Survivorship Curves, Food webs, and Interactions

Community Continuum

- As with many things in biology, communitities are more of a continuum than a discreet entity (migrating birds will move between and through communities over time), and one community can be continuous with another (the graduation of forest into grassland)

Energy

- Energy enters ecosystems as light and leaves primarily as heat - plants convert light energy into matter, and maintain or grow themselves, and other organisms consume this mater and metabolize it to generate energy - trophic levels are defined according to the source of energy for the organism

Use of food webs

- Food webs are simplified versions of community interactions, but they can stillbe used to help build models of interactions and predict effects of changes Ex: they can help predict effects of removing predators or increasing the number of herbivores. Such efforts are increasingly important for conservation and with problems like invasive species

The challenge in making generalizations about communities

- No two communities will be exactly alike even if we have the same number of organisms of each species of the same size/age in the same position - It is a challenge therefore to be able to generalise to a point in order to make useful comparisons/understand systems, without being so sweeping as to eliminate the subtlety. - We wish to capture the relationship between large numbers of species to see the outcome in system behaviour

Abstract vs. Concrete Community

- The concrete form of a community refers to it at a given point, or components of it that exist. - Abstract community: more generalized version Concrete: a specific individual tree, or the community as it is now Abstract: The fact that the community has contained trees before and will in the future is more abstract, as is the existence of this community at different times.

Chain and Food Webs

- These are typically short (few chains have more than 3 to 4 links) or the system would run out of energy to support higher trophic levels - However any one species may potentially feed on dozens of different food items, so food chains may be combined to form food webs

Definition: Communities as groups of species

- This is a useful definition only in that it is relatively simple to define - it ignores the components of space - a community should represent a location (an time) as much as it does the organisms within it

Levels of complexity

- We can look at ecology of organisms at many different levels - At a large scale we can look at global systems (currents) or entire ecosystems (deserts), down to a single species or the population level of a species, or even individual organisms. - One common intermediate is the ecological community

Community Structure

A community may show organization (structure) in various ways: -Species composition - Temporal -Trophic

Interactions: Mutualism

An interaction where both organisms benefit Ex: pollination. The honey bird guides the honey badger (ratel) to a bees nest.

Energy Transfer

As organisms consume on another, energy is transfer between them - Some energy is lost to movement, metabolism, growth, and some is not digested - As a result, at each trophic level, there is less energy available for the one above, hence there are a lot of plants, and few carnivores in a given ecosystem or community.

Ecological Pyramids

As with trophic pyramids, the numbers and interactions can be represented in a variety of forms. Most typically these represent number of individuals, energy, or biomass

Food Chains

As with webs individual links can be determined by amount of energy, strength of interactions etc.

Definition of Community

Between the population/species level, and the ecosystem level - A community may be described as a collection of species or a collection of populations, and interactions between these collections over time, and in a given space - This can be made more specific as required (the bird community in this forest for the last two years)

Demographic Transition

Birth/Death rate interact as the population changes

Interactions through time and space

Communitities change over time (succession) and can be long term (millions of years) or daily (intertidal zones) and individuals may live for days or centuries. - Different species occupy landscapes at very different scales through many still interact (krill vs. Whale) - A solution to this issue is to move away from the space/time dimension and instead consider the interaction between organisms.

Interactions: Parasitism

Consumption of part or all of an organism by another - In addition to 'traditional' definitions, one might consider browsing to be parasitism by a herbivore on a plant One is harmed, One benefits

Defining trophic levels

Definitions are not as clear cut as we would like - Carnivorous plants are both primary producers and secondary consumers - Omnivores are primary, secondary, and even tertiary consumers - Detritivores consume all organisms, but only dead parts.

Community Structure: Species Composition

Each community has a characteristic set of species in terms of patterns and/or relative abundances - At any given time a community will have a pool of species represented. The total number of species will indicate the richness. - There may also be dominant species which set the sate of the community (oaks in an oak forest). May be a keystone species.

Community Structure: Temporal

Each community has a different cycles of activity that vary daily and seasonally - Activities may be diurnal (light), nocturnal (night), or crepuscular (during twilight) - Seasonal changes based on sunlight, temperature, rainfall etc.

Community Structure: Trophic

Each community has different patterns of energy transfer

Food webs

Food chains and webs illustrate relationships (what eats what) between species or populations within a community - They demonstrate consumer-resource interactions ex:predation, herbivory, parasitism - Represents energy transfer within the community

Guilds

Food webs can potentially be enormously complicated. Single species or populations may be aggregated into 'guilds' of similar functional types for simplicity - a guild may be based on trophic level (large secondary consumers) or more general biology (shade loving plants) or systematics (passerines - perching birds)

Tracing food webs

How can pathways be indentified and ideally quantified? -observational data of feeding - examination/analysis of waste (bones/carapace, DNA fragments) - Isotope tracking - ecotron

Chain shortening

However, while natural selection may lengthen chains, energy loss and reduced populations means that chains will also be under selection pressure s to shorten - if a super-predator takes out all the prey in the trophic level below it, it will have to move onto the next level below and so on - Selection may therefore potentially increase and decrease chain lengths

Dependent vs. Independent for Mortality

If density increases and mortality increases then it is density dependent.

