Publications

2012
Garnas JR, Houston DR, Ayres MP, Evans C. Disease ontogeny overshadows effects of climate and species interactions on population dynamics in a nonnative forest disease complex. Ecography [Internet]. 2012;35 :412-421. doi: 10.1111/j.1600-0587.2011.06938.xAbstract

Biotic threats to trees often arise from interactions among two or more species, frequently insects and fungi, that function together to defeat host defenses, secure resources and colonize new hosts. Feedbacks among plant enemies can have large effects on host population and disease dynamics, either by promoting stabilizing negative feedbacks or contributing to positive feedbacks that can destabilize populations and permit outbreaks. Feedbacks can be rapid and direct (e.g. within trees or among years) or can arise from slowly developing changes in host resource quantity or quality at the scale of for- est stands or landscapes. Climate may also influence system dynamics by altering feedbacks within or among species or through density independent effects. We evaluated major drivers of population dynamics of beech bark disease (BBD), an important forest disease in eastern deciduous forests of North America, using data from 28 study sites in the eastern United States monitored for up to 14 yr between 1979 and 1992. Both primary causal agents of BBD – the introduced felted beech scale Cryptococcus fagisuga and native fungi Neonectria spp. – showed strong simple density dependence in all study populations. Surprisingly, densities of scale insects and fungi had little or no effects on population growth rates of the other, despite their habit of living in close physical relationships. For both insects and fungi, ecologically important features of the density dependent functions (slope, carrying capacity and density independent variance) were variable across sites. Climatic effects on density-dependent functions (and scatter around them) were evident but generally weak and variable. The most striking predictor of patterns in density dependence was duration since establishment of BBD in the region. Apparently BBD alters forests over decades in ways that strengthen self-regulation among causal agents without eliminating or even dramatically reducing host populations.

McPhee K, Garnas JR, Drummond F, Groden E. Homopterans and an invasive red ant, Myrmica rubra (L.), in Maine. Environmental Entomology [Internet]. 2012;41 :59-71. doi: 10.1603/EN11046Abstract

Myrmica rubra (L.), is an invasive ant that is spreading across eastern North America. It is presently found in over 40 communities in Maine and areas in Vermont, New Hampshire, Massachusetts, New York, and several provinces in the Canadian Maritimes and Ontario. In addition to disrupting native ant faunas, invasive ants also have been shown to influence homopteran abundance and species composition. We conducted surveys of Homoptera in infested and noninfested sites and conducted manipulative experiments to quantify the effects of M. rubra on homopteran abundance and composition in the summers of 2003, 2006, and 2007 on Mount Desert Island, ME. In 2003, Homoptera family-level richness was higher in infested sites compared with noninfested sites with two out of three sampling methods. Homopteran abundance in infested compared with noninfested sites depended upon the site. The sites with the highest population of M. rubra were associated with significant differences in Homoptera population abundance. In 2006 and 2007, two out of three host plants sampled had significantly higher abundances of the aphids, Aphis spiraephila Patch and Prociphilus tessellatus Fitch. An ant exclusion field experiment on the native plant, meadowsweet (Spiraea alba Du Roi), resulted in higher abundances of A. spiraephila with M. rubra tending compared with native ant tending. A predator exclusion field experiment was conducted on meadowsweet using adult ladybeetles, Hippodamia convergens Guérin-Méneville, larval green lacewings, Chyrsoperla carnea Stephens, and no predators. Predator impacts on aphid populations were reduced in the presence of M. rubra with C. carnea and moderately reduced with H. convergens.

2011
Garnas JR, Ayres MP, Liebhold AM, Evans C. Subcontinental impacts of an invasive tree disease on forest structure and dynamics. Journal of Ecology [Internet]. 2011;99 :532-541. doi: 10.1111/j.1365-2745.2010.01791.xAbstract

1. Introduced pests and pathogens are a major source of disturbance to ecosystems world-wide. The famous examples have produced dramatic reductions in host abundance, including virtual extirpation, but most introductions have more subtle impacts that are hard to quantify but are potentially at least as important due to the pathogens’ effects on host reproduction, competitive ability and stress tolerance. A general outcome could be reduced host abundance with concomitant increases in the abundance of competitors.

2. Beech bark disease (BBD) is a widespread, fatal affliction of American beech (Fagus grandifolia), currently present in c. 50% of beech’s distribution in eastern North America. Despite high adult mortality, beech remains a dominant component of the forest community.

3. Employing spatially extensive data from the national Forest Inventory and Analysis program of the United States Forest Service, we show that forests have changed dramatically in the presence of BBD. Within the 2.3 million km2 range of beech, size-specific mortality was 65% higher in the longest-infected regions, and large beech (>90 cm diameter at breast height) have declined from c. 79 individuals km−2 to being virtually absent. Small stem beech density was dramatically higher (>350%) such that infested forests contain a roughly equivalent cross-sectional (basal) area of beech as before BBD.

