Int J Biometeorol (1988) 32:205-216 meteorology. An extralimital population in a warm climatic outpost: the case of the moth Idaea dilutaria in Scandinavia.
Int J Biometeorol (1988) 32:205-216
meteorology An extralimital population in a warm climatic outpost: the case of the moth Idaea dilutaria in Scandinavia Nils Ryrholm Department of Zoology, Section of Entomology, Uppsala University, Box 561, S-751 22 Uppsala, Sweden
Abstract. The moth Idaea dilutaria Hiibn. (Geometridae) has an isolated population on the Kullaberg peninsula in southern Sweden. Investigations of the local and micro-climate on the peninsula showed that the local distribution range of the moth coincided with the areas of warmest climate, supporting the hypothesis that the Kullaberg population is dependent for its survival on the warm climate of this area and that the species here is a "thermal relict" from a previously warmer climatic period. Key words: Isolated population - Geometrid moths - Local and mikro-climate - Solar radiation - Thermal relicts
Europe for I. dilutaria and T. conigerum (Skou 1984; Holm 1977). A few specimens of D. pallipes have been observed associated with saw mills in northern Germany (80-100 years ago), and there is one record from southern Denmark in 1974 (Bangsholt 1975; B. Ehnstr6m and V. Mahler personal communication). However, these records probably represent individuals imported with the timber. Apart from the fact that these species are mostly ground-living, small and with low mobility, they have very different life patterns. The larvae of L dilutaria spend August to June feeding on the ground on leaf litter with low food value, and those of D. pallipes live in dead branches and logs
Introduction In northern Europe there are many isolated populations of plant and animal species. Most of these disjunctly distributed species have been interpreted variously as (i) sub-arctic relicts of the last ice age (Cassagnau 1959; Holdhaus 1954; L6ve and L6ve 1963; Udvardy 1969) or (ii) thermal relicts of warmer postglacial periods (Ahl6n 1984; de Lattin 1967; Lundqvist 1968; Udvardy 1969), which are now restricted to a few localities with suitable habitats and particularly favourable climate. Some of the most restricted and isolated arthropod populations in Scandinavia occur on the southern slopes of the Kullaberg Peninsula in NW Scania (Fig. I). The disjunct species found on Kullaberg include the moth Idaea dilutaria Hfibn. (Lepidoptera, Geometridae), the beetle Danacea pallipes Panz. (Coleoptera, Melyridae), and the spider Theridion conigerum Sim. (Araneae, Theridiidae). Kullaberg lies around 800 to 1000 km (or more) north of the limits of the continuous distributions of these species (Fig. 2) and is the only known locality in northern
Fig. 1. Mean annual temperatures in southern Sweden. Kullaberg is within the small square (revised from Eriksson 1982)
206 mainly consists of red-grey, fine grained gneiss (Forsell 1962; Hede 1964). The peninsula is orientated W N W - E S E , with very steep slopes along the entire north side and parts & t h e southern side (Figs. 3, 9), especially close to the sea. The inclinations of the lower south-facing slopes west of M611e (see Fig. 9 for names) vary from 35 ~ to 48 ~ and those west of Ransvik vary from 25 ~ to 35 ~ Almost all the steepest slopes lie below 65 m above sea level. In the eastern, higher part of Kullaberg most of the inclinations of the south-facing slopes are between 20 ~ and 27 ~ and in the lower eastern part the steepest slopes are between 30 ~ and 40 ~ (Figs. 3, 9). o
Fig. 2. The European distribution of Idaea dilutaria. The northern limit of the continuous distribution of Danaeea pallipes is slightly north of the drawn line. Theridion conigerum is known mainly from south and central France and Czechoslovakia
from August to April (or May), while the spider T. conigerum lives entirely on the ground. If the isolated arthropod populations on Kullaberg are "thermal relicts" and occur on the peninsula because of a locally favourable climate, then these species can be expected to be associated with the warmest microclimate within the area. In the present study I have investigated the local and micro-climate of the Kullaberg peninsula and the distribution pattern of L dilutaria in order to test the prediction that the species should be preferentially associated with the warmest microclimate within the area. Site description Geography. Kullaberg is a promontory 9 km long and 1-1.5 km wide of pre-Cambrian origin in N W Scania (Figs. 1, 3), which
Vegetation. The south side is covered mainly by oak (Quercus petraea Liebl., Q. robur L.) and beech (Fagus sylvatica L.) woodland with small, irregularly spaced clearings. Leaf expansion in these woods normally starts at the end of April or in early May. The northern and northeastern parts of the peninsula are covered by (partly planted) coniferous stands consisting of a mixture of local and introduced species (Kraft 1982). The outer tip and some areas on the higher tops of the peninsula were deforested by a hurricane in 1980; before this event these areas were covered with pine.
