Chapter 4 Ecosystems: How they change
Yellowstone fires 1988 • Started Naturally - burned like crazy • Burned from May/June till September • Recovery was quicker than expected.
Exponential Growth • Constant percent rate of growth • Increasing numbers of growth • Doubling time stays steady
Population change • Most Populations approximate a S-curve • Some become J-curves • 1) repeated • 2) stabilize into S curve
Carrying Capacity • The maximum in a particular population that can be sustained in an environment • A realized S-curve tends to fluctuate around the carrying capacity
Density Dependent • These are limiting factors that increase their resistance the higher the density gets • Water • Food • Territory • disease
Density Independent • Do not depend on the density of a population • Floods • Fires • Temperature extremes
Critical Number • Below a certain # some populations experience an extreme change in survival • Flocks, packs, etc…. • Threatened: rapid population decline • Endangered: near or below critical number
Two problems • Pop grow too fast • J-curve boom and bust • Population decreases • Approaches critical number and is threatened or endangered
Top-Down v. Bottom-up • Top-Down population regulation • Control of a prey species by its predator: Lions eat zebras • Bottom-up • Control of a species by a limiting factor or resource - often a food supply, especially with primary consumers
How come all of the Moose did not get eaten? • Wolves cannot take down the large and healthy moose • As we’ll see, herd thinning can be good for the moose, as well.
Parasites • “A parasite can work in conjunction with a predator to control a given herbivore population: Parasitic infection breaks out in a dense population of herbivores; individuals weakened by infection are more easily removed by predators, leaving a smaller, but healthier, population (p 87).”
Plant-Herbivore dynamic • Overgrazing: St. Matthew Island • 29 Islands introduced in 1944 with no predators
Keystone species • A species with a crucial role in maintaining ecosystem biotic structure • Ex. Seastars eat mussels, when removed in a study the mussels were able to decrease the diversity by eating everything.
Competition • 2 kinds • Interspecifc: between two different species • Intraspecific between members of the same species
Intraspecific competition Territoriality For food, mates, limits mating, but can encourage dispersal Self Thinning causes growth to be spread out. Can happen to animals too Effects Population regulation, natural selection
Interspecific Competition Consider Plant Distribution in riparian woodlands Sycamore and red maple along riverbanks Oaks and pines and higher elevations OR - Plants might get nutrients from different depths Bottom line is:drives diversity in communities
Interspecific Animals • Competitive Exclusion principle • Consider: Paramecia in flasks • Resource partitioning • Barnacle ranges in tidal regions • Bottom line: elimination of competitive species and driving of natural selection
Introduced Species • Rabbits: • Introduced to Australia in 1859. • No Natural enemies led to overgrazing • Controll has been attempted with a virus, but adaptation occurred.
Introduced Species • American Chestnut • Chestnut blight accidentally introduced with some chinese chestnut trees • American chestnut almost decimated • Ecologically oak filled the niche, but much economic damage
Introduced Species • Pests • Agricultural - japanese beetles, fire ants, gypsy moths • Cats can help with rodents, but also remove songbirds • Bottom line - Globalization is increasing introduction
Evolution • Selective pressure - includes all factors of environmental resistance • Reminder (p84): • Lack of food, water, habitat, • Adverse weather • Predators, disease, parasites, competition
Adaptations to the environment • Dealing with the climate • Obtaining food or water • Avoiding becoming food • Finding or attracting mates • Migration/dispersal
Limits of change • When the environment changes • Adapt - Different than human response of the same name • Migration • Extinction (can be local) • MAD • Move, adapt, or die
Keys to Survival • 1) Geographic distribution: more widespread=more likely to survive (as a species) • 2) Specialization: Koala v. Crow • 3) Genetic variation in the gene pool: affects ability to respond to change • 4) Reproductive rate Mammals v. insects
Evolution of Species • New Species results from • Mutation - the “raw material” of natural selection • Reproductive Isolation • Natural Selection/Genetic Drift
Ecosystems Disturbed • Ecological Succession • Primary: On bare rock - Lichen, moss, bigger, bigger • Secondary: after a disturbance such as fire, humans, flooding, hurricane - Already has a nutrient layer and usually seeds and nearby animals
Landscapes and disturbance • Disturbance encourages diversity • Disturbance is often localized and uneven
Fire Climax Ecosystems • Fire now recognized as a natural part of many ecosystems cycle of disturbance and renewal • Thins underbrush • Results in less damaging fires • Renews soil • Germinates seeds (Like lodgepole pine)
Humans in this context • Ecosystem Management • Integrated ecosystem view • Revised as new research is available • Incorporates human element • Has goal of sustainibility
Human stress on ecosystems • Big list on page 110 • We all depend on nature • Humans cause change - may improve human lives at expense of ecosystems: soil, air, water, biodiversity, food supply • Pressures likely to increase • Technology may be part of the solution