Exploitative Interactions: Predation, Herbivory, Parasitism, and Disease Chapter 14

Outline Introduction Complex Interactions Exploitation and Abundance Population Fluctuations Models Refuges Prey Density Size

Introduction Exploitation: Interaction between populations that enhances fitness of one individual while reducing fitness of the exploited individual. Predators kill and consume other organisms. Parasites live on host tissue and reduce host fitness, but do not generally kill the host. Parasitoid is an insect larva that consumes the host. Pathogens induce disease.

Parasites That Alter Host Behavior Spiny-headed worms (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 (positive phototaxis), which is usually indicative of shallow water, and thus closer to predators. Only when the worm reaches the appropriate stage in life.

Parasites That Alter Host Behavior In a terrestrial example, a spiny-headed worm infects a pill bug. Infected pill bugs leave shelter to wander out in the open where they are eaten by starlings.

Parasites That Alter Host Behavior Experiments showed that infected isopods were more likely to be eaten by starlings. Likely due to behavior.

Parasites That Alter Host Behavior Rust fungus Puccinia monoica manipulates growth of host mustard plants (Arabis spp.).

Parasites That Alter Host Behavior 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.

Parasites That Alter Host Behavior The combination of the yellow color and sugary fluids attracts pollinators. Carry rust spermatia (fungal reproductive cells) to other pseudo-flowers. Host plant generally dies. Check out this recent blog-post by Carl Zimmer on this subject!

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

Entangling Exploitation with Competition Adelina lives as an intracellular 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.

Exploitation and Abundance Predators, parasites, and pathogens influence the distribution, abundance, and structure of prey and host populations.

Herbivorous Stream Insect and Its Algal Food Lamberti and Resh studied influence of caddisfly larvae (Helicopsyche borealis) on algal and bacterial populations on which it feeds. Results suggest larvae reduce the abundance of their food supply.

Herbivorous Stream Insect and Its Algal Food In a follow up study, a set of tiles was raised off the stream bed in a way that prevented colonization of Helicopsyche, but not other invertebrates.

Herbivorous Stream Insect and Its Algal Food The results show that bacterial & algal populations were reduced on the streambed tiles as compared to the elevated tiles. Helicopsyche reduces populations of its food.

Introduced Cactus and Herbivorous Moth Mid 1800’s: prickly pear cactus Opuntia stricta was introduced to Australia. Established populations in the wild with no natural enemies. Government sought an insect herbivore to reduce the population. Moth Cactoblastis cactorum found to be effective predator. Also disperses pathogens Reduced by 3 orders of magnitude in 2 years. Equilibrium between the two.

A Pathogenic Parasite, a Predator, and Its Prey Foxes in Sweden infected with mange mites in 1975. Results in hair loss, skin deterioration, & death. Spread throughout Sweden in a decade. Population of foxes reduced by 70%.

A Pathogenic Parasite, a Predator, and Its Prey Ecologists studied the effects of population reduction of foxes on their prey. Prey species population sizes increased following the reduction of foxes.

Dynamics Predator-prey, host-parasite, and host-pathogen relations are dynamic. Temporal dynamics – populations of predators and prey are not static, they cycle in abundance over time.

Cycles of Abundance in Snowshoe Hares and Their Predators Snowshoe Hares (Lepus americanus) and Lynx (Lynx canadensis) both have extensive trapping records that allow us to study population sizes over the past 200 years. 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. Suggested long term studies.

Population Fluctuations The data show that lynx and hare populations fluctuate with a 10 year cycle.

Snowshoe Hares - Role of Food Supply Hares live in boreal forests dominated by conifers. Dense growth of understory shrubs. In winter, they 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.

Snowshoe Hares - Role of Food Supply Shoots produced after heavy browsing can increase levels of plant chemical defenses. Reducing usable food supplies.

Snowshoe Hares - Role of Predators Lynx (Classic specialist predator) Coyotes & other generalist predators may also play a large role. Predation can account for 60-98% of mortality during peak densities.

Snowshoe Hares - Role of Predators Complementary: Hare populations increase, causing food supplies to decrease. Starvation and weight loss may lead to increased predation, all of which decrease hare populations.

Experimental Test of Food and Predation Impacts A large-scale, long-term experiment was designed to sort out the impacts of food and predation on snowshoe hare population cycles. Populations of all three trophic levels need to be studied simultaneously.

Population Cycles in Mathematical and Laboratory Models Mathematical and laboratory models offer population ecologists the opportunity to manipulate variables that they cannot control in the field.

Population Cycles in Mathematical and Laboratory Models The Lotka-Volterra model assumes the host population grows exponentially, and population size is limited by parasites, pathogens, and predators.

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.

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.

Laboratory Models Utida found reciprocal interactions in adzuki bean weevils, Callosobruchus chinensis, over several generations. Gause found similar patterns in P. aurelia. Most laboratory experiments have failed in that most have led to the extinction of one population within a relatively short period.

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

Refuges Gause attempted to produce population cycles with Paramecium 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.

Refuges Huffaker studied six-spotted mite Eotetranychus sexmaculatus and predatory mite Typhlodromus occidentalis. Separated oranges and rubber balls with partial barriers to mite dispersal.

Refuges Typhlodromus (pred) crawls while Eotetranychus (prey) balloons. Provision of small wooden posts to serve as launching pads maintained population oscillations spanning 6 months.

Variety of Refuges - Space Spatial refuges – places where members of the exploited population have some protection from predators and parasitoids. Burrows Trees Air Water or land

Variety of Refuges - Numbers Living in a large group provides a “refuge.” Predator’s response to increased prey density: Prey consumed x Predators = Prey Consumed Predator Area Area Wide variety of organisms employ predator satiation defense. Prey can reduce individual probability of being eaten by living in dense populations.

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.

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

Predator Satiation by Periodical Cicadas 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.

Size As A Refuge If large individuals are ignored by predators, then large size may offer a form of refuge.

Size As A Refuge Large mussels are eaten infrequently by sea stars. If mussels can avoid predation long enough to reach 10-12 cm, it will be immune from most sea stars.

Size As A Refuge Peckarsky observed mayflies (Family Ephenerellidae) making themselves look larger in the face of foraging stoneflies. In terms of optimal foraging theory, large size equates to lower profitability.