How Rapidly Are We Losing Hosts And Parasites

Estimates for the loss of biodiversity use a variety of methods to compare current rates of species extinction against background rates (May et al., 1995; Regan et al., 2001). All of these methods suggest that we are entering a period of mass extinction that is directly comparable to the mass extinctions recorded in the fossil record. Poulin and Morand (2004) used the proportion of threatened hosts in each major vertebrate taxon to estimate the potential threatened number of parasitic species. We have modified their projection to consider different levels of host specificity (Table 4.2). Poulin and Morand's original calculation assumed a direct correspondence between the proportion of parasites threatened and the proportion of hosts threatened. This figure was then adjusted by the degree of host specificity of the parasites. Koh et al. (2004) performed a similar analysis, using more sophisticated models on select groups of hosts and parasites for which they acquired good data on host-use patterns. All s

TABLE 4.2 Percentage of Vertebrate Species Listed as Threatened by IUCN Red List and the Estimated Numbers of Parasitic Helminth Species That This Puts at Risk of Extinction (upper) [Poulin and Morand (2004), Who Assume That the Proportion of Parasite Species at Risk Equals the Proportion of Hosts at Risk], and Proportion of Parasites at Risk When Corrected for Different Levels of Host Specificity Exhibited by Each Parasite Taxa in Each Host Taxa (lower)

TABLE 4.2 Percentage of Vertebrate Species Listed as Threatened by IUCN Red List and the Estimated Numbers of Parasitic Helminth Species That This Puts at Risk of Extinction (upper) [Poulin and Morand (2004), Who Assume That the Proportion of Parasite Species at Risk Equals the Proportion of Hosts at Risk], and Proportion of Parasites at Risk When Corrected for Different Levels of Host Specificity Exhibited by Each Parasite Taxa in Each Host Taxa (lower)

Host Species (% of host species listed as threatened)

Chondrichthys

Osteichthys

Amphibia

Reptilia

Aves

Mammalia

Parasite Species

(2)

(2)

(2)

(3)

(11)

(U)

Total

No. of parasite species at risk

Trematoda

1

117

23

113

1,085

409

1,748

Cestoda

27

89

6

33

1,546

510

2,211

Acanthocephala

25

3

6

86

33

153

Nematoda

3

53

53

192

1,007

328

1,636

Trematoda

1

18

4

64

365

203

656

Cestoda

16

14

1

15

655

270

971

Acanthocephala

2

0

1

10

8

20

Nematoda

1

5

10

90

307

54

468

Totals (%)

18 (1.13)

39 (0.28)

16 (0.38)

170 (1.48)

1,338 (3.95)

535 (4.60)

2,115 (2.75)

of their data suggest that the relationship between host extinction and parasite species extinction is concave, with parasites (and other dependent species) lost more rapidly than their free-living host species. However, the two groups of parasites that they examined (lice and pinworms of primates) both have very high host specificities, so we would expect quite a tight matching between host extinction and parasite extinction.

The estimates of parasite species extinction rate that Poulin and Morand initially produced failed to account for patterns of host specificity (upper section of Table 4.2) and produced high estimates for loss rates of parasite diversity. When we take host specificity into account, parasitic species seem to go extinct at a lower rate than the host species (lower section of Table 4.2); only «3% of helminths («2,000 species among 75,000 total) would then seem to be endangered. If our estimates of net parasitic helminth diversity are low by as much as a factor of four, then there could be as many as 10,000 threatened parasitic helminth species. All of this suggests that we are likely to lose considerable numbers of parasitic helminth species before we have had time to obtain specimens that might be identified and classified.

The numbers for parasitic helminth diversity calculated by Poulin and Morand (Table 4.1) suggest that the bulk of parasitic helminth diversity occurs in birds. The majority of these species will have complex life cycles and thus will also depend on host species at lower trophic levels to complete their life cycles. For example, most of the trematode species also require a snail species in which they undergo asexual reproduction, and many will then pass through another intermediate host that will be a prey item in the diet of the bird that acts as the definitive host in which the parasite reproduces sexually. Although the trematode may be able to use a diversity of different bird species as a definitive host, it will most likely be specific to the snail host. As we will show in the next section, projected avian extinctions imply that the spatial patterns of avian loss will be a major driver of the loss of parasite diversity.

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