Ecosystem and Conservation Ecology

Main notes

An ecosystem is a community of living organisms together with the nonliving environment they interact with. This chapter asks how ecosystems run, where their energy comes from, how it moves, and how water and nutrients cycle through them. It then surveys the biomes that organize life on land and the biodiversity crisis now threatening them, along with the conservation strategies used in response. It builds on the population and community ecology of earlier chapters and connects to applied fields from agriculture to climate policy.

Primary Productivity

Every ecosystem is powered by autotrophs, also called producers, which capture light energy and turn it into chemical energy through photosynthesis. Gross primary productivity (GPP) is the total rate at which producers fix carbon through photosynthesis. Net primary productivity (NPP) is GPP minus the energy the producers burn in their own cellular respiration, so NPP equals GPP minus R. Net primary productivity is the energy that remains for consumers, and it is the real measure of how much new biomass an ecosystem can support.

Productivity varies enormously across ecosystems. Tropical rainforests, estuaries, coral reefs, and wetlands rank among the most productive on Earth, while deserts, tundra, and the open ocean rank among the least productive per unit area. The open ocean contributes a large share of global photosynthesis only because it covers so much area. The main limiting factors are light, water, temperature, and nutrient availability; adding the limiting factor raises productivity until another factor takes over.

Common Mistake: Using GPP and NPP as if they were the same number. NPP is always the smaller value, because every producer burns part of its product in respiration; when ecologists say a forest produces a certain amount, they almost always mean NPP.

ELI-10

Think of a gardener who bakes bread from the wheat she grows. Her gross productivity is the whole loaf, and her net productivity is what is left after she eats her own slice. The neighbors can only share the leftover part. The energy that remains after a producer feeds itself is the only energy that everyone else in the ecosystem can use.

Trophic Levels and Energy Flow

Energy passes through ecosystems along feeding relationships. A food chain is a linear sequence of who eats whom, and the trophic levels are the feeding positions in that sequence. Producers occupy the first trophic level, primary consumers or herbivores the second, secondary consumers the third, and tertiary consumers the fourth. Decomposers, mainly fungi and bacteria, break down dead organisms and waste at every level, releasing nutrients and returning energy as heat. Most ecosystems contain many interwoven chains, described as a food web.

Only about 10 percent of the energy in one trophic level transfers to the next; the rest is lost as heat and unusable waste. The rule explains why food chains rarely run past four or five links and why there are far more producers than top predators. Ecologists draw ecological pyramids of energy, biomass, and numbers to show this narrowing toward the top. Energy flows one way, entering as sunlight and leaving as heat, so the sun must continually resupply it.

1. Producers capture sunlight and store it as chemical energy.
2. Primary consumers eat producers and keep about 10 percent of the energy.
3. Secondary consumers eat primary consumers, keeping 10 percent of that.
4. Tertiary consumers eat secondary consumers, keeping 10 percent of that.
5. Decomposers break down remains at every level and release the rest as heat.

Common Mistake: Saying energy is recycled in an ecosystem. Energy flows one way, from sun to heat, and is never reused; matter cycles through the system again and again.

ELI-10

Imagine ten dollars being handed down a line of people. The first person keeps the ten dollars, passes one dollar along, and burns the other nine as heat. The next person passes only a dime onward and burns the rest. After a few hands almost nothing is left, and that is why food chains are short.

Carbon Nitrogen Phosphorus and Water Cycles

Matter moves through ecosystems in biogeochemical cycles, circular paths that carry carbon, nitrogen, phosphorus, and water between organisms and large storage pools called reservoirs. Unlike energy, matter is never created or destroyed; the same atoms are reused on loops that run on very different time scales.

The carbon cycle moves carbon through the atmosphere, oceans, living things, soils, and rocks. Photosynthesis pulls carbon dioxide out of the air, and cellular respiration, decomposition, and combustion return it. The ocean is the largest active reservoir, absorbing and releasing carbon dioxide, while fossil fuels, wood, and marine sediments store carbon for millions of years before it is buried. Burning of fossil fuels has raised atmospheric carbon dioxide from about 280 ppm before the industrial era to above 420 ppm today, the fastest such rise in geological history.

The nitrogen cycle converts inert nitrogen gas, which makes up about 78 percent of the atmosphere, into forms organisms can use. Most organisms cannot use N2 directly, so the cycle depends on a sequence of microbial conversions.

1. Nitrogen fixation: bacteria and lightning convert N2 gas into ammonia.
2. Nitrification: soil bacteria oxidize ammonia into nitrite, then nitrate.
3. Assimilation: plants take up nitrate and ammonia and build proteins and DNA.
4. Ammonification: decomposers release ammonium from dead tissue and waste.
5. Denitrification: anaerobic bacteria convert nitrate back into N2 gas.

Nitrogen-fixing bacteria include free-living soil species and Rhizobium, which lives in nodules on the roots of legumes. Industrial fertilizer production now fixes as much nitrogen as all natural processes combined. Runoff of excess nitrogen and phosphorus into lakes and coastal waters causes eutrophication, explosive algal growth that suffocates other life when the algae die and decay.

The phosphorus cycle is the slowest and simplest of the four because phosphorus has no significant gas phase; it moves mainly through rock, soil, water, and living tissue. Weathering releases phosphate from rocks, plants take it up, animals get it from food, and decomposition and excretion return it to soil and water, where much of it washes into the ocean and eventually becomes seafloor sediment. Tectonic uplift slowly raises those sediments into new rock, closing a loop that takes millions of years. Phosphorus is frequently the limiting nutrient in freshwater systems, so even small additions can trigger eutrophication.

