Values of mutual information I (L; E) and the barycenter “G” for every species.
The chapter provides data from a survey carried out on water beetles in various freshwater ecosystems in Tunisia as a Mediterranean country of considerable diversity. Studies dealing with these insects are fragmentary not only in comparison with the European fauna but also in comparison to other zoogeographical areas. A compiled checklist of beetle species collected from Tunisia is given with an insight on new recorded species. Diversity, altitudinal distribution, and geographical pattern of water beetles in Northern Tunisia are discussed with regard to other Mediterranean areas. They include various chorotypes related to the history of the Mediterranean basin. Several species are threatened and require conservation. According to the criteria of the IUCN, several water beetle species can be included in the list of threatened species.
- water beetles
Water beetles are holometabolous insects characterized by a strongly sclerotized body with the forewings hardened into elytra . They occur in a wide variety of habitats, living in virtually every kind of fresh- and brackish-water habitat, from the smallest ponds to lagoons and wetlands and from streams to irrigation ditches and reservoirs . They exhibit high species richness in the Mediterranean area and are primarily found in the ecotone between land and inland waters . They are of great ecological interest as bioindicators of the quality of limnic ecosystems, the type of water, and habitats in danger . Tunisia, a Mediterranean country, has important water beetles diversity. Studies dealing with these insects are fragmentary not only in comparison with the European fauna but also in comparison to other zoogeographical areas [5–16]. Synonymy of species is established by following the “
The fauna of North Africa could be considered as originating from the passage of Euroasian species to the African continent as a result of plate tectonics, leading to the connection of the two continents. The Mediterranean and the Atlantic were later connected (in the Pliocene), thereby isolating the two continents . Water beetles of Tunisia include various chorotypes related to the history of the Mediterranean basin of which Tunisia is a part. The northern part of Tunisia includes two mountain ranges: the Tell (Kroumir and Mogods Mountains) and the Dorsale (Châambi Range reaching the Cap Bon Peninsula) . Tunisia has a humid to Saharan climate. The humid area is limited to the Kroumir Mountains . Annual rainfall decreases from the north to the south, with most of the rainfall in winter . Water resources are unevenly distributed within the country; the northern part, covering an area of only 17% of the territory, has 60% of the total water resources . This highly influences the water permanence and, therefore, the phenology, composition, and distribution of the water beetles’ communities. We analyze the faunistic, chorological, and phenological aspects of the aquatic coleopteran species in the study area.
As the species distribution is determined by a set of ecological and historical filters acting on several spatial and temporal scales , we analyze the assemblage of aquatic beetles in response to environmental variables characterizing the explored streams.
Understanding patterns in biological diversity along major geographical gradients is an important topic in ecology. Garrido Gonzalez et al.  reported that the species distribution of water beetles was greatly influenced by altitude affecting the characteristics of aquatic settings. We tested if such finding is similar for the coleopteran fauna in Northern Tunisia.
2. Knowledge status of water beetles
Previous to the exploration of Northern Tunisia during the last years, very little was known of the water beetle fauna of Tunisia. So far as can be ascertained, there are only a few published notes on their biogeography, and the other a compiled checklist of 214 species collected by [5–16, 27–34]. Compilation of studies focusing on taxonomy of water beetles of Tunisia indicated a checklist of 236 species taking into account the eventual synonymous. The result of our recent researches is a list of 123 species, including all that I have found on the Northern Tunisia (Figure 1 and Table 1). It is mainly the result of monthly collecting surveys over the course of a year (May 2005–April 2006), supplemented by some species collected by friends while working at other groups of insects. We considered only the water beetles’ families sampled in the recent survey. About 1420 species in about 40 genera belong to Hydraenidae Mulsant, 1844 and are encountered on all continents, and some inhabit even the Subantarctic Islands, where only a few insects are able to cope with the hostile climatic conditions . The Hydraenids of Tunisia comprise 57 species. Recently, we sampled 24 species including four recorded for the first time in Tunisia:
|Species||PH||CH||D||I (L;E)||“G”||Species||PH||CH||D||I (L;E)||“G”|
Elmids occur on all the continents with about 1330 species in 146 genera. Members of this family are generally living in lotic habitats, and very few species are encountered on lake shores or in ponds, whereas Dryopids are represented by about 300 species in 33 genera occurring in all biogeographical regions, except for the Australian continent. Larvae are generally riparian or terrestrial; adults of about 75% of the species are regarded as aquatic (lotic and lentic habitats), and the remaining ones are riparian or terrestrial (humicolous and arboricolous) . Studies on Elmidae and Dryopidae [13, 15, 31] reported 18 species. We collected nine species (Table 1), four of them are new recorded from Tunisia;
Helophoridae is a monogeneric family with about 185 species, more or less confined to the Holarctic Realm . Most species seem to prefer standing shallow water with plenty of organic debris, such as edges of small-to-medium sized water bodies . Thirteen species of
An updated checklist of the aquatic adephagan Coleoptera includes a total of 90 species, of them 57 were sampled in the study area (three Gyrinidae, six Haliplidae, one Paelobiidae, one Noteridae, and 46 Dytiscidae).
