Browsers, grazers, miners and suckers. From the tiniest caterpillar, to the most majestic red deer stag, herbivores – the animals that eat plants – have a huge effect on vegetation throughout Scotland’s native forests. Indeed, most of the Earth’s terrestrial ecosystems are heavily influenced by these animals.
The action of herbivores, known as herbivory, differs from predation in that predators generally kill the animal they eat, whereas plants usually survive after being fed upon by herbivores. Also, a plant can support a large population of herbivores, as there are so many parts of it which can be eaten: leaves, buds, bark, wood, stem, sap, flowers, pollen, nectar, roots, fruits and seeds. Take a rowan tree (Sorbus aucuparia) as an example. A deer might browse some emerging buds, twigs and leaves in spring. A host of insects may feed on the nectar and pollen from its flowers, caterpillars of several moths make mines in the leaves, and in the autumn fieldfares (Turdus pilaris) and their relatives will gorge on the berries. These are just a fraction of the herbivores the tree may support.
There are a myriad of herbivore species in the forest and their interactions with plants are far-reaching and complex. Below we will look at some of the main herbivore groups. Using examples of the more obvious and influential species and their effects, we will gain a glimpse into the fascinating world of herbivory.
Insects have been referred to as major architects of the plant world. Although much of the attention given to forest insects has been related to pest outbreaks in monoculture plantations, native forests are more diverse and generally less vulnerable to pest attacks. On the whole it is not in an insect population’s interest to completely kill its host. When insects do kill plants, it is often the weak ones, and this contributes to a stronger, healthier gene pool, a diverse forest structure, and allowing more light to reach the forest floor.
Caterpillars, including those of the pine looper moth (Bupalus piniaria) and the pine sawfly (Neodiprion sertifer) feed on the needles of Scots pine (Pinus sylvestris), but their numbers are controlled by predators such as wood ants (Formica aquilonia and F. lugubris). The frass, or excrement, and other organic matter dropped by the caterpillars enriches the forest floor.
Bees and butterflies such as the pearl-bordered fritillary (Boloria euphrosyne) are among the more obvious forest insect herbivores. Feeding on sugar-rich nectar provided by flowers, they spread pollen between plants. Along with the provision of berries, this is a good illustration of how plants have adapted to utilise herbivores for their mutual benefit.
Less visible are the various tiny insects which ‘mine’ leaves. These live inside the leaf itself and move around as they consume the cellulose there. Some insects, particularly the true bugs such as aphids, suck liquids from plants. There are a range of insect (and other invertebrate) herbivores which induce plants to produce galls. Laying their eggs in the plant’s tissues, they cause an abnormal growth in the plant which provides protection and shelter for the insect larva. Some of the spangle galls – small discs found on the underside of the leaves of oaks (Quercus spp.) – are induced by a tiny wasp (Neuroterus quercusbaccarum). These do little harm to the tree, and provide food for birds such as blue tits (Parus caeruleus).
The main mammal herbivores in the forest include red deer (Cervus elaphus), roe deer (Capreolus capreolus), the red squirrel (Sciurus vulgaris), wood mice (Apodemus sylvaticus), voles (Microtus agrestis) and hares (Lepus spp.). In the past, there were also other large mammalian herbivores, but they are now extinct in Britain (see below). Each species has (or had) its own unique feeding habits, adding to the complexity and richness of the Caledonian Forest.
Deer play a vital role in the forest, as their grazing and browsing helps to create a diverse structure in the vegetation. It is quite natural that some tree seedlings are browsed, and this can help produce glades, and promotes interesting growth forms on the trees as they mature. However, in large areas of the Highlands the grazing pressure is so intense that native forest is unable to regenerate at all. The high deer population at present (combined with the effects of sheep and other livestock) means that a very large proportion of seedlings are being overgrazed. Bitten back year after year, the young trees eventually die, leading to an unnaturally low proportion of woodland cover.
