Human influences Notes

How Do Humans Influence The Environment?

1. Very few ecosystems remain completely isolated from human effects, which differ from that of other organisms:
   1. Most species modify ecosystems locally.
   2. Humans modify ecosystems across regions and continents.
   3. Changes often occur faster than ecosystems can adapt.

Why Is The Scale Of Human Activity Ecologically Significant?

1. Scale determines the severity of ecological impact.
   1. Small-scale disturbances are often reversible.
   2. Large-scale disturbances affect entire systems at once.
2. Human activities operate at unusually large scales.
   1. Forests are cleared across thousands of square kilometres.
   2. Rivers are altered across whole catchments.
   3. Oceans are used as shared global resources.
3. Large-scale change reduces ecosystem resilience.
   1. Recovery depends on nearby unaffected areas.
   2. When disturbance is widespread, recovery sources are lost.

Why Does The Speed Of Human Change Matter?

1. Ecosystems require time to adjust to change.
   1. Populations adapt over many generations.
   2. Species distributions usually shift gradually.
2. Human-driven change is often rapid.
   1. Habitats can be altered within years or decades.
   2. Species may not reproduce or adapt quickly enough.
3. Rapid change increases extinction risk.
   1. Behavioral and physiological adjustments lag behind change.
   2. Populations decline before adaptation is possible.

How Do Humans Simplify Ecosystems?

1. Human activities often reduce ecosystem complexity.
   1. Diverse habitats are replaced with uniform landscapes.
   2. Many species are replaced by a few dominant ones.
2. Simplified ecosystems are less stable.
   1. Fewer species perform fewer ecological roles.
   2. Loss of one species has larger consequences.
3. Simplification increases vulnerability to disturbance.
   1. Disease spreads more easily.
   2. Environmental stress has stronger effects.

How Do Humans Alter Species Distributions?

1. Humans move species beyond their natural ranges.
   1. Transport through trade and travel.
   2. Deliberate introduction for agriculture or pest control.
2. Some introduced species become invasive.
   1. Natural predators or competitors may be absent.
   2. Population growth can be rapid.
3. Invasive species disrupt existing interactions.
   1. Native species may be outcompeted.
   2. Food webs can be altered.

How Do Human Activities Fragment Habitats?

1. Continuous habitats are divided into smaller patches.
   1. Roads, farms, and cities break up ecosystems.
   2. Populations become isolated from one another.
2. Fragmentation affects populations even without total habitat loss.
   1. Smaller populations are more vulnerable to decline.
   2. Movement between populations is restricted.
3. Fragmentation reduces genetic diversity.
   1. Inbreeding becomes more likely.
   2. Ability to adapt decreases over time.

Why Are Human Impacts Uneven Across Ecosystems?

1. Ecosystems differ in sensitivity.
   1. Island and polar ecosystems have low resilience.
   2. Highly diverse ecosystems may absorb change more effectively.
2. Human pressure is unevenly distributed.
   1. Urban and agricultural regions experience intense modification.
   2. Remote regions experience less direct pressure.
3. Impact depends on both pressure and sensitivity.
   1. Small disturbances can cause major change in sensitive systems.
   2. Larger disturbances may be absorbed in more resilient systems.