effect of water level fluctuations on wading bird foraging habitat use at an irrigation reservoir,...

Effect of Water Level Fluctuations on Wading Bird Foraging Habitat Use at an IrrigationReservoir, Lake Kerkini, GreeceAuthor(s): Anastasios Dimalexis and Myrto PyrovetsiSource: Colonial Waterbirds, Vol. 20, No. 2 (1997), pp. 244-252Published by: Waterbird SocietyStable URL: http://www.jstor.org/stable/1521690 .

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Effect of Water Level Fluctuations on Wading Bird Foraging Habitat Use at an Irrigation Reservoir, Lake Kerkini, Greece

ANASTASIOS DIMALEXIS AND MYRTO PYROVETSI

Department of Ecology, School of Biology, Aristotle University, U.P. Box 119, 540 06 Thessaloniki, Greece Internet: [email protected]

Abstract.-Lake Kerkini, a Ramsar site in Macedonia, Greece, is an irrigation reservoir with water level fluctuations exceeding 5 m annually. We determined the effect of water level fluctuations on the area of available Ciconi- iformes' foraging habitat at low, intermediate and high lake stages. Log-linear analysis assessed the changes in the patterns of foraging habitat use by Great Egrets, Little Egrets and Grey Herons in response to the interactions of foraging habitat type and lake stage. At low lake stage Ciconiiformes mainly foraged in non-vegetated habitats or in lacustrine and palustrine habitats with submerged vegetation, while at intermediate lake stages they used palustrine habitats with emergent or submerged vegetation. At high lake stage birds shifted to riverine habitats that constituted the main available foraging grounds. The coexistence of three wetland systems enhances habitat heterogeneity at Lake Kerkini and enables large heron populations to thrive, despite the relatively small foraging area compared to other Mediterranean wetlands. Only Great Egrets seem to face constraints related to the limited availability of marshes-habitats linked to the species feeding and nesting requirements-during the breeding period. Received 22 August 1996, accepted 22 March 1997.

Key words.-Ardea alba, Ardea cinerea, Ciconiiformes, Egretta garzetta, foraging habitat, Greece, impoundment, Lake Kerkini, wading birds.

Colonial Waterbirds 20(2): 244-252, 1997

Irrigation reservoirs can provide extreme water level fluctuations; their water regimes are determined by agriculture's seasonal de- mand for irrigation water. Large wading birds of the family Ciconiiformes may be di- rectly affected by water level fluctuations that severely restrict their ability to feed suc- cessfully. In this sense, wading birds can be used as indicators of ecological change in wetland ecosystems (Kushlan 1993), and their study may provide useful guidelines concerning management of impoundments.

At Lake Kerkini, a Ramsar site (IUCN 1987), water levels are managed so that the amplitude of fluctuations exceeds 5 m. The effect of such water management practices on the wetland's biological wealth (Pyrovetsi and Papastergiadou 1992, Crivelli et al. 1995a), has been a subject of debate among scientists and decision-makers for the last 2 decades. The latter have used Kerkini's biodi- versity as a positive example in an attempt to justify alterations in other natural wetlands of Greece. During the last decade, several field studies concerning colonial waterbirds have been carried out at Kerkini (Jerrentrup 1990, Pyrovetsi 1990, Tsachalidis 1990, Dimalexis and Pyrovetsi 1993, Pyrovetsi and Papazahar-

iadou 1995, Crivelli et al. 1995b), but no research has been conducted on the effect of the water regime on the availability and use by birds of the various foraging habitats. The goal of this study was to investigate how waterbird populations overcome the pressure of water regime on their foraging grounds and to draw conclusions concerning future management of the lake. The specific objectives were to investigate the effect of water level fluctuations on the area of the available Cico- niiform foraging habitat, and determine changes in the patterns of foraging habitat use by Ciconiiformes in response to water level fluctuations.

Our research focused on Grey Heron (Ardea cinerea), Great Egret (A. alba) and Lit- tle Egret (Egretta garzetta), which breed at Lake Kerkini and exhibit considerable seasonal variation in their population. Grey Herons are present at Kerkini year-round, and their breeding population exceeds 200 pairs. Great Egrets overwinter at Kerkini in varying numbers, usually over 200 individuals. A small population of these birds (3-4 breeding pairs and some immatures) remains in the wetland during the breeding season, making Kerkini the second most im-

244



WATER LEVELS AND HERON FORAGING HABITAT

portant breeding site of the species in Greece. The Little Egret is a migrant, with over 400 pairs breeding at Kerkini. The bulk of Little Egret's population leaves the wetland by the end of October.

