Download Life Under Your Feet: Measuring Soil Invertebrate Diversity

TIEE
Environmental Biology
BIO 212
Teaching Issues and Experiments in Ecology - Volume 3, April 2005
Life Under Your Feet: Measuring Soil Invertebrate Diversity
What Happens
In this lab, you will measure the diversity of soil invertebrates that are collected from two
different forest stands: 1) a broad-leaved deciduous forest and 2) a stand of pine and spruce.
Our Question
1. How does different vegetation affect soil invertebrate diversity?
Last week you collected leaf/humus samples from the forest floor of two different forest stands.
A) Mixed Hardwoods, B) Norway Spruce. Upon returning to campus, my lab assistant put each
of the samples in a Berlese-Tullgren funnel. Berlese funnels are used to extract invertebrates
from soil and leaf litter (detritus). They work on the principle that insects and other invertebrates
that normally live in soil and detritus will respond negatively to light and dry conditions.
Therefore, a light source is used to force the invertebrates to move downward, where they will
fall into a funnel and then into a container of ethanol (for preservation).
Adapted from - TIEE, Volume 3 © 2005 - Richard L. Boyce and the Ecological Society of America.
Teaching Issues and Experiments in Ecology (TIEE) is a project of the Education and Human Resources
Committee of the Ecological Society of America (http://tiee.ecoed.net).
page 2
Environmental Biology
BIO 212
Lab Objectives
At the conclusion of the lab, you will
1. Learn about the incredibly diverse invertebrate community found in soil (literally under
their feet).
2. Learn how to quantify diversity of different soil invertebrate communities. This will
prepare you to quantify the diversity of 3 forest stands that you will visit over the next 3
weeks.
3.
Generate hypotheses about underlying ecological processes that consider the factors that
affect soil invertebrate diversity in different environments.
Background
Diversity and the measurement of diversity are central to many issues in ecological research as
well as for applying ecology to real world problems. Every textbook in ecology devotes
considerable description and explanation of species diversity, species richness, and species
evenness. Community ecologists use measures of diversity to study and explain ecological
patterns in many different types of communities.
In terrestrial ecosystems, litter decomposition has important effects on processes such as nutrient
cycling and community structure. Decomposition is affected by the type and quality of litter,
climate, the edaphic conditions (including soil temperature, hydration, and chemistry), and the
community of decomposer organisms (Swift et al. 1979).
Figure - Interactions among factors that control litter
decomposition (from Swift et al. 1979).
This model shows the relationships among the three factors that govern litter decomposition
rates: the Biota (structure and activity of the biotic soil food webs, i.e., microbes, invertebrates,
vertebrates), the Physico-chemical environment (climate, habitat, edaphic factors, i.e.,
contributions from the non-living environment); and Resource Attributes (primarily plant
species diversity and tissue chemistry, i.e., contributions from the living environment). Many
studies have shown how both the living and the non-living environments affect soil community
structure and diversity (Swift et al. 1979, Elliott et al. 1980, Ingham et al. 1982, Freckman &
Virginia 1997).
TIEE: EXPERIMENTS
Life Under Your Feet: Soil Invertebrate Diversity
page 3
Each of these 3 factors influences the others.
For example, decomposition of plant litter that is high in lignin and/or low in nutrients and is
therefore difficult to decompose (resource quality) leads to dominance by fungal-feeding groups
in the soil food web (namely, some taxa of nematodes, mites and Collembola), whereas easily
broken-down litter is decomposed primarily by bacteria, which is reflected higher up the food
chain (Coleman & Crossley 1996). And soil community diversity is at least partially determined
by plant community diversity (Siemann et al. 1998). So in this case, the living environment is
determining the soil community.
On the other hand, recent work suggests that composition and biodiversity of soil organisms
itself may have a greater effect on decomposition than has been previously recognized (González
and Seastedt 2001, Wardle and Lavelle 1997, Wardle et al., 2003), especially in tropical
ecosystems. So in this case, the soil biota is the driving force of the Physico-chemical
environment and therefore the Resource Attributes in the Swift et al. (1979) model above.
