The food chain in Cayuga Lake. Autotrophic plankton (algae and cyanobacteria) fix the energy of the sun, heterotrophic plankton feed on them, and both are consumed by smelt. The smelt are eaten by trout, with about a fivefold loss in fixed energy. For humans, the amount of smelt biomass is at least five times greater than that available in trout, although humans prefer to eat trout.
Why does it take so many calories of algae to support so few of humans? Answer: At each link in the food chain, only a small fraction of the energy at one level is converted into mass of organisms at the next level. Much energy is dissipated as heat or excreted.
Ecological pyramids. Ecological pyramids measure different characteristics of each trophic level. (a) Pyramid of numbers. (b) Pyramids of biomass, both normal (top) and inverted (bottom). (c) Pyramid of energy.
How can the existence of inverted pyramids of biomass be explained? Answer: In the inverted pyramid, the primary producers reproduce quickly and are eaten quickly, so that at any given time, a small population of primary producers exist relative to the heterotroph population.
Trophic cascades. Streams with trout have fewer herbivorous invertebrates and more algae than streams without trout.
Why do streams with trout have more algae? Answer: Because the trout eat the invertebrates which graze the algae. With fewer grazers, there is more algae.
Four-level trophic cascades. Streams with large, carnivorous fish have fewer lower- level predators, such as damselflies, more herbivorous insects (exemplified by the number of chironomids, a type of aquatic insect), and lower levels of algae.
What might be the effect if snakes that prey on fish were added to the enclosures? Answer: The snakes might reduce the number of fish, which would allow an increase in damselflies, which would reduce the number of chironomids and increase the algae. In other words, lower levels of the food chain would be identical for the "snake and fish" and "no fish and no snake" treatments. Both would differ from the enclosures with only fish.
Bottom-up effects. At low levels of productivity, herbivore populations cannot be maintained. Above some threshold, increases in productivity lead to increases in herbivore biomass; vegetation biomass no longer increases with productivity because it is converted into herbivore biomass. Similarly, above another threshold, herbivore biomass gets converted to carnivore biomass. At this point, vegetation biomass is no longer constrained by herbivores, and so again increases with increasing productivity.
Why are there portions of the curves where vegetation biomass does not increase as productivity increases? Answer: Because the increase in vegetational mass is all eaten by herbivores and is translated into greater herbivore biomass.
Bottom-up effects on a stream ecosystem. Increases in the amount of light hitting the stream lead to increases in the amount of vegetation. However, herbivore biomass does not increase with increased productivity because it is converted into predator biomass.
Why is the amount of light an important determinant of predator biomass? Answer: More light means more photosynthesis. More plant material means more herbivores, which translates into more predator biomass.
Effect of species richness on ecosystem stability and productivity. (a) One of the Cedar Creek experimental plots. (b) Community stability can be assessed by looking at the effect of species richness on community invasibility. Each dot represents data from one experimental plot in the Cedar Creek experimental fields. Plots with more species are harder to invade by non-native species, which occur in surrounding agricultural fields.
How could you devise an experiment on invasibility that didn't rely on species from surrounding areas? Answer: Introduce them yourself. For example, each spring, you could place a premeasured number of seeds of a particular invasive species in each plot. Such an experiment would have the advantage of more precisely controlling the opportunity for invasion, but also would be less natural, which is one of the advantages of the Cedar Creek study site: the plots are real ecosystems, interacting with their surrounding environment in natural ways.
Factors that affect species richness. (a) Productivity: In fynbos plant communities of mountainous areas of South Africa, species richness of plants peaks at intermediate levels of productivity (biomass). (b) Spatial heterogeneity: The species richness of desert lizards is positively correlated with the structural complexity of the plant cover in desert sites in the American Southwest. (c) Climate: The species richness of mammals is inversely correlated with monthly mean temperature range along the west coast of North America.
(a) Why is species richness greatest at intermediate levels of productivity? (b) Why do more structurally complex areas have more species? (c) Why do areas with less variation in temperature have more species? Answer: (a) Perhaps because an intermediate number of predators is enough to keep numbers of superior competitors down. (b) Perhaps because there are more habitats available and thus more different ways of surviving in the environment. (c) Hard to say. Possibly more stable environments permit greater specialization, thus permitting coexistence of more species.