STUDY AREA

We conducted the study between 1991 and 1994 at Lake Kerkini (41013'N, 23?08'E), in Macedonia, Greece. Kerkini occupies an area of approximately 72

km' and drains a 10,800 km2 watershed (Psilovikos et al. 1992). The lake was created in the early 1930s by the construction of dikes and dams in the Strymon River floodplain. In 1982 a higher dam and larger dikes were constructed to overcome sedimentation problems with the lake. The river upstream of the lake was aligned and diked, and a new water management plan, of larger water level fluctuations, was imposed on the system. The northern part of the reservoir remained undiked, form- ing a mosaic of wetland habitats (Fig. 1).

The lake has a maximum depth of 10 m and is char- acterized by seasonal fluctuations of its water level. In- duced flooding begins in February, and, until the end of

LAKE KERKINI

:;'I! Marshes

.***. Ponds l* * *j UI

1.. Wet Meadows

:*; Riparian Forest

N\

0 1 2 Km Dam Km Dam

--i Seasonally Flooded Area Figure 1. Map of Lake Kerkini depicting the major wading bird habitats.

245

':'::s: River



COLONIAL WATERBIRDS

May, the water level rises about 5 m. In lateJune the Stry- mon River, the main feeder of Lake Kerkini, often dries out, due to irrigation upstream. Furthermore, since water demands from the reservoir during this period are very high, its water level rapidly drops until late Septem- ber, when the flow from the river is restored. During periods when the water recedes, the reservoir bottom becomes exposed and surface water is confined to topographic depressions (ponds). In early autumn the maximum depth of the lake is less than 5 m, and the flooded area drops to 4,500 ha. More details about Lake Kerkini can be found in Pyrovetsi and Papastergiadou (1992) and Crivelli et al. (1995a).

METHODS

In 1993 and 1994 we recorded foraging habitat use of Grey Heron and Great Egret, two long-legged species, and Little Egret, a short-legged species. Biweekly counts of foraging individuals were conducted in spring and early summer, and monthly counts during the rest of the year. We followed standard routes covering the major foraging grounds, which are located mainly at the northern part of the lake and along part of the Strymon River. Foraging birds were distinguished from roosting individuals either by their activity or their posture. Ob- servations were made in early morning from the perim- eter of the lake and the river banks, using a telescope (20 x 60). A total of 384 observations of Great Egret, 2,124 observations of Little Egret and 1,308 observations for Grey Heron were made. For each foraging in- dividual, we recorded foraging depth in relation to tarsal exposure, described foraging habitat (distance from the shore, substrate, vegetation cover) and noted the approximate location of the site on a 1 km2 grid map of the lake. Lake water levels were obtained from the water gauge station at the dam.

Log-linear frequency analysis (Sokal and Rohlf 1981) was performed to describe the direct and interactive effects of habitat and lake stage on habitat use for each of the 3 heron species. We pooled data from both years to maximize sample sizes and environmental variability and to avoid 0 values in the resulting matrices. We began by calculating saturated models that included main effects and interactions. Likelihood-ratio chi-square was used to test the significance of main effects and interactions in the models. At the end point, interactions were excluded to obtain expected values when only the main effects were added to the analysis (using the backward selection method in SPSS software). Parameter estimates and z- values were used to assign the level of significance for each effect. Significance was assigned if P < 0.05, except when otherwise stated in the text.

The various habitat types in the Kerkini wetlands were identified and classified using a hierarchical struc- ture, focusing on substrate, water regime, vegetation form and dominant plant or animal species (Cowardin et al. 1979). Classification was conducted during the low- water period. Habitat types were identified using dominant features during the spring and early summer grow- ing season (Cowardin et al. 1979) and by the existing literature (Babalonas and Papastergiadou 1989, Papa- nastasis 1989, Papastergiadou 1990, Papastergiadou and Babalonas 1992, Crivelli et al. 1995a). The result was a map of the various lake habitats which was over-laid on the 1 km2 grid to assign the proper habitat type to the sites where herons foraged. We then grouped the habi-

