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Chapter 53

BIOLOGY 1M03 Chapter 53: Biology Chapter 53

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Ben Evans

1 Biology Chapter 53: Community Ecology Species Interaction  A biological community consists of interacting species, usually living within a defined area  To study species interaction, biologists focus on analyzing the effects on the fitness of individuals involved  Four categories of interaction o -/- relationship known as competition o +/- relationship known as consumption and parasitism o +/+ relationship termed mutualism o +/0 relationship termed commensalism ▪ Ex. Birds that follow moving army ants in the tropics ▪ As ants march along forest, they hunt insects and small vertebrates ▪ As they do, birds follow and pick off prey species that fly or jump out of the way of the ants ▪ The birds are commensals that benefit from the association (+), but have no measurable impact on the ants (0)  Section focuses on three key themes o Species interaction may affect the distribution and abundance of a particular species ▪ Changes in species interactions often explain short-term changes in population size and distribution o Species act as agents of natural selection when they interact ▪ Deer are fast and agile in response to natural selection exerted by major predators • Speed and agility of deer, in turn, promote natural selection that favours wolves and cougars that are fast and have superior eyesight and senses of smell • Changes in species interactions lead to long-term changes in characteristics of populations, a phenomenon called coevolution, in addition to having short-term impacts on population size o Outcome of interactions among species is dynamic and conditional ▪ Consider relationship between army ants and bids that follow them, which is usually commensal ▪ If bird attacks start to force other insects into path of ants, then both benefit and relationship becomes mutualistic ▪ If birds begin to steal prey that would otherwise be taken by ants, relationship becomes parasitic  Competition o Competition is a -/- interaction that occurs when different individuals use the same resources and when those resources are limiting 2 o Competition that occurs between members of same species is called intraspecific competition ▪ Intraspecific competition for space, sunlight, food and other resources intensifies as population’s density increases ▪ A major cause of density-dependent growth o Interspecific competition arises when individuals from different species use the same limiting resources o There are many types if interspecific competition ▪ Consumptive competition occurs when individuals consume the same resources (ex. trees competing for same water and nutrients) ▪ Pre-emptive consumption exists when one species makes space unavailable to other species ▪ Overgrowth competition happens when one species grows above another (ex. large fern overgrown other individuals and is shading them) ▪ Chemical competition takes place when one species produces toxins that negatively affect another species ▪ Territorial competition arises when a mobile species protects its feeding or breeding territory against other species (ex. grizzly bears drive off black bears) ▪ Encounter competition occurs when two species interfere directly for access to specific resources (ex. spotted hyenas and vultures fight over kill) o Using the Niche Concept to Analyze Competition ▪ A niche can be thought of as the range of resources that the species is able to use or the range of conditions it can tolerate ▪ Interspecific competition occurs when niches of two species overlap o What Happens When One Species Is a Better Competitor ▪ G.F. Gause claimed it is not possible for species with the same niche to coexist ▪ This hypothesis is called the competitive exclusion principle and was inspired by a series of experiments Gause did with Paramecium ▪ When Gause placed small populations of P. caudatum and P. Aurelia in separate laboratory cultures, both specie exhibited logistic growth ▪ But Gause showed that when two species are put in the same culture together, only the P. Aurelia population exhibits a logistic growth pattern, P. caudatum is driven to extinction ▪ His results are a product of asymmetric competition ▪ When asymmetric competition occurs, one species suffers a much greater fitness decline than the other species does ▪ Under symmetric competition, each of the interacting species experiences a roughly equal decrease in fitness ▪ If asymmetric competition occurs and the two species have completely overlapping niches, then the stronger competitor is likely to drive the weaker competitor to extinction 3 ▪ But if the niches do not overlap completely, then the weaker competitor should be able to retreat to an area of non-overlap ▪ In cases like this, an important distinction arises between a species’ fundamental niche, which is the combination of resources or areas used or conditions tolerated in the absence of competitors and its realized niche, which is the portion of resources or areas used or conditions tolerated when competition occurs o Experimental Studies of Competition ▪ Joseph Connell observed that there were two species of barnacles with distinctive distributions in an intertidal rocky shore ▪ Barnacle larvae are mobile, but adults live attacked to rocks ▪ The adults of one species, Chthamalus stellatus, occurred in an upper intertidal zone, while the adult of the other species, Semibalanus balanoides, were restricted to a lower intertidal zone ▪ The upper zone is a more severe environment for barnacles because it is exposed to air for longer periods at low tide each day ▪ The young of both species were found together in the lower intertidal zone ▪ To explain observations, Connell hypothesized that adult Chthamalus were competitively excluded from the lower intertidal zone ▪ The alternative hypothesis is that adult Chthamalus are absent from the lower intertidal zone because they do not thrive in the physical conditions