Survivorship Curves

If we plot out mortality over time we can see the changing effects of and how this influences the population and its structure. This is a survivorship curve. • In particular it is useful to see how things are changing during development or ageing.

What causes rates of death?

Importantly mortality may be linked to population structure and density. • Mortality may be density dependent - the rate of death is linked to the size of the population. The higher the density, the greater the rate of mortality. • This is often a result of intraspecific competition and fecundity will also typically decrease with increasing density (more pressure on the individuals).

Relative Density

May be measured by a number of different factors. For example: • Number of animals seen per day, number of calls herd, number of pellets or latrines found, number trapped per unit effort, amount of bait taken etc. • Direct observations of the targets are not necessarily needed.

Rule of Tens

On average, only 10% of the energy makes it from one trophic level to the next. ex: 1000 calories stored by producers --> 100 for primary consumers --> 10 for secondary consumer --> 1 for teritiary - However the starting values are also low: plants only harvest about 2% of the light energy they receive

Trophic levels

Primary Producer: Plants and other photsynthesising organisms. They are adding energy/matter to the system Primary Consumers: organisms that consumer primary producers (herbivores) Secondary Consumers: Organisms that consume primary consumers (predators)

Scramble Vs. Contest competition

Scramble: Resources are available to all competitors and is not monopolised -- resources are limited & may lead to starvation. Below K there is minimal competition, then it becomes extreme and huge numbers die. Contest: a form of competition where there is a winner and a loser and where resources can be attained completely or not at all. Competition increases steadily with increasing population. Losers don't necessarily die (e.g. may not get a breeding site).

Energetics

Since there is a finite amount of energy available in an ecosystem and energy limits the lengths of food chains, it seems reasonable that energetics may be strongly linked to diversity - There should be only so many ways the available energy can be partitioned into niches and viable populations species - The mere presence of a plant should provide an opportunity for a herbivore and thence a predator etc. immediately lengthening food chains.

Keystone species/Dominant Species

Species in a community that other (or even the community as a whole) depends on. Their loss will lead to significant losses of others. - They can be massively influential (A large tree can provide leaves and seeds as food for animals, be used by parasites, and by other organisms for shelter, and the waste produced is used by decomposers. The size and scale of the tree can form microhabitats for other species.) - Entire populations of many species may be reliant on a single organism in this way

Interactions: Predation

The complete consumption of one organism by another. - Some forms of herbivory fall under this definition (when the whole plant is consumed) One is harmed, One benefits

Indicator species

These are species which can provide information about a community and can be used as a proxy or marker for the likely condition of the community ex: trout require high oxygen content of clear water. thus their presence gives a clear indication of the environment

Definition: Community as a collection of populations

This definition often applies better to animals than plants. Since the former can move, they are less constrained by the physical environment and tend to interact more. Thus plant communities tend to be determined by the environment and animals by the flora. - Plant community data typically looks at number of individuals within species whereas animal data focuses on the presence of species.

How do we represent energy transfer?

Transfer of energy or the trophic levels of a system can be represented in a number of ways - pyramids of numbers -pyramids of biomass - food chains or webs

Survivorship Curve Types

Type I: are characterized by high age-specific survival probability in early and middle life, followed by a rapid decline in survival in later life. They are typical of species that produce few offspring but care for them well, including humans and many other large mammals. Type II: curves are an intermediate between Types I and III, where roughly constant mortality rate/survival probability is experienced regardless of age. Some birds and some lizards follow this pattern. Type III curves: the greatest mortality (lowest age-specific survival) is experienced early in life, with relatively low rates of death (high probability of survival) for those surviving this bottleneck. This type of curve is characteristic of species that produce a large number of offspring (see r/K selection theory). This includes most marine invertebrates. For example, oysters produce millions of eggs, but most larvae die from predation or other causes; those that survive long enough to produce a hard shell live relatively long.

Compartmentalizing

Webs can be compartmentalized into groups where interactions are strong, but between interactions are week ex: grass-zebra-lion are linked to but largely seperate from acacia-caterpillar-finch - Larege webs may be separated by this interaction occuring periodically (scares food leads to prey shift). This may increase resistance - effects to one compartment won't likely disrupt the rest of the web

Intraspecific competition in agriculture

• Agriculture typically places together huge numbers of individuals of the same species that are in direct competition for limited resources. • Normally nutrients are cycled within systems, but in agriculture we remove the nutrients when we take out the crop. Plants and seeds will not now die in place and fertilise the soil for the next generation - they are gone • This removal introduces a temporal element to intraspecific competition. • Different generations will compete with each other heavily unless the nutrients are replaced - fertilisers. • The soil will become exhausted if nutrients are not returned. • Crop rotation helps this with different levels of use by different plants, but may introduce interspecific competition.

Birth vs. Death

• Although there may be consistent mortality, organisms will also be reproducing and increasing the population. • How do these two intersect?

