2.4. Assessment of major biodiversity components

By evaluating the overall situation of the Lesser Prespa Lake wetlands we were able to identify the encroachment of reeds over the largest part of the areas flooded in spring by the rising lake waters as a basic pressure that leads to an impoverishment of the overall lake ecosystem diversity, basically depriving several large and rare waterbirds of their feeding habitats, thus limiting their population growth, as well as negatively affecting the spawning success of several phytophilous fish.  Hence, decisions converged on intervention with management to try and clear some areas of reeds and create more open shallow water spaces for birds and fish.  Right from the start we envisaged that management would have to take the form of mowing, destroying or grazing reeds in certain places that would probably have to change from year to year. 

Assessing target waterbirds populations 

One of our fundamental concerns before starting any management intervention was to record the status of the nine species of waterbirds that we suspected were most likely to have been affected by the shrinkage of their feeding grounds, i.e. the two species of pelican, pygmy cormorant, four heron species, glossy ibis and ferruginous duck. The use of feeding sites by the target species, as well as their population sizes, were assessed initially in 2017, but because that year had been very dry, we extended the assessment to the next, moderately wet, year, 2018. Methods used included counts from vantage points between the end of January to May, “flight-line” counts at the colonies and counts of nests on photos taken by drone. 

The breeding population size of Dalmatian pelicans was estimated at 1,266 pairs and breeding success at 0.73 young per nest. The great white pelican breeding population was estimated at 588 pairs and the breeding success at 1.2 young per nest. We also counted 2,051 pygmy cormorant nests, 135 great white egret nests, 100 little egret nests, 89 night heron nests, 31 squacco heron nests and 1 glossy ibis nest. The populations of the four latter species were lower in 2018 than in 2017. The breeding success for great white egrets in 2018 was 2.2-2.5 young per nest, much higher than that in 2017 (1 young/nest). The breeding population of the ferruginous duck was estimated at 4-8 pairs.   

As regards the frequency of use of the foraging sites, our results showed that the overall species richness in 2018 (24 species) was almost three times higher than in 2017 (9 species), while the mean abundance in 2018 (106 individuals per sampling) was more than twice that of 2017 (46 individuals per sampling).  Most species and higher densities were observed at sampling sites in the northern part of the lake, between Slatina Laimou and Koula. During May-June high percentages of all species except the night heron used feeding sites on the shores of Great Prespa Lake, while practically 100% of night herons fed at Lesser Prespa Lake. Our overall results confirmed that the priority for management should be given mainly to areas in the northern part of Lesser Prespa Lake.   

Figure 1: Graphic representation of the results of the monitoring scheme on habitat use by target waterbirds as foraging sites in Lake Lesser Prespa during 2018. The diameter of the circle indicates the number of species recorded in each site. The colour of each site represents a range abundance (yellow=very low, red=very high)

Assessing other biodiversity components

We acknowledged, however, that the reedbeds/open water dynamic complex interface system is a habitat used by a plethora of other organisms too. Through our knowledge of the population trends and life history traits of target waterbirds and fish we could easily anticipate the effects of management on them, but we had little insight into what this would possibly mean for other organisms using this habitat.  Our main concern had been two things: a. to avoid intervention in particular areas which might have a high importance for some species; and b. to ensure that the overall mosaic of undisturbed and managed areas we would aim to create would not have negative implications for any groups of species. We decided to focus on plants and vertebrates because from a review of the available body of knowledge and understanding there were not even slight indications implying that such kind of management could pose any hazard to invertebrate reed communities. We thus chose plants, amphibians, reedbed passerines and small mammals, as well as the European otter, all organisms spending the whole, or large parts, of their lives in this reed-wet meadow-shallow water transition habitat.   

In his study of plants and their communities, Fotiadis (2017) concluded that there was higher floristic richness and more vegetation types in areas managed with mowing or/and grazing. In the absence of management, monospecific communities of Phragmites australis and Typha angustifolia prevailed.  He concluded that only a few small areas around patches of habitat type 3150 (Natural eutrophic lakes with Magnopotamion or Hydrocharition - type vegetation) should stay out of any management regime, in order to protect the habitat from waves. 

Figure 2: recorded floristic diversity in Lesser Prespa Lake (yellow: very low species presence, light orange: low species presence, orange: medium species presence, red: high species presence).

