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What are the aims of the survey?
Each survey is unique and the aims should determine the survey methodology. The aims influence many components of the survey, including
- sampling units and sampling intensity
- taxonomic priorities
- timing of sampling
- taxonomic resolution
- appropriateness of scale
- which site attributes are surveyed
- monitoring change over time
Sampling units and sampling intensity
A range of sampling techniques will be described in following topics.
The choice of techniques and sampling intensity will be determined by
the aims of the survey.
For example, if you were surveying an area to determine if vegetation had re-established following disturbance, then you may simply determine the presence or absence of vegetation on the rehabilitated site. However, if you were surveying the area to determine whether ecosystem restoration was occurring, you would undertake a floristic survey of that site, determine the age-structure and reproductive status of those species and a survey of the fauna to determine whether a functional community was re-establishing.
Taxonomic priorities
The aims should be clearly stated so that the survey design can focus
on the species of most interest. Sometimes this choice will be obvious,
such as autoecology studies. Commonly, however, the researcher will have
to make decisions about the taxonomic groups that should be surveyed to
fulfil the stated aims. For example, imagine being requested to design
a survey to measure the effects of an introduced weed at a site. You would
have to prioritise the groups of biota that you wish to sample (e.g. plants,
birds, mammals, invertebrates), which species within these groups should
be sampled, and whether other factors should be measured (e.g. soil nutrients,
pH etc.).
This choice may be assisted by gathering knowledge on the ecological function of the ecosystem under investigation. In Kakadu, a project investigating the effects of Para Grass (an introduced pasture grass) on the floodplain biota is being undertaken by staff atCharles Darwin University. At the outset, researchers had to prioritise the taxa for survey. Previous investigations into Para Grass have suggested that it displaces many native plant species which are food resources for Magpie Geese. Therefore, the survey focussed on the changes in plant species, and the abundance and distribution of Magpie Geese on the floodplain.
In some cases, an indicator species may be identified that represents the status or composition of a community. A common species that is particularly vulnerable to some form of landuse activity could be chosen as the 'indicator' species in a broadscale survey aimed to determine the effect of that activity. For example, butterflies, ants or frogs have been advocated as indicators of environmental health in various settings.
In the past, faunal surveys have often been limited to descriptions of the vertebrates and vegetation. Attention is increasingly being given to the recognition of invertebrates in biological surveys. They are the major component of the fauna, and play a fundamental role in ecosystem function. Although special expertise is required for their identification, the aims of a survey may not be satisfied without an assessment of the invertebrate fauna.
Timing of Sampling
The timing of sampling is also greatly influenced by the aims of the
survey. This is particularly relevant for surveys conducted in seasonal
environments such as the wet-dry tropics.
If the aim of a study is to determine the bird species in a tropical savanna, then surveys may need to be done during both the wet and dry seasons to ensure that seasonal migrants, such as Black Kites and Dollarbirds, are detected.
Similarly, if the aim of a survey is to measure biodiversity, this may be underestimated if surveys are not undertaken in both the wet and the dry seasons.
Taxonomic resolution
If the aims of the study are to determine structural differences between
sites or habitats, then taxonomic identifications may be unnecessary.
Measurements of total biomass or cover of vegetation (regardless of taxonomic
composition) may be all that are required. On the other hand, if the survey
aims to detect more subtle differences it may be necessary to identify
flora or fauna to the level of species. In the latter case, the cost of
the survey may be considerably higher due to the extra time that may be
required for higher taxonomic resolution.
Appropriateness of scale
It is crucial that the survey is conducted at an appropriate scale to
achieve the aims of the study. If insufficient attention is paid to choosing
the appropriate scale then the survey may provide misleading or useless
information with respect to the goals of the survey. This refers to both
the scale of individual sampling units (quadrats, transects etc) and to
the spatial arrangement of sites in the study region.
For example, a survey of the biota in areas of different types of floodplain grasses was conducted using patches of grass habitats of up to 1 ha. While this scale was appropriate for the vegetation and relatively sedentary organisms such as aquatic invertebrates, it proved to be too small for birds such as Magpie Geese, which range and forage at a much larger spatial scale. Larger-scale survey techniques such as aerial surveys would probably be more appropriate to survey Magpie Geese.
These cautions apply equally to both the temporal and spatial scale of the survey.
Other attributes of the site
Collecting information on site attributes such as soil properties and
geology, increases the information base for that site. However collecting
and analysing this information increases the cost of the survey.
For example, the Australian Soil and Land Survey Handbook lists 150 soil attributes. A strategic decision must be made about which of these attributes will assist in meeting the study's aims. Similarly, a range of vegetation attributes can be collected during a survey. A decision must be made about the level of accuracy required. In another example, measuring canopy cover using a camera with a hemispherical lens is time-consuming, yet very accurate and repeatable. By contrast, an estimate of canopy cover using a densiometer is far less time consuming, but also less accurate.
The attributes that are collected should be determined by the survey aims.
Monitoring change over time
This can include changes between seasons, years, or over much longer
periods of time. Monitoring change over time is more complicated than
a 'once-off' survey. There are additional things to consider in the sample
design. These depend on
- the hypotheses being tested
- the nature of the variables being measured
- sampling techniques
- time-frame of the study.
For example, you may need to choose between using fixed sample plots, or randomly sampling new plots during each subsequent visit. The statistical analyses to be used and sampling technique will influence this decision, e.g. destructive sampling will necessitate a fresh plot to be sampled on subsequent visits.
A decision must be made regarding the appropriate time period between samples and how much time is required to detect change. Also, you may need to take into consideration inter-annual variability in environmental variables (e.g. rainfall).
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Please Read |
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3.1 What would you do if you were the reserve manager and... (a) Your management objective was to maintain stable or increasing populations of Ptychosperma bleeseri on two reserves? Given the following points, what survey could be established to monitor this, and what difficulties might you face?
(b) Your management objective was to maintain stable or increasing cover (% cover) of Wild Rice in three populations on two reserves.
(c) Your management objective was to estimate tree density in a savanna before and after a fire?
Please send your responses to your lecturer, who will give you
feedback to see if you are on the right track. Your responses and
feedback will then form the basis of your next online tutorial. |
