Ecosystem Interdependence, Bioaccumulation, Invasive Species & Sampling – Comprehensive Study Notes

  • Interdependence of Organisms (Section 7.3)

• Definition of interdependence
– Every population in an ecosystem relies on others for survival; changes ripple through food chains, webs, and wider ecological networks.
– Key resources that link populations: food, shelter, disease vectors, mutualistic services (e.g., seed dispersal, pollination).

• Feeding relationships
– Producers (green plants, algae) manufacture organic sugars by photosynthesis.
– Herbivores consume producers; carnivores/omnivores consume animals; decomposers recycle dead biomass and wastes.
– Food chains/webs are simplified diagrams scientists use to represent these links; they capture energy flow but hide real-world complexity.

• Population dynamics & balance
– "Population" = number of individuals of one species in a defined area.
– Size of one population affects (and is affected by) sizes of others.
– Typical sigmoid (S-shaped) growth curve phases:
• Lag phase – slow increase because few breeding pairs.
• Exponential phase – rapid growth; birth ≫ death.
• Deceleration phase – growth slows when food, space, or health become limiting.
• Crash/die-back – sharp decline if resources are over-exploited, pollution or disease strikes.
– Illustrative case: deer moved to a predator-free island. Abundant food → exponential growth; overgrazing → starvation and population crash.

• Predator–prey cycles
– Many predators are generalists (e.g., jaguar eats > 85 prey species) so no single prey is exterminated.
– When a predator specialises, cycles are easier to see (wolves ↔ caribou).
• Caribou rise ⇒ more food, wolf births rise.
• Intense predation ⇒ caribou fall ⇒ wolf starvation/reduced breeding ⇒ wolf numbers fall.
• Reduced predation ⇒ caribou rebound. Cycle length depends on gestation times & resource renewal.

• Disease & parasites
– High density → faster transmission of infectious agents.
– Parasites (tapeworms, ticks, fleas) feed on a host, weakening but seldom killing immediately; they regulate large populations and can finish off small ones already in decline.

• Biodiversity link
– High species richness spreads risk; if one link weakens, alternatives exist (diet switches, alternate pollinators, etc.), so overall population sizes stay more stable.

Bioaccumulation & Pesticides (Section 7.4)

• Pests cost ≈ 30 % of global crop yields; farmers respond with pesticides (chemical toxins that kill insects, weeds, fungi, molluscs, rodents, etc.).

• Types of pesticide toxins
– Non-persistent → degrade quickly (photolysis, biodegradation).
– Persistent → resistant to breakdown; fat-soluble; bioaccumulate.

• Bioaccumulation
– Definition: progressive increase in concentration of a substance inside an organism over time because intake > excretion/metabolism.
– Along a food chain this is magnified (biomagnification) because predators eat many prey.
– Classic pattern: \text{[toxin]}{\text{top predator}} \gg \text{[toxin]}{\text{producer}}.

• Three exemplar chains

  1. River run-off → small fish → bigger fish → heron; heron receives lethal dose.

  2. Sprayed crops → insects & mice → sparrowhawk.

  3. Marine path: land run-off → plankton/small fish → larger fish → seals → polar bear (top dose).

• Ecosystem-level impacts
– Death/decline of top predators breaks control of prey populations, causing prey irruptions and stress on producers.
– Shift in species composition (e.g., mesopredator release, competitive exclusion).
– Ethical/policy debate: balance human food security vs. biodiversity health; impetus for integrated pest management and stricter pesticide regulation.

Native, Introduced & Invasive Species (Section 7.5)

• Terminology
– Native (endemic/indigenous): evolved or arrived without human aid; part of original ecosystem.
– Introduced (non-native/exotic): transported by humans intentionally or accidentally.
– Invasive: introduced species whose rapid spread harms native biota, economy, or human health.

• Why invasives prosper
– Enemy-release: few/no predators, pathogens, or competitors in new range.
– Life-history traits: high fecundity, broad diet, tolerance of varied conditions, rapid growth, effective dispersal.

• Case studies
– Cane toad (Rhinella marina): introduced for pest control; toxic skin deters predators; omnivorous; decimates native amphibians, reptiles, northern quoll populations.
– Water hyacinth (Eichhornia crassipes): ornamental pond plant; extraordinarily fast vegetative reproduction; forms dense mats → blocks light, reduces dissolved O₂, clogs waterways, kills fish & invertebrates; costly to remove.

