Unintended Consequences of Invasive Predator Control in an Australian Forest: Overabundant Wallabies and Vegetation Change

Over-abundance of native herbivores is a problem in many forests worldwide. The abundance of native macropod wallabies is extremely high at Booderee National Park (BNP) in south-eastern Australia. This has occurred because of the reduction of exotic predators through an intensive baiting program, coupled with the absence of other predators. The high density of wallabies at BNP may be inhibiting the recruitment of many plant species following fire-induced recruitment events. We experimentally examined the post-fire response of a range of plant species to browsing by wallabies in a forest heavily infested with the invasive species, bitou bush Chrysanthemoides monilifera. We recorded the abundance and size of a range of plant species in 18 unfenced (browsed) and 16 fenced (unbrowsed) plots. We found the abundance and size of bitou bush was suppressed in browsed plots compared to unbrowsed plots. Regenerating seedlings of the canopy or middle storey tree species Eucalyptus pilularis, Acacia implexa, Allocasuarina littoralis, Breynia oblongifolia and Banksia integrifolia were either smaller or fewer in number in grazed plots than treatment plots as were the vines Kennedia rubicunda, Glycine tabacina and Glycine clandestina. In contrast, the understorey fern, Pteridium esculentum increased in abundance in the browsed plots relative to unbrowsed plots probably because of reduced competition with more palatable angiosperms. Twelve months after plots were installed the community structure of the browsed and unbrowsed plots was significantly different (P = 0.023, Global R = 0.091). The relative abundance of C. monilifera and P. esculentum contributed most to the differences. We discuss the possible development of a low diversity bracken fern parkland in Booderee National Park through a trophic cascade, similar to that caused by overabundant deer in the northern hemisphere. We also discuss its implications for broad scale fox control in southern Australian forests.     

Resource availability and plant diversity explain patterns of invasion of an exotic grass

Aims In this study, we examine two common invasion biology hypotheses—biotic resistance and fluctuating resource availability—to explain the patterns of invasion of an invasive grass, Microstegium vimineum.Methods We used 13-year-old deer exclosures in Great Smoky Mountains National Park, USA, to examine how chronic disturbance by deer browsing affects available resources, plant diversity, and invasion in an understory plant community. Using two replicate 1 m 2 plots in each deer browsed and unbrowsed area, we recorded each plant species present, the abundance per species, and the fractional percent cover of vegetation by the cover classes: herbaceous, woody, and graminoid. For each sample plot, we also estimated overstory canopy cover, soil moisture, total soil carbon and nitrogen, and soil pH as a measure of abiotic differences between plots.Important findings We found that plant community composition between chronically browsed and unbrowsed plots differed markedly. Plant diversity was 40% lower in browsed than in unbrowsed plots. At our sites, diversity explained 48% and woody plant cover 35% of the variation in M. vimineum abundance. In addition, we found 3.3 times less M. vimineum in the unbrowsed plots due to higher woody plant cover and plant diversity than in the browsed plots. A parsimonious explanation of these results indicate that disturbances such as herbivory may elicit multiple conditions, namely releasing available resources such as open space, light, and decreasing plant diversity, which may facilitate the proliferation of an invasive species. Finally, by testing two different hypotheses, this study addresses more recent calls to incorporate multiple hypotheses into research attempting to explain plant invasion.     

Indirect interactions between browsers and seed predators affect the seed bank dynamics of a chaparral shrub

Interactions between herbivores and seed predators may have long-term consequences for plant populations that rely on persistent seed banks for recovery after unpredictable fires. We assessed the effects of browsing by deer and seed predation by rodents, ants and birds on the densities of seeds entering the seed bank of Ceanothus cuneatus var. rigidus, a maritime chaparral shrub in coastal California. Ceanothus produced many more seeds when protected from browsers in long-term experimental exclosures than did browsed plants, but the seed densities in the soil beneath browsed and unbrowsed Ceanothus were the same at the start of an intensive one-year study. The density of seeds in the soil initially increased in both treatments following summer seed drop: while densities returned to pre-drop levels within a few weeks under browsed plants, soil seed densities remained high for 5–8 months beneath unbrowsed plants. Rodent abundance (especially deer mice) was higher near unbrowsed plants than >30 m away, and rodents removed Ceanothus seeds from dishes in the experimental plots. At least in the short term, rodent density and rates of seed removal were inversely related to the intensity of browsing. Our data have management implications for maintaining viable Ceanothus populations by regulating the intensity of browsing and the timing, intensity and frequency of fires.Electronic Supplementary Material Supplementary material is available to authorised users in the online version of this article at .     

