Estimation of the Effects of Deer in High Country

Forest and Bird, Issue 105, 1 August 1952, Page 5

Estimation of the Effects of Deer in High Country

r I ’HIS is a digest of a paper under the above title by Mr. R. I. Kean, biologist of the Wildlife Division of the Internal Affairs Department. The full paper was published in N.Z. Science Review and reprinted in Wildlife Publication No. 13, and the digest has been prepared, with the author’s helpful comments, in the office of this Society, in the belief that members would welcome a report of an authoritative investigation into the effects of deer.

When deer form a part of a balanced ecosystem, they are kept. in check by predators, which are assisted by secondary factors such as disease and deficiencies of the various environments. Under such conditions, over-population by deer, if it occurs at all, would be rare. But such ecosystems are now few, being restricted almost to remote areas. With a general reduction of predators, deer over-population is common, with essentially the same consequences in any country irrespective of whether deer are ‘indigenous members of the local fauna or not. The most difficult problem is posed, not by the dense population, which can be reduced by shooting, but, as will be explained later, by the already reduced one which is shown by continued soil erosion to require still further reduction. Success in deer control can only be attained through intimate knowledge of the deer, coupled with careful study of the reactions of the vegetation to browsing and trampling. The clearest insight into deer problems can perhaps be gained by tracing the process of establishment of a hyopthetical deer colony in wet mixed forest. There is no reason to expect that red deer will ever be exterminated in this country. Of the nine species of deer known to be established in New Zealand, only one, the red deer (Cervus elaphus Linn.) is a national problem, and accordingly this species is the only one considered in this article. Deer Herds Deer do not wander at large. As a rule they restrict themselves to country with which they have become familiar. Physiological Differences Different physiological requirements appear to be responsible for the separation of stags from hinds and young deer. During the greater part of the year, the hind is either pregnant or accompanied by a calf, so protection from climatic severity and rough travelling becomes important. The stag is a more robust animal, largely impervious to cold and dampness, but requiring good supplies of calcium and protein in an environment usually poorly provided with these essentials. His needs are determined first by the annual, extremely rapid antler and neck development, followed by building up for the active rut period when little food is taken. He needs nutritious food, and for this reason, as well as for innate tendencies, he wanders far, thus acting as a pioneer in extending the deer range, while hinds and young occupy the central opened out and tracked portions in which the most highly preferred foods have been depleted. Lower Climax Forest Observations show that deer have a preference for country with open forest rising to rolling grassland where there is scope for free movement.

Heavy forest is not favoured. During the early occupation of the country, however, they .will inhabit climax forest as the shrubs and ferns therein provide good food, but as these are eaten out and the area becomes more open, it is little used for feeding. Provided light and soil conditions are suitable, unpalatable or browse-tolerant plants then begin to replace lost species, and the forest shows signs of recovery. Opening out of the forest produces a general reduction of humidity, and the forest ground cover changes from a fern basis to a herbaceous one. Although the most palatable plants become reduced, depletable foods become replaced by permanent food —grasses, sedges and herbs —resulting in an increase in the long term deer-carrying capacity of the forest. The lower climax forest thus is generally little vulnerable under the conditions described especially when growing at an elevation (generally under 2000 ft.) where a wide range of plants is available for regeneration including a number of low palatability species such as -the fern Blechnum discolor (crown fern) or shrubs of the Pseudozvintera (horopito) and Myrtus species or Suttonia australis (mapou). Upper Climax Forest Climax forest at higher altitudes is more vulnerable. Not only are the available plant species much fewer in number, but the fern and shrub species most useful as ground cover, Polystitchum vestitum (prickly shield fern), Leptopteris superba (Prince of Wales fern), Astelia spp., Coprosma foetidissima (hupiro) and Olearia colensoi (tupare) rank in palatability at least high enough for them to be regarded as staple deer foods, and also deer use at this elevation is very much greater than at lower levels. t It has been stated that, except at high altitude, climax forest is relatively invulnerable to low deer densities, but deer in high numbers will largely denude the forest of both the shrub layer and the ground cover. Litter is removed from slopes by the flow of surface water and soil is then exposed, during rain, to the impact of the heavy drip, falling without interception from the tree canopy twenty to thirty feet above. Under such conditions soil loss is rapid, often resulting in exposure of loose stones which move slowly down-hill, effectively preventing regeneration, irrespective of browsing pressure. Food Resources The best food combining both high average palatability and availability for deer is found in subclimax communities such as occur on slip faces, stream margins, forest blow-downs, etc. In New Zealand typical species occurring in these communities are Blechnum procerum (kio kio), Polypodium diversifolium, Asplenium bulbiferum, Microlaena

