Chapter 12 Parasitism © 2002 by Prentice Hall, Inc. Upper Saddle River, NJ 07458

Outline Parasites feed on a host, but generally do not kill it Hosts have evolved many defenses (e.g., immune responses) against parasites Models show that the rate of spread of diseases is govern by the density of susceptibles in the population, the transmission rate of the disease and the length of life of the infected host

Outline Parasites can substantially decrease host population size Parasites can affect the structure of host communities Parasitoids help in biological control by reducing the density of pests

Defining Parasites Parasite: a predatory organism that feeds off another but generally does not kill it Host: prey of a parasite Parasitoid: Cases where the host does not survive but one host is insufficient for the development of the parasitoid

Defining Parasites Some parasites live with their host most of their lives (e.g., tapeworms) Some parasites drop off after prolonged periods of feeding (e.g., ticks, leeches)

Defining Parasites Are mosquitoes and Wilde beasts parasites? Some parasites are parasitic on other plants –Holoparasites: lack chlorophyll, and are totally dependent on another plant for water and nutrients –Hemiparasites: photosynthesize, but do not have a root system, so they rely on the host for this function Ex. mistletoe

Defining Parasites Monophagous: parasites that feed off one to three closely related species Polyphagous: parasites that feed off many host species

Defining Parasites Ectoparasites: live on the outside of the host's body (e.g., fleas and ticks) Endoparasites: live inside the host's body (e.g., tapeworms and bacteria)

Defining Parasites Haustoria: plant parasite outgrowths that penetrate inside a host plant to tap into it's nutrient supply Use of multiple hosts: fluke 肝蛭 (Figure 12.3) Adult flukes produce eggs inside a cow. The eggs are passed in the cow’s feces. Adult lancet fluke Life cycle of lancet fluke, Dicrocoelium Dendriticum Ants eat the “slime balls.” Some of the flukes migrate into the ant’s brain, causing it to climb to the tip of a blade of grass where it can be eaten by a cow. Snails eat the fluke eggs; later the eggs hatch in the snail’s intestine. The eggs hatch and asexually produce offspring. The offspring are passed from the snail in “slime balls.”

Defining Parasites Parasites outnumber free-living species 4 to 1 (Figure 12.4) Fish Birds Mammals True bugs Beetles Flies Wasps Butterflies and moths Trees (95) Average number of parasite species per host

Defense Against Parasites Cellular defense reactions –Eggs of parasatoids are rendered inviable by encapsulating them Immune responses in vertebrates –Phagocytes may engulf and digest small alien bodies, and encapsulate and isolate larger ones –Hosts may develop a "'memory,"' that may make then immune to reinfection

Defense Against Parasites Defensive displays or maneuvers –Actions intended to deter parasites Grooming and preening behavior –Behavior intended to remove parasites

Modeling Parasitism Differ from models of predation and herbivory –Life cycles of many parasites involves intermediate hosts –Models of parasite population dynamics generally describe the population growth rate by the average number of new disease cases

Modeling Parasitism For microparasites, the number of infected hosts is the most important factor Rp = NBL –Rp = number of infected hosts, with p for parasite and R for net reproductive rate Transmission threshold; Rp = 1 For disease to spread; Rp > 1 For disease to die out; Rp < 1 Microparasites are transferred from host to host –N = density of susceptible hosts in population –B = transmission rate of disease –L = average period over which the infected host remains infectious

Modeling Parasitism –Generalizations (cont.). As L increases so does Rp If diseases are highly infectious, Rp increases Large populations of susceptible hosts promotes the spread of diseases

Modeling Parasitism Critical threshold NT where Rp = 1 –NT is an estimate of the number of susceptible hosts needed to maintain the parasite population at a constant size –NT = 1 / BL »If B is large, N is small »If B or L are small, the disease can only persist only in a large population

Number of cases per 3-month interval (thousands) Year Modeling Parasitism Many diseases undergo periodic cycles –Ex. Measles 麻疹 in England (Figure 12.5) Peaks occur because host immunity is developed New births lead to new susceptible hosts, and cycle repeats

Modeling Parasitism Parasites spread by a vector –Lifecycle of both parasite and vector become important in controlling diseases –Ex. Farmers use insecticides to kill aphids, which transmit viral diseases to crops (rather than chemicals to kill the parasite) –Ex. Yellow fever was eradicated in the US by inoculation rather than eradication of all mosquitoes

Parasites Affect Host Populations Using biological control to study the effects of parasites on hosts –Ex. Hawkins 1999: Biological control of pests, especially by parasitoids, was greater in exotic, simplified, managed habitats than in natural habitats –Control is most often exerted by a single parasitoid species, in contrast to natural systems, which require a suite of generalized enemies –Thus, biological control projects can not providerigorous evidence of the importance of parasites in natural systems

