Biological invasions dramatically affect the distribution, abundance and reproduction of many native species. Because of these ecological effects, exotic species can also influence the evolution of natives exposed to novel interactions with invaders. Evolutionary changes in natives in response to selection from exotics are usually overlooked, yet common responses include altered anti-predator defenses, changes in the spectrum of resources and habitats used, and other adaptations that allow native populations to persist in invaded areas. Whether a native population is capable of responding evolutionarily to selection from invaders will depend on the demographic impact of the invader, the genetic architecture and genetic variability of the native population and potentially the history of previous invasions. In some cases, natives will fail to evolve or otherwise adapt, and local or global extinction will result. In other cases, adaptive change in natives may diminish impacts of invaders and potentially promote coexistence between invaders and natives. Here, we review the evidence for evolutionary responses of native species to novel community members. We also discuss how the effects of introduced species may differ from those caused by natural range expansions of native species. Notably, introduced species may come from remote biotas with no previous evolutionary history with the native community. In addition, the rate of addition of introduced species into communities is much greater than all but the most extreme cases of historical biotic exchange. Understanding the evolutionary component of exotic/native species interactions is critical to recognizing the long-term impacts of biological invasions, and to understanding the role of evolutionary processes in the assembly and dynamics of natural communities.

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Evolutionary responses of natives to introduced species: what do introductions tell us about natural communities?

For millions of years the distribution of the world's biota has been restricted by oceans and other natural barriers. During the last 100 years, human activities, especially international travel and trade, have circumvented these barriers and species are invading new continents at an increasing rate. Biological invasions of insect, plants, and fungal pest species often cause substantial disturbance to forest ecosystems and as well as severe socioeconomic impacts. The invasion process is composed of three phases: arrival, establishment, and spread. Arrival occurs when a species is initially transported to the new area (e.g. transportation to a new continent). Establishment is essentially the opposite of extinction; it is the process by which a population becomes abundant enough to prevent extinction. Spread is the process by which a species expands its range into adjoining uninfested areas. Management of pest invasions focuses on preventing arrival, establishment, or spread. We present case histories that illustrate the invasion process via details of the arrival, spread, impact, and management of selected exotic forest pests. Biological invasions are probably the most significant environmental threat to the maintenance of natural forest ecosystems in North America and elsewhere. The magnitude of this problem necessitates increased efforts to reduce the incidence and impacts of pest invasions. FOR. SCI. MONOGR. 30:1-49. ADDITIONAL

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Invasion by Exotic Forest Pests: A Threat to Forest Ecosystems

Phylogenetic and population genetic methods that compare nucleic acid variation are being used to identify species and populations of pathogenic fungi and determine how they reproduce in nature. These studies show that asexual or sexual reproductive morphology does not necessarily correlate with clonal or recombining reproductive behavior, and that fungi with all types of reproductive morphologies and behaviors can be accommodated by a phylogenetic species concept. Although approximately one fifth of described fungi have been thought to be asexual and clonal, recent studies have shown that they are also recombining. Whether a particular pathogen reproduces clonally or by recombination depends on factors relating to its biology and its distribution in space and time. Knowing the identity of species and populations and their reproductive modes, while taking a broad view of pathogen behavior in space and time, should enhance the ability of pathologists to control pathogens and even predict their behavior.

THE EVOLUTION OF ASEXUAL FUNGI: Reproduction, Speciation and Classification

As climate changes, the effects of forest diseases on forest ecosystems will change. We review knowledge of relationships between climate variables and several forest diseases, as well as current evidence of how climate, host and pathogen interactions are responding or might respond to climate change. Many forests can be managed to both adapt to climate change and minimize the undesirable effects of expected increases in tree mortality. We discuss four types of forest and disease management tactics – monitoring, forecasting, planning and mitigation – and provide case studies of yellow-cedar decline and sudden aspen decline to illustrate how forest diseases might be managed in the face of climate change. The uncertainties inherent to climate change effects can be diminished by conducting research, assessing risks, and linking results to forest policy, planning and decision making.

