Feeding behaviour of Red Deer (Cervus elaphus) offered Sitka Spruce saplings (Picea sitchensis) grown under different light and nutrient regimes

1. In addition to exhibiting preferences for particular plant species, vertebrate herbivores select particular individuals of these species whilst leaving others undamaged. This pattern of diet selection may reflect differences in the chemical composition (and hence nutritional quality) between individual plants, and/or variability in the physical constraints on intake rate, such as plant structure.
2. An experiment was conducted to test the effects of environmental manipulations on the morphology and chemical composition of Sitka Spruce saplings, and to evaluate the consequences for herbivory by Red Deer. Fertilizing the trees increased tree height, branch span and leader length, twig width and needle width, and decreased the concentrations of total phenolics, condensed tannins, fibre and lignin but monoterpene content was not altered. Shading also reduced phenolic and tannin concentrations.
3. When the fertilized and shaded trees were offered to deer in feeding trials, the probability of a tree being visited by a deer and the biomass removed were influenced by tree morphological variables, as were the bite rate and intake rate of the deer. More biomass was removed from larger trees.
4. Once the effects of tree morphology had been taken into account, there was no effect of the fertilizer and shade treatments on deer browsing behaviour that could be attributed to changes in chemical composition of the trees. The relationship between intake rate and bite size at each tree varied between individual deer, but the functional response relationship between intake rate and bite size was not influenced by the treatments applied to the trees.
5. Tree morphology may have a larger influence on deer feeding behaviour than tree chemical composition.     

Age-dependent shift in response to food element composition in Collembola: contrasting effects of dietary nitrogen

Developing leaves that are soft, with high concentrations of resources, can be particularly vulnerable to herbivore damage. Since a developing leaf cannot be very tough, given the constraints of cell expansion, the major form of protection is likely to be chemical defence. We investigated changes in concentration of herbivore resources (protein, carbohydrates and water) and putative defences (total phenolics, tannin activity, cyanogenic glycosides, alkaloids, cell wall, and leaf mechanics) across five leaf development stages of the soft-leaved Toona ciliata M. Roem. and the tough-leaved Nothofagus moorei (F. Muell.) Krasser. Chemical defences were predicted to be more highly developed in young than expanded leaves of both species, and to decline more in expanded leaves of N. moorei, which become tough and strong at maturity, than in the softer expanded leaves of T. ciliata. Resources and defences were dynamic within the developing leaves. Highest concentrations of protein were recorded in young leaves in both species, and highest levels of non-structural carbohydrate were recorded in young leaves of T. ciliata. Allocation to defence varied in both amount and type across leaf stages. In T. ciliata, there was an increase in chemical defence in expanded leaves (tannin activity, alkaloids). However, in N. moorei, increasing strength and toughness of developing leaves coincided with decreasing chemical defence, consistent with our hypothesis. For phenolics, this decrease was partly due to dilution by cell wall, but cyanogenic glycosides were present in young leaves and absent in fully mature leaves. These results are consistent with leaf toughness acting as an effective anti-herbivore defence, thereby reducing the need for investment in chemical defence.     

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.     

Growth,chemical responses and herbivory after simulated leaf browsing in <Emphasis Type="Italic">Combretum apiculatum</Emphasis>

Vegetative and chemical responses to simulated leaf browsing during the growth season, and their subsequent effect on herbivory, were studied on Combretum apiculatum Sonder (Combretaceae) in Botswana. Treatments (50% and 100% leaf and shoot apex removal) were performed just before the shoot growth curve levelled out, and responses recorded 3 months later, just before leaf fall. Compared to controls, defoliation treatments, removing apical dominance, reduced growth in tree height and increased shoot mortality, although the production of lateral shoots increased. At the end of the trial, there was no difference in total length of annual shoots between treatment groups. Significant refoliation occurred only after 100% defoliation. Refoliated leaves were smaller and the 100% defoliated trees had a lower final leaf biomass. Total leaf biomass production was, however, equal for all treatment groups. Refoliated leaves contained higher levels of N, lower levels of acid-detergent fibre (ADF) and total phenolics, and showed a trend towards lower levels of condensed tannins, compared to leaves on control trees. Such chemical changes may be due to either carbon stress or to younger physiological age of new leaves. In spite of the observed potential increase in food quality, we found no evidence of increased levels of insect or ungulate herbivory on refoliated leaves, which, at least for insect herbivory, may be explained by the reduction in temporal availability of leaves. We conclude that the single severe defoliation was not detrimental to C. apiculatum in the short-term, although the resource loss and induced compensatory growth may produce negative effects during subsequent growth seasons.     

