Density dependent and independent factors

1. Environmental Ecology Lab
Stream Ecology:
Density Independence Versus Density
Dependence In Streams
Presented By- Mamun Md.
Master’s Degree
Dept.- Bioscience and Biotechnology
Major Field-Biodiversity and Environmental Ecology
Student Id-201650783
Session -2016,Spring

2. Environmental Ecology Lab
Contents:
 Introduction
 Characterization of Stream systems
 Objectives
 Density Independent Mechanism
 Density Dependent Mechanism
 Density-dependent factors
 Density-independent factors
 Implication of density dependence
 Existence of density dependent factors in stream systems
 Experimental evaluation of various density dependent
mechanisms in stream systems
 Support density independent regulation as the major factor
regulating organisms in stream systems by Researchers
 Dominance of Density Independence – Is It valid?

3. Environmental Ecology Lab
Contents:
 Testing Stream Systems for Density Regulating Mechanisms
 Functional group concept
 How density dependence can conduct by using Grazers?
 Several Problems arise during study of density dependent
interactions by using grazers
 Mechanisms of detritus in stream ecosystems
 Stream detrital dynamics model
 Conclusion

4. Environmental Ecology Lab
Introduction:
♣ Descriptive Science
♣ Taxonomy has been a major thrust
♣ Describe basic structure of stream communities
♣ Life histories of some species
♣ Examine functional relationships within stream community
♣ Formalization of the functional group concept, RCC concept
♣ Recognition the importance of detritus and instream autotrophy

5. Environmental Ecology Lab
Characterization of stream systems:
♣ Annual floods
♣ Occasional catastrophic floods
♣ Pulsed periods of detrital input
♣ Seasonal temperature changes
 bed of the stream is subject to both erosion and deposition

6. Environmental Ecology Lab
Objectives:
♣ To know about density independence factors
♣ To gain knowledge about density dependent factors

7. Environmental Ecology Lab
What are the density independent and
density dependent mechanisms?

8. Environmental Ecology Lab
Density independent mechanism:
♣ Catastrophic mechanism which act independently of the density
of the population being affected
♣ Like, a spate or sudden freeze
♣ After passing that event population contain exponential growth.
Density dependent mechanism:
♣ Change in intensity as a function of population density
♣ Like, predation , competition and parasitism.
♣ When the population increases , the influence of density
dependent mechanisms also increases
♣ Reduction in population growth = intrinsic rate of population’s
growth

9. Environmental Ecology Lab
Density-dependent factors:
A factor that depends on population size is called a density-
dependent factor.
Density-dependent factors include:
♣ Competition
♣ Predation
♣ Parasitism
♣ Disease
♣ Territoriality
♣ Waste Accumulation
♣ Migration

10. Environmental Ecology Lab
Density-independent factors:
Density-independent factors affect all populations in similar
ways, regardless of the population size.
Examples of density-independent factors
include:
♣ Unusual Weather
♣ Natural Disasters
♣ Seasonal Cycles
♣ Certain Human Activities—such As Damming Rivers
And Clear-cutting Forests
♣ Fire
♣ Flood Weather
♣ Volcanic Eruption
♣ Chemical Pesticides

11. Environmental Ecology Lab
Implication of density dependence:
♣ Population is controlled by food supplies or floods
♣ Concern reproductive strategies, diversity and the general
applicability of equilibrium based ecological theory
♣ More applied scale – management strategies , impact
assessment and prediction
♣ Assemblages of opportunists rearrange the community
structure( Paine, 1966,1669)

12. Environmental Ecology Lab
Existence of density dependent factors in stream
systems:
♣ Impact of predaceous flatworms in restructuring a stream
community ( Macan,1962)
♣ Factors influencing the distribution of lotic Trichoptera
(Edington, 1965)
♣ Physical factors set as distributional template and territorial
behavior was the ultimate regulating mechanism(Edington, 1965)
♣ Support the competitive exclusion principle(Allan, 1975)
♣ Microhabitat segregation found between two congeneric
harpacticoid copepods (Shiozawa,1978)

13. Environmental Ecology Lab
Experimental evaluation of various density
dependent mechanisms in stream systems:
♣ Manipulation of invertebrate in cages and conclude that
movements were density dependent (Peckarsky,1979)
♣ Competitive interactions among benthic grazers (McAuliffe and
Hart)
♣ Found significant differences in leaf pack decomposition
rates(Oberndorfer et.al)
♣ Perhaps the best known working hypothesis is the excess
production hypothesis(Waters 1961, 1965, 1966)

14. Environmental Ecology Lab
♣ Excess production hypothesis relates behavioral drift to
assimilation and secondary production above the carrying
capacity
♣ Manipulated food level in artificial streams and significantly
found more drift with lower concentrations(Hildebrand, 1974)
♣ No significant change in stream benthos after trout were
excluded from Colorado stream (Allan, this volume)