Effects of competition,herbivory and substrate disturbance on growth and size structure in pin cherry (Prunus pensylvanica L.) seedlings

We examined separate and interactive effects of intraspecific competition, vertebrate browsing and substrate disturbance on the growth and size structure of pin cherry (Prunus pensylvanica L.) in the first two seasons of growth after clearcutting, in a hardwoods forest in New Hampshire, United States. Over the 15-month study period, 97.5% of 1801 individuals survived, and mean plant height increased from 4-fold at high density to 5-fold at low density. Relative height growth was significantly lower at higher plant densities in two of the three growth periods examined. Vertebrate browsers (moose and deer) significantly preferred taller plants. Browsed plants had higher relative height growth following browsing than unbrowsed plants (compensatory growth) at low and intermediate densities. The degree of compensation declined with density and compensation was not significant at the highest density level. At low and intermediate densities, plants browsed early in life regained height dominance through compensatory growth; they failed to regain dominance at high density. Because compensatory growth tended to offset the effects of size-selective browsing, there was no difference in the degree of size inequality between browsed and unbrowsed plots. However, size inequality increased with plant density. Substrate disturbance caused by logging had no significant effects on either relative height growth or size inequality. The slope of the relationship between relative height growth and initial height increased significantly with density and time, and was higher in unbrowsed than in browsed plots, suggesting that competition among plants was size-asymmetric. Despite the preference of browsers for large plants, there was a clear net growth advantage for plants of large initial size, when the effects of competition, browsing and compensatory growth were combined. The interactive effects of density and browsing demonstrate the importance of a multifactorial approach to the analysis of individual plant performance and population structure.     

Ungulate browsing in winter reduces the growth of <Emphasis Type="Italic">Fraxinus</Emphasis> and <Emphasis Type="Italic">Acer</Emphasis> saplings in subsequent unbrowsed years

Browsing by ungulates has become a hotly debated issue in many European mountain forests in the past century. Acer pseudoplatanus L. and Fraxinus excelsior L. are broadleaved tree species that are preferentially browsed by roe deer (Capreolus capreolus) in central Europe. We analyzed growth data from shaded saplings of both tree species to quantify the extent to which height growth after game browsing is reduced in subsequent, unbrowsed years in forest stands. Sixty saplings of F. excelsior and A. pseudoplatanus from forest stands at Albisriederberg (Switzerland) were available to us that had been dissected into pieces that then were split in the middle for counting tree rings and assessing ungulate damage. We fitted the von Bertalanffy growth equation to these height growth data and included a reduction factor for winter browsing. Both tree species showed significantly reduced height growth in unbrowsed years after one to several browsing events in winter, and this effect increased with the number of browsing events. Saplings with a high growth rate showed a higher growth reduction. After winter browsing, height growth of A. pseudoplatanus saplings was less affected in unbrowsed years than that of F. excelsior saplings. We conclude that browsed saplings of these species in forest stands are not able to compensate browsing-induced height loss, but that height differences between browsed and unbrowsed saplings probably increase over time. A comparison between our analysis and the parameters estimated using equations published by Eiberle for predicting age at 130 cm height suggests that our parameter values are rather conservative estimates of the growth reduction effect after winter browsing. Neither F. excelsior nor A. pseudoplatanus show a distinct pattern in browsing-induced growth reduction with respect to soil moisture, nutrient level, and altitude. We thus conclude that our results are likely to be valid for a wide range of forested sites.     

The influence of deer browsing on the reproductive biology of Canada yew (Taxus canadensis marsh.)