avenaceae (bush oat grass), Danthonia cunninghamii (bush tussock), Uncinia spp. (hook grass), Coprosma australis (raurekau), Nothopanax spp. (five finger spp.), Aristotelia racemosa (wineberry), and Melicytus ramiflorus (mahoe). It is these areas, together with the upper forest scrub belt and the tussock grassland, that absorb deer which at first fed in climax forest. Consequences of Deer Use and Population Changes Deer in small numbers at first have little obvious effect in forest, natural regeneration being sufficient to hide the worst of their depredations. As the herds increase, however, with the large stags eating wastefully and destroying a large amount of plant food while only eating the titbits, the forest appears, generally rather suddenly, to be badly denuded, deer become visible in large numbers and the impression is given of a sudden large influx of deer. Heavy deer population can be reduced without solving the high country soil erosion problem, for two reasons, (a) deer do not occupy country uniformly, but tend to concentrate in limited areas and (b) they will persist in frequenting eaten out slopes. These areas, which in the aggregate make up a large proportion of the steeper forest land are continually stripped and trampled. Tall Tussock Grassland High altitude tussock grassland is not invulnerable. Danthonia flavescens (snow grass), although not of high palatability, is a staple deer food. Destruction of tussock by browsing is relatively slow, but it is accelerated by deer concentrating upon individual tussocks, so it is not unusual to find, in one locality, eaten tussocks together with healthy, little-reduced tussocks. It seems likely that eating off the tips of tussock exposes more palatable portions below and it is reliably reported that deer will pull tussock to pieces, eating the soft bases and rejecting the leaf blades. On the colder slopes the dominant plant association, Carpha albina—Oreobalus pectinatus, is little affected by browsing but is vulnerable to trampling

which opens up the way to sapping by rain, frost and snow, and the rapid seepage into the soil aggravates the trouble. Erosion In the steep high-rainfall districts the commonest form of geographical erosion is by landslip. The scar heals fairly rapidly and passes through varying plant succession although a climax association may not have time to develop before slipping again occurs. Deer (and opossums) increase the incidence of slipping both by destroying plant cover and by increasing the under cutting of slopes caused by scouring due to increased and faster run-off. Lower Rainfall Country . The foregoing descriptions have given a generalised picture of developments in high-rainfall country. Low-rainfall country such as beech or grassland presents a simpler situation. These areas favour rapid spread of deer of either sex- owing to the easily reduced scrub layer. Forest food resources are limited, but the Nothofagus seedlings and the adventitious shoots produced during the pole stage of development probably have higher food values than mature beech leaves. Usually deer in such areas have a grassland economy, and forest has a supplementary role. A light deer population is able to keep a forest of this type denuded, but deer here are very much more easily destroyed than in high rainfall areas. Soil erosion problems are essentially similar in both wet and dry areas, but in the latter, scouring and consequent slipping are generally less. Validity of Conclusions The evidence on which the preceding paragraph is based has been drawn in the main from the Tararua Range which has changed during the past twenty years, section by section, from stable dense forest to depleted and rapidly eroding deer range. The Tararua Range has been regarded as a standard area, with which it has been possible to compare most of the deer country of New Zealand either from brief personal visits, or from written and oral

reports of wildlife field officers. Observation and deduction during this period have been aided by evidence derived from the adjacent Haurangi State Forest which had been eaten out many years before. Population and Control From experience with deer and deer country, it is possible for a fairly sound estimate to be made of the number of deer which can be shot in one year on any large land area, but an accurate estimate of deer population, according to present indications, is impossible in New Zealand. Under most forest conditions, restriction of visibility, and deer movement make estimates unreliable, but probably the greatest source of error is caused by deer not being readily seen until population density has reached saturation point. During the early days of organised deer control in New Zealand, a deer district would be shot intensively until numbers were reduced to an apparently low level, and then spelled while work was carried out in another area of high population. Rapid increase of deer again in the first area was thought to be due mostly to immigration from inaccessible

back country. Some immigration certainly occurred, because numbers dropped on some unshot country, but it is the writer’s belief that repopulation was due largely to the natural increase of a large hidden population. In other words, shooting considerably reduced the visible surplus, but it had much less effect upon actual population numbers. This phenomenon of sudden concealment is only the reverse of the deer irruption process previously described. Some natural control of deer can occur in beech forest if adjacent grassland becomes covered with snow deep enough to inhibit movement, for winter deaths of deer of all classes are reported, but survivors again flourish with change of season. Except in mineral-deficient country, which is rare, malnutrition of deer seems hard to substantiate, and deer in eaten-out country seem to be able to maintain good condition on scanty diets of withered herbs, fallen leaves, adventitious shoots and a little grass and sedge; the persistence of numbers of deer in eatenout areas is regarded as evidence that food is seldom a limiting factor, and that topography, cover, and altitude weigh more heavily in the constitution of a good deer area.