One approach to this complex topic of ecotoxicologic effects is to organize effects according to biological levels of organization. One may consider effects, in ascending order, at the subcellular (molecular and biochemical), cellular, organismal, population, community, and ecosystem levels of organization. Ecotoxicology deals with, theoretically at least, all species, and in line with other aspects of natural resource management, the primary concern is one of sustainability. The policies and regulations surrounding chemical effects in natural ecosystems are designed to protect ecological features such as population dynamics, community structures, and ecosystem functions.
Molecular and Biochemical Effects
This lowest level of organization includes fundamental processes associated with the regulation of gene transcription and translation, biotransformation of xenobiotics, and the deleterious biochemical effects of xenobiotics on cellular constituents including proteins, lipids, and DNA.
Gene Expression and Ecotoxicogenomics
Xenobiotics can affect gene transcription through interactions with transcription factors and/or the promoter regions of genes. In the context of environmental toxicology, perhaps the most studied xenobiotic effects involve ligand-activated transcription factors. These intracellular receptor proteins recognize and bind specific compounds, thus forming a complex that binds to specific promoter regions of genes, thereby activating transcription of mRNAs, and ultimately translation of the associated protein.
The dominant natural ligand for this nuclear receptor is estradiol (E2). Binding of E2 with estrogen receptor (ER) produces a complex that can then bind to estrogen response elements (ERE) of specific genes that contain one or more EREs, thereby causing gene transcription. Genes regulated in this manner by E2–ER play various important roles in sexual organ development, behavior, fertility, and bone integrity.
A number of chemicals can serve as ligands for ER; in most cases these “xenoestrogens” activate gene transcription acting as receptor agonists. Some of these xenoestrogens include diethylstilbestrol (DES), DDT, methoxychlor, endosulfan, surfactants (nonyl-phenol), some PCBs, bisphenol A, and ethinyl E2, a synthetic estrogen observed in municipal effluents and surface waters. Environmental exposures to these chemicals are sufficient to perturb reproduction or development.
Aryl Hydrocarbon Receptor
The aryl hydrocarbon receptor (AHR) is a member of the basic helix–loop–helix Per ARNT Sim (bHLH-PAS) family of receptors/transcription factors that is involved in development, as sensors of the internal and external environment in order to maintain homeostasis, and in establishment and maintenance of circadian clocks. Characterized genes that are upregulated by the AHR system code for enzymes involved in the metabolism of lipophilic chemicals, including organic xenobiotics and some endogenous substrates such as steroid hormones. These enzymes include mammalian CYP1A1, 1A2, and 1B1 and their counterparts in other vertebrates, glutathione transferase, glucuronosyltransferase, alcohol dehydrogenase, and quinone oxidoreductase.
Some ubiquitous pollutants that act as AHR ligands and markedly upregulate gene transcription via the AHR–ARNT signaling pathway include the polycyclic aromatic hydrocarbons (PAHs) and the polyhalogenated aromatic hydrocarbons (pHAHs). In general, pHAH-type AHR ligands are more potent AHR ligands and enzyme inducers than PAHs.
Genomics and Ecotoxicogenomics
Ecotoxicogenomics has great potential for elucidating impacts of chemicals of ecological concern and ultimately for playing an important role in ecological risk assessments (ERA) and regulatory ecotoxicology. Specific areas to which this emerging field can contribute include prioritization of chemicals investigated in ERA, identification of modes of action of pollutants, identification of particularly sensitive species, and effect prediction at higher levels of organization.
Acetylcholinesterase (AChE) degrades the neurotransmitter acetylcholine, and controls nerve transmission in cholinergic nerve tracts. The widely used organophosphate and carbamate classes of insecticides kill by inhibiting AChE, and this mechanism is operative for “nontarget” organisms including invertebrates, wildlife, and humans. Of particular ecological concern have been the ingestion of AChE-inhibiting insecticides with food items or granular formulations (mistaken as seed or grit) by birds and aquatic animal exposures from agricultural run-off. In addition to enzyme inhibition, chemicals can damage proteins in other ways, including oxidative damage and the formation of stable adducts similar to those formed with DNA.
