Biology Reference
In-Depth Information
14.
Adaptation of Animals to Alien Predators,
Parasites, and Disease Agents
“Everything else being equal, there is an advantage to parasites that carry vir-
ulent alleles, as these parasites are able to infect a broader spectrum of hosts
than parasites carrying avirulent alleles. Similarly, there is an advantage to
hosts that carry resistant alleles, as these hosts are able to resist infection from
a broader array of parasites than hosts carrying susceptible alleles.”
—A GRAWAL AND L IVELY 2002.
The brown trout ( Salmo trutta ), native to Europe, was introduced to many
rivers in New Zealand beginning in the late 1860s and now occurs in
most streams and lakes throughout the country. This fish has greatly
affected the distribution of native fish belonging to the genus Galaxiids in
many streams. Galaxiids and brown trout rarely occur together at the same
location. In most cases, galaxiids prosper only in waters inaccessible to
brown trout. In the Taieri River system of the South Island, for example,
galaxiids occur only in stream headwaters above waterfalls that are high
enough to prevent access by brown trout (Townsend 1996).
Brown trout and galaxiids differ in feeding behavior.Trout are daytime
feeders that forage in the water column and locate prey primarily by
vision. Galaxiids are nocturnal substrate feeders that rely primarily on
mechanical stimuli for prey recognition. Larvae of the mayfly Nesameletus
ornatus are an important prey of both fish in the Taieri River.Where only
galaxiids occur, mayfly nymphs are active on rock surfaces both day and
night and also drift in the water column. In brown trout waters, however,
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