tat types, into broader-scale habitat categories and ag- gregated the observations of foraging individuals according to these categories and 3 lake-stage categories. We avoided assigning more detailed habitat categories and lake stages in order to insure reasonable sample sizes. We distinguished 4 foraging habitat categories: 1) "marshes," including the seasonally flooded littoral area of the lake-dominated by Cynodon dactylon, Paspalum paspalodes and Xanthium strumarium-and the seasonally flooded palustrine habitats at the northern part of the wetland - dominated by pleustophytes such as Salvinia natans, Spirodella polyrhiza-and some patches of reedbeds (Phragmnites australis); 2) "ponds," defined as isolat- ed bodies of water with submerged (Ranunculus fluitans) or emergent (Typha angustifolia, Phragrnites australis) vegetation; 3) "riverine," habitats, including the Strymon River and surrounding irrigation canals); and 4) "wet meadows," defined as seasonally inundated areas with dense emergent grasses that grow in spring and early summer, dominated by Paspalum paspalodes, Trifoli- um subterraneum and Ranunculus scleratus (Fig. 1).

We classified water levels into 3 lake stages of more or less equivalent duration, based on the annual duration of flooding: 1) low, with water levels below 32.5 m Above Sea Level (ASL) (above this level most palustrine habitats become inundated); 2) intermediate, with water levels between 32.5 and 34 m ASL, close to the aver- age annual water level; and 3) high, with water levels exceeding 34 m ASL, during which all palustrine habitats are inundated. Daily records of lake water level for the period 1991-1994 were used to estimate the mean annual duration and mean water level of each stage.

From our observations and the literature (Kushlan 1976, 1981; Erwin 1983) we chose a water depth of 30 cm as a threshold depth below which herons were able to forage. Although cases have been described in the literature (Smith 1995) of Ciconiiformes foraging in much deeper areas by perching or aerial foraging, we found these behaviors uncommon and insignificant. The area of the various foraging habitat categories was estimated for the mean water level of each lake stage, using a depth contour map of the lake, aerial photographs (1:15000) and a topographic map (1:10000) (Vouvalidis 1991). For the river, a foraging area 5 km upstream from the colony site was estimated, since our observations and those of Custer and Osborn (1978) and Hafner and Fasola (1992) suggest that breeding herons mainly forage within this distance of the colony.

RESULTS

Duration of Flooding and Availability of Heron Foraging Habitat

During the 4-year period 1991-1994, with a mean annual water level of 33.23 ? SD of 1.66 m ASL, mean water level at each lake stage and mean duration of flooding were as follows. The low lake stage had a mean water level of 31.43 ? SD of 0.64 m ASL and lasted for 129.5 ? SD of 61.8 days annually, from September until January. The intermediate lake stage had a mean water level of 33.23 + SD of 0.43 m ASL and lasted for a total of

246




116.3 ? SD of 38.1 days, from February until early April and from late July until the end of August. The high lake stage had a mean water level of 35.23 ? SD of 0.62 m ASL lasting 117 ? SD of 24.1 days, from April until late July (Fig. 2).

The foraging area available to herons fluctuated seasonally, relative to lake water levels. At low lake stage, marshes covered more than two thirds of the available heron foraging habitat because of the extensive marshy areas of the unconsolidated shores. With the exception of the palustrine floating vascular aquatic bed, which was virtually dry and thus unavailable during this period, all other marshy habitats attained their maximum area. Ponds and the river also had maximum areas available for foraging by herons. Wet meadows were dry and thus unavailable to foraging wading birds during most of the low water period (Fig. 3).

At intermediate water levels, marshes were reduced to about one-third of their maximum area but still accounted for the largest portion of the available foraging hab-

36.0

35.5

.J 35.0 co < 34.5 E _ 34.0 0) > 33.5

0 33.0

3 32.5 0)

X 32.0

31.5

31.0

itat. Most of the littoral seasonally flooded mudflats were not accessible to herons for

foraging because they were flooded to a depth of more than 1 m. The other marshy habitats, including the palustrine floating vascular aquatic beds, were covered by water shallow enough to permit heron foraging. The availability of ponds was reduced because most of the rooted vascular palustrine wetland was partially inundated by water deeper than 30 cm. The river foraging area remained unchanged, while wet meadows became available for foraging as they were gradually flooded.

