a review of habitat changes in the camargue: an assessment of the effects of the loss of biological...

Biological Conservation 70 (1994) 39-47 © 1994 Elsevier Science Limited

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ELSEVIER

A REVIEW OF HABITAT CHANGES IN THE CAMARGUE: AN ASSESSMENT OF THE EFFECTS OF THE LOSS OF

BIOLOGICAL DIVERSITY ON THE WINTERING WATERFOWL COMMUNITY

Alain Tamisier Centre d'Ecologie Fonctionnelle et Evolutive, CNRS, 34033 Montpellier, France

&

Patrick Grillas Station Biologique de la Tour du Valat, Le Sambuc, 13200 Aries, France

(Received 17 May 1993; revised version received 18 October 1993; accepted 6 December 1993)

Abstract For the last 50 years, drastic changes have occurred in the Camargue. The loss of c. 40,000 ha of natural areas (33,000 ha of wetlands) was related to the extension of agriculture, salt exploitation and industry. On most of the remaining wetlands (mostly private hunting estates), management involved division of the marshes into smaller dyked units and large inputs of freshwater, resulting in a decrease in mean water salinity and an increase in duration of flooding. The original unpredictable variations in water level and water salinity, a characteristic of Mediterranean seasonal marshes, has been replaced by a tendency towards predictable permanent freshwater marshes. Consequently the species composition of aquatic habitats has changed from diversified Mediterranean to monospecific continental-type communities. The higher plant biomass favours use by wintering waterfowl which are, in turn, probably limited by high hunting pressure. These changes result in losses of biological diversity at the intra- and inter-habitat, as well as at the Mediter- ranean and continental levels. New concepts o f conservation should be proposed which take into consideration the biodiversity of the delta as a whole, including private properties, instead of isolated management plans for reserves only.

Keywords: habitat loss, water regime, wintering waterfowl, Mediterranean France, management, conservation.

INTRODUCTION

At the 1962 MAR Conference (Olney, 1965), wetlands were emphasized as important and endangered habitats. Interest in these aquatic habitats continued to grow, at first because of their high productivity (Odum & Odum, 1960), then due to their additional function in terms of biodiversity. This interest was supported by ornithologists and conservationists (Finlayson & Moser, 1991), who regularly use aquatic bird numbers

39

as a basis for wetland evaluation (Atkinson-Willes, 1972; Scott, 1980; Boyd & Pirot, 1989). The Ramsar Convention, signed in 1971, provides an international framework for the definition and protection of wetlands.

The physical loss of wetlands is worldwide, usually associated with the extension of agricultural, industrial and urban zones. Wetlands are frequently selected for recla- mation because they are relatively cheap to drain or infill. Mediterranean wetlands are submitted to additional constraints due to tourist resorts, a general shortage of freshwater and a weak interest in conservation. For instance, in Greece 61% of wetlands (300 000 ha) have disappeared over the last 70 years (Handrinos, 1992).

At the same time, qualitative changes have occurred in wetlands mostly because of the development of human activities: waste water discharge, extensive use of agricultural land around wetlands and in the catch- ment, and nutrient enrichment for pisciculture lead to increasing eutrophication (Lieutaud et al., 1989; H. Krist, pers. comm.). Furthermore the management of wetlands for hunting, conservation or educational purposes usually involves inputs of freshwater which do not follow the natural cycle.

The objective of this paper is to analyse the quantitative and qualitative changes which have occurred during recent decades on the natural ecosystems and landscapes of the Camargue, a highly protected area on the Mediterranean French border. This analysis is based on a series of data sets, most already published, which allow an assessment of trends at both local and regional scales.

SITE DESCRIPTION, MATERIAL AND METHODS

The Camargue extends over 145000 ha of alluvial landscape at the mouth of the Rh6ne River on the Mediterranean. As a delta it has long been a dynamic system with regular water and sediment inputs from the

40 Alain Tamisier, Patrick Grillas

Wetlands

Salt pans Managed ~

Freshmarsh - -

~ ) ' Aries / ~ Unmanaged

( ~ Limits of reserves and protected areas

, , i Gulf of FOS 0 5 10 km

Fig. 1. Map of the Camargue. Reserves and protected areas extend over 19 000 ha, mostly wetlands: (1) National Reserve (13 000 ha); (2) Conservatoire du Littoral; (3) Department; and (4) private Tour du Valat Reserve. Other areas are dry, mostly

agriculture.

