SGCEP SCIE 1121 Environmental Science Spring 2012 Section 20531 Steve Thompson: steventhompson@sgc.edu http://www.bioinfo4u.net/ 1
Populations, communities, Remember... and dynamics Population: a group of members of the same species living in an area, and... Community: populations of different species living together in an area. Let s start with how populations change in their number of individuals. 2
Population growth Some real, real easy math... Populations grow with births and immigration; They decline with deaths and emigration: Therefore... (Births + Immigration) (Deaths + Emigration) = Change in population number (positive or negative) Equilibrium means: (Births + Immigration) = (Deaths + Emigration) However, population growth is often not equal to zero. Population growth rate is the amount the population has changed divided by the time it had to make those changes. 3
Exponential growth Species can increase their population exponentially under favorable conditions. Exponential increase: 2, 4, 8, 16, 32, 64... The doubling time remains constant. For example, if it takes two days to go from 8 to 16 individuals, it would still take two days to go from 1,000 to 2,000 individuals. Such growth is called explosive, and... graphs out as a J-shaped curve. Population growth curves show how populations grow (or shrink), and are used to find things like: How fast a population could grow; How many individuals there are now; What the future population size could be;... 4
Population growth curves And some example scenarios... Exponential growth i.e. Disease can do this too! This one is logistic. 5
The equation s not that bad. The per capita rate of increase (or decrease) of a population is called r. G is the number of individuals added per unit time. And... N is the number of individuals at the start of a given time interval. Therefore,... G = rn This is exponential growth. 6
Outcomes of population increase Exponential growth often results from unusual disturbances, such as the... Introduction of a foreign species, a suddenly changed habitat, the elimination of a predator, arrival in a new habitat, etc. So the population continues to grow and then it crashes and dies off. Versus, more commonly... Logistic growth: some process slows growth so it levels off near carrying capacity. This... Results in an S-shaped curve, which... Levels off at K (the carrying capacity). As the population approaches K, growth slows. The population remains steady and growth = 0 The maximum rate of population growth occurs halfway to K. 7
Those processes are called environmental resistance. Abiotic and biotic factors cause mortality (death) Prevents unlimited population growth. Environmental resistance: the biotic and abiotic factors that may limit a population s increase. Biotic: predators, parasites, competitors, lack of food. Abiotic: unusual temperatures, moisture, light, salinity, ph, lack of nutrients, fire. Environmental resistance can also lower reproduction, which slows population growth... Loss of suitable habitat, pollution; Changed migratory habits of animals. 8
Reproductive strategies The interplay of environmental resistance and biotic potential (capability to reproduce) drives the success of two reproductive strategies: r-strategists (population growth rate-selected species): produce lots of young, but leave their survival to nature. They usually have... Rapid reproduction, rapid movement, short life spans, small bodies; and are... Adapted to a rapidly changing environment. They have Boom-and-bust populations. Often called weedy or opportunistic. An example is the housefly. 9
Versus.... K-strategists K-strategists (carrying capacity-selected species) have a much lower reproductive capacity. They... Care for and protect their young; Live in a stable environment already populated by the species; Are... Larger, longer lived, and well-adapted to normal environmental fluctuations. Their populations fluctuate around carrying capacity. They are... Also called equilibrial species. For example, elephant, California condor. 10
Life histories go right along. Life history: the progression of changes in an organism s life. This includes... Age at first reproduction, length of life, etc. Can be visualized in a survivorship graph. Type I survivorship: low mortality in early life. Most live the bulk of their life span (e.g., humans). Type III survivorship: many offspring that die young. Few live to the end of their life (e.g. oysters, dandelions). Type II survivorship: intermediate survivorship pattern (e.g. squirrels, many birds, coral). K-strategists usually have a Type I pattern; r- strategists show Type III. All are extremes. 11
Survivorship curves More K- selected like More r- selected like 12
Predictable pattern in species There is a predictable pattern to the way human activities affect species. r-strategists become pests when humans change an area. For example... Houseflies, dandelions, cockroaches increase. K-strategists become rarer or extinct with change. For example... Eagles, bears, and oaks decline. An exception: rare opportunistic species (rselected) so separated from new habitat... That they cannot succeed, despite high biotic potential. 13
Why does a population grow? A population s size depends on the interplay between its biotic potential and the environmental resistance it encounters. A population s biotic potential remains constant. But... Environmental resistance changes. Population balance is a dynamic balance Additions (births, immigration) and subtractions (death, emigration) occur continually. The population may fluctuate widely or very little. A population is at equilibrium when populations restore their numbers and the ecosystem s capacity is not exceeded. 14