The influence of deer can also be seen on more mature trees. Bark is sometimes stripped in order to access the more nutritious inner bark, and in parts of the forest (and even some parks and gardens), a definite ‘skirt’ can be seen around some trees, where deer have browsed the lower branches. This can give the trees the appearance of having been trimmed with a hedge trimmer!
Although they are much smaller than deer, red squirrels can also be significant as herbivores. By one estimate, a single squirrel can eat the seeds from 20,000 Scots pine cones in the course of a year. They also feed on broadleaved tree seeds such as acorns and hazelnuts, and aid the regeneration and dispersal of these trees, through their caching of the seeds for the winter. Not all of the stored seeds are recovered and some grow on to become new trees, often at a considerable distance from the parent.
Birds are among the most mobile herbivores, and their feeding habits can be crucial for seed dispersal . The capercaillie (Tetrao urogallus) helps to spread the seeds of plants such as blaeberry (Vaccinium myrtillus), and it grazes on pine needles in the tops of ‘granny’ pines, contributing to the trees’ unique growth forms. Another native pinewood bird, the Scottish crossbill (Loxia scotica), has evolved its characteristic crossed beak so that it can prise open pine cones and extract the seeds from inside.
Members of the thrush family are well-known for their fondness for berries. Redwings (Turdus iliacus) among others distribute rowan berries considerable distances. Clusters of these trees are often found growing beneath Scots pines, where perching birds deliver the seeds in their droppings. Rowan can also be found by rocky perches, far from any other tree cover, which highlights the importance of birds in expanding the forest. The mistle thrush (Turdus viscivorus) is a major distributor of holly berries (Ilex aquifolium), and it is possible that a concentration of holly on the south shore of Loch Beinn a Mheadhoin in Glen Affric is the work of this bird.
Jays (Garrulus glandarius) rely on acorns in the autumn. They bury thousands for storage, and while they remember the locations of many of them, some are inevitably left, and grow into mature oaks. It is interesting to note that jays may be seen more readily in Glen Moriston, which has a considerable number of oaks, than just to the north, in Glen Affric, where there are very few oaks.
Many insectivorous bird species play a role in regulating herbivory, by feeding on the insects which eat the leaves of trees and other plants.
Impacts on plants
The impact of herbivores on plants can be huge, but it is often difficult to measure accurately, as the influence varies depending on what part of the plant is eaten. For example, if a deer eats 10 grams of birch buds, the long-term effect is greater than if it were to eat 10 grams of mature birch leaves. This is because the potential growth from the buds would have been lost. Moderate levels of herbivory can actually stimulate plant growth, and make them more vigorous.
Herbivores and plant evolution
As well as affecting the distribution and vigour of plants in the forest ecosystem in the short-term, constant pressure from herbivores over millions of years has forced plants to evolve a variety of defences. Obviously plants are not nearly as mobile as most animal species, but they are far from defenceless.
Physical defences such as spines or thorns can be an effective way of deterring certain hungry mammals. Holly and thistles (Cirsium spp.) are familiar examples. The nuts of hazel (Coryllus avellana) have a hard shell which helps to protect the nut itself, and grasses contain high levels of silica (the substance from which sand and glass are formed), which wears down the teeth of herbivores.
Chemical defences are not as immediately obvious, but are certainly very powerful. Plants can produce a wide range of toxic chemicals, such as tannins and alkaloids. Tannins act by inhibiting the absorption of proteins by herbivores, which can eventually lead to malnourishment and death. Oak trees contain high levels of this substance, to protect their leaves and other parts from herbivore attack. Interestingly, the tannin from oak was once much in demand for tanning leather.
Research in the United States has suggested that some trees may actually release chemical signals (pheromones) when under heavy insect attack, to stimulate the production of defensive chemicals in trees elsewhere in the forest. Does this process take place in the Caledonian Forest? It would certainly be an interesting area for research.
Timing can be an effective defence strategy. Some deciduous tree species will avoid bursting into leaf at exactly the same time as their neighbours, to avoid the risk of coinciding with a ‘boom’ caterpillar emergence.
Another major strategy is spatial defence – that is, keeping vulnerable tissues out of reach of hungry herbivores, so the very ‘treeness’ of trees – a stem which generally keeps the foliage high off the ground – is partly a defence.
