🐠Ecotoxicology Unit 8 – Ecosystem–Level Effects of Toxicants

Key Concepts and Definitions

• Ecotoxicology studies the effects of toxicants on ecosystems, including their components, interactions, and functions
• Toxicants are substances that have adverse effects on living organisms and can be natural or anthropogenic (pesticides, heavy metals)
• Bioaccumulation occurs when an organism absorbs a toxicant at a rate faster than it can eliminate it, leading to an increase in concentration over time
• Biomagnification is the process by which toxicant concentrations increase as they move up the food chain due to repeated bioaccumulation
• Population dynamics refer to changes in population size, structure, and distribution over time, which can be affected by toxicants
• Community-level impacts involve changes in species composition, diversity, and interactions within an ecosystem due to toxicant exposure
• Ecosystem functions include nutrient cycling, energy flow, and primary production, which can be disrupted by toxicants
• Ecological risk assessment evaluates the likelihood and severity of adverse effects on ecosystems caused by toxicants

Toxicants and Their Sources

• Toxicants can be classified as organic (pesticides, PCBs) or inorganic (heavy metals, acids)
• Point sources of toxicants include industrial discharges, sewage treatment plants, and mining operations
  • These sources release toxicants directly into the environment at a specific location
• Non-point sources are diffuse and include agricultural runoff, atmospheric deposition, and urban stormwater
  • These sources are more challenging to control and regulate due to their widespread nature
• Natural toxicants can be produced by organisms (algal toxins) or result from geological processes (volcanic emissions)
• Anthropogenic toxicants are created by human activities and include synthetic chemicals, byproducts of industrial processes, and pharmaceuticals
• The persistence of a toxicant in the environment depends on its chemical properties, such as solubility, volatility, and reactivity
• The bioavailability of a toxicant determines its potential for uptake and accumulation by organisms

Ecosystem Components and Interactions

• Ecosystems consist of biotic (living) and abiotic (non-living) components that interact with each other
• Producers (plants) convert sunlight into chemical energy through photosynthesis, forming the base of the food chain
• Consumers (animals) obtain energy by feeding on producers or other consumers and can be classified as herbivores, carnivores, or omnivores
• Decomposers (bacteria, fungi) break down dead organic matter, releasing nutrients back into the ecosystem
• Abiotic factors include temperature, pH, moisture, and nutrient availability, which influence the distribution and abundance of organisms
• Interactions among organisms include competition, predation, parasitism, and mutualism, which can be affected by toxicants
• Food webs depict the complex feeding relationships within an ecosystem and can be used to trace the movement of toxicants
• Keystone species play a disproportionately large role in maintaining ecosystem structure and function, and their loss can have cascading effects

Bioaccumulation and Biomagnification

• Bioaccumulation occurs when the rate of toxicant uptake exceeds the rate of elimination, leading to an increase in concentration within an organism over time
• Factors affecting bioaccumulation include the toxicant's lipophilicity (fat solubility), the organism's metabolic rate, and the duration of exposure
• Biomagnification is the process by which toxicant concentrations increase as they move up the food chain due to repeated bioaccumulation
  • Organisms at higher trophic levels (top predators) tend to have the highest toxicant concentrations
• Bioconcentration is the accumulation of a toxicant from the surrounding environment (water, air) into an organism's tissues
• Bioaccumulation factors (BAFs) and bioconcentration factors (BCFs) are used to quantify the extent of accumulation relative to the toxicant's concentration in the environment
• Trophic transfer efficiency determines the amount of toxicant passed from one trophic level to the next and influences the degree of biomagnification
• Persistent organic pollutants (POPs) are particularly prone to bioaccumulation and biomagnification due to their stability and lipophilicity (DDT, PCBs)

Effects on Population Dynamics

• Toxicants can affect population dynamics by altering survival, reproduction, and growth rates
• Acute toxicity occurs when organisms are exposed to high concentrations of a toxicant over a short period, often resulting in mortality
• Chronic toxicity involves long-term exposure to lower concentrations, which can lead to sublethal effects such as reduced growth, impaired reproduction, and increased susceptibility to disease
• Endocrine disruptors (BPA, DDT) can interfere with hormone signaling, leading to reproductive abnormalities and population declines
• Toxicants can cause shifts in age structure by disproportionately affecting certain life stages (larvae, juveniles)
• Population recovery after toxicant exposure depends on factors such as the severity of the impact, the species' reproductive potential, and the availability of unaffected individuals for recolonization
• Toxicants can alter population genetics by selecting for resistant individuals, leading to changes in allele frequencies over time

