BROWN HARES ( L E P U S EUROPAEUS) ORFORD BEACH, 1964-6
ON
D . A. WHITE
(East Suffolk Warden Naturalist of the Nature Conservancy)* BROWN Hares (Lepus europaeus) and Rabbits (Oryctolagus cuniculus) are the largest and most conspicuous animals resident on Orford Beach. Their activities appear to have been a major influence on the development of Vegetation, but since the first appearance of myxomatosis on the beach in 1958 the Rabbit population has been insignificant. Brown Hares, however, have flourished in the period 1958-67, although immediately after the severe winter of 1962-3 they were said by local people to be very scarce. Throughout this period the Nature Conservancy has monitored Vegetation changes in a small number of 10 m. X 10 m. plots, half of which are fenced to exclude Hares and Rabbits. A long-term study of the populations of these two species would help to explain the Vegetation changes. My pilot study from April 1964 to October 1966, yielded information about longevity, population and weight of Brown Hares on Orford Beach. The Site Orford Beach is the peninsular part of the OrfordnessHavergate National Nature Reserve, on the coast of Suffolk, and it is ideally suited for counting and live-marking Hares. T h e peninsula, lying south of a partially Hare-proof fence at grid reference TM(62)430480, (see FIG. 1) is 6J kilometres long, up to 300 metres wide and has a total area at high tide of about 100 hectares. T h e northern end stems from the wider peninsular of Orfordness, which is joined to the mainland kilometres away by the Slaughden isthmus. T h e spit lies between the North Sea and the River Ore, which is nowhere less than 100 metres wide and is subject to tidal currents of up to 7 knots. T h e sampled area is composed predominantly of coarse shingle with minor flanking areas of alluvial mud. Intermittent Vegetation of open-g round and field-layer types is presumably productive enough to sustain a number of small mammals and lagomorphs. It is possible, however, that the Hare population studied subsists in part on the more luxuriant Vegetation which occurs in arable and cattle-grazed marshes at Orfordness and beyond the tidal barrier. Counting Between January 1964 and October 1966, while paying frequent visits (fortnightly on average) to the beach, I obtained five total counts of the Hare population in March-April and •Now Deputy Regional Officer for South Wales, T h e Nature Conservancy, Pias Gogerddan, Aberystwyth.
50
Transactions of the Suffolk Naturalists',
Vol. 14, Part 1
Plan of cage, site map inset
October, representing respectively spring and autumn. On each occasion a group of sixteen to twenty-one beaters lined up at approximately 20 metre intervals across the northern end of the beach and walked slowly to the tapering distal end, counting all Hares which escaped northwards and driving others into a cage put up at the narrowest part of the beach. On the whole this was a satisfactory method because most Hares fled. None of the leverets thought to be present got up, however; presumably they, and perhaps some adults, lay unseen as the beaters passed. In the 100 metres immediatelv before the cage, the beaters, despite their close spacing (5 to 10 metres) had difficulty in counting Hares which broke back with great agility or entered the sea.
BROWN HARES ON ORFORD BEACH
51
Sampling The beaters concentrated on counting but on each occasion herded a sample of the population into the cage. Here we sexed and weighed 116 Hares captured alive or accidentally killed and released ninety-five individuals with ear-markings. Farmers on the mainland and the gun-club at Orfordness were requested to report observations of tags. Figure 1 shows the design of the temporary fence and cage into which we drove the Hares. It consisted of wire-netting supported at a minimum height of 4 feet on driftwood poles and weighted down by driftwood logs and pebbles. The cage ( 6 x 2 metres), built over a hollow in the shingle was lined with litter for the captives. Having discovered that Hares can wriggle through 3" mesh, I used 1" mesh for the cage and 2\" mesh for the fence; no Hare got through or over this barrier. It was, however, imperfectly designed for holding a large number of animals undamaged. Many Hares, having unsuccessfully scanned the whole length of netting, escaped by doubling back. We captured only the few which took cover beneath the litter in the cage when the beaters closed the door. Several Hares damaged the skin of their noses as they dashed headlong into the wire or tried to bite through it, and one or two animals died in the cage, possibly as a result of suffocation, exhaustion or shock. Using spring balances, we weighed the captives to the nearest l lb. in sacks of known weight. For ear-marking, I mainly used " S t i g s " but also tested Partridge wing-tabs and tattoo-marks. Numbers tattooed on the pinna were not detectable six months afterwards. Stigs are plastic ear-tags (1" in diameter, engraved with small numbers) fastened through holes punched in the pinna. They persisted well, some for at least two years, although at least one Hare is known to have lost its stig. Incidentally, numbers painted on the stigs could be identified in the field under good conditions, i.e., a stationary Hare with ears erect, at less than 50 metres with a telescope x 2 5 . Partridge wing-tabs, which are small metal clips engraved with small numbers, were easier to fit than stigs and seemed as persistent.