Rh6ne and the sea. Dykes have been built for many centuries along the arms of the river to limit these inputs (Stouff, 1986). At the end of the 19th century, a network of dykes isolated the central part (85 000 ha) from both fluviatile and marine inundations. Consequently a major change occurred in the hydraulic situation. Before dyke construction, duration of flooding as well as water salinity were primarily controlled by climatic conditions: because of a hydric deficit between mean annual evapo- ration (1.2 m) and rainfall (0.6 m), we can assume that water salinity was high and that most of the marshes dried up in summer. During this century, the duration of flooding and water salinity have been progressively influenced by the pumping of large quantities of freshwater (c. 400 millions m 3 in 1990, Heurteaux et al., 1992) for irrigation, larger canals and a general increase in the control of the hydrological system by man.

The Camargue (Fig. 1) includes a National Reserve (13000 ha) and several national, regional and private reserves or protected areas of smaller extent, managed by the Conservatoire du Littoral, the D6partement and the Tour du Valat Foundation (total surface area c. 6000 ha). The National Reserve, created in 1927, is currently managed by an NGO--the Soci6t6 Nationale de Protection de la Nature. World-famous, the Camargue is often considered to be the best protected French natural area.

Among the wetlands of the Camargue three zones (Fig. 1) can be distinguished:

(1) Saltpans extending over 25000 ha close to the seashore, filled with sea water from April to September for salt production and drying out thereafter (Britton & Johnson, 1987).

(2) Oligohaline marshes distributed over about 40 000 ha in an arc above the central Vaccar+s system, and managed mostly for hunting, and to a lesser extent for tourist or conservation goals. Usually the marshes are dried out in spring and flooded with fresh water in July before the arrival of migrating birds and the beginning of the hunting season. Water comes through the agricultural irrigation system and occasionally by specific canals. In order to improve the water control these marshes are most often parcelled by dykes.

(3) The central basin of the Vaccar6s with its adjacent temporary marshes--most of them brackish-- covering most of the National Reserve, and totally free of dykes over about 11 000 ha. The Vaccar6s lagoon itself receives some drainage water from the surround- ing agricultural fields and wetlands of zone 2, but its adjacent marshes are not directly dependent on this supply. Although there is no comprehensive analysis of the original water regime of the Camargue before dyke construction, the marshes of the National Reserve are considered as being closest to its original physical conditions.

We used four distinct criteria to evaluate the changes which have occurred in the Camargue as a consequence of the increasing water control.

Habitat loss in the Camargue 41

Physical conditions of the habitats Aerial photographs were used to measure the changes in area of wetlands and other natural habitats from 1942 to 1984 (Lemaire et al., 1987; Tamisier, 1990a). Natural habitats are distinguished from man-made (agricultural, salt-pans, industrial zones) where as much of the ecosystem energy as possible is concentrated on specific production (rice, maize, salt, etc.).

Water-regime in the wetlands The water level of the wetlands was evaluated according to a visual scale from 0 to 5: 0, dry; 1, some water present; 2, half full; 3, wide beaches along the borders; 4, full; 5, overflowing.

This evaluation (Water Level Index -- WLI) was made from an aircraft in mid-August from 1980 to 1991 on a sample of 67 marshes and lagoons over the whole of the Camargue. The WLI values of the water- managed wetlands of zone 2 (n = 50) were compared to those of the unmanaged wetlands of the National Reserve (n = 17) considered as a control. August WLI values were used since that month marks the end of the climatic drying season and the beginning of the artificial flooding period.

