Chapter 55 Community Ecology 55 1 Species Interactions Fitness is the ability of an individual to survive and reproduce fertile offspring Commensalism is when one species benefits another but other species are unaffected Competition is when both species compete for resources and therefore the fitness for both species is lowered Consumption is when one organism eats or absorbs nutrients from another This increases the consumer s fitness but decreases the other s fitness cid 127 Mutualism is when both species interact in a way that benefits the others in terms of fitness Species interactions can affect the distribution and abundance of a particular species Species act as agents of natural selection when they interact Coevolution is when two species interact in a way that influences the other s adaptations over time A coevolutionary arms race is when species interact and create a repeating cycle of reciprocal adaptation The outcome of interactions between species is conditional and dynamic Interactions between individuals may change between time and space nCommenalism Competition Commenalism is the least studied type of interaction because it is hard to quantify no fitness change and it is also very conditional Competition between members of the same species is called intraspecific competition cid 127 Major cause of density dependent growth Competition between members of different species is called interspecific competition Different species using the same limiting resources Using the Niche Concept to Analyze Interspecific Competition A niche is a range of resources a species can use and the range of conditions it can endure Interspecific competition occurs when the niches of two species overlap Symmetric competition is when both species suffer similar loss of fitness What Happens When One Species is the Better Competitor Asymmetric competition occurs when one species fitness declines significantly more than the other and the amount of decline depends on the amount of The competitive exclusion principle states that two species that occupy the same niche in the same space can not coexist because one will outcompete the overlap in their niches other A fundamental niche is the total range of conditions a species can survive in A realized niche is the portion of a fundamental niche that a species occupies given limiting factors such as competition with other species nExperimental Studies of Competition in Nature The most common way to study competition in nature is to remove one competitor from the environment Fitness Trade offs and Competition If individuals are extremely good at competing for a certain resource then they are most likely not good as good at enduring drought situations warding off disease or preventing predation A fitness trade off is an inevitable compromise in adaptation Mechanisms of Coexistence Niche Differentiation Because competition is detrimental to both species involved there is strong natural selection occurring on both species to avoid it An evolutionary change in traits reduces niche overlap and therefore the amount of competition Niche differentiation is an evolutionary change in resource use caused by competition over generations also called resource partitioning The evolutionary change that occurs in species traits and enables them to exploit different resources is called character displacement Character displacement allows Niche differentiation to happen An example of Niche differentiation is adapting to eat new seeds An example of character displacement is reduced beak size Consumption There are three major types on consumption cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 Parasitism A parasite organism living in or on a species consumes small amounts of tissue or nutrients from another individual called the host Predation Predator kills and eats all or most of another individual called the prey Researchers can use this term to refer to carnivores but it can also be used to describe an individual eating plants especially seeds containing entire Herbivory herbivores eat plant tissue Can happen over a long period of timw embryos Constitutive Defences Natural selection strongly favors traits that allow individuals to avoid being eaten Traits like these are called standing or constitutive defenses because they are present even in the absence of predators cid 127 Mimicry is when one species evolves to look or sound like another species Batesian mimicry is when a harmless or palatable species adapts to look like a dangerous or poisonous species cid 127 Muelerian mimicry is when two dangerous or unpalatable species resemble one another predators are more likely to learn not to eat these if they look similar Inducible Defenses Constitutive defenses can be very effective but they also consume a lot of energy and resources to maintain them Inducible defenses are used only when a prey is in danger These are efficient energetically but it takes time to produce them These can also be lost if a predator leaves the habitat Predators are usually efficient enough to reduce prey populations to well below carrying capacity Why Don t Herbivores Eat Everything Why is the World Green cid 127 Meta analysis is a study where there is a large amount of data collected to answer a specific question study of studies Two Theories Top down hypothesis herbivore populations are limited by predation and disease Predators and parasites remove herbivores that eat plants so much plant material remains uneaten Bottom up limitation hypothesis plant tissues offer poor nutrition and are well defeded Plants have less than 10 of the nitrogen content by weight than animal tissue does Plants also have weapons such as thorns prickles hairs or potent poisons that deter herbivores Both bottom up and top down limitations effectively limit herbivory Adaptation and Arms Races cid 127 When predators and prey or herbivores and plants interact over a long period of time coevolutionary arms races occur Using Consumers as Biocontrol Agents Integrated pest management strategies to maximize crop or forest productivity while also minimizing insecticide usage and other harmful compounds Introducing a new species to a community can have effects on other organisms within that community Mutualism The Role
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