6.change In Community Structure&function

CHANGES IN COMMUNITY STRUCTURE & FUNCTION

• community is in a state of flux – changing in time • temporal changes are due to major perturbations of the system & changes are part of the pattern of natural communities Short-term cycles in community structure: • community is composed of subcommunities which replace each other in a regular pattern over time • diurnal changes • tidal flow in littoral communities • over a longer time scale – changing weather conditions, changing seasons of the year

• pattern of domination in plant community show a cyclical change because of some separation of growing season, flowering time Shifts in community structure: colonisation & extinction • the principles of ecological balance of islands (MacArthur & Wilson, 1963, 1967): -Rate of colonisation & extinction were simple functions of the number of species already on island, so that the rate of immigration of new species decreases as the number of established species increases - extinction increase with species number - extinction rate is a function of island size & species number - the equilibrium between colonisation & extinction is different for islands of different size

Immigration rate Extinction rate

Ŝ Number of species on island

Succession • The non-seasonal, directional continuous pattern of colonization & extinction on a site by populations • Directional & additional progressive change in community structure & functional • All the minor changes of structure & operation accumulate over time so that the community itself develops a new emphasis & a different function • A process resulting in the greater stability of the community & the climax community is considered the most stable community that can exist in that particular environment • Communities from a variety of different starting points proceed towards a very limited number of end-communities or climax state

i. Degradative succession • occur over a relatively short time scale • the environmental conditions become so altered that the original coloniser can no longer exist – becomes extinct • the new coloniser – better suited to the changed environment – outcompetes the original species • the community of organisms occupying a particular environment at any one time have a significant impact upon environment, which become less suitable for them & more suitable for other species • e.g. the community of scavengers & decomposers which occupy the decaying carcasses of animals • Degradative succession terminate because the resource is completely metabolized & mineralized

Heterotrophic (Degradative) succession: Succession on a degradable source; most living biomass is animal, fungal, or microbial material

ii. Autogenic succession • successions which occur on newly exposed landforms & in the absence of gradually changing abiotic influence • primary succession: the sequence of species on landform not previously been influenced by a community • secondary succession: Subsequent sequence of species on an area where the vegetation has been partially or completely removed, but where well-developed soil & seed & spores remain Facilitation: • a process where an early successional species may so alter conditions or the availability of resources in a habitat that the entry of new species is made possible

Primary succession: sequence of communities developing in a newly exposed habitat devoid of life (eg. succession on bare rock, newly deposited sand

Secondary succession (different from glossary): sequence of communities taking place on sites that have already supported life (eg. oldfield succession, clearcut forests, burned areas, etc.).

• changes in the abiotic environment are imposed by the developing community • the entry & growth of the later species depends on earlier species preparing the ground •particularly important in primary successions where conditions are initially severe Inhibition • the opposite of facilitation, characterized by species which inhibits its replacement by another • the succession only occurs only because the species which dominate early are more susceptible to the rigours of the physical environment

• all species resist invasions of competitors • late species colonize & grow to maturity when individuals of the early species are killed • the early species are killed by local disturbance caused by extreme physical conditions or the action of predators • the early colonists among the trees usually have efficient seed dispersal, the late colonists have larger seeds, poorer dispersal & long juvenile phases Tolerance • a predictable sequence is produced because different species have different strategies for exploiting resources • later species are able to tolerate lower resource levels & can grow to maturity in

the presence of early species, eventually outcompeting them The mechanics of succession: •

3 models of the mechanisms which underlie successions: i) facilitation, ii), tolerance iii) inhibition

•

vital attributes that determine their place in succession:

i. Method of recovery after disturbance ii. The ability of individuals to reproduce in the face of competition • 2 alternatives that might increase the fitness of an organism in a succession: i. The species reacts to the competitive selection pressures & evolves characteristics which enable it to persist longer in the succession

ii. It may develop more efficient mechanisms of escape from the succession, & discover & colonize suitable early stages of succession elsewhere Changes in the characteristics of individual organisms associated with different successional stages: Characteristic of Developmental organism stages

Mature stages

Niche style

Broad

Narrow

Size of organism

Small

Large

Life cycles

Short, simple

Long, complex

Growth form Rapid growth (selection pressure for)

Feedback control

Production (selection for)

Quality

Quantity

Disturbance opens up a relatively large space FACILITATION

Individuals of any species in the succession could establish & exist as adults under the prevailing conditions

Only certain ‘pioneer’ species are capable of becoming established in the open space

TOLERANCE Modification of the environment by early occupants makes it more suitable for recruitment of ‘late successional’ species

Modification of the environment by early occupants has little or no effect on subsequent recruitment of ‘late successional’ species

INHIBITION

In time, earlier species are eliminated through competition for resources with established ‘late successional’ adults

The sequence continues until the current resident species no longer facilitate the invasion & growth of other species &/or no species exists that can invade & grow in the presence of the resident