4. There was no evidence for compensation by sugar maple or other co-occurring tree species via increased recruitment or adult survivorship at the landscape scale. Overall, community composition remained roughly unchanged as a result of BBD.

5. Surprisingly, trajectory of stand dynamics (shifts in stem density and mean tree size reflecting normal stand maturation (self-thinning) or retrogression (more abundant, smaller trees over time)) did not differ between affected and unaffected regions. Variance in stand dynamics was greater in afflicted forests, however, indicating that predictability of forest structure has been diminished by BBD.

6.Synthesis. Forests of eastern North America have shifted to increased density and dramatically smaller stature – without notable change in tree species composition – following the invasion of a novel forest disease. Our results reinforce the conclusion that introduced diseases alter fundamental properties of ecosystems, but indicate that the spectrum of potential effects is broader than generally appreciated.

2009
Dukes JS, Pontius J, Orwig D, Garnas JR, Rodgers VL, Brazee N, Cooke B, Theoharides KA, Stange EE, Harrington R, et al. Responses of insect pests, pathogens, and invasive plant species to climate change in the forests of northeastern North America: What can we predict?. Canadian Journal of Forest Resources [Internet]. 2009;39 :231-248. doi: 10.1139/X08-171Abstract

Climate models project that by 2100, the northeastern US and eastern Canada will warm by approximately 3–5 °C, with increased winter precipitation. These changes will affect trees directly and also indirectly through effects on “nuisance” species, such as insect pests, pathogens, and invasive plants. We review how basic ecological principles can be used to predict nuisance species’ responses to climate change and how this is likely to impact northeastern forests. We then examine in detail the potential responses of two pest species (hemlock woolly adelgid (Adelges tsugae Annand) and forest tent caterpillar (Malacosoma disstria Hubner)), two pathogens (armillaria root rot (Armillaria spp.) and beech bark disease (Cryptococcus fagisuga Lind. + Neonectria spp.)), and two invasive plant species (glossy buckthorn (Frangula alnus Mill.) and oriental bittersweet (Celastrus orbiculatus Thunb.)). Several of these species are likely to have stronger or more widespread effects on forest composition and structure under the projected climate. However, uncertainty pervades our predictions because we lack adequate data on the species and because some species depend on complex, incompletely understood, unstable relationships. While targeted research will increase our confidence in making predictions, some uncertainty will always persist. Therefore, we encourage policies that allow for this uncertainty by considering a wide range of possible scenarios.

dukes2009_responses_of_insect_pests_pathogens.pdf
2007
Garnas JR, Drummond FA, Groden E. Intercolony aggression within and among local populations of the invasive ant, Myrmica rubra (Hymenoptera: Formicidae), in coastal Maine. Environmental Entomology [Internet]. 2007;36 :105-13. LinkAbstract

Myrmica rubra L. was introduced into New England in the early 20th century, and at present, has a patchy distribution in parts of northeastern North America, including records from 31 communities in Maine. M. rubra is highly polygynous, and colonies reproduce vegetatively, forming dense local populations where conditions are favorable. Using mobile nests and baited arenas in a series of field aggression bioassays, we tested patterns of internest tolerance within and among local populations on Mt. Desert Island, ME. We found that foragers originating from fragments of the same colony or from neighboring nests retained a high level of intraspecific tolerance over several months, whereas significant intercolony aggression among workers was present between colonies within the same local patch separated by approximately 10 m. Within populations, aggression score values were found to increase linearly with internest distance within a site. Aggression was highest between colonies from spatially different populations on the island and was higher still when nests were assayed against colonies at an off-island site 70 km away in Castine, ME. These data strongly suggest a multicolonial organization within and among local populations of M. rubra in parts of its introduced range. These findings contradict the loss of intraspecific aggression and unicolonial social structure over large geographic areas that have previously been observed in other invasive ant species, particularly Linepithema humile Mayr.

garnas2007.pdf
2005
Groden E, Drummond FA, Garnas J, Franceour A. Distribution of an invasive ant, Myrmica rubra (Hymenoptera: Formicidae), in Maine. Journal of Economic Entomology [Internet]. 2005;98 :1774-84. pdfAbstract

Introduced populations of the north temperate ant species, Myrmica rubra (L.), have become pestiferous in various locations in the northeastern United States, particularly in coastal communities in Maine. Native populations of this ant are widely distributed throughout northern Europe and western Asia; however, nest densities in its native range do not usually reach the high levels observed for many introduced populations. This aggressive ant readily stings, and because of its high densities, homeowners continually encounter nests at a frequency that interferes with use of their properties. Surveys were conducted in Maine from 2001 through 2004 to determine the statewide extent of current infestations. Nests in established populations in coastal Maine were sampled from April through September to assess seasonal changes in the density and composition of colonies. Similarities and differences between introduced populations of M. rubra in Maine and published reports of this species in Europe are discussed. Museum records of this species in North America were also researched and are presented.

groden-2005-distribution_of_an_invasive_ant_m.pdf

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