Chapter 4- SPECIES INTERACTIONS AND COMMUNITY DYNAMICS

Competition for scarce resources and Predation are major factors in evolution and adaptation.

Competition

Law of competitive exclusion: no two species will occupy the same niche and compete for exactly the same resources in the same habitat for very long.

One species will have a competitive edge, and will gain a larger share of resources. Other species will migrate to a new area, become extinct, or change its behavior in a way to minimize competition. Process of niche evolution is called resource partitioning. Niche specialization can create behavior separation that allows subpopulations of a single species to diverge into separate species.

For what do organisms compete?

• Energy and matter in usable forms.
• Space
• Specific sites for life activities

Intraspecific competition: competition among members of the same species

Interspecific competition: competition among members of different species

• Competition among animals may not be in the form of fighting. In fact, many animals tend to avoid fighting if possible as it is not worth getting injured.
• Intraspecific competition can be especially intense because members of the same species have the same space and nutritional requirements.
• Plants have developed mechanisms to cope with intraspecific competition.
  • Seedlings unable to germinate in the shady conditions created by parent plants.
  • Plants disperse seeds to other sites by water, air, or animals.
  • Plants secrete substances that inhibit the growth of seedlings near them.
• Animals have developed mechanisms to cope with intraspecific competition.
  • Varied life cycles ( e.g. different habitats and feeding in juvenile and adult invertebrates)
  • Occupy different ecological niches.
  • Territoriality: intense form of intraspecific competition in which organisms define an area surrounding their home site or nesting site and defend it.
• These mechanisms (plant and animal):
  • Help allocate resources of an area by spacing out the members of a population
  • Promote dispersal into adjacent areas.

Predation

Predator: an organisms that feeds directly upon another living organism, whether or not it kills the prey to do so.

All forms of organisms which feed on living things can be considered predators:

• Carnivores
• Herbivores
• Omnivores
• Parasites
• Pathogens

Exceptions include scavengers, detritivores, and decomposers (which feed on dead things)

Predation is a potent and complex influence on the population balance of communities involving:

• All stages of the life cycles of predator and prey species
• Many specialized food-obtaining mechanisms
• Specific pre-predator adaptations that either resist or encourage predation

Predators play a role in evolution by

• Preying most successfully on the slowest, weakest, least fit members of their target population
• Reducing competition
• Preventing excess population growth
• Allowing successful traits to become dominant in the prey population

Coevolution: process in which species exert selective pressure on each other

Prey species evolve many protective or defensive adaptations to avoid predation.

Predators evolve mechanisms to overcome the defenses of their prey.

Keystone Species

Keystone species: species or set of species whose impact on its community or ecosystem is much larger and more influential than would be expected from mere abundance.

Both top predators (e.g. wolves) as well as less conspicuous species (e.g. tropical figs and some microorganisms) play essential community roles.

Often a number of species are intricately interconnected in biological communities so that it is difficult to tell which is the essential key.

The filter-feeding Krill is a keystone species in the complex food web of the Antarctic.

Symbiosis

Symbiosis: the intimate living together of members of two or more species.

In contrast to predation and competition, symbiotic interactions between organisms can be non antagonistic. Symbiotic relationships often entail some degree of coadaptation or coevolution of the partners, shaping, or at least in part, their structural or behavior characteristics (mutualistic coadaptation). Commensalism: type of symbiosis in which one member clearly benefits and the other is neither benefited nor harmed. Cattle and cattle egrets Many mosses, bromeliads, and other plants growing on trees. Mutualism: association in which both members of the partnership benefit. Lichens (combination of fungi and a photosynthetic partner) Parasitism: type of symbiosis in which one member benefits and the other is harmed.

Defensive Mechanisms

Many plants and animals have toxic chemicals, body armor, and other defensive adaptations to protect themselves from competitors or predators.

• Arthropods, amphibians, snakes, and some mammals produce noxious odors or poisonous secretions
• Plants also produce a variety of chemical compounds that make them unpalatable or dangerous to disturb
  • Poison ivy
  • Stinging nettles

Batesian mimicry: harmless species will evolve colors, patterns, or body shapes that mimic species that are unpalatable or poisonous.

• Wasps and longhorn beetle

Muellerian mimicry: two species, both of which are unpalatable or dangerous have evolved to look alike so that when predators learn to avoid either species, both benefit.

Species also evolve amazing abilities to avoid being discovered.

• Insects that look exactly like dead leaves or twigs

Predators use camouflage to hide as they lay in wait for their prey.

• Scorpion fish