INTRODUCTION TO ANIMAL ECOLOGY
CHAPTER 1 – WHAT IS ECOLOGY?
Definitions of ecology
There are different definitions of ecology:
- The first definition was given by Haeckel (1866) = ecology is the comprehensive science of the
relationship of the organism to the environment.
- At a point in time, plant and animal ecology were separated, so each part had its own definition.
- Definition in the book = ecology is the scientific study of the distribution and abundance of
organisms, the interactions that determine them, and the relationships between organisms and the
transformation and flux of energy and matter.
Ecologists try to explain, understand, describe and predict; the explanations they give can be:
- Proximate an explanation that tells what is happening “here and now”;
- Ultimate explanation that tells how does an organism developed the characteristics that it has.
(it focuses more on the evolution)
Three general points to consider:
1. Ecological phenomena occur at different scales: time scales, space scales and biological scales
2. Ecological evidence comes from different sources
3. Ecology relies on scientific evidence.
1. Ecological phenomena occur at different scales: time scales, space scales and biological scales.
Time scales = ecologists study phenomena over different time periods (days, months, years)
- Ecological succession it is the successive and continuous colonisation of an area by a certain
species population, accompanied by the extinction of another species in that area. This
phenomenon can be studied over a certain period (time scale).
- The amount of time/time scale needed to study a phenomenon depends on what question you are
investigating. However, many ecological studies last less time than what is most appropriate
because long studies are expensive.
Spatial scales = ecologists study phenomena at different space levels
Biological scales = ecologists study levels from individual organisms, to populations, to communities, to
ecosystems and global biosphere:
- Organism = individual
o Organism ecology = it studies how individuals are affected by their environment and what
are their physiological and behavioural responses to the environment
- Populations = groups of individuals of the same species in a certain location
o Population ecology = it studies how the number of individuals of a certain species change
in a certain time and place
- Communities = all populations of (different) species present in a certain location
o Community ecology = it studies what controls the diversity of species in a certain area
- Ecosystems = include the community of organisms and the physical environment in which they live.
, o Ecosystem ecology = it studies the functioning of entire ecosystems (lakes, forests,
wetlands) or other portions of the planet in terms of energy and material inputs and
outputs.
- Biosphere = the totality of all life interacting with the physical environment at global scale.
From this, three generalities emerge:
- The characteristics observed at a certain level are the result of what occurs at lower levels
- In order to understand the causes of a certain characteristics at a certain level, a scientist needs to
look at the next lower level of organisation (e.g. to understand the cause of a certain phenomenon
in a population, you have to look at what happens at organism level)
- Characteristics observed at a certain level of organisation can be predicted without completely
understanding the functioning at lower levels.
2. Ecological evidence comes from different sources
Ecologists use different methods to make their studies:
- Observations they observe what changes occur in the organism/population/community etc, and
often compare these changes with different areas (e.g. they observe how the abundance of a
species changes over time and space)
o Comparative field observations used to compare data from different areas
- Experiments can be done in the lab or in the field
o Manipulative field experiments conducted to test hypothesis about a phenomenon;
they have an advantage compared to comparative field observations because confounding
variables are maintained constant during the same experiment, so they are eliminated.
o Experiments in the lab are often the best way to provide an answer to a question
- Mathematical models they are used to predict or simulate scenarios or situations about
ecological interactions, function and structure. Models are useful in case we do not have real data,
but we can use it to make predictions.
3. Ecology is based on scientific evidence
Ecology relies on obtaining estimates from representative samples. Thanks to the use of statistics,
ecologists can attach a level of confidence to their conclusions (a level of confidence/confidence interval
tells how much your results are similar to reality). Many ecological studies replicate the measurements of
interest many times, to increase the probability to have a result that is significant and realistic. However,
replication can be expensive and time consuming.
1.3 Ecology in practice
An introduction of an exotic fish species to New Zealand – investigation on multiple biotic scales:
A study was conducted on the introduction of an exotic fish to streams in New Zealand: the brown trout. It
was introduced in New Zealand in 1867 and formed populations in many streams and rivers, often at the
expense of (alle spese di) native fish species of the genus Galaxias. Microalgae growing on rocks are at the
basis of the food chain in most New Zealand streams. Mayfly nymphs and other invertebrates eat them,
and are in turn eaten by fish. The behaviour of nymphs differs depending on if they are in streams with
Galaxia or with brown trout. In one experiment, nymphs collected from a trout stream were more active
,during the night, while those collected from a Galaxias stream were active both day and night. Trouts use
mainly vision to catch prey, while Galaxias use mechanical cues. This means that nymphs in a trout stream
are more at risk of predation during daylight, when the trout can see better and is most active.
Another experiment tested if brown trouts can affect the stream food web differently from the Galaxias.