Community-Level Impacts

• Community-level impacts involve changes in species composition, diversity, and interactions within an ecosystem due to toxicant exposure
• Toxicants can cause direct mortality of sensitive species, leading to shifts in community structure
• Indirect effects occur when toxicants alter the abundance or behavior of one species, which then affects other species through trophic interactions (predator-prey relationships, competition)
• Keystone species are particularly important in maintaining community structure, and their loss can have cascading effects on other species
• Toxicants can reduce species diversity by eliminating sensitive species or favoring tolerant ones, leading to a simplification of the community
• Invasive species may be more tolerant to toxicants than native species, allowing them to outcompete and displace native populations in contaminated environments
• Changes in community composition can alter ecosystem functions, such as nutrient cycling and primary production
• Recovery of communities after toxicant exposure depends on the severity of the impact, the availability of unaffected species for recolonization, and the restoration of habitat quality

Ecosystem Function Alterations

• Ecosystem functions are the processes that maintain the flow of energy and cycling of nutrients within an ecosystem
• Primary production is the conversion of sunlight into chemical energy by producers (plants) and can be reduced by toxicants that inhibit photosynthesis or cause plant mortality
• Nutrient cycling involves the transfer of essential elements (carbon, nitrogen, phosphorus) between biotic and abiotic components of an ecosystem
  • Toxicants can disrupt nutrient cycling by altering microbial communities or changing the chemical form of nutrients
• Decomposition is the breakdown of dead organic matter by decomposers (bacteria, fungi) and can be impaired by toxicants that affect microbial activity or alter litter quality
• Toxicants can change the rates of ecosystem processes, such as respiration and denitrification, by affecting the organisms responsible for these functions
• Alterations in ecosystem functions can lead to changes in ecosystem services, such as water purification, carbon sequestration, and flood control
• The resilience of an ecosystem to toxicant exposure depends on its ability to maintain or recover essential functions after disturbance

Assessment and Monitoring Methods

• Ecological risk assessment is a process that evaluates the likelihood and severity of adverse effects on ecosystems caused by toxicants
  • It involves problem formulation, exposure assessment, effects assessment, and risk characterization
• Biomonitoring uses living organisms (bioindicators) to assess the health of an ecosystem and detect the presence of toxicants
  • Bioindicators can be species, communities, or biological processes that are sensitive to environmental changes
• Biomarkers are measurable changes in biological systems (molecular, cellular, physiological) that indicate exposure to or effects of toxicants
  • Examples include enzyme activity, DNA damage, and stress protein production
• Environmental monitoring involves the regular measurement of physical, chemical, and biological parameters to track changes in ecosystem health over time
• Toxicity tests are conducted in the laboratory to determine the effects of toxicants on individual organisms or simple communities under controlled conditions
  • These tests can provide information on acute and chronic toxicity, as well as sublethal effects
• Field studies are used to assess the impacts of toxicants on ecosystems under natural conditions and can involve manipulative experiments or observational surveys
• Modeling techniques, such as population viability analysis and ecological risk models, can predict the long-term effects of toxicants on ecosystems based on available data

Case Studies and Real-World Examples

• The Deepwater Horizon oil spill (Gulf of Mexico, 2010) released millions of barrels of oil, affecting marine ecosystems and causing widespread mortality of fish, birds, and marine mammals
  • Long-term impacts included reduced biodiversity, altered community structure, and impaired ecosystem functions
• The Chernobyl nuclear accident (Ukraine, 1986) released radioactive contaminants that accumulated in plants and animals, leading to genetic mutations and population declines
  • Exclusion zones around the site have become unintentional wildlife refuges, demonstrating the resilience of ecosystems in the absence of human disturbance
• Acid rain, caused by the emission of sulfur and nitrogen oxides from fossil fuel combustion, has led to the acidification of lakes and streams, reducing biodiversity and altering nutrient cycling
  • Recovery of affected ecosystems has been observed following the implementation of emission control regulations
• The use of DDT as a pesticide in the mid-20th century caused widespread declines in bird populations due to eggshell thinning and reproductive failure
  • The ban on DDT in many countries has allowed for the recovery of affected species, such as the bald eagle and peregrine falcon
• Eutrophication, the excessive growth of algae and aquatic plants due to nutrient pollution (phosphorus, nitrogen), can lead to oxygen depletion, fish kills, and changes in community structure
  • Management strategies, such as nutrient load reduction and wetland restoration, have been used to mitigate the effects of eutrophication in aquatic ecosystems