Sotiropoulos et al. (2017) studied the amphibians and acknowledged that, although in some cases reedbed management proved to be beneficial for amphibian diversity and populations, the existing knowledge was still limited. They assessed the diversity and densities of amphibians in 13 pre-selected parts of the reedbeds in both lakes, in order to estimate their population status and produce the knowledge necessary to inform the proper selection of management sites and the consequent evaluation of the applied management. The study added several new data for the area of Prespa. Amongst them, several new locality records were added for the Macedonian crested newt (Triturus macedonicus). In general, differences regarding species composition and densities of the larval stages of amphibians were recorded, with reedbeds showing higher diversity indices compared to adjacent habitats. On the other hand, increased larval densities were observed in the wet meadows. The results showed that sites exhibiting an increased diversity of amphibians consisted of a mosaic of habitats, since different species of amphibians occupy different niches. Therefore, a proper management strategy that would create open shallow areas and at the same time preserve patches of undisturbed reeds, was expected to increase the diversity of amphibians and to ensure the conservation of the Macedonian crested newt in the area.

Bounas (2017) made the census of passerine birds in the reedbeds of Lesser Prespa Lake. Reedbeds are important habitats for birds, including some rare and vulnerable species. Censuses of birds at 22 census points were used to assess the densities and diversity of passerine birds in reedbeds at the margins of the lake. A total of 54 species were recorded, but analyses were limited to nine species that are obligate reed-dwellers, at least for some part of their annual cycle. The great reed warbler (Acrocephalus arundinaceus) was the most common species, followed by the reed warbler (Acrocephalus scirpaceus). The great reed warbler also showed the highest relative abundance, dominating most of the surveyed sites. The bearded reedling (Panurus biarmicus) was more common in the areas of Agios Achilleios, Vromolimnes and Ekvoli, whereas the reed warbler showed the highest population density in the Vromolimnes area. The highest species richness was observed in Slatina, Opaya and Platy, while on the other hand Dhaseri, Agios Nikolaos and Latsista had the lowest species richness. Sedge warbler (Acrocephalus schoenobaenus) was far more common in the past, while the marsh warbler (Acrocephalus palustris) was found – almost exclusively – in the northern part of the lake. Under the scope of management, it seemed that areas of low passerine diversity, dominated by the great reed warbler, such as the reedbeds of Pyli, as well as parts of the area from Mikrolimni to Bouskani, would make reasonably good candidates. Great reed warbler abundance would most probably be slightly affected, while opportunities for other species to colonise the areas could arise. It would be advisable to avoid cutting reeds in areas of high densities of resident birds such as the bearded reedling and reed bunting, as well as in sites of already high bird diversity such as Slatina, Opaya and Platy. 

Figure 3: Graphical summary of the relative abundance of every species observed in each site. Numbers correspond to site codes (see upper leftcorner)

Theodoropoulos (2017) explored the spatial distribution of the otter (Lutra lutra). The study followed the standard methodology for the otter, based on a systematic sampling survey for field signs of otters. More light was shed on the distribution and possible breeding sites of the otter in the study area. The species was found to occupy all available habitats in Lesser Prespa Lake. Ensuring that a sufficient number of reedbed patches – of the appropriate extent and siting – remained undisturbed, was of major conservation significance in order to maintain a healthy otter population. Fourteen “plausible” female otter territories were identified in the central and northern sections of the lake. Efforts were then concentrated on identifying the apparently most suitable spots to host breeding sites along those 14 “plausible territories”, that would be potentially at risk from mowing interventions. Taking all relevant parameters under consideration, a map was produced delineating the suggested “otter friendly management areas”, that were based on the best of our knowledge and understanding of the species’ requirements.

Further reading: 

Please, refer to the initial biodiversity survey reports in the following links: 

Waterbirds: Assessment of the population status, habitat use and phenology of the target waterbird species   

Floristic diversity (in Greek, with English summary): Survey on the floristic diversity of Lesser Prespa lake

Amphibians (in Greek, with English summary): Assessment of the diversity of amphibians in reedbeds Lesser (Mikri) and Great (Megali) Prespa lake 

Passerine birds: Census of passerine birds in the reedbeds of Lake Lesser (Mikri) Prespa   

Small terrestrial mammals: A baseline survey of small terrestrial mammals along the reedbeds of Lesser Prespa lake 

Otter: Study on the presence of the Otter Lutra lutra  in Lesser Prespa Lake