Ecosystems & Conservation Decisions — Chapter review · Life Science: Biology, Grade 10

A population is all the members of one species in an area (for example, all the gray squirrels in a forest); a community is all the different populations living together there (the squirrels, oaks, hawks, fungi, and every other species combined).

A population is limited to a single species — such as every gray squirrel in the forest — while a community includes every population of every species sharing that space. The community is essentially all the living things interacting in one area; add the nonliving surroundings and you have an ecosystem.

400 rabbits.

Exponential doubling follows N = N₀ × 2ⁿ. Here N₀ = 25 and n = 4, so N = 25 × 2⁴ = 25 × 16 = 400 rabbits. In reality, limiting factors would slow this J-curve into an S-curve long before the numbers grew this large.

(a) density-dependent, (b) density-independent, (c) density-dependent, (d) density-independent.

Density-dependent factors get stronger as a population becomes more crowded: disease spreads faster in a dense herd (a), and competition for food intensifies when many individuals share limited resources (c). Density-independent factors strike regardless of crowding: a hard freeze (b) and a wildfire (d) can kill the same fraction of a sparse or a dense population.

Oak tree: autotroph. Deer and hawk: heterotrophs (consumers). Soil fungus: decomposer (a type of heterotroph).

The oak tree makes its own food by photosynthesis, so it is an autotroph (producer). The deer and hawk both get energy by eating other organisms, so they are heterotrophs. The soil fungus feeds on dead material, breaking it down and recycling nutrients, so it is a decomposer — a specialized heterotroph.

8,000 kcal for primary consumers; 800 kcal for secondary consumers.

Primary consumers receive about 10% of the producers' energy: 80,000 × 0.10 = 8,000 kcal. Secondary consumers receive about 10% of that: 8,000 × 0.10 = 800 kcal. Roughly 90% is lost as heat or used for life processes at each step.

About 15,000 kcal.

There are three energy transfers from the producers up to the top predator, and each step keeps only about 10% (÷10). Running the rule backward multiplies by 10 for each step down: 15 × 10 × 10 × 10 = 15,000 kcal. It takes a huge energy base at the bottom to support just 15 kcal at the top.

Photosynthesis; the carbon is built into sugars that become part of the plant's tissues and can then travel up the food web.

During photosynthesis, plants absorb carbon dioxide and use light energy to combine it with water, building sugar molecules. That carbon becomes part of the plant's own body, and when animals eat the plant, the carbon moves up the food web.

Nitrogen gas (N₂) has a strong triple bond that plants can't break, so they can't absorb it from the air. Nitrogen-fixing bacteria (and lightning) convert N₂ into ammonia and nitrates that plant roots can absorb.

Atmospheric nitrogen exists as N₂, whose two atoms are held by a very strong triple bond. Plants have no way to break that bond, so they cannot use nitrogen gas directly. Instead, nitrogen fixation — carried out mainly by bacteria such as Rhizobium in legume root nodules, plus some fixation by lightning — converts N₂ into ammonia and nitrates. Plants absorb those nitrates from the soil and build them into proteins and DNA.

The added nutrients trigger an algae bloom; when the algae die, decomposer bacteria multiply and use up the dissolved oxygen; without oxygen, the fish suffocate. This is eutrophication.

The fertilizer adds a surge of nitrogen (and phosphorus) to the lake, fueling an explosive bloom of algae across the surface. The bloom eventually dies, and decomposer bacteria feed on the mass of dead algae, multiplying rapidly. As they respire, these bacteria consume the oxygen dissolved in the water. The dissolved-oxygen level drops so low that fish and other aquatic animals can no longer breathe and die off. This whole nutrient-driven chain of events is called eutrophication, and it shows how disrupting the nitrogen cycle can destabilize an entire ecosystem.

Habitat loss, pollution, invasive species, and climate change. Habitat loss (and fragmentation) causes the most extinctions.

Human activity threatens biodiversity mainly through habitat loss and fragmentation, pollution, invasive species, and climate change (overharvesting is often added as a fifth). Of these, destroying and fragmenting habitat removes the places species need to live and is the single largest driver of species loss.

10 ppm. Because the toxin concentrates about tenfold at each level, top predators accumulate the highest, most dangerous doses.