• Management attempts
– Chemical herbicides/pesticides = temporary & environmentally risky.
– Mechanical removal = labour-intensive, expensive.
– Biological control (see Section 7.6) – mixed success; requires careful risk assessment.

Invasive Species Disrupting Ecosystems (Section 7.6)

• Guam & the brown tree snake (Boiga irregularis)
– Arrival: ~1940s via cargo from Papua New Guinea.
– Traits: nocturnal, arboreal, egg-eating, no natural predators on Guam.
– Outcome: 10/12 native bird species extinct; by 2012 ≈ 2 million snakes.
– Trophic cascade (see Fig 7.6 B):
• Bird loss → spiders no longer preyed upon, plus reduced competition for insects → spider boom.
• Seed dispersal collapse (> 50 % of tree species relied on birds) → seeds now fall under parents; regeneration limited; long-term threat to forest structure.
• Snakes shift to lizards/insects, keeping snake numbers high.

• Biological pest control concept
– Definition: \text{Biological control} = \text{intentional use of a living organism to reduce a pest population to acceptable levels}.
– Success example: prickly pear cactus (Opuntia spp.) invaded 24 million ha in Australia; introduction of Cactoblastis cactorum moth (larvae feed inside cactus) from 1926 → > 99 % reduction by 1933; today maintained at low equilibrium.
– Criteria for success: host-specific agent, high reproductive rate, survival in target climate, minimal non-target impacts.

• Risks & ethical considerations
– Biocontrol agent may itself become invasive (e.g., cane toad fiasco).
– Need for rigorous ecological risk assessment, quarantine trials, post-release monitoring.
– Balancing preservation of native biodiversity against unintended harm.

Sampling Ecosystems with Quadrats (Section 7.7)

• Purpose
– Estimate population size/density & distribution of sessile or slow-moving organisms; detect invasion fronts; monitor management outcomes.

• Quadrat basics
– Frame of known area (square/rectangle/circle); common sizes: 0.25\,\text{m}^2, 1\,\text{m}^2.
– Suitable for plants, lichens, corals, worm casts, molluscs, droppings, nests.

• Random sampling protocol

  1. Map study area; overlay coordinate grid.

  2. Use random number generator to pick x,y coordinates → eliminates conscious bias.

  3. Place quadrat, count target species using consistent inclusion rule (e.g., count individuals rooted inside or touching top/right edges only).

  4. Repeat ≥ 3 times (preferably 10–20) to improve reliability; calculate mean and sometimes variance/standard error.

• Key calculations
– Mean per quadrat: \text{mean} = \frac{\sum \text{counts}}{n{\text{samples}}}. – Scaling to whole habitat: \text{Total} = \left(\frac{A{\text{habitat}}}{A_{\text{quadrat}}}\right) \times \text{mean count}.
– Example (earpod wattle):
• Counts = 3, 0, 1, 4, 2 (five 0.25 m² quadrats). \text{mean}=\frac{10}{5}=2.
• Habitat = 100\,\text{m}^2. Number of quadrats possible: \frac{100}{0.25}=400.
• Estimated plants = 400 \times 2 = 800.

• Reliability & replication
– Greater sample size reduces sampling error.
– Replicate over time to detect trends (e.g., 6-month resurvey after seedling removal).
– Cross-check identification skills; consistent counting rules.

• Management applications
– Data justify community action (mechanical removal, herbicide spot-treatment, public awareness).
– Adaptive management: monitor → act → monitor again → adjust strategy.

Overarching Connections & Implications

• Interdependence, bioaccumulation, and invasions are intertwined:
– Removing predators via toxins or invasives destabilises food webs.
– Loss of biodiversity lowers ecosystem resilience to disease, climate shifts, and further invasions.

• Mathematical & scientific practices
– Use of population curves, mean calculations, scaling factors.
– Importance of random sampling, replication, and statistical reasoning for reliable ecological inference.

• Ethical & practical debates
– Food security vs. environmental health (pesticide use).
– Deliberate introductions (for pest control, aesthetics, agriculture) vs. precautionary principle.
– Role of humans as ecosystem managers; need for integrated, evidence-based solutions.

• Key vocabulary
– Interdependence, population, producer, consumer, decomposer, predator–prey cycle, biodiversity, pesticide, persistent toxin, bioaccumulation, biomagnification, native, introduced, invasive, biological pest control, quadrat, random sampling.