Ecology
Ecology
Study of interactions of organisms with each other and with their physical environment
Hierarchical organization
Organism - a unique individual
Population - group of potentially interbreeding individuals (i.e., species)
Community - all populations within an ecosystem (i.e., multiple species)
Ecosystem - community + physical environment
Biosphere - ecosystems of the Earth
Food webs
Trophic levels
Competition
Law of Competitive Exclusion - No two species will occupy the same niche and compete for exactly the same resources for an extended period of time
–One will either migrate, become extinct, or partition the resource and utilize a sub-set of the same resource
•Given resource can only be partitioned a finite number of times
Resource partitioning
Apparent competitors may actually have slightly different niches
Species may use resources in a different way or time
Minimizes competition and allows coexistence
Law of Competitive Exclusion
No two species can occupy the same niche indefinitely when resources are limiting
Competition
Interspecific - Competition between members of different species
Intraspecific - Competition among members of the same species
–Often intense due to same space and nutritional requirements
Growth rate
Growth rate is an important factor affecting competition among plants
In favorable environments, the main competitive force is a struggle for light
Spring ephemerals have evolved to exploit small window of opportunity
Growth rate between competitiors
Affected by differences in:
–Height
–Leave arrangement
–Crown shape
–Energy allocation to roots versus leaves (R:S ratio)
–Photosynthetic efficiency
–Water use efficiency
Alleolopathy
Some plants produce chemicals that inhibit the growth of others
Black walnut
Hemlock
Rye grass
Sorghum
Mutualisms
Both species benefit from the interaction
Mycorrhizae
Mutualistic relationship between plants and fungi occuring in many vascular plants
Fungi increase plant’s ability to capture water and essential nutrients, especially phosphorus
Fungi provide protection against attack by pathogenic fungi and nematodes
Fungi receive carbohydrates and vitamins
Mycorrhizae
Association between plant and fungus
Can strongly influences growth and survival
Acacia and ants
Ants get nectar and a place to live
Ants provide protection and reduction in competition
Herbivory - The act of consuming plants
Types of herbivores
Above ground
–Mammals
•Ungulates
•Rodents (Many are seed predators)
–Insects - Beetles, caterpillars, aphids, thrips, ants, etc.
–Mollusks - Slugs, snails
Below ground
–Burrowing mammals
–Many larval insects
–Nematodes
Herbivores
Large variety in how things feed
–Chewers
–Borers – feed from the inside of the plant
•Leaf, stem, and root borers
–Suckers – drain phloem
–Gall-Formers – cause plant to create housing, and then use some type of feeding
All these types of feeders can be found either above or below ground
Allows for specialization and niche differentiation among herbivores
Why is the world green?
Despite the large number and variety of herbivores, when you look out, you see lots of plants.
–On average, < 20% of plant growth is consumed by herbivores
Why is the world green?
–Self-regulation of herbivores (unlikely)
–High rates of predation limits herbivore number (only partly likely)
•Predators usually cause a shift in what types of plants are found, not if they are found
–Plants are bad food (likely)
Types of plant defenses
Structural
–Thorns and Needles
•Generally increase animal handling time
–High lignin (carbon) content
•Decrease digestibility, N value
–Leaf toughness
•Makes it hard for small inverts to feed
–High silica content (in grasses)
•Increases wear on ungulate teeth, increases cost of feeding
Defensive associations
–ant/acacia
Chemical defenses
Chemical defenses
Secondary plant compounds
–Metabolic products not directly related to plant metabolism
–Alkaloids, terpenes, phenolics, resisns, steroidals, cyanogens, glycosides, etc.
Storage
–In cells, released when broken
–In specialized epidermal glands – secreted as a contact poison
Qualitative vs quantitative defenses
–Alkaloids (< 2% dry weight) vs. phenolics (up to 60% dry weight in leaves)
Constitutive vs. induced
Phytoalexins
Phytoalexins are lipid compounds that appear in response to fungal or bacterial elicitors
Animal counter measures
There is an “evolutionary arms race”
Specialization by herbivores
–Alter feeding strategy to avoid toxins
–Develop ability to detoxify
–Develop immunity
Results in highly toxic defenses working against most herbivores, but failing against specialists
Behavior to avoid defense
Effects on individual plants
Can reduced fitness
Big debate over whether plants can over-compensate
–Have increased growth with herbivory compared to without
How could herbivory help?
–Reduce self-shading
–If old leaves eaten, may reduce maintenance costs without altering photosynthetic ability
–N deposition
Recent review shows that vast majority of plants are hurt by herbivory
–But there are lots of exceptions
Population dynamics
Herbivores can have dramatic effects on plant populations.
