Influence of parasitism on community structure

The incidence of a parasite, like that of parasites may drive other types of exploiter, may determine vulnerable host whether or not a host species occurs in species extinct an area. Thus, the extinction of nearly

50% of the endemic bird fauna of the Hawaiian Islands has been attributed in part to the introduction of bird pathogens such as malaria and bird pox (van Riper et al., 1986); and changes in the distribution of the North American moose (Alces alces) have been associated with the parasitic nematode Pneumostrongylus tenuis (Anderson, 1981). Probably the largest single change wrought in the structure of communities by a parasite has been the destruction of the chestnut (Castanea dentata) in North American forests, where it had been a dominant tree over large areas until the introduction of the fungal pathogen Endothia parasitica, probably from China.

Like grazers and carnivores, parasites can cause more subtle effects too. In many streams in Michigan, USA, larvae of the herbivorous caddis-fly Glossosoma nigrior play a key role in the community because their foraging maintains attached algae at very low levels, with negative consequences for most other stream herbivores a microparasite with subtle direct and indirect effects in a stream community

Figure 19.19 (left) (a) Spider species richness plotted against total number of individuals (all censuses) in the presence and absence of lizards at three heights in the vegetation. For a given number of individuals, enclosures without lizards (•) contained a greater number of spider species than enclosures with lizards (o) except low in the vegetation. (b) Mean proportion of censuses in which each web spider was recorded per enclosure in the presence (□) and absence of lizards (■). Error bars ± SD. (After Spiller & Schoener, 1998.)

Lisard Transfromation

(Kohler, 1992). G. nigrior is subject to sporadic outbreaks of a highly specific microsporidian microparasite, Cougourdella, which result in dramatic whole stream reductions in G. nigrior density that may be maintained for years. In Seven Mile Creek, for example, the mean G. nigrior density was 6285 per m2 in the 10 generations before a parasite outbreak in 1990 but it averaged 164 per m2 for the next decade (Figure 19.20). The decline of G. nigrior leads to increased abundance of its food resource (Figure 19.21a). As a result, several herbivores (Figure 19.21b-d), including a species that was previously absent or extremely rare (Figure 19.21e), increased in abundance after the streams had experienced a parasite-caused decline in G. nigrior. Thus, by reducing the abundance of the competitively dominant herbivore, the parasite increased herbivore equitability (one aspect of species diversity) and may have been responsible for an increase in species richness. This example, therefore, has the hallmarks of parasite-mediated coexistence The parasite was also responsible for further effects - increased algal abundance seems to have resulted in more fine particulate dead organic matter (through sloughing off of algal cells) fueling an increase in the density of filter-feeders (Figure 19.21f), and the increase in abundance of vulnerable herbivore species (G. nigrior is relatively invulnerable to predators) led to increased densities of predaceous caddis-flies (Rhyacophila manistee) and stoneflies (Paragnetina media) (Figure 19.21g).

In terrestrial ecosystems, too, there are apparent examples of parasitemediated coexistence. For example, the malarial parasite Plasmodium azurophilum infects two Anolis lizards on the Caribbean island of St Martin. One of the lizards, thought to be the competitive dominant, is widespread throughout the island while the other is only found in a limited area. Schall (1992) reported that the superior competitor was much more likely to be infected

Figure 19.20 Glossosoma nigrior density in Seven Mile Creek, Michigan, and the percentage of the population infected (prevalence) by Cougourdella. (After Kohler, 1992.)

by the parasite and, intriguingly, the two species only coexisted where the parasite was present. Once again, though, this is far from a universal pattern. For example, the invading grey squirrel (Sciurus carolensis) is displacing the resident red squirrel (S. vulgaris) throughout much of its range in Britain. At least part of the reason seems to be that the invader has brought with it a para-pox virus that has little discernible effect on the grey squirrel but a dramatic adverse effect on the health of the native red squirrel (Tompkins et al., 2003).

Brood parasites (see Section 12.2.3), such as brown-headed cowbirds (Molothrus ater), might also be expected to affect the composition or richness of the communities in which they operate. De Groot and Smith (2001) made use of a cowbird reduction program in a pine (Pinus banksiana) forest in Michigan (designed to protect one of the cowbird's hosts, the endangered Kirtland's warbler Dendroica kirtlandii) to investigate whether the songbird community as a whole was affected by a reduction in the density of the brood parasite. Their results provided no support for parasite-mediated coexistence, nor was there a change in community composition or an increase in the representation of songbird species known to be unsuitable as hosts for cowbirds.

Parasites may sometimes influence community composition not by altering the outcome of competitive interactions but through an impact on a key member of the community that acts as an ecosystem engineer (sensu Jones et al., 1994, 1997). The juvenile stages of the trematode Curtuteria australis encyst in the foot of cockles, Austrovenus stutchburyi, and impair the burrowing ability of the cockles. This results in heavily infected cockles remaining stranded at the surface of the sediments, where they are easy prey parasite-mediated coexistence in Caribbean lizards...

... but not in songbirds parasites that influence species that are themselves strong community interactors

Figure 19.21 Mean densities of (a) attached algae (cells cm-2), (b-e) herbivorous insects (number m-2) and (f) filter-feeders (number m-2), in relation to Glossosoma nigrior density (high, before a parasite outbreak; low, during a parasite outbreak) in six streams. Lines connect data points for each of the six streams; the points with error bars (± 1 SE) are the overall means. (g) Predator densities before and after a parasite-induced reduction of G. nigrior in Silver Creek (dashed lines show mean densities before and after the collapse). (After Kohler & Wiley, 1997.)

(d) Antocha 10,000

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Rhyacophila manistee Paragnetina media

1988 1990f1992 1994 1996 1998 2000 2002 Glossosoma collapse Year for oystercatchers, the trematode's definitive host (Thomas & Poulin, 1998; Mouritsen, 2002). Cockles, the dominant bivalves in New Zealand soft-sediment intertidal zones, are normally buried 2-3 cm under the sediment surface. But in areas of intense parasitism, large numbers protrude from the sediment or lie on its surface, increasing surface heterogeneity and changing patterns of water flow and sedimentation. Mouritsen and Poulin (2005) manipulated the density of surface cockles by creating plots with 30 or 100 surface cockles added to compare with control plots with naturally few cockles at the surface. After 6 months, there were significantly more species of macrofaunal invertebrates (polychaetes, molluscs, crustaceans, etc.) in the treatments with added surface cockles and the densities of a variety of taxa were greater in these experimental plots (Figure 19.22).

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  • aziz
    Are cockles herbivores?
    7 years ago

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