Table of Contents
Introduction: Ecology Starts with the Individual
Ecology often looks at big systems—forests, lakes, the whole biosphere—but all of these are built from individual organisms living in specific places. This chapter focuses on how single organisms (plants, animals, fungi, microorganisms) are affected by, and interact with, their immediate environment.
Later chapters in ecology will zoom out to populations, ecosystems, and the biosphere. Here, we stay mostly at the organism level and build the foundation for understanding environmental influences in more detail.
The Environment of an Organism: Habitat and Niche
Every organism lives in a particular place and plays a particular “role” in its surroundings. Two key ideas help describe this:
- Habitat
The physical place where an organism lives.
Examples: a pond, a soil layer, the bark of a tree, a cow’s intestine, a human scalp. - Ecological niche (only the basic idea here)
Describes how an organism “makes a living” in its environment: - What it eats or how it gets energy
- When it is active
- How it reproduces
- How it deals with temperature, light, water, and other factors
- How it interacts with other organisms (predator, prey, parasite, mutualistic partner, etc.)
You can think of the habitat as the address and the niche as the profession and lifestyle of an organism.
Levels of Organization: From Individual to Ecosystem
Although this chapter focuses on individuals, it is useful to know where they fit in the larger picture:
- Individual organism – a single living being
- Population – all individuals of the same species in a given area
- Community – all populations of different species living together in an area
- Ecosystem – the community plus the non-living environment (water, soil, climate, etc.)
At the core of all these levels is the individual organism, which:
- Experiences conditions and resources directly.
- Must survive long enough to grow and reproduce.
- Responds to environmental changes (for example by moving, changing physiology, or dying).
Environment as Conditions and Resources
From the organism’s point of view, the environment can be roughly split into:
- Conditions – environmental factors that an organism experiences but does not “consume”.
Examples: temperature, pH, salinity, wind, current, day length. - Resources – things that are used up or made unavailable to others when consumed.
Examples: food, water (for drinking/uptake), light (for photosynthesis), mineral nutrients, space for nesting or rooting.
This distinction is important:
- A fish can use up oxygen in water (making it less available to others) → oxygen acts partly like a resource.
- Temperature cannot be used up → it is a condition, but it still strongly influences survival and performance.
The Individual’s Perspective: Survival and Fitness
From an ecological viewpoint, the success of an organism in its environment can be described by two related ideas:
- Survival and performance
Can the organism: - Maintain its basic functions (metabolism, repair, osmoregulation)?
- Grow and develop?
- Avoid being killed by predators or harsh conditions?
- Fitness (biological fitness)
Not about strength or health in a human sense, but about reproductive success: - How many viable offspring does an individual contribute to the next generation, relative to others?
The environment influences fitness by:
- Affecting survival (extreme temperatures may kill some individuals).
- Influencing growth and body size (which may affect mating success).
- Changing access to resources (food shortage can reduce reproduction).
Environmental conditions rarely remain perfectly stable, so organisms are constantly challenged to maintain their functions and reproduce despite fluctuations.
Environmental Heterogeneity: Patchy and Variable Worlds
Natural environments are rarely uniform. They are:
- Patchy in space
Even over small distances, conditions can differ: - Light is higher at forest edges than inside dense forest.
- Soil moisture may be higher in a small depression than on a ridge.
- Oxygen may be lower in deeper water than near the surface.
- Variable in time
Over time, conditions shift: - Daily (day–night temperature cycles, light–dark cycles)
- Seasonally (winter cold, summer drought)
- Irregularly (storms, floods, heat waves, fires)
Individual organisms must cope with:
- Short-term fluctuations (for example, a hot midday, a dry week).
- Long-term patterns (for example, winter vs. summer, wet vs. dry seasons).
Different species are adapted to different patterns of variability. For instance:
- Desert plants often endure long dry periods and take rapid advantage of rare rains.
- Many temperate-zone animals plan their life cycle around strong seasons (migration, hibernation, reproduction timing).
Strategies for Dealing with the Environment
Organisms do not simply endure the environment passively. They use strategies that can be grouped, in a simplified way, into three broad types:
1. Avoidance
The organism experiences harsh conditions less, or not at all.
- Spatial avoidance
Moving to more favorable areas: - Migration (birds flying to warmer regions in winter)
- Daily vertical movements in lakes (plankton moving deeper by day, near surface at night)
- Temporal avoidance
Becoming active only when conditions are favorable: - Nocturnal activity to avoid daytime heat
- Seeds that stay dormant in the soil until moisture or temperature is right
- Diapauses or resistant stages (resting eggs, cysts) in many invertebrates
Avoidance is especially common in mobile animals, but even nonmoving organisms (like plants) can use dormant stages to “wait out” bad times.
2. Tolerance (Physiological Coping)
The organism stays in place and endures varying conditions through internal adjustments.
- Short-term physiological responses
- Sweating or panting to cool down (in mammals and birds)
- Closing stomata in leaves to reduce water loss (plants)
- Changing heart rate or breathing rate with temperature or activity
- Longer-term physiological or anatomical traits
- Thick fur or feathers for insulation
- Antifreeze proteins in some fish and insects to prevent ice crystal formation in body fluids
- Special water storage tissues in succulents (e.g., cacti)
Tolerance ranges differ greatly between species. Some microbes thrive in boiling hot springs, while others die at slightly above room temperature.
3. Adaptation (Evolutionary Change)
Over many generations, populations may evolve features that better fit their typical environment.
For individual organisms in one lifetime, it is important to recognize:
- They do not evolve; they inherit traits from parents.
