Community Ecology

What Is Community Ecology?

An ecological community is defined as a group of actually or potentially interacting species living in the same place. A community is bound together by the network of influences that species have on one another. Inherent in this view is the notion that whatever affects one species also affects many others -- the "balance of nature". We build an understanding of communities by examining the two-way, and then the multi-way, interactions involving pairs of species or many species.  A community is made up of populations of different species, or animals, plants, fungi, and bacteria, living in the same area

A. Predation

The most obvious form of species interaction is when one species eats another, predation being the technical term for this unfortunate fate. The predator is the organism that does the eating, the prey the one that gets eaten. A simple way to depict who eats what is by drawing a food chain, with the arrows pointing in the direction the food is going, i.e. to the predator. For example, take this food chain found off the shore of Nanortalik. The algae gets eaten by the herring, which in turn gets eaten by the cod, which gets eaten by the seal, which is devoured by the orca.

The organisms at the bottom of food chains (or far left when drawn horizontally), usually plants, are called producers. They produce food by converting carbon in the atmosphere, i.e. through photosynthesis. The herbivores that eat the plants , or the algae in this case, are called primary consumers (like the herring), and the carnivores that eat the primary consumers are secondary consumers (like the cod). Tertiary consumers such as seals eat secondary consumers, and so on. The orca is said to be 'at the top of the food chain' because it has no natural predators. It is considered a quaternary consumer, or top predator.

Food webs, like the one in the picture here, are used to show how the food chains in an ecosystem are interconnected.

For example, from the diagram we can see that the herring mentioned above is also part of other food chains involving the humpback whale or the Greenland shark as secondary consumers.

Another example of a food chain in this ecosystem would be:

algae --> small invertebrates --> sand eels --> Greenland shark --> Orca whale

1. Criptic coloration or camouflage that makes prey difficult to spot. For example in the picture below which shows the camouflage of a canyon tree frog that equates the color of his body with the background color of the tree he inhabits.

2. Aposematic coloration that warns predators to stay away from prey. For example in the picture below which shows the color of a bright and striking arrow frog that means that the frog warns its predator that it is poisonous.

3. Mimicry

• Batesian:One species is harmful the other is non-harmful, Non-harmful evolved similar colouration so predators think its monarch
• Mullerian: Both species are harmful and benefit from higher encounter rate and learning by predators. Similar colouration so predator only has to learn to avoid one thing (both bee species)

Predation and Herbivory

Predation requires one individual, the predator, to kill and eat another individual, the prey (Figure 3). In most examples of this relationship, the predator and prey are both animals; however, protozoans are known to prey on bacteria and other protozoans and some plants are known to trap and digest insects (for example, pitcher plant) (Figure 4). Typically, this interaction occurs between species (inter-specific); but when it occurs within a species (intra-specific) it is cannibalism.
Cannibalism is actually quite common in both aquatic and terrestrial food webs (Huss et al. 2010; Greenwood et al. 2010). It often occurs when food resources are scarce, forcing organisms of the same species to feed on each other. Surprisingly, this can actually benefit the species (though not the prey) as a whole by sustaining the population through times of limited resources while simultaneously allowing the scarce resources to rebound through reduced feeding pressure (Huss et al. 2010). The predator-prey relationship can be complex through sophisticated adaptations by both predators and prey, in what has been called an "evolutionary arms race." Typical predatory adaptations are sharp teeth and claws, stingers or poison, quick and agile bodies, camouflage coloration and excellent olfactory, visual or aural acuity. Prey species have evolved a variety of defenses including behavioral, morphological, physiological, mechanical, life-history synchrony and chemical defenses to avoid being preyed upon (Aaron, Farnsworth et al. 1996, 2008).

Figure 3.  Crocodiles are some of the evolutionarily oldest and dangerous predators.

Figure 4: A carnivorous pitcher plant. A carnivorous pitcher plant that preys upon insects by luring them into the elongated tube where the insects get trapped, die and are then digested.

Another interaction that is much like predation is herbivory, which is when an individual feeds on all or part of a photosynthetic organism (plant or algae), possibly killing it (Gurevitch et al. 2006). An important difference between herbivory and predation is that herbivory does not always lead to the death of the individual. Herbivory is often the foundation of food webs since it involves the consumption of primary producers (organisms that convert light energy to chemical energy through photosynthesis). Herbivores are classified based on the part of the plant consumed. Granivores eat seeds; grazers eat grasses and low shrubs; browsers eat leaves from trees or shrubs; and frugivores eat fruits. Plants, like prey, also have evolved adaptations to herbivory. Tolerance is the ability to minimize negative effects resulting from herbivory, while resistance means that plants use defenses to avoid being consumed. Physical (for example, thorns, tough material, sticky substances) and chemical adaptations (for example, irritating toxins on piercing structures, and bad-tasting chemicals in leaves) are two common types of plant defenses (Gurevitch et al. 2006)

Figure 5: Sharp thorns on the branch of a tree, used as anti-herbivory defense.