Major basis of bio-control
Prickly pear introduced into Australia in 1839
By 1925 was in 240,000 km2
Released moth from S.America in 1925
Big effect
Herbivory and species diversity
Moderate herbivory can increase plant diversity
Intermediate Disturbance
–Low rate: Dominated by best competitor
•Wins in asymmetric competition for light, keeping others out
–High rate: Dominated by most resistant
–Inter: Mixture of both
Effects of disturbances
Disturbances can result in the long –term maintenance of species diversity
Disturbance in communities is the norm and not the exception
Disturbances
In the short term, disturbances can cause local extinctions of previously dominant species
What happens following disturbance?
Ecological succession
Change in the composition of species over time
Primary Succession
Ecological succession
Primary succession - A community begins to develop on a site previously unoccupied by living organisms
Pioneer Species
Species that colonize barren habitats
Lichens, small plants with brief life cycles
Improve conditions for other species who then replace them
–Process of environmental modification (facilitation) by organisms is called ecological development
Primary succession
Ecological succession
Secondary Succession - An existing community is disrupted and a new one subsequently develops at the site
–Old field succession
Succession
Clements: continuous, directional change in the species composition of a community … leading to a single ultimate community.
Current: not continuous, directional, nor is there a single endpoint
–Sequential change in the relative abundances of species following disturbance
Climax community
Stable array of species that persists relatively unchanged over time
Succession does not always move predictably toward a specific climax community; other stable communities may persist
Cyclic changes
Cyclic, nondirectional changes also shape community structure
Equilibrium or Disclimax Communities - Never reach stable climax because they are adapted to periodic disruption
Tree falls cause local patchiness in tropical forests
Fires periodically destroy underbrush in sequoia forests
Terrestrial biomes
Biomes - Areas sharing similar climate, topographic and soil conditions, and roughly comparable communities
•Most terrestrial biomes are identified by the dominant plants
Biomes
Temperature and precipitation are among the most important determinants in biome distribution
Tropical Moist Forests
Humid tropical regions support one of most complex and biologically rich biomes
Ample rainfall and uniform temperatures
Tropical Moist Forests
Tropical Rainforests - More than 140 cm annual rainfall with warm-hot temperatures year-round
–90% nutrients tied up in living organisms
–Rapid decomposition and nutrient cycling
–Thin soil cannot support continued cropping and cannot resist erosion
Rainforests
Limiting resource is light
Dominance of trees and arboreal species
Tree exploiters
–Epiphytes
–Lianas
Canopy gap dynamics are important
Tropical Seasonal Forests
Semi-evergreen and partly deciduous forests tending toward open woodlands and grassy savannas
–Characterized by distinct wet and dry seasons with hot temperatures year-round
Savannas
Open with widely spaced trees
Seasonal rainfall
Savannas/Tropical seasonal forests
Lots of shade-intolerant perennial grasses
Fire adapted
Leaves small
Deserts
Characterized by low moisture levels and precipitation that is infrequent and unpredictable
Wide daily and seasonal temperature fluctuations
Plants and animals exhibit water conservation characteristics
Soils are easily disturbed by human activities and slow to recover
Desert plant adaptations
Annual plants well represented
Succulents
CAM photosynthesis
Large taproots
Small leathery/hairy leaves/spines
Ability to drop leaves
Photosynthetic stems
Grasslands
Communities of grasses and seasonal herbaceous flowering plants
–Herbs require disturbance
No trees due to inadequate rainfall
Large daily and seasonal temperature fluctuations
Frequent grass fires
Historic grazing by roaming herds of large ungulates
Historic conversion to farmland
Broad-Leaved Deciduous Forest
Areas of warm summers and cool winters
Temperate regions support lush summer plant growth when water is plentiful
–Deciduous leaves are an adaptation to freezing temperatures and lack of water
Eastern half of US was covered with broad-leaf deciduous forest when European settlers arrived
–Much of that was harvested a century ago for timber
Mediterranean
Characterized by warm, dry summers and cool, moist winters
Evergreen drought resistant thickets
Fires are a major factor in plant succession
–Referred to as chaparral in California
•Biodiversity hotspot threatened by human development
–Referred to as thorn scrub in Africa
Conifer forests
Cone-bearing
Plants reduce water loss by evolving thin, needle-like evergreen leaves with thick waxy coating
Fire often plays role in maintenance
Humans rely on conifer forests for forest products
Temperate conifer forests
Temperate mixed forests of SE US, Eastern Europe
Temperate conifer forests of western North America
Conifer Forests
Boreal Forest - Northern ConiferForest
–Broad band of mixed coniferous and deciduous trees between 45° and 60° N latitude
–Moist and cool climate with abundant streams and wetlands
Taiga - Northernmost edge of boreal forest
–Species-poor
–Harsh climate limits productivity
Tundra
Treeless
Low-lying shrubs and grasses
Permafrost
Very short growing season, with cold harsh winters
Damage slow to heal
Low diversity and productivity
Global warming threatens tundras
Air pollution accumulates at high latitudes
Oil and gas drilling