10o% T 90% t

80% ~-

70% JI-

6O%

5O%

4O%

30%

20%

:9: 8

7

8 Artificial habitats

atural abltats

1o%

0% 1942 1953 1976 1984

Fig. 2. The quantitative loss of natural habitat, from 1942 to 1984, concerns 40,000 ha, of which 33,000 ha were wetlands (modified after Lemaire et al. 1987). Natural habitats are permanent marshes (1), temporary marshes (2), Arthrocnemum fields (3), freshwater marshes (4), pastures (5) and woods (6). Artificial habitats are salt marshes (7), agricultural fields (8)

and industrial zones (9).

Aquatic plant communities The artificial flooding of marshes with freshwater results in a longer flooded period and in lower salinity. These two variables have been shown to be the most important environmental factors controlling plant distribution in the Camargue (Molinier & Tallon, 1970; Verhoeven, 1979; Grillas, 1990) and in other Mediter- ranean marshes (Rivas-Martinez et al., 1980; Montes et al., 1982; Morgan & Boy, 1982; Lorenzoni, 1985; Duarte et al., 1990; Garcia et al., 1993). Experiments were conducted in the Camargue in order to separate the effects of water regime on plant communities from those of salinity. In this analysis both field and experimental data sets are used to analyse the impact of management on wetland habitats in the Camargue. The relationship between plant communities and physical factors was studied in the field by Verhoeven (1980), van Wijk (1989) and Grillas (1990) in freshwater and brackish marshes. The impact of salinity and flooding date was studied experimentally on samples of natural plant communities (Grillas, 1992; Bonis et al., 1993; Grillas et al., 1993). These communities were obtained from sediment cores (diameter 0-08 m) from marshes transferred with their natural seed bank in containers. The method is fully described and discussed in Grillas et al. (1992a).

(Dervieux et al., 1980). From the total monthly values (September-March), we calculated a mean annual index of abundance of waterfowl populations for the last 27 years.

RESULTS

Physical changes

Quantitative loss of habitat (Fig. 2) The analysis of aerial photographs showed a drastic change between 1942 and 1984; in 1942, 67% of the Camargue was covered by natural habitats: marshes, ponds, 'sansouies' (local name for saline marshes generally covered by perennial halophytes, ,4rthrocne- mum spp.), meadows and woods, the remaining being divided into agriculture and salt pans. In 1984, the situation had reversed, with 56% occupied by artificial areas (agriculture, salt pans and industry) and 39% by natural habitats; 40 000 ha of natural habitat were lost, of which 33 000 ha were wetlands. Today, natural habitats extend over 58 000 ha (50 000 ha wetlands). These changes occurred mainly during the 1960s and 1970s (Lemaire et al., 1987; Tamisier, 1990a). The present situation is thought to be not very different from that found in 1984 (ARPE, 1992).

Wintering waterfowl Aerial monthly censuses have been conducted by the senior author over the whole of the Camargue, provid- ing basic data from 1964 to 1991 on the numbers and specific composition of the waterfowl populations (ducks Anatidae and coots Fulica atra) that winter in the Camargue (Tamisier, 1989). The accuracy of these counts was tested by photographs on field controls

Changes in water regime (Fig. 3) The mean Water Level Index in August was found to be higher in managed wetlands (WLI = 2.94) than in the uncontrolled wetlands of the National Reserve (WLI = 1-73, Kolmogorov-Smirnov test 0.670, p < 0.05). In August 81% of the managed wetlands are flooded (more than half-full: WLI in the range 3-5) in contrast to only 39% of uncontrolled wetlands. At the


b~

o>, g g

I.l_

Managed 50

Unmanaged

40

30,

ii 10"

0 : HI-1 : o 1 2 3 4 5

Water level index

Fig. 3. Frequency distribution of the values of Water Level Index (1-5, from dry to overflowing) of managed marshes and marshes without water control. Data collected in mid-

August from 1980 to 1991.

same time 30% of the temporary wetlands of the National Reserve are totally dry as compared With only 7% of the managed wetlands. Furthermore the annual variability of the WLI was much lower in managed wetlands (coefficient of variation = 0.06) than in the more natural wetlands of the National Reserve (0.33). Managed wetlands are no longer dependent on rainfall; no significant correlation was found between their WLI annual values and rainfall in the period May-August for the corresponding year. In contrast this correlation was significant for the National Reserve wetlands (r 2 = 0.479, p < 0.05). Finally, no significant trend could be found in the WLI of managed wetlands during the study period; by 1982 these wetlands were already only marginally affected by climatic conditions.

