← All chapters

Field notes · chapter 56

Dynamics of Ecosystems

pp. 1264–1286 · Raven Part VIII

Best quiznot started
56.1

Biogeochemical Cycles

pp. 1265–1271
56.2

The Flow of Energy in Ecosystems

pp. 1271–1276
56.3

Trophic-Level Interactions

pp. 1276–1280
  • Because food chains exist, a species in one trophic level can affect more than one other level. Primary carnivores affect the animals they eat and, indirectly, the plants or algae those prey were eating. Watch How Wolves Change Rivers on YouTube, from Sustainable Human
  • A trophic cascade is the process by which effects exerted at an upper trophic level flow down to influence two or more lower levels. The effects themselves are called top-down effects. Watch How Wolves Change Rivers on YouTube, from Sustainable Human
  • A bottom-up effect runs the other way: an effect that flows up the chain from the primary producers to higher trophic levels, usually starting with a change in primary productivity. Watch Energy Flow in Ecosystems on YouTube, from Bozeman Science
  • Top-down effects are easiest to confirm experimentally in freshwater systems, where sections of a stream can be enclosed with mesh and predators added or excluded.
  • New Zealand stream experiment (3 levels): brown trout — predators on invertebrates — were added to some mesh enclosures. After 10 days, invertebrate numbers in the trout enclosures were only about two-thirds of the no-fish numbers (≈2900 vs ≈4500 per m²). Watch How Wolves Change Rivers on YouTube, from Sustainable Human
  • The cascade completed itself one level down: algal biomass was about five times greater in the trout enclosures (≈1.95 vs ≈0.4 µg chlorophyll a/cm²). Fewer grazers meant more algae.
  • Northern California experiment (4 levels): in stream enclosures the principal primary carnivores were damselfly nymphs. Fish that prey on nymphs were added to some enclosures. Watch How Wolves Change Rivers on YouTube, from Sustainable Human
  • The four-level result flips the sign: fish reduced damselfly nymphs → herbivorous insects (chironomids) increased → algal biomass decreased. Adding one more level reverses the effect on the bottom level — this contrast between fig 56.14 and fig 56.15 is a favorite exam item. Watch How Wolves Change Rivers on YouTube, from Sustainable Human
  • Large-scale cascades are harder to verify experimentally than enclosure studies, and their workings are less thoroughly known — yet several are accepted by most ecologists.
  • The classic large-scale case: sea otters, sea urchins, and kelp forests on the West Coast of North America. Otters eat urchins; urchins eat young kelp and inhibit kelp forest development. Watch Community Ecology II: Predators - Crash Course Ecology #5 on YouTube, from CrashCourse
  • Two states of the otter–urchin–kelp system: abundant otters → few urchins → well-developed kelp forest; sparse otters → many urchins → impaired kelp forest. Watch Community Ecology II: Predators - Crash Course Ecology #5 on YouTube, from CrashCourse
  • Orcas (killer whales) added a level on top: in recent years they have preyed intensively on otters, driving otter numbers down. The recent hypothesis is that this orca diet switch is pushing the ecosystem into the low-otter, urchin-dominated state.
  • Human activity produces top-down effects mainly by removing top-level carnivores — the reverse of adding a predator in an enclosure. Watch How Wolves Change Rivers on YouTube, from Sustainable Human
  • Aldo Leopold described the pattern in A Sand County Almanac long before trophic cascades were formally articulated: state after state extirpating its wolves, new "wolfless mountains" wrinkled with deer trails, every edible bush and seedling browsed to death, and trees defoliated to the height of a saddle horn.
  • Barro Colorado Island (created when the Panama Canal flooded a hilltop) lost its large predators — jaguars and mountain lions. Smaller predators normally held in check — monkeys, peccaries, coatimundis, armadillos — became extraordinarily abundant and eat almost anything they find. Watch How Wolves Change Rivers on YouTube, from Sustainable Human
  • On Barro Colorado, ground-nesting birds are the most vulnerable, many species have declined, and at least 15 bird species have vanished from the island entirely — a cascade measured as local extinction.
  • In the oceans, overfishing has cut large predatory fish such as billfish and cod to an average of about 10% of their previous numbers in virtually all regions. Watch How Wolves Change Rivers on YouTube, from Sustainable Human
  • The cod cascade continues downward: in some regions cod prey such as certain shrimp and crabs have become many times more abundant, with further effects at still lower levels.
  • Bottom-up effects occur when enhanced primary productivity flows upward, increasing food for herbivores and then for the carnivores that eat them. Predicting them requires attention to the life histories of the species present. Watch Energy Flow in Ecosystems on YouTube, from Bozeman Science
  • Stage 1 of the bottom-up model: when primary productivity is low, producers cannot support meaningful herbivore populations, so producer biomass rises with productivity — nothing is cropping it.
  • Stage 2: above one threshold, herbivore populations establish and grow. Now the increase in productivity is entirely devoured by herbivores, so herbivore biomass rises while producer biomass stays flat.
  • Stage 3: above another threshold, herbivores are numerous enough to support primary carnivores. Further productivity increases now go into carnivore biomass; herbivore biomass stays roughly constant because carnivores crop the increase.
  • Stage 3 has a twist that students miss: with herbivores held in check by carnivores, producer biomass is released and rises again with productivity. So at high productivity, producer and carnivore biomass rise while herbivore biomass stays flat.
  • The key to reading the model is keeping productivity (a rate) separate from standing crop biomass (an amount at one moment). Biomass can stay flat while productivity climbs.
  • Eel River experiment (northern California) tested the model: enclosures in a river excluded large fish (secondary carnivores) and each got a roof — some clear, some tinted to varying degrees — so enclosures differed in incoming sunlight. Watch Energy Flow in Ecosystems on YouTube, from Bozeman Science
  • Eel River results matched the prediction: productivity was highest under clear roofs and lowest under dark ones; as illumination and productivity rose, primary producer biomass rose and carnivore biomass rose, while the sandwiched herbivore level did not increase much.
  • Manipulating light rather than adding predators is what makes the Eel River study a bottom-up experiment — the perturbation is applied at the bottom of the chain.
  • Direction is the whole distinction: if the manipulation is at the top (add or remove a predator), it is top-down / trophic cascade; if it is at the base (change light, nutrients, or productivity), it is bottom-up. Both can operate in the same ecosystem at the same time. Watch How Wolves Change Rivers on YouTube, from Sustainable Human
56.4

