Ecology – Biology 3500
Dr. Adams
COMMUNITIES and ECOSYSTEMS UNIT
Chapter 16: Species Abundance and Diversity -- Community Structure
Definition of Community
Guilds (used mostly by zoologists); growth
(life) forms
(used mostly by botanists)
Species Abundance: relative abundance in a community -- most
species are moderately
abundant; fewer species are very abundant or very
uncommon (see Fig. 16.3).
The Lognormal distribution for abundance:
LARGE samples tend to show this
distribution (see Preston's Canadian moth data on my
website). This distribution of species
undoubtedly is influenced by many biological factors,
but its utility is in its predictive
value.
Species Diversity: defined by two factors -- 1. the
number of species in a community, or
species richness, and 2. the
relative abundance of species, or species evenness.
Quantitative Index of species diversity -- the Shannon -
Wiener index (will use in Lab 13):
s
H' = -
Σ pi
ln pi where p
= the proportion of a particular species, & s = # of species
i =1
This index gives a quick comparison number
for species diversity in different locations; see
sample
calculations on page 357.
Rank-Abundance curves -- see examples on pages 351 -
352
(will also use in Lab 13)
Important question: Has any
habitat been sampled for ALL known species? Will mention
ATBI
initiative in the Great Smokey Mountain NP.
Environmental Complexity -- Species diversity higher where
environmental complexity is higher.
Remember yet again (!) MacArthur's
warbler study (see Fig. 16.9 & 16.10).
More complexity = more possible
"niches." Studies have shown a positive correlation between
environmental
complexity and species diversity in groups such as: mammals, lizards, reef
fish, marine
gastropods, plantkton. Most of these represent animal groups, whose
diversity
is largely
dependent on PLANT diversity. So what about plants and phytoplankton?
Plant (and algal) diversity: for
various species of plants, which largely compete for the same
resources, there seems
to be a violation of the competitive exclusion principle . . . so how
do so many
plant species occupy a single community? Turns out that very specific
needs are
quite different
between plant/algal species.
Specific studies -- In
both aquatic and terrestrial habitats, there is nutrient heterogeneity.
1.
Tilman and algae: see Fig. 16.11. Algae require very specific
balance of certain minerals.
2. Lebo and particulate concentrations (nitrates/silicates) in lakes --
Fig. 16.12.
3.
Jordan: Soil quality (nitrates, water, etc.) and species/community
differences in the
tropical rain forest; see Figs. 16.13 & 16.14.
A significant trend: As nutrient
availability INCREASES, plant/algal diversity in the community
DECREASES. WHY? What this
also means is that fertilization will DECREASE diversity (see
Fig. 16.16 for long-term
feritilization study at Rothamsted, England). Also see this with
both
above ground (mushroom) and below ground (mycorrhizal)
fungal diversity with increased
nitrogen application (which also results in more acidic conditions). So what
happens is a shift
from
efficient nitrogen utilizers
to a few acid-tolerant, high soil fertility, competitive species.
Disturbance and Diversity
For several chapters, we've been talking about several
influences on species numbers taking place
(competition, predation, parasitism,
etc.) resulting in some sort of equilibrium state (K). Clearly,
this is NOT what conditions are like
in the "real" world.
Disturbance defined -- understand that, just as
"stable" is different for different organisms,
"disturbance" is similarly varied in
what it means to different organisms. Indeed, what we might
consider "disturbance" (fluctuating
temperature/salinity) could be part of the natural conditions of
certain habitats (temperate
biomes/estuaries). Basically, any event (in time) that disrupts the
community/ecosystem such that
resource/substrate availability changes and, in turn, makes it
possible (at least temporarily) for
new individuals to get a foothold.
Disturbance can be characterized by two factors:
frequency and intensity.
Intermediate Disturbance hypothesis -- see figure
16.18
Connell suggests that high
diversity is a consequence of continuously changing conditions, and
that
intermediate levels of disturbance promote the highest diversity. WHY
does this make
some sense?