Each step up the food web multiplies the concentration by about 10, so two levels up gives 0.1 × 10 × 10 = 10 ppm — a hundredfold increase over the plankton. Since top predators eat many contaminated prey and the toxin doesn't break down, biomagnification loads the greatest concentration into the animals at the top of the food web, which is why species like eagles and large fish are hit hardest.

(a) It grows nearly exponentially at first, since the usual limiting factors are absent; (b) native fish decline from added predation and competition, and the effects can ripple through the food web; (c) managers could remove the invasive fish, restrict boat/bait transfers to prevent spread, or protect and restore native habitat.

(a) Freed from the predators, parasites, and competitors that limited it in its home range, the invasive fish faces few density-dependent brakes, so its population can climb along a steep, nearly exponential J-curve. (b) As its numbers explode, it competes with native fish for food and space and preys on them directly, driving native populations down; because food webs are interconnected, species that depended on those natives — or that the invader now outcompetes — can be affected too, lowering biodiversity. (c) Because the lake's usual limiting factors don't apply to the newcomer, managers must supply the 'brakes' themselves: physically removing or culling the invasive fish, preventing its spread to other lakes by cleaning boats and banning its use as bait, and restoring native habitat to help native populations recover. This capstone ties together population growth, limiting factors, species interactions, and conservation.

850

The level at which growth stops is the best available estimate of carrying capacity under the conditions observed, which may change if resources change.

They compete, and several outcomes are possible: one species may be excluded locally, or the two may divide the resource by feeding at different heights, times, or on different insect sizes, which reduces direct competition. Which occurs depends on how similar their requirements are, whether variation exists that allows divergence, and whether the resource is abundant enough to support both. Measuring where and when each species actually feeds would distinguish exclusion from resource division.

Response criteria

• State a biologically accurate conclusion.
• Show the data, mechanism, or calculation that supports it.
• Include units, conditions, or limits where relevant.

Final count is 80+20−12+9−5 = 92. Change is +12, or 12/80×100 = 15%, under the stated boundary and interval.

Final count is 80+20−12+9−5 = 92. Change is +12, or 12/80×100 = 15%, under the stated boundary and interval.

Response criteria

• Use the stated measurements or conditions, with correct units.
• Connect the result to the biological mechanism or evidence relationship.
• Identify the assumption, model limit or further evidence requested.

9 %

45 / 500 × 100 = 9 percent, close to the commonly cited approximation of about 10 percent.

In a simple chain, losing the single prey species removes the predator's entire food supply, so the predator's population collapses. With three prey species, the predator can shift its feeding to the remaining two, so its population may decline without collapsing. That alternative pathway makes the web more able to absorb the loss, although pressure on the remaining prey species increases, which can cause further changes. Redundancy in feeding relationships therefore spreads the effect rather than eliminating it.

Response criteria

• State a biologically accurate conclusion.
• Show the data, mechanism, or calculation that supports it.
• Include units, conditions, or limits where relevant.

The outputs are 1,200 kJ and 96 kJ. The stated transfer fractions define this comparison; ten percent is an approximation, not a compulsory constant.

The outputs are 1,200 kJ and 96 kJ. The stated transfer fractions define this comparison; ten percent is an approximation, not a compulsory constant.

Response criteria

• Use the stated measurements or conditions, with correct units.
• Connect the result to the biological mechanism or evidence relationship.
• Identify the assumption, model limit or further evidence requested.

8 units

220 − 212 = 8 units added per year; a reservoir grows whenever inputs exceed outputs, however small the difference.

Atoms are conserved and can be reused indefinitely, so carbon, nitrogen, and phosphorus pass repeatedly between organisms, air, water, and soil, with decomposers returning elements from dead material to inorganic forms producers can take up. Energy is conserved in total but degrades at each transformation, with part released as heat that disperses and can no longer do cellular work. An ecosystem therefore needs no continuing input of matter but does need a continuing input of energy, which is why sunlight must keep arriving.

Response criteria

• State a biologically accurate conclusion.
• Show the data, mechanism, or calculation that supports it.
• Include units, conditions, or limits where relevant.