On yet a third hand, the soil Biota can directly affect the Resource Attributes. De Deyn et al.
(2003) showed that soil fauna enhanced succession and diversity in a grassland community.
Soil invertebrates play important roles in soil communities. Some directly consume detritus,
others consume detritivores, whereas others are higher-level carnivores that can indirectly
control decomposition by their effects on lower levels of the food web (see Soil Food Web figure
on the next page).
Soil invertebrates are clearly affecting litter decomposition rates, soil aeration, nutrient
mineralization, primary production, and other ecosystem services related to soil ecosystem
function and agroecological conservation (e.g., Six et al. 2002). With interest in global climate
change has come the realization that soil biota may strongly affect soil CO2 sequestration and
release, which is a critical variable in climate change models. Agroscientists and restoration
ecologists have found that soil biota play critical roles in toxic chemical and metal mobility and
remediation; they directly affect disturbed ecosystem recovery/ ecological restorations that occur
after fire, UV-B exposure, post-urbanization, and herbicide-stressed soils (e.g., Lal 2002).
Bioprospectors carry out the search for novel antibiotics and other drugs among the billions of soil
microorganisms. Soil invertebrates are also recognized for their role in mediating or determining
belowground interactions among plants. Because they are often prey for vertebrates such as birds
and mammals, they have vital roles in the food chains that include those animals.
Environmental Biology
Relationships between soil food web, plants, organic matter, and birds and mammals. Image courtesy of USDA
Natural Resources Conservation Service, http://soils.usda.gov/sqi/soil_quality/soil_biology/soil_food_web.html
page 4
BIO 212
TIEE: EXPERIMENTS
Life Under Your Feet: Soil Invertebrate Diversity
page 5
What you will do in lab on Wednesday
1) Brief presentation on how to use a microscope.
2) Brief presentation on species richness, evenness and diversity.
3) Sort through collection vial.
a. Sort organisms by morphotype and count the number of them.
b. Try to identify each morphotype
c. Your instructor will try to determine if your organisms are either detritivores or
predators.
4) Calculate a) Species Richness and b) Species Diversity
Species Richness and Species Diversity. Species richness is the total number of different
species in a sample/community. Each species, however, is not likely to have the same number of
individuals in that community. One species might be represented by 1000 indivudals, and
another by 200, and a third by a single individual. The distribution of individuals among species
is called species evenness, or species equitability. Evenness is maximized when all species have
the same number of individuals. Species diversity is a combination of richness and evenness; it
is species richness weighted by species evenness, and there are formulae that permit the diversity
of a community to be expressed in a single number.
Diversity calculated using the Shannon-Wiener index (H´), one of the most popular. The
Shannon-Wiener index measures both richness (the number of species) and evenness, or how
evenly individuals are distributed among species. High values of H´ denote high biodiversity.
Shannon’s index is advantageous over simply counting the total number of different species,
because the latter is greatly affected by sampling effort (plot size and total number of individuals
sampled). The greater the sample, the more rare species you find. H´ is superior because it is
calculated from proportions, as you will see, and rare species contribute very little. Therefore,
this index is relatively insensitive to the random inclusion or omission of rare species that
happens with any sampling effort. The equation for Shannon’s index is:
S
H ′ = −∑ ( pi )(log pi
)
i =1
or
H’ = -[p1 log(p1) + p2 log(p2) + p3 log(p3) + p5 log(p4) + p5 log(p5)]
where the pi’s are the proportion of all observations in the ith species category, and S is the total
number of species. Shannon’s index is unitless and has no true biological meaning. Very
common species contribute much more than do rare species to the numerical “diversity”
estimate.
page 6
Environmental Biology
BIO 212
Consider Community A below. Note that although there are a total of 5 species in the sample,
species A accounts for 70% of the observations. In fact H’ for this 5 species sample is only about
2.74 “equivalently common species.” So the diversity of this 5 species community is the same as
that of a community with (not quite) 3 species with all the same abundance. This community has
a fairly low diversity because it is dominated by species A.