The story is not one-sided, however. Over long periods of time herbivores themselves have had to adapt to tackle plants’ defences. Rodents such as the red squirrel have front incisors able to tackle the shells protecting energy-rich hazelnuts.
Deer and cattle have evolved a complex digestive system. Cellulose is a major structural component of plants. A substance called cellulase is required to break it down, and while vertebrates are unable to produce their own cellulase, certain microbes can. All animals have these micro-organisms in their digestive tracts, but some, known as ruminants, have a specially-adapted stomach called a rumen, which has evolved for this job. These large herbivores have also developed high-crowned teeth to resist the wear from eating silica-rich grasses.
Past and future: the role of herbivores in ecological restoration
Studying the effects of large mammals is relevant in restoring our native forest ecosystems. In the past, a fully functioning ecosystem had a wide range of herbivores that were crucial in keeping the forest diverse and healthy. Centuries ago, Scottish forests were home to the moose (Alces alces), aurochs (Bos primigenius) (the huge, prehistoric ancestor of today’s domestic cattle), reindeer (Rangifer tarandus), beaver (Castor fiber) and wild boar (Sus scrofa), as well as the current complement of herbivores. Each had its own unique effect on the forest.
Dutch ecologist Frans Vera suggests that grazing animals would have played a more significant role than has previously been assumed. No one is certain what Scotland’s prehistoric forests would have looked like, but the extensive woodland had a very diverse structure (including some open areas), and was influenced by the wide range of large herbivores and their complex interactions. In addition, the population and distribution of the herbivores themselves was affected by their predators. Clearly the extirpation of some of our native herbivores and their predators has had major knock-on effects for the Caledonian Forest.
There have been some innovative projects involving the use of large mammals in European conservation, many of which are relevant because the insights they offer may help with ecological restoration in the Highlands. One of these is Oostvadersplassen in the Dutch polders. There, a range of different herbivores are being used to try and mimic prehistoric grazing patterns. These include large Heck cattle, which have been bred to closely resemble the auroch. There are also konik ponies which are similar to the prehistoric wild horse known as the tarpan (Equus caballus gmelini). Another example is the ancient Polish forest of Bialowieza, which has a dynamic mosaic of woodland and open ground maintained by herbivores such as the reintroduced population of European bison (Bison bonasus).
In native woodland in Glen Garry, south of Glen Affric, Forestry Commission for Scotland have reduced grazing pressure so effectively that birch regeneration has been extremely dense. Highland cattle have now been introduced to the site to help create a more natural vegetation structure, and their browsing and trampling action is helping to open up some areas and break up the ground. As far as can be judged within a few years, Scots pine seedlings seem to be benefiting from this intervention.
At their Corrimony Nature Reserve, the RSPB are also using cattle to keep an area open for black grouse (Tetrao tetrix). These birds utilise leks, in which the males perform ritual courtship displays, and if those areas become too overgrown the population would suffer. It is exciting to think that grouse may have depended on aurochs in a similar way.
The role of the European beaver (Castor fiber) is currently topical with the proposed trial reintroduction in Argyll. These herbivores have a key role in creating habitats for a wide range of other forest organisms. They help to create standing dead wood by ring-barking some trees, as well as flooding some areas. This provides habitat for a host of dead wood-dependent insects and fungi, as well as for hole-nesting birds such as the great spotted woodpecker (Dendrocopos major). Beavers also keep wetland areas open, to the benefit of many other species.
Wild boar are omnivores, although much of their feeding may still be classed as herbivory. Roots and tubers form an important part of their diet and the boars’ rooting behaviour helps to create seedbeds for trees and expose food for birds.
Herbivores play a fundamental role in keeping the forest healthy and diverse. Human influences over the millennia have shifted the balance to the point where we have lost some herbivores, and now have excessive populations of others, which has a profound effect on the vegetation. Developing a fuller understanding of the role of herbivory, and re-establishing a more natural herbivore fauna, are key steps in restoring our native forests.