Ecology – Biology 3500
COMMUNITIES and ECOSYSTEMS UNIT
Chapter 16: Species Abundance and Diversity -- Community Structure
Definition of Community
Guilds (used mostly by zoologists); growth forms (used mostly by botanists)
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). This
distribution of species undoubtedly is influenced by many biological factors, but perhaps its
utility is in its predictive value.
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):
H' = - Σ pi ln pi where p = the proportion of a particular species, & s = # of species
This index gives a quick comparison number for species diversity in different locations; see
sample calculations on page 355.
Rank-Abundance curves -- see examples on pages 355 (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 and species/community differences in the tropical rain forest;
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). 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 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. 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
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 Webs -- can be incredibly complex (in terms of who eats whom) trophic interactions
Obviously, some interactions will be strong, others weak, in terms of influence on community
structure. (See bottom of page 373; interactions with the Blue Tits)
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 if not
collapse the community. For instance, some predators may 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, 19%
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. 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 species, 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? 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
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 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. Biomass
production from inorganic sources with a source of energy (sun, in most cases).
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 temperature and moisture
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 may 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, lowest in deep open
ocean (Fig. 18.9) WHY? (Big hint: n & l) In the Baltic Sea, nitrogen is a limiting factor to
overall productivity (Fig. 18.10); nitrogen seems to be an important factor in saline environments.
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 397-398 (see Fig. 18.12). Notice that the top level consumer has
alternating effects (increase-decrease) on successively lower trophic levels to a point. 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 (too heavy and plants have
reduced ability to recover).
Trophic levels -- Energy losses limit the number of trophic levels. Trophic dynamics leads to
energy and biomass pyramids, which we will discuss more later (see Fig. 18.16 & .17, as well
as diagram attached to this handout).