there ▪ Connell tested these hypotheses by removing a number of rocks that had been colonized by Chthamalus from the upper intertidal zone and transplanting them into the lower intertidal zone ▪ He screwed the rocks onto place and allowed Semibalanus larvae to colonize them ▪ Once the colonization period was over, Connell divided each rock into two groups ▪ In one half he removed all Semibalanus that were in contact with or next to a Chthamalus ▪ Allowed Connell to document Chthamalus survival in absence of competition with Semibalanus and compare it with survival during competition • Common experimental strategy (one competitor is removed and response by remaining species is observed) ▪ Connell’s results support hypothesis of competitive exclusion ▪ In unmodified areas, Semibalanus killed many of the young Chthamalus by growing against them and lifting them off the substrate ▪ Chthamalus survival was much higher when all Semibalanus were removed o Mechanisms of Coexistence: Fitness Trade-Offs and Niche Differentiation 4 ▪ Why haven’t Semibalanus and other superior competitors taken over the world ▪ The key here is that the ability to compete for a particular resource is only one aspect of an organism’s niche ▪ If individuals are extremely good at competing for a particular resource, then they are probably less good and enduring drought conditions, warding off diseases or preventing predation ▪ In the case of Semibalanus and Chthamalus, fitness trade-off is rapid growth and success in competing for space versus ability to endure harsh physical conditions of upper intertidal ▪ Semibalanus are fast-growing and large ▪ Chthamalus grow slowly, but can survive long exposures to air and to intense sun and heat ▪ Neither species can do both things well so fitness trade-offs limit the ability of superior competitors to spread ▪ Because competition is a -/- interaction, strong natural selection on both species to avoid it ▪ An evolutionary change in traits reduces the amount of niche overlap and thus the amount of competition ▪ This change in resource use is called niche differentiation or resource partitioning ▪ The change in species’ traits is called character displacement ▪ The fundamental idea is that competition exerts natural selection and that the characteristics of species change in a way that reduces competition  Consumption o Occurs when one organism eats another o Three major types of consumption ▪ Herbivory takes place when herbivores consume plant tissue ▪ Parasitism takes place when parasites consume relatively small amounts of tissue or nutrients from another individual, called the host • Often occurs over long period of time • Not all parasitism involves consumption, social parasites in birds and insects lay their eggs in other species’ nests and induce them to raise the young ▪ Predation occurs when a predator kills and consumes all or most of another individual • Consumed individual is called the prey o How Do Prey Defend Themselves ▪ Standing or constitutive defences are always present • Prey may hide, run, swim away when they sense predator • Many species find safety in numbers • Other prey species spray toxins or employ weaponry 5 • Mimicry o Occurs when one species closely resembles another species o When harmful prey species resemble each other, Mullerian mimicry is said to occur o To explain existence of Mullerian mimics, biologists propose that the existence of similar-looking dangerous prey in the same habitat increases the likelihood that predators will learn to avoid them o In this way, Mullerian mimicry should reduce likelihood of dangerous individuals being attacked o Batesian mimicry occurs when predators avoid the harmless mimics because they mistake them for a dangerous prey • Key point here is that prey have adaptations that reduce their likelihood of becoming victims • These adaptations are responses to natural selection exerted by predators • Constitutive defences are expensive in terms of energy and resources that must be devoted to producing and maintaining them however ▪ Therefore, many prey species have inducible defences, traits that are produced only in response to presence of a predator ▪ Inducible defences are efficient energetically, but are slow ▪ Ex. Blue mussels • Predation on mussels by crabs was high in an area of the estuary with relatively slow tidal currents, but low in an area of the estuary with relatively rapid tidal currents • Researchers hypothesized that if blue mussels possess inducible defences, then heavily defended prey individuals should occur in low-flow area where predation is higher, but not in high-flow area, where water movement reduces the number of crabs present • To test this hypothesis, biologists had two tanks, both containing seawater, but one with a crab that ate on fish, but not mussels and one with no crab • The tank with the crab allowed researchers to measure shell growth in mussels that were “downstream” of the crab • As predicted, the mussels exposed to a crab in this way developed significantly tougher shells than did mussels not exposed to crab • These results suggest that even without direct contact, mussels can sense the presence of crabs and increase their investment in defences 6 • In a similar experiment, investigators compared mussels that were exposed to water running through broken mussel shells versus intact, but empty mussel shells • They recorded a significant increase in shell thickness in the tank downstream from the broken shells • This result supports the hypothesis that mussels can detect presence of predators from molecules released by broken shells o Are Animal Predators Efficient Enough to Reduce Prey Populations ▪ Research supports hypothesis that species interactions have a strong impact on evolution of predator and prey populations ▪ Prey are typically smaller than predators, have larger litter sizes and tend to reproduce at a younger age ▪ As a result, they have a much larger intrinsic growth ratmax) ▪ If prey reproduce rapidly and are also well defended, it is not