Temperature and climate. The western coastal areas of Scandinavia have comparatively high winter temperatures, with monthly winter means of around 0 ~ (Johannessen 1970; Laaksonen 1979). The high winter temperatures are associated with mild autumns and comparatively sunny and early springs, which give a long growing season along the SW coast of Scania and also the highest annual mean temperatures found in Sweden (Fig. 1). This part of Scania, together with the eastern and southeastern parts of Denmark, has the highest annual mean temperatures found at this latitude (Liljequist 1970; Wall6n 1970).
Materials and methods Climatic measurements. The air temperature was measured from May 1983 to September 1984 using a short-legged, double roofed, Stevenson screen containing a maximum-minimum thermometer and a biophenometer, which samples the temperature every 10th rain and stores the accumulated data (see Ryrholm 1988). The screen was situated in a small clearing 50 m
Kattegalt
Fig. 3. Distribution of open ground (shaded) and sample points (dots); open circles, sample points added November 1983 ; filled triangle, SMHI station; open triangle, field station
207 Table 1~ Mean air temperatures on Kullaberg Mean air temperature, ~ C SMHP, lighthouse (1951-1980) 1983
1984
SMHP, lighthouse (1983 1984)
South slope (1983-1984)
Difference (1983-1984)
May June July August September October November December January February March April May June July August
10.2 14.7 16.1 16.1 13.3 9.2 5.0 1.8 - 0.4 - 1.0 1.1 4.9 10.2 14.7 16.1 16.1
10.5 14.4 17.4 17.2 13.9 9.9 4.6 1.7 0.9 - 0.2 0.5 5.7 11.4 14.0 15.7 17.0
13.2 15.5 19.6 19.1 14.5 10.6 5.3 1.7 1.5 0.6 2.3 7.7 13.7 15.5 18.3 19.1
2.7 1.1 2.2 1.9 0.6 0.7 0.7 0.0 0.6 0.8 1.8 2.0 2.3 1.5 2.6 2.1
Year mean
7.6
7.9
9.2
1.3
" Swedish Meteorological and Hydrological Institute
above sea level (asl) in the eastern part of the L dilutaria distribution area (Fig. 3), shaded (in the summer) by surrounding trees except between 12.00 h and 15.00 h. During April 1984 a thermograph was also used in a separate screen. No deviations between the data from the two instruments were found. The data from the southern side were compared with corresponding data from an SMHI (Swedish Meteorological and Hydrological Institute) station at the lighthouse on the western tip of Kullaberg (Fig. 3). The Pallman invert sugar method (Pallman et al. 1940; Berthet 1960; Lee 1969; Jones 1972) was used to measure temperatures very close to the ground (mean ground temperature). A sucrose solution (pH 2.3), based on a potassium oxalate buffer, pH 2.4, and the sucrose recipe given by Berthet (1960) was used. One solution batch was mixed and " b o t t l e d " for each m o n t h and then immediately frozen. The probes were kept frozen until use. After use, they were immediately frozen on dry ice and kept cold until analysed. A system of 98 "invert sugar" measurement points (6 outside the map) was arranged to cover the entire peninsula (Fig. 3). Measurements were made from June 1983 to March i985. A probe (one 5 ml test-tube) 50 x 15 ram, filled with sucrose solution, was placed at each sample point. At 3 points, double sugar probes were used, and the two Stevenson screens contained double or triple probes. Double probes were also placed out (and compared each m o n t h with ground temperature measurements) at a microclimatic field station outside Lurid, Scania (see Loman 1986), 80 km southeast