Eco Exam 3

Community

A group of species occurring together in a location

Processes that control how organisms interact

Herbivory, Parasitism, Predation, Mutualism and Competition

Competition

Contest to acquire a limited resource. One ind. or species limits the other.

Intra-specific Competition

Competition between members of the same species. General trend of lower survival, reproduction with more conspecifics (slower growth).

Inter-specific competition

Competition between members of two different species. Usually due to resource exploitation.

Resource Exploitation

Two individual or species exploit the same resource, so if one depletes it, it hinders the others ability to acquire said resource.

Interference Competition

direct, aggressive action by on ind. or species that limits the resource access of the other.

resource comp. vs interference comp.

hard to differentiate, requires studies

Asymmetric competition

A limits resources for B while B has little effect on A. Disproportionate to individual size.

Symmetric competition

Two individuals simultaneously deplete a resource and each is limited by the other’s presence. Proportionate to individual size.

Intra vs inter comp

Intra is typically stronger due to individuals of the same species having nearly identical resource requirements

Intra-model

dn/dt = rmaxN(K-N/K)

where dn/dt is ind. added, rmax rate of growth, N is current pop size, and K is carrying cap.

Inter-model

dn/dt= rmax*N1(K1-N1/K1)

what if comp. from another species limits species 1’s ability to reach it’s K?

Add N2 to the formula!

dn/dt= rmax*N1((K1-N1)-N2/K1)

if N2>0 then dn/dt=0 before N1=K

assumes that both models use identical resources!

How to fix assumption from:dn/dt= rmax*N1((K1-N1)-N2/K1)

add alpha!

dn/dt= rmax*N1((K1-N1)-aN2/K1) (also known as Lotka Voltera comp. model)

alpha

competition coefficient that represents the strength of comp, if:

a<1: a single ind. of species 2 limits species 1 LESS than a single ind. of species 1 limits itself

a>1: a single ind. of species 2 limits species 1 MORE than a single ind. of species 1 limits itself

Niche

Set of variables that determines where a species can persist. Distinct from habitat (env. where a species is found).

If two species have identical niches? (or one is nested completely in the other)

Assume one will always outcompete and exclude the other. Also known as the competitive exclusion principle.

If two species have at least partially different niches?

Both should be able to co-occur

How does niches influence community assembly?

Species sorting

Species sorting

two species with very similar niches, when their geographic range meets, growth stops. allopatry only.

if niches sufficiently distinct then can continue to grow when geographic ranges meet. sympatry possible

Niche-based hypothesis for species co-existence

Species can only co-exist because they have segregated niches

Ex. of different niches

Foraging locations, nest site locations, foraging method, etc.

Niche segregation in plants?

All plants need the same resources (sun, water and nutrients) so niches are controlled with TRADE-OFFS!

Trade-offs in H2O deficits

Increased root biomass, narrow leaves, reflective hairs, etc.

Trade-offs in O2 deficits

Anaerobic respiration in roots and aerenchyma (specialized, spongy plant tissue containing large air spaces or channels), etc.

trade-offs with nutrients

if nutrients abundent, plants that grow faster are favored. In more limited nutrient environments slow growing plants are favored. Plants used to nut can’t handle when they don’t have them.

Environments are patchy?

spatially heterogenous (not uniform), can also be temporally heterogeneous like seasons or over longer periods of time.

Succession

Predictable replacement/turnover of plant species over time after a disturbance.

Disturbance

Death of biomass over a period of time much shorter than it takes the biomass to re-grow. More resources now available.

Traits of a species that typically colonizes first after disturbance?

Wide seed dispersal, seed persistence and tolerance of harsh abiotic conditions (if disturbance was severe).

Why don’t early colonizers remain dominant in the absence of disturbance?

Local conditions change! Early colonists change the environment for later successional species.

Inhibition

Earlier stage of succession reduce the success of later species.

like covering the entire ground so no other plants can grow?

Facilitation

Earlier stages of succession increase the success of later species.

How do local conditions change?

Over time and spatially on a landscape.

Spatial Heterogenity

In the successional stage, allows species with different niches to coexist in the same landscape.

How do environmental conditions vary?

Over-space: heterogenity

Over-time: seasonally, and over a longer time: succession

How do we know if species are more different than we might expect by chance?

Comparing Correlative data of niches and co-existence:

1.) observe x trait differences between co-existing species

2.) randomly shuffle and re-assign species to communities

3.) compare the x differences between the randomly paired species and the actually co-existing species.

more the numbers differ, the stronger the evidence that competition influences communities

though correlation doesn’t equal causation, other processes could be at play

Neutral theory

species enter communities due to chance dispersal events. Differences in morphology, physiology, etc. among a species in a community are due to random chance.

Modern co-existing theory

Based in part on ‘rare species advantage’ - if a rare species’ population can grow quickly and invade a site occupied by a common competitor, this facilitates co-existence

common species limit their own population growth rate before they can exclude rarer species

when co-existing species are grown together, population growth should be reduced more strongly by density of conspecifics than by density of other species (α < 1) –> classic density-dependence should be common

Field experiment results for competition exclusion between competitors

Shows that comp. often limits the abundance of competitors but few should total competitive exclusion

Invasive competitors?

Invasive animal species appear to have NEVER driven competitors to extinction. possibly because they limit themselves as abundance gets high. though most invasions are recent, and this may be a long, slow process

predators and pathogens DEFINITELY can drive prey extinct though

Hawaii Bird Experiment?

As the number of exotic bird species in Hawaii increases, a lower proportion of new introductions are successful. Could be due to niches filling up or overall less resources available.

Evidence against competition?

Locations with greater native species richness also have greater numbers of introduced species. Due to amount of resources available overall.

Niche-based mechanisms and neutral process?

are probably acting in all communities, and the goal of empirical research should be to determine the relative importance of these factors in driving community assembly on a case by case basis