Impacts of these changes on aquatic vegetation in individual marshes Changes in the physical environment of the marshes have led to important modifications in the characteristics of the plant communities. Experimental and field results showed a sharp increase in plant biomass with increased duration of flooding (Fig. 4). The effect of duration of flooding appears to be particularly important in the range 15-30 weeks/year. Above 30 weeks, biomass does not vary consistently with duration of flooding and other factors probably interfere (e.g. nutrients). This trend was expected because reduced duration of flooding implies reduced duration of the growing season. However, the pattern is particularly significant in Mediterranean seasonal marshes where drought occurs in summer. When flooding does not last longer than 35 weeks, it implies that most of the growing season occurs during the colder months. When flooding lasts longer (up to permanent), the total biomass of the plant communities is ranked among the highest for freshwater aquatic macrophytes (Westlake, 1982). In the experimental results (Grillas, 1992) most of the species exhibited the same trend of a strong increase in biomass with the increased duration of flooding. However, the species composition of the plant

t N I000

E 0"~ 8 0 0

O3 GO 6 0 0 . ( 2

E 0 4 0 0 .

o _ . . Q

" ~ 2 0 0 .

o F-- o

o : a ~ .

1 0

• Experiment o Field data (1) n Field data (2)

Field data (3)

A

I , I , I 20 30 40 50

Duration of flooding (weeks)

Fig. 4. Effect of duration of flooding on total plant biomass in oligohaline marshes in the Camargue, from experimental and field data; experimental data from Grillas (1992), standard deviation of 10 replicates per date; field data (1) from Grillas and Duncan (1986), standard deviation for five replicates; field data (2) from Van Wijk (1988); field data (3) from

Bonis (1993).

communities was strongly affected because species varied greatly in their sonsitivity to the flooding date (Grillas, 1992).

Salinity also controls biomass and production (Fig. 5). The experimental reduction of salinity resulted in an increase in the total biomass of the plant communities. However, in the range of salinities tested (0-6 g/litre C1-), the impact on primary production was weaker than that of the water regime. There was no significant effect of salinity between 0 and 2 g/litre CI-, and the effect between 2 and 6 g/litre CI varied according to the specific composition of the communities (Grillas et a/., 1993). In the field, total biomass varies according to salinity; at low salinity (0.2 to 1.2 g/litre CI-), it ranges in permanent marshes from 275 to 550 g/m 2 (Grillas & Duncan, 1986; van Wijk, 1988), reaching the highest values for freshwater aquatic macrophytes (Westlake, 1982). At higher salinities (3-32 g/litre C13, total plant biomass was found to lie in the range 60-288 g/m: (Verhoeven, 1980).

O4 E 500

if) ffl

~ I00 o c~

-6 O

O

t -

10

O

[-JA & D • O B l i E [] Z~C O F

' I , I , I , I , I

2 4 6 8 10 Sal ini ty ( tota l sal ts g / I )

Fig. 5. Effect of salinity on the mean total biomass (n = 12) of six experimental communities of submerged macrophytes from seasonal freshwater (open symbols) and oligohaline

marshes (closed symbols).


0 F ie ld

• Expe r . 1

1 0 • Expe r . 2

0 • 8 o "6 ,~ . o _ T e l , , o o o

~'1 6 -

O

~ 4- ..o

E = 2. I c ~ o z 0

0 , I , I , I ~ I ~ I 0 1 0 2 0 3 0 4 0 5 0

Duration of flooding (weeks)

Fig. 6. Effect of duration of flooding on number of species of the aquatic submerged macrophyte communities, from experimental data (mean and standard deviation of 10 replicates:

from Grillas (1992) and field data, from Grillas (1990).