B. Symbiosis: Mutualism, Commensalism and Parasitism

Symbiosis is an interaction characterized by two or more species living purposefully in direct contact with each other. The term "symbiosis" includes a broad range of species interactions but typically refers to three major types: mutualism, commensalism and parasitism. Mutualism is a symbiotic interaction where both or all individuals benefit from the relationship. Mutualism can be considered obligate or facultative. (Be aware that sometimes the term "symbiosis" is used specifically to mean mutualism.) Species involved in obligate mutualism cannot survive without the relationship, while facultative mutualistic species can survive individually when separated but often not as well (Aaron et al. 1996). For example, leafcutter ants and certain fungi have an obligate mutualistic relationship. The ant larvae eat only one kind of fungi, and the fungi cannot survive without the constant care of the ants. As a result, the colonies activities revolve around cultivating the fungi. They provide it with digested leaf material, can sense if a leaf species is harmful to the fungi, and keep it free from pests (Figure 6). A good example of a facultative mutualistic relationship is found between mycorrhizal fungi and plant roots. It has been suggested that 80% of vascular plants form relationships with mycorrhizal fungi (Deacon 2006). Yet the relationship can turn parasitic when the environment of the fungi is nutrient rich, because the plant no longer provides a benefit (Johnson et al. 1997). Thus, the nature of the interactions between two species is often relative to the abiotic conditions and not always easily identified in nature.

Leaf cutter ants.

Figure 6: Leaf cutter ants.

Leaf cutter ants carrying pieces of leaves back to the colony where the leaves will be used to grow a fungus that is then used as food. The ants will make "trails" to an acceptable leaf source to harvest it rapidly.

Commensalism is an interaction in which one individual benefits while the other is neither helped nor harmed. For example, orchids (examples of epiphytes) found in tropical rainforests grow on the branches of trees in order to access light, but the presence of the orchids does not affect the trees (Figure 7). Commensalism can be difficult to identify because the individual that benefits may have indirect effects on the other individual that are not readily noticeable or detectable. If the orchid from the previous example grew too large and broke off the branch or shaded the tree, then the relationship would become parasitic.

Epiphytic bromeliads that grow on the limbs of large tropical rainforest trees.

Figure 7: Epiphytic bromeliads that grow on the limbs of large tropical rainforest trees.

The bromeliads benefit by occupying space on the limb receiving rain and sunlight, but do not harm the tree.

Parasitism occurs when one individual, the parasite, benefits from another individual, the host, while harming the host in the process. Parasites feed on host tissue or fluids and can be found within (endoparasites) or outside (ectoparasites) of the host body (Holomuzki et al. 2010). For example, different species of ticks are common ectoparasites on animals and humans. Parasitism is a good example of how species interactions are integrated. Parasites typically do not kill their hosts, but can significantly weaken them; indirectly causing the host to die via illness, effects on metabolism, lower overall health and increased predation potential (Holomuzki et al. 2010). For instance, there is a trematode that parasitizes certain aquatic snails. Infected snails lose some of their characteristic behavior and will remain on the tops of rocks in streams where food is inadequate and even during peaks of waterfowl activity, making them easy prey for the birds (Levri 1999). Further, parasitism of prey species can indirectly alter the interactions of associated predators, other prey of the predators, and their own prey. When a parasite influences the competitive interaction between two species, it is termed parasite-mediated competition (Figure 8). The parasite can infect one or both of the involved species (Hatcher et al. 2006). For example, the malarial parasite Plasmodium azurophilum differentially infects two lizard species found in the Caribbean, Anolis gingivinius and Anolis wattsi. A. gingivinius is a better competitor than A. wattsi but is susceptible to P. azurophilum, while A. wattsi rarely contracts the parasite. These lizards are found coexisting only when the parasite is present, indicating that the parasite lowers the competitive ability of A. gingivinius' (Schall 1992). In this case, the parasite prevents competitive exclusion, therefore maintaining species diversity in this ecosystem.

Multiple conceptual models of species interactions that involve parasites.

Figure 8: Multiple conceptual models of species interactions that involve parasites.

The + and – indicate positive and negative influence, respectively, between resources, hosts, predators and parasites.

C. Competition
Occurs when an individual is denied resources (food, water, light, territory, space, mates)

• Exploitative – use of resources by one individual deprives others (species A use up resource before species B can use them)
• Interference – one individual prevents others access to resources (direct interaction b/w competitors)
• Gause’s Experiments: Two paramecium species cannot live together but can live alone (species with the same niche cannot coexist)
• Removal Experiment: Remove one species, evidence of comp. if other species expands
• Fundamental Niche: Habitat/resource use in absence of competitors
• Realized Niche: Restricted habitat/resource use in the presence of competitors (could be same size or smaller then fundamental niche, never bigger)
• Character Displacement: Differences between species are exaggerated in areas of overlap (we see differences when they live together, they start using different pools of resources

D. Community Structure

Species Richness: Number of species

Species Evenness: Describes how equally distributed individuals in the community are across species

Species Diversity: An index composed of richness and evenness; Intermediate Disturbance Hypothesis: Species richness greatest at intermediate disturbance