Patterns in space and time

Question 1

Community ecology

Answer 1

Community ecology is the study of the community level or organisation, rather than a spatially or temporally defined unit
Areas of land and volumes of water contain assemblages of different species, in different proportions and doing different things
These communities have properties that are the sum of the properties of the individual denizens, plus their interactions. The interactions make the community more than the sum of its parts – synergistic emergent properties
Coexisting individuals of a single species possess characteristics 9e.g density, sex, ratio, birth and death rates, immigration and emigration) that are unique to populations
Activities at the population level have consequences for the next level up (community)
Community ecology seeks to understand the manner in which groupings of species are distributed and the way these groupings are influenced by the abiotic environment and species interactions
Science at the community level poses daunting problems – large and complex data base, search for patterns in the collective emergent properties of the community, repeated consistencies or groupings of similar growth forms in different places, repeated trends along different environmental gradients, scales and hierarchies
A community can be defined at any scale within a hierarchy of habitats, broad patterns on a global scale - e.g temperate forest biomes

Question 2

Community description

Answer 2

Species richness – total number of species
Requires skilled taxonomists, only a subsample is counted, number of species then depends on number of samples/volume of habitat sampled, most common species represented first, increasing sample size adds rarer species to the list
Once a plateau of species richness has been reached, an appropriate number of subsamples to represent the community have been taken

Question 3

Diversity indices

Answer 3

Diversity consists of species richness and abundance or equitability
In closely defined communities counts of individuals of each species may suffice
Interested in all animals in a woodland, their disparity in size means simple counts would be misleading, biomass per species per unit area
Simpsons index – first calculate Pi, the abundance of a given species divided by the total abundance of species in an area
1. Probability that 2 randomly selected individuals belong to the same species, value between 0 and 1, higher D = lower density
s
D = Σ Pi² where S = total number of species in a community, and Pi = the proportion of the ith species
2. Probability that 2 randomly selected individuals belong to the same species, value between 0 and 1, higher value = higher density
1 – D
3. Simpsons reciprocal index – value starts at 1, higher value = higher diversity, maximum value is the number of species in the sample
1 / D
Shannon-Weiner diversity index – similar to Simpsons as it uses species richness and abundance, measures uncertainty in predicting the species identity of an individual that is taken at random
s
H’ = - Σ PilnPi. where S = total number of species in a community, and Pi = the proportion of the ith species

Question 4

Rank abundance diagrams

Answer 4

Provide a more complete picture of species abundance and distribution within a community, each species is ranked (1 being most common), proportion of each individuals plotted against rank

Question 5

Community patterns in space

Answer 5

Communities do not always have sharp boundaries, rather they occur along gradients (of conditions or resources), altitude, moisture, salinity, light
Classification – assumes communities consist of discrete entities, groups of communities that are similar, e.g cluster analysis diagram
Ordination – allows communities to be organised on a graph so that those most similar in species composition and abundance appear closest together, those that differ considerably are placed further apart, can plot samples (sites) or species, e.g Detrended Correspondence Analysis (DECORANA)
Problems of boundaries in community ecology – often blurred, ecotones, transition zones, aquatic and terrestrial boundaries appear sharpened but frogs and otters frequently cross, many aquatic insects spend their larval lives in the water and their adults lives in the air/on land, on land, sharp boundaries between vegetation types on acidic and basic rock types or where serpentine rocks are juxtaposed, still minerals leach across and boundaries become blurred
Community conceived as a superorganism – members tightly bound together, now and in evolutionary history, individuals, populations and communities bore a relationship to each other resembling that between cells, tissues and organisms
In contrast, individualistic concept – relationships of coexisting species are simply the results of similarities in the requirements and tolerances partly due to chance, current view is close to the individualistic concept
The relative abundance of species varies in space and time
A species will only occur where and when:
- It is capable of reaching a location
- Appropriate conditions and resources occur
- Competitors, predators and parasites do not preclude it
A temporal sequence in appearance and disappearance of species requires conditions, resources and enemies to vary with time
Community composition may shift against a background of seasonal change
Sometimes external actors drive community change e.g siltation on a salt marsh leads to its replacement by a forest
Sometimes temporal patterns are a reflections of changes in key resources e.g the sequences of organisms associated with faeces or dead bodies
Disturbance is a relatively discrete event that removes organisms or disrupts the community by
- Influencing the availability of space or food resources
- Changing the physical environment
Common in natural environments
- Forests; wind, lightening, elephants, lumberjacks, age of tree
- Grassland; frost, burrowing animals, grazers
- Coral reefs/rocky shores; tidal waves, hurricanes, battering by logs or moored boats, careless fins of scuba divers
Two kinds of community responses to disturbance according to the type of competitive relationships exhibited by the component species
Founder controlled – species equally able to colonise an opening left by a disturbance
Dominance controlled – some species are competitively superior so that an initial coloniser of an opening cannot maintain its presence
Early species are good colonisers and fast growers
Later species can tolerate lower resource levels and grow to maturity in the presence of early species, eventually outcompeting them

Question 6

Ecological succession

Answer 6

Ecological succession – the non-seasonal, directional and continuous pattern of colonisation and extinction on a site by species populations
Primary succession occurs on exposed landforms uninfluenced by a previous community e.g lava lows and sand dunes
Secondary succession occurs when vegetation of an area is removed, but soil and seeds/spores remain and a sequence of species regenerates e.g loss of trees of abandonment of old farmland
Primary successions can take hundreds of years, but on recently denuded rock surfaces in the marine subtidal zone, succession can take place in a decase
Temporal succession is often represented by a community gradient in space
Caution is advised – in northern temperate areas vegetation may still be undergoing recolonisation and responding to climatic variation following the last ice age
Plants often provide most of the biomass and physical structure of communities, not just primary producers, decompose slowly, accumulating as leaf litter or peat, trees dominate numerous communities because they accumulate dead material, upon which they hold leaf canopies above other vegetation
Coral reef growth and structure drive successions through the accumulation of the calcified skeletons, provide vacant areas for occupation, determine reef communities – high biodiversity
Animals can passively follow vegetation successions
Some animals can control successionary processes

Question 7

Primary succession on coastal sand dunes

Answer 7

1. Plants like sea splurge that are adapted to harsh conditions next to the sea (salt, few minerals)
2. Marram grass stabilises the soil
3. Mosses and lichens establish. Ridges have slacks where rushes and wetland plants grow
4. Shrubs grow in dune scrub, small mammals live here
5. Woodlands grow, climax community