Big Idea 4 - Ecology Ch. 53 – community ecology

Levels of Organization To study relationships within the biosphere, ecologists have organized it into smaller pieces. The levels are: Organism (a single individual – one fish, for example) Population (when organisms of a single species share the same geographic location at the same time) Biological community (a group of populations that interact and occupy the same space at the same time) Ecosystem (a biological community plus all of the abiotic factors that affect it) ex – trees plus water and soil 5. Biome (a large group of ecosystems that share the same climate and have similar communities) Ex – marine or desert biomes 6. Biosphere (all the biomes on Earth combined)

Communities A community is a group of organisms of different species that live in a particular area

Overview Ecologists call relationships between species in a community interspecific interactions Interspecific interactions affect species survival and reproduction Examples are competition, predation, herbivory, symbiosis (parasitism, mutualism, and commensalism), and disease

Ecological Niches An organism’s niche is the specific role it plays in its environment All of its uses of biotic and abiotic resources in its environment Example: oak tree in a deciduous forest Provides oxygen to plants, animals, etc. Home for squirrels Nesting ground for blue jays Takes water out of the soil Etc., etc. Ecologically similar species can coexist in a community if there are one or more significant differences in their niches

Fundamental v. Realized Niche Fundamental Niche includes resources an organism could theoretically use (if no competition) Realized Niche includes resources it actually does use given competition from other species.

Resource partitioning Resource partitioning is differentiation of ecological niches enables similar species to coexist in a community Similar species develop ways to partition/divide resources in order to coexist.

Interspecific Competition Competition between organisms of different species occurs when species compete for a resource in short supply can lead to competitive exclusion (local elimination of a competing species) The Competitive Exclusion Principle: Two species with similar needs for the same limiting resources cannot coexist in the same place Niches may overlap but they may not be identical.

Interspecific Interactions There are 4 major interspecific interactions (symbiotic relationships): Parasitism (includes predation and herbivory) Competition Commensalism Mutualism

Predation (and Parasitism) (+ -) The interaction is beneficial to one species and detrimental to the other Predation: When a predator eats its prey Some feeding adaptations of predators are claws, teeth, fangs, stingers, and poison Example in picture:

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

Plant Defenses Against Herbivores “Plants Fight Back!” Plants have 2 major mechanisms by which they defend themselves against being eaten Mechanical Defenses Thorns, hooks, etc.

Plant Defenses Against Herbivores Chemical Defenses Produce chemicals that are distasteful or harmful to an herbivore Morphine (opium poppy) Nicotine (tobacco)

Animal Defenses Against Predators Animals defend themselves against predators passively (hiding) or actively (fleeing) Cryptic coloration (camouflage) makes prey difficult to spot Aposematic coloration (warning coloration)warns predators not to each animals that may be toxic or may sting.

Animal Defenses Against Predation Mimicry When one species “imitates” or “mimics” another Batesian mimicry When one edible or harmless species mimics an bad-tasting (unpalatable) or harmful species Example: hawkmoth mimics a snake

LE 53-7 Green parrot snake Hawkmoth larva

Animal Defenses Against Predation Mimicry Mullerian mimicry Two species, both of which are unpalatable (taste bad) or harmful, resemble each other Example: monarch butterfly (unpalatable) and queen butterfly (unpalatable) resemble each other

Parasitism One organism (the parasite) gets its nourishment from another organism (the host), which is harmed in the process Usually do not kill their hosts Endoparasites: Live within host tissues (tapeworms) Ectoparasites: Feed on external surfaces (mosquitoes)

Mutualism (++) relationship Both partners benefit from the relationship “You scratch my back, I’ll scratch yours” Examples: Ants & acacia tree tree provides high protein food in beltian bodies & habitat for nests inside thorns; ant protects against predators Mycorrhizae-fungal extentions on plant roots Plant gets increased water/nutrition, fungi gets food Hummingbirds & flowers Hummingbirds get food, flowers can reproduce

Commensalism Commensalism – one organism benefits and the other is neither helped nor harmed Ex – shark and remora fish; sea anemones and clownfish

Dominant & Keystone Species In general, a few species in a community exert strong control on that community’s structure Two fundamental features of community structure are species diversity and feeding relationships Dominant Species: Species in a community that have the highest abundance or highest biomass One hypothesis suggests that dominant species are most competitive in exploiting resources Another hypothesis is that they are most successful at avoiding predators American Chestnut tree and the chestnut blight Keystone Species: Important to a community because of their ecological roles (niches), not by numbers In contrast to dominant species, keystone species are not necessarily abundant in a community Wolves control deer populations Bees help pollinate local plants Elephants eat small trees, which preserve grasslands Prarie dogs eat grass which keeps water in the soil and prevents evaporation

LE 53-17 100 80 Otter number (% max. count) 60 40 20 Sea otter abundance 400 300 Grams per 0.25 m2 200 100 Sea urchin biomass 10 8 Number per 0.25 m2 6 4 2 1972 1985 1989 1993 1997 Year Total kelp density Food chain before killer whale involvement in chain Food chain after killer whales started preying on otters

Ecosystem “Engineers” (Foundation Species) Some organisms exert influence by causing physical changes in the environment that affect community structure Some foundation species act as facilitators that have positive effects on survival and reproduction of some other species in the community For example, beaver dams can transform landscapes on a very large scale

