Organisms are adapted to live within certain ranges of environmental conditions. Tolerance limits are the maximum or minimum conditions, such as temperature or moisture, that an organism can survive. Since many environmental factors affect survival, it is useful to consider critical factors that limit a species' growth or expansion.
Evolution is gradual change of organisms by natural selection. Natural selection refers to a higher rate of survival and reproduction among individuals that happen to have advantageous traits. Environmental conditions can exert selective pressure by making some traits more advantageous than others.
An ecological niche is usually described as its ecological role in a community; a niche can also be the place or set of environmental conditions in which an organism lives. Generalist species can occupy a range of habitats and ecological roles or environmental conditions. Highly specialized species occupy narrower niches.
Resource partitioning occurs when species adapt to use a single resource differently.
Species interact in many ways. Some general classes of interaction include predation, parasitism, symbiosis, and competition. All of these interactions can exert selective pressure, as organisms develop defenses against predators or parasites, as they develop traits that improve competitiveness, or as they develop mutually beneficial interactions. Both interspecific (between species) and intraspecific (within a species) competition can lead to changes in traits or behavior.
Defensive mechanisms can include Batesian mimicry, in which a harmless species looks like a dangerous one, and Müllerian mimicry, in which two dangerous species look like each other, and thus both discourage predation.
Primary productivity, or the rate of biomass accumulation, is a basic characteristic of communities. Abundance and species diversity are also important characteristics.
Complexity refers to the number of species at each trophic level and the number of trophic levels in a community. Many ecologists believe that complexity contributes to stability in an ecosystem, or resilience to abrupt change such as fire, flood, or drought. Others believe that complex communities can be less resilient than simple ones.
Edges, where contrasting conditions meet, are important features in biological communities. Ecotones, or zones of transition, have great diversity. Edges also reduce habitat quality for interior species.
Primary succession occurs when pioneer species occupy areas previously lacking living things. Secondary succession occurs when an existing community is disrupted and a new, different community develops.
The idea of a climax community is a stable community that appears to be the culmination of successional processes.
A contrasting idea is that species occur individualistically, each according to its ability to colonize an area.
Introduced species are one of the greatest modern threats to biological diversity and ecosystem complexity. When introduced species are free of predators, they can become abundant and cause significant damage to ecosystems.
