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Exploitation: Predation, Herbivory, Parasitism, and Disease

Chapter 14

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Introduction

• Exploitation: Interaction between populations that enhances fitness of one individual while reducing fitness of the exploited individual. Predators (herbivores or carnivores) kill

and consume other organisms. Parasites live on host tissue and reduce

host fitness, but do not generally kill the host.

Pathogens cause infectious disease.

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Overview:

• Prey defense adaptations

• Parasites Effects

• Predator-Prey Cycles

• Refuges and immigration for persistence.

• Predator Satiation

• Biocontrol

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Figure 53.5 Camouflage: Poor-will (left), lizard (right)

Camouflage

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Mullerian mimic (wasp) Batesian mimic (fly)

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“Predation” on plants - herbivory

• Physical defenses Thorns, silica

• Chemical defenses Toxins and tastes Alkaloids (very diverse) Polyphenolics (tannins)

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Parasites

• Parasites can have multiple developmental stages, each with its own host.

• Parasitoid: larvae develops in host (e.g. wasps)• Parasitism can be based in parasite behavior.• Parasite infection can modify host behavior for

the benefit of the parasite.• Parasites can change competition outcomes of

host and its competitor.• Parasites of predators can benefit prey.

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Schistosoma Life Cycle

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Figure 53.x2 Parasitic behavior: A female Nasonia vitripennis laying a clutch of eggs into the pupa of a blowfly (Phormia regina)

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Brood Parasitism

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Momma wood thrush has fallen for all babies being cute.

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Parasites That Alter Host Behavior

• Spring-Headed Worm (Acanthocephalans) changes behavior of amphipods in ways that make it more likely that infected amphipods will be eaten by a suitable vertebrate host. Infected amphipods swim

toward light, which is usually indicative of shallow water, and thus closer to predators.

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Parasites That Alter Host Behavior

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Parasites That Alter Host Behavior

• Rust fungus Puccinia monoica manipulates growth of host mustard plants (Arabis spp.). Puccinia infects Arabis rosettes and

invades actively dividing meristemic tissue. Rosettes rapidly elongate and become

topped by a cluster of bright yellow leaves.

Pseudo-flowers are fungal structures including sugar-containing spermatial fluids.

– Attract pollenators

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Parasites That Alter Host Behavior

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Entangling Exploitation with Competition

• Park found the presence/absence of a protozoan parasite (Adeline tribolii) influences competition in flour beetles (Tribolium).

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Entangling Exploitation with Competition

Adelina lives as an intercellular parasite. Reduces density of T. castaneum but

has little effect on T. confusum. T. castaneum is usually the strongest

competitor, but with the presence of Adelina, T. confusum becomes strongest competitor.

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Predation Influenced by Disease(Lindstrom et al., 1994)

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Cycles of Abundance in Snowshoe Hares and Their Predators

• Snowshoe Hares (Lepus americanus) and Lynx (Lynx canadensis). Extensive trapping records. Elton proposed abundance cycles driven

by variation in solar radiation. Keith suggested overpopulation theories:

Decimation by disease and parasitism. Physiological stress at high density. Starvation due to reduced food.

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Population Fluctuations

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Snowshoe Hares – Observational Evidence

• Food - in winter, browse on buds and stems of shrubs and saplings such as aspen and spruce. One population reduced food biomass

from 530 kg/ha in late Nov. to 160 kg/ha in late March.

• Predators -Lynx (Classic specialist predator) Coyotes may also play a large role.

Predation can account for 60-98% of mortality during peak densities.

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Snowshoe Hares – Experimental Evidence

• 3 control plots, 6 experimental plots Added food (2) or fertilizer (2) or removed

predators (2)

• Predator removal and food supplementation increased hare density

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Population Cycles in Mathematical and Laboratory Models

• Lotka Volterra assumes host population grows exponentially, and population size is limited by parasites, pathogens, and predators:

dNh/dt = rhNh – pNhNp

• rhNh = Exponential growth by host population. Opposed by:

p = rate of parasitism or predation. Nh = Number of hosts.

Np = Number of parasites or predators.

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Population Cycles in Mathematical and Laboratory Models

• Lotka Volterra assumes parasite/predator growth rate is determined by rate of conversion of food into host offspring minus mortality rate of parasitoid population:

dNp/dt = cpNhNp-dpNp

• cpNhNp = predator rate of production from exploited hosts.

• pNhNp = Rate at which predators consume hosts.

• c = Conversion factor (host to predator)• d = Death rate of predator.

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Model Behavior

• Host exponential growth often opposed by exploitation. Host reproduction immediately translated

into destruction by predator. Increased predation = more predators. More predators = higher exploitation rate. Larger predator population eventually

reduces host population, in turn reducing predator population.

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Model Behavior

• Reciprocal effects produce oscillations in two populations. Although the assumptions of eternal

oscillations and that neither host nor exploiter populations are subject to carrying capacities are unrealistic, L-V models made valuable contributions to the field.

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Wasp and Weevil

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Refuges

• To persist in the face of exploitation, hosts and prey need refuges.

• Gause attempted to produce population cycles with P. caudatum and Didinium nasutum. Didinium quickly consumed all Paramecium

and went extinct. (Both populations extinct) Added sediment for Paramecium refuge.

Few Paramecium survived after Didinium extinction.

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Refuges and Immigration

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Size As A Refuge

• If large individuals are ignored by predators, then large size may offer a form of refuge. Many predators are gape-limited – they

cannot open their mouths wide enough to capture prey larger than a certain size

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Predator Satiation by an Australian Tree

• Synchronous widespread seed and fruit production is known as masting. Janzen proposed that seed predation is a

major selective force favoring mast crop production.

O’Dowd and Gill determined synchronous seed dispersal by Eucalyptus reduces losses of seeds to ants.

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Predator Satiation by Periodical Cicadas

• Periodical cicadas Magicicada spp. emerge as adults every 13-17 years. Densities can approach 4x106 ind / ha.

• Williams estimated 1,063,000 cicadas emerged from 16 ha study site. 50% emerged during four consecutive

nights. Losses to birds was only 15% of

production. http://news.nationalgeographic.com/news/2004/03/0329_040329_cicadas.ht

ml#mainCicadas in PA

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Biocontrol of Unwanted Species

• Introduced Cactus and Herbivorous Moth Mid 1800’s:prickly pear cactus Opuntia

stricta was introduced to Australia. Established populations in the wild.

Government asked for assistance in control.

Moth Cactoblastis cactorum found to be effective predator.

– Reduced by 3 orders of magnitude in 2 years.

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Exploitation and Abundance

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Biocontrol

• Use of one or more beneficial organisms to control pests Classical – introduce predator from pest’s

original country Inoculative – release of natural enemies Innundative – Mass-releases of natural

enemies• Predators, parasitoids, pathogens,

herbivores• http://www.nysaes.cornell.edu/ent/biocontrol/