14.16: Carrying Capacity in Marine Communities

Carrying capacity is the stable number of individuals in a community. Carrying capacity has limiting factors including living space, food availability, and the physical and biological factors (previously discussed). For example prey and predator populations have limits within a geographic area.

Example: Raise the temp 10 degrees C for a group of poikilothermic organisms with a limiting factor of food and hold other factors constant -- what is the most likely result?

Same problem but with homeothermic organisms?

Changes in physical and biological factors create opportunity and misfortune/bad luck, operating under natural selection (Adapt, move, or die!)

Local die-offs happen frequently. Die offs occur because of seasonal changes (warm vs. cold), changes on food supply, predation, major storms, or any of the other natural physical or biological factors that change in an environmental setting. Species that survive these events have adaptations that allow them to survive, such has having abilities such as migration, hibernation, or producing seeds or eggs that can survive and create a new generation of offspring even when all adult members of a species are wiped by a seasonal or catastrophic event. Many species will migrate from suitable birthing grounds in one region to another location with the capacity to feed a population in another. Examples include migrations of many birds, whales, and other animals that migrate from the tropical regions in the winter season, to polar regions when the receive the greatest sunlight (summer in the northern hemisphere; winter in the southern hemisphere).

Survival or Extinction (Past and Present)

When all viable reproducing members of a species or community of species die off, it is extinction. Extinction is a natural process. Most fossils preserved and recovered from past geologic time periods represent species that do not exist in the modern world. These ancient species have either undergone extinction, or through time (many generations), have produced offspring that have adapted and changed into new species with new characteristics. For example, members of a species may become geographically isolated from a greater population. Examples include:

● species that are trapped and then isolated islands in the oceans (such as Hawaiian Islands).
● species trapped near isolated sources of water when climates change from wet to desert conditions.
● species that become isolated on nearshore land masses by sea-level rise.
● species that survive on isolated mountain ranges that are separated from a large regional population when climate change occurs.
● species surviving patches of unglaciated land that were not destroyed advancing continental glaciers.
● streams river systems become blocked or change course, isolating or shifting a population.

In many cases, when ice ages occurred, some species survived in a refugia. A refugia is an area where special environmental circumstances have allowed a viable population of a species to survive after extinction occurred in the surrounding area.
Species that survive in refugia can become the only survivors where the rest of the species could be wiped out by disease, predation, or other environmental catastrophes.

Exotic and Invasive Species

The name exotic species applies to any plant or animal species introduced into an area where they do not occur naturally. Most plants and animals sold in plant and pet stores are exotic species. Humans have both intentionally and unknowingly introduced exotic species into environments that native species are unprepared to cope with. However, some of these species can become invasive species when they escape, reproduce successfully, and spread through a new environmental setting. These species can consume, threaten, and displace native species. Examples include introduction of species sold in pet and plant stores that have either escaped or have been intentionally turned loose into a natural setting. Many countries now have laws and enforcement are trying to cope with the sale and the prevention of the introduction invasive species. Note that not all all exotic species are considered invasive species; most exotic species would not survive without some human intervention. Examples of invasive species include the introduction of lionfish introduced into the Caribbean Sea (Figure 14.27), pythons and other snakes into south Florida's Everglades and other coastal areas, carp and other "sport fishing" species into lakes and rivers practically everywhere. Everywhere humans have moved around the world, particularly in the last two centuries, humans have unintentionally brought invasive species with them. Examples include rats, feral cats and pigs, cockroaches, mosquitoes, snakes, grasses and other weeds, and many other invasive species of plants and animals (both aquatic and terrestrial). In many places around the world invasive species are contributing to widespread habitat destruction and the displacement or annihilation many species, and including extinctions.

Figure 14.27. Lionfish, native to the tropical Pacific, were introduced the Caribbean by pet owners who purchased them and then decided to turn them loose when they didn't want them anymore. In the tropical parts of the Atlantic Ocean basin, these toxic beasts have no predators to control their population, and they are contributing to wiping out populations of some native species.

Unfortunately, human activities are now the primary cause of extinctions in many parts of the world. Global environmental changes, expansion of agricultural lands and urban development, and irresponsible exploitation are driving forces of extinction.

In our modern world, zoos, arboretums, and wilderness preserves are becoming the only refugia for many species.

Collectively, we must face the fact that in most places around the world, without sustainability, humans are both exotic and/or invasive species!