45.6 Community Ecology

Community Interactions and Dynamics

Dynamics of predator-prey relationships

Predator-prey relationships are one of the most fundamental forces shaping community structure. Predators regulate prey populations through direct mortality, and prey availability in turn controls predator numbers, since predators depend on sufficient prey for survival and reproduction.

The Lotka-Volterra model describes the cyclic fluctuations that result from this interdependence:

1. When predators are scarce, prey populations grow because predation pressure is low.
2. More prey means more food for predators, so predator populations begin to increase.
3. Rising predator numbers drive prey populations back down through increased consumption.
4. With less prey available, predators face food scarcity and their populations decline.
5. The cycle then repeats, producing characteristic oscillating curves over time.

These effects don't stop at two species. Trophic cascades occur when changes at one trophic level ripple through the food web. For example, removing wolves (top predators) from Yellowstone led to elk overgrazing on willows and aspens, which altered plant community structure and even affected stream bank erosion.

Keystone species are organisms whose impact on community structure is disproportionately large relative to their abundance. Sea otters are a classic example: they prey on sea urchins, keeping urchin populations in check. Without otters, urchin populations explode and decimate kelp forests, collapsing the habitat for many other species.

Food webs map out these complex feeding relationships and energy flow within a community, showing how predator-prey interactions connect across multiple trophic levels.

Adaptations against predation and herbivory

Organisms have evolved a wide range of defenses against being eaten. These fall into three main categories.

Physical defenses make organisms difficult to consume:

• Spines (cacti), thorns (roses), and tough exoskeletons (beetles) physically deter attackers
• Camouflage helps organisms blend into their surroundings (leaf insects resemble the foliage they rest on)
• Mimicry involves resembling another species. King snakes, which are harmless, have color patterns similar to venomous coral snakes, discouraging predators from attacking

Chemical defenses discourage consumption through toxicity or bad taste:

• Monarch butterflies accumulate toxic compounds from milkweed during their larval stage, making them unpalatable to birds
• Plants produce secondary metabolites like tannins (oak trees) that make tissues hard to digest, or alkaloids (caffeine in coffee plants) that are toxic to herbivores

Behavioral defenses help organisms avoid or escape predators:

• Fleeing or hiding (rabbits darting into burrows)
• Forming groups (wildebeest herds), which reduces each individual's risk through the dilution effect and increases the chance that at least one member spots a predator
• Aposematic coloration uses bright warning colors to advertise toxicity. Poison dart frogs, for instance, display vivid reds, blues, and yellows that signal danger to would-be predators

Competitive exclusion principle in communities

The competitive exclusion principle states that two species with identical ecological niches cannot coexist indefinitely in the same habitat. Over time, the stronger competitor will drive the other to local extinction.

So how do similar species manage to coexist? Through resource partitioning, where species specialize on different resources or microhabitats. This is also called niche differentiation. Darwin's finches on the Galápagos Islands are a textbook example: different species evolved distinct beak shapes to exploit different food sources (seeds of different sizes, insects, cactus fruits), allowing them to share the same islands.

Character displacement is the evolutionary mechanism behind resource partitioning. When closely related species overlap geographically (are sympatric), natural selection favors individuals whose traits differ from the competing species. Over generations, this leads to divergence in morphology, behavior, or ecology that reduces niche overlap. In Galápagos finches, beak sizes differ more between species where they coexist on the same island than where each species lives alone.

Interspecific competition is a major force shaping which species are present in a community and how abundant they are.

Types of symbiotic relationships

Symbiosis refers to close, long-term interactions between species. There are three main types, defined by whether each partner benefits or is harmed.

Mutualism (+/+): Both species benefit.

• Pollination: Plants produce nectar to attract pollinators like bees. The bee gets food, and the plant gets its pollen transferred to other flowers for reproduction.
• Nitrogen fixation: Legumes such as soybeans host nitrogen-fixing bacteria (genus Rhizobium) in root nodules. The bacteria convert atmospheric $N_2$ into usable ammonia for the plant, and the plant supplies carbohydrates to the bacteria.

Commensalism (+/0): One species benefits; the other is unaffected.

• Epiphytes like orchids grow on tree branches, gaining better access to sunlight without harming the host tree.
• Remora fish attach to sharks via a suction disc, gaining transportation and scraps of food without measurably affecting the shark.

Parasitism (+/−): The parasite benefits at the expense of the host.

• Tapeworms live in the digestive tract of animals, absorbing nutrients and causing malnutrition and weight loss in the host.
• Mistletoe is a parasitic plant that taps into a host tree's vascular tissue to extract water and nutrients, potentially reducing the tree's growth and survival.

Parasites generally don't kill their hosts outright (that would eliminate their resource), but they reduce host fitness, growth, or reproductive success.

Formation of community structure

Ecological succession is the process by which community composition changes over time in a predictable sequence. There are two types, distinguished by starting conditions.

Primary succession occurs in areas with no pre-existing soil or life, such as newly formed volcanic islands or land exposed by retreating glaciers:

1. Pioneer species like lichens and mosses colonize bare rock. Lichens produce acids that slowly break down rock, beginning soil formation.
2. As thin soil accumulates, grasses and herbaceous plants move in, adding organic matter and building soil depth.
3. Shrubs and small trees establish as soil fertility improves, gradually shading out the earlier species.
4. Mature forest develops as large trees create a complex canopy, supporting diverse understory vegetation.

Primary succession is slow, often taking hundreds or thousands of years.

Secondary succession occurs after a disturbance (fire, logging, abandoned farmland) in areas where soil and seed banks already exist, so it proceeds much faster:

1. Grasses, forbs, and herbaceous plants quickly colonize the disturbed area and rebuild soil organic matter.
2. Shrubs and fast-growing trees like aspen establish, outcompeting the herbaceous species.
3. Shade-tolerant trees like maple gradually replace the early colonizers and form a mature canopy.

A few patterns hold across both types of succession:

• Early successional species (dandelions, fireweed) are fast-growing, produce many seeds, and tolerate harsh conditions.
• Late successional species (oak, hickory) grow more slowly, compete well for resources, and tend to be more specialized.

The climax community is the relatively stable endpoint of succession, determined largely by regional climate and soil conditions. Examples include temperate deciduous forests, prairies, and marshes. A climax community persists until a major disturbance resets the process.

Community Resilience and Conservation

Biodiversity strengthens community stability in two key ways. Functional redundancy means multiple species perform similar ecological roles, so the loss of one species doesn't collapse an entire function. Response diversity means species within a functional group react differently to disturbances, so the community as a whole is more likely to withstand environmental change.

Habitat fragmentation breaks continuous habitat into isolated patches, disrupting species interactions, limiting movement between populations, and reducing overall biodiversity. This is one of the most significant threats to community structure in conservation biology.