Summary Browsed Canada yew (Taxus canadensis) populations have a higher proportion of males and a lower proportion of monoecious plants than unbrowsed yew populations. The proportion of monoecious plants increases with time following protection from browsing suggesting that deer browsing causes male-biased sex expression in Canada yew. In contrast, results from comparing browsed and unbrowsed populations, exclosure studies, and browse simulation experiments indicate that strobilus ratios and phenotypic gender of browsed yews may be female-biased. In part, these results correspond to the influence of size on sex expression in Canada yew; small yews tend to be male, but if monoecious, have female-biased strobilus ratios. Large yews are monoecious, but have male-biased strobilus ratios. There is, however, no consistent relationship between size and gender in Canada yew, suggesting that in some circumstances, yews shift allocation to female function in response to browsing.     

Interactions among elk, aspen, galling sawflies and insectivorous birds

Using two years of observational and experimental data, we examined the hypothesis that browsing by elk on aspen indirectly affects the distribution of a leaf-galling sawfly, which in turn affects insect diversity and foraging patterns of insectivorous birds. We found that: i) in an analyses of 33 arthropod species, the presence of sawflies significantly increased arthropod richness and abundance by 2 X and 2.5 X, respectively. ii) browsing by elk reduced sawfly gall abundance such that 90% of the galls were found on unbrowsed aspen ramets. iii) insectivorous birds attacked 60–74% of the galls on unbrowsed shoots compared to 11% on browsed shoots. When leaf-galler abundance was experimentally held constant on browsed and unbrowsed shoots, predation by insectivorous birds did not differ significantly. This result suggests that browsing affects the patterns of avian predation by altering the distribution of a galling insect. These data argue that bottom-up, top-down, and lateral factors can act in concert to affect the distribution of a galler, structure arthropod communities and affect predation by insectivorous birds.     

Trait‐mediated indirect interactions: Moose browsing increases sawfly fecundity through plant‐induced responses

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Different facets of tree sapling diversity influence browsing intensity by deer dependent on spatial scale

Browsing of tree saplings by deer hampers forest regeneration in mixed forests across Europe and North America. It is well known that tree species are differentially affected by deer browsing, but little is known about how different facets of diversity, such as species richness, identity, and composition, affect browsing intensity at different spatial scales. Using forest inventory data from the Hainich National Park, a mixed deciduous forest in central Germany, we applied a hierarchical approach to model the browsing probability of patches (regional scale) as well as the species‐specific proportion of saplings browsed within patches (patch scale). We found that, at the regional scale, the probability that a patch was browsed increased with certain species composition, namely with low abundance of European beech (Fagus sylvatica L.) and high abundance of European ash (Fraxinus excelsior L.), whereas at the patch scale, the proportion of saplings browsed per species was mainly determined by the species’ identity, providing a “preference ranking” of the 11 tree species under study. Interestingly, at the regional scale, species‐rich patches were more likely to be browsed; however, at the patch scale, species‐rich patches showed a lower proportion of saplings per species browsed. Presumably, diverse patches attract deer, but satisfy nutritional needs faster, such that fewer saplings need to be browsed. Some forest stand parameters, such as more open canopies, increased the browsing intensity at either scale. By showing the effects that various facets of diversity, as well as environmental parameters, exerted on browsing intensity at the regional as well as patch scale, our study advances the understanding of mammalian herbivore–plant interactions across scales. Our results also indicate which regeneration patches and species are (least) prone to browsing and show the importance of different facets of diversity for the prediction and management of browsing intensity and regeneration dynamics.     

Large herbivores affect the stoichiometry of nutrients in a regenerating woodland ecosystem

Previous studies on the effects of herbivores on nutrient cycling have given little consideration to the relationship between soil nitrogen (N) and phosphorus (P) availability. Here we examined how browsing by red deer influences the relative availability of N and P in a regenerating woodland ecosystem. We found that removal of browsing by fencing for 14 years led to a shift from N toward P limitation of the dominant tree species Betula pubescens . This was evidenced by a significant increase in foliar N:P ratio of B. pubescens as a result of removal of browsing; mean N:P ratio of foliage from browsed areas was 13.2 suggesting that trees growing in browsed areas were N limited, whereas foliage from unbrowsed areas had a mean N:P ratio of 15.8, suggesting that these areas were more P limited. Further evidence of a shift toward P limitation in unbrowsed areas came from the finding that root uptake of labelled ³²P was significantly greater in roots collected from unbrowsed than browsed trees. Soil phosphatase activity did not significantly differ between browsed and unbrowsed areas. Our data indicate therefore that herbivores have the potential to significantly affect the stoichiometry of N and P in forest ecosystems.     