With Juncus Without Juncus LE 53-19 8 6 Number of plant species 4 2 With Juncus Without Juncus Salt marsh with Juncus (foreground) Conditions

Ecological Succession Ecological succession is a change in the species that live in a given area over a period of time One community replaces another Primary succession = occurs in places where soil is not yet formed (bare bedrock) Secondary succession = occurs in places where there is soil, but where some disturbance has eliminated the previous community

Ecological Succession The first organisms to inhabit an area undergoing succession are known as pioneer organisms These are usually small organisms (bacteria, lichens, algae, etc.) The ecosystem goes through a number of stages, with each new stage usually consisting of larger organisms than the last one Once a community has become stable and is not changing much, it is known as a climax community

Ecological Succession cont. Early-arriving species and later-arriving species may be linked in one of three processes: Early arrivals may facilitate appearance of later species by making the environment favorable They may inhibit establishment of later species They may tolerate later species but have no impact on their establishment

Causes of Ecological Succession There are 3 major causes of ecological succession: Human Activities - logging, mining, development, etc. Natural Disasters/Disturbances - fires, volcanic eruptions, etc. 3. Natural Competition Among Species - Fictitious example: - turtles and frogs both eat crickets - frogs are faster, turtles are slower - frogs eat more crickets, turtles starve - turtle population dies out, frog population gets bigger

Species Diversity Species diversity of a community is the variety of organisms that make up the community It has two components: species richness relative abundance Species richness is the total number of different species in the community Relative abundance is the proportion each species represents of the total individuals in the community

Species diversity cont. Two communities can have the same species richness but a different relative abundance A community with an even species abundance is more diverse than one in which one or two species are abundant and the remainder are rare

Trophic Structure Trophic structure is the feeding relationships between organisms in a community It is a key factor in community dynamics Food chains link trophic levels from producers to top carnivores

A terrestrial food chain LE 53-12 Quaternary consumers Carnivore Carnivore Tertiary consumers Carnivore Carnivore Secondary consumers Carnivore Carnivore Primary consumers Herbivore Zooplankton Primary producers Plant Phytoplankton A terrestrial food chain A marine food chain

Food Webs A food web is a series of connected food chains with complex trophic interactions

Limits on Food Chain Length Each food chain in a food web is usually only a few links long Two hypotheses attempt to explain food chain length: the energetic hypothesis and the dynamic stability hypothesis

Limits on food chain links cont. The energetic hypothesis suggests that length is limited by inefficient energy transfer Most data support the energetic hypothesis The dynamic stability hypothesis proposes that long food chains are less stable than short ones

Bottom-Up and Top-Down Controls The bottom-up model of community organization proposes a unidirectional influence from lower to higher trophic levels In this case, presence or absence of mineral nutrients determines community structure, including abundance of primary producers

The top-down model proposes that control comes from the trophic level above In this case, predators control herbivores, which in turn control primary producers

Long-term experimental studies have shown that communities can shift periodically from bottom-up to top-down controls Pollution can affect community dynamics Biomanipulation can help restore polluted communities

What Is Disturbance? A disturbance is an event that changes a community, removes organisms from it, and alters resource availability Fire is a significant disturbance in most terrestrial ecosystems It is often a necessity in some communities

LE 53-21 Before a controlled burn. A prairie that has not burned for several years has a high propor-tion of detritus (dead grass). During the burn. The detritus serves as fuel for fires. After the burn. Approximately one month after the controlled burn, virtually all of the biomass in this prairie is living.

The intermediate disturbance hypothesis suggests that moderate levels of disturbance can foster higher diversity than low levels of disturbance The large-scale fire in Yellowstone National Park in 1988 demonstrated that communities can often respond very rapidly to a massive disturbance

LE 53-22 Soon after fire. As this photo taken soon after the fire shows, the burn left a patchy landscape. Note the unburned trees in the distance. One year after fire. This photo of the same general area taken the following year indicates how rapidly the com-munity began to recover. A variety of herbaceous plants, different from those in the former forest, cover the ground.

Human Disturbance Humans are the most widespread agents of disturbance Human disturbance to communities usually reduces species diversity Humans also prevent some naturally occurring disturbances, which can be important to community structure

Biogeographic factors affect community diversity Equatorial-Polar Gradients Species richness generally declines along an equatorial-polar gradient and is especially great in the tropics The greater age, climate, and amount of light found in tropical environments may account for the greater species richness

Biogeographic factors affect community diversity cont. Climate is likely the primary cause of the latitudinal gradient in biodiversity Two main climatic factors correlated with biodiversity are: solar energy and water availability They can be considered together by measuring a community’s rate of evapotranspiration Evapotranspiration is evaporation of water from soil plus transpiration of water from plants

Area Effects The species-area curve quantifies the idea that, all other factors being equal, a larger geographic area has more species Species richness on islands depends on island size, distance from the mainland, immigration, and extinction

Rivet and Redundancy Models The rivet model suggests that all species in a community are linked in a tight web of interactions It also states that loss of even a single species has strong repercussions for the community The redundancy model proposes that if a species is lost, other species will fill the gap Community hypotheses and models represent extremes; most communities probably lie somewhere in the middle