Biodiversity and Ecosystem Stability

pp. 1280–1283
  • Species richness is simply the number of species present in a community. Ecologists have long debated what differences in richness do to a community. Watch Biodiversity Research at Cedar Creek on YouTube, from Institute on the Environment, University of Minnesota
  • Ecosystem stability is the property of remaining relatively constant in composition and being able to resist disturbance. The long-standing hypothesis is that species-rich communities are more stable. Watch Biodiversity Research at Cedar Creek on YouTube, from Institute on the Environment, University of Minnesota
  • The hypothesis has been tested most elegantly at the University of Minnesota's Cedar Creek Natural History Area. Watch Biodiversity Research at Cedar Creek on YouTube, from Institute on the Environment, University of Minnesota
  • Cedar Creek design: 207 small rectangular plots of prairie, each 8–16 m², monitored for 11 years. In each plot researchers counted plant species and measured total plant biomass.
  • Cedar Creek result 1 — stability: plots with more species showed less year-to-year variation in biomass.
  • Cedar Creek result 2 — drought resistance: in two drought years, the decline in biomass was negatively related to species richness. More species meant a smaller drought hit.
  • Cedar Creek result 3 — confirmation by manipulation: plots deliberately seeded with different numbers of species reproduced the pattern, with richer plots showing greater year-to-year biomass stability over 10 years. Seeding rather than observing rules out the objection that stable places simply attract more species.
  • Cedar Creek result 4 — invasibility: when seeds of nonnative plants were added, the ability of those species to establish was negatively related to species richness. Invasibility is used as another measure of stability. Watch Biodiversity Research at Cedar Creek on YouTube, from Institute on the Environment, University of Minnesota
  • The Scientific Thinking experiment in full. Question: does species richness affect the invasibility of a community? Hypothesis: successful invasion will be lower where richness is greater. Experiment: add seeds of the same number of nonnative plants to plots differing in plant species number.
  • Scientific Thinking result: although the number of successful invaders is highly variable, species-rich plots are on average invaded by fewer species — the fitted line slopes downward. Note the honest caveat: the scatter is wide, so richness predicts invasion only loosely.
  • A separate Cedar Creek study of 147 plots varying in species number measured growth and soil nitrogen uptake: the more species a plot held, the greater the nitrogen uptake and the greater the total biomass produced. Diversity appeared to raise productivity, not just stability.
  • Laboratory confirmation: artificial 1 m² ecosystems were built in growth chambers with controlled temperature, light, air currents, and gas concentrations, stocked with 9, 15, or 31 species (lower-diversity sets being subsets of the higher). Biomass produced and CO₂ consumed both tracked species richness.
  • The main objection: the more species you add, the greater the chance of including one highly productive species by luck. Under that argument the gain comes from a particular species, not from richness itself. Watch Biodiversity Research at Cedar Creek on YouTube, from Institute on the Environment, University of Minnesota
  • Overyielding is the test that answers the objection: plot productivity must exceed that of the single most productive species grown in isolation. Recent work at Cedar Creek and elsewhere has found evidence of overyielding, supporting the claim that richness itself raises productivity and stability. Watch Biodiversity Research at Cedar Creek on YouTube, from Institute on the Environment, University of Minnesota
  • Chapter 55 covered two other richness factors — loss of keystone species and moderate physical disturbance. This chapter adds three more: primary productivity, habitat heterogeneity, and climatic factors. Watch Community Ecology II: Predators - Crash Course Ecology #5 on YouTube, from CrashCourse