Low levels of disturbance lead to the strongest competitors excluding others
(think of a
climax forest), and high levels of disturbance remove a number of species
which
require some
time to establish themselves. Intermediate frequency of disturbance
allows a
lot of
colonization without competitive exclusion over the long run. Remember the
"ruderals
vs.
competitive species" discussion in Chapter 12??
Example: in the Intertidal
Sousa --
boulder size and frequency of wave displacement
Big boulder = infrequent movement (1% turned over/month); Intermediate (9%); and
Small boulder = frequent movement (42%). The "infrequent" (big) boulders
supported
1 - 3 species for the most part, the "frequent" (small) boulders supported
mostly 1 species,
and the intermediate boulders typically supported 3 - 5 species.
Example: Prairie Dogs in the
Grasslands
Whicker and
Detling's work -- turnover of soil and plants; aeration of soil
Chapter 17 -- Species Interactions and Community Structure
Community (Food) Webs -- can be incredibly complex trophic interactions
Obviously, some interactions will be
strong, others weak, in terms of influence on community
structure.
(See top of page 368; interactions with the reed plant Phragmites)
Indirect Interactions -- influencing another species indirectly
through yet a third:
Certain commensalisms can be indirect --
beaver/cottonwood/leaf beetle example
Apparent competition is indirect -- example:
two prey species "share" a predator; increases in
one prey
species increases the number of predators, and, in turn, impacts other prey
species
Keystone species -- those feeders (consumers) or those
being fed upon (can be producers or
lower order consumers) that have the
absolute strongest influence on community structure,
inordinately when compared to other
species, and whose absence would radically alter the
stability, and ultimately diversity, of the system. These are NOT
typically the most abundant;
indeed, keystone species are virtually never the most abundant. For
instance, some predators
help keep prey species below levels
where they might competitively exclude others, allowing
for coexistence and not exclusion, thereby maintaining or increasing
diversity.
Paine and marine communities: as diversity of
plankton communities increase, proportion which
are predators also increases (in
Atlantic continental shelf plankton community: 81 species, 16%
predators; in Sargasso sea: 268
species, 39% predators). When comparing temperate to
tropical intertidal communities,
there was one top predator in each (a sea star), but many more
mid-level predators in the tropical
intertidal feeding on more prey species (see Fig. 17.9).
Experimental removal of sea stars: Paine
found that removal of the top sea star (Pisaster) in the
temperate intertidal resulted in a
loss of invertebrate diversity of nearly half -- from 15 to 8
species. Within just a few
months, one barnacle species became dominant, but was crowded
out within a year by mussels and
another barnacle, which became the dominant two species.
Similar results were shown with
removal of the top sea star (Stichaster) in a New Zealand
intertidal community (20 to 14
species) with a mussel increasing coverage significantly (See Fig.
17.10). When Paine additionally removed a
vigorously competitive brown alga as well, the results
were even more dramatic, with the mussel
becoming even more abundant.
Lubchenco: Algae (Enteromopha/Chondrus), Littorina snails, Carcinus
crabs, and seagulls
In tide pools (that remain
submerged), diversity highest with intermediate Littorina densities. In
emergent habitats, highest
diversity is with lowest Littorina density. WHY? You need to
under-
stand the interactions
between ALL listed "players" in the community.
Power and fish in the Eel River in northern
California: at the base of the food web is the alga
Cladophora, fed on by the
larvae of chironomid midges. If remove top predators (roaches
[minnows] and steelhead trout), the
algal growth blooms. Or, in other words, when these fish
are present, the algal mats decrease
significantly by midsummer. WHY? Fish feed on midge
larval predators.
So, what is a keystone species? A species whose
influence is disproportionate to its biomass. What
this means is that a keystone
species is NOT the same as a dominant (most abundant) species.
Mutualistic Keystones -- Cleaner fish example; Ants (as
seed-dispersers) example
The cleaner wrasse (Labroides dimidiatus) can remove
and eat 1200 parasites a day from client
fish species.
Disappearances/removals of the wrasse reduced fish species richness by nearly
25%, while additions of wrasses to
wrasse-free communities increased the richness by 25%
(four month timespan in Egypt's Ras
Mohammed Nat'l Park) (Bshary, 2003).