Modification of the environment by early occupants makes it less suitable for recruitment of ‘late successional’ species

As long as earlier colonists persist undamaged and/or continue to regenerate vegetatively, they exclude or suppress subsequent colonists of all species

If external stresses are present, early colonists may be damaged or killed & be replaced by species which are more resistant

The evolution of natural lakes is normally from oligotrophic to eutrophic This results from the delivery of sediment and nutrients to the lake, which slowly causes the lake to fill in and become shallower. This generally causes an increase in temperature and biologic productivity. An increase in sediment delivery to lakes can accelerate the eutrophication process because of nutrients that bind to fine sediments. Continued filling with sediments leads to advanced eutrophication, swampy or marshy conditions, and finally total infilling of the prior lake environment Retrogressive succession or rejuvenation of lakes: techniques by which lakes are restored & their aging process are postponed

a)

b)

Old lake bed New lake bed

Old lake bed c)

d)

New lake bed

New soil level Old water bed Old lake bed

Successional change during the gradual silting of a freshwater lake

Changes proceed from early developmental stages towards climax: a) A progressive increase in the total organic matter of the community b) A tendency for nutrients to be increasingly bound up within organisms rather than free or extrabiotic c) There is increased nutrient conservation & slow nutrient loss d) The decomposer element of the community becomes larger & more important e) The community itself becomes more diverse in terms of species number & balance f) It becomes more spatially diverse & heterogenous g) The whole community becomes more complex – biotic interrelationships diversify h) Flow of material around the community becomes increasingly slower

• the community shifts from the rapidly growing, changing & expanding system characteristic of pioneer communities, to a slower, more stable & fully exploited system • niche-specialisation increase, competition from other organisms in the increasingly complex community becomes more intense • organisms of early successional communities tend to be relatively shortlived, but have relatively rapid generation times & massive reproductive potential • organisms characteristic of later seres tend to be longer-lived & have a much reduced reproductive output

• successional changes within the community are caused by a change in the species composition of the primary plants •any change in the species pattern of the producers will have repercussions in species composition throughout the rest of the associated community • succession may proceed by continued modification of the physical & chemical environment by an existing community, & its subsequent replacement in this altered environment by a new community more suited to the new conditions • the immutable features (geological, soil & substrate structure, climatic features etc) which cannot be changed by biotic intervention will block the chain of succession)

Effects on environment Community n

Changed environment

Colonisation by new 1ºplants 1º plants 2º plants Animals

Allows immigration of new 2ºplants Allows immigration of new animal spp. Community n + 1

Climax community

Schematic representation of the process of succession

Physiological characteristics of early & late successional plants Attribute

Early successional plants

Late successional plants

Well dispersed

Poorly dispersed

Seed germination enhanced by light, fluctuating T, high NO3-

Yes

No

Light saturation intensity

High

Low

Light compensation point

High

Low

Efficiency at low light

Low

High

Photosynthetic rates

High

Low

Respiration rate

High

Low

Transpiration rates

High

Low

Stomatal & mesophyll resistance

Low

High

Resistance to water transport

Low

High

Recovery from resource limitation

Fast

Low

Seed dispersal in time

Major climax formations •

the major climax community types of the world are called biomes

•

biomes are general groupings of similar communities

•

these groupings are based upon the distinctive life forms of the important species in each community

•

in terrestrial regions, dominant vegetational types are usually used to characterize biomes

• i. -

the principal biomes of the world: Tundra North of the Arctic Circle A very short growing season Extremely low temperatures during winter

- Under such harsh conditions, only a few species of animals are found -Most of the mammals are burrowing species - some of the herbivores feed in the tundra zone only during the short time of production of grasses & lichens - plant life is represented by many lichens species, mosses & low-growing flowering plants - food chains & webs are relatively simple - there is a great amount of annual moisture but much of it is tied up in snow or permafrost or lost by evaporation – relatively little water is available to tundra organism

ii. Coniferous forest or taiga • located at the south of the tundra, around the polar region of the Northern Hemisphere • the spruces, firs, pines & other conifers are able to grow at these latitudes because of the increase in available annual moisture • the stresses of subarctic winter bring photosynthetic production to a virtual halt • animal life in this biome is relatively poor in species

Coniferous forest or taiga

iii. Temperate deciduous forest • rainfall in this biome is sufficient to support broadleafed shrubs & trees, eg. the great hardwood forests of eastern North America & Europe • the species is subject to considerable temperature fluctuations & a cyclic alternation of photosynthetic production • Temperate deciduous forests are located in the mid-latitude areas which means that they are found between the polar regions and the tropics. • The deciduous forest regions are exposed to warm and cold air masses, which cause this area to have four seasons. • The areas in which deciduous forests are located get about 750 to 1,500 mm of precipitation spread fairly evenly throughout the year.

