Transcript Predation
Predation Great White Shark and Fur Seal Predator-Prey Interactions Food Preferences • A preference exists if the proportion of a type of food is higher in the animal’s diet than it is in its proportion in the environment • Ranked preference – predator preferentially eats prey which is most valuable (usually in terms of calories) • Balanced preference – predator eats prey items that provide integral parts of a balanced and mixed diet (usually in terms of nutrients and vitamins) Pied Wagtail Caribou feeding in winter Preferences • Fixed preference – predator eats certain items regardless of what else is available – can be fixed in terms of species consumed or energy consumed Edible mussel – Mytilus edulis Shore crab and edible mussels Backswimmers Asellus aquaticus – prey for backswimmers Guppy as predator Guppy prey – tubificid worms and fly larvae Eurasian oystercatcher Search image – this image has dashes arranged as L, as T, and plus signs – can you find the Ts? Bluegill sunfish Effects on populations • Population regulation refers to the tendency of a population to decrease in size when above a particular level, and to increase in size when below that level. Population regulation can only occur as a result of one or more density dependent processes acting on birth or death rates. • Population abundance is determined by the combined effects of all factors and processes that influence population size, whether they are density dependent or density independent. Predator effect on individual prey Mink Muskrat Arctic Ground Squirrel – Predator population is self-limited Red Grouse in Heather – Predator population is self-limited Predator Switching Regulates Prey Population Bank vole Tawny Owl Cinnabar Moth and Caterpillar on Ragwort Tansy Snowshoe hare and Lynx Lynx Ruffed Grouse Snowshoe hare Sea Otter Sea Urchin Kelp Forest Sea Otter eating Sea Urchin in Kelp Forest Comparison of kelp and urchin biomass with and without sea otters Sea Urchin Barren Kelp forest ecsystems with and without sea otters Plant Resource Defense • Qualitative defense - highly toxic substances, small doses of which can kill predators • high nutrient environment/fast growth (high turnover in plants) - use toxins (plant secondary compounds) that often require N, expensive to make (must be replaced often), but can be made rapidly - cyanide compounds, cardiac glycosides, alkaloids - small molecules Plant Resource Defense • Quantitative defense - substances that gradually build up inside an herbivore as it eats and prevent digestion of food • low nutrient environment/slow growth (low turnover in plants) - primarily use carbon structures - wood, cellulose, lignin, tannins large molecules - makes plant hard or unpleasant to eat (woodiness, silica), but plants are slow to make these defenses Evolutionary “Arms” Races Monarch and milkweed Evolutionary “Arms” Races Evolutionary “Arms” Races California garter snake Pacific newt Other Plant Defenses Include: • mechanical defenses - plant thorns and spines deter many vertebrate herbivores, but may not help much against invertebrate herbivores • failure to attract predators - plants somehow avoid making chemicals which attract predators • reproductive inhibition - some plants such as firs (Abies) have insect hormone derivatives which if digested, prevent successful metamorphosis of insect juveniles • masting - the synchronous production of very large numbers of progeny (seeds) by trees of one species in certain years Eurasian Jay with Acorn Masting Masting Fagus sylvaticus – European Beech Dipterocarp distribution Dipterocarp trees Beech seeds and boring moth Lyme’s disease life cycle Masting and Human Health - Lyme’s Disease Induced Defenses • Another aspect of plant defenses is that plants do not always have tissues loaded with defensive chemicals - in many plants, defensive chemicals are only produced when they are needed, usually after the plant has experienced some herbivory - this is an induced defense Impact of Herbivores Is Not Uniformly Experienced Aphids attacking Alfalfa Spotted Alfalfa Aphid Induced defenses in Birch Trees Induced defenses in Birch Trees Induced defenses in Birch Trees Rubus prickles Acacia depanolobium Plant defenses are developed at a cost to fitness when: 1. Organisms evolve more defenses if they are exposed to much damage and fewer defenses if cost of defense is high 2. More defenses are allocated within an organism to valuable tissues that are at risk 3. Defense mechanisms are reduced when enemies are absent and increased when plants are attacked mostly true for chemicals not structures 4. Defense mechanisms are costly and cannot be maintained if plants are severely stressed by environmental factors