Limits on populations The carrying capacity may not explain what limits a population. Population density: number of individuals per unit area. The higher the density, the more likely a factor (e.g., crowding or disease) affects the population. Some factors (e.g., a tornado) keep a population from increasing, but have nothing to do with the density of the organisms. 15
Density dependence and independence Density-dependent factor: increases with increased population density. Includes... Predation, disease, food shortage, etc. Logistic growth occurs when populations become more crowded (approach carrying capacity). Density-independent factor: one whose effects are independent of the density of the population. Spring freeze, flood, earthquake, fire, etc. Is not involved in maintaining population equilibrium in the logistic growth. 16
Regulating a population Only density-dependent factors can regulate a population (keep it in equilibrium). Top-down regulation: control of a population (species) by predation. Bottom-up regulation: control of a population occurs as a result of scarcity of a resource (food). Factors controlling a population determine the effects on an ecosystem of adding or removing a species. Removal of a species can affect species that don t directly interact with it. 17
Critical number Critical number: the minimum population base allowing the survival and recovery of a population. For instance,... A pack of wolves, flock of birds, school of fish, etc. The group is necessary to provide protection and support to all members. If a population falls below this number: Surviving members become more vulnerable; Breeding fails; and... Extinction is almost inevitable. 18
Community interactions Relationships between species may be positive (helpful), negative (harmful), or neutral for each species. Predation: one member benefits, the other is harmed; Includes parasitism, herbivory. Competition: both species are harmed. Either... Interspecific competition: between different species; or... Intraspecific competition: between the same species. Mutualism: both species benefit. Commensalism: One species benefits, the other is not affected at all. 19
Major types of interactions between species 20
Predation Predator: the organism that does the feeding. Prey: the organism that is fed upon. Predator-prey interaction: carnivores (meat eaters) eat herbivores (plant eaters). Herbivores feed on plants. Parasites feed on hosts (a type of predation). Parasite: an organism (plant or animal) that feeds on its prey, usually without killing it. Host: the organism that is being fed upon. 21
Parasitic organisms... Include tapeworms, disease-causing bacteria, viruses, fungi. Parasites affect host populations in a densitydependent way. Increased population density makes it easier for parasites and their vectors (carriers of the parasite) to find new hosts. Pathogens: bacteria and viruses that cause disease. No real ecological difference from other parasites. They are just... Highly specialized parasites. 22
Regulation of prey Predators can regulate herbivore numbers (top-down control). For example... Moose crossed to Isle Royale, in Lake Superior. Several years later, wolves also reached the island. Without wolves, the moose population grew rapidly. Low environmental resistance for the moose. The wolf population also increased and preyed on moose. Low environmental resistance for the wolves. Fewer moose (high environmental resistance) resulted in fewer wolves (high environmental resistance). 23
Predator-prey relationships 24
Other factors affect populations Wolf predation wasn t the only factor affecting the moose. Other things mattered... Deep snow limited access to food; A tick infestation caused significant mortality. A sharp decline in moose popuations kept wolf populations low because there were... Not enough calves to catch. Wolves can t catch a mature moose in good condition. Predator-prey relationships involve top-down (on the prey) and bottom-up (on the predator) regulation. Parasites weaken hosts, making them more vulnerable to predation. 25
Overgrazing If herbivores eat plants faster than they can grow, plants are depleted and animals eventually suffer. Reindeer were brought to St. Matthew Island. At first, they were healthy and wellnourished. They ate up the lichens. A few years later, they were malnourished. With little food and harsh weather, almost the entire herd starved. No population can escape the ultimate limitations set by environmental resistance. 26
Plant-herbivore interactions 27
Predator removal Eliminating predators upsets plantherbivore relationships. White-tailed deer in the U.S. were originally controlled by wolves, mountain lion, bears. But these were killed because they were thought to threaten livestock and humans. Deer populations can get so large they overgraze the area. Humans control numbers through hunting. 28
Keystone species Removal of one species can create a cascade of effects... Impacting far more than just the other species they interact with. For example... Sea stars eat mussels in rocky intertidal zones. Removing sea stars allows mussels to crowd out all other species, reducing diversity. Keystone species: play a crucial role in maintaining ecosystem biotic structure. They... Moderate other species that would take over; thereby allowing other, less-competitive species to flourish. 29
Competition is... Interactions where both species are harmed. They compete for a scarce resource. Species that compete have overlapping niches. Over time, there is pressure to reduce the overlap. 30
Competition can be... Intraspecific competition: competition between members of the same species. This... Occurs over resources. And is all about... Territory: an area defended by an individual or group. Is vigorously defended. Most defense is intimidation serious fights are rare. Organisms fight to protect an area for nesting, establishing a harem, or food resources. Lack of territories: a density-dependent limitation on a population. 31
