Explain ecosystem equilibrium and succession, and describe how human activities (climate change, habitat loss, pollution, invasive species, and extinction) reduce biodiversity (Ohio's Learning Standards for Science, Biology, B.DI.2 / B.DI.3).

What this topic is asking

Ohio standards B.DI.2 (equilibrium and disequilibrium) and B.DI.3 (Loss of Diversity: climate change, Anthropocene effects, extinction, invasive species) close the ecology module by asking how ecosystems stay stable and how humans disrupt them. The Ohio Biology EOC turns this into items where you explain a human impact, interpret data on a changing ecosystem, or predict the effect of a disturbance such as an invasive species. The crosscutting idea is stability and change. This brings together biodiversity and the cycling of matter, since human activity disrupts both.

Equilibrium and disequilibrium

An ecosystem in equilibrium is relatively stable: the populations, energy flow, and matter cycles stay roughly in balance over time, even though individual populations rise and fall. Equilibrium is dynamic, not frozen; it means the system stays within a stable range.

A disturbance can push an ecosystem into disequilibrium, a disrupted state. Disturbances can be natural (a fire, a flood, a drought, a storm) or human-caused (clearing a forest, introducing a species, a pollution spill). After a disturbance, an ecosystem may recover toward a new equilibrium, or, if the disturbance is severe or biodiversity is low, it may not fully recover.

Ecological succession

Ecological succession is the gradual, predictable change in a community over time, often as it recovers from a disturbance or colonizes new ground. It moves an ecosystem toward a more stable, mature community.

• Primary succession begins on bare rock or ground with no soil (after a volcanic eruption or a retreating glacier). Pioneer species (such as lichens) start to build soil, and a community develops slowly over a long time.
• Secondary succession begins where a disturbance has cleared the community but soil remains (after a fire, flood, or abandoned farmland). Because soil and some seeds survive, it is faster than primary succession: small fast-growing plants return first, then shrubs, then trees.

Succession shows how an ecosystem can restore stability after disequilibrium, gradually rebuilding a complex, balanced community.

Biodiversity and stability

The link from the biodiversity topic matters here: greater biodiversity generally makes an ecosystem more stable and resilient. A diverse ecosystem has more feeding relationships and backup roles, so it absorbs disturbances and recovers better. A low-diversity ecosystem is more fragile, so the loss of a single key species can destabilize it. This is why the human impacts that reduce biodiversity also threaten stability.

Human impacts that reduce biodiversity

Ohio standard B.DI.3 focuses on how human activities cause loss of diversity. The main impacts:

• Climate change. Burning fossil fuels and deforestation add greenhouse gases (especially carbon dioxide) to the atmosphere, warming the planet. This shifts species' ranges, disrupts timing of life cycles, bleaches coral, and stresses ecosystems.
• Habitat loss and destruction. Clearing land for farming, cities, and roads (deforestation, urbanization) removes the places organisms live, the leading driver of species decline.
• Pollution. Contamination of air, water, and soil (chemicals, plastics, fertilizer runoff causing algal blooms) harms organisms and disrupts cycles of matter.
• Overexploitation. Taking organisms faster than they reproduce (overfishing, overhunting, overharvesting) drives populations down.
• Invasive species. Non-native species introduced to a new area where they have no natural predators can spread rapidly, outcompeting or preying on native species, altering food webs and reducing native biodiversity.
• Extinction. The above pressures raise extinction rates far above the natural background. Each extinction permanently removes a species, lowering biodiversity and potentially destabilizing the ecosystems that depended on it.

These act together (the Anthropocene, the era in which humans are the dominant influence on the environment) to reduce both genetic and species diversity.

Try this

Q1. State the difference between primary and secondary succession. [2]

• Cue. Primary succession begins on bare ground with no soil (and is slow); secondary succession begins where soil remains after a disturbance (and is faster).

Q2. Explain why an invasive species often causes more harm in its new habitat than in its native one. [2]

• Cue. In the new habitat it has no natural predators or controls, so it spreads unchecked and outcompetes or preys on native species, reducing native biodiversity.