Temperate decidous forest

iv. Chaparral • characterized by low, shrubby vegetation with thick, evergreen leaves, whose toughness & waxy coatings make them resistant to prolonged periods of drought •Climate in this biome is very dry for most of the year, with low total precipitation in the form of winter rains & occasional summer thunderstorm •Chaparral is characterized as being very hot and dry. As for the temperature, the winter is very mild and is usually about 10 °C. Then there is the summer. It is so hot and dry at 40 °C that fires and droughts are very common. •The chaparral biome is found in a little bit of most of the continents - the west coast of the United States, the west coast of South America, the Cape Town area of South Africa, the western tip of Australia and the coastal areas of the Mediterranean.

The chaparral

•this chaparral biome vegetation is fireadapted – depends upon periodic fastburning fires to perpetuate itself & allow reproduction of the various species of shrubs at the expense of trees • chaparral is an example of a firemaintained climax community

v. Grassland • Grasslands are generally open and continuous, fairly flat areas of grass. • They are often located between temperate forests at high latitudes and deserts at subtropical latitudes. • Grasses vary in size from 2.1 m (7 ft) tall with roots extending down into the soil 1.8 m (6 ft), to the short grasses growing to a height of only 20 to 25 cm (8 to 10 in) tall. These short grasses can have roots that extend 1 m (about 3 ft) deep. • sufficient rainfall to support a heavy growth of herbaceous plants, esp. bunch grasses & spreading grasses (species with underground stems) • extensive temperate plains areas of North America, Eurasia, Australia

Grassland

vi. Desert • characterized by very low rainfall, a high evaporation rate, high diurnal temperatures & low nocturnal temperatures • desert shrubs & other plants have waxy, drought resistant leaves • 1º production is low - 1º consumption by animal life is low • because of the cool nights, dew is a significant source of moisture for desert organisms • desert plants can absorb dew & translocate its moisture from their leaves to their stems & roots

Desert

Desert biomes are the driest of all the biomes. The temperature also varies greatly depending on the location of the desert. Since desert conditions are so severe, the plants that live there need to have adaptations to compensate for the lack of water.

vii. Tropical savanna • characterized by a mixture of dry-adapted herbaceous plants, shrubs • e.g. acacias in Africa & palms in South America • rather large mixed herds of 1º consumers characterize the savanna fauna • Most savanna grass is coarse and grows in tufts with intervening patches of bare ground. • Scattered, individual trees or small groves of trees are common. The umbrella shaped Acacia tree is a notable species of the Savanna biome. • Trees do not dominate the biome because the small amount of high sun rainfall is not enough to sustain such vegetation

Tropical Savanna

viii. Tropical deciduous forest • rainfall becomes heavier on the margins of a tropical savanna & support regular forest • this biome is subjected annually to a dry season-wet season environmental cycle • with uniformly warm temperatures all year, the growth of plants is affected only by seasonal distribution of the rainfall • The trees are considerably taller here than in thornscrub and the columnar cacti rarely penetrate the canopy • The plants and animals in this community are sensitive to cold temperatures near freezing which defines the limits of northward dispersal for many tropical species of plants and animals.

Tropical deciduous forest

• in the annual dry season such forest lose most leaves & then leaf out again at the onset of the rainy season • most species exhibit special adaptations for passing the dry season in much the same manner as temperate-zone species pass the winter

xi. Tropical rainforest • rainfall tends to be more or less evenly distributed throughout the year • the mean annual temperature lies between about 68ºF and 82ºF, with very little seasonal fluctuation • neither temperature nor water is a limiting factor on plant or animal growth • biotic interactions are very important as limiting factors

• Very high biotic diversity The total biological diversity of only a few square kilometers of rich tropical rainforest can exceed that of entire regions in the temperate zone. • Most of the plants and animals of the world are found in the complex mosaic of natural communities that make up this biome.

Tropical Rainforest Biome

Extreme Niche Specialization Ecological relationships within tropical forests are extremely complex and often quite specialized. Structural Complexity At best, a temperate forest has four definable levels of vegetation development - the canopy, a weakly developed sub-canopy, a shrub layer, and a ground layer a typical tropical rainforest will demonstrate many layers, including species that emerge above the main canopy, a primary canopy layer, one or more well-developed sub-canopy layers, and poorly developed shrub and ground-layer vegetation, primarily as a result of reduced light levels below the many canopy levels.

• there are more different kinds of organisms in this region than in any other biome • there are fewer individuals of any one species per unit area than in other biomes • the rainforest canopy becomes essentially continuous in some places so that the forest floor receives little light; hence undergrowth is limited • where the canopy is broken, the forest is much more complex & trees of various heights present a layered or storied picture • most plants are evergreen & may exhibit ‘drip tips’ on the ends of their leaves, apparently an adaptive feature to speed runoff of rain water