Chemical and mechanical defense against herbivory in two sympatric species of desert Acacia

Abstract. Most African Acacia trees occur in semi-arid savannas, but some species grow in deserts at the periphery of their range in northern Africa and the Middle East. We studied Acacia raddiana and A. tortilis in the southern Negev desert of Israel. According to the resource availability hypothesis, plants in poor habitats invest heavily into anti-herbivore defense because losses to browsers are costly when growth rates are low. Few plants invest simultaneously in different categories of defense. This suggests that trade-offs exist, although little is known about how individuals allocate resources to defence in natural populations. In a field experiment, we studied chemical and mechanical defences as a response to differential browsing for more than 10 yr. Both Acacia species increased spinescence but there was no clear response in the production of total polyphenols, condensed tannins, and protein-precipitating tannins (only A. raddiana may increase secondary compounds under very high browsing levels). There was an age effect in spinescence but not in secondary compounds for both species, with younger trees investing more into anti-herbivore defense than older individuals. We were unable to find negative correlations between traits of chemical and mechanical defense. We conclude that inducibility and age effects suggest a cost for mechanical but not necessarily for chemical defense. Contrary to the predictions of the resource availability hypothesis, concentrations of secondary compounds were not higher in this desert environment than in savannas.     

Ontogenetic changes in anti-herbivore defensive traits in leaves of four Mediterranean co-occurring <Emphasis Type="Italic">Quercus</Emphasis> species

Investment in anti-herbivore defence in tree species has been one of the priority research topics in plant terrestrial ecology during the last decades. However, despite considerable experimental effort, interspecific differences in the ontogenetic trends in the investment in defence are still a matter of debate, as to date experimental evidence is contradictory. In the present work, insect herbivory levels were measured in seedlings and mature trees of four co-occurring Mediterranean Quercus species with differing leaf life spans, as well as several leaf characteristics that can determine herbivore preference. The measured leaf traits included nitrogen (N), fibre (cellulose, hemicellulose and lignin), total phenolic contents, leaf mass per unit area (LMA) and leaf thickness. The leaves of seedlings had a lower LMA and leaf thickness and lower concentrations of N and cellulose, but higher concentrations of lignin and phenols than those of mature trees. However, the loss of leaf area tended to be more severe for seedlings than for mature trees, although the differences were only significant for deciduous species. This constitutes a confirmation of the strong effects of physical traits on herbivore preferences. The greater resource limitations for defensive mechanisms in seedlings with respect to mature trees would explain that at intraspecific level we do observe a compromise between chemical and physical defences. As a result, seedlings rely on chemical rather than on physical defences.     

The induction of spinescence in European holly leaves by browsing ungulates

Evidence is presented which suggests that the spinescence of leaves of European holly, Ilex aquifolium, deters feeding by ungulates and is induced by browsing. Spinescence decreased as leaf size increased; hence, spinescence may be achieved by reducing adult leaf size. Holly shrubs with very spiny leaves were browsed less often than less spiny shrubs. In the absence of browsing ungulates during a one year period, the spinescence of leaves of holly shrubs significantly decreased. Browsed shrubs exhibited reduced annual shoot growth, increased branching, and produced smaller leaves with high spinescence. The regrowth on browsed branches of holly trees was characterized by increased leaf spinescence relative to unbrowsed branches. Hence, the induced response was localized, thereby reducing the ability of browsing ungulates to exert selective pressures on holly trees.     