Parasites Affect Host Populations Effects of introduced parasites on natural systems –Chestnut blight in the Appalachian Mountains of North America Virtually eliminated chestnut tree (Figure 12.6) Year Density of stems per hectare

Parasites Affect Host Populations –Chestnut blight in the Appalachian Mountains of North America Introduced in New York in 1904 In Britain, 25 million elm trees (out of 30 million) were wiped out by the disease between 1960s and the 1990s (Figure 12.7)

Parasites Affect Host Populations –Rinderpest, A virus with at least 47 natural artiodactyls hosts, most of which occur in Africa The virus belongs to a class known asmorbilliviruses, which includes measles and distemper Spread by food and water contaminated by dung of sick animals Can be fatal to certain animals (buffalo, eland, kudu, and warthogs)

Parasites Affect Host Populations –Rinderpest, (cont.). Major epidemic swept through Africa in the 1890s, leaving vast areas uninhabited by certain species 80% of hoofed stock died. Disease traveled 5,000 km in eight years Brought under control in the 1960s, through the use of cattle vaccinations –Endangered species

Parasites Affect Host Populations –Endangered species 1. Many endangered species are threatened by diseases from domestic animals (Table 12.2)

Parasites Affect Host Populations –Endangered species (cont.). Ex. The demise of the marsupial wolf in Tasmania was because of a distemper-like disease obtained from domestic dogs Some endangered species have been given vaccinations to protect them from disease –Mountain gorillas were vaccinated for measles

Parasites Affect Host Populations Natural systems –Massive mortality of big horn sheep from infection by lungworms (Protostrongylus stilesi and P. rushi ) Predisposes animals to pathogens, which cause pneumonia

Parasites Affect Host Populations –Massive mortality of big horn sheep from infection by lungworms (Protostrongylus stilesi and P. rushi ) (cont.). Infection rates of 91% and mortalities of 50-75% have been reported

Parasites Affect Host Populations –Colorado pine tree plantations and mistletoe. Mistletoe can cause 30% loss in extractable timber –Saline marshes in North America and the plant parasite, Cuscuta salina (Figure 12.8a)

Parasites Affect Host Populations –Saline marshes in North America and the plant parasite, Cuscuta salina (cont.) Infects the most common plant in California marshes, Salicorniavirginica thus promoting the growth of two other species, Limonium and Frankenia (Figure 12.8b) Uninfected Infected With Cuscuta SalicorniaLimoniumFrankenia (b) Plant mass (g)

Parasites Affect Host Populations – Parasite removal experiments Fuller and Blaustein (1996) compared the survivorship of parasite infected and uninfected free-living deer mice –Conducted in large outdoor enclosures –Decreased over-winter survivorship for those deer mice infected with the protozoan Eimeria arizonensis –Contamination spread through the digestion of contaminated feces

Parasites Affect Host Populations Hurtrez-Bousses et al. (1997) reduced the number of blowfly larvae parasites in young blue tits in Corsica –Blowfly larvae suck blood from chicks, causing anemia and high mortality – Removal was accomplished by removing nests from nest boxes, and microwaving the nests to kill the parasites, and then returning the nests and chicks

Parasites Affect Host Populations Hurtrez-Bousses et al. (1997) reduced the number of blowfly larvae parasites in young blue tits in Corsica (cont.). –Chicks from microwaved nests were found to have greater body weight at fledging (Figure 12.9) Mass at fledging (g) % nest failure

Parasites Affect Host Populations Stiling and Rossi (1997) manipulated parasitic infection levels of a gall- making fly on a coastal plant, Borrichia frutescens, on isolated islands off the coast of Florida –Low rates of parasitism treatment »Allowed potted plants on one island to be colonized by gallflies

Parasites Affect Host Populations –Low rates of parasitism treatment (cont.). »Allowed potted plants on one island to be colonized by gallflies »Plants were removed before parasitoids could find them – High rates of parasitism treatment »They left plants on the island longer, to allow the parasites to colonize the galls

Parasites Affect Host Populations –Results: High degree of parasitism of gallflies resulted in a significant reduction in the number of new galls (Figure 12.10) May JuneJulyAugSeptOctNov 1995 MayJuneJulyAugSeptOctNov 1995 High parasitoids Low parasitoids Galls per 200 terminals (a) (b) Percentage parasitism

Parasites Affect Communities Parasites affect the presence or absence of various species in a community

Parasites Affect Communities The meningeal brainworm Parelaphostrongylus tenuis –Usual host is the white-tailed deer, which is tolerant of the infection