The final store is 50+4×(12−9)=62 units. Twelve units/year is an input flux, not the stock or net annual gain; the net gain is three units/year.

The final store is 50+4×(12−9)=62 units. Twelve units/year is an input flux, not the stock or net annual gain; the net gain is three units/year.

Response criteria

• Use the stated measurements or conditions, with correct units.
• Connect the result to the biological mechanism or evidence relationship.
• Identify the assumption, model limit or further evidence requested.

30 %

(312 − 240) / 240 × 100 = 30 percent. The comparison population's 18 percent decline is what makes the increase attributable to protection rather than to a favourable period.

The conclusion needs a comparison: an unprotected population or an earlier period monitored with the same method, so that the rise can be separated from a general trend. Alternative explanations include favourable weather or food supply across the whole region, a change in counting method or effort that detects more individuals, immigration from outside the protected area, and natural fluctuation in a population that varies year to year. Without a comparison, none of these can be ruled out.

Response criteria

• State a biologically accurate conclusion.
• Show the data, mechanism, or calculation that supports it.
• Include units, conditions, or limits where relevant.

The raw count rises 50%, but changed detection can contribute. Use comparable effort/methods, suitable reference sites and repeated population/condition indicators; estimate detection where appropriate before attributing recovery.

The raw count rises 50%, but changed detection can contribute. Use comparable effort/methods, suitable reference sites and repeated population/condition indicators; estimate detection where appropriate before attributing recovery.

Response criteria

• Use the stated measurements or conditions, with correct units.
• Connect the result to the biological mechanism or evidence relationship.
• Identify the assumption, model limit or further evidence requested.

Track native plant cover across years and matched sites; it does not alone measure species diversity, soil function, or all populations.

3

36 / 12 = 3 times as long, indicating lower resilience in the second community.

Resistance is how little a community changes when a disturbance occurs; resilience is how quickly and fully it returns afterwards. Resistance is measured as the size of the initial drop — for example the percentage of biomass or species lost immediately after a storm. Resilience is measured as the time taken to recover to a stated fraction of the pre-disturbance level, such as years to reach 95 percent of former biomass. A community can be low in resistance and high in resilience, dropping sharply and recovering fast, so the two must be reported separately.

Response criteria

• State a biologically accurate conclusion.
• Show the data, mechanism, or calculation that supports it.
• Include units, conditions, or limits where relevant.

It has low resistance for this disturbance/output but relatively rapid recovery over the stated interval. Output recovery alone does not establish restored species composition or every ecological function.

It has low resistance for this disturbance/output but relatively rapid recovery over the stated interval. Output recovery alone does not establish restored species composition or every ecological function.

Response criteria

• Use the stated measurements or conditions, with correct units.
• Connect the result to the biological mechanism or evidence relationship.
• Identify the assumption, model limit or further evidence requested.

Use consistent sampling, comparison sites, repeated seasons, movement or reproduction evidence, and consideration of weather and other interventions.

500 $/ha

450,000 / 900 = 500 dollars per hectare, against 700 for Plan B — though cost per hectare alone does not establish which plan conserves more.

Hectares are not interchangeable. A cheaper hectare may be of lower habitat quality, poorly connected to other protected land, or less used by the species of concern, while an expensive hectare may contain the only breeding sites. Cost per hectare also ignores ongoing costs of management and monitoring, the risk that protection fails, and who bears the cost. A defensible comparison states what each plan is expected to achieve for the species or community concerned, with what uncertainty, and at what total cost over time.

Response criteria

• State a biologically accurate conclusion.
• Show the data, mechanism, or calculation that supports it.
• Include units, conditions, or limits where relevant.

Their areas are 10,000 m² and perimeters 400 versus 500 m. Boundary exposure and width differ, but species, habitat quality, connectivity, disturbance, costs and goals determine the useful comparison.

Their areas are 10,000 m² and perimeters 400 versus 500 m. Boundary exposure and width differ, but species, habitat quality, connectivity, disturbance, costs and goals determine the useful comparison.

Response criteria

• Use the stated measurements or conditions, with correct units.
• Connect the result to the biological mechanism or evidence relationship.
• Identify the assumption, model limit or further evidence requested.