Human civilization is the dominant force shaping the planet’s ecosystems, leading many scientists to refer to the current era as the Anthropocene, or the “Age of Humans.” This designation reflects the unprecedented scale at which human activities are altering Earth’s physical, chemical, and biological systems. The rate of these changes has intensified in recent decades, affecting the atmosphere, oceans, land, and life globally. The size of the human population and the intensity of our activities exert pressure on the biosphere, resulting in a significant impact on animal populations and their natural surroundings.
Conversion and Fragmentation of Natural Habitats
The most visible impact on wildlife is the physical destruction or alteration of habitats, primarily driven by land-use conversion. Habitats that were once continuous are cleared for agriculture, urban expansion, and large infrastructure projects, such as roads and dams. This process reduces the total area available to species and introduces habitat fragmentation, splitting large ecosystems into smaller, isolated patches.
The isolation created by fragmentation severely restricts the movement of animals, preventing them from accessing necessary resources or finding mates. For example, a major highway cutting through a forest divides a population into two smaller, vulnerable groups that cannot interbreed, leading to a loss of genetic diversity.
The division of a large habitat also increases the amount of “edge,” exposing the interior of the remaining patches to harsher conditions. These “edge effects” subject forest interiors to higher light, wind, and temperature fluctuations, altering the microclimate. This makes it difficult for species adapted to the stable, cool, and humid core to survive. When habitats become too small or too isolated, they can no longer support the range of species that once lived there.
Contamination of Ecosystems
Wildlife populations face threats from chemical and material contamination that degrades the quality of the remaining environment. Industrial runoff, untreated sewage, and the widespread use of agricultural pesticides and fertilizers introduce pollutants into water and soil systems. This includes non-biodegradable substances like Persistent Organic Pollutants (POPs) and heavy metals such as mercury, which resist natural breakdown.
These toxic chemicals enter the food web through two interconnected processes: bioaccumulation and biomagnification. Bioaccumulation occurs when an individual organism absorbs toxins faster than it can excrete them, causing the substance to build up in its tissues over its lifetime. Since many pollutants are fat-soluble, they concentrate in the fatty tissues of living organisms.
Biomagnification amplifies this effect as the toxins move up the food chain. A predator consumes many prey items, accumulating the total toxic load from those smaller organisms, resulting in increasingly higher concentrations at each trophic level. For example, apex predators like orcas have been found to have high levels of polychlorinated biphenyls (PCBs) in their blubber. This concentration of toxins in top predators can lead to severe health issues, including reproductive problems and immune system suppression.
Direct Pressure on Wildlife Populations
Human activities exert direct pressure on animal populations through the harvesting and removal of individual organisms. This exploitation rapidly depletes populations and is a major driver of global biodiversity loss, independent of habitat loss or pollution. Commercial activities, particularly in marine environments, often lead to overexploitation, such as the collapse of major fish stocks like the Atlantic cod.
Overfishing removes vast numbers of reproductive-age fish, changing the size and maturity rate of the remaining population and destabilizing the marine food web. When a species is reduced to such low numbers that it can no longer perform its ecological role, it is considered “functionally extinct.”
On land, unsustainable hunting and the illegal wildlife trade, or poaching, target specific species for body parts, luxury items, or the exotic pet trade. These pressures create population bottlenecks, reducing genetic diversity and making the remaining animals more vulnerable to disease or environmental changes.
Altering Global Systems and Species Dynamics
Human actions are altering fundamental global systems, creating pervasive effects that influence species dynamics across the entire planet. The release of greenhouse gasses, primarily from burning fossil fuels, causes climate change, which shifts temperatures and alters weather patterns that animals rely on for migration and breeding cycles. Rising atmospheric carbon dioxide also contributes to ocean acidification, where the ocean absorbs excess $\text{CO}_2$, lowering the water’s $\text{pH}$.
This chemical change makes it more difficult for calcifying organisms, such as corals, oysters, and pteropods, to build and maintain their shells and skeletons. Ocean acidification, combined with warming waters, can also favor “nuisance” species like algae and jellyfish, which thrive in the changed conditions, potentially leading to significant shifts in marine community composition.
Compounding these systemic changes is the human-facilitated introduction of non-native species, often through global trade and travel. These invasive species are introduced into new habitats where they lack natural predators. They can rapidly outcompete native animals for resources, predate on them, or introduce new diseases. Warming temperatures and altered conditions caused by climate change can further enable the spread of these invasive species into new ranges, fundamentally changing the dynamics of the local ecosystem.