In the Light of Evolution Volume II: Biodiversity and Extinction (2008) / Chapter Skim
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4 Homage to Linnaeus: How Many Parasites? How Many Hosts?--ANDY DOBSON, KEVIN D. LAFFERTY, ARMAND M. KURIS, RYAN F. HECHINGER, and WALTER JETZ
4 Homage to Linnaeus: How Many Parasites? How Many Hosts?--ANDY DOBSON, KEVIN D. LAFFERTY, ARMAND M. KURIS, RYAN F. HECHINGER, and WALTER JETZ
Pages 63-82
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From page 63... ...
We have no credible way of estimating how many parasitic protozoa, fungi, bacteria, and viruses exist. We estimate that between 3% and 5% of parasitic helminths are threatened with extinction in the next 50 to 100 years.
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From page 64... ...
Recent studies of food webs suggest that ≈75% of the links in food webs involve a parasitic species; these links are vital for regulation of host abundance and potentially for reducing the impact of toxic pollutants. This implies that parasite extinctions may have unforeseen costs that impact the health and abundance of a large number of free-living species.
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From page 65... ...
Erwin's initial estimates suggested there might be as many as 30 million species of beetles in the world's tropical forests [considerably more than the 20,000 species initially estimated by John Ray (1627–1705) and cataloged by Linnaeus in Systema Naturae (Linnaeus, 1735)
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From page 66... ...
HOW MANY PARASITE SPECIES? Rohde (1982)
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From page 67... ...
. The issue of cryptic species will significantly distort estimates of global parasite species richness based on extrapolations from host specificity and mean numbers of parasites observed per host species.
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From page 68... ...
(43,945) Mean parasite species per host species Trematoda 0.12 2.04 1.27 1.06 3.24 1.61 Cestoda 2.71 1.57 0.27 0.39 3.67 1.89 Acanthocephala -- 1.01 0.19 0.42 0.72 0.28 Nematoda 0.48 1.49 2.82 2.15 3.32 3.90 Mean host specificity Trematoda 2.00 6.35 5.40 1.77 2.97 2.01 Cestoda 1.69 6.38 4.75 2.21 2.36 1.89 Acanthocephala -- 14.95 6.74 12.50 8.35 4.32 Nematoda 2.67 10.28 5.27 2.12 3.28 6.07 Estimated global species richness Trematoda 51 5,831 1,170 3,773 9,862 3,714 24,401 Cestoda 1,352 4,466 283 1,112 14,058 4,637 25,908 Acanthocephala -- 1,226 140 212 779 301 2,658 Nematoda 152 2,631 2,662 6,389 9,150 2,979 23,963 Total 1,555 14,154 4,225 11,486 33,849 11,631 76,930
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From page 69... ...
There are many more fish species in the tropics, so we might initially expect there to be more parasite species as well. But, if high host diversity in the tropics leads to low densities of each host species, then some host-specific parasites might be unable to maintain viable populations in their low-density tropical hosts, in which case host-specific parasites and their hosts could exhibit reverse gradients of species diversity.
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From page 70... ...
However, consideration of the trophic links of the parasitic species significantly changes our perception of how ecological food webs are structured. The standard ecological food web is normally considered to be a trophic pyramid, with primary producers on the bottom, fewer species of herbivores on the next level, and even fewer predatory species higher up (Lindeman, 1942)
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From page 71... ...
. We suspect that the food-web structure observed in salt-marsh communities is common to most natural ecological communities, with parasite species comprising ≈40% of the local species diversity but exerting significant stabilizing forces that hold together the structure of much of the free-living web.
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From page 72... ...
, 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)
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From page 73... ...
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.
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From page 74... ...
A major future challenge is to examine how the pattern of parasitic helminth diversity maps onto this pattern of host diversity. Our null expectation is that the two patterns should be concordant, but the high levels of host species diversity per order (and per family)
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From page 75... ...
If similar latitudinal patterns occur in avian orders and genera, and if parasites are responsible for driving significant components of sexual selection that lead to host speciation, then we might expect complex patterns of geographical variation in parasite diversity at the taxonomic level of host order and family. Unfortunately, the parasite data with which to test these hypotheses are unavailable.
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From page 76... ...
The dots illustrate number of avian species, lightest shading denotes range change due to climate change, and black illustrates land-use change due to agricultural expansion.
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From page 77... ...
Although a parasite species that can use a range of host species will not go extinct if one of its hosts species declines to extinction, it is likely that the abundance and geographical range of a parasite species will decline as each potential host species is lost or itself declines in range and abundance. This suggests that parasitic species will tend to decline at a faster rate than their hosts.
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From page 78... ...
. REGULATION OF HOST POPULATIONS AND RELATIVE ABUNDANCE IN COMMUNITIES Parasites create a diversity of links in food webs that at first sight may appear atypical, but they are not unusual in nature -- more than 75% of links in natural food webs probably involve parasites (Lafferty et al., 2006b)
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From page 79... ...
, infected hosts of species A cannot infect species B; the complementary case operates for the lower matrix, where both species of pathogen infect both species of host. The main consequence of host species sharing nonspecific parasite species is that several elements of the interactions matrix have to be converted (across the main diagonal)
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From page 80... ...
The trematode and acanthocephalan species that are recorded as adult worms from scores of vertebrate host species often depend entirely on a single species of mollusk or amphipod that serves as their intermediate host. Thus, snails or other invertebrates that invade natural ecosystems and replace crucial host species within the complex life cycles of parasites may lead to losses of parasite diversity that cascade throughout the food web.
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From page 81... ...
. If significant increases in extinction rates now apply to birds, mammals, amphibians, and fish, then it is almost inevitable that extinction rates in host-specific parasite species are increasing at least concomitantly.
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From page 82... ...
If the major job of conservation biologists is to maintain fully functional food webs, then it is crucial that we consider parasites as a vital and necessary component of biodiversity. It is then but a small step to acknowledge that these animals are well worth conserving.
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