Biology · AP Biology
Ecology
energy and populations in communities
Ecology studies how organisms interact with one another and with their surroundings, and how energy and matter move between them.
The pyramid in the figure is a model ecosystem. Producers capture 20,000 kcal of energy per square metre each year. Only about a tenth of that reaches the herbivores that eat them, a tenth of theirs reaches the carnivores, and so on. The rest leaves each level as heat (red).
The chapter climbs the levels of organization. An individual responds to its environment and balances an energy budget. A population, the individuals of one species in one area, grows until resources run short. A community, all the populations in an area, is shaped by how species eat, compete with and help one another. An ecosystem adds the nonliving parts: water, soil, air, light.
The last pages ask what keeps an ecosystem stable, and what happens when it is disrupted.
behaviour and physiology that answer a signal
Organisms detect changes in their environment and respond, by behaviour or by physiology, in ways that tend to keep them alive and reproducing.
Plants respond too. In phototropism a shoot grows toward light; in photoperiodism a plant flowers when the night reaches a certain length. Among animals, a taxis is movement directed toward or away from a stimulus, like a moth flying to a light. A kinesis changes only the speed or the turning rate of undirected movement. Being active by night (nocturnal) or by day (diurnal) and the fight-or-flight response are responses too.
The figure. Pill bugs in a choice chamber walk fast and fairly straight on dry paper, and slowly, turning often, on moist paper. None of them aims for the moisture. Each one simply lingers there, about 70% of its time in this model, so the moist half fills up. That is a kinesis.
Testing the result. After 10 minutes a class counts 15 of 20 bugs on the moist side. The null hypothesis, no preference, predicts 10 and 10:
With two categories there is 1 degree of freedom, and the critical value at p = 0.05 is 3.84. Since 5.0 > 3.84, reject the null hypothesis: the bugs are not spread at random.
Communication. Animals exchange information by visual, audible, tactile, electrical and chemical signals (a pheromone is a chemical one). A honeybee’s waggle dance tells nestmates where food is; an alarm call warns of a predator. Signals change the behaviour of the receiver.
Behaviour and fitness. Natural selection favours innate and learned behaviours that raise survival and reproductive success. Cooperative behaviour, such as meerkats taking turns as lookouts, helps the population survive.
endotherms, ectotherms and body size
Every organism needs energy to keep its cells organized, to grow and to reproduce. Over time, the energy it takes in must cover the energy it spends.
Endotherms (birds and mammals) keep a steady body temperature with heat from their own metabolism. Ectotherms (most fish, amphibians, reptiles and invertebrates) lack efficient internal heating and take heat from their surroundings, as a lizard basking on a rock does. At the same body mass an ectotherm needs far less food, but it is sluggish when it is cold.
Body size. Metabolic rate per unit of body mass falls as mass rises: a small animal has more surface for each gram of body, loses heat faster, and must burn fuel faster to replace it. The curve is an empirical fit for mammals, 3.4M0.75 watts for the whole body, so 3.4M−0.25 watts per kilogram:
| animal | mass | whole body | per kg |
|---|
| mouse | 0.02 kg | 0.18 W | 9.04 W/kg |
| rabbit | 2 kg | 5.7 W | 2.86 W/kg |
| human | 70 kg | 82 W | 1.18 W/kg |
| horse | 500 kg | 360 W | 0.72 W/kg |
| elephant | 4,000 kg | 1700 W | 0.43 W/kg |
An elephant uses far more energy in all than a mouse, but each kilogram of mouse uses 21 times as much as a kilogram of elephant. The red arrows in the figure show the heat from each kilogram.
Net gain or loss. An organism that takes in more energy than it uses stores the excess, as fat or starch, and grows. One that takes in less burns its stores and loses mass; if that continues, it dies. Reproduction is costly, so many organisms breed only when energy is plentiful, in a particular season.
about a tenth passed on, the rest lost as heat
Energy enters most ecosystems as sunlight, passes from organism to organism as food, and leaves as heat. It flows one way; it does not cycle.
Autotrophs (producers) capture energy from the physical world: photosynthetic organisms from light, and chemosynthetic bacteria and archaea, at deep-sea vents for example, from inorganic chemicals. Heterotrophs (consumers) get energy by eating other organisms. Decomposers such as fungi and bacteria break down dead matter and waste.
