Nevertheless, it is still important for its heuristic value in defining the
way in which questions on habitat selection should be approached. How-
ever, it is doubtful that flawed models can teach us how to think about
nature, and clinging to accepted wisdom is often the most important
obstacle to the development of new ideas and models.
IFD assumes that, in any environment, potential habitats vary in
resource quality, and it predicts how animals in the population (com-
petitors) will choose among the habitats. In this model, competitors are
‘ideal’ in that they have perfect knowledge of their environment and know
which habitats have the highest gains, and they are ‘free’ to enter into
any habitat because the resources are not defended in any way. The
model predicts that competitors will distribute themselves in proportion
to habitat profitabilities such that their distribution in the environment
will be aggregated (more competitors in some habitats than others) but the
fitness of each competitor will be similar (Fretwell and Lucas, 1970). In a
very fundamental way, this theoretical model applies directly to parasite
habitat selection. Parasites have an ideal knowledge of their living
environments, which developed over long evolutionary periods of
intimate host/parasite contact. In parasites, habitat-selection decisions are
so fine-tuned that the responses have become genetically fixed. Clearly,
then, these fixed behaviour patterns should represent optimal solutions to
the problem of habitat selection. However, parasites in the small intestine
do not distribute themselves in accordance with the IFD model (Sukhdeo,
1991, 2000; Sukhdeo and Bansemir, 1996).
InT. spiralisinfections, the adults remain at the site of larval estab-
lishment in the anterior small intestine (Sukhdeo and Meerovitch, 1980).
The distribution of these worms in the gut is aggregated (Tyzzer and
Honeij, 1916; Roth, 1938; Gursch, 1949), but it is not an IFD, specifically
because individual reproductive fitness (fecundity) is not similar at all
locations (Sukhdeo, 1991). Worms inhabiting the poorer habitats should
move to the better habitats until density-dependent effects reduce the
fitness of these good sites to that in the poorer sites. The non-IFD
distributions inT. spiralismay be due to constraints on the parasite’s
ability to move freely, because it is an intracellular parasite. However,
similar non-IFD distributions are seen in the habitat selection behaviour
ofH. polygyrus(Bansemir and Sukhdeo, 1996). These worms are mobile
and easily migrate around in the gut; worms transplanted into the most
terminal ileal regions can return upstream to the duodenum within 24 h,
a distance of more than 30 cm (Sukhdeo and Bansemir, 1996). Yet, in
low-level infections where there are few competitors, worms will often
establish in suboptimal locations, even when the best habitats (those
associated with highest fecundity) are completely vacant (Sukhdeo,
2000). Such behaviours are not easy to explain, because there are obvious
penalties on reproductive success for choosing poor locations.
IFD and other models based on optimal foraging (MacArthur and
Pianka, 1966) have not fared well in studies in the free-living world either
(Seger and Stubblefield, 1996; van der Steen, 1998). The problem may lieIntestinal Nematode Parasites of Vertebrates 231