Vertical zonation of browse quality in tree canopies exposed to a size-structured guild of African browsing ungulates

We investigated whether the food quality of tree foliage for African savanna browsers varies across the feeding height range of the guild. This was to address the question of why giraffes (Giraffa camelopardalis) generally feed at a higher level in the canopy than is accessible to all other browsers. We defined a giraffe browse unit (GBU) as the length of twig corresponding to the average "bite" taken by giraffes from two staple browse plants: Acacia nigrescens and Boscia albitrunca. We sampled at three study sites in South Africa in the late dry season, at each site clipping GBUs at three heights above ground: 0.5 m, 1.5 m and 2.5 m; these representing the levels typically browsed by small, medium and large-bodied browsing ungulates respectively. For each GBU we measured leaf dry mass, total N, neutral detergent fibre and condensed tannin, using near-infrared spectroscopy calibrated by conventional laboratory analyses. We found no differences between height levels with regard to leaf chemistry concentrations, but leaf biomass per GBU was significantly higher at the 1.5-m and 2.5-m levels than at the 0.5-m level. The larger browsers thus gain a bite-size advantage by browsing above the reach of the smaller species. A likely reason for the reduced leaf biomass per GBU at the low browsing level is the tendency for small browsers to pluck individual leaves from shoots, while large browsers prune off whole shoots. We contend that our findings are analogous to those from parallel studies on the grazing guild, and are consistent with the hypothesis that the smaller members of ungulate guilds competitively displace the larger ones from shared feeding sites when resources become restricted. A prediction of this hypothesis is that the smaller members of each guild drive the grazing succession from behind and maintain browsing height stratification from below.     

Common allometric response of open-grown leader shoots to tree height in co-occurring deciduous broadleaved trees

Background and Aims

Morphology of crown shoots changes with tree height. The height of forest trees is usually correlated with the light environment and this makes it difficult to separate the effects of tree size and of light conditions on the morphological plasticity of crown shoots. This paper addresses the tree-height dependence of shoot traits under full-light conditions where a tree crown is not shaded by other crowns.

Methods

Focus is given to relationships between tree height and top-shoot traits, which include the shoot''s leaf-blades and non-leafy mass, its total leaf-blade area and the length and basal diameter of the shoot''s stem. We examine the allometric characteristics of open-grown current-year leader shoots at the tops of forest tree crowns up to 24 m high and quantify their responses to tree height in 13 co-occurring deciduous hardwood species in a cool-temperate forest in northern Japan.

Key Results

Dry mass allocated to leaf blades in a leader shoot increased with tree height in all 13 species. Specific leaf area decreased with tree height. Stem basal area was almost proportional to total leaf area in a leader shoot, where the proportionality constant did not depend on tree height, irrespective of species. Stem length for a given stem diameter decreased with tree height.

Conclusions

In the 13 species observed, height-dependent changes in allometry of leader shoots were convergent. This finding suggests that there is a common functional constraint in tree-height development. Under full-light conditions, leader shoots of tall trees naturally experience more severe water stress than those of short trees. We hypothesize that the height dependence of shoot allometry detected reflects an integrated response to height-associated water stress, which contributes to successful crown expansion and height gain.     

African Elephants Loxodonta africana Amplify Browse Heterogeneity in African Savanna

There is a growing concern that the feeding habits of the African elephant, which include pushing over, uprooting and snapping trees, may have a negative impact on other herbivores. Browsed trees are known to respond by either increasing production (shoots and leaves) or defence (secondary compounds). It is not clear, however, what proportion of the browsed biomass can be made available at lower feeding heights after a tree is pushed over or snapped; thus, it is also unclear how the forage quality is affected. In a field survey in Kruger National Park, South Africa, 708 Mopane trees were measured over four elephant utilization categories: snapped trees, pushed‐over trees, uprooted trees and control trees. The elephants' impact on the leaf biomass distribution was quantified, and the forage quality (Ca, P, K and Mg, N, digestibility and condensed tannin [CT] concentrations) were analyzed. Pushed‐over and uprooted trees had the maximum leaf biomass at lower heights (<1 m), snapped trees at medium heights (1–2 m) and control trees at higher heights (>2 m). In all three utilization categories, the minimum leaf biomass was seven times higher than it was for control trees at a height of below 1 m. Leaf nitrogen content increased in all three categories and was significantly higher in snapped trees. CT concentrations increased slightly in all trees that were utilized by elephants, especially on granitic soils in the dry season. The results provide the insight that elephants facilitate the redistribution and availability of browse and improve the quality, which may positively affect small browsing herbivores.     

