Lecture 22 - Community Interactions

Communities A community is a group of different species living close enough together for potential interaction. Communities differ in their species richness, the number of species they contain, and the relative abundance of different species. Ecological niche...

Communities A community is a group of different species living close enough together for potential interaction. Communities differ in their species richness, the number of species they contain, and the relative abundance of different species. Ecological niche Specialists: Niche: An organism’s use of biotic and Have narrow niches and abiotic resources. specific needs, extremely good at what they do but vulnerable when conditions change. Includes habitat, food selection, role in energy and nutrient flow, interactions with other individuals. Generalists: The ecological niche of a species is Have broad niches and use essentially the position (or the standing) a wide array of habitats of that species in its ecosystem. and resources and can live in many different places. Ecological niche Ecologically similar species can coexist in a community if there are one or more significant differences in their niches. Resource partitioning  A species’ fundamental niche is the niche potentially occupied by that species.  A species’ realized niche is the niche actually occupied by that species.  As a result of competition, a species’ fundamental niche may differ from its realized niche. Species Interactions Species interactions Interspecies interactions are interactions between species in a community. Interspecific interactions can affect survival and reproduction of each species. Effects can be summarized as positive (+), negative (−), or no effect (0). Competition Intra and interspecific competition (-/-) can lead to the exclusion of one species (one species outcompete another). There are two types of competition: 1. Interference 2. Exploitation Competition Interspecies competition occurs when species compete for a resource in short supply. It includes competitive exclusion whereby two species competing for the same limiting resources cannot coexist in the same place.  It results in elimination of a competing species. It also includes the creation of fundamental niches. Competition Interspecific competition causes realized niches:  Not fundamental niches  Ex: the presence of one barnacle species limits the realized niche of another species. Competition also results in character displacement:  Evolution of differences in morphology and resource use as result of resource partitioning.  Ex: the bottom feeding stickleback fish is much larger than the surface feeding stickleback fish. Predation Predation refers to an interaction in which one species, the predator, kills and eats the other, the prey. Some feeding adaptations of predators are claws, teeth, fangs, stingers, and poison. Prey display various defensive adaptations. Behavioral defenses include hiding, fleeing, forming herds or schools, self-defense, and alarm calls. Animals also have morphological and physiological defense adaptations. Avoiding Predation Avoiding Predation Mimicry can also be used by predators to approach prey. Herbivory Herbivory refers to an interaction in which an herbivore eats parts of a plant or alga. It has led to evolution of plant mechanical and chemical defenses and adaptations by herbivores. Symbiosis Symbiosis refers to a relationship where two or more species live in direct and intimate contact with one another: Mutualism (+/+): an interaction that benefits both species. Commensalism (+/0): one species benefits and the other is neither harmed nor helped (these are difficult to document). Parasitism (+/-): one organism (parasite) derives nourishment from its host, which is harmed in the process. Facilitation Interspecies facilitation refers to an interaction in which one species positively affects another species without direct contact. Ex: The black rush makes soil more hospitable for other plant species. Community Structure Predation In general, a few species in a community exert strong control on that community’s structure. Two fundamental features of community structure are: 1. Species diversity 2. Feeding relationships (trophic levels) Community Structure 1. Species Diversity Species diversity of a community is the variety of organisms that make up that community. Two components:  Species richness: The number of different species in the community.  Relative abundance: The proportion each species represents out of all the individuals in the community. Two communities can have the same species richness but a different relative abundance Community Structure 1. Species Diversity Greater diversity leads to greater stability! Communities with higher diversity are:  More productive (produce more biomass) and more stable in their productivity.  Better able to withstand and recover from environmental stresses.  More resistant to invasive species, organisms that become established outside their native range. Biodiversity Biodiversity describes the variety of life on earth More biodiversity is often good More resilient ecosystems (redundancy) More organisms to complete different tasks in the ecosystem We have a few different ways to measure Species Species Relative biodiversity Richness Evenness Abundance How many If one How much of different species one species 4 kinds of dominates Beetles there is Total species there thedominate, compared to are community pretty good 1 3 2 4 another 1 not overall 10 30 20 40 evenness or 0 100 Your turn Work with a buddy to determine the species richness and species evenness of these two communities Community Structure Keystone species Keystone species exert strong control on a community by their ecological roles, or niches. They are not necessarily abundant in a community, but they have a large impact on species richness when removed. They belongs to one of the consumer groups.