- What they can do is adjust within the limits that their genes allow (for example, increase red blood cell count at high altitude).
From an organism-level perspective, adaptation appears as:
- Traits that seem well-suited to a particular environment (e.g., webbed feet in aquatic birds).
- Behaviors that are effective in a certain habitat (e.g., burrowing in desert rodents).
Later chapters will explore adaptation and evolution in more detail; here, the key point is that the match between organism and environment is often the product of past selection.
Trade-Offs and Constraints
No organism can be perfectly adapted to all conditions. Instead, each faces trade-offs:
- Energy and time are limited:
- Energy spent on temperature regulation is not available for growth or reproduction.
- Producing many small offspring may reduce investment in each one’s survival.
- Specialization vs. generalization:
- Specialists are very effective under a narrow range of conditions (for example, a parasite that can live in only one host species).
- Generalists can survive under a broader range of conditions but may never be optimal under any one of them (for example, omnivores that eat many food types).
For a single organism, these trade-offs show up in:
- How large it grows and how quickly.
- How many offspring it has and how much care it provides.
- Whether it endures harsh conditions or avoids them.
Life Cycles and the Environment
An organism’s life cycle (the sequence from birth to reproduction to death) is closely linked to environmental patterns.
Important aspects include:
- Timing of reproduction
Many organisms reproduce when conditions give their offspring the best chance: - Plants may flower when pollinators are abundant.
- Amphibians may breed during rainy seasons when water bodies are available.
- Developmental stages in different environments
Some species use different habitats at different life stages: - Many insects: eggs and larvae in water, adults in air/land.
- Amphibians: aquatic larvae (tadpoles), terrestrial adults.
- Resting or dispersal stages
Species often produce special stages to survive or spread: - Seeds that can survive drought or cold.
- Spores in fungi and some plants.
- Cysts or resistant eggs in some protists and invertebrates.
From the organism’s perspective, each life stage faces its own environmental challenges and may be adapted differently (for example, larvae may be more sensitive to pollutants than adults).
Environmental Stress and Limits of Tolerance
Each individual has:
- A range of conditions within which it can live (tolerance range).
- Within that, a narrower optimum range where it functions best (maximum growth, highest reproduction, fastest movement, etc.).
From the viewpoint of an individual:
- Near the optimum: performance is high, and resources can be used efficiently.
- Near the limits: performance drops, stress responses increase, and the risk of death rises.
Common effects of environmental stress at the organism level:
- Reduced growth and reproduction.
- Increased susceptibility to disease or predators.
- Behavioral changes (seeking shade, reduced activity, hiding).
Later sections in this part of the course (“Tolerance Range and Ecological Potency” and the chapters on specific abiotic and biotic factors) will analyze these limits more systematically. Here it is enough to recognize that:
- Each organism has boundaries beyond which life is not possible.
- Environmental stress pushes individuals closer to these boundaries.
Interaction with Other Organisms: The Individual in a Biological Context
While this chapter emphasizes the physical environment, no organism lives in isolation from other living beings. Even for a single organism, other organisms are part of its immediate environment:
- Competitors: individuals that use the same resources (food, light, space).
- Predators and prey: relationships where one organism eats another.
- Parasites and hosts.
- Mutualistic partners: for example, pollinators visiting flowers, or gut bacteria aiding digestion.
For an individual, these interactions can:
- Increase survival (e.g., protection in herds, cleaning by symbiotic partners).
- Decrease survival (e.g., being infected by parasites).
- Change access to resources (e.g., being outcompeted for food or territory).
Specific types of biotic interactions will be examined in a later chapter. Here the main idea is that other organisms are as much part of the environment as temperature and water.
Human Influence on the Immediate Environment of Organisms
Humans alter the conditions and resources that organisms experience directly:
- Habitat modification
- Forest clearing, urbanization, agriculture change available space, light, and shelter.
- River regulation and dam building affect water flow, oxygen content, and temperature.
- Chemical changes
- Pollutants (pesticides, heavy metals, plastics) can be toxic or disrupt physiology.
- Nutrient enrichment (fertilizers) can favor some species (algae) and harm others (through oxygen depletion).
- Climate change
- Alters local temperatures, precipitation patterns, and the frequency of extreme events.
- Shifts the conditions beyond the tolerance limits of some organisms, forcing them to move, adapt, or die.
For each individual organism, these changes can mean:
- Higher stress.
- A mismatch between inherited adaptations and the new conditions.
- New interactions, such as exposure to unfamiliar competitors or predators.
Later chapters will explore these human impacts in more detail at population and ecosystem levels. At the organism level, they translate into immediate challenges for survival and reproduction.
Summary: Key Ideas About Organisms in Their Environment
- Each organism lives in a specific habitat and has an ecological niche describing its way of life.
- The environment provides both conditions (e.g., temperature, pH) and resources (e.g., food, water, light).
- Natural environments are patchy and variable in space and time; organisms face both short-term fluctuations and long-term patterns.
- Individual organisms cope through avoidance, tolerance, and through inherited adaptations shaped by evolution.
- Trade-offs and constraints mean that no organism can be perfect under all conditions; specialization and generalization both have costs and benefits.
- An organism’s life cycle, including timing and developmental stages, is strongly tuned to its environment.
- Environmental stress pushes individuals toward the limits of their tolerance, reducing performance and survival.
- Other organisms form a crucial part of the environment, shaping an individual’s access to resources and risk of death.
- Human activities increasingly modify the environments that individual organisms experience, often rapidly and dramatically.
These ideas provide the basis for the following chapters, which will examine tolerance ranges, specific environmental factors, and interactions in more detail.