Low populations in exploited species
As populations of certain species become smaller, for example due to overfishing, their genetic diversity shrinks, and inbreeding predisposes them to deleterious phenotypes. There is a threshold for how low a population can get before it can no longer be saved from extinction. This differs between species, and there are species which are inherently of low genetic diversity without having problems.
Recovery of populations following exploitation involves letting them breed without interference to their previous, high level of individuals, and thus refresh their genetic diversity.
The only difference between the founder effect and genetic bottlenecks is the way in which the new genetic pool is formed. In the founder effect the new pool is formed when a few individuals from a population become geographically isolated, while in genetic bottlenecks the new gene pool is formed when only a few individuals from a population survive a mass disaster, or are the only ones to breed.
The effect is the same: the genetic variation of the new population is decreased compared to the original population.
Habitat loss impacts species richness
Different species occupy different niches in a habitat. As such, there are edge species that thrive at the boundary between habitats, and interior species that reside in the heart of the habitat. Damage to habitats at their edges can create habitat fragmentation that impacts edge species.
Edge species can subsequently retreat into the heart of the habitat and overlap their niche with the interior species, at their expense.
Strategies to prevent this poor outcome include creating links to habitat corridors that can allow a continuum of animal movement, and a greater area for feeding, mating and habitat recolonisation following extinction.
Introduced species disrupt indigenous species
Intentionally or by accident, humans have introduced species to new locations throughout history. Introduced species can settle into a new habitat and over time become naturalised species, or outcompete indigenous (native) species, in which case they are called invasive species.
Since introduced species are plucked out of their original environment with its associated predators, parasites and pathogens, they can find themselves unstoppable in their invasion of the new land. Introduced species have often displaced their native counterparts, as seen with the North American eastern grey squirrel displacing the native red squirrel in parts of Europe where it has been introduced by humans.
The newcomer can outcompete for resources the native species, outright predate it, or make sweet love and hybridise with them. Examples of hybridisation include the mallard duck with the Hawaiian duck, and the Californian Tiger salamander with the Barred Tiger salamander. The hybrid salamander has a lower death rate than either parent species.
A seemingly small increase of several degrees Celsius can have vast effects on the Earth’s crop plants, insect pests and wild plants and animals.
For example, the life cycle of many insect pests is tightly regulated by temperature. A very finely tuned heating up or cooling down triggers development and reproduction. The result of warming is a faster life cycle which means that instead of one generation arising yearly, there might be two or three generations arising yearly instead. This poses problems for the protection of crop plants.
Another example is the redistribution of wild animals. Changes in temperature cause migrations towards the poles of the Earth, and increased desertification at the equator.
The susceptibility of various parts of the world to be desertificated has also been projected:
Challenges in modelling the effects of climate change on species and ecosystem diversity include a lack of knowledge about the total species that exist in given habitats (many are yet to be discovered and studied), a poor understanding of species distribution (because of sparse ecological surveys), and lack of insight into the attributes that influence niches of many species (that is, what affects their potential niches).