Tradeoffs with growth limit host range in complex life-cycle helminths

Parasitic worms with complex life cycles have several developmental stages, with each stage creating opportunities to infect additional host species. Using a dataset for 973 species of trophically transmitted acanthocephalans, cestodes, and nematodes, we confirmed that worms with longer life cycles (i.e. more successive hosts) infect a greater diversity of host species and taxa (after controlling for study effort). Generalism at the stage level was highest for ‘middle’ life stages, the second and third intermediate hosts of long life cycles. By simulating life cycles in real food webs, we found that middle stages had more potential host species to infect, suggesting that opportunity constrains generalism. However, parasites usually infected fewer host species than expected from simulated cycles, suggesting generalism also has costs. There was no tradeoff in generalism from one stage to the next, but worms spent less time growing and developing in stages where they infected more taxonomically diverse hosts. Our results demonstrate that life cycle complexity favors high generalism, and host use across life stages is determined by both ecological opportunity and life history tradeoffs.