Table of Contents
Biotic environmental factors are the influences that living organisms exert on one another. In contrast to abiotic factors (such as light, temperature, or water), biotic factors are dynamic: they change as populations grow, decline, or alter their behavior.
In this chapter, the focus is on:
- How organisms affect each other’s survival, growth, and reproduction.
- Typical types of biotic interactions.
- How these interactions can limit or promote the distribution and abundance of species.
Types of Biotic Interactions
Biotic factors arise from interactions between:
- Individuals of the same species (intraspecific interactions).
- Individuals of different species (interspecific interactions).
These interactions can be classified according to whether they have positive, negative, or neutral effects on the partners:
- Both partners benefit: mutualism ($+ / +$).
- One benefits, the other is harmed: predation, parasitism, herbivory ($+ / -$).
- Both are harmed: competition ($- / -$).
- One benefits, the other is (almost) unaffected: commensalism ($+ / 0$).
- One is harmed, the other is unaffected: amensalism ($- / 0$).
- Essentially no effect on either: neutralism ($0 / 0$, rare and mostly theoretical).
These interactions act as “environmental forces” that influence where organisms can live successfully and how their populations develop.
Intraspecific Interactions
Intraspecific (within-species) interactions are especially important because individuals of the same species have similar needs and use similar resources.
Intraspecific Competition
Members of a species compete for limited resources such as:
- Food
- Water
- Space
- Mates
- Nesting or resting sites
Key points:
- Competition tends to intensify as population density increases.
- It can limit growth, reduce reproduction, and increase mortality.
- It often leads to territorial behavior, dominance hierarchies, and dispersal of individuals.
Examples:
- Plants of the same species growing densely compete for light, water, and nutrients; weaker individuals may die (self-thinning).
- Tadpoles of the same frog species may grow more slowly when crowded, due to limited food and accumulation of waste.
Intraspecific competition is a central mechanism for density regulation in populations and influences how individuals are spaced (e.g., evenly distributed territories vs. aggregations).
Social Interactions and Group Living
Many animals interact socially within their species and form groups (packs, flocks, herds, colonies).
Advantages (potential biotic benefits):
- Better protection from predators (more eyes, group defense).
- More efficient foraging (sharing information about food sources).
- Cooperative breeding or care of young.
Costs (biotic “pressures” within the group):
- Competition for food and mates.
- Spread of diseases and parasites.
- Risk of conflicts and injuries.
Dominance hierarchies (e.g., “pecking order” in chickens) can:
- Reduce the frequency of serious fights by clarifying access to resources.
- Influence reproductive success (dominant individuals often reproduce more).
In social insects (ants, bees, termites), highly developed division of labor and communication systems (pheromones, dance language in honeybees) shape the internal biotic environment of the colony.
Reproductive Interactions: Mates and Parental Care
Biotic factors also arise from interactions related to reproduction:
- Mate choice: preferences for certain traits (e.g., bright plumage, courtship songs) can influence which individuals reproduce (sexual selection).
- Mating systems (monogamy, polygyny, polyandry) determine how reproductive opportunities are shared.
- Parental care (feeding, protection, teaching) affects offspring survival and thus the next generation’s success.
These interactions can drive the evolution of traits that are advantageous in social and reproductive contexts but may have costs in other environments (e.g., conspicuous ornaments that attract both mates and predators).
Interspecific Interactions: Competition
Interspecific Competition and the Niche
When different species rely on similar limited resources, they may compete. This can affect:
- Growth rates
- Survival
- Reproductive success
- Spatial distribution of species
The concept of the ecological niche (covered in more detail elsewhere) is key to understanding interspecific competition:
- Each species has a niche defined by its resource use and role in the ecosystem.
- If two species’ niches overlap strongly, they may not coexist stably; one may exclude the other (competitive exclusion).
Outcomes of interspecific competition:
- Local extinction of one competitor.
- Resource partitioning: competing species use different parts or aspects of the resource (different foraging times, microhabitats, or food sizes).
- Character displacement: evolutionary changes that reduce niche overlap (e.g., beak size differences in closely related bird species living together).
Competition is thus both a limiting factor and a driver of specialization and diversity.
Predation, Herbivory, and Parasitism
Interactions where one organism benefits at the expense of another are widespread and powerful biotic factors.
Predation
Predators kill and eat other organisms (prey). This includes classical predators (e.g., wolves feeding on deer) and many smaller interactions (e.g., ladybugs eating aphids).
Effects on prey populations:
- Direct reduction of numbers via killing.