of Kullaberg. Each tube was attached to and partially sheltered by a 20 cm long piece of V-shaped moulding. These " s t i c k s " were put into the ground, orientated to avoid direct solar radiation on the testtube and with the bottom of the tube resting on the ground surface. All probes were placed on an open ground surface of at least 2 x 2 m in area. Shaded sites were avoided. Field adapted values of the velocity constant (Kr) for the solution were calculated by correlating the arithmetic means from the stations with the values obtained from the correspond-
ing sugar probes. This was done to reduce errors caused by temperature fluctuations (see Lee 1969). Kr was calculated by linear regression and then corrected for variation due to month, year and exposure time. The mean deviation from the '~ means was 0.52_+0.38 ~ C and did not differ significantly from 0 ( t = 1.349, n = 40). Most of this variation was a result of differences between the batches of sugar solution. The temperature was then calculated as : T = (5133.1886)/(13.8479-- log Kr) and K r = l/t log (Ro - R~/R~-- R~) Where: T = temperature in Kelvin t =inversion time for the solution Ro = initial rotation angle of the sucrose solution Re = rotation angle at time t R ~ = rotation angle of the completely inverted sugar solution When measured at 436 nm Ro and R~o were 50.0 and - 1 7 . 5 ~ respectively. The accuracy of the method was tested by comparing the internal differences of all double and triple probes. The mean of absolute differences between the pairs was 0.28 _+0.60 (99% confidence interval). To fill some gaps, another 15 sample points were added in November 1983 (Fig. 3). F r o m 1982 to 1984, 15 separate temperature measurements were performed to confirm the other results. Isotherms based on the combined results were drawn onto the geotopographical map of Elvhage (1983). Wind velocity and direction were measured on 6 occasions on the western and southeastern parts of the peninsula using an anemometer (1.5 m above the ground). The measurements were made during NW, W, SW, S, SE and N winds.
L dilutaria distribution. The distribution of L dilutaria was assessed during the flight season (July) in 1982-1986, by searching
208 Table 2. Weekly minimum temperatures and temperature means on Kullaberg, November 1983-May 1984 Period
Mean temperature (o C)
Minimum temperature (~ C) Lighthouse
Southslope
Difference
-6.11 -13.11 -20.11 -28.11
6.4 -2.0 -1.0 0.0
6.5 -1.8 -1.0 0.0
0.1 0.2 0.0 0.0
8.5 5.5 4.7 3.3
9.2 6.3 5.4 4.1
0.7 0.8 0.7 0.8
-3.12 -11.12 -17.12 -23.12 -30.12
-5.8 -5.2 -3.5 -3.0 -1.2
-5.5 -5.2 -3.6 -3.1 -1.1
0.3 0.0 -0.1 -0.1 0.1
-1.2 2.8 -0.7 1.5 3.4
-1.0 3.1 -0.7 1.5 3.4
0.2 0.3 0.0 0.0 0.0
-6.01 -14.01 -21.01 -31.01
0.5 -5.6 -4.6 -6.5
0.4 -5.6 -4.5 -6.4
-0.1 0.0 0.1 0.1
3.6 1.6 1.3 -1.2
3.4 1.6 1.7 -0.3
-0.2 0.0 0.4 0.9
-5.02 -11.02 -26.02
-0.8 -1.6 -4.5
-0.6 -1.3 -4.3
0.2 0.3 0.2
1.6 1.1 -1.3
2.5 2.5 ~0.7
0.9 1.4 0.6
-5.03 -11.03 -15.03 -25.03
-1.6 -2.1 -2.4 -4.0
-1.4 -2.0 -2.5 -3.8
0.2 0.1 -0.1 0.2
0.8 0.8 1.2 -0.8
1.7 2.7 2.8 1.6
0.9 1.9 1.6 2.4
-1.04 -7.04 -23.04 -28.04
-0.5 -0.5 0.2 3.0
-0.5 -0.3 0.5 3.4
0.0 0.2 0.3 0.4
2.1 4.3 6.0 7.5
3.2 5.1 7.7 10.5
1.1 0.8 1.7 3.0
-8.05 -13.05 -20.05 -1.06
5.0 3.5 9.3 10.0
5.5 3.8 9.6 10.5
0.5 0.3 0.3 0.5
8.9 7.8 12.5 14.1
11.4 11.1 14.5 15.8
2.5 3.3 2.0 1.7
the slopes and mapping all observations of adults. Suitable habitats were searched at 2-3 day intervals.