In contrast in Community B below, note that all 5 species are equally abundant; consequently H’
equals 5 species. Thus, this is a high diversity community – as high as it can get for a 5 species
community.
Species 1
Species 2
Species 3
Species 4
Species 5
(p1)
(p2)
(p3)
(p4)
(p5)
H’
Community A
70
10
10
5
5
100
Community B
20
20
20
20
20
100
0.4344
0.699
Since we now have numbers that describe each community, it is possible to carry out a statistical
test to compare the Shannon-Wiener indices from two different communities provided that you
have numerous replicates. I might try that with the data from both lab sections – just like I did
with the data from sampling Gill-over-the-Ground.
H’ = [p1 log(p1) + p2 log(p2) + p3 log(p3) + p5 log(p4) + p5 log(p5)]
To calculate H’, you first need to know what p is.
p = proportion (%) that any one species contributes to the total, expressed as a decimal.
For Community A, for species 1, p1 = 70/100 = 0.70
log(p1) = log of 0.7.
So for each species, you need to calculate p x log(p) If you know MS-EXCEL, this is fairly easy
to calculate. I will show you how later this semester for your terrestrial communities lab report.
For today, you will use the following formula:
H’ = [(N log N) - Σ[ni log ni]]/N
Where N = to total number of organisms in the sample
And ni = the number of speciesi
To make your calculations even easier, I’ve also posted a table that has the values for all of the
(N log N) and (ni log nI) combinations up N or ni = 490.
TIEE: EXPERIMENTS
Life Under Your Feet: Soil Invertebrate Diversity
Species 1 (p1)
Species 2 (p2)
Species 3 (p3)
Species 4 (p4)
Species 5 (p5)
Total organisms
H’
page 7
Community A
n1 = 70
n2 = 10
n3 = 10
n4 = 5
n5 = 5
N = 100
0.4344
From the N log N table.
N log N = 100 x log(100) = 200
Species 1 = n1 log(n1) = 70 x log(70) = 129.57
Species 2 = n1 log(n2) = 10.0
Species 3 = n1 log(n3) = 10.0
Species 4 = n1 log(n4) = 3.495
Species 5 = n1 log(n5) = 3.495
H’ = [(200) - Σ[129.57 + 10 + 10 + 3.495 + 3.495]]/100 = 0.4344
Questions for Further Thought and Discussion
1. Do the soil invertebrate communities differ between the two forest types?
2. Do you think leaf litter quality is influencing soil invertebrate communities, or could it be
the soil itself? Consult a soil survey map.
3. According to the indices, which community has greatest taxa diversity? Do the measures of
S, and H´ give different perspectives on total diversity? Why?
4. Which factor is influencing your value of H’ the most, species richness, or species
evenness?
5. Where in the soil food web do most of your invertebrates fall (what is their trophic level –
are they detritivores or predators)? What could this be telling you about the quality of litter,
richness of the site, etc.?
6. What direct effects are your soil invertebrates likely to have on large animal (vertebrates)
food chains? Could the different communities you have found explain some of the
differences?
page 8
Environmental Biology
BIO 212
Number of Observations
Species
Annelida (Earthworm)
Gastropoda (snails)
Isopoda
Coleoptera (ground beetles)
Hymenoptera (ants, wasp)
Collembola (springtails)
Microcoryphia (bristletails)
Tysanura (silverfish)
Proturans
Diplura
Pseudoscorpions
Araneae (spiders)
Diplopoda (millipede)
Chilopoda (centipede)
Chilopoda (centipede)
Diptera (flies)
Total
Site 1 - Coniferous
Site 2 – Deciduous
TIEE: EXPERIMENTS
Life Under Your Feet: Soil Invertebrate Diversity
page 9
page 10
Environmental Biology
BIO 212
TIEE: EXPERIMENTS
Life Under Your Feet: Soil Invertebrate Diversity
page 11