Ants are responsible for 30% of seed dispersal in South
African natural areas. Invading
Argentine ants, which do not disperse
seeds, have displaced the natural ants in some places.
Large-seeded species do not
recruit much at all in communities with the Argentine ant;
instead, these seeds are eaten by
rodents or destroyed by fire.
Many native pollinators in the U.S. have been excluded
by honeybees. Reason for concern?
Chapter 18 -- Primary Production and Energy Flow
Productivity -- Know primary, gross primary,
and net primary production; also secondary
production. Biomass comes
from inorganic molecules w/ a source of energy (sun, mostly).
Net primary = Gross primary -
producer respiration (producers own energetic needs).
Remember trophic levels: producers, primary
consumers (herbivores), second + level
consumers (carnivores).
Omnivores and detritivores. Above primary consumers, many
species can act at more than one
trophic level.
Terrestrial -- largely limited by temp., moisture, and nutrients
(Have we heard that before?)
Actual Evapotranspiration (AET): measure of the amount
of moisture lost from the landscape
(evaporation) and from plants
(transpiration); AET highest in warm moist places, and
productivity is highest in these
places as well. Works across ecosystems, and within the same
ecosystem (tallgrass to shortgrass
prairie in east to west gradient) across a temp/precip
gradient (see Figs. 18.2 & 18.3).
Soil Fertility (see Fig. 18.5) also plays a role in
explaining variability in productivity under similar
temp/precip regimes. Increased
nutrient (P, N, etc.) availability, not surprisingly, increases
productivity (remember, though, that
increased nutrient availability can DECREASE diversity)
Aquatic -- nutrient, light (for most) and, to a lesser extent,
temperature driven
Higher nutrient availability, particularly phosphorus (and
nitrogen), increases algal biomass and
productivity in lakes. When
lake system fertilized with P, N and C, we see exactly what we
would expect -- increased
productivity.
Marine productivity -- highest in shallow seas along
continental margins and along equator where
there is significant upwelling;
marine production is lowest in deep open ocean (Fig. 18.8) WHY?
(Big hint:
n & l) In the Baltic Sea, nitrogen is a limiting factor to overall productivity (Fig. 18.9);
nitrogen
seems to be an important factor in saline environments.
Primary Producer Diversity
A couple of straightforward
relationships:
Higher terrestrial plant species richness
results in higher above ground production and biomass
Higher aquatic algal diversity similarly results in higher nitrate uptake and
algal biomass
So far, we've looked at effects of physical and chemical factors on primary
production -- these are
the so-called bottom-up controls; next we look at the
influences of higher trophic levels -- the
consumers, what we call top-down controls.
Consumer influences --
Top-down controls suggest the trophic cascade
effect, where effects of the top-level
consumers cascade down through the
food web. Remember the effects of the top level fish on
algal productivity in the Eel
River? This is exactly what we are talking about here. Another example
is shown on pages 392-393
(see Fig. 18.12). Notice that the top level consumer has alternating
effects
(increase-decrease; Fig. 18.13) on successively lower trophic levels to a point. This is
basically what we were talking about with the algae/Littorina snails in the tide pool example above.
Also understand, however, that this
type of cascade is more likely to take place in ecosystems
with lower diversity/complexity (WHY?) and seems to be easily detectable in
some aquatic eco-
systems, though, certainly at the top levels, we can see direct effects of
the predators on at
least the next level down (remember also the lynx/hare/willow example).
Grazer communities -- in the African Serengeti.
Rainfall (obviously) increases
savanna productivity. Interestingly, grazing does so as well, by a
phenomenon called compensatory growth
(we talked about such plant responses previously).
Such growth was highest under
intermediate grazing conditions(disturbance; hmm, where have we
heard about intermediate distrubance before?); too heavy and plants have reduced ability to recover.
Secondary Production
Trophic levels -- Energy losses (indicating a certain level of
ecological efficiency) limit the number
of trophic
levels. Trophic dynamics leads to energy and biomass
pyramids, which we will
discuss more later (see Fig. 18.18
& as
well as a diagram attached to this handout).