Status and trends
Invasive alien species represent significant threat to biodiversity, which intensity and scope is still being researched.
Since 2009 the number of recorded alien species in Croatia has increased but complete analysis of their number, introduction, distribution and impact on ecosystems still does not exist. Lists of invasive alien species for different taxonomic groups are only partial. The most comprehensive one is the preliminary list of invasive alien plants. Among 606 species and subspecies
of alien plants recorded for Croatia (what is about 11.9% of the total Croatian vascular flora), 64 of them are invasive. As it is assumed that there could be even more than 1500 alien plants in Croatia, the number of registered invasive ones will most certainly rise. The majority of invasive alien plants originate from North and South America. They have been registered on almost 50% of Croatian territory, most of them being distributed in big towns and sea ports as well as along the large lowland rivers. During last two decades, the largest number of new alien species is connected to Mediterranean area, especially islands. Generally, alien plants occur in most cases in agricultural and artificial areas.
Species
Abutilon theophrasti Medik. Erigeron annuus (L.) Pers. subsp. strigosus (Mühlenb. ex Willd.) Wagenitz
Acer negundo L. Euphorbia maculata L.
Ailanthus altissima (Mill.) Swingle Euphorbia prostrata Aiton
Amaranthus retroflexus L. Galinsoga ciliata (Raf.) S.F.Blake
Ambrosia artemisiifolia L. Galinsoga parviflora Cav.
Amorpha fruticosa L. Helianthus tuberosus L.
Angelica archangelica L. subsp. archangelica Impatiens balfourii Hooker f.
Artemisia annua L. Impatiens glandulifera Royle
Artemisia verlotiorum Lamotte Impatiens parviflora DC.
Asclepias syriaca L. Juncus tenuis Willd.
Aster squamatus (Spreng.) Hieron. Lepidium virginicum L.
Bidens frondosa L. Nicotiana glauca Graham
Bidens subalternans DC. Oenothera biennis L.
Broussonetia papyrifera (L.) Vent. Oxalis pes-caprae L.
Carpobrotus edulis (L.) N.E.Br. in Phillips Panicum capillare L.
Cenchrus incertus M.A.Curtis Panicum dichotomiflorum Michx.
Chamomilla suaveolens (Pursh) Rydb. Parthenocissus quinquefolia (L.) Planchon
Chenopodium ambrosioides L. Paspalum dilatatum Poir.
Conyza bonariensis (L.) Cronquist Paspalum paspalodes (Michx.) Scribn.
Conyza canadensis (L.) Cronquist Phytolacca americana L.
Table 3. Preliminary List of Invasive Alien Plants in Croatia
Species
Conyza sumatrensis (Retz.) E.Walker Reynoutria japonica Houtt.
Cuscuta campestris Yuncker Reynoutria sachalinensis (F.S.Petrop.) Nakai in T. Mori
Datura innoxia Mill. Robinia pseudoacacia L.
Datura stramonium L. Rudbeckia laciniata L.
Diplotaxis erucoides (L.) DC. Solanum eleagnifolium Cav.
Duchesnea indica (Andrews) Focke Solidago canadensis L.
Echinocystis lobata (Michx.) Torr. et Gray Solidago gigantea Aiton
Eleusine indica (L.) Gaertn. Sorghum halepense (L.) Pers.
Elodea canadensis Michx. Tagetes minuta L.
Epilobium ciliatum Raf. Veronica persica Poir.
Erigeron annuus (L.) Pers. subsp. annuus Xanthium spinosum L.
Erigeron annuus (L.) Pers. subsp. septentrionalis (Fernald et
Wiegand) Wagenitz Xanthium strumarium L. subsp. italicum (Moretti) D.Löve
Pressure of invasive alien algae in the Adriatic Sea is increasing due to global climate change. The tropical green algae Caulerpa taxifolia and Caulerpa racemosa var. cylindracea are among the most invasive, posing significant threat to sea grass meadows. C. taxifolia was observed at three locations in
1994 and 1996 but it remained restricted to only one site with decreasing trend since 2009, possibly because of relatively low winter temperatures. C. racemosa (Fig. 10.) was first found in autumn 2000 close to the Hvar Island in Middle Adriatic and by the end of 2010 it was observed at 99 locations.
Figure 10. Caulerpa racemosa. Photo: A. Žuljević Figure 11. Gambusia holbrookii. Photo: A. Duplić
For different groups of animals some lists of alien species exist but without specifying their invasive influence. Among freshwater fish, 25 alien species have been recorded for Croatia, which makes about 70 % of the total alien freshwater species recorded in Europe. Most of them have adapted to
Species
Ameiurus melas Hypophthalmichthys nobilis Oncorhynchus mykiss
Ameiurus nebulosus Lepomis gibbosus Oreochromis niloticus
Carassius auratus Micropterus salmoides Percottus glenii
Carassius gibelio Morone saxatilis x M. chrysops Piaractus brachypomus
Coregonus lavaretus Neogobius fluviatilis Pseudorasbora parva
Coregonus peled Neogobius gimnotrachellus Salmo salar
Ctenopharyngodon idella Neogobius kessleri Salvelinus alpinus
Gambusia holbrookii Neogobius melanostomus Salvelinus fontinalis
Hypophthalmichthys molitrix
Table 4. List of Alien Freshwater Fish Species in Croatia. Species that are recorded in this reporting period are marked bold.