Results
Temperature Except during December and the first half of January, the south-facing slopes on average had higher mean temperatures than the adjacent coastal areas (Tables 1, 2). The thermal conditions during the winter were the result of low solar altitude and the low number of sunshine hours (Fig. 4 and Tables 3, 4). During,this period, only small differences were observed between the air and mean ground temperatures (Fig. 5 and Tables 1, 2). From the end of January, when solar altitude and the number of sunshine hours rapidly increased (Fig. 4, Table 3), there was a distinctly faster temperature increase on the southern slopes (Table 2). This increase was first observed and was most pronounced for the ground temperatures (Figs. 5, 6). As spring progressed the heat influx rapidly in-
Lighthouse
Southslope
Difference
creased, especially on steep, south-facing surfaces close to the sea (Tables 2, 4), leading to an increase in the difference between the slopes west of M611e and the adjacent areas (Figs. 5, 6, 7 and Tables 1, 2). The largest monthly increase in ground temperature took place in April, with mean temperatures reaching summer values 0 > 1 0 ~ C) on the south side (Fig. 7 a), and the largest increase in air temperature was recorded in May (Tables 1, 2). As a result of the comparatively high solar altitude during the summer, the difference in heat influx between slopes of different inclinations was reduced (Fig. 4, Table 4), leading to a rel/ttively even but strong overall heating. In September, solar altitude and the number of sunshine hours decreased markedly (Fig. 4, Table 3) and an overall temperature decline was observed (Fig. 8b, Table 1). In the autumn, further reduction in solar height and sunshine hours gave a gradual decrease of heat influx (Fig. 4, Table 3). The solar radiation again became more concentrated on the steeper slopes (Table 4). In October, both air and mean ground temperatures dropped below 10~ (Fig. 9, Ta-
209 S
'eh
Fig. 4. Solar altitudes and possible duration of sunshine in western Scania, March-September (revised from Mattsson 1961) 2
~i
a~
Table 3. Monthly and annual means of bright sunshine hours in NW Scania 1961-1975 (estimated from SMHI) January February March April
40 60 130 195
May June July August
250 280 260 230
September 165 Total for October 100 Year: 1795 November 50 December 35
ble 1) a n d the difference between the m e a n air a n d g r o u n d t e m p e r a t u r e s decreased. O n the lower a n d steeper slopes west o f M611e, however, the t e m p e r atures still r e m a i n e d higher t h a n at adjacent sites (Fig. 9, T a b l e 1). This difference lasted until early D e c e m b e r (Tables 1, 2). T h e m i n i m u m air t e m p e r a t u r e s in general were slightly higher o n the lower slopes t h a n elsewhere, with the greatest differences in spring a n d s u m m e r (Tables 2, 5). D u r i n g July 1984 there was a signific a n t difference between the s o u t h slope a n d the S M H I station (Table 5, P < 0 . 0 0 1 6 ) . T h e higher m e a n s f o u n d on the slopes were thus n o t only a result o f the higher d a y - t i m e t e m p e r a t u r e s b u t also, at least in part, o f higher night t e m p e r a t u r e s . This difference was m o s t p r o b a b l y caused b y r a d i a t i o n o f stored h e a t f r o m r o c k o u t c r o p s (see Fig. 11). T h e w a r m e s t climate was restricted to a zone a l o n g the steeper, l o w e r p a r t s o f Barakullen, G a s talgm (see Fig. 9 for n a m e s ) 10-75 m asl a n d the slopes west o f R a n s v i k 25-75 m asl (Figs. 5-9, 11). All these slopes are characterized by their relative p r o x i m i t y to the sea, slope angles o f between 30 ~
Table 4. Yearly variation in global radiation means for clear
days, on south-facing surfaces of different inclination in southern Sweden, Whm-2 (after SMHI) Inclination
0~
10~ 20~ 30~ 40~
January February March April May June July August September October November December
839 1192 1930 2537 3580 4290 5503 6175 6993 7330 7601 7790 7110 7303 5804 6204 4050 4557 2310 2767 1066 1430 599 863
1559 3191 4915 6539 7576 7905 7498 6570 5175 3383 1865 1148
1880 3564 5392 6873 7735 7995 7640 6836 5605 3937 2255 1436
2151 3970 5760 7029 7674 7836 7510 6864 5840 4280 2549 1653
50~ 60~ 90~ 2360 4264 6008 6984 7395 7447 7170 6717 5957 4532 2779 1829
2511 4435 6015 6739 6906 6849 6661 6412 5894 4663 2932 1953
2563 4308 5283 5113 4621 4327 4316 4588 4839 4296 2914 2011