The influence of deer browsing on the reproductive biology of Canada yew (Taxus canadensis marsh.)

Summary Canada yew (Taxus canadensis) populations currently browsed by white-tailed deer (Odocoileus virginianus) or browsed by deer in the past had significantly lower production of male strobili, female strobili, and seeds than unbrowsed yew populations. Exclosure studies showed that protected yews produced significantly more male and female strobili than unprotected yews, but only after several years of protection. Seed production did not respond as readily to protection from deer perhaps because of reduced pollination levels in browsed yew populations. Previously unbrowsed yews were clipped at different levels of removal of available browse (control (no removal), 25%, 50%, 75%, and 100% removal) to simulate deer browsing. Reduction in male strobilus production was linearly related to clipping intensity in three years of observation. Female strobilus production was significantly reduced only at the 100% level of removal. Intermediate levels of clipping may have even stimulated production of female strobili. Analysis of covariance, with previous year's branch production as the covariate, showed no significant effect of clipping on male strobilus production except in the 100% removal group. Female strobilus production showed no such covariance with branch production. Effects of clipping on seed production could not be reliably assessed in 1984 and 1985 due to low seed production. Seed production in 1986 was significantly reduced only in the 100% removal group. Field observations of deer browsing of Canada yew indicate that 100% levels of removal are typical of natural levels of browsing.     

The role of moose <Emphasis Type="Italic">Alces alces</Emphasis> L. in boreal forest – the effect on ground beetles (Coleoptera,Carabidae) abundance and diversity

The indirect effect of moose Alces alces browsing on ground beetle's abundance and diversity was investigated by pitfall trapping in a mixed coniferous forest in Vestfold County (59°19′ N, 9°50′ E, Norway), during the summer of 2002. Three areas with different browsing pressures, ranging from non- to medium- and heavily browsed were chosen and dry weight of bilberry Vaccinium myrtillus and humidity at ground level were measured in the three locations. As predicted, the gradient analyses showed that browsing by moose influenced the composition of carabid fauna, and that browsing intensity and humidity covaried with the most important gradient in carabid species composition found across the three locations. Species that live in light stands with rather dry soil, were more often captured in the browsed areas, whereas shade tolerant and hygrophilous species were more abundant in the non-browsed area. The carabid abundance increased significantly with increasing browsing pressure. According to our predictions, the diversity at trap level (α-diversity) was higher in the highly browsed area. Conversely, species turnover (β-diversity) decreased with browsing intensity. On the other hand, the rarefaction analysis showed that the regional species richness (γ-diversity) was considerably higher in the medium browsed area than in the heavily browsed one, which is consistent with the intermediate disturbance hypothesis. This study indicates that moose, by reducing the bilberry that constitutes the field layer in summer, affect carabid species composition and might be capable to reshape the whole ecosystem in our study area by a cascade effect.     

Effects of kangaroo rat exclusion on vegetation structure and plant species diversity in the Chihuahuan Desert

Long-term (1977–90) experimental exclusion of three species of kangaroo rats from study plots in the Chihuahuan Desert resulted in significant increases in abundance of a tall annual grass (Aristida adscensionis) and a perennial bunch grass (Eragrostis lehmanniana). This change in the vegetative cover affected use of these plots by several other rodent species and by foraging birds. The mechanism producing this change probably involves a combination of decreased soil disturbance and reduced predation on large-sized seeds when kangaroo rats are absent. Species diversity of summer annual dicots was greater on plots where kangaroo rats were present, as predicted by keystone predator models. However, it is not clear whether this was caused directly by activities of the kangaroo rats or indirectly as a consequence of the increase in grass cover. No experimental effect on species diversity of winter annual dicots was detected. Our study site was located in a natural transition between desert scrub and grassland, where abiotic conditions and the effects of organisms may be particularly influential in determining the structure and composition of vegetation. Under these conditions kangaroo rats have a dramatic effect on plant cover and species composition.     

