Different methods, different results: an assessment of tests for nested subset patterns in parasite communities

TIMI, J.T.; POULIN, R.

Viterbo, Italia

Simposio; 7th Internationsl Symposium on Fish Parasites; 2007

Universitá degli studi della Tuscia y Societá Italiana di Parassitologia.

The search for nested subset patterns has become a powerful tool for understanding the processes shaping communities in insular habitats. In particular, it has been applied widely to parasite communities to study the way in which parasite species are distributed among host individuals in a host population. Using RANDOM1, the first algorithm available to study nested patterns, all early studies concluded that nestedness was the exception rather than the norm in parasite communities. In contrast later studies, using instead the nestedness temperature calculator (NTC), found that nested subset patterns were very common in parasite communities. Recently, a new algorithm to calculate the nestedness of presence-absence matrices, the binary matrix nestedness temperature calculator or BINMATNEST, has been proposed, and it is becoming imperative to assess these different algorithms and re-evaluate how common nestedness really is in parasite communities. Here, using data on a total of 31 helminth parasite communities of fish hosts, we show that applying the NTC yields consistently more significant nested patterns than when RANDOM1 is used on the same data. The use of BINMATNEST produced results that depended entirely on the choice of null model. To provide a benchmark, a straightforward comparison between the observed frequencies of co-occurrences of species with those expected from their prevalence under random assembly was also made for each community, as a simple and basic test for non random structure. This test indicates that random structure occurs in practically all communities, even those where one or more of the three nestedness analyses found a significant pattern. We compare the assumptions of different methods and null models, and conclude that nestedness is indeed rare within component communities of parasites, and also make recommendations regarding the appropriate choice of null models in nestedness analyses. Financial support by grants from CONICET (PIP 5996/06) and Universidad Nacional de Mar del Plata (15/E 303)