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 • 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.
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
Entangling Exploitation with Competition • Park found the presence/absence of a protozoan parasite (Adeline tribolii) influences competition in flour beetles (Tribolium). • 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.
Herbivorous Stream Insect and Its Algal Food • Lamberti and Resh studied influence of caddisfly (Helicopsyche borealis) on algal and bacterial populations on which it feeds. • Results suggest larvae reduce the abundance of their food supply.
Exploitation and Abundance • 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.
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.
Snowshoe Hares - Role of Food Supply • Live in boreal forests dominated by conifers. • Dense growth of understory shrubs. • 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. • 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 may also play a large role. • Predation can account for 60-98% of mortality during peak densities. • Complementary: • Hare populations increase, causing food supplies to decrease. Starvation and weight loss may lead to increased predation, all of which decrease hare populations.
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 / predation. • Nh = Number of hosts. • Np = Number of parasites / predators.
Population Cycles in Mathematical and Laboratory Models • Lotka Volterra assumes parasite/predator growth rate is determined by rate of conversion of food into offspring minus mortality rate of parasitoid population: dNp/dt = cpNhNp-dpNp • cpNhNp = Conversion rate of hosts into offspring. • pNhNp = Rate at which exploiters destroy hosts. • C = Conversion factor
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. • 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.
Refuges • Huffaker studied six-spotted mite Eotetranychus sexmaculatus and predatory mite Typhlodromus occidentalis. • Separated oranges and rubber balls with partial barriers to mite dispersal. • Typhlodromus crawls while Eotetranychus balloons. • Provision of small wooden posts to serve as launching pads maintained population oscillations spanning 6 months.
Protection in 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. • 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. • 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.