Species richness of the submerged macrophyte communities was found also to be affected by the duration of flooding of the marshes in both field and experimental results. The number of species was lower when duration of flooding was short, it increased to a maximum at 15-35 weeks of flooding/year and decreased when flooding exceeded 35 weeks (Fig. 6). This pattern is consistent with the changes in diversity across a stress or disturbance gradient (Grime, 1979). Except when duration of flooding is very short the plant communities in seasonal marshes are made up of four to nine species (from the genera Zannichellia, Ruppia, Ranunculus, Callitriche, Chara, Tolypella). These species constitute communities which are characteristic of the coastal marshes of the Mediterranean region (e.g. Verhoeven, 1979; Rivas-Martinez et al., 1980; Martinez-Taberner & Moya, 1991). They are replaced in permanent marshes by Potamogeton pectinatus or Myriophyllum spicatum, two subcosmopolitan species which tend to form monospecific stands in freshwater (0-3 g/litre CI ) (Molinier & Tallon, 1970; Van Wijk 1989; Grillas, 1990) and by Ruppia cirrhosa at higher salinities (> 10-15 g/litre CI-) (Molinier & Tallon, 1970; Verhoeven, 1979). The first two species are often considered as weeds in aquatic ecosystems because of their high production. P. pectinatus is particularly sought by managers of hunting marshes because it is a key food for waterfowl. These two species have therefore greatly increased their abundance in the Camargue with the development of rice cultivation, which promoted the introduction of huge quantities of freshwater in the delta, part of which finds its way into the marshes.

These results highlight the fact that the primary production in natural seasonal marshes of the Camargue is strongly limited by summer drought and salinity (e.g. Molinier & Tallon, 1974; Verhoeven, 1979). The increased duration of flooding, combined with low salinity, has had the effects desired by managers of the hunting marshes, as it has released the plant communities from the primary constraints of high salinity and summer drought, and considerably increased primary

productivity. This has been achieved at the cost of species diversity. Management of marshes to maximize biomass has further long-term consequences as large quantities of nutrients are introduced into the marshes with irrigation water (El Habr, 1987; de Groot, 1992). Altogether this further enhances production and, in combination with the effects of the modification of the water regime, leads to accelerated eutrophication of the marshes. Denitrification, which is very effective under the Mediterranean climate (El Habr, 1987; El Habr & Golterman, 1990), contributes to the loss of nitrogen from the system. However, the irreversible accumulation of phosphorus in the sediment constitutes a long- term threat to the marshes (De Groot, 1992). The negative impacts of the accumulation of organic matter in the sediment of the marshes, resulting from increased primary production, have been shown to be detrimental to the growth of submerged macrophytes in Camargue marshes (Van Wijck et al., 1992).

Summer flooding and lower salinity also facilitate the introduction of exotic species in the Camargue. Pas- palum paspalodes has been introduced from Central America with seed stocks due to rice cultivation (Molinier & Tallon, 1974). This species does not constitute a major threat to the native flora because it is very effectively controlled by grazing. In contrast Ludwigia grandiflora constitutes at the moment a major threat to the native flora and to the irrigation/drainage network as it rapidly covers the water surface, blocking canals and restricting light penetration in the marshes (Grillas et al., 1992b).

Biological impacts of the changes at the Camargue scale

Wetland functioning and plant communities Except on the largest part of the National Reserve, almost all wetlands of the Camargue are managed in order to attract the maximum number of waterbirds, mostly for hunting, but partly for conservation and tourist purposes. Consequently the seasonal marshes have disappeared, most being replaced by artificially flooded areas or permanent marshes. This has occurred progressively over the centuries with the development of the ancient hydrographic network of canals, but it was intensified by the change from windmill to steam- engines and electric pumping stations (for irrigation but now also for drainage). It was accelerated by the building of a network of small dykes around and across the marshes, since it is easier to control smaller areas. It does not mean that all marshes remain under water all year long but that they all are under control. The marshes receive polluted water either directly from the Rhone river or as drainage water from ricefields, and are spared regular summer drought. Furthermore, this water management has been recently extended to 'sansouires': enclosed by small dykes, they are artificially inundated in order to increase the size of hunting areas.