Short-Term Effects of Cattle Browsing on Tree Sapling Growth in Mountain Wooded Pastures

Browsing by livestock has been identified as an important factor preventing tree regeneration in wooded pastures. Two field experiments were performed to investigate the effects of cattle browsing on tree sapling growth in a mountain-wooded pasture. Two size classes (ca. 12–17 cm and 41–59 cm) of each of 4 species (Picea abies, Abies alba, Acer pseudoplatanus and Fagus sylvatica) were exposed to zero, low and high grazing intensities. We measured the proportion of saplings browsed and the effect of browsing on growth. A mowing treatment within the zero grazing intensity treatment showed no significant effect on sapling growth. One percent of the large saplings but 25% of the small saplings escaped browsing. Saplings were better protected when surrounded by taller vegetation. The proportion of saplings browsed was not significantly different among species although evergreen tree saplings lost a larger proportion of biomass than deciduous species when browsed. Under grazing, average current-year shoot production and total aboveground biomass of all species were significantly reduced. Browsing effects tended to be smaller at the lower grazing intensity. Because the deciduous species were least reduced in aboveground growth, especially under the low grazing intensity, we conclude that at least in short-term, deciduous species are less affected by cattle browsing.     

Differential response of trees and shrubs to browsing and pruning: the effects on Pistacia growth and gall-inducing aphids

The effects of long term browsing on growth and morphology of P. atlantica trees and P. palaestina shrubs, and on the colonization of these hosts by their respective gall-inducing aphid guilds, were studied in natural vegetation pastures in Israel. To simulate apical dominance release by browsing, trees and shrubs were pruned and observed one year later. P. atlantica responded strongly to browsing, producing a crown of dense and sharp dry branches and thorn-like shoots, and pruning, compensating or overcompensating for the lost biomass in the studied morphological variables. Morphological responses of P. palaestina were less evident: we found no compensation in most variables. Three species of aphids produced more galls on browsed than on control shoots of P. atlantica. Two species induced more galls on pruned shoots. The five aphid species on P. palaestina colonized browsed and unbrowsed shoots equally, and produced fewer galls on pruned shoots. The role of apical dominance in architectural responses of trees and shrubs to browsing and pruning, as related to resource availability and timing, is discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

Deer Browsing,Forest Edge Effects,and Vegetation Dynamics Following Bottomland Forest Restoration

Vegetation composition and forest stand development are frequently mediated by browsing herbivores. These relationships have received little attention in a forest restoration context even though White‐tailed deer (Odocoileus virginianus) is likely to influence these agriculture, forest, and restored ecosystem mosaic landscapes. Tree species composition, herbaceous vegetation, and deer browsing patterns were assessed 5 and 7 years following bottomland hardwood forest restoration on a 526‐ha site in the Cache River watershed in southern Illinois, United States. Light‐seeded tree species (Fraxinus pennsylvanica, Acer negundo, Liquidambar styraciflua, and Platanus occidentalis) of volunteer origin dominated the woody vegetation component, with especially high stocking and density near existing forest cover and potential seed sources. At more distant locations, presumably planted Quercus spp. were more likely to dominate and were the only tree species found in 15% of plots in year 7. Quercus stocking increased over the course of the study, constituting 7% of trees during year 7. Deer herbivory was associated with reduced stem height and disproportionately impacted seedlings of Quercus palustris and Celtis spp. Our results suggest that deer browsing influences forest stand composition and density as a function of distance from the nearest forest edge. Herbaceous vegetation had little impact on early stand development. Continued spread of the exotic and invasive Lonicera japonica and potential mortality of F. pennsylvanica due to an anticipated Emerald ash borer (Agrilus planipennis) epidemic, combined with low stand density and delayed canopy closure, may result in persistent overstory gaps and compromise long‐term restoration success.     