Each feeding step is a trophic level. A food chain follows one path of feeding; a food web shows all of them.
| level | energy stored (kcal/m²/yr) |
|---|
| Producers | 20,000 |
| Primary consumers | 2,000 |
| Secondary consumers | 200 |
| Tertiary consumers | 20 |
Only about 10% of the energy in one level becomes biomass in the next. The rest is used in cellular respiration and lost as heat, or never eaten, or not digested and passed on to decomposers. The figure’s cubes have volumes to scale: each is a tenth of the one below. The top level gets 0.10000000000000002% of what the producers stored, which is why food chains rarely have more than four or five levels.
Primary productivity is the rate at which producers store energy. Gross primary productivity is all that photosynthesis captures; net primary productivity is what is left after the producers’ own respiration, NPP = GPP − R, and is what consumers can eat.
Example. Two bottles of pond water start at 8.0 mg/L of dissolved O2. After a day the one kept in light reads 9.5 mg/L and the one kept dark reads 6.8 mg/L. In the dark there is only respiration: R = 8.0 − 6.8 = 1.2. In light, net productivity is 9.5 − 8.0 = 1.5. So GPP = 1.5 + 1.2 = 2.7 mg O2/L per day.
carbon, nitrogen, phosphorus and water
Energy flows through an ecosystem and is lost, but its atoms stay. Carbon, nitrogen, phosphorus and water move between living things and nonliving reservoirs over and over: biogeochemical cycles.
Carbon is in every carbohydrate, lipid, protein and nucleic acid. Photosynthesis takes CO2 out of the air into sugar (black, in the figure); consumers take it in by eating. Cellular respiration, by producers, consumers and decomposers, returns it as CO2. The ocean dissolves CO2 and releases it (blue). Burning fossil fuels returns, in decades, carbon that was buried for millions of years (red), so atmospheric CO2 rises.
Nitrogen is in proteins and nucleic acids. The air is 78% N2, which most organisms cannot use. Nitrogen fixation by bacteria, some living in the root nodules of legumes, turns N2 into ammonia (NH3, as NH4+ in soil). Nitrification by other bacteria turns ammonium into nitrate (NO3−). Plants take up ammonium and nitrate; decomposers return ammonium from dead matter (ammonification); denitrification by bacteria returns N2 to the air.
Phosphorus is in nucleic acids, ATP and phospholipids. It has no important gas phase: it enters ecosystems as phosphate weathered from rock, passes through food webs, and settles into sediments that become rock again. It is slow, so it often limits growth.
Water evaporates from oceans and lakes and transpires from leaves, condenses and falls as precipitation, and returns as runoff and groundwater.
| cycle | main reservoir | into living things |
|---|
| carbon | atmosphere CO2, ocean, rock | photosynthesis |
| nitrogen | atmosphere N2 | fixation, then uptake |
| phosphorus | rock and sediment | uptake of phosphate |
| water | ocean | uptake by roots |
unlimited resources, exponential growth
A population is the individuals of one species living in one area. It grows by births and shrinks by deaths (and by migration, set aside here).
where N is the population size and B and D are births and deaths per unit of time. With unlimited food and space and no disease, each individual contributes at the species’ highest rate, the maximum per capita growth rate rmax:
The growth rate is proportional to the size, so the more there are, the faster they add more: exponential growth, a J-shaped curve. In the figure the red line is the slope of the curve, dN/dt. It steepens as N rises.
The figure. 20 individuals with rmax = 0.4 per year. After 10 years there are 1,092. Each dot on the right is ten of them.
Example. When this population reaches 500:
Exponential growth happens when resources are plentiful: bacteria in fresh broth, a species newly arrived in an empty habitat, a population recovering after a crash. It never lasts.
the logistic model and carrying capacity
As a population grows, each individual gets less food, space and shelter. Growth slows and stops at the carrying capacity K, the largest population the habitat can sustain.
The factor (K − N)/K is the fraction of capacity still unused. Near N = 0 it is close to 1 and growth is nearly exponential; at N = K it is 0; above K it is negative and the population shrinks. The curve is S-shaped: logistic growth.