Absolute Density

• An exact count of number of individuals in area. Absolute counts are very difficult, and usually estimates are required. • Typically done via quadrat surveys, or markrecapture programs.

Competition

• Competition describes interactions between organisms over a limited resource (space, food, water, mates etc.). If there is no limit, there will be no competition! • Both sides must suffer if they cannot get the resource in question, or there is not a competition (lions don't compete with leopards for grass). • May lead to competitive exclusion of the other organisms / species.

Dispersion Patterns

• Density measures or estimates do not reveal how the organisms are dispersed in the environment. • They may been evenly distributed (uniform), clumped together or randomly dispersed.

Human Population Increase

• In recent decades, the human population has been growing exponentially. At what point will it terminate? • Only 12% of the land area is even suitable for agriculture - mostly in temperate zones. • But, this assumes no decrease in soil fertility or other issues (climate change, increasing desertification etc.), and we need to retain wild habitats (rainforests etc.). • The limit to human populations might lie away from food and be linked to water

Quality vs Quantity

• Intraspecific competition affects parameters other than pure numbers. • The best possible performance of an organism (ideal conditions, no competition) is likely to be very different to the reality, especially compared to multiple individuals. e.g. A given amount of water etc. may be enough to grow one full-sized tree, or three half-sized ones. Each is doing worse in the latter, but there is more net tree

Methods of estimating population density

• Methods of estimating population density will vary according to the size and mobility of the organisms at hand. Small birds will be much harder to track than say elephants. • Measures may be absolute, or merely relative - attempting to determine which area has a greater density etc.

Life History Strategies

• Organisms typically adopt one of two major reproductive strategies representing different approaches to the problem and will result in different ecological effects: • r selection strategy is having very numerous young, but with little investment per individual. • K selection strategy is having few offspring but investing heavily in each one. • These will lead to very different population structures of organisms and growth rates. - These two types are extremes and not the only options available - there are intermediates, but most can be categorised as one or the other.

Inverse density dependence

• The patterns of fecundity and mortality may be such that while at high densities, population may be controlled, at low densities they may be driven to extinction. • Such patterns are very important for conservation of rare species. Some suffer when not in sufficient numbers e.g. flamingos. • At densities above K, mortality > fecundity. The reverse is true below K. • Below U, M greatly exceeds F leads to extinction.

Theory vs. Reality

• Theoretical extrapolations and mathematical models are of course very important for understanding the world, but the observation or implementation can be very different. • For example the threshold is very hard to determine in real conditions. • Unlikely to be simply those that get resources and those that don't, but very much a continuum. Plus multiple resources at stake, not all based on something as simple as food.

Population Density

• This is of course a population size over a given area. • Estimates of population density are useful as they help us as ecologists quantify populations and environments. • They also allow comparisons between populations / areas. Two populations of organisms may be identical in size, but very differently distributed.

Population Effects

• Those with an r strategy will have a population structure that is heavily biased to juveniles (for at least some periods!), a K strategy will have a population biased to adults. • This will also be influenced by development time - a pair of elephants may only produce a baby every few years, but by taking a decade or more to mature, there will still be more juveniles than adults in the population.

Mark-Recapture

• Used on animals that move or are hard to count accurately. e.g. mice. • Over a period of time, animals are captured, recorded, and then released. • In a second sampling period, more animals are captured and tagged. The proportion of those caught on both occasions, can given an indication of the size of the population.

Interspecific Competition

•Competition between species. • Species may compete directly or indirectly. • Direct competition may occur over a single specific resource e.g. two animals competing over a carcass. • Indirect may occur by e.g. both wolves and bear hunt and eat deer so every one removed by wolves is not available to bears even if they never meet

Limits to Population growth

•Populations cannot grow indefinitely. Naturally organisms want to maximise their reproductive output (i.e. most offspring that themselves reproduce) but this will ultimately be limited. • Overpopulation, diseases, lack of food, predators etc. will limit the population and stop growth, and can even lead to a crash. • This was the fundamental idea of Thomas Malthus and formed a major part of Darwin's thoughts on natural selection.

Quadrats

•Randomly place down quadrats in representative areas. • Perform an absolute count of the population within that area. • Scale up from here known the typical population per unit area within the quadrats. • May also be done with transect lines instead of quadrats

Issues with mark-recapture

•Samples taken must match each other. It's no good doing one in summer and one in winter. • Samples over short periods can assume a lack of immigration / births, but over longer times will cover the whole population. • Animals can be trap shy, or trap happy, potentially confounding the data.

Intraspecific competition

•There are two theoretical extremes of intraspecific competition: scramble competition and contest competition. • Scramble competition envisions all individuals scrambling for the resource with none likely getting enough to sustain them. • Contest competition however has a deciding factor (e.g. a direct fight b/w individuals or a binary resource) as to who gets the resources with winners getting enough.

What causes mortality?

•This can be for a wide range of reasons (e.g. predation, starvation, disease), but we are primarily interested in the ecological effects and influences. • How are these changes linked to life history, and interactions from the environment (e.g. climate) or other species (e.g. predator-prey interactions, diseases)?