultivation and husbandry, unintentional actions that foster immigrant populations until they are self-perpetuating and uncontrollable. Whatever the cause, biotic invaders can in many cases inflict enormous environmental damage: (1) Animal invaders can cause extinctions of vulnerable native species through predation, grazing, competition, and habitat alteration. (2) Plant invaders can completely alter the fire regime, nutrient cycling, hydrology, and energy budgets in a native ecosystem and can greatly diminish the abundance or survival of native species. (3) In agriculture, the principal pests of temperate crops are nonindigenous, and the combined expenses of pest control and crop losses constitute an onerous "tax" on food, fiber, and forage production. (4) The global cost of virulent plant and animal diseases caused by parasites transported to new ranges and presented with susceptible new hosts is currently incalculable. Identifying future invaders and taking effective steps to prevent their dispersal and establishment con- stitutes an enormous challenge to both conservation and international commerce. Detection and management when exclusion fails have proved daunting for varied reasons: (1) Efforts to identify general attributes of future invaders have often been inconclusive. (2) Predicting susceptible locales for future invasions seems even more problematic, given the enormous differences in the rates of arrival among potential invaders. (3) Eradication of an established invader is rare, and control efforts vary enormously in their efficacy. Successful control, however, depends more on commitment and continuing diligence than on the efficacy of specific tools themselves. (4) Control of biotic invasions is most effective when it employs a long-term, ecosystem- wide strategy rather than a tactical approach focused on battling individual invaders. (5) Prevention of invasions is much less costly than post-entry control. Revamping national and international quarantine laws by adopting a "guilty until proven innocent" approach would be a productive first step. Failure to address the issue of biotic invasions could effectively result in severe global consequences, including wholesale loss of agricultural, forestry, and fishery resources in some regions, disruption of the ecological processes that supply natural services on which human enterprise depends, and the creation of homogeneous, impoverished ecosystems composed of cosmopolitan species. Given their current scale, biotic invasions have taken their place alongside human-driven atmospheric and oceanic alterations as major agents of global change. Left unchecked, they will influence these other forces in profound but still unpredictable ways.

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Biotic invasions: causes, epidemiology, global consequences, and control

. Much confusion exists in the English-language literature on plant invasions concerning the terms ‘naturalized’ and ‘invasive’ and their associated concepts. Several authors have used these terms in proposing schemes for conceptualizing the sequence of events from introduction to invasion, but often imprecisely, erroneously or in contradictory ways. This greatly complicates the formulation of robust generalizations in invasion ecology.

Based on an extensive and critical survey of the literature we defined a minimum set of key terms related to a graphic scheme which conceptualizes the naturalization/invasion process. Introduction means that the plant (or its propagule) has been transported by humans across a major geographical barrier. Naturalization starts when abiotic and biotic barriers to survival are surmounted and when various barriers to regular reproduction are overcome. Invasion further requires that introduced plants produce reproductive offspring in areas distant from sites of introduction (approximate scales: > 100 m over  6 m/3 years for taxa spreading by roots, rhizomes, stolons or creeping stems). Taxa that can cope with the abiotic environment and biota in the general area may invade disturbed, seminatural communities. Invasion of successionally mature, undisturbed communities usually requires that the alien taxon overcomes a different category of barriers.

We propose that the term ‘invasive’ should be used without any inference to environmental or economic impact. Terms like ‘pests’ and ‘weeds’ are suitable labels for the 50–80% of invaders that have harmful effects. About 10% of invasive plants that change the character, condition, form, or nature of ecosystems over substantial areas may be termed ‘transformers’.

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Naturalization and invasion of alien plants: concepts and definitions

Humans move species beyond their native ranges both deliberately and inadvertently, and many of these species become established and spread in their new habitat. The list of established introduced species grows annually, as does the number of them that cause significant economic and ecological effects. One recent and notorious example in North America is the Eurasian zebra mussel which like many other aquatic organisms entered in the ballast water of ships, and like many others spread rapidly once it arrived. The invasion of zebra mussels is unusual in the magnitude of its economic consequences; the mussels grow and reproduce rapidly, covering river and lake bottoms and municipal and industrial water inlets. The cost of clearing blocked intake pipes has been calculated to be approximately US$2 billion (Office of Technology Assessment, 1993). Zebra mussels also alter populations of algae and the concentrations of nutrients in whole ecosystems (Caraco et al., 1997), and they are continuing to spread in rivers, lakes, and canals throughout North America. We suggest that biological invasions by notorious species like the zebra mussel, and its many less-famous counterparts, have become so widespread as to represent a significant component of global environmental change. This point has been made before (eg Elton, 1958), but is not widely appreciated, even by the global change research community or by those who study and/or work to control biological invasions. In part, this lack of appreciation reflects the fact that our perception is limited spatially it is possible to document the presence and importance of biological invasions almost anywhere, but more difficult to perceive that invasions are almost everywhere. In part, it may also reflect a narrow view of global environmental change, one that emphasizes climate change (global warming) at the expense of other, equally significant components of human-caused global change. In this paper, we place biological invasions in context with other human-caused global environmental changes; briefly describe the global extent of biological invasion; illustrate the consequences of particular invasions as they affect human health and wealth, and/or the functioning and biological diversity of natural ecosystems; discuss interactions between biological invasions and other components of global change; and describe ways that society can prevent, manage, and/or cope with invasions.

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Introduced species: a significant component of human-caused global change

What attributes make some plant species more invasive

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 animal-mediated 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 world's 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 conductive for the establishment of various alien/alien 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.

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Plant invasions--the role of mutualisms.

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.

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Marcel Rejmánek

Papers

Naturalization and invasion of alien plants: concepts and definitions

Introduced species: a significant component of human-caused global change

What attributes make some plant species more invasive

Plant invasions--the role of mutualisms.

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