Biodiversity

Biodiversity Concept

Evolution (long-term change)

Factors of short-term change

• Understanding the patterns of and controls on distribution of organisms in aquatic habitats is essential to the study of ecology, particularly in the fields of conservation biology and fisheries management.

• Species over-exploitation, habitat destruction, and introduction of exotic (alien) species by human activities has lead to dramatic community alterations and species extinction (locally and globally).

Biodiversity• Measures of biological diversity help define patterns and infer controls

on community structure over various scales:

– spatially (globally to between and within habitats).

– temporally (evolutionary to seasonal)

• These measures permit monitoring of ecosystem stability and/or impacts from outside disturbance (e.g. human activities).

• Species Richness (S)– Total number of species in an area.

• Evenness (or equitability; E):– Degree of equal representation for each species.

• Shannon-Weaver Index (H’)– Incorporates information on both S and E.– H’ increases when either S or E increases.

j

S

jj ppH ln'

1

Where p is the proportion of species j to the total of all individuals (= Nj / N)

Where lnS is the maximum diversity; or maximum evenness for S species.

Biodiversity over Spatial Scales

Within-Habitat (α diversity) versus Between-Habitat (β-diversity)

• Consider the two sets of four ponds A-D and E-H.

• Overall diversity of each set is similar.

• Set A-D has lower α diversity; one species per habitat dominated community.

• Set E-H has lower β diversity; little difference in community between habitats.

Global Scale

Ecoregions: classification of large geographic areas based on their distinct assemblages of natural communities.

Information on organisms and abiotic characteristics are considered.

Presently, only particular animal taxa (fish, amphibians, crayfish, mussels) are used for distinguishing ecoregions.

North America has been divided into 76 ecoregions.

(1999)

Evolution as the Source of Biodiversity• Uninterrupted time and reproductive isolation are key to evolution of new species.

• Few freshwater ecosystems have fulfilled this criteria (contrast marine ecosystems) due to climate variation (e.g., glaciations).

• Most freshwater ecosystems have “cosmopolitan” species (wide spread geographically), and few have many “endemic” species (unique to a particular habitat).

• Tectonic lakes (deep and old) have a much greater proportion of endemic species as compared to glacier lake.

• Compare Lake Baikal (high endemic crustacean diversity) and the African Rift Lakes (high endemic teleost diversity).

• Both show examples of adaptive radiation (many species from a single founder).

Baikal Gammarids (amphipods)

Tanganyika Cichlidea family

Short-term Variation in Diversity1) Habitat diversity (many types in a single ecosystem).

2) Size of habitat (positive relationship with diversity).

3) Connectivity of habitats (ecotones; colonization conduits).

4) Sources of recruitment (dormancy and dispersal).

5) Species interactions (specialize to avoid competition; niche).

6) Productivity (timing and location coincident with recruitment).

Species space spawning activity to limit competition.

Phytoplankton Diversity• Phytoplankton require light, CO2 (inorganic carbon) and nutrients (P, N, etc.) to

grow through photosynthesis; most aquatic environments are nutrient limited.

• Many species competing for the same nutrient resources in the same areas should lead to competition and ultimately competitive exclusion.

• Instead, MANY different species of plankton co-exist at once. This has been termed “The Paradox of the Plankton.”

Disturbance• One mechanism proposed to explain this paradox is the fact

that lake conditions are not in a state of equilibrium for more than 1 month before the system is disturbed; it would take longer than this for 1 species to become dominant.

• Disturbances can be difficult to characterize (vary in magnitude from slight shifts from equilibrium to punctuated events.– Lakes, groundwaters less prone to major disturbance events; but

experience seasonal changes.– Streams, rivers, & wetlands experience regular disturbance (flooding,

drying, etc.)

• Systems prone to disturbance are less likely to achieve a classic “equilibrium” state (climax community); rather “dynamic equilibrium” is more normal.

Succession• Succession is the sequence of species colonizing newly available

habitat and niches.

• The sere (sequence of specific organisms) is based on an organism’s characteristics for colonization (recruitment), growth rate, resource competition, predator avoidance, physicochemical tolerances, disease resistance, and relative community scale.

• Over time, the habitat may become modified so to favor the next organisms in the sere (e.g. nutrient depletion shifts competition).

• Stages of Succession:– Early invaders: rapid reproducers and colonizers (r selection)– Mid- to late-succession: Better long-term competitors (K selection)– Maximum diversity occurs during mid-succession stages, as both early-

stage and late-stage species are present and competing for resources.

• Disturbance and succession within a larger ecosystem will favor an increase in diversity up to some limit.

Intermediate-Disturbance Hypothesis

Theoretical Relationship Between Diversity and "Disturbance"

Frequency of DisturbanceIntensity of Disturbance

Bio

tic

Div

ersi

ty

competition (K)

recruitment/ colonization (r)

Long-Term Lake Succession“Lake Aging”

• Over thousands of years, a newly formed lake will eventually fill with sediments and return to a more terrestrial state, regardless of trophic state. (30m lake at 1 mm/y will take 30,000 y to fill)

• Although many exceptions exist; hypothetically lake succession proceeds from oligotrophic → mesotrophic → eutrophic → senescence (marsh) → terrestrial.

• Over decadal scale a subclimax may be observed.

• Mean depth, lake size and watershed size and fertility are major facts on controlling the timing of lake succession.

• Catastrophic change in watershed, climate, or nutrient loads can rapidly shift subclimax state.

• Some manmade impacts on trophic state have been demonstrated to be reversible when appropriately mitigated (i.e. rejuvenation).