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Climate change and forest diseases

Phytophthora ramorum, causal agent of sudden oak death, is an emerging plant pathogen first observed in North America associated with mortality of tanoak (Lithocarpus densiflorus) and coast live oak (Quercus agrifolia) in coastal forests of California during the mid-1990s. The pathogen is now known to occur in North America and Europe and have a host range of over 40 plant genera. Sudden oak death has become an example of unintended linkages between the horticultural industry and potential impacts on forest ecosystems. This paper examines the biology and ecology of P. ramorum in California and Oregon forests as well discussing research on the pathogen in a broader management context.

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Phytophthora ramorum: integrative research and management of an emerging pathogen in California and Oregon forests.

After a full century in North America, the blister rust epidemic has yet to stabilize, continuing to spread into warmer and drier areas previously considered climatically inhospitable. The disease apparently has no environmental limits wherever white pines and Ribes spp. cohabit and will eventually become pandemic. Although much timber value has been lost, more severe long-term damage is disruption caused to ecosystems by altered patterns of natural succession. During the last half of the century just past, development of genetic resistance superceded other direct control measures-mainly Ribes spp. eradication and antibiotics-which proved ineffective and/or unfeasible in large areas of the white pine range, especially in the West. Several mechanisms of complete (major gene) and partial resistance are common to at least several white pine species. Although North American populations of rust have low genetic variability overall, rust genotypes with specific virulence to major resistance genes exist in some local demes at high frequencies. The challenge will be to package and deploy resistance genes in ways that will dampen sudden increases in rust races of wide virulence. New introductions of blister rust from its gene center in Asia remain the gravest threat to genetic improvement programs.

https://apsjournals.apsnet.org/doi/pdfplus/10.1094/PHYTO.2003.93.8.1044

White pine blister rust in north america: past and prognosis.

A dominant gene for resistance to white pine blister was indicated by Mendelian segregation in full-sib families of western white pine parent trees selected for phenotypic resistance in six heavily infected stands in the Western Cascades of Oregon and Washington. Seedlings were artificially inoculated three times between 1959 and 1964 and observed for development of stem infection. Segregation at this locus (Cr2) occurred in only two of the six parent populations sampled: one a natural stand, Champion Mine (CM), and the other a plantation of unknown seed origin. At CM, reduced penetrance of this gene was expressed by altered Mendelian ratios (mostly less-than-expected resistant phenotypes) in families of specific combinations of certain parents, indicating the presence of modifier genes with effects that ranged from mild to almost complete suppression of Cr2. Between 1968 and 1994, an apparent shift in virulence at CM caused all of the resistant selections to become infected and die. Recent inocu...

https://apsjournals.apsnet.org/doi/pdfplus/10.1094/PHYTO.1999.89.10.861

A major gene for resistance to white pine blister rust in Western white pine from the Western cascade range.

Four of eight white pine species native to western North America surveyed for resistance to white pine blister rust by artificial inoculation showed classical hypersensitive reactions (HR) at frequencies ranging from very low to moderate. Mendelian segregation, indicating a single dominant allele for resistance (Cr3), was observed in southwestern white pine (Pinus strobiformis), as it was previously in sugar pine (P. lambertiana, Cr1) and western white pine (P. monticola, Cr2). HR was present at a relatively high frequency (19%) in one of five bulk seed lot sources of limber pine (P. flexilis), and was also presumed to be conditioned by a single gene locus, by analogy with the other three species. HR was not found in whitebark pine (P. albcaulis), Mexican white pine (P. ayacahuite), foxtail pine (P. balfouriana), or Great Basin bristlecone pine (P. longaeva), but population and sample sizes in these species may have been below the level of detection of alleles in low frequency. When challenged by...

https://apsjournals.apsnet.org/doi/pdfplus/10.1094/PHYTO.2002.92.3.278

Genetic specificity in the white pine-blister rust pathosystem.