The figure. The same start as 8.3, 20 individuals with rmax = 0.4, now with K = 800. The grey curve is exponential growth with the same rmax.
| N | 200 | 400 | 600 | 800 |
|---|
| dN/dt per year | 60 | 80 | 60 | 0 |
|---|
Growth is fastest at N = K/2 = 400: 0.4 × 400 × ½ = 80 per year.
Density-dependent factors grow stronger as the population gets denser: competition for food and nesting sites, predation, disease and parasites spreading from host to host, and the build-up of waste. They are what hold a population near K. Density-independent factors strike regardless of density: a freeze, a drought, a fire, a flood.
how species affect one another
A community is all the populations living in one area. Its structure is the species it holds, how many there are of each, and how they interact.
| interaction | species 1 | species 2 | example |
|---|
| Competition | − | − | two plant species drawing on the same soil water |
| Predation | + | − | a lynx eating a hare |
| Herbivory | + | − | a deer browsing shrubs |
| Parasitism | + | − | a tapeworm in a host’s gut |
| Mutualism | + | + | a bee pollinating a flower it feeds from |
| Commensalism | + | 0 | a barnacle riding on a whale |
(+ helped, − harmed, 0 unaffected.) In mutualism both gain; in commensalism one gains and the other is unaffected. Predation, herbivory and parasitism all feed one species at the other’s expense.
Competition. Two species that need exactly the same limited resource cannot coexist for long: one excludes the other (competitive exclusion). Species that overlap often divide the resource, feeding at different heights or times, and so coexist (resource partitioning).
Predator and prey. When prey are plentiful, predators eat well and their numbers rise. More predators eat more prey, and the prey fall. With little to eat, the predators fall, and the prey recover. In the figure the prey (black) peak at year 2.5; the predators (red) peak 2.5 years later, a quarter of the 10-year cycle behind.
Interactions shape a community more than any one species does: a predator can keep a superior competitor in check and so let more species persist.
why many species make a sturdier ecosystem
Ecosystems with more biodiversity are generally more resilient: they withstand and recover from disease, drought or other change better than simple ones.
Measuring diversity. Diversity depends on how many species there are and how evenly the individuals are spread among them. Simpson’s diversity index combines both:
where n is the number of individuals of one species and N the total of all species. It runs from 0 (one species) toward 1 (many species in even numbers).
The figure. Two plots, each with 20 plants of the same 4 species.
A blight kills the red species. Plot A keeps 75% of its plants; plot B, dominated by that one species, keeps 15%. Genetic diversity within one population works the same way: a field of genetically identical crops can be wiped out by one disease.
Keystone species. Some species have an effect far larger than their numbers. Sea otters eat sea urchins, which graze kelp; where otters were hunted out, urchins multiplied and stripped the kelp forests, and the many species living in the kelp went with them. When a keystone species is removed, the ecosystem often collapses.
Producers, at the base of every food web, and abiotic factors such as water, nutrients and temperature also set how much diversity an ecosystem can hold.
invaders, human change and natural events
An ecosystem shaped over long times can change quickly when something new arrives or conditions shift.
Invasive species. A species carried to a new place may find resources and no predators, parasites or competitors that keep it in check at home. It grows exponentially and takes over a niche. Zebra mussels, native to the region of the Black and Caspian Seas, reached the Great Lakes in ships’ ballast water in the 1980s. They filter phytoplankton from the water, leaving less for zooplankton and native mussels, and they settle on native mussels’ shells and smother them.
The figure shows a model lake web: as the red invader grows, the phytoplankton shrink, the native mussels almost vanish, and the loss climbs the web to the fish.
Human activity changes environments faster than many populations can adapt: habitat destroyed or broken into fragments, overharvesting, pollution and climate change.
Eutrophication. Fertilizer and sewage add nitrogen and phosphorus to lakes and coasts. Algae bloom, then die; decomposers breaking them down use up the dissolved oxygen; fish and other animals suffocate. The result is a hypoxic “dead zone.”
Biomagnification. A persistent toxin such as DDT or methylmercury is stored in fat rather than excreted. Each consumer eats many prey and keeps their toxin, so its concentration rises at each trophic level and is highest in top predators. DDT thinned the eggshells of eagles and pelicans.
Natural events disrupt too: volcanic eruptions, El Niño, floods and fires, and, over geological time, moving continents and asteroid impacts that caused mass extinctions. After each, surviving lineages diversified into the emptied niches.
← Natural selection