During high floods in late spring, heron foraging habitat was confined to the north- eastern part of the wetland. At this stage, marshes occupied a very limited area: the artificial rocky shore, small patches of seasonally flooded mudflats and the persistent emergent palustrine wetland. Ponds com- prising rooted vascular palustrine aquatic bed were covered by deep water, and thus the only available pond habitats were those of persistent emergent palustrine wetland.

J F M A MJ J A S O N D

Month Figure 2. Mean daily water levels from 1991 to 1994 at Lake Kerkini. Horizontal dotted lines represent the three lake stages (low <32.5 m ASL, intermediate 32.5-34 m ASL, high >34 m ASL), and the vertical lines represent the corresponding periods.

247



COLONIAL WATERBIRDS

[ Wet Meadows

El River

3 Ponds

a Marshes

245(72.05)

100-

90(58.06)

U I

Low Intermediate

5(15.4E

High

Lake stages

Figure 3. Areas of available heron foraging habitat at low, intermediate and high water levels. Numbers in pa- rentheses represent percentages of each habitat catego- ry.

River and wet meadow habitats remained available to foraging herons.

Patterns of Habitat Use

Log-linear analysis revealed that all 3 species altered their pattern of habitat use significantly in response to lake water levels. The main effects for habitat categories showed that Great Egrets (partial likelihood ratio X23 = 139.53, P < 0.0001) used marshes signifi-

cantly more and wet meadows significantly less than expected. Ponds and riverine habitats were used less than expected, but not significantly so. Little Egrets (partial likelihood ratio %23 = 58.01, P < 0.0001) used marshes and wet meadows significantly less and the other two categories significantly more than expected. Grey Heron (partial likelihood ratio X23 = 378.34, P < 0.0001) used wet meadows significantly less and the other categories significantly more than expected.

Patterns of habitat use could not be satis- factorily explained, however, using only the main effects of habitats and lake stages. Ob- served and expected values varied significantly only when the main effects of habitat and lake stage were included in the analysis. Tests for k-way effects for each of the 3 species indicated that interactions between lake stage and habitat significantly influenced the distribution of foraging individuals (P < 0.0001; see Tables 1-3). Deviations from the general pattern of habitat use, attributed to the effect of interactions, were exhibited by each species.

Great Egrets significantly altered their pattern of habitat use in response to changing lake stages (Table 1). In general, the species foraged primarily in marshy habitats, which accounted for the largest proportion of observations at all lake stages. At low lake stages, the species foraged mostly at marshes and more particularly at the littoral lacustrine part of Kerkini; marshes were used significantly more than expected. By contrast, ponds were used significantly less than ex-

Table 1. Observed and expected values of study-wide patterns of habitat use in relation to lake stage for Great Egret (likelihood ratio for the interaction habitat*lake stage X26 = 50.47, P < 0.0001; asterisks indicate cells responsible for significant pattern of variation, attributed to the effect of interaction).

Habitat Categories Lake Stage (m ASL) Marshes Ponds River Wet Meadows Observed Total

<32.5 103/80* 15/37 26/26 14/15 158 (65.1)b (9.5) (16.5) (8.9)

32.5-34 77/97 72/46* 28/32 15/18 192 (40.1) (37.5) (14.6) (7.8)

>34 15/17 3/8* 9/6 7/3 34 (44.1) (8.8) (26.4) (20.6)

aObserved/expected values when interaction effect is excluded. bRow percentage corresponding to observed count.

350 -

300

250

200 -

C150 - <

50-

It I

248




Table 2. Observed and expected values of study-wide patterns of habitat use in relation to lake stage for Little Egret (likelihood ratio for the interaction habitat*lake stage X26 = 43.147, P < 0.0001; asterisks indicate cells responsible for significant pattern of variation, attributed to the effect of interaction).


<32.5 94/98a 172/124' 132/149 79/106* 477 (19.7)b (36.1) (27.7) (16.5)

32.5-34 145/135 164/172 185/206 165/146* 659 (22) (24.9) (28.1) (25)

>34 196/202 218/258' 348/309* 226/219 988

(19.8) (22.1) (35.2) (22.9)


pected during low lake stages. At intermediate lake stages, however, Great Egrets used ponds significantly more than expected, although the largest proportion of the species' foraging attempts were observed in marshes. At high lake stages there was no significant shift in the pattern of foraging habitat use.

