Brian M. Connolly

Direct and indirect effects of plant litter on a seed–pathogen interaction in Bromus tectorum seed banks

The naturally occurring fungal seed pathogen, Pyrenophora semeniperda , reduces the seed bank of Bromus tectorum but the role of plant litter in this seed–pathogen interaction is unexplored. To investigate the direct and indirect effects of litter on this interaction, we first collected field seed-bank samples from low and high Bromus litter patches. From these data, we explored the relationship between litter depth, seed-bank density and seed mortality from P. semeniperda . Second, we manipulated the fungal stages (conidial spores and mycelium) in/on the litter through sterilization techniques, to measure the direct effect of litter on seed death. Third, for indirect effects, we manipulated litter levels and held seed density and inoculum constant to determine whether Bromus litter could modify the seed zone microsites to favour disease. We found that seed-bank samples from high-litter patches contained higher field-killed seed densities compared with low-litter patches, although the percent difference of disease between litter patch types varied among sites and years (e.g. 80% to 46%). In testing the direct effects of litter on the seed–pathogen interaction, we found that litter can act as a direct inoculum source for the pathogen in the early summer but decreases in disease transmission by the following spring when the litter naturally is in contact with seeds. Investigating indirect effects, we found four times as many pathogen-killed seeds in high-litter treatments as compared with low-litter treatments when inoculum loads and seed densities were held constant. In addition, we found that litter influences the seed–pathogen interaction through density-dependent disease transmission. Our findings demonstrate the ecological importance of litter in semi-arid environments as it influences disease levels of a seed pathogen by direct and indirect means.

Induced defences in plants reduce herbivory by increasing cannibalism

Plants are attacked by myriad herbivores, and many plants exhibit anti-herbivore defences. We tested the hypothesis that induced defences benefit tomato plants by encouraging insects to eat other members of their species. We found that defences that promote cannibalism benefit tomatoes in two ways: cannibalism directly reduces herbivore abundance, and cannibals eat significantly less plant material. This previously unknown means of defence may alter plant–herbivore dynamics, plant evolution and pathogen transmission.Plants can induce anti-herbivore defences through the synthesis of proteins that reduce feeding. Induction of these defences in tomato plants also reduces herbivore damage by promoting cannibalism among insect larvae.

Plants eavesdrop on cues produced by snails and induce costly defenses that affect insect herbivores

Although induced defenses are widespread in plants, the degree to which plants respond to herbivore kairomones (incidental chemicals that herbivores produce independent of herbivory), the costs and benefits of responding to cues of herbivory risk, and the ecological consequences of induced defenses remain unclear. We demonstrate that undamaged tomatoes, Solanum lycopersicum, induce defenses in response to a kairomone (locomotion mucus) of snail herbivores (Helix aspersa). Induced defense had significant costs and benefits for plants: plants exposed to snail mucus or a standard defense elicitor (methyl jasmonate, MeJA) exhibited slower growth, but also experienced less herbivory by an insect herbivore (Spodoptera exigua). We also find that kairomones from molluscan herbivores lead to deleterious effects on insect herbivores mediated through changes in plant defense, i.e., mucus-induced defenses of Solanum lycopersicum-reduced growth of S. exigua. These results suggest that incidental cues of widespread generalist herbivores might be a mechanism creating variation in plant growth, plant defense, and biotic interactions.