Oxidative stress has been defined as the point at which production of ROS exceeds the capacity of antioxidants to prevent damage. Numerous environmental contaminants act as prooxidants and enhance production of ROS. The resulting oxidative damage can account wholly or partially for toxicity. Mechanisms by which chemicals enhance ROS production include redox cycling, interactions with electron transport chains (notably in mitochondria, microsomes, or chloroplasts), and photosensitization. Redox cycling chemicals include diphenols and quinones, nitroaromatics and azo compounds, aromatic hydroxylamines, paraquat, and certain metal chelates, particularly of copper and iron.
Photosensitization is an important mechanism in aquatic systems. Ultraviolet (UV) radiation (specifically UVB and UVA) can penetrate surface waters to varying depths, depending on the wavelength of the radiation and the clarity of the water. The UV radiation generates ROS and other free radicals via excitation of photosensitizing chemicals, including common pollutants of aquatic systems.
ROS can drive redox status to a more oxidized state, potentially reducing cell viability. These ROS-mediated impacts and others have been associated with several human diseases including atherosclerosis, arthritis, cancer, and neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. With the exception of cancer, the role of ROS in specific diseases in wildlife has received little attention. It is reasonable to assume that oxidative stress accounts in part for the toxicity of diverse pollutants to free-living organisms.
Cancer is an important health outcome associated with chemical exposures in wildlife. In the context of ecotoxicology, the most widely studied form of damage has been the formation of stable DNA adducts, particularly by PAHs. PAHs must be activated to reactive metabolites to form these adducts.
Cellular, Tissue, and Organ Effects
Most free-living organisms routinely experience energy deficits. For example, food resources are often scarce during the winter for many animals, which adapt by conserving energy (by hibernating or lowering metabolism) or by storing energy beforehand (as is the case for many migratory birds). Thus, the effects of pollutants on mitochondrial energy metabolism can be of particular importance to wildlife.
Lysosomes, which are involved in the degradation of damaged organelles and proteins, sequester many environmental contaminants, including metals, PAHs, and nanoparticles. The accumulation of xenobiotics by lysosomes can elicit membrane damage, which warns of pathological effects in both invertebrates and vertebrates.
Chemical effects on nuclei have been examined in ecological contexts. Micronuclei are chromosomal fragments that are not incorporated into the nucleus at cell division, and chemical exposures can markedly increase their frequency. Elevated micronuclei numbers have been observed in erythrocytes in fish and in hemocytes in clams from a PCB-polluted harbor.
An important target organ in ecotoxicology of nonmammalian aquatic vertebrates and many invertebrates is the gill, which is the major site of gas exchange, ionic regulation, acid–base balance, and nitrogenous waste excretion. Gills are immersed in a major exposure medium for these animals (surface water), so metabolically active epithelial cells are in direct contact with this medium. They also receive blood supply directly from the heart. Common structural lesions in gills include cell death (via necrosis and apoptosis), rupture of the epithelium, hyperplasia, and hypotrophy of various cell pop-ulations that can lead to lamellar fusion, epithelial swelling, and lifting of the respiratory epithelium from the underlying tissue.
Chemical pollution of the environment does not generally attain levels sufficient to outright kill wildlife. The ecotoxicologic concerns are the long-term, chronic impacts of chemicals on organismal variables such as reproduction and development, behavior, and disease susceptibility, and how such impacts parlay into effects at the population and higher levels of organization. However, mortality is an endpoint in exposure studies.
Reproduction and Development
Contaminant effects on development are often difficult to discern in field studies, due to the small size of embryos and the fact that developmental impacts are generally either lethal or greatly reduce survival. Because early life stages of most organisms are generally more sensitive to xenobiotics than other life stages, developmental impacts merit careful attention by ecotoxicologists.
Chlorinated hydrocarbons continue to generate concerns although many (DDT and other insecticides, and PCBs) have had their production and use sharply curtailed. The dioxins (TCDD) and coplanar PCBs compromise cardiac development, among other effects in vertebrates, and these developmental perturbations are largely receptor-mediated and dependent on binding of the chemical (such as TCDD) with the AHR.
Hydrocarbons, in large part PAHs, associated with oil spills, contaminated sediments, paper mill effluents, and creosote used for wood treatment have profound developmental effects in fish embryos. In many cases, the effects observed visually appear similar to those observed in fish embryos exposed to dioxins and coplanar PCBs, and include malformed hearts (“tube hearts”), craniofacial deformities, hemorrhaging, and edema of the pericardium and yolk sac, the latter resulting in a distended, faintly blue yolk sac that gives this syndrome the name “blue sac disease.”