A comparative study of the phytochemistry of two African Rain Forests

A comparative analysis of aspects of the secondary chemistry of plants from the Kibale Forest, Uganda, and the Douala-Edea Forest Reserve, Cameroon (93 species in all) has shown mean concentrations of tannins and other phenolics to be significantly greater in both young and mature foliage from Douala-Edea than in comparable taxa from Kibale. The differences remain significant when analysis was restricted to common species only. Chromatographic analysis confirmed that the proportion of species from Douala-Edea yielding tannin breakdown products was significantly greater than that from Kibale. In contrast, the proportion of species whose leaves gave definite alkaloid-positive Dragendorff reactions was significantly higher in the Kibale site. Within each site, mature leaves of the great majority of species yielded either tannins or alkaloids but the presence of detectable quantities of both types of compound was rare. In each site distributions of the two classes relative to one another departed significantly from independence at the O < 0.005 level. Nutrient analyses of vegetation also showed striking differences between the two sites; the Ugandan material appearing to be more nutrient-rich. The distribution patterns of nutrients and of secondary metabolites between the two sites are discussed in relation to current hypotheses concerning strategies of defence chemistry among plant communities. The data suggest that in Douala-Edea, which is characterized by very sandy and acid soils, the common elements of the flora invest heavily in the production of high concentrations of tannins and other phenolics. According to current hypotheses, these are the class of secondary compounds whose characteristics are most suitable to defence of vegetation growing on poor soils, and/or in species-poor stands, and in which leaves are likely to be long-lived.     

Variation in defence strategies in two species of the genus Beilschmiedia under differing soil nutrient and rainfall conditions

The relationships between various leaf functional traits that are important in plant growth (e.g., specific leaf area) have been investigated in recent studies; however, research in this context on plants that are highly protected by chemical defences, particularly resource-demanding nitrogen-based defence, is lacking. We collected leaves from cyanogenic (N-defended) Beilschmiedia collina B. Hyland and acyanogenic (C-defended) Beilschmiedia tooram (F. M. Bailey) B. Hyland at high- and low-soil nutrient sites in two consecutive years that varied significantly in rainfall. We then measured the relationships between chemical defence and morphological and functional leaf traits under the different environmental conditions. We found that the two species differed significantly in their resource allocation to defence as well as leaf morphology and function. The N defended species had a higher leaf nitrogen concentration, whereas the C-defended species had higher amounts of C-based chemical defences (i.e., total phenolics and condensed tannins). The C-defended species also tended to have higher force to fracture and increased leaf toughness. In B. collina, cyanogenic glycoside concentration was higher with higher rainfall, but not with higher soil nutrients. Total phenolic concentration was higher at the high soil nutrient site in B. tooram, but lower in B. collina; however, with higher rainfall an increase was found in B. tooram, while phenolics decreased in B. collina. Condensed tannin concentration decreased in both species with rainfall and nutrient availability. We conclude that chemical defence is correlated with leaf functional traits and that variation in environmental resources affects this correlation.     

Secondary metabolites and nutrients of woody plants in relation to browsing intensity in African savannas

Carbon-based secondary metabolites (CBSMs) are assumed to function as defences that contribute to herbivore-avoidance strategies of woody plants. Severe browsing has been reported to reduce concentrations of CBSMs and increase N concentrations in individual plants, causing heavily browsed plants to be characterised by N-rich/C-poor tissues. We hypothesised that concentrations of condensed tannins (CT) and total polyphenols (TP) should decrease, or N increase, in relation to increasing intensity of browsing, rendering severely browsed plants potentially more palatable (increased N:CT) and less N-limited (increased N:P) than lightly browsed ones. We sampled naturally browsed trees (taller than 2 m) of four abundant species in southern Kruger National Park, South Africa. Species-specific relationships between N:CT, CT, TP and P concentrations and increasing browsing intensity were detected, but N and N:P were consistently invariable. We developed a conceptual post-hoc model to explain diverse species-specific CBSM responses on the basis of relative allocation of C to total C-based defence traits (e.g. spines/thorns, tough/evergreen leaves, phenolic compounds). The model suggests that species with low allocation of C to C-based defence traits become C-limited (potentially more palatable) at higher browsing intensity than species with high allocation of C to C-based defences. The model also suggests that when N availability is high, plants become C-limited at higher browsing intensity than when N availability is low.     