Changes in Trap Temperature as a Method to Determine Timing of Capture of Small Mammals

Patterns of animal activity provide important insight into hypotheses in animal behavior, physiological ecology, behavioral ecology, as well as population and community ecology. Understanding patterns of animal activity in field settings is often complicated by the need for expensive equipment and time-intensive methods that limit data collection. Because animals must be active to be detected, the timing of detection (e.g., the timing of capture) may be a useful proxy for estimation of activity time. In this paper, we describe a new method for determining timing of capture for small mammals. In our method, two small temperature loggers are positioned in each trap so that one logger registers the internal temperature of a live-trap at set intervals while the other logger simultaneously records external trap temperature. We illustrate the utility of this technique using field data from live-trapping of deer mice, Peromyscus maniculatus, one of the most ubiquitous, widely distributed small mammals in North America. Traps with animals inside registered consistent increases in internal trap temperature, creating a clear, characteristic temperature deviation between the two data loggers that can determine trap entry time within a very narrow time window (e.g., 10 minutes). We also present pilot data to demonstrate the usefulness of the method for two other small-mammal species. This new method is relatively inexpensive, robust to field conditions, and does not require modification of traps or wiring of new devices. It can be deployed as part of common live-trapping methods, making it possible to assay the timing of capture for a large number of animals in many different ecological contexts. In addition to quantifying timing of capture, this approach may also collect meaningful temperature data and provide insight into the thermal costs of animal activity and relationships between environmental conditions and the time of an animal’s capture.

Seedling responses to decreased snow depend on canopy composition and small-mammal herbivore presence

Winter is becoming warmer and shorter across the northern hemisphere, and reductions in snow depth can decrease tree seedling survival by exposing seedlings to harmful microclimates. Similarly, herbivory by small mammals can also limit the survival and distribution of woody plants, but it is unclear whether winter climate change will alter small-mammal herbivory. Although small-scale experiments show that snow removal can either increase or decrease both soil temperatures and herbivory, we currently lack snow-removal experiments replicated across large spatial scales that are needed to understand the effect of reduced snow. To examine how winter herbivory and snow conditions influence seedling dynamics, we transplanted Acer saccharum and Tsuga canadensis seedlings across a 180 km latitudinal gradient in northern Wisconsin, where snow depth varied seven-fold among sites. Seedlings were transplanted into one of two herbivory treatments (small-mammal exclosure, small-mammal access) and one of two late-winter snow removal treatments (snow removed, snow unmanipulated). Snow removal increased soil freeze-thaw frequency and cumulative growing degree-days (GDD), but the magnitude of these effects depended on forest canopy composition. Acer saccharum survival decreased where snow was removed, but only at sites without conifers. Excluding small mammals increased A. saccharum survival at sites where the smallmammal herbivore Myodes gapperi was present. Excluding small mammals also increased T. canadensis survival in plots with < 5 cm snow. Because variation in canopy composition and M. gapperi presence were important predictors of seedling survival across the snow-depth gradient, these results reveal complexity in the ability to accurately predict patterns of winter seedling survival over large spatial scales. Global change scenarios that project future patterns of seedling recruitment may benefit from explicitly considering interactions between snow conditions and small-mammal winter herbivory.

Interactive Effects of Contact Fungicide and Cold Stratification on the Germination Rate for Five Dominant Temperate Tree Species

Fungicide application facilitates seed and seedling survival, but these pesticides may also negatively affect germination. Moreover, it is unclear how fungicide may interact with influential environmental conditions, such as cold stratification, to affect tree seed germination. We examined how fungicide and five different cold stratification durations of increasing length influenced tree seed germination (i.e., germination fraction and germination rate) for five important species commonly found in temperate North American forests (Abies balsamea, Acer saccharum, Picea glauca, Pinus resinosa, and Pinus strobus). Greater cold stratification durations increased A. saccharum germination fraction, had positive effects on P. strobus and A. balsamea germination fractions, and decreased germination for P. glauca. Longer cold stratification reduced time to germination for all species, reinforcing the importance of this pregermination condition in tree seed phenology. We also found that although Captan 50W fungicide had no effect on the germination fraction for any species, it delayed the germination of P. glauca and P. strobus at specific stratification durations. We suggest that delays in tree seed germination—mediated by fungicide application, reductions in stratification duration driven by warmer winters, or both— could hinder seedling survival and performance with long-term effects on the vigor of tree seedlings used for transplanting.