Territoriality as an advantage It protects a population from the possibility of everyone getting some resources, but nobody getting enough to survive. Without territoriality, when resources are scarce... Every encounter could end in a potentially lethal fight. Plus... All members would get only a part of what they need and could die. Competition lowers fitness and production of offspring. However,... Territoriality lowers the direct effects of competition. 32
Not everyone gets a territory. Individuals unable to claim a territory... Are often young and may obtain a territory later. They... May disperse, opening up new habitats to the species. Or they... May die. Territoriality is most likely in K- strategists. It is... An adaptation that helps organisms disperse and stabilizes populations. 33
Intraspecific competition s impact Short-term impact on a population: it leads to density-dependent regulation of a population... Through territoriality, and... Through self-thinning: crowded organisms (e.g., trees) become less numerous as they get bigger. Long-term impact on a population: it leads to long-term changes as the species adapts to its environment: Those better able to compete, survive, and reproduce. And... Their superior traits are passed on to successive generations. 34
Interspecific competition Competitive exclusion principle: species cannot survive competition if they compete directly in many respects. Especially... In simple habitats with species needing the same resources. But species do occupy the same area without becoming extinct. How is this? It s because abiotic and biotic conditions in an environment vary in space and time. Adaptations of species to specific conditions allows it to thrive and overcome its competitors in one location or time, but not in another. 35
Environments are heterogeneous. Variable environments support species with different niches. Competitors in the same habitat usually have different niches. For instance... Woodpeckers eat insects; but other birds may be specialized to eat seeds. Space can also be a resource. Resource partitioning: the division of a resource and specialization in different parts of it. With more intense competition, resources are even further divided. 36
For example: resource partitioning in warblers each bird has a preferred feeding zone. 37
Mutualism An arrangement between two species where both benefit. Examples of mutualism: Pollinators (e.g., bees) receive nutrition while plants receive pollination; Fungi on roots (mycelium): the fungus gets nutrition, the plant gets easier intake of soil nutrients. Lichens are made of a fungus and an algae. The anemone fish protects the anemone from predation by the butterfly fish, and the anemone protects the fish. 38
Commensalism 39 Commensalism: one species benefits; the other is unaffected. More rare than others. E.g.... Buffalo stir up insects that cattle egrets eat. Or... Orchids live on trees but do not harm or feed off of them. Amensalism: one species is harmed; the other is unaffected. This is... Usually accomplished by natural chemical compounds. E.g.... Black walnut trees produce a chemical that kills other plants. Symbiosis: two species live close to each other. This is huge in nature! Can be beneficial (mutualism) or harmful (parasitism).
Evolution as a force for change Predation and competition keep populations under control: Individuals with qualities that lower impacts of negative interactions survive. Predators and prey become adapted to each other. Intraspecific competition leads to improved adaptations to the environment. Interspecific competition promotes adaptations in competitors. They specialize in exploiting a resource. Resource partitioning allows resource sharing. 40
Selective pressures Most young plants and animals do not survive. Selective pressures: environmental resistance factors affect which individuals survive and reproduce. For instance... Predators, parasites, drought, lack of food. Animals with traits that protect them or allow them to escape can survive and reproduce. Predators function as a selective pressure: favoring survival of traits that enable prey to escape predation. Food is a selective pressure: predators with keen eyesight or swift speed survive. 41
Humans have introduced thousands of species from foreign ecosystems, some accidentally, others deliberately, changing community and population relationships, usually for the worse. This causes... Economic losses in the U.S. of $138 billion/year! For example: rabbits were introduced into Australia for sport shooting; without natural enemies, the population exploded, devastating the environment. 42 Introduced species Kudzu, a vine introduced to control erosion and for cattle fodder, has invaded and climbed over forests.
Introduced species often become serious pests, e.g... Japanese beetles, fire ants, love bugs, gypsy moths; Chestnut blight, spotted knapweed, purple loosestrife, cogongrass, wisteria, hydrilla; Goats and pigs on islands; Domestic and feral cats; Zebra and quagga mussels; Ctenophore jellyfish; A lack of competition allows the invasive species to displace native species. The problem is increasing due to expanding world trade and travel. 43
Implications for human management Ecologists study populations and communities: To increase our understanding of the world; To better manage natural resources by protecting declining species and controlling introduced pest species; To study our impacts on populations. Keystone species are such an integral part of the ecosystem. Removing them can cause the ecosystem to collapse. For example, removing beavers (by overhunting and habitat destruction) hurt wetland meadow species. 44
Next time... Looking into ecosystems more carefully. 45