Three treatments were made: no fish, only Galaxias present and only trout present. Algae and
invertebrates were allowed to colonise the artificial channels of the test for 12 days before introducing fish.
After other 12 days, algae and invertebrates were sampled; results showed that the trout reduced
invertebrate biomass significantly, but the Galaxias had no effect on it. In the trout treatment, there were
also more algae probably because of the less invertebrates present that could eat them. Researchers then
investigated what are the consequences for the ecosystem in terms of photosynthesis and energy flow in
the food chain photosynthesis by algae was 6 times higher in the trout stream; this resulted in more
energy to support the entire food chain, and the invertebrates that eat algae produced new biomass in the
trout stream 1.5 time faster than in the Galaxias stream. Consequently, there was more food for trout so
also trouts produced new biomass 9 times faster than Galaxias. In conclusion, streams with trouts are
more productive.
If the goal is to benefit fishers, having trouts in streams is better because there is higher production. Of
course, invasive species like the trout are an increasing problem and often have a negative influence on
both ecological function and other issues related to human well-being.
Why Asian vultures were heading for extinction:
In 1997, vultures in India and Pakistan had dramatic declines in numbers of G. bengalensis and G. indicus.
This was a big concern because vultures have an important role in removing dead bodies of large animals.
Without vultures, wild dogs and rats have more carcasses to eat so their populations grow and also the
chance they spread diseases. In addition, if dead animals are not quickly removed by vultures, there is more
risk of contamination of water and spread of disease by flies. Researchers found that dead vultures suffered
from visceral gout (accumulation of uric acid in the body) followed by kidney failure. The vultures dying
from this condition contained residues of the drug diclofenac. They were contaminated after eating
carcasses of domestic animals treated with diclofenac.
Researchers built a mathematical model to predict the behaviour of vulture populations. The model
specified birth and death rates of the population, and how diclofenac can affect survival based on the
probability that an adult vulture would eat from a contaminated carcass. The researchers posed this
question what proportion of carcasses (C) would have to contain lethal doses of diclofenac to cause
declines in vulture populations? the answer found with the model was 1 contaminated carcass in 135
carcasses was enough. Consequently, the government have taken action to ban the use of diclofenac. This
is a good example of how mathematical models are useful in exploring situations for which we have little or
no real data.
CHAPTER 2 - EVOLUTIONARY ECOLOGY
2.1 Evolution by natural selection
, Darwin and Wallace à they agree on the fact that natural diversity is a result of evolution in which natural
selection favoured some variants in the species through a “struggle for existence”. Darwin realised that
every species must suffer destruction during some period of its life, while in other periods the numbers of
that species will start to grow exponentially.
The theory of evolution by natural selection is based on a series of truths:
1. Individuals that form a population of a species are not identical
2. Some of the variation (genetic variation, difference) between individuals is heritable, so it can be
passed on to descendants
3. All populations could grow at a very high rate, but in fact most individuals die before being able to
reproduce, and most reproduce less than their maximum potential.
4. Different individuals produce different numbers of descendants , so they do not all contribute
equally to the next generations. Those that contribute the most have the greatest influence on the
heritable characteristics of the next generations.
The individuals that determine the direction of evolution (because they produced more offspring), were
best able to survive the risks and hazards of their environment.
Evolution = change in the heritable characteristics of a population or species, over time. This change is
inevitable. Evolution could be divided into microevolution processes, which refer to the small changes that
occur from one generation to the next, and macroevolution, which refers to changes on the long term.
Evolutionary mechanisms:
- Genetic drift à mechanism in which allele frequencies of a population change over generations by
chance.
- Migration migration of individuals into a new population can change the allele frequencies in
that population, e.g. by introducing new alleles.
- Mutation of genes
- Selection occurs when the survival or mating success of animals is different because they have
different genotype (they have different fitness)
- Non-random mating when individuals choose the genotype or phenotype of their mate based
on their own phenotype. With positive assortment, individuals choose a mate that is similar to
themselves. In negative assortment, they choose a mate that is different from themselves.
2.2 Evolution within species
Geographical variation within species:
Since a species can experience different types of environments/habitats, we can expect that natural
selection favoured different variants of a species in different environments. This would create differences
between populations of the same species. (However, this is not always the case.)
These are some mechanisms related to evolution within species:
- Counter gradient interaction it occurs when genetic differences in a species counteract/oppose
environmental effects. For example, when common frogs living at different latitudes were reared in
a common environment, it was observed that frogs coming from higher latitudes (colder
environments) developed faster. This phenomenon was not observable in nature, but only in lab.
- Hybridisation production of offspring from parents of different variety or breed
- Reciprocal transplant experiments used to test if organisms have evolved to become specialised
to live in their local environment. This is done by comparing their performance when they are kept