Parasites Affect Communities The meningeal brainworm Parelaphostrongylus tenuis (cont.). –All other cervids and the pronghorn antelope are potential hosts

Parasites Affect Communities –All other cervids and the pronghorn antelope are potential hosts (cont.). Worm causes severe neurological damage

Parasites Affect Communities –The worm makes the white-tailed deer a potential competitor with other cervids, because they can not survive in the same area as the white-tailed deer. This phenomenon is known as apparent competition

Parasites and Biological Control Not all parasites are detrimental to humans Many are used to protect crops from pests: Biological control

Parasites and Biological Control Many are used to protect crops from pests: Biological control (cont.). –Only 16% of classical biological control would qualify as economic successes

Parasites and Biological Control Many are used to protect crops(cont.). –Organisms used in biological control, are released in a 'hit or miss' technique: Just release a bunch of parasites and predators, and hope that one of them does the job.

Parasites and Biological Control Many are used to protect crops(cont.). – New techniques: Ex. novel parasite-host associations

Parasites and Biological Control Many are used to protect crops(cont.). –Review of 548 control projects: the more parasites that were releaseds, the lower the rate of establishment

Parasites and Biological Control Necessary attributes of a good agent of biological control (Huffaker and Kennett 1969) –General adaptability to the environment and host –High search capacity

Parasites and Biological Control Necessary attributes of a good agent of biological control cont.) –High rate of increase relative to the host's –General mobility adequate for dispersal

Parasites and Biological Control Necessary attributes of a good agent of biological control cont.) – Minimal time lag effects in responding to changes in host numbers

Parasites and Biological Control Methods affecting the success of biological control (Stiling 1990) –Factor of greatest importance: climatic match between the control agent's locality of origin and the region in which it will be released

Parasites and Biological Control Methods affecting the success of biological control (Stiling 1990)(cont.). –Importance of climatic variation was underscored by another review of biological failures (Stiling 1993)

Parasites and Biological Control –Importance of climatic variation was underscored by another review of biological failures (Stiling 1993)(cont.). Climate accounted for 34.5 % of the failures (Figure 12.11) Death by natural enemies Lack of alternative hosts Prey has refuge Wrong strain of host Wrong climate Other reasons % reasons in failure in biocontrol

Parasites and Biological Control Risks of biological control –Reduction in native Hawaiian lepidopterans was partly due to wasp species introduced for biological control of lepidopteran crop pests

Parasites and Biological Control Risks of biological control –Reduction in native Hawaiian lepidopterans (cont.) Wasps attacked target exotic species but also non-target native species

Parasites and Biological Control –Reduction in native Hawaiian lepidopterans (cont.) Stresses the importance of more narrowly focused release, rather than the traditional "hit and miss" technique

Parasites and Biological Control Risks of biological control –Problems with crops While it is of interest to insure that the agent does not adversely affect the crop, however, non-crop plants are not as vigorously tested

Applied Ecology Importance of parasites to plants and animals Five main categories of disease-causing organisms that most plants and animals are susceptible to (Table 1)

Applied Ecology Leading causes of human death by disease, worldwide (Table 2)

Summary The true definition of parasite is problematic. Parasites may include many species that feed on plants, plus more traditional parasites such as tapeworms, leeches, bacteria, viruses and parasitoids that attack animals. 80% of all life forms are considered parasitic

Summary The presence of various parasite defense mechanisms is testament to the importance of parasitism in nature

Summary Mathematical models suggest that effective parasites will keep their hosts alive as long as possible, thereby facilitating the transmission of parasites to additional hosts.

Summary The huge influence of introduced diseases, such as chestnut blight and Dutch elm disease, provide evidence for the severe effects that parasites can have on a host's population and host density

Summary Parasites of insects can often be used as control mechanisms against crop and forest pests. This technique is termed biological control. Finding the attributes of successful biological control agents is valuable. Unfortunately, biological control agents can have a significant impact on non-target natural populations

Discussion Question #1 Discuss some of the main differences between microparasites, macroparasites, parasitoids, predators, and herbivores in terms of their life history strategies.

Discussion Question #2 Why can we eradicate some diseases, such as yellow fever, through vaccinations, while we have not been able to eradicate diseases such as malaria?

Discussion Question #3 Can we ever expect chemical pesticides to be replaced entirely by biological control agents? Explain.

Discussion Question #4 Parasites are usually very small organisms, capable of passing through screen enclosures. With this in mind, how might you design an experiment to remove parasites from a population and compare that population's survival rates with those of an unmanipulated control population?

Discussion Question #5 Do you think that biological control would be more successful on islands or on mainland continental areas? Against native or exotic pests?