Although Little Egrets significantly altered their pattern of habitat use in response to changing lake stages, they seemed to be more foraging generalists than Great Egrets, exploiting all habitat categories during each lake stage (Table 2). At the low lake stage the species used ponds more and wet meadows significantly less than expected; the latter were nearly dry during most of this period. Since Little Egrets breed but do not overwinter at Kerkini, the low lake stage for this species corresponds only with the period of water recession. During intermediate lake stages the species used wet meadows signifi-

cantly more than expected. At high lake stages a shift to riverine habitats appeared, while ponds were used significantly less than expected.

Grey Herons predominantly foraged in marshes and ponds during low lake stages, while their use of riverine habitats increased with rising water levels (Table 3). The species used ponds significantly more than expected at low and intermediate lake stages, although these habitats were used significantly less than expected at high lake stages. Marsh- es were also used more than expected at low lake stages and significantly less than expected at high lake stages. By contrast, riverine habitats were used significantly less than expected at low lake stages but were used significantly more than expected during high lake stages. Wet meadows accounted for the smallest proportion of Grey Heron foraging attempts at all lake stages.

Table 3. Observed and expected values of study-wide patterns of habitat use in relation to lake stage for Grey Heron (likelihood ratio for the interaction habitat*lake stage z26 = 237.13, P < 0.0001; asterisks indicate cells responsible for significant pattern of variation, attributed to the effect of interaction).


<32.5 160/117*' 137/94* 50/133* 17/20 364 (44)b (37.6) (13.7) (4.7)

32.5-34 167/172 162/138* 175/197 33/30 537 (31.1) (30.2) (32.6) (6.1)

>34 93/131' 38/105' 254/149* 22/22 407 (22.9) (9.3) (62.4) (5.4)


249



COLONIAL WATERBIRDS

DISCUSSION

Water management practice is the driv- ing force determining habitat availability and variability at Kerkini, as is the case in other locations (Kushlan 1986, Bancroft 1989, Smith et al. 1995). Seasonality and the magnitude of water level fluctuations affect habitat features such as vegetation form (Ri- chardson et al. 1995), water depth, prey availability and degree of isolation from human disturbance, creating a mosaic that changes in time and space. Because of water management, unpredictability and ephemerality are the dominant characteristics of Kerkini wetlands. These features, along with the coexistence of 3 wetland systems (lacustrine, palustrine and riverine systems) in such a limited area, lead to increased habitat heterogeneity, a factor which can enhance species diversity in such ecosystems (Burger et al. 1982, Hafner and Fasola 1992).

Fluctuations of lake water level appear related to patterns of heron foraging habitat use. From September until February, water draw-down results in the drying out of the northern part of the wetland and the con- centration of the remaining surface water in small depressions such as ponds. During this period, the area of available foraging habitat reaches its maximum. The 2 long-legged species (Grey Heron and Great Egret) primarily foraged in marshy habitats of the lacustrine seasonally flooded mudflats which, at that time, constituted the most extensive foraging grounds. Lack of submerged vegetation in these areas enabled these 2 predominantly fish-eating species to search for prey more effectively than in more vegetated sites. How- ever, unlike the Grey Heron, which also sought food in ponds, Great Egrets seemed to avoid ponds, perhaps because of the pres- ence of submerged vegetation that required tactile foraging. This is in agreement with our previous results on resource partitioning of the 3 species at the same wetland (Dimal- exis and Pyrovetsi 1993); the Great Egret appeared to be a solitary open-water forager, preferring deeper areas without vegetation cover at a distance from the shore. Little Egrets invested the majority of their foraging

attempts in ponds, but they also used riverine habitats (mainly irrigation canals) during low lake stages.

Intermediate water levels occur during two different periods yearly: when the water level starts to rise in early spring and during water draw-down in late summer. Marshes and ponds lose about half of their area compared with the low lake stages. On the other hand, wet meadows become available as they are flooded by the rising water. Great Egrets foraged mainly in marshes and ponds, and Grey Herons used the river as well. Prey abundance in ponds is expected to increase during spring, because of higher water tem- peratures, and in late summer, due to water recession; as the water recedes, fish and other aquatic organisms migrate into ponds where their densities increase, providing a highly concentrated and readily harvestable energy source for wading birds (Kushlan 1976, Smith et al. 1995). This situation may explain the significant trend of all 3 species toward pond use. Little Egrets used wet meadows as well, where they could seek prey by tactile foraging.