Following these changes, the Camargue is losing a heterogeneous aquatic ecosystem where dates of inun-

44 Alaot Tamisier, Patrick Grillas

60

40

20

0

Annual index

160

120

10080 ~ ~ V /N ~ , . - ~ ',

o * o I o o , o o * o o o , I , o o • • • • • , • • • • ° . . . . . . • . . . . , . . . . . . . • •

64-65 69-70 74-75 79-80 84-85 89-90

Mean of maximum

180

160

140

120

100

8O

60

4O

2O

0

Fig. 7. Changes in the size of the wintering populations of duck and coot according to a mean annual index calculated from monthly counts (September-March) from 1964-65 to 1990-91. Mean maximum values of ducks (× 1 000) for three successive

periods of 10, 10 and seven years.

dation and drought, duration of submersion, water level and water salinity were previously highly variable both in time and space. Most of the originality--in terms of species composition and functioning--of the Camargue's wetlands relied on this variability and unpredictability. We are today facing a homogeneous and highly predictable system, where aquatic communities are dominated by one or two subcosmopolitan species (M. spicatum and P. pectinatus) and a higher biomass is made available to waterfowl. These changes have also been observed in the microfauna of the marshes where five species of copepods considered as characteristic of seasonal marshes in the 1960s have been replaced by a single cosmopolitan species (Dagan, 1991).

Wintering waterfowl populations (Fig. 7) Over the last 27 years, the size index of the population of ducks and coots wintering in the Camargue changed roughly according to three periods: the first (10 years) with variable values of the mean annual index (variation coefficient = 34.95%), the second (10 years) with regular decreasing values (rate of decrease -- -0.768, p < 0-1) and the third (seven years) when the index increased again (rate of increase -- 0.960, p < 0.01). These changes concerned almost all species. Simultane- ously, the mean annual maxima for ducks calculated for the three periods decreased regularly: 142000, 128000 and 98000 respectively (not significant). It varies more irregularly for coots (35 000, 28 000, 33 000 (not significant). These numerical changes are not clearly understood. They are not significantly correlated with rainfall, either at the community level or at any specific or guild level. The number of birds are no longer correlated with the total area of ricefields in the Camargue, which are intensively exploited by some species of ducks, mostly mallard Anas platyrhynchos and teal A. crecca.

The increased number of wintering birds that have been observed over the last seven years cannot be directly related to the recent changes in salinity of the Vaccar6s lagoon which occurred at about the same time and resulted in the development of rich beds of

Zostera nohii (Heurteaux, 1992). More birds were at- tracted to the Vaccar+s lagoon, but the overall changes in the Camargue do not coincide, either in numbers or in species, with the change on the Vaccar6s scale. More generally it appears that no significant trend occurred for the whole period .(1964-65 to 1990-91). Conse- quently, the physical (quantitative loss of habitat) and biological changes which have occurred in the Camar- gue over the last 30 years seem not directly to control the size and the species composition of the waterfowl populations.

The relative stability in numbers of waterfowl wintering in the Camargue contrasts also with the general decline of 46% observed over the last 15-20 years of those populations which winter on the scale of the Mediterranean basin as a whole (Van Vessem et al., 1992). Long-term changes observed in palearctic or nearctic species cannot be easily explained by factors operating on the breeding habitats (Bell & Owen, 1990). It is more likely that relationships exist with environmental factors which act on the wintering areas, mostly loss and degradation of wetland habitat and hunting (Rtiger et al., 1986; Van Vessem et al., 1992). In this context, the long-term stability of numbers in the Camargue is surprising. It can be interpreted as the result of two opposing factors. On the one hand, the increase in total plant biomass and a longer availability of marshes through the season, both caused by the longer duration of flooding in autumn, has increased the carrying capacity of the Camargue for wintering waterbirds. On the other hand, the loss of aquatic habitat associated with a high disturbance and kill from hunting (150000 ducks killed per year, Tamisier, 1991) and a regular increase in hunting pressure (60% for the last 20 years, Tamisier, 1992) have lowered this carrying capacity. These opposing effects of trophic level and disturbance on the bird populations have not yet been conclusively demonstrated, but they probably counter-balanced one another during the study period. This could explain the increase of the mean population size during recent years whilst the mean maximum values continue to decrease: birds take advantage of