a n d 45 ~, o a k - b e e c h forest v e g e t a t i o n with small clearings, patches with o p e n r o c k a n d orientations between SSE a n d SSW. T h e p e r i o d o f the investigation was climatically representative w h e n c o m p a r e d with the m e a n temp e r a t u r e s for 1951-1980 (Table 1). T h e o b s e r v e d difference in m e a n t e m p e r a t u r e s between the stations is therefore p r o b a b l y also fairly r e p r e s e n t a tive.
Wind K u l l a b e r g is exposed to winds f r o m m o s t directions. T h e prevailing winds are f r o m the west.
210 Kattegatt |
Fig. 5. Mean ground temperatures, November 1983February 1984
Katlegatt
Fig. 6. Mean ground temperatures, March 1984
However, great local variation occurs as a result of the topography. All the lower parts of the slopes west of M611e were protected from NW-N-E winds. The slope west of Ransvik was exposed to winds from W N W to SE. The lower slopes of Gastalgm and Barakullen were exposed to winds from WSW to SSE, but only S winds (which rarely occur) gave stronger turbulence. Thus, there was only a weak influence of the wind at ground level on these slopes.
Species distribution Idaea dilutaria was restricted to two separate areas (Fig. 10), and the largest observed extension of distribution occurred in 1982 and 1983 (in 1984-1986 the distribution of this species on Kullaberg was even more restricted). In 1982-1983 the species was found within a 350 m long strip from west of Ransvik (25-55 m asl), and in a 500 m long strip running from the western part of the south-facing
211 Kattegatt
a Kattegatt
Fig. 7a-b. Mean ground temperatures, a April 1984; b May 1984
slope of Gastalgm to the steep eastern limit of Barakullen (10-60 m asl). The mean ground temperature was significantly higher within the distribution limits of L dilutaria than at the other south-facing sites with the same habitat type (Table 6). The other two species with range-margin populations on Kullaberg, the beetle D. pallipes and the spider T. conigerum, appear to have even more restricted distribution ranges than I. dilutaria. As far as is known, D. pallipes is confined to the lower slopes of Gastalgm and Barakullen, probably within the range of L dilutaria. T. conigerurn is only found on the slopes west of Ransvik (S. Almquist, personal communication), and is thus restricted to
the western part of the range of L dilutaria. The overall geographical distribution of these two species suggests that they are dependent on a warm climate. However, their precise temperature requirements have not been investigated. Discussion
The climate of the southern slopes of Kullaberg is unique within northern Europe. This climatic uniqueness is due to a combination of macroclimatic and topographical characteristics, which enhances energy influx and raises local temperatures. Winters are mild, with mean temperatures close
212 Kattegatt lta
a Kattegatt
b
to 0 ~ C during the coldest month. The number of sunshine hours is high for northern Europe (Table 3), especially in late winter and spring, giving the southern side of the peninsula an extraordinarily early and rapid start to the growing season. The proximity of the sea not only buffers low temperatures in autumn and winter, but also considerably increases the energy influx of sunshine from low solar altitudes by reflection towards the lower, steeper slopes (see Sellers 1965). The concave, south-facing slopes have inclinations of 30045 ~, which allow the establishment of vegetation but are also steep enough to receive up to nearly 3 times more energy on sunny days compared with level ground in autumn to spring (Table 4). Out-
Fig. 8a-b. Mean ground temperatures. a August 1984; b September 1984
crops of bare rocks on the vegetated slopes function as heat stores on sunny days and re-radiate energy during the evening and night, thus elevating the minimum temperatures at least during late spring-autumn (Fig. 11, Tables 2, 5). The annual mean air temperature observed on the southern slope of Kullaberg was higher than those found along the northern distribution limit of L dilutaria in northern Austria and southernmost Germany (see Walltn 1977). This difference is mainly due to the colder winter temperatures in Austria and Germany. However, during the rest of the year the temperature characteristics of southernmost Germany and the southern slopes of Kullaberg do resemble each other, especially the
213 Kattegatt
"I~7o ,
56018 ,
Skdlderviken
i
Fdgelv~ken .