- Indirect effects: changes in behavior and habitat use to avoid predators (e.g., feeding at safer times, using cover).
Effects on predator populations:
- Predator numbers depend on prey availability.
- Predator and prey densities can show cyclical fluctuations (population cycles).
Ecological consequences:
- Predators can prevent prey populations from exceeding the carrying capacity of their environment.
- They can “shape” prey behavior, morphology (e.g., defensive spines, camouflage), and life histories.
- They may maintain species diversity by limiting competitively dominant prey species.
Herbivory
Herbivores feed on plants or algae. Unlike many predators, they often do not kill their food organisms outright (grazing, browsing) but still reduce plant fitness.
Biotic effects:
- Reduction of plant biomass and reproductive output.
- Selective feeding can change plant community structure (some species are more heavily grazed than others).
- Plants respond with defenses:
- Structural: thorns, tough leaves.
- Chemical: toxins, bitter substances.
Plant–herbivore relationships can be highly specialized (e.g., insects feeding only on one plant species) or generalist.
Parasitism
Parasites live in or on a host organism and obtain resources at the host’s expense, usually without immediately killing it.
Types:
- Ectoparasites: on the surface (e.g., ticks, lice, some fungi).
- Endoparasites: inside the host (e.g., tapeworms, many protists).
- Microparasites (e.g., bacteria, viruses) and macroparasites (e.g., worms, arthropods).
Biotic effects:
- Reduced host growth and reproduction.
- Increased host mortality, especially when stressed by other factors.
- Altered host behavior (some parasites manipulate host behavior to enhance their transmission).
Parasites can also regulate host populations and influence community composition, often in subtle ways.
Mutualism and Commensalism
Not all close interactions are harmful; many are beneficial to one or both partners.
Mutualism
In mutualistic interactions, both partners benefit.
Examples of ecological importance:
- Pollination: flowering plants and their pollinators (insects, bats, birds); plants gain reproduction, pollinators gain nectar or pollen.
- Seed dispersal: animals transport seeds (inside their digestive tract or stuck to their body) while gaining food.
- Nutrient exchange: mycorrhizal fungi on plant roots increase nutrient and water uptake; plants provide sugars.
- Gut symbionts: microorganisms in animal digestive tracts aid in digestion; they receive a stable habitat and nutrients.
Mutualisms can be:
- Obligate: partners cannot survive or reproduce successfully without each other.
- Facultative: partners benefit but can live independently.
Mutualistic interactions can strongly influence where species can live (e.g., some plants only thrive where their mycorrhiza partners occur).
Commensalism and Amensalism
Commensalism ($+ / 0$):
- One species benefits; the other is (as far as can be measured) neither helped nor harmed.
- Example: small fish following larger fish and feeding on leftovers; birds nesting in trees that are not significantly affected.
Amensalism ($- / 0$):
- One species is harmed; the other is unaffected.
- Example: large animals trampling vegetation but gaining no food or shelter from the trampled plants.
Although hard to demonstrate precisely (small effects can be difficult to detect), such relationships illustrate that not all associations fit neatly into the more common categories of competition, predation, or mutualism.
Biotic Factors and Species Distributions
Biotic environmental factors influence:
- Whether a species can establish in a new area.
- Which habitats within an area are actually occupied.
- How abundant a species becomes.
Important aspects:
- A habitat may be physically suitable (appropriate abiotic conditions), but a species may still be absent or rare due to strong predation, intense competition, or lack of mutualistic partners.
- Introduction of new species (e.g., invasive species) can drastically alter existing biotic interactions, sometimes causing declines or extinctions of native species.
- Changes in one species’ population can cascade through a food web, affecting many others (trophic cascades).
Thus, the “living environment” created by other organisms is as crucial as the nonliving environment in shaping ecological patterns.
Dynamic Nature of Biotic Environmental Factors
Because organisms grow, reproduce, move, and die, biotic factors are:
- Variable in time (daily, seasonally, across years).
- Heterogeneous in space (different in microhabitats, across regions).
- Responsive to changes in abiotic conditions (e.g., climate shifts change species’ ranges and thereby their interactions).
Key consequences:
- Populations and communities are constantly evolving systems.
- Ecological relationships can change qualitatively (mutualisms turning into parasitisms or vice versa under different conditions).
- Human activities that alter species abundances (overfishing, habitat fragmentation, introduction of non-native species) modify these biotic factors and can destabilize ecosystems.
Biotic environmental factors, together with abiotic factors, form the complex network of constraints and opportunities that organisms face in their environment.