3. Ecological traits of water beetles
3.1. Diversity and geographical pattern of water beetles
Species were categorized into three groups according to their adult phenology, following the approach of Valladares and Garrido ; permanent species (found over the course of the year), frequent species (encountered in three seasons), and seasonal species (occurring only during one or two seasons). The phenology of species is based on the presence of the adults since the capture of larvae is sporadic and requires an appropriate methodology . The distribution of species in the studied areas in a transect from west to east took into account the differences in geological (landform localization) and hydrological (basin connectivity) characteristics. Four distributional categories of the water beetle species are distinguished according to the areas in which each species occurs (Figure 1 and Table 1); 19 species occurring only in the Kroumir and Mogods Mountains (G1), mainly Hydraenidae and Hydroporinae (Dytiscidae) that are pollution-sensitive and rheophilous collected primarily from montane streams of the Aïn Draham region that are safe from any anthropogenic activities, 15 species also occurring east of the
Beetles living in freshwater are strongly influenced by physicochemical and biological factors . The chronology of appearance of the sampled species may be attributed to the flow of rivers, which may be temporary. The temporal appearance of species also is affected by a spatial variation that can hide the chronology of their emergence in each stream. Indeed, the study sites belong to different bioclimatic regions that largely influence water permanence, trophic factors, and hence, population dynamics (intensity of drift and migration and distribution of the fauna according to the availability of prey and competition).
The species richness of water beetles follows seasonal fluctuation (Figure 2). The frequency and abundance of their adults attenuate considerably and even are absent when the environmental conditions become unfavorable in winter because of decreased water temperature, high turbidity, and the reduction of the aquatic vegetation as food, and refuge for the benthic community. The seasonal succession of species in temporary waters affects trophic structure, adaptations to drought, and traits common to most successful taxa, including highly flexible life cycle, temperature-dependent development, diapause or otherwise protected eggs, and high dispersal ability . Among the main factors affecting community structure are runoff from agricultural areas, vegetation cover, and water chemistry . Except for Noteridae, water beetles have terrestrial pupae. The life cycles of these species may include larval or adult terrestrial stages that minimizes the likelihood of their occurrence in the aquatic environments. In groups such as Dytiscidae and Elmidae, larvae and adults are aquatic. In other groups, such as Scirtidae, only larvae are aquatic. Hydraenidae and the hydrophilid (Hydrochinae and Helophorinae) have only adults as the aquatic stage ; they live as larvae in a dry cocoon in an excavated cavity above the water level, and can leave and return to it as adults , which minimizes the chance of capture. The overall phenology of the water beetle species reveals highest abundance and frequency in summer and in autumn. The phenological categories revealed a predominance of permanent species compared with frequent and seasonal species, with slight differences in abundance levels. Permanent and frequent species follow the overall phenological pattern, whereas the seasonal species show a spring maximum of abundance, which decreases toward a winter minimum .
