Herbivory: the consumption of plant parts (generally leaves and roots) by animals

Transcription

1 Herbivory: the consumption of plant parts (generally leaves and roots) by animals >25% of all species on earth are herbivores >50% of all organisms are plant and herbivores, so their interactions have profound implications for ecological and evolutionary processes Herbivores: chewing insects (most abundant) Herbivores: sucking insects Herbivores: mining insects Herbivores: galling insects Herbivores: chewing vertebrates important in grasslands but less so in forests (e.g. biomass of bison equals biomass of grasshoppers) 1

2 Impact of herbivores on plants : ecological time scale 1. Reduces growth and reproduction of individual plants a) 10% of leaf area eaten per year (same as investments in reproduction) b) high variance in rates of herbivory between years, between different species of plants and between different plant parts (more later) Impact of herbivores on plants : ecological time scale 2. Changes competitive outcomes between plant species a) edible species are generally at a disadvantage (Utah bunch grasses) b) introduced species may have an advantage if they have left their herbivores behind (40% of California flora is introduced) c) biocontrol: to control introduced plants, their herbivores are also introduced (tamarisk in Utah, prickly pear cactus in Australia) 2

3 Prickly pear (Opuntia) was brought to Australia from S. America as a potted plant in 1800 s. By 1925, almost 100,000 sq. miles of eastern Australia was dense stands Top: 1926 dense stand of cactus Bottom: 1929 after attack by the introduced moth (Cactoblastis) Why is the world green? Why haven t the herbivores eaten up all the plants? Herbivores are kept below their K 1. due to predators 2. due to plant defenses 3

4 Plants are defended : physical defenses (spines, hairs, toughness) Plants are defended : phenology (timing) Young leaves are nutritious, tender and vulnerable to herbivores. Synchronous leaf production may satiate herbivores or occur at times when herbivores are at low abundance Temperate zone (winter) Tropical forests (aseasonal) 4

5 Plants are defended : mutualisms with protective ants Ants provide defense against herbivores in exchange for food (nectar) Plants are defended : mutualisms with protective ants In more specialized mutualisms, ants provide defense against herbivores and vines in exchange for food (nectar and protein) and housing Acacia with yellow food bodies, ants live in swollen thorns 5

6 Plants are defended : mutualisms with protective ants In more specialized mutualisms, ants provide defense against herbivores and vines in exchange for food (nectar and protein) and housing Cecropia provides purple food bodies and ants living inside hollow stems Plants are defended : mutualisms with protective ants In more specialized mutualisms, ants provide defense against herbivores and vines in exchange for food (nectar and protein) and housing Cecropia provides purple food bodies and ants living inside hollow stems Ants cut vines Tree without ants covered in vines 6

7 Plants are defended : mutualisms with protective ants Ants provide defense against herbivores in exchange for food Fly - seed predator Plants are defended : chemical defenses secondary metabolites because they are not used in primary metabolism but are synthesized for their defensive attributes Alkaloids: nitrogen containing compounds that affect nervous system of herbivores Examples: caffeine, nicotine, morphine, cocaine, novocaine, quinine 7

8 Plants are defended : chemical defenses Tannins: the most abundant and widespread class of secondary metabolites; they bind with proteins; can be 35% of dw of leaf tissue Plants are defended : chemical defenses Terpenes: aromatic compounds (give the smells of mints, pines etc) 8

9 Plants are defended : chemical defenses Cardiac glycosides: cause heart attacks in vertebrates. Monarch butterflies feeding on milkweed sequester the cardiac glycosides for their own defense Plants are defended : physical, phenological and chemical defenses Hairs, spines, toughness Synchronous leaf flushing Mutualisms with ants Tannins Terpenes Alkaloids Saponins Hormone analogs: ecdysone stops molting so caterpillars are crushed inside of their own exoskeleton Cyanogenic glycosides (clover) are cleaved into sugar and cyanide upon crushing of the leaf Toxic proteins and non-protein amino acids 9

10 Coevolution between herbivores and plants HO O H 2 N O O O OH HO O OH HO OH Coevolutionary arms race between plant and herbivore Time 1 # of plants # of herbivores Time 2 # of plants # of herbivores Nicotine concentration Nicotine tolerance Direction of selection 10

11 Chemical Arms Race (Co-evolution) Plants evolve novel defenses Herbivores evolve detox mechanisms Plants evolve novel defenses Selection favors increases in the amount of a compound or novel compounds such that the diversity and effectiveness of chemical defenses is increasing Chemical Arms Race (Co-evolution) H + H N - COO HO Summary: 1. Individual plants that have new or more effective defenses (with a genetic basis) will be more successful (higher fitness) => tendency for the defensive trait to increase in the plant population over time 2. Individual herbivores that have the genetic capability to cope with the newly evolving plant defenses will have higher fitness 3. Therefore plant defenses and herbivore detoxification ability are under continual reciprocal selection (= co-evolution ) 11