The Croatian ichthyofauna of the Adriatic Sea also includes alien fish species. There are 25 alien fish species recorded in the Adriatic so far but only 10 have been recorded more than once, so it is difficult to define their status in the Adriatic and if some of them may have established populations. One of the causes of their entry into the Adriatic is changes in climate and oceanographic processes. New fish species arrive into the Mediterranean via the Gibraltar and the Suez Canal. Those
new habitats but some of them have been recorded only occasionally. In this reporting period there are 6 new species, only one of them - Neogobius gymnotrachelus with established breeding populations.
that reach the Mediterranean via the Suez Canal from the Red Sea are called “Lessepsian migrants” (Fig. 12.). Some of these species are strong predators and can have a direct impact on the food chains in the Adriatic ecosystem. Accidental introduction, escape from aquaculture or aquariums, and ballast waters are also important causes of the entry of alien fish species into the Adriatic.
Figure 12. Bluespotted cornetfish (Fistularia commersonii) - Lessepsian migrant. Photo: A. Žuljević
Species
Caranx crysos Holacanthus ciliaris Plectorhinchus mediterraneus
Caranx ronchus Lagocephalus lagocephalus lagocephalus Saurida undosquamis
Cyclopterus lumpus Lagocephalus sceleratus Siganus luridus
Elates ransonnetti Lobotes surinamensis Siganus rivulatus
Enchelycore anatina Mycteroperca rubra Sphyraena chrysotaenia
Epinephelus coioides Pagrus major Stephanolepis diaspros
Equulites (Leiognathus) klunzingeri Paraexocetus mento Terapon theraps
Fistularia commersonii Paranthias furcifer Tylosurus acus imperialis
Hemiramphus far
Table 5. List of Alien Sea Fishes in Croatia. Underline - more than one record
The most problematic species of invasive reptiles is the Red-eared terrapin (Trachemys scripta elegans) (Fig. 14.), which negatively affects the native European pond terrapin (Emys orbicularis). A research following its colonization of the lakes in protected park Maksimir in Zagreb was undertaken in the period 2006-2012. The number of this alien species was rising constantly on account of the native terrapin, from 65 in 2006 to 206 in 2012, while the native species decreased from estimated 11-12 to only 2 specimens.
In 2009, 16 invasive alien invertebrates in freshwater ecosystems have been recorded, but today this number is probably higher than 20. The most known are the Zebra mussel (Dreisenna polymorpha), the Killer shrimp (Dikerogammarus villosus) and Signal crayfish (Pacifastacus leniusculus). Among the most known terrestrial invasive alien invertebrates are Harlekin beetle (Harmonia axyridis) (Fig. 13.), Chestnut gall wasp (Dryocosmus kuriphilus), butterfly Geranium bronze (Cacyreus marshalli), spider (Mermessus trilobatus) and snail Red slug (Arion rufus).
Some invasive species cause serious damages to the economy and pose a threat to human health. It is estimated that the Zebra mussel (Dreisenna polymorpha) overgrowing parts of hydropower plant Varaždin on the Drava River, caused additional costs for its removal of 200,000€ during the four years period. Besides, cleaning of one channel overgrown with the Canadian waterweed (Elodea canadensis) cost cca 70.000€
through five years while in the summer of 2008 this species caused damage worth 500,000€ through the loss of 2% of yearly electricity production.
The most common invasive species causing human health problems are the Common ragweed (Ambrosia artemisiifolia) (Fig. 15.) because of allergy reaction of many people as well as the Tiger mosquito (Aedes albopictus) (Fig. 16.) that was first registered in 2004 in Zagreb and today is distributed throughout the most part of Croatia.
It should be noted that there are many species in Croatia that are not alien for the country but are considered alien species for certain ecosystems or regions where they can become invasive and suppress native species. Such are some freshwater fishes like the Catfish (Silurus glanis), the Pike (Esox lucius), the Carp (Cyprinus carpio) and the Zander (Sander lucioperca) which are indigenous to the Danube basin but can have negative effect when they are translocated, intentionally or unintentionally to the Adriatic Basin. Similar situation is with the game species Wild boar (Sus scrofa) that has spread on many Croatian islands.
Figure 13. Harlekin beetle Photo: B. Krstinić
Figure 14. Red-eared terrapin Photo: G. Laušić
Figure 15. Common ragweed. Photo: I. Boršić
Figure 16. Tiger mosquito. Photo: James Gathany, www.wikimedia.org