Results Individual records of all Hares captured, weighed, and aged are tabulated in Figure 2 (2A for bucks, 2B for does), while Figure 3 is a summary table of the total counts, percentage samples examined, average weights of the sample and estimated biomass of each population. The total counts, judging from the beaters' detailed reports, are very close to the true populations, excluding leverets, except in October 1964, when a dozen or more Hares squeezed through a piece of 3" mesh netting and were not counted.
52 Transactions of the Suffolk Naturalists', Vol FIG. 2A
Buck weights and ages (* died on day of count. (*) died between counts) WE1GHT (to i Ib.) Minimum (months) Buck Spring Autumn Autumn Spring Autumn age uihen last No. 1.4.64. 23.10.64.15.10.65. 1.3.66. 14.10.66.re-examined 1 81 7t 2 3 81 7t 4 5 5 6 6i 5 7 8 51I 5 9 10 41 18 11 +1 12 61 13 51 14 61 (*) 15 6 18 16 6i 17 7| 7 18 7 19 20 5i 21 41* 22 6 23 6 6 15 24 8i 25 9t 26 6 27 91 6 15 28 71 6 12 29 6i 7t 5 30 51 12 7t 31 7t 5 32 61 33 51 34 7i 5t 5 35 () 36 7! ( 37 51* 6 38 ? 39 61 6 12 7t 40 7 41 42 6 * 43 6t 44 5f 45 6| 46 51 47 61 48 7J 49 51 50 7} 7 51 52 61 53 61* 54 6 55 6t 56 3 31 57 41 58 59 5? 60 41 61 6 62 6 63 51 Average weight 7 5t 61 61 6t (to i Ib.)
BROWN HARES ON ORFORD BEACH
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FIG. 2B Doe weights and ages
(* died on day of count. R lost tag, 1 metal, 2 stig) WE1GHT (to i Ib.) Minimum (months) Spring Autumn Autumn Spring Autumn age when last 1.4.64. 23.10.64.15.10.65.11.3.66. 14.10.66.re-examined
Average wcight Uo l Ib.)
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Transactions of the Suffolk Naturalists', Vol. 14, Part 1 FIG. 3
Summary Table Spring 1.4.64.
Autumn 23.10.64.
Autumn 15.10.65.
Spring 11.3.66.
Autumn 14.10.66.
61
95
165
146
182
Sample as % of total count
53%
22%
23%
20%
13%
Average weight of sample (to i lb.)
6i
ti
7t
6i
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Biomass (lb./ha.) of population
3-8
6-4
12-0
9-5
100
Total count
The biomass figures have been obtained by dividing the product of the total count and the average weight of the sample by the area sampled. A.
POPULATION, W E I G H T AND BIOMASS CHANGES
April to October, 1964: the population growth-rate was at least 58%. Although two out of four individuals re-examined had lost weight, the average weight of the sample increased by 8%. Consequently the biomass rose markedly. Since there are no 1965 spring data, a direct comparison with the 1964 autumn figures is not possible. October, 1964 to October, 1965: the net gains in population and average weight over this period were 74% and 7% respectively. Four out of five individual weights increased. The net increase in biomass probably masks a fall in winter, followed by a very steep rise in summer. October, 1965 to March, 1966: the population declined by 16% and there were parallel declines in all seven individual weights, in average weight (10%) and consequently in biomass. April, 1964 to March, 1966: over this two year period, the spring figures show a population growth-rate of 139%, slight increases in average weight and three individual weights and an upward trend in biomass. March to October, 1966: the population growth-rate was 25% but, intriguingly, only one out of three individuals re-examined had gained weight and the average weight dropped by 16%. The median weight, too, feil to its lowest record. The seasonal rise in biomass is negligible. October, 1965 to October, 1966: in this period, while the population continued to rise, the average weight and all five individual weights declined and the biomass failed to reach its 1965 peak.
BROWN HARES ON ORFORD BEACH
B.