The species exhibiting an autumnal peak of abundance are
The geographical distribution was analyzed by chorotype based on distributional patterns that are deduced from a comparative analysis of geographical ranges of species . The fauna of Tunisia, as well as all of North Africa, is a heritage of Eurasian and Afrotropical elements. However, during the Pliocene, the isolation from Europe blocked the arrival of several European lineages. The desertification of the Sahara during the Holocene impeded the northward movement of Afrotropical species. These two facts explain the relative poverty of the water beetle fauna compared to less isolated zoogeographic regions in the world . The chorological category corresponding to each species is given in Table 1. The most important chorotypes are Europeo-Mediterranean (19.2%), North African (15.8%), West-Mediterranean (12.5%), Turano-Europeo-Mediterranean (12.5%), and Mediterranean (8.3%). The number of endemic species is low, about 2.5% of the total fauna. For the species of the genus
The fauna of North Africa probably originated from the passage of Euroasian species to the African continent as a result of plate tectonics (in the Tortonian), leading to the connection of the two continents. The Mediterranean Sea and the Atlantic Ocean were later connected (in the Pliocene), thereby isolating the faunas of the two continents . The origin of the water beetle fauna in Northern Tunisia reflects the history of the Mediterranean basin. During the secondary era, the coastal massifs of the Rif, as far as the Kroumir, were an emerged part of a continent or more probably an archipelago, the Tyrrhenid, which spread over what is now the western Mediterranean between Spain and Italy . The Eocene and Oligocene transgressions reduced the Tyrrhenid to the Betico-Rifan massif separated from Europe by the North-Betican trough and from Africa by the South-Rifan trough and by some islets near the Kabylie. These lands remained emerged up to the present time. North Africa was joined to Eurasia at the end of the Miocene, and the Mediterranean, thus enclosed, dried up. During the Pliocene, a transgression covered the Tyrrhenid and the Strait of Gibraltar divided the Betico-Rifan into two parts . These events provide a hypothesis explaining the chorological aspects of the current water beetle fauna of Tunisia and North Africa.
3.2. Altitudinal distribution of water beetles
The northern part of Tunisia comprises several orographic areas: the
The species richness of water beetles decreases with increasing altitude. This may be explained by the fact that some species present in lowland streams were not found at higher altitudes (Table 1). Furthermore, it can also be attributed to the fact that the majority of sampled sites were in the first altitudinal level. Also, there was a decrease in new species with the accumulation of new records. Five species were newly recorded in the mid-altitude level, whereas 25 species disappear in it. Sixteen species were added in the high altitude level to those of the mid-altitude level, with the disappearance of 18 species; seven species were new, and 69 were absent in the high altitude level in comparison with the low altitude level.
The water beetles in Tunisia are poorly studied not only in comparison with the European fauna but also with other zoogeographical areas. The present conducted survey aims at bettering the knowledge on this freshwater fauna. The species of richness of water beetles is updated by new records; it can be bettered through samples from central and southern Tunisia. This checklist seems to better the knowledge of the diversity of the water beetles’ habitats and provides a solid basis for further research, focusing on macroinvertebrates in order to better direct monitoring conservation projects, and to asses the effects of anthropogenic activities on these fragile ecosystems. The fauna of Tunisia, as well as all of North Africa, is a heritage of Eurasian and Afrotropical elements. However, during the Pliocene, the isolation from Europe blocked the arrival of several European lineages. The desertification of the Sahara during the Holocene impeded the northward movement of Afrotropical species. These two facts explain the relative poverty of the water beetle fauna compared to less isolated zoogeographic regions in the world . These aquatic insects are heterogeneous in their local and world distribution. Their assemblage is structured by physicochemical parameters. They include indicator species (water quality, altitude. etc.). Altitude could be considered among the physical factors that affect distribution of stream macroinvertebrate communities, but its effect is also combined with other environmental variables such as temperature, substratum, water flow, and stream geomorphology, particularly in streams extending along altitudinal gradients . Water permanence and depth were considered by Williams et al.  among the main environmental variables explaining invertebrate assemblage structure. The present study was restricted to the northern part of Tunisia. The sampled sites ranged between 1 and 714 m, which is a rather limited altitudinal gradient. In many geographical areas, 714 m would hardly be considered “high altitude.” This can make difficult a comparison with other areas in which the habitats typical of the Tunisian “lowland” are found above such an altitude. Therefore, the distribution of the species has to be related to the distribution of habitats, as the same species can be found at different altitudes in different areas, depending on where suitable habitats are found. Further sampling is required to confirm these results, especially in higher mountains in central and southern Tunisia, since those in central Tunisia rise to 1544 m.