clear whether predators should be able to kill enough of them to reduce prey population significantly ▪ Some results suggest that they can (wolf vs. moose) ▪ Other experiments support the hypothesis that predators play a role in density-dependent growth of prey populations ▪ Data available indicate that in many instances, predators are efficient enough to reduce prey population o Why Don’t Herbivores Eat Everything ▪ Top-down control hypothesis • States that herbivore populations are limited by predation and disease • Predators and parasites remove herbivores that eat plants ▪ Poor-nutrition hypothesis • Contends that plants are a poor food source in terms of nutrients they provide • Plant tissue have less than 10% of nitrogen found in animal tissues • If growth and reproduction of herbivores are limited by availability of nitrogen, then their populations will be low and impact of herbivory relatively slight • Herbivores could eat more plant material to gain nitrogen, but at a cost – they would be exposed to predation and expend more energy processing food ▪ Plant-defence hypothesis • Holds that plants defend themselves effectively enough to limit herbivory • Most plant tissues are defended by weapons such as thorns, prickles or hairs, or by potent poisons • No animal species, without help from protists or bacteria, digest cellulose or lignin, which are components of wood 7 • Natural selection should favour plants that evolve an ever- changing suite of compounds to deter the ever-changing array of herbivores they face ▪ All three factors are important in limiting the impact of herbivory, although the particular mix of factors will vary from plant species to plant species and from habitat to habitat o Adaptations and Arms Race ▪ When predators and prey or herbivores and plants interact over time, coevolutionary arms races result • Consumers evolve traits that increase their efficiency, in response, prey evolve traits that make them unpalatable or elusive, which leads to selection on consumers for traits that counter the prey adaptation and so on o Can Parasites Manipulate their Hosts ▪ To thrive, parasites have to be transmitted to new hosts ▪ To a parasite, an uninfected host represents uncolonized habitat, teeming with resources ▪ Consider species of land snails that are parasitized by flatworms, specifically by flukes in the genus Leucochloridium ▪ When the flukes have matured and are ready to be transmitted to their next hose, a bird, they burrow into the snail’s tentacles and wriggle ▪ In addition, infected snails become attracted to light, even though uninfected snails avoid sunlit areas and prefer dark, shady environments ▪ When infected snails move out of the shade into the open and glide about with wriggling tentacles, they are more easily spotted and consumed by bids ▪ Biologists suggest that the worms manipulate the behaviour of the snail and that the change in snail behaviour makes the parasite more likely to be transmitted to a new host  Mutualism o Mutualisms are +/+ interactions that involve a wide variety of organisms and rewards o Ex. Many species of bees visit flowers to harvest nectar and pollen ▪ Bees benefit because nectar is used as a food source for adult bees and pollen is fed to larvae ▪ Flowering plants also benefit because in the process of visiting flowers, foraging bees carry pollen from one plant to another and accomplish pollination o Ex. Ants in the genus Crematogaster, which live in acacia trees ▪ Ants in live in bulbs at the base of acacia thorns and feed on small structures that grow from tree branches ▪ These ants protect the tree by attacking and biting herbivores and by cutting vegetation from the ground below the host tree ▪ Plants have high protein bodies that ants can feed on 8 o Important to note that even though mutualism benefits both species, the interaction does not involve individuals from different species being altruistic to each other o Judith Bronstein described mutualism as “a kind of reciprocal parasitism; each partner is out to do the best it can by obtaining what it needs from its mutualist at the lowest possible cost to itself” o Not surprising that some species cheat on mutualistic systems o Ex. Deceit pollination occurs when certain species of plants produce a showy flower, but no nectar reward ▪ Pollinators have to be deceived to make a visit and carry out pollination ▪ Evolutionary studies show that deceit pollinators evolved from ancestral species that did provide a reward ▪ Over time, a +/+ interaction evolved into a +/- interaction  Because the costs and benefits of species interaction are fluid, an interaction between the same two species may range from parasitism to mutualism to competition  Table 53.1 Community Structure  How Predictable are Communities o Frederick Clements promoted the view that biological communities are stable, integrated and orderly entities with a highly predictable composition o Hypothesis was that species interaction are so extensive and coevolution is so important that the groups of species called communities have become highly integrated and interdependent units of nature o Argued that communities develop by passing through a series of predictable stages dictated by extensive interactions among species and that this development culminates in a stable final stage known as a climax community o According to Clements, nature of the climax community is determined by the area’s climate and does not change over time o Henry Gleason, in contrast, contended that the community found in a particular area is neither stable nor predictable o He claimed that plant and animal communities are ephemeral associations of species that just happen to share similar climatic requirements o Mapping Current and Past Species’ Distributions ▪ If communities are predictable assemblages, then the ranges of species that make up a particular community should be congruent ▪ The same group of species should almost always be found growing together ▪ H
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