SUCCESSION

SUCCESSION

Which one defines climax community?

A climax community, in the traditional succession concept, is a relatively stable, self-perpetuating assemblage that maintains its broad composition under the prevailing climate and soil conditions. Long-lived organisms reproduce successfully within the community, nutrient cycles are comparatively tight, and small population changes do not automatically reset succession. This does not mean the community is motionless: individuals die and recruit, species abundances fluctuate, and patches are created by local disturbance. Nor must it have the highest primary productivity; mature biomass carries high maintenance respiration, so net production may be lower than in a young, rapidly growing stage. Pioneer species dominate early stages rather than the climax. Modern ecology has modified the single-climax idea because landscapes experience recurrent disturbances, environmental change, and alternative stable states. A better contemporary interpretation is a persistent late-successional state within a dynamic mosaic. Even with that qualification, “stable and self-perpetuating” captures the intended contrast with short-lived, rapidly changing pioneer communities and explains why it defines the conventional climax.

Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 20

View Details

SUCCESSION

What determines succession pattern?

Successional patterns emerge from interactions among species, dispersal, and abiotic environmental factors. Dispersal determines which seeds, spores, larvae, or adults reach a site, creating historical contingency and priority effects. Abiotic filters such as moisture, temperature, salinity, light, soil, and disturbance decide which arrivals can survive. Species then facilitate, tolerate, inhibit, consume, compete with, or form mutualisms with one another, altering establishment and replacement. Because these processes interact, identical disturbances need not produce identical endpoints. A nitrogen-fixing pioneer may improve poor soil, for example, but only if it arrives and can tolerate the climate. Genetic drift, mutation, and evolution can alter populations over longer times, yet they do not by themselves explain the immediate community sequence. The combined description is therefore more complete than any single driver. It also reflects modern succession theory, which treats trajectories as contingent and potentially multiple rather than a predetermined march toward one climax. Arrival, filtering, and interaction form the mechanistic sequence underlying the keyed content.

Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 20

View Details

SUCCESSION

In primary succession, soil forms due to:

Soil development during primary succession begins through rock weathering combined with the activity of pioneering organisms such as lichens. Temperature fluctuations, freezing, water, and abrasion physically break rock, while carbonic and organic acids chemically dissolve minerals. Lichen fungi adhere to the surface, retain moisture, and secrete acids; their photosynthetic partners provide organic carbon. Dust and small mineral particles become trapped, and dead microbial and lichen material adds the first organic matter. This mixture gradually stores more water and nutrients, enabling mosses and vascular plants to establish. Organic decay contributes, but it cannot act alone before organisms have produced organic material, and it does not supply the mineral fraction. Earthworms generally require an existing soil with organic resources, so they are later contributors rather than initial agents. Fire may expose substrate but does not normally construct soil on bare rock. The combined action of weathering and pioneers therefore explains both mineral production and organic enrichment during the earliest stages.

Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 20

View Details

SUCCESSION

An example of autogenic succession is:

Grasses colonizing a sand dune can initiate autogenic succession because the organisms themselves modify the habitat and influence later community development. Grass roots bind mobile sand, while stems reduce wind speed and trap additional particles. Dead leaves and roots add organic matter, microbes release nutrients, and improved water retention makes the substrate less hostile. These changes permit herbs, shrubs, and eventually trees to establish where regional climate allows. The succession is autogenic to the extent that biological engineering drives the transition, although wind deposition and storms remain allogenic influences. A forest recovering after fire is classified mainly as secondary succession initiated by an external disturbance. Colonization of a new volcanic island and bare rock are examples of primary succession, but those descriptions alone do not specifically demonstrate organism-caused environmental modification. Sand-dune grasses provide the clearest mechanism among the choices: colonists stabilize and enrich their own substrate. Their positive feedback between vegetation and soil development illustrates how ecosystem engineers can redirect physical conditions and produce a sequence of community replacement.

Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 20

View Details

SUCCESSION

Which term describes modification of environment by organisms?

Autogenic change is environmental modification caused by organisms within the community, and it can drive succession. Plants create shade, roots alter soil structure, litter adds organic matter, nitrogen-fixing microbes enrich nutrient supply, and sphagnum can acidify wetland water. These changes modify the establishment and competitive success of later species. Allogenic change originates outside the community, such as volcanic deposition, flooding, erosion, climate shifts, or altered fire regimes. “Catagenic” and “progenetic” are not the standard contrasting terms for organism-driven succession in this context. Autogenic processes can facilitate successors, as when pioneers build soil, or inhibit them, as when a closed canopy excludes light-demanding seedlings. Thus, the term identifies the source of the environmental change, not whether its ecological effect is positive or negative. Feedback is central: organisms respond to conditions and then alter those same conditions, changing future community composition. This causal loop explains why modification by organisms is described as autogenic.

Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 20

View Details

SUCCESSION

Which of the following is true?

In the classic succession framework, later or climax communities commonly contain greater structural complexity and support more species than pioneer communities, making high biodiversity the best statement provided. Early colonists are usually light-demanding rather than shade-tolerant, because they exploit open habitats. Pioneer species typically grow rapidly, disperse well, and reproduce early, so low growth is not their usual syndrome. A climax community is described as relatively stable, not inherently unstable, although modern ecology recognizes continuing disturbance and multiple dynamic states. As succession proceeds, accumulated soil, vegetation layers, dead wood, and trophic interactions create additional niches for plants, animals, fungi, and microorganisms. Diversity does not always increase monotonically or peak at the latest stage: competitive exclusion, fire suppression, nutrient enrichment, or environmental stress can produce different patterns. Accordingly, “high biodiversity” is a textbook tendency rather than an absolute definition. It remains scientifically stronger than the alternatives because habitat heterogeneity and longer colonization time generally expand species occupancy as a community matures.

Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 20

View Details

SUCCESSION

Which factor is most important in primary succession?

Primary succession is distinguished by the initial absence of developed soil, so the keyed phrase “soil presence” is scientifically mismatched with the process as written. Soil formation, rather than pre-existing soil, is one of the most important early developments. Weathering of rock, dust deposition, microbial activity, lichens, and accumulation of dead organic matter create a substrate that stores water and nutrients and permits larger plants to establish. Temperature and light strongly affect colonists and weathering, but neither alone universally controls all primary succession. Competition usually becomes more important after several species have arrived, not at the earliest stage. If the intended meaning was “development or availability of soil,” then soil is indeed the central factor governing transition from pioneers to later vegetation. However, among the literal choices, no answer accurately states that primary succession begins without soil. This distinction also separates primary succession from secondary succession, where existing soil, seed banks, roots, and microbes greatly speed recovery after disturbance.

Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 20

View Details

SUCCESSION

Which succession occurs in an area where life previously existed?

Secondary succession occurs where a community previously existed and a disturbance removes organisms without eliminating all soil and biological legacies. Forest regrowth after fire, abandoned agriculture, storm damage, or logging are common examples. Remaining seed banks, roots, spores, microbes, soil structure, and nutrients accelerate recovery compared with primary succession. Primary succession starts on newly exposed or sterilized substrate with little or no developed soil, as on fresh lava, glacial till, or bare rock. “Autogenic” and “allogenic” describe causes of successional change rather than whether prior life existed. Autogenic change is produced by organisms themselves, such as shading or soil modification; allogenic change is driven by external forces such as sediment deposition or climate. Either type of driver can influence a secondary sequence. The retained ecological memory is the tested distinction: because the site formerly supported life, colonists do not need to build an ecosystem entirely from mineral substrate. This usually makes secondary recovery faster, although severe disturbance, isolation, or invasive species can redirect it.

Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 20

View Details

SUCCESSION

Which ecological change is least reversible?

Extinction is effectively irreversible because it eliminates the last living members of a species and therefore its unique evolutionary lineage and gene pool. Once no viable individuals or preserved reproductive material remain, ordinary ecological processes cannot recreate that species. Evolution might eventually produce organisms with similar traits, but not the same historical lineage. Succession can reverse or restart after disturbance, invasion can sometimes be controlled or eradicated, and evolutionary change may be altered by future selection or gene flow, though none is always easily reversible. Extinction therefore represents the strongest permanent loss among the choices. Its ecological effects may cascade when the vanished species was a pollinator, predator, ecosystem engineer, or dominant producer. Local extinction can sometimes be reversed by recolonization or reintroduction from another population; global extinction cannot. Cryopreserved cells or seeds complicate the boundary if viable material remains, but most extinct species lack such reserves. The loss of all reproducing individuals is the biological mechanism making extinction qualitatively less reversible than changes in community composition.

Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 20

View Details

SUCCESSION

In inhibition model of succession:

The inhibition model gives early occupants a negative effect on later colonists. Once pioneers establish, they monopolize space, light, nutrients, or attachment sites, or they release inhibitory chemicals, thereby preventing other species from recruiting successfully. Succession proceeds mainly when residents die, are damaged, or are removed by consumers or disturbance, creating openings that another species can occupy. The first colonist need not prepare the site for its successor and may even be a long-lived species, so the sequence depends strongly on arrival order and priority effects. In facilitation, the sign of the interaction is opposite: early species improve conditions for later ones. Tolerance allows later species to establish without either requiring help or being completely excluded, followed by replacement through competitive traits. Saying later species inhibit early ones reverses the defining causal direction. Therefore, early species preventing later ones expresses the mechanism of inhibition. Rocky intertidal communities, where established organisms pre-empt limited surface area, provide a classic context in which such priority effects can shape succession.

Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 20

View Details

SUCCESSION

Which best characterizes r-selected species in early succession?

High fecundity is characteristic of many r-selected species that dominate early succession. Disturbed habitats are open, temporary, and unpredictable, so natural selection favours organisms that reproduce quickly and produce many offspring capable of reaching new patches. Seeds or propagules are often small, dispersive, and short-generation, allowing rapid population increase when light and nutrients are abundant. Investment in numerous offspring usually trades off against large body size, prolonged parental care, durable tissues, and late reproduction. Long life span and slow maturation are more often associated with species in stable, competitive later stages, although real organisms fall along continua rather than into two fixed categories. Early colonists also tend to grow fast and tolerate physical stress but may be poor competitors under shade. The “r” refers to the intrinsic rate of population increase in population models. Producing many offspring raises that potential rate, explaining mechanistically why high fecundity fits the opportunities and risks of newly disturbed successional environments.

Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 20

View Details

SUCCESSION

The process of succession is driven by:

Species interactions are major drivers of succession because organisms change both the physical environment and one another’s probability of establishment and survival. Pioneer plants may facilitate successors by building soil, adding nitrogen, or providing shade. Residents may instead inhibit newcomers through competition, allelopathy, or occupation of space. In tolerance pathways, later species establish independently but ultimately prevail because they survive low light or resources better. Herbivory, predation, mutualism, and decomposition also redirect trajectories. The keyed choice highlights this biotic mechanism, but succession is not driven by interactions alone. Disturbance creates or resets sites, dispersal determines which species arrive, and abiotic conditions filter which can persist; the workbook itself later recognizes this fuller combination. Mutation and evolution can influence traits over longer periods, while “migration only” omits establishment filters and local dynamics. Therefore, species interactions are the best offered driver, provided they are understood as operating with disturbance, dispersal, and environmental constraints rather than as an exclusive cause.

Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 20

View Details