Inter- and intra-plant variations in nitrogen,tannins and shoot growth of <Emphasis Type="Italic">Sclerocarya birrea</Emphasis> browsed by elephants

Impacts of elephants (Loxodonta africana africana) on woody vegetation has attracted substantial attention for decades, but plant-level responses remain a gap in the understanding of savanna ecology. Marula (Sclerocarya birrea caffra) forms an important part of elephant diets. We investigated the relationships between browsing intensity and shoot/leaf size, nitrogen (N) and condensed tannin (CT) concentrations in upper and lower canopies of male and female marula individuals in Hluhluwe-iMfolozi Park, South Africa. Browsing intensity (54%) did not differ between sexes, suggesting no preference by elephants for either sex. Females had higher [CT] than males and tannin decreased with increasing browsing intensity in both sexes. In lightly or moderately browsed trees, [CT] was controlled by unmeasured factors such that within-tree impacts of browsing were more variable in lightly/moderately browsed than heavily browsed trees. There was little change in [N] up to ~60% browsing intensity, but [N] increased dramatically at higher intensity. Shoots and leaves on broken branches in the lower canopy were larger (2.5 and 1.2 times, respectively) than those on unbroken branches in either upper or lower canopies. Chemical responses were systemic and potentially influence browsing among trees, while growth responses were strongly localised and potentially influence browsing within trees. Although marula trees are able to compensate vigorously for browsing at the scale of individual organs, trees may become progressively carbon-deficient and have their lives shortened if total plant growth is negatively affected by chronic browsing, e.g. near permanent water.     

Effects of red deer on tree regeneration and growth in Aorangi Forest, Wairarapa

New Zealand forests have been substantially modified by introduced red deer over the past century. New Zealand’s indigenous forest managers need to know if regeneration of palatable tree species can be restored following control or eradication of browsing ungulates. Aorangi Forest, Wairarapa, suffered dramatic changes in forest understorey composition by the 1950s after more than seven decades of colonisation by red deer (Cervus elaphus), feral goats (Capra hircus) and pigs (Sus scrofa). Since then feral goats have been eradicated, but red deer and pigs persist throughout Aorangi Forest despite ongoing recreational hunting. This study uses data from paired fenced and unfenced plots established at seven sites in Aorangi Forest between 1981 and 1987, and re-measured in 2004, to show the effects of ungulates on tree (≥ 2 cm diameter at breast height) regeneration and growth. Our results show that browsing by red deer has prevented regeneration of kanono (Coprosma grandifolia), a highly palatable, fast-growing sub-canopy hardwood tree. Deer reduced the growth of mahoe (Melicytus ramiflorus) trees, probably by directly browsing epicormic shoots. The regeneration of other less palatable sub-canopy trees (e.g. porokaiwhiri, Hedycarya arborea), and slower-growing canopy species (e.g. hinau, Elaeocarpus dentatus and rewarewa, Knightia excelsa) appears to have been unaffected by deer browsing. Accordingly, tree species composition does not appear to have been greatly affected by browsing from deer populations in Aorangi Forest over the past two decades.     

Long-term defence of fruit by Mistle Thrushes Turdus viscivorus

In an area of south-central England Mistle Thrushes Turdus viscivorus regularly defend fruit-hearing plants (four kinds of tree, ivy and mistletoe) against intruders from autumn to early spring. Holly Ilex aquifolium is by far the most important of the plants defended. To be suitable for defence, a tree must be free-standing and of moderate size. Observations were carried out over three winters, one mild, one very severe and one intermediate. During the mild winter hollies were successfully defended until the Mistle Thrushes gave up active defence with the onset of breeding activities. During the very severe winter nearly all defended hollies were overwhelmed by invading flocks of other thrush species during the very cold spells. Mistle Thrushes in possession of a fruit supply conserve it by feeding on undefended fruit near-by if it is available, and to a large extent on the ground when conditions are suitable. Defended fruit trees are generally avoided by other species living in the vicinity. Hence depletion of fruit from defended trees is much slower than from undefended trees, and consequently the fruit may last for several months and provide a food supply in dry weather in spring and early summer for Mistle Thrushes and other species.
Fieldfares and Blackbirds show similar, but less well developed, fruit-guarding behaviour. Fruit-defence is clearly related to the bird's size, but other physical and behavioural adaptations may be involved. The most important characteristic of holly fruit making it suitable for defence, apart from the dimensions and spatial relationship of the tree, is its ability to last for many months without significant deterioration.     

