Woody plant secondary chemicals increase in response to abundant deer and arrival of invasive plants in suburban forests

Plants in suburban forests of eastern North America face the dual stressors of high white-tailed deer density and invasion by nonindigenous plants. The combination of chronic deer herbivory and strong competition from invasive plants could alter a plant’s stress- and defense-related secondary chemistry, especially for long-lived juvenile trees in the understory, but this has not been studied. We measured foliar total antioxidants, phenolics, and flavonoids in juveniles of two native trees, Fraxinus pennsylvanica (green ash) and Fagus grandifolia (American beech), growing in six forests in the suburban landscape of central New Jersey, USA. The trees grew in experimental plots that had been subject for 2.5 years to factorial treatments of deer access/exclosure X addition/no addition of the nonindigenous invasive grass Microstegium vimineum (Japanese stiltgrass). As other hypothesized drivers of plant secondary chemistry, we also measured non-stiltgrass herb layer cover, light levels, and water availability. Univariate mixed model analysis of the deer and stiltgrass effects and multivariate structural equation modeling (SEM) of all variables showed that both greater stiltgrass cover and greater deer pressure induced antioxidants, phenolics, and flavonoids, with some variation between species. Deer were generally the stronger factor, and stiltgrass effects were most apparent at high stiltgrass density. SEM also revealed that soil dryness directly increased the chemicals; deer had additional positive, but indirect, effects via influence on the soil; in beech PAR positively affected flavonoids; and herb layer cover had no effect. Juvenile trees’ chemical defense/stress responses to deer and invasive plants can be protective, but also could have a physiological cost, with negative consequences for recruitment to the canopy. Ecological implications for species and their communities will depend on costs and benefits of stress/defense chemistry in the specific environmental context, particularly with respect to invasive plant competitiveness, extent of invasion, local deer density, and deer browse preferences.

Human and environmental associates of local species-specific abundance in a multi-species deer assemblage

Understanding how habitat, landscape context, and human disturbance influence local species-specific deer density provides evidence informing strategic management of increasing deer populations. Across an extensive (187 km2) heterogeneous forest-mosaic landscape in eastern England, spatially explicit density surface models of roe deer Capreolus capreolus and introduced muntjac Muntiacus reevesi were calibrated by thermal imaging distance sampling (recording 1590 and 400 muntjac and roe deer groups, respectively, on 567 km of driven transects). Models related deer density to local habitat composition, recreational intensity, and deer density (roe deer models controlled for muntjac density and vice versa) at a local grain across 1162 composite transect segments, incorporating geographical coordinates accounting for spatial autocorrelation. Abundance of both species was lower in localities with more grasslands (inter-quartile, IQ, effect size: roe −2.9 deer/km2; muntjac −2.9 deer/km2). Roe abundance (mean = 7 deer/km2, SD = 6) was greater in localities with more young stands (IQ effect size, + 1.3 deer/km2) and lower at localities with more recreationists (−1.1 deer/km2). Muntjac density (mean = 21 deer/km2, SD = 10) was greater in localities with more recreationists (+ 2.4 deer/km2), with more mature (≥ 46 years) stands (+ 1.5 deer/km2), or calcareous soil (+ 7.1 deer/km2). Comparison of models incorporating candidate variables and models comprising geographical coordinates only shows candidate variables to be weak predictors of deer densities. Adapting forest management to manipulate habitat and recreational access may influence local deer densities, but only subtly: effect sizes are not sufficient to mitigate deer impacts through planting vulnerable tree crops in areas avoided by deer. Effective culling remains the most viable management option.

Experimental evidence for opposing effects of high deer density on tick-borne pathogen prevalence and hazard