Browser selectivity alters post-fire competition between <Emphasis Type="Italic">Erica arborea</Emphasis> and <Emphasis Type="Italic">E. trimera</Emphasis> in the sub-alpine heathlands of Ethiopia

Mammalian herbivores have the potential to alter the competitive relations of woody species, if consumption is unevenly distributed between species. At elevations above 3,500 m in the southern Ethiopian highlands, vegetation is dominated by Erica arborea and E. trimera. Both species can potentially grow into short trees, but are burnt on a rotation of 6–10 years, and regenerate by re-sprouting from belowground lignotubers. The regenerating scrub is heavily browsed by cattle. We set up browsing exclosures at three burnt sites to quantify the impact of browsing over a 3-year period. When protected from browsing, E. trimera had similar or better height growth than E. arborea, but in browsed vegetation, E. arborea instead grew taller. Browsing was more intense on E. trimera in the first years after fire, indicating a difference in palatability between the species. We checked if browse quality differed, by analysing shoot contents of acid detergent fibre (ADF), protein, phenolics and tannins. Contrary to expectations, the preferred E. trimera contained more ADF, less protein and had a higher tannin activity than E. arborea. Although the vegetative growth of E. arborea is favoured relative to E. trimera under high browsing pressure, rapid change in abundance would not be expected, since short-interval fire will repeatedly eradicate any gains in vegetative growth. However, within the typical fire return interval of less than 10 years, E. trimera barely reach a reproductive state, whereas E. arborea flower profusely. Under the current regime of fire and browsing, this may in the long run be more important than differences in height growth, leading to a gradual increase in the proportion of E. arborea.     

Patterns of use and damage by exotic deer on native plant communities in northwestern Patagonia

Ungulate overabundance can strongly alter composition and structure of forest communities through selective damage independently of the herbivory history of the recipient system. In the early 1900s, red and fallow deer (Cervus elaphus and Dama dama, respectively) were introduced to northwestern Patagonia. We studied patterns of relative habitat use, damage, and species selection by introduced deer populations in three major plant communities that have the longest history of invasion in the region. We also evaluated community structure and composition characteristics as possible explanatory factors for the observed patterns of browsing, fraying, and bark-stripping. At the species level, exotic deer browsed more than expected on two evergreen species (Austrocedrus chilensis and Schinus patagonicus) and two spiny species (Colletia hystrix and Dasyphyllum diacanthoides), while one dominant shrubland tree (Lomatia hirsuta) was selected for fraying and bark-stripping. Browsing was the more widespread damage in all plant communities, while fraying and bark-stripping occurred at low incidence. Furthermore, species identity was found as the main driver of deer damage over plant community type. Finally, variation in damage and in habitat use was explained by community structure and composition characteristics. Bark-stripping and fraying are best predicted by community composition, whereas browsing and habitat use are best predicted by structural properties including understory cover, cover of fallen logs, and tree density. The patterns of damage and the association with community structure characteristics reported here provide insight for developing management strategies.     

Increased thorn length in Acacia depranolobium —an induced response to browsing

Summary I report here longer thorns induced by large mammal herbivory on the tree Acacia depranolobium. I compared trees that had been browsed by domestic goats to trees protected from goat browsing. Thorns on browsed branches within the reach of goats (<125 cm above the ground) were significantly longer than thorns from higher branches on the same browsed trees, and significantly longer than branches at similar heights on unbrowsed trees. It appears that increased thorn length was an induced response to large mammal herbivory in Acacia depranolobium, both among and within individual trees.     

Chemical and morphological variation of Mediterranean woody evergreen species: Do plants respond to ungulate browsing?

Abstract. Plant defense theories suggest that chemical or structural defences should be maximized when and where browsing is most likely to occur. We tested this hypothesis on four evergreen woody species growing in a Mediterranean area with a high density of ungulates. In this system, levels of browsing are more intense in the winter (due to the lack of annual plants) and young foliage is often preferred. Therefore we predicted that the chemical defences of these species, namely their phenolic content, would vary with leaf age, season and damage intensity. In addition, we tested whether ungulates preferentially selected species containing lower phenolic levels, and also whether browsing induced either chemical or morphological changes in damaged plants. Phenolic levels varied greatly between plant species; ungulates browsed preferentially on the species with the lowest phenolic levels. No difference in phenolic content was found between browsed and unbrowsed trees. Morphological changes in heavily browsed trees included an increase in shoot and leaf density and a net decrease in leaf size. We suggest that for Mediterranean plants, which have evolved under high browsing pressure from large mammals, the production of small leaves and dense shoots in response to browsing might decrease ungulate foraging efficiency and hence reduce the rate of further damage as effectively as high levels of chemical defence.