more available resources by arriving relatively earlier and leaving later in the season. Furthermore, it is suggested that, because of the local excessive hunting pressure, the long-term stabilization of the population size occurs at a lower level than that determined by the trophic carrying capacity of the area (Tamisier, 1991, 1992). This hypothesis is in accordance with the independence of the wintering population size from environmental factors, as demonstrated previously.

DISCUSSION

This set of data highlights the drastic changes which have occurred in the Camargue during the last 50 years, especially the loss of 33 000 ha of wetlands and degradation of the remaining wetlands outside the National Reserve. These recent changes in the Camargue represent an acceleration and intensification of the centuries-old processes of transformation of the delta. They lead to a simplification of the aquatic ecosystem already mentioned by Aguesse and Marazanof (1965), where original variability and unpredictability have been replaced by a tendency towards predictable permanent freshwater on most of the wetlands. The result is a loss in alpha diversity (Whittaker, 1972), both in space (at the marsh scale) and time throughout the annual cycle. There is also a loss in gamma diversity within the Camargue since the mosaic tends to be replaced by standard identical habitats. This loss of diversity also concerns the western Mediterranean region as a whole since similar trends towards artificialization and simplification have already been observed at the most important wetland areas of the region: Ebros delta (Camara et al., 1977) and the Marismas del Guadalquivir (Rivas-Martinez et al., 1980) in Spain, Lake Ichkeul in Tunisia (Tamisier, 1990b), El Kala wetland complex in Algeria (Skinner & Smart, 1984). Finally the loss of gamma diversity reaches a continental scale since the observed changes lower the degree of separation between Mediterranean and continental wetlands.

At the trophic level of waterfowl, these changes have a distinct effect. The increase in total plant biomass as well as the higher availability and predictability of marshes makes the Camargue more attractive to the birds. However, as a corollary of these changes, hunting limits this availability so that the population size of duck and coot wintering in the Camargue is no longer determined solely by local environmental variables; these birds cannot be considered as indicator species of the area, as usually thought (Morrison, 1986), though they do remain viable criteria for identifying wetlands of international importance (Boyd & Pirot, 1989).

These changes in wetland area and functioning in the Camargue are all the more surprising as the area is considered to be one of the better protected wetland areas of the Mediterranean region. They demonstrate that human activities, even here, are dominated by eco- nomics so that the forces for habitat degradation are greater than those for conservation.

The long-term conservation of the natural characteristics and biodiversity of the Camargue requires recon- sideration of the management of the area. It is essential that the variability of the marsh water regime and salinity be restored, both on the geographic scale (to restore a complex mosaic of habitats) and between sea- sons and years. The unpredictability of these variations should also be restored since they have been shown to be of considerable importance in the maintenance of the diversity of plant communities (Bonis, 1993). This implies a lower primary productivity of the marshes which would in turn have consequences at two successive levels. First, from an ecological point of view, energy losses would be expected at the higher trophic levels and particularly for waterbirds and domestic live- stock. Secondly, from an economic point of view, net losses also have to be expected since hunting, grazing and tourism depend on this management. In consequence, the conservation of the biodiversity of the Ca- margue, along with its Mediterranean characteristics, requires the implementation of a management plan for the delta as a whole instead of isolated management plans for the reserves only. To put this into action effectively probably requires new tools for financial compensation of private landowners. Some of these tools are gradually becoming available through the revision of the Common Agricultural Policy and Agro- Environmental measures of the European Union. In other respects, this new concept of conservation for the Camargue should also take into consideration the changes which are linked to tectonic movements (subsi- dence) and to the direct consequences of global change (rise in sea level) which both constitute a major threat in the long term.

ACKNOWLEDGEMENT

We wish to thank O. Dehorter and J. Skinner for use- ful comments and help in graphs and English transla- tion respectively.

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