The water cycle, also called the hydrologic cycle, is driven by solar energy. Water evaporates from oceans, lakes, and soil, transpires from plant leaves, condenses into clouds, and falls as precipitation, then runs off or soaks into groundwater before evaporating again. Water is the solvent that carries every other cycle, so a healthy water cycle is a prerequisite for all the others.

ELI-10

Think of one water bottle shared by a whole soccer team. The same bottle keeps coming back to each player, and it never leaves the field. Nutrients in nature travel the same way, on loops that take seconds or millions of years. Nothing new is created, and everything is reused.

Biomes

A biome is a large region whose climate, mainly temperature and precipitation, selects a characteristic set of plants, which in turn shape the animals that live there. Moving from the equator toward the poles, or from sea level up a mountain, produces the same sequence of biomes, because both latitude and altitude change climate in predictable ways. Oceanic and freshwater systems are grouped into aquatic biomes such as estuaries, coral reefs, and the open ocean, where light, temperature, and salinity play the roles rainfall plays on land.

ELI-10

Biomes are like the neighborhoods of a world weather map. A town that gets rain all year looks one way, and a town that almost never gets rain looks completely different. Plants are the best clue to the neighborhood, because they must match the local rain and temperature exactly. Where the climate is similar on different continents, the plant neighborhoods look similar even when the species inside them are different.

Biodiversity Loss

Biodiversity is the variety of life measured at three levels: genetic diversity within species, species diversity in a community, and ecosystem diversity across landscapes. All three are falling. Species have always gone extinct, so the question is the rate: the background extinction rate, the natural pace between mass events, is roughly one to five species per million species per year. Current extinction rates are about 100 to 1,000 times faster than that, and many biologists call the present era the sixth mass extinction.

The causes are human activities, often summarized with the mnemonic HIPPO: habitat loss, the largest single cause as forests, wetlands, and grasslands are converted or fragmented; invasive species that outcompete or eat natives; pollution; human population growth and its demands; and overexploitation such as overfishing and poaching. Climate change adds a global pressure that shifts or destroys habitats faster than many species can adapt or move. Biodiversity loss erodes ecosystem services, the benefits people get free from nature, including pollination, clean water, flood control, and medicines. It also removes the genetic raw material that lets crops and wild populations adapt to change.

Common Mistake: Concluding that extinction is natural and therefore no cause for concern. Background extinction is real, but the current rate is roughly 100 to 1,000 times faster; each species lost is gone forever and narrows the options of every ecosystem that depended on it.

ELI-10

Think of a toolbox holding a thousand different screwdrivers. Losing a few every century barely matters, but losing about a hundred every year soon means no screw can be turned. Species are disappearing at roughly that accelerated pace. Every lost species is a tool that can never be replaced, because evolution takes millions of years to build a new one.

Conservation Strategies

Conservation biology applies ecology to protect species, habitats, and the services they provide. Protected areas, such as national parks, reserves, and marine protected areas, now cover about 17 percent of land and 8 percent of the ocean, and the global 30 by 30 target calls for 30 percent of both by 2030. Because protected land is often fragmented into islands, wildlife corridors connect reserves so animals can move, find mates, and track shifting climates. Ex situ conservation protects species outside their natural habitat in zoos, aquaria, botanical gardens, and seed banks such as the Svalbard Global Seed Vault, which preserves crop varieties against disaster. Captive breeding followed by reintroduction has pulled species back from the brink, most famously the California condor, which fell to 22 wild birds in 1982 and has climbed back above 500 through breeding and release programs. Restoration ecology takes the opposite direction, repairing damaged ecosystems directly by replanting forests, reconnecting rivers, removing invasive species, and rebuilding wetlands.

Conservation also works through law and economics. Treaties such as CITES regulate wildlife trade, national laws like the US Endangered Species Act protect listed species and their habitat, and ecotourism and carbon markets give intact ecosystems an economic value that competes with clearing them. Community-based conservation often outlasts top-down protection, because people protect what they benefit from.

High-Yield:

• Net primary productivity, not gross, is the energy available to consumers.
• Only about 10 percent of energy moves up each trophic level, so food chains stay short.
• Energy flows one way and is lost as heat, while matter cycles through biogeochemical cycles.
• Current extinction rates are roughly 100 to 1,000 times the background rate.
• Habitat loss is the single biggest cause of biodiversity decline.

ELI-10

Think of a museum that stores a copy of every painting and a nursery that raises baby birds until they are strong enough to fly. The stored copy does not replace the original, but it keeps the art alive if the original is lost. Conservation does the same thing, saving seeds, eggs, and young animals while also repairing the forests and wetlands where they belong. Saving the home itself works best, and connecting the homes lets the animals move around as the weather changes.

Quick Review

• Gross primary productivity is total photosynthesis; net primary productivity is what remains after producer respiration.
• Only about 10 percent of energy passes to the next trophic level; the rest leaves as heat.
• Carbon and nitrogen cycles have large atmospheric phases, while the phosphorus cycle has none and is the slowest.
• Nitrogen fixation, nitrification, ammonification, and denitrification are the microbial conversions of the nitrogen cycle.
• Biomes follow climate, so latitude and altitude repeat the same plant patterns.
• Current extinction rates run roughly 100 to 1,000 times the background rate.
• Conservation combines protected areas, corridors, ex situ programs, and restoration.

Key terms

Key terms are emphasized and defined within the main notes.

Important formulas or processes

See the formulas, procedures, and process blocks in the main notes where applicable.

Common mistakes

See the labeled common-mistake callouts in the main notes where present.