As discussed in an earlier blog, the Banggai cardinalfish, which are considered to be a vulnerable or threatened species in their original home of the Banggai islands, is an introduced species to Lembeh (the exact story about how this happened varies, from a broken seine net to a transport being late and an exporter just dumping his already packed fish in the harbor). After introduction, they have subsequently spread in the strait and around Lembeh island, and even up to the Bangka island. The fact that these cardinalfish are beautiful and very timid fish, and their bleak conservation status in their origin, could lead one to assume that their introduction into the Lembeh strait would be applauded. However, that is certainly not the case. Why is that so?
Introduced species are causing some of the most severe problems in conservation of native species. Introduced species can act in three main ways to cause such problems.
First of all, introduced species can bring diseases, that the species themselves are adapted to, to new places where the native species have little or no defense against the disease. Examples of this are abundant. In the US, an introduced chestnut caused the chestnut blight, which eradicated large tracts of woodland. Much the same is happening in Europe, where elms are disappearing due to a disease brought in by an ornamental variety of a foreign elm. However, as marine diseases spread easily, and the distance between Lembeh and Banggai isn´t that long, diseases brought in by the Banggai cardinals are not likely to be a major issue in Lembeh, as the same diseases should be present in both places.
Second, introduced species can be predators on native species, driving these to extinction. Feral dogs in Australia, Nile perch in Lake Victoria and brown tree snakes in Guam are excellent examples of such problems. In marine environments, the ongoing spread of lionfish in the Caribbean, where small reef fish which are not adapted to lionfish are pushed to very low densities, is a prime example. However, as Banggai cardinalfish mainly feed on copepods, which are very abundant tiny planktonic crustaceans, it is highly unlikely that the feeding of the Banggai cardinalfish in Lembeh will lead to any problems at all for the prey of the cardinalfish.
Third, and finally, introduced species can compete with native species for fundamental resources. That is exemplified by rabbits in Australia, which were so efficient herbivores that some native species went extinct. Also, mountain sheep in the US is occasionally displaced by introduced donkeys, which thrive on food on less quality than the sheep are able to handle. This is probably the real issue with the cardinalfish. Young Banggai cardinalfish take shelter in sea urchins, branching corals and sea anemones.
However, every sea anemone in Lembeh is home to one of a number of species of anemonefish. The number of anemone fish in each anemone is dependent on the size of the anemone, and dominant anemone fish aggressively limit immigrants to the anemone by displacing surplus fish. Thus, there are two major effects the Banggai cardinalfish can have on the anemone fish. First of all, anemone fish defend their anemone against Banggai cardinalfish. However, as the cardinalfish at times number in the hundreds close to an anemone, any specific cardinalfish will not be to bothered by the anemone fish, while the anemone fish will spend a lot of energy trying to get the Banggai cardinalfish to move away. The other issue is related to the fixed number of fish allowed in each anemone. If this number is based on a resource that both anemone fish and Banggai cardinalfish use, such as plankton, the Banggai cardinalfish could well outcompete the anemone fish.
So do we see any decline in the number of anemone fish in the strait? I simply do not know. With hindsight, it is obvious that the density of anemone fish should have been determined early in the process of the Banggai cardinalfish introduction, and if I could I would kick myself somewhere in the seat area for not doing it the first time I came here. But it was not done, and now we will have to wait far longer before we really know.