In late spring, during high water, marshy habitats are greatly reduced in area, while ponds are available only at the northwestern part of the wetland. Spawning fish populations concentrate along the river banks (Tsa- chalidis 1990, authors' pers. obs.), providing ideal foraging habitat-increased abundance of energy-rich fish-in close proximi- ty to the breeding colonies. Grey Herons and Little Egrets primarily used the river during this period (May-June), which coincides with the peak of heron breeding at Kerkini. Great Egrets continued using marshes proportion- ally more than the other habitats. The pres- ence of the river enables heron populations to overcome the decline of the available foraging habitat area. It appears that reduced quantity of foraging habitat is balanced by the increased quality of that which remains. Hafner and Fasola (1992) estimated that a minimum of 800 ha within a radius of 5 km around a breeding colony may be consid- ered critical for the size of a breeding colony in the Mediterranean region. Nevertheless, it is likely that Kerkini-because of its high

250




productivity (Tatarakis 1995), spatial and temporal habitat heterogeneity, and the availability of high quality prey-may be able to support a much higher heron population per area of foraging habitat than most other Mediterranean wetlands.

On the other hand, the palustrine system, of great value to wading birds since it provides them with nesting and foraging habitat, has been subjected to a serious decline because of current hydrological management. Half of the riparian forest of broad-leaved deciduous and needle-leaved evergreen species has been lost during the past 15 years, and its regeneration has gradually declined since a new water management practice was established in 1982 (Pyrovetsi and Papastergiadou 1992, Crivelli et al. 1995a). The abrupt and high rise in water level also causes the loss of many nests each year (Tsachalidis 1990, Crivelli et al. 1995b). When the water levels of the study period are compared with those of the previous 4 years (1987-1990), it becomes evident that there has been a significant increase in the mean duration of flooding for the areas over 32.5 m ASL. Extended and deep flooding, together with annual draw-down of water, negatively influences most emergent wetland plant communities, including wet meadows. These factors, combined with the drying out of the river almost every summer, decrease the area of foraging habitat available to wading birds and may constitute a critical limit for their breeding populations (Custer and Osborn 1978, Kushlan 1978).

In summary, our 3 study species exhibited different habitat use patterns in response to lake water fluctuations. Great Egret appeared to be heavily dependent on the pres- ence of marshes at all lake stages. This finding differs from Smith's (1995) in the Ev- erglades, where Great Egrets exploited a number of different foraging habitats. We may assume that the limited marsh area, es- pecially that of reedbeds, is probably a rea- son for the small breeding population of the species compared to those of the Grey Her- on and Little Egret. Both nesting and feeding requirements of Great Egrets are linked with marshy habitats. The Grey Heron was

more of a generalist in habitat use and foraged at marshes, ponds and the river, de- pending on lake stage. Neither Great Egrets nor Grey Herons foraged considerably at wet meadows, probably because of the reduced fish availability there. Little Egrets, foraging either on fish or invertebrates, used all habitats in all lake stages, but showed reduced marsh use, probably because of water-depth constraints.

We recommend that hydrological regula- tion and ecosystem management at Kerkini ensure the availability of all wetland types, with their internal heterogeneity, while providing for the multiple uses of the lake (pro- tection from floods, irrigation, conservation of biological diversity, fishing and grazing). Special care should be given to the preserva- tion of the palustrine habitats, which fulfill most of the nesting and foraging requirements of herons and which would be threat- ened by any further increase of the water depth. Management practices should also ensure that the amplitude of water level fluctuations be lessened, by increasing the minimum water level and keeping the limit of maximum flooding well below 36 m ASL. This will secure waterbird breeding and foraging habitat. Continual flow of the Strymon River should also be maintained during the summer and visitors should be regulated on the river area each spring to reduce disturbance to foraging birds. Construction of seminatural wetlands with regulated water levels at the borders of the flooded area could provide appropriate conditions for the reestablishment of patches of emergent vegetation and improve foraging opportuni- ties for heron species.

ACKNOWLEDGMENTS

We are thankful to Dr. Stefanos Sgardelis for his help in data analysis. We are also in debt to Jeff P. Smith, Donald McCrimmon and an anonymous reviewer for their constructive comments on the manuscript.

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