15. Environmental Ecology Lab
Support density independent regulation as the
major factor regulating organisms in stream
systems by Researchers:
♣ Factors controlling benthic invertebrates in running waters and
90% discussion was density independent
mechanisms(Hynes,1970)
♣ No community of running water may considered as climax and
a climax community implies equilibrium conditions and Violence
of flow is the main regulatory mechanisms(Margalef, 1960)
♣Headwater stream communities as assemblages of
opportunists(Patrick,1972)
♣ Opportunists inhabits in headwater streams of temperate
area(Stout and Vandermeer,1975)

16. Environmental Ecology Lab
Dominance of Density Independence – Is
It valid?

17. Environmental Ecology Lab
Yes , it is valid due to four reasons.
1. Major factor regulating stream community structure
2. High diversity that exists in temperate streams
3. Support highly variable distribution of stream benthos
4. Predominantly a descriptive science

18. Environmental Ecology Lab
1. Major factor regulating stream community
structure:
♣ Effects of three catastrophic floods caused major restructuring
of the stream ecosystems( Hynes, 1972)
♣ Snow melt with ice jam effect downstream community
structure in several kilometers( Shiowzawa)
♣Summer cloud bursts caused catastrophic changes in
community structure
♣Long term dynamics of a single community is nonexistent and
density dependence , like density independence ,need to operate
continually(macArthur,1972; Wiens,1977)

19. Environmental Ecology Lab
2. High diversity that exists in temperate
streams:
♣ Diversity may develop from a patchy environment induced by
density independent factors
♣ Few major contradictions noted by MacArthur when compare
with tropical diversity
♣ Inferior species increase the community diversity
♣ Intermediate disturbances are important factors in controlling
diversity in both tropical coral reefs and tropical forests and
undergoing succession by uncolonized cells or patches
( Connell,1978and 1979)

20. Environmental Ecology Lab
3. Support highly variable distribution of stream
benthos :
♣ Support correlative field relationship depend on physical
factors
♣ Most familiar example , sampled a “ relatively uniform riffle “
and estimate community density with 95% Confidence(Needham
and Usinger, 1956)
♣ Improved the quantitative nature of sampling gear( Waters and
knapp,1961; Mundie ,1971)
♣ Refined statistical methodology(Elliott,1977)
♣Benthic sampling depend on plot type sampling technique ,one
square foot

21. Environmental Ecology Lab
♣ Increasing the area during sampling on heterogeneous
conditions improved the precision of Data
♣ processing of samples in the laboratory can be simplified by
statistics( Cochran,1963;green ,1979 et.al) and equipment
(Waters,1969;Mundie 1971)
♣ Plotless techniques ( Shiozawa,1978) and reduced sampler
area improve resolution of habitat grain (Elliott,1977)

22. Environmental Ecology Lab
4. Predominantly a descriptive science:
♣ Basic approach is sample survey
♣ sample numbers converted to densities or biomass and give a
picture about stream community structure
♣ After advent of computer and statistical packages data seta
gain more validity and interpretations obtain more legitimacy

23. Environmental Ecology Lab
Two basic problems arise with statistical
methodology:
♣ First, As we measure the parameters through our intuition
which is biased by our own limited ability in perceiving all
potential causal factors
♣ Stream ecosystems analyzed by using two components .
I. Abiotic components( often field observed)
II. Biotic components( often laboratory derived)
♣ Second, Sampling survey can not prove causality
♣ Statistics based on sample surveys do not make definitive
conclusions about causality

24. Environmental Ecology Lab
Testing Stream Systems for Density Regulating
Mechanisms:
♣ Formulation of logical hypotheses
♣Mathematical modelling(give precise predictions and
opportunity of formulating biological relationships and observing
“noiseless “ system changes according to these relationships)
How hypotheses can be tested?
♣ Very extensive sampling-type surveys
♣ Experimental manipulation
 Experimental manipulation is probably the best approach for
the stream researchers
 Here causality can be demonstrated

25. Environmental Ecology Lab
How benthic community regulated by the
factors?
♣ Not all members of the benthic community are regulated by
the same factors
♣some are regulated by predation , others by food , space or
density independent factors
♣ The community include organisms of different trophic levels
and different functional groups( Cummins,1974;cummins and
Klug,1979) across these levels
♣In a community the manipulation of one factor may only
influence a few species and the other species might show no
effect

26. Environmental Ecology Lab
Functional group concept:
♣ Functional designations are mutable(Merritt and
Cummins,1978)
♣Organisms may specialize differentially within separate
communities( Fox and Morrow,1981) and thus have different
functional roles in those communities.
♣Within a community a given species may perform several
functions

27. Environmental Ecology Lab
How density dependence can conduct by using
Grazers?
♣High densities of mountain suckers (Pantosteus platyrhynchus)
occurred in some pools and were in low densities in others in the
Raft River ,Idaho
♣ The riffles adjacent to pools containing high sucker densities
had low filamentous algal cover
♣Mountain suckers are grazers( Scott and Crossman,1973) and
helps in structuring the local algal communities
♣Exclusion of grazers from the tiles resulted in an increase in
algal biomass( Lamberti and Resh,1980)