Inducing condensed tannin production in Colophospermum mopane: Absence of response to soil N and P fertility and physical damage

The effects of available soil N and P and the effect of simulated browsing (leave removal) on foliar condensed tannin (CT) concentration were tested on young Colophospermum mopane (J. Kirk ex Benth) J. Leonard (Mopane) plants. Although clear differences in growth occurred between different levels of soil N, no differences in foliar CT concentration were found. Changes in available soil P and physical damage did not affect the plant growth or chemical composition. The complete absence of response of CT concentration to physical damage and soil nutrients may be related to the age of the trees in this study. It is proposed that existing theories on the interaction between soil properties and carbon based defences in trees are expanded, to include the potential responses of young trees to different soil nutrient levels.     

Effects of simulated winter browsing on mountain birch foliar chemistry and on the performance of insect herbivores

Winter browsing by mammalian herbivores is known to induce a variety of morphological and physiological changes in plants. Browsing has been suggested to decrease the carbohydrate reserves in woody plants, which might lead to reduced tannin production in leaves during the following summer, and consequently, to increased herbivore damage on leaves. We conducted a clipping experiment with mature mountain birch trees and measured the effects of clipping on birch growth, leaf chemistry and toughness, as well as on the performance of insect herbivores. Leaves grew larger and heavier per unit area in the clipped ramets and had a higher content of proteins than leaves in the control trees. Clipping treatment did not affect the total content of sugars in the leaves (mg g^?1), suggesting that a moderate level of clipping did not significantly reduce the carbohydrate pools of fully‐grown mountain birch trees. Furthermore, the contents of proanthocyanidins (condensed tannins) and gallotannins were slightly higher in the leaves of clipped ramets, contrary to the hypothesis of reduced tannin production. The effects of clipping treatment on leaf and shoot growth and on foliar chemistry were mainly restricted to the clipped ramets, without spreading to untreated ramets within the same tree individual. The effects of clipping on leaf characters varied during the growing season; for instance, leaf toughness in clipped ramets was higher than toughness in control trees and ramets only when leaves were mature. Accordingly, clipping had inconsistent effects on insect herbivores feeding at different times of the growing season. The generally small impact of clipping on herbivore performance suggests that the low intensity of natural browsing at the study area, simulated by our clipping treatment, does not have strong consequences for the population dynamics of insect herbivores on mountain birch via enhanced population growth caused by browsing‐induced changes in food quality.     

Browsing in a heterogeneons savanna

Large herbivores generally depend on and interact with a food resource that is heterogeneous at different spatial scales. Plants allocate resources to rapid growth or to defence mechanisms depending on the availability of resources relative to loss of resources from herbivory. Herbivores select food and feeding habitats in order to maximize intake rate of nutrients and digestible energy, while avoiding chemical and structural deterrents. To optimize foraging, herbivores select habitats and food items in a hierarchical way, and different attracting and deterring factors may govern selection at different scales. We studied the impact of twig biting by a guild of indigenous browsers in three vegetation types in a semi-arid savanna in Botswana. The heaviest browsing pressure was in the vegetation type richest in preferred plant species, although that type was also richest in defended species. There were large differences in relative utilization between plant species, and ranking of species was roughly similar in the different vegetation types. Browsing pressure varied between species from almost 0-30%. Overall, spinescent trees were less browsed than non-spinescent ones, and evergreen species were less browsed than deciduous ones. In two of the three vegetation types there was a negative correlation between browsing pressure on a species and its frequency. There was a high incidence of rebrowsing, and once a tree had been browsed, the probability that it would be browsed again increased. The results largely agree with predictions based on the resource availability hypothesis, the scarcity accessibility hypothesis and recent theories on the significance of plant defences and on plant's response to browsing and the subsequent response by herbivores on the plant's responses.     

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.