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LONGEVITY ( F I G . 2 )
Tagged doe number 2, already mature (pregnant) when first captured in April 1964 was thirty months older when recaptured for the fourth time in October, 1966. T h e tagging should reveal more records of longevity in due course.
Discussion For discussion's sake it is assumed that the total counts are accurate and the population growth-rates are not affected by diurnal migration. T h e long-term upward trend in both autumn and spring population indicates infilling of an environment previously more or less vaeated. T h e population trough may have been due to the severe winter of 1962-3. There is no local evidence relating the subsequent increase in Hares to the decrease in Rabbits. Either migration, or mortality and natality in a resident population, or both would account for the long-term trends and seasonal fluctuations observed.
A.
MIGRATION
Hares could migrate between Orford Beach and Orfordness overland; and between the spit and the mainland either overland at Slaughden or by swimming the River Ore. Information about the latter possibility shows, but not conclusively, that Hares swim the River Ore only when pursued on the mainland or on the spit. Hares are generally absent from Havergate Island in the River Ore but they occur occasionally after shoots on the mainland (R. Partridge, pers. comm.). Of the many Hares seen swimming from the beach during counts, most soon returned or, regrettably, drowned; only eight landed on the mainland about 200 yards away. I saw only one Hare enter the water when not pursued; it headed for the mainland but after swimming about 70 metres in twelve minutes turned back and relanded on the beach. No tags have been recovered from the mainland. T h e evidence of overland migration is also largely negative. Hares do go from Orford Beach to Orfordness on foot but out of ninety-nine tags used none has been recovered by the Orfordness gun-club, let alone from the mainland. T h u s although the population counted is not necessarily confined to Orford Beach, it does seem that the tagged hares were territorial in some way. The sampling may have favoured Hares from the distal part of the spit at the expense of those further north. There is no evidence against summer immigration from Orfordness but, since the territorial behaviour would probably be similar throughout the spit, it seems likely that the population growth-rates on Orford Beach apply throughout the spit and are not due to seasonal migration.
56 B.
Transactions of the Suffolk Naturalists', Vol. 74, Part 1 RESIDENT POPULATION D Y N A M I C S
Observations suggest that at least part of the population is resident in the sampling area. Hares occur on Orford Beach at all times of year and about 25% of the tagged ones turned up in the area again at least once alive or dead. Breeding is proved because I saw leverets on two occasions; I probably overlooked many others. Even if the whole population were resident on Orford Beach throughout the year, the growth-rates over six month periods can be reasonably explained wholly by seasonal excess of natality over mortality and vice-versa. For example, the growth-rates in the first and final six month periods represent respectively 1 -4 and 0-5 surviving offspring per doe. True natalities, allowing for mortality, would of course be higher. It is interesting to note that the lower "natality" coincides with peak breeding population. C.
W E I G H T AND BIOMASS
The uniform decline in individual weights in the winter of 1966 is the only seasonal trend that is both clear and expected. While one might expect individuals to gain weight in summer it is not difficult to explain anomalous losses of weight by referring to such factors as pregnancy, recent feeding, excess age or poor health. Average weight would be expected not to show clear seasonal patterns because in summer adults which had gained weight since the winter might be more or less balanced by juveniles. This is borne out by the two spring figures being similar while the three autumn figures are divergent. The autumn samples may, by chance, not fairly represent the weight-classes available but it is not clear which of the averages are the more accurate. The 1966 sample appears to be anomalous mainly because it contains a number of Hares, juveniles presumably, weighing less than 5i lbs. while this weight-class is barely represented in the 1964 and 1965 samples. It is stränge however that juveniles should be so prominent in 1966 when the growth-rate was relatively low. The anomalous biomass in autumn 1966 may be significant. It is partly due to a notable lack of heavy Hares in the sample, reflected in the relatively low median weight. The biomass curve suggests that, when the carrying capacity of the beach is approached, there might be a negative relationship between population density and average weight. Without further work this possibility should be stated no more strongly because the biomass figures are few and governed by many variables. The idea of a density factor operating, however, is supported by one independent result already noted, i.e., that population recruitment in 1966, despite the greater adult population, was no higher than in 1964 and consequently the growth-rate was lower in 1966.
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Acknowledgements I a m indebted to the Earl of Cranbrook, whose interest stimulated this study and to all those whose co-operation made it possible, i.e., the beaters—staff and boys from local secondary modern schools; our boatman M r . R. Partridge, local landowners, and my colleagues in the Nature Conservancy.