The potential impacts of environmental contaminants on immune systems that render organisms more susceptible to disease are of great concern. Numerous laboratory studies have demonstrated chemical impacts on immune systems in animals of ecological relevance. These include pesticides in amphibians, PCBs in channel catfish, heavy metals in rainbow trout, PAHs in bivalves, and flame retardants (polybrominated diphenyl ethers) in American kestrels.
Relatively subtle effects on behaviors associated with mating and reproduction, foraging, predator–prey interactions, preference/avoidance of contaminated areas, and migration have potentially important ramifications for population dynamics. In some cases, the biochemical mechanisms underlying behavioral effects have been elucidated, which may assist our understanding of these issues and provide useful biomarkers for behavioral toxicants in field studies.
Chemicals causing behavioral effects in wildlife are known to be neurotoxicants. Behavioral effects of insecticides have been observed in fish. For example, impacts of the organophosphate diazinon on olfactory-mediated behaviors such as the alarm response and homing in the Chinook salmon have been observed, as well as similar thresholds for the effects of another organophosphate (chlorpyrifos) on swimming and feeding behaviors and on AChE inhibition in coho salmon (O. kistich). Mercury, particularly as methylmercury, comprises another potent neurotoxin that has been shown to perturb behavior in wildlife.
Environmental contaminants not generally thought of as neurotoxicants have also been shown to perturb behavior. For example, cadmium and copper have been shown to impact olfactory neurons and associated behaviors (preference/avoidance to chemicals, including pheromones) in several fish species. Copper exposure in zebrafish also led to loss of neurons in the peripheral mechanosensory system (“lateral line”), which could lead to altered behaviors associated with schooling, predator avoidance, and rheotaxis (physical alignment of fish in a current). Clearly, numerous mechanisms of chemical toxicity can result in behavioral impacts, including direct toxicity to neurons, alterations in hormones that modulate behaviors, and impaired energy metabolism. Also, impaired behavior may comprise a sublethal impact with substantive ecological consequence.
Beginning in the 1960s, numerous cases of cancer epizootics in wildlife that are associated with chemical pollution, particularly in specific fish populations, have been reported in North America and northern Europe. As in humans, cancer in these animals occurs largely in relatively older age classes and therefore is oftentimes considered a disease unlikely to directly impact population dynamics or other ecological parameters. However, this may not always be the case, particularly in species that require many years to attain sexual maturity and/or have low reproductive rates.
Lifestyle is a major contributor to differential cancer susceptibility; benthic (bottom-dwelling) species such as brown bullhead (Ameriurus nebulosus) and white sucker (Catostomus commersoni) in freshwater systems and English sole (Parophrys vetulus) and winter flounder (Pseudopleuronectes americanus) in marine systems generally exhibit the highest cancer rates in polluted systems. The bulk of chemicals in these systems associated with cancer epizootics, such as PAHs, PCBs, and other halogenated compounds, reside in sediments; benthic fish live in contact with these sediments and prey in large measure on other benthic organisms.
It is noteworthy that many reports of elevated cancer rates in free-living animals occur in fish, with few reports of potentially chemically related cancers to our knowledge in other vertebrates. It is likely that elevated exposures play an important role in the relatively high frequency of reports of cancers in benthic fish; relative inherent sensitivities among mammals, birds, reptiles, and amphibians, and fish are unclear.
A population is a collection of individuals of the same species that occupy the same space and within which genetic information can be exchanged. Population ecotoxicology covers a wide range of topics with core research themes being (1) epidemiology of chemical-related diseases, (2) effects on general population qualities including demographics and persistence, and (3) population genetics.
The level of belief warranted for possible contaminant-related effects in nonhuman populations is assessed by applying routine epidemiological methods. Rules of thumb for gauging the level of belief warranted by evidence that emerged from human epidemiology are also applied in population ecotoxicology (Table 29–1).
Table 29–1 A summary of one popular set of rules of thumb for assessing plausibility of a causal association in an ecological epidemiology. ||Download (.pdf)
Table 29–1 A summary of one popular set of rules of thumb for assessing plausibility of a causal association in an ecological epidemiology.