Oresur~.d
~
(
/r
12~ 1 krrl
Fig. 9. Mean ground temperatures, October 1984
Table 5. Minimum temperatures (~ C) on Kullaberg, July 1984 Date
J8 19 20 21 22 23 24 25 26 27 28 29 30 31
Prevailing wind at min. temperatures
Minimum temperatures
Direction
Speed, m/s
Lighthouse
WNW NNW WSW W WNW WNW WNW WSW WNW NNW NW WNW WNW SE
8 5 5 14 11 9 6 9 5 8 7 11 6 11
14.6 16.4 14.3 13.0 13.2 13.1 13.9 13.4 14.7 14.5 14.0 14.0 14.5 15.5
15.5 16.3 14.9 13.2 13.4 13.5 14.5 14.3 15.3 15.3 15.0 14.7 14.9 16.3
I4.3
14.8
Period mean
, South slope
rapid increase in temperature from March to May. This similarity in temperature development indicates that the climatic regimes found here are close to the minimum requirements of L dilutaria.
Both the significant temperature difference between the distribution sites of L dilutaria and adjacent, apparently suitable habitats along the southern side (Table 6), and the restriction of all three species populations to the absolutely warmest areas indicate a strong dependence on the microclimate. The only " w a r m " area where none of the species occurred was on the slope below Kulla mosse (see Fig. 9 for names). However, this area was covered by a pine plantation until the hurricane in 1980 and is now vegetated by grasses and juniper bushes (Juniperus communis L.). Parts of this slope have recently been planted with small oaks, and L dilutaria should at least be able to colonize this area during favourable periods, when the oaks have grown up to form more continuous cover.
The length of the season of potential activity (growing season) and the winter temperatures are of crucial importance for L dilutaria (N. Ryrholm, unpublished results). Experiments of survival and activity in constant temperatures show that larvae can survive at a temperature of - 4 ~ for 1.5 months but that small larvae become inactive at temperatures below 3 ~ to 4 ~ C and larger (4-5 in-
214
Kattegatt Params
Fig. 11. Infrared scanning photograph of Kullaberg, March 20, 1969, 19.20 h (18.20 UTC); the light tones are the warmest areas. From JO Mattsson et al. (unpublished results)
star) larvae at below 2 ~ to 3 ~ C. These results suggest that L dilutaria larvae are not usually exposed to harmful temperatures on Kullaberg during normal winters. This suggestion is corroborated by the observed reduction in temperature differences between the L dilutaria habitats and adjacent areas in December and January (Tables 1, 2). If minim u m winter temperature were the main restricting factor, then the population would either have a larger distribution or it would already have become extinct. During very severe winters, however, the minimum temperatures will reduce the chances for survival of the larvae (N. Ryrholm, unpublished results). The most likely explanation to the observed distribution is the extended season (caused
by the early spring and the prolonged autumn) which characterizes the L dilutaria habitats. The length of the period with temperatures of approximately 3~ C and above is probably the main factor limiting the distribution of L dilutaria in northern Europe. The period with temperatures of above 3~ should therefore be a good estimate of the length of the larval growing season that is available to this species. My results indicate that larvae on the south-facing slopes should normally be able to remain active until the beginning of December and at least sometimes again from February onwards (Table 2). Since the temperature difference is even more pronounced at the ground level, the growing season is probably even longer on the
215 Table 6. Mean monthly temperature (~ C) on the south side
of Kullaberg Month
Habitaff Mean SD
N
p