The equilibrium model of island biogeography. (a) Island species richness reaches an equilibrium (black dot) when the colonization rate of new species equals the extinction rate of species on the island. (b) The equilibrium shifts depending on the rate of colonization, the size of an island, and its distance to sources of colonists. Species richness is positively correlated with island size and inversely correlated with distance from the mainland. Smaller islands have higher extinction rates, shifting the equilibrium point to the left. Similarly, more distant islands have lower colonization rates, again shifting the equilibrium point leftward. (c) The effect of distance from a larger island, which can be the source of colonizing species, is readily apparent. More distant islands have fewer species compared to nearer islands of the same size.
Do islands with the same number of species necessarily have the same extinction rate? Answer: No. Examine figure 55.20b. The equilibrium number of species can be the same for two islands, but the rates of extinction and colonization can be different due to island size and distance to the mainland.
1). Over land, most of the water in the atmosphere results from ___________, whereas over the oceans, most of the water in the atmosphere results from ____________.
2). Which of the statements about groundwater is not accurate?
a). In the U.S., groundwater provides 50% of the population with drinking water.
b). Groundwaters are being depleted faster than they can be recharged.
c). Groundwaters are becoming increasingly polluted.
d). Removal of pollutants from groundwaters is easily achieved.
3). The largest store of carbon molecules on earth is in
a). the atmosphere.
b). fossil fuels.
c). marine sediments.
d). living organisms.
4). Some bacteria have the ability to "fix" nitrogen. This means
a). they convert ammonia into nitrites and nitrates.
b). they convert atmospheric nitrogen gas into biologically useful forms of nitrogen.
c). they break down nitrogen-rich compounds and release ammonium ions.
d). they convert nitrate into nitrogen gas.
5). The phosphorous cycle differs from the water, carbon, and nitrogen cycles in that
a). the reservoir for phosphorous exists in mineral form in rocks rather than in the atmosphere.
b). phosphorous is far more abundant than water, carbon, or nitrogen.
c). phosphorous is less important to biological systems than water, carbon, or nitrogen.
d). phosphorous, once used by an organism, does not cycle back to the environment.
6). In the Hubbard Brook experiments, which of the following situations did not occur?
a). The undisturbed forests were effective at retaining nutrients.
b). The deforested areas gained nitrogen.
c). The deforested areas lost more water to runoff than the undisturbed forests.
d). The deforested areas lost more minerals to runoff than the undisturbed forests.
7). What percentage of the energy that hits a plant's leaves is converted into chemical (food) energy?
8). According to the trophic cascade hypothesis, the removal of carnivores from an ecosystem may result in
a). a decline in the number of herbivores and a decline in the amount of vegetation.
b). a decline in the number of herbivores and an increase in the amount of vegetation.
c). an increase in the number of herbivores and an increase in the amount of vegetation.
d). an increase in the number of herbivores and a decrease in the amount of vegetation.
9). Experimental evidence from the Cedar Creek Natural History Area supports the hypothesis that
a). species richness is related to community stability.
b). human activities are upsetting the trophic cascade.
c). bottom-up effects can regulate the number of top carnivores in a system.
d). some aquatic ecosystems have inverted biomass pyramids.
10). The equilibrium model of island biogeography suggests all of the following except
a). larger islands have more species than smaller islands.
b). the species richness of an island is determined by colonization and extinction.
c). smaller islands have lower rates of extinction.
d). islands closer to the mainland will have higher colonization rates.
Answer: c Test Your Visual Knowledge
1). Using this table relate productivity to species richness.
Answer: Highly productive ecosystems can support more species than less productive ecosystems. The tropical rainforests have the greatest net primary productivity (NPP) and also have the greatest diversity. In comparison, the deserts have low NPP and low species diversity.
Apply Your Knowledge
1). Nitrogen is a limiting nutrient to plant growth in most ecosystems. To increase plant production, fertilizers that contain nitrogen (and phosphorous) are used. What effects might this influx of nitrogen have on ecosystems?
Answer: Increased nitrogen has allowed for increased plant growth in areas where plant growth was normally limited. This has had a major impact on many ecosystems. This is most obvious in lakes where runoff of fertilizers containing nitrogen and phosphorous have stimulated plant growth and caused lakes to become choked with aquatic vegetation.
2). Many communities in the U.S. struggle with issues of deer overpopulation. Explain how human activities have created this situation.
Answer: Humans have removed most of the deer's normal predators. This has helped deer populations to grow. Additionally, humans have fragmented forests and subsequently created habitats that deer are able to exploit more readily than large contiguous forests. This has also contributed to an increase in deer populations.