Background Identifying the mechanisms driving disease risk is challenging for multi-host pathogens, such as Borrelia burgdorferi sensu lato (s.l.), the tick-borne bacteria causing Lyme disease. Deer are tick reproduction hosts but do not transmit B. burgdorferi s.l., whereas rodents and birds are competent transmission hosts. Here, we use a long-term deer exclosure experiment to test three mechanisms for how high deer density might shape B. burgdorferi s.l. prevalence in ticks: increased prevalence due to higher larval tick densities facilitating high transmission on rodents (M1); alternatively, reduced B. burgdorferi s.l. prevalence because more larval ticks feed on deer rather than transmission-competent rodents (dilution effect) (M2), potentially due to ecological cascades, whereby higher deer grazing pressure shortens vegetation which decreases rodent abundance thus reducing transmission (M3). Methods In a large enclosure where red deer stags were kept at high density (35.5 deer km−2), we used an experimental design consisting of eight plots of 0.23 ha, four of which were fenced to simulate the absence of deer and four that were accessible to deer. In each plot we measured the density of questing nymphs and nymphal infection prevalence in spring, summer and autumn, and quantified vegetation height and density, and small mammal abundance. Results Prevalence tended to be lower, though not conclusively so, in high deer density plots compared to exclosures (predicted prevalence of 1.0% vs 2.2%), suggesting that the dilution and cascade mechanisms might outweigh the increased opportunities for transmission mechanism. Presence of deer at high density led to shorter vegetation and fewer rodents, consistent with an ecological cascade. However, Lyme disease hazard (density of infected I. ricinus nymphs) was five times higher in high deer density plots due to tick density being 18 times higher. Conclusions High densities of tick reproduction hosts such as deer can drive up vector-borne disease hazard, despite the potential to simultaneously reduce pathogen prevalence. This has implications for environmental pathogen management and for deer management, although the impact of intermediate deer densities now needs testing. Graphical abstract

Experimental Evidence for Opposing Effects of High Deer Density on Tick-Borne Pathogen Prevalence and Hazard

Background: Identifying the mechanisms driving disease risk is challenging for multi-host pathogens, such as Borrelia burgdorferi s.l., the tick-borne bacteria causing Lyme disease. Deer are tick reproduction hosts but do not transmit B. burgdorferi s.l., whereas rodents and birds are competent transmission hosts. Here, we use a long-term deer exclosure experiment to test three mechanisms for how high deer density might shape B. burgdorferi s.l. prevalence in ticks: increased prevalence due to higher larval tick densities facilitating high transmission on rodents (M1); alternatively, reduced B. burgdorferi s.l. prevalence because more larval ticks feed on deer rather than transmission-competent rodents (dilution effect) (M2), potentially due to ecological cascades, whereby higher deer grazing pressure lowers vegetation which decreases rodent abundance thus reducing transmission (M3).Methods: In a large enclosure where red deer stags were kept at high density (32.5 deer/km²), we used an experimental design consisting of eight plots of 0.23ha, four being fenced to simulate the absence of deer and four that were accessible to deer. In each plot we measured the density of questing nymphs and nymphal infection prevalence in spring, summer and autumn and quantified vegetation height and density, and small mammal abundance Results: Prevalence tended to be lower, though not conclusively so, in high deer density plots compared to exclosures (predicted prevalence of 1.0% vs 2.2%), suggesting that the dilution (M2) and cascade (M3) mechanisms might outweigh the increased opportunities for transmission (M1). Presence of deer at high density led to lower vegetation and fewer rodents, consistent with an ecological cascade. However, Lyme disease hazard (density of infected I. ricinus nymphs) was five times higher in high deer density plots due to tick density being 18 times higher.Conclusion: High densities of tick reproduction hosts such as deer can drive up vector-borne disease hazard, despite the potential to simultaneously reduce pathogen prevalence. This has implications for environmental pathogen management and for deer management, although the impact of intermediate deer densities now needs testing.

Nutritional niche separation between native roe deer and the nonnative fallow deer—a test of interspecific competition

On an evolutionary time scale, competition for food drives species formation by genetic adaptations to the environment and subsequent niche separation. On a short-term scale, animals use different strategies to meet their nutritional requirements, which ultimately influence their fitness. Understanding these adaptations in herbivores is especially important in temperate climates where animals have adapted both physiologically and behaviorally to seasonal variations in order to meet their nutritional requirements. The aim of this project was to investigate temporal variation in chemical composition of rumen content between two coexisting species of large herbivores, the native roe deer (Capreolus capreolus L.) and the introduced fallow deer (Dama dama L.), as well as a potential effect of competition on niche separation (interspecific differences in rumen nutrient composition). We analyzed 345 rumen samples collected from animals at one 95 km2 estate, Koberg, in southwestern Sweden. Based on samples from all seasons, temporal variation in nutrient composition and interspecific differences between the two deer species were investigated under two contrasting fallow deer population densities. Results revealed that nutrient composition varied between species and across seasons. Roe deer had a higher proportion of rumen protein compared to fallow deer, with the highest proportions in spring. In contrast, fallow deer had a higher proportion of rumen hemicellulose compared to roe deer in spring, while no differences in nutrient composition between species could be found in fall. Overall, there were greater differences between the two species when fallow deer density was high and competition likely more pronounced than when fallow deer density was low. The results from this study can be used to understand interspecific competition and how it fosters niche separation between coexisting large herbivores.