Kinloch, B. B., Jr., Sniezko, R. A., and Dupper, G. E. 2003. Origin and distribution of Cr2, a gene for resistance to white pine blister rust in natural populations of western white pine. Phytopathology 93:691-694. The distribution and frequency of the Cr2 gene for resistance to white pine blister rust (Cronartium ribicola) in western white pine (Pinus monticola) was surveyed in natural populations of the host by inoculation of open-pollinated seedlings from 687 individual seed parents from throughout most of the species’ range. Because Cr2 is dominant and results in a conspicuous hypersensitive reaction (HR) in pine needles, the phenotype can readily be detected in offspring of susceptible seed parents fertilized by unknown Cr2 donors in the ambient pollen cloud. Gametic frequencies of Cr2 were thus determined as the proportion of total challenged seedlings that were pollen receptors exhibiting the Cr2 phenotype. Zygotic frequencies, the proportion of seed parents with progeny that segregated in Mendelian ratios for the Cr2 phenotype to the total number of parents, were a complementary, though less precise, measure. Cr2 frequency was rare overall, ranging from 0.004 to 0.008 in the Sierra Nevada to about 0.001 in the central Cascade Range; it was undetectable further north in the Cascades, as well as in the Rocky Mountains and Coast Mountains of the United States and Canada. The diminishing frequency of Cr2 from the southern and central Sierra Nevada northward mirrors that of Cr1 in sugar pine (P. lambertiana) and points to this region as the origin of both genes. We rationalize that this coincidence may have resulted from protection that these genes may have conferred on both species to an endemic pine stem rust congeneric with C. ribicola (C. occidentale) in recent geologic epochs. White pine blister rust, caused by Cronartium ribicola, was introduced into western North America almost a century ago and has had a devastating impact on western white pine (Pinus monticola), a species of major economic and ecological importance. Up to 95% of once nearly pure stands have been destroyed in the northern Rocky Mountains and the effect on natural regeneration is altering patterns of succession (10). Genetic resistance is considered the most feasible method to control the disease and help restore this once magnificent species to its former ecological role. Discovery of Cr2, a major gene for resistance to white pine blister rust in western white pine, was made in a small, naturally regenerated stand in the Western Cascades of Southwest Oregon (8). Known as the Champion Mine site, this stand is in an environment unusually favorable for spread and intensification of white pine blister rust, lying near the top of a steep, moist, narrow canyon with a northwest aspect that supports abundant stands of Ribes bracteosum and other alternate host Ribes spp. Successive epidemic waves of rust since about 1940 eliminated most of the white pines, leaving a naturally selected residual population of highly resistant phenotypes. Most of this parental population was subsequently determined to carry Cr2 in heterozygous form (8). The distribution of this gene elsewhere in western white pine populations is unknown, but is of both theoretical and practical interest because its use to mitigate the impact of white pine blister rust will depend on locating and selecting Cr2 parents adapted to local site conditions. We undertook this investigation to map the distribution and frequency of Cr2 throughout the range of western white pine in order to assess the feasibility of deploying this gene for practical control of the disease, and also to adduce evidence as to its geographic and evolutionary origin. A comparable study of the Cr1 gene in sugar pine (P. lambertiana) has been made (4).

https://apsjournals.apsnet.org/doi/pdfplus/10.1094/PHYTO.2003.93.6.691

Origin and distribution of Cr2, a gene for resistance to white pine blister rust in natural populations of western white pine.

The gametic frequency of a dominant allele (R) for resistance to white pine blister rust, a disease caused by an introduced pathogen (Cronartium ribicola), in natural populations of sugar pine was ...

Bohun B. Kinloch

Papers

White pine blister rust in north america: past and prognosis.

A major gene for resistance to white pine blister rust in Western white pine from the Western cascade range.

Genetic specificity in the white pine-blister rust pathosystem.

Origin and distribution of Cr2, a gene for resistance to white pine blister rust in natural populations of western white pine.

Distribution and frequency of a gene for resistance to white pine blister rust in natural populations of sugar pine