28. Environmental Ecology Lab
Several Problems arise during study of density
dependent interactions by using grazers:
♣ First, the influence of seasonality on density dependent
interactions
♣ Factors such as canopy, angle of the sun , turbidity and water
temperature vary seasonally and influence the production rate
of autotroph and instream primary production will not remain
constant (Hickman,1974)
♣ Second, many aquatic organisms grow in cohorts
♣ Biomass is confounded with time

29. Environmental Ecology Lab
Use of production rates avoid these two
problems:
♣ Excess production hypothesis not only invokes density
dependence as a regulating mechanism, but also converts the
mechanism into one related to energy flow and production
♣ Drift acts as a mortality factor
♣Probability of mortality is higher in drifters
(Allan,1978;Waters,1972)
♣ Measurement of primary and secondary production is not
easy and normally conducted in unmanipulated field conditions

30. Environmental Ecology Lab
Mechanisms of detritus in stream ecosystems:
♣ Most detritus is introduced in Autumn leaf fall in stream in
temperate climates
♣ Leaves are colonized and turn into food for detritivores
(Cummins,1974)
♣ Carrying capacities increase by annual pulsing of leaf litter
♣ Subsequent increase of CPOM and then transfer into MPOM
and FPOM , respectively and then into DOM
♣ Leachate and other DOM flocculate (Lock and Hynes, 1975 and
1976) and make up a significant portion of fines(Boling
et.al.,1975)

31. Environmental Ecology Lab
♣ FPOM fluctuate annually ( Hynes et.al., 1974)
♣ High degree of age and species specificity in the filter net size
implies some degree of specialization to the food resources being
exploited ( Wallace et al., 1977)
♣ Periods of food scarcity (Weins,1977) are most likely to occur
during the periods of decline of the pulses
♣Shredder community can also be predicted by using K of leaf
material

32. Environmental Ecology Lab
Stream detrital dynamics model:
♣Predict changes in leaf standing stock with different storage
capacities of the stream systems
♣ Rate of leaf decomposition in a stream is related to factors like
leaf species, shredder abundance , water temperature and
location of the leaf material in the stream(cummins et.al;1973
and so on.
♣ if detritus decomposition and processing is location specific
(Meyer,1980) then the standing stock of leaf material will be a
function of the diversity locations.
♣ Decomposition slower in reducing environments
♣ depositional regions(pools) contain low amount of oxygen but
erosional regions(riffles) contain high amount of oxygen because
of turbulence

33. Environmental Ecology Lab
♣ Simulation was run with continuous litter input and the
results behave as anticipated (Fig.1 and 2)
♣ Low riffle ratio (high pool) stored more material than
the high riffle system
♣ Litter input is pulsed , not continuous and the levels of
material resulting in streams with differing storage
capacities

34. Environmental Ecology Lab
♣ A simple function(Dixon,1976) was used which allowed a
changing rate of leaf fall ( Fig.3)
♣ Systems with high proportions of riffles tend to have more
pulsed leaf litter standing stock in the areas. Input of the leaves
from terrestrial system and subsequent leakage is dominant
(Fig.4)
♣Systems with low riffle ratio (Fig.5) and high pool ratios have
less pulsed leaf litter standing stocks
♣ impact of leakage from the system is buffered by leakage to
the riffle from the pool storage(Fig.6)

35. Environmental Ecology Lab
♣ This model is still being modified and data collected from
valley Creek, Minnesota and have some assumptions
♣ While general predictions are still accommodated, storage
capacities and leakage rates differ more than originally assumed
♣ These variations will probably increase the differences
between riffle –pool ratios rather than diminish them

36. Environmental Ecology Lab
♣ Density dependence will be more likely in the high riffle ratio
systems
♣ Here leaves for shredding should become limiting rapidly
♣ High storage capacity streams have more gradual leakage to
the riffles
♣Assimilation ratio of riffles to pools related to total secondary
production rates
♣Increased secondary production from increased riffle area may
reduced storage capacity of stream

37. Environmental Ecology Lab
♣ Leaf pack decomposition approximates a negative exponential
curve(Petersen and Cummins,1974)
♣ If leaf pack size sets K, K will eventually approach zero
♣ Shredder curve shows an initial colonization pulse and
subsequently , smoothly tracks the declining K
♣ Drift occurs in a diel pattern , the feeding activity patterns of
drifting organisms are likely to be diel also
♣ Oscillations of K depend on microorganisms growth rates with
time and nutrients

38. Environmental Ecology Lab
Conclusion:
♣ Streams are complex systems
♣ Their complexity does not make them
impossible subjects for study of density
dependent and independent factors
♣ It based on sound hypothesis and
experimental design
Thank you for kind attention