1. Strength of association
How strong the association is between the possible cause and the effect, for example, a very large relative risk
2. Consistency of association
How consistently is there an association between the possible cause and the effect, for example, consistent among several studies with different circumstances
3. Predictive performance
How good is the prediction of effect made from the presence/level of the possible cause
4. Monotonic trend
How consistent is the association between possible cause and effect to a monotonic trend (i.e., either a consistent increase or decrease in effect level/prevalence with an increase in exposure)
5. Inconsistent temporal sequence
The effect, or elevated level of effect, occurs before exposure to the hypothesized cause
6. Factual implausibility
The hypothesized association is implausible given existing knowledge
7. Inconsistency with replication
Very poor reproducibility of association during repeated field assessments encompassing different circumstances or repeated formal laboratory testing
Some species populations fluctuate within a range of densities. These fluctuations are characteristic of the species' strategy for maintaining itself in various types of habitats and toxicant exposure could potentially change this range. Combined with decreases in population densities driven by external forces such as weather events, these toxicant-induced modifications of the average population densities and dynamics can increase the risk of a population's density falling so low that local extinction occurs. Toxicants can change a species population's vital rates, such as age- and sex-dependent death, birth, maturation, and migration rates. Combined, these changes determine the population density and distribution of individuals among ages and sexes during exposure.
Individuals of the same species often are grouped into subpopulations within a habitat and all of these subpopulations together comprise a metapopulation (Figure 29–2). Subpopulations in the metapopulation have different levels of exchange and different vital rates that depend on the nature of their habitat. Spatial distances and obstacles or corridors for migration influence migration among patches; habitat quality determines vital rates.
Metapopulations are composed of subpopulations that differ in their vital rates and tendency to exchange individuals. In this illustration, subpopulation A occupies a keystone habitat. The loss of subpopulation A would devastate the metapopulation. Also, loss of the migration corridor between subpopulations A, B, and D would devastate the metapopulation. In contrast, the loss of subpopulation F would not influence the metapopulation to the same degree.
The genetics of exposed populations are studied to understand changes in tolerance to toxicants and to document toxicant influence on field populations. Some populations have the capacity to become more tolerant of toxicants via selection. Genetic qualities are also used to infer past toxicant influence in an exposed population. Another piece of evidence demonstrating past toxicant influence on populations can be a change in genetic diversity. A drop in genetic diversity in populations is thought to be an adverse effect because genetic diversity is required in populations to evolutionarily adapt to environmental changes. Toxicants can influence genetic diversity by purely stochastic means.
An ecological community is an interacting assemblage of populations occupying a defined habitat at a particular time. Populations in a community interact in many ways and, because these many interactions are complex, a community has properties that are not predictable from those of its component populations.
Communities take on characteristic structures as predicted by the Law of Frequencies: the number of individual organisms in a community is related by some function to the number of species in the community. Ecotoxicants can alter the resulting community structure in predictable ways by either directly impacting the fitness of individuals in populations that make up the community or by altering population interactions.
Recently, structural and functional qualities in communities have been combined to generate multimetric indices such as the Index of Biotic Integrity (IBI). Ecological insight is used to select and then numerically combine community qualities such as species richness, health of individual animals in a sample, and the number of individuals in a sample belonging to a particular functional group, such as number of piscivorous fish. The IBI score for a study site is calculated and compared with that expected for an unimpacted site in order to estimate its biological integrity.
Another central theme in community ecotoxicology is toxicant transfer during trophic interactions. Toxicant concentrations can decrease (biodiminution), remain constant, or increase (biomagnification) with each trophic transfer within a food web. Metals that biomagnify are mercury and the alkali metals, cesium and rubidium. Zinc, an essential metal that is actively regulated in individuals, can exhibit biomagnification or biominification depending on whether ambient levels are below or above those required by the organism to function properly.
Most individuals in a community can feed on different species depending on their life stage, seasons, and relative abundances of prey species. These trophic interactions are best described as occurring in a trophic web, not a trophic chain.
Ecosystems, the functional unit of ecology, are composed of the ecological community and its abiotic habitat. The ecotoxicologist is interested in understanding how toxicants diminish an ecosystem's capacity to perform essential functions and to understand toxicant movement within different ecosystem components enough to assess exposure.
Conventional ecosystem studies involve descriptions of contaminant concentrations and movements in easily defined ecosystems such as lakes, forests, or fields. Some toxicants, especially those subject to wide dispersal by air or water, cannot be completely understood in this framework, so a landscape scale might be chosen instead. As an example, acid precipitation might be examined in the context of an entire watershed, mountain range, or even a continental region. Still other ecotoxicants require a global context in order to fully understand their movements and accumulation. As an example, hexachlorobenzene concentration in tree bark collected worldwide showed a clear latitudinal gradient.