Ecological niches

Biological Species Definition

 Species definition:

 Organisms in a group or population

that share enough genetic similarity

to interbreed in nature, and produce

viable, fertile offspring

 Note: fertile means the offspring also

have the ability to sexually reproduce

Biological Species Definition

 Lions and tigers are categorised as different species but can reproduce

 They produce a tigon or liger, but these hybrid offspring are not fertile

so they can’t reproduce themselves

 This process is unlikely to occur in nature, but there are recorded examples, such

as brown bears and polar bears, and dolphins with other dolphin/whale

species

Problems With The Species Definition

 Organisms that reproduce asexually, such as

bacteria, are in effect clones

 Organisms with variation that are spread

over a large geographic area, such as gulls

in the arctic circle

 Some species can successfully breed and

produce fertile offspring, but are considered

separate species – for example domestic

dogs, dingos, and wolves

Ecological Niche

 Each species fills a role within an ecosystem, which is referred to as its

ecological niche

 This role includes:

 Spatial habitat – where it lives

 Mode of respiration – its oxygen

requirements

 Mode of nutrition – how it gets food

 Modes of interaction – how it interacts

with other species

Fundamental and Realised Niches

 Some species may not be able to occupy their entire niche due to the

presence or absence of other species

 The fundamental niche is the entire set of conditions under which an organism

can survive and reproduce (where it can live)

 The realised niche is the set of conditions actually used by a given organism

after interactions with other species are taken into account (where it does live)

Fundamental and Realised Niches

 Chthamalus (a type of

barnacle) is generally only

found in the high tide zone -

where it is best adapted to

survival / most competitive, this

is its realised niche

 However, if a neighbouring

type of barnacle (Balanus) is

removed Chthamalus can

actually occupy the entire

intertidal zone - its

fundamental niche

Niche Differentiation

 Niche differentiation describes the way by which competing species use

the environment differently in order to exist

 If two species with identical niches compete, two distinct outcomes are

possible:

 Competitive exclusion: One species will use the resources more efficiently and

drive the other species to local extinction

 Resource partitioning: Two species will alter their use of the niche to avoid direct

competition, allowing for co-existence

Competitive Exclusion Principle

Resource Partitioning

Modes of Respiration

 Species can be placed in three categories based on their oxygen

requirements: (Obligate = by necessity Facultative = occurring optionally)

Category Requirements Examples

Obligate aerobes Require a continuous oxygen supply so only

live in oxic environments

Animals and plants, some bacteria (such

as the skin bacteria Micrococcus luteus)

Obligate anaerobes Inhibited or killed by oxygen so only live in

anoxic environments

Clostridium tetani (tetanus bacteria),

methanogenic archaea

Facultative anaerobes Use oxygen if available so can live in oxic

and anoxic environments

Escherichia coli (a gut bacteria),

Saccharomyces cerevisiae (yeast), some

polychaete marine worms

Trophic Levels

 An organism's trophic level refers to where its niche is situated in a food

chain

 Producers always occupy the first trophic level in a feeding sequence

 Primary consumers feed on producers and hence occupy the second

trophic level

 Further consumers (e.g. secondary, tertiary, etc.) may occupy subsequent

trophic levels

Modes of Nutrition

 Autotrophic nutrition

 Organisms synthesise organic material from inorganic sources provided by their

environments, either by photosynthesis (photoautotrophs) or chemosynthesis

(chemoautotrophs)

 Autotrophs are producers in an ecosystem, they produce their own energy through

chemical processes and are the first level of the food chain

 Heterotrophic nutrition

 Organisms which derive their energy from consuming organic matter, either by

holozoic nutrition (ingesting organic matter) or saprotrophic nutrition (absorbing

externally digested organic matter)

 Heterotrophs are consumers and decomposers in an ecosystem, and are found in

the higher levels of the food chain

Modes of Nutrition

Saprotrophic Nutrition

 Decomposers are organisms that break down organic matter and play

an important role in food webs and nutrient cycling

 Detritivores obtain their organic nutrients from detritus (decomposing plant and

animal matter) by internal digestion

 For example: earth worms, millipedes, sea stars

 Saprotrophs obtain their organic nutrients from breaking down dead organisms

by external digestion and then absorbing the nutrients

 For example: fungi, soil bacteria

Decomposers

 Detritivore  Saprotroph

Mixotrophic Nutrition

 Autotrophs make their own carbon

compounds from simple substances,

including carbon dioxide, and heterotrophs

obtain their carbon compounds from other

organisms

 Some unicellular eukaryotes (protists) are

able to use both methods, which is referred

to as mixotrophic nutrition

 Euglena gracilis (images to the right) have

chloroplasts and carry out photosynthesis

when light is sufficient, but can also feed on

detritus and smaller organisms by endocytosis

Modes of Nutrition

Populations and Communities

 A population is an interacting group of individual organisms of the same

species living in a given area

 The members of a population normally interbreed with each other and are

reproductively isolated from other populations

 This isolation is often due to geographic barriers such as the sea between two

islands, a river, a mountain range etc

 A community is a group of populations living together in an area and

interacting with each other

 This includes plants, animals, fungi and bacteria, and can consist of hundreds, or

even thousands, of species

Symbiosis

 Symbiosis is any type of close, long-term

biological interaction between two different

biological organisms in a community

 Obligatory symbiosis: where symbiosis is

essential for life

 Facultative symbiosis: symbiosis is beneficial for

at least one organism, but not essential

 Interspecific interactions occur between

members of different species

 Intraspecific interactions occur between

members of the same species

Types of Symbiosis

 Commensalism

 One species benefits while the

other neither benefits nor is harmed

 Mutualism

 Both participating species benefit

 Parasitism

 One species benefits but the other

is harmed

 Pathogenicity

 One species causing disease in

another species

 Predation

 One species kills and consumes

another organism

 Herbivory

 Consumers feeding on producers

 Competition

 The use of the same limited

resource by two organisms or

species in the same area

Intraspecific Competition and Cooperation

 Competition

 Unless a resource is abundant, members in a population of the same species will

frequently compete with each other for the that resource

 Some members of the population will be more successful competing for the

resource, so are more likely to survive and reproduce

 This is a key factor in natural selection and the evolution of that species over time

 Cooperation

 Individuals in a population may develop cooperative interactions, which have

strong advantages because all individuals may benefit

 It is less common in plants, and more common in social animals