  • Productivity and richness are not linearly related. Richness is greatest at intermediate levels of productivity — the hump-shaped curve seen in South African mountain plant communities. Watch Biodiversity Research at Cedar Creek on YouTube, from Institute on the Environment, University of Minnesota
  • The explanation for the hump: at low productivity there are few herbivores, so superior plant competitors eliminate most other plants. At high productivity there are so many herbivores that only grazing-resistant plants survive. Maximum coexistence sits at intermediate productivity and intermediate herbivory.
  • That hump is also the standing counterargument to the "tropics are diverse because they are productive" hypothesis: past the peak, more productivity should lower richness.
  • Habitat (spatial) heterogeneity: a spatially varied abiotic environment contains many habitat types — different soil types, for instance — and can accommodate more species than a homogeneous one. Watch Biodiversity Research at Cedar Creek on YouTube, from Institute on the Environment, University of Minnesota
  • Heterogeneity propagates upward: varied environments support more plant species, and animal richness tends to mirror plant richness. Desert lizard richness in the American Southwest tracks the structural complexity of the plants at each site.
  • Climatic factors cut both ways in theory. A seasonal environment might hold more species because different species are favored at different times of year; a stable environment might hold more because it permits specialists that could not survive fluctuation.
  • The data support the stability side: along the West Coast of North America, mammal species richness is inversely correlated with the local monthly mean temperature range — the wider the swing, the fewer mammal species.
  • Species diversity gradient: for many kinds of organisms, richness increases steadily from the Arctic to the tropics. It has been documented in plants, birds, mammals, and reptiles.
  • The scale of the bird gradient (fig 56.21): fewer than 100 bird species at arctic latitudes versus more than 600 in southern Central America.
  • The ecologists' difficulty is not producing a plausible explanation for tropical diversity but choosing among the many plausible explanations. Five are commonly discussed.
  • Hypothesis 1 — evolutionary age. The tropics have persisted through long uninterrupted evolutionary time while temperate regions were repeatedly glaciated, so complex interactions had time to coevolve.
  • Counterevidence to hypothesis 1: pollen in soil cores shows tropical forests contracted to small refuges surrounded by grassland during glaciations, so long-term tropical stability has been exaggerated. Phylogenies also suggest many lineages evolved in the tropics and then colonized temperate zones, so tropical richness may reflect a longer diversification period.
  • Hypothesis 2 — increased productivity. More solar radiation plus a year-round growing season raise photosynthetic activity. Picture total resources as a pie and each species' niche as a slice: a bigger pie fits more slices. The counterpoint is the hump-shaped curve above.
  • Hypothesis 3 — stability and constancy of conditions. Less seasonal variation encourages specialization, subdividing niches to partition resources and avoid competition, yielding more and narrower niches. Almost all field tests support this one, reporting more narrow niches in tropical communities.
  • Hypothesis 4 — predation. Predation appears more intense in the tropics; heavier predation reduces the importance of competition, permits greater niche overlap, and so allows more species to coexist.
  • Hypothesis 5 — spatial heterogeneity. Tropical forests are structurally complex and create many microhabitats. The tall vertical column of vegetation filters light into a wide range of frequencies and intensities, producing many distinct light environments.
  • Exam framing for 56.4: know the definitions (richness, stability, invasibility, overyielding), the Cedar Creek four results, the three richness factors with their curve shapes, and the five tropical hypotheses with which one the field data favor. Watch Biodiversity Research at Cedar Creek on YouTube, from Institute on the Environment, University of Minnesota
56.5

Island Biogeography

pp. 1283–1284