Notesale: Turn your study into money

Document Preview

Extracts from the notes are below, to see the PDF you'll receive please use the links above

---

Factors limiting distributions

Aim: to examine the factors that determine species distributions
Why are organisms of a particular species present in some places and absent in others? – easy way
to answer is to ask what factos limit the distribution or range of a species
...

Abiotic limiting factors
Terrestrial environments




Most important are temp and water – mainly control distribution
Other major factors are light, gases, available nutrients
Not always limiting

Aquatic environments



Most important are light and nutrients
Others are temp, salinity, dissolved oxygen, waves and currents

Terrestrial ecosystems: abiotic limiting factors
Temperature








Water

Major influence on organisms ability to undertake activity
Daily seasonal rhythms control daily and seasonal activity
Can act on any stage of the life cycle, limiting distribution through its effects on survival etc
...
g
...
g
...
(POT = water
demand)
Water circulates – through transpiration/evaporation, rain etc
...

– main losses are through leaching
Nitrogen is different – all organisms require nitrogen, 78% of gas in the atmosphere is
nitrogen, but few organisms can use it in this form – nitrogen fixers (bacteria/algae) live in
soil or on plants are very important – plants will have symbiotic relationship with nitrogen
fixers
Nutrients may be in soils but unavailable to plants due to factors such as pH
Organisms can be limited by too much of a nutrient as well as too little
Calcium is important – plants either like or dislike – so is a very limiting nutrient factor
Calcicole = calcium loving




Calcifuge = dislike calcium
Some plants have particular requirements

Liebigs ‘law’ of the minimum









Under ‘steady state’ conditions the essential material available in amounts most closely
approaching the critical minimum needed will tend to be the limiting one
In terrestrial ecosystems the most likely nutrients to be limiting are nitrogen, potassium and
phosphorus, which is why they are fertilisers most commonly applied to soils
...
They were exploited during the 19th century as a
source of phosphorus
...

Guano Wars: The Latin American countries achieved their independence from Spain in the
war of 1814 to 1826
...


Aquatic ecosystems
In aquatic systems phytoplankton (microscopic, free floating organisms – algae, seaweed) undertake
most of the photosynthesis taking carbon dioxide from the water and releasing oxygen into the
water – forming the beginning of the food chain
The spatial separation of light and nutrients often limits the productivity of aquatic ecosystems
Biggest limit is that the nutrients sink of the bottom (after organisms die etc
...
) occur over a range of values
Within this gradients, species will have an upper and lower range of tolerance where the
species can be present
If the conditions are ideal the abundance will be perfect
Species usually function most efficiently over a limited part of each gradient
...
Plant seeds can lie dormant in the soil for many years (seed bank)

Biotic factors limiting distribution




Intrinsic characteristics such as the dispersal strategies and the behaviour of the organism
(e
...
habitat preferences)
...


Intrinsic characteristics
Dispersal and barriers


Organisms do not necessarily occupy all their potential range and if transported outside
either normal range they may be able to survive, reproduce and spread
...
Strategies – feathery to catch wind, sticky or
hooked seeds to attach to animals, package (in fruit etc
...
Birds, mammals and invertebrates are in some cases capable of active
long-distance dispersal

Barriers to spread






For terrestrial organisms the oceans are the major barrier to spreading, other barriers
include climatic (desert) or topographic (mountains)
The ability of organisms to cross barriers is shown by the occurrence of widely scattered
areas of occuptation
disjunct distributions
the organism may have crossed the barrier due to a chance combination of favourable
circumstances – e
...
high winds or floating rafts of vegetation
the barrier may not always be present or in the same location – e
...
due to continental drift,
climate change, sea-level change (the animals crossed when there was no existing barrier)

Predation




includes carnivores and herbivores - plant predators
there is evidence to suggest that amongst animals the nature and abundance of food is the
most important factors influencing presence and size of populations
e
...
many are territorial
...

This allows members of the group access to a wider range of resources than would be
possible for an individual or pair
...
This particularly applies to insects many of which live only
on one species of plant for part of their life cycle (e
...
caterpillars)
...

Prey evolve mechanisms to avoid predators – e
...
being nocturnal, cryptically coloured,
poisonous
Predators need to keep up to survive, so an evolutionary arms race develops
Predators need to hunt in an efficient manner, balancing their energy against the energy
benefits

Competition




The hardships which arise from the proximity of neighbours
Intraspecific competition – within a species
Interspecific competition – between species

competition occurs over resources:




Plants – light, nutrients, water, plus pollinators
Animals – water, food, space
The less interspecific competition the more species an area can support

Reducing competition
Temporal and spatial separation:




Predators in the same habitat many hunt at different times (owls, hawks)
Ground flora plants in woodland may come out in leaf/flower before the development of a
tree canopy (to avoid competition for light)
Resources of a habitat may be divided between species by the restriction of each species to
part of it
...
These are also
commonly Used as a competitive device, known as allelopathy
...


Parasites




An organism that is dependant on another, at the expense of the host
May live on (ecto) or inside (endo) the host organism
A plant parasite – Toothwort is a root parasite and is often parasitic on hazel

Commensalism




One species benefits, while the other is not affected
...
Insects and browsing mammals should
eat the leaves and branches, slowing the growth of the Acacias and allowing fast-growing,
competing vegetation to out shade them
...
The ants live
inside inflated thorns at the base of leaves and receive additional food (nectar) from the plant

---