concepts Flashcards
Optimal foraging theory (OFT)
Animals maximise the benefits of food gathering (energy intake) while minimizing cost (energy, time, risk). This is also done via optimal diet model and patch choice model. Limitation = assumes they have complete knowledge of whole environment. There are also factors such as competition, social behaviour and environmental unpredictability.
Exploiter-mediated coexistence
When species that might otherwise compete directly for the same resource can coexist because of the presence of a shared predator or parasite (‘the exploiter’). This works as regulation and stops one species from being dominant or outcompeting another. It also allows multiple species to share the same habitat. Limitation: could be too effective and lead to overexploitation, or also predator specialization.
Trophic cascade
Top-down effects that predators exert on lower levels of a food chain/web, which can influence a whole ecosystem. When species predate, it indirectly affects the abundance of lower levels.
Sampling effect
The more species, the more likely there is a highly competitive species. This species is a more efficient user of resources which leads to higher productivity, which means there are less unconsumed recourses and fewer opportunities for new species.
Portfolio effect
The more species there are to spread the variability across, the more stable the ecosystem. Like in business, the more diverse the investment portfolio, the less susceptible it is to market volatility.
Complementarity effect
Each species has a combination of resources at which it performs best. Variation means that each species covers a different part of the habitat, but no species can fully exploit the full range of conditions.
Adaptive radiation
Important for ecological diversity. The diversification of a lineage into species that exploit a variety of different resource types and differ morphologically or physiologically. Adaptive radiation may be responsible for many of the species in isolated communities (over immigration). This could be since many locations are difficult to colonize. And they may find ‘empty niches’ anyway.
Rarity
Evolutionary age: (older species have bigger ranges, they have had a longer time to expand from their point of origin, this could also predict range size).
Relative dispersal ability: (better dispersers have better ranges, like plants for example with birds and wing-dispersed seeds). Wing loading (wing size relative to body size), wing loading predicts range size (Enochrus).
Niche breadth: (specialists would have a smaller niche/habitat than generalists, this will be due to physiological differences which are fundamental to setting niche breadths). Like thermal range for example (Calosi et al. (2010)). Temperature is the best predictor of latitudinal range extent.
Biogeographic accident: (down to where they evolved, and how easy it is to leave that place, for example in island endemics). Island endemics should have bigger ranges, but what restricts them is that they evolved on the islands. But if they could get out, their ranges would be big.
Ice ages summary
Recent history = repeated cycles of glacial and interglacial periods.
Ecological and evolutionary consequences = latitudinal diversity gradient, conc. of endemic species at low latitudes (Rapoports rule- narrow range endemics aren’t evenly disributed), communities at higher latitudes often of relatively recent origin.
All affect relative responses to ongoing global change.
Albido effect
build up of snow and ice cools earth even more due to the reflection into the environment.
Periodic changes in characteristics of the earth’s orbit
Eccentricity (96000 year periodicity) = more elliptical orbit, greater seasonal contrasts in solar radiation. Obliquity (42000 year periodicity) = angle of tilt, when the angle is greater the seasons are more marked, summers in both hemispheres receive more energy from sun, and winters less. Precession (21000 periodicity) = effects strengths of seasons, similar in both hemispheres, to more marked in one or other.
Taxa vs latitude: local, regional + temporal
Local explanations- competition + predation. Structural complexity = more niches for more species (high diversity). Doesn’t explain why more species co-occur (more regional focus).
Regional- regional richness determines local richness, (balance between speciation + extinction).
Time- lower extinction rates at lower latitudes (or higher speciation rates due to more gens of organisms in real time) compared to in higher latitudes with stronger seasons. = faster mutations and adaptations.
climatic stability = lower extinction rates.
latitude diversity gradient (LDG)
Most common during ‘icehouse’ earth (glaciation). Few cases of modern-style LDG. During the global greenhouse conditions, tropical biotas were at higher latitudes. Pleistocene ice cover 3MYA- present, is more extensive in the northern hemisphere, there are more species dropping off now (isopods gastropods, bivalves). But still same in Southern hemisphere (more taxa). Arctic is isolated by its own current. Productivity + speciation rates aren’t necessarily mutually exclusive.
Ecosystem functioning
Energy and nutrient cycling, key trophic processes (primary and secondary production), factors underpinning ecosystem processes (ecological succession, partly predictable change in communities of how they are composed over time).
Ecosystem services: subset of processes and functions beneficial to humans. Like CO2 fixation, O2 release, soil formation, water purification, climatic regulation, N2 fixation, pollination.
Niche complementarity
Doing different things in niches which complement each other, more overlap as more resources are used. functional redundancy = if species leave/die, so the system can continue due to overlap, but then it will stop at a critical point. Idiosyncrasy is all over the place, not just about the no. of species but what the species are and their functional traits within the system. Not all species are equal in contributing to ecosystem functioning.
Keystone species
Species whose influence on the community is greater than would be predicted from their abundance/biomass.
Studied via: patterns in nature, field manipulation, mesocosm studies (tank in lab, cotton experiment). None of these methods are able to describe all results.
Life history
Adaptations of an organism that influences aspects of its biology.
- Reproduction (no. of offspring, timing of reproduction, sex ratio of offspring, gestation period)
- Growth (size at birth, maturity, growth rates).
- Mortality (lifespan, mortality schedules (mayfly has short life span, but have mass emergence and then die-off)).
Darwinian fitness
Lifetime reproductive success of an individual. Selection for optimal combos of life history traits.
-Eg: ocean sunfish have low chances of encountering partners, so they have a long life with a high reproductive output (300 million eggs).
Evolutionary constraints + trade-offs
Change in one trait increases fitness, but results in a change in another trait that decreases fitness. Organisms aim to max reproductive success given constraints, resource allocation.
-Physical + developmental constraints (allometric relationship).
-Selection pressure can vary among life cycle stages (declines with age)
-Biotic interactions (herbivores)
Reproductive effort- allocation of time, energy + resources for offspring production/care.
Grouping by LH vs functional traits
LH: to understand evolutionary origins of different life histories, predict how species might respond, identify species vulnerable to anthropogenic pressures, why some species are useful in human-altered environments.
Functional traits: (physiological, morphological, behavioural traits influencing performance/fitness). Can influence the ability of species to respond to changing or novel environments.
LH strategies (r vs k-selected)
r = pr capita rate of increase (expanding populations, faster LH (more allocation to reproduction)).
K = carrying capacity of pop (stable populations, slower LH (more allocation to survival)).
r-selected species
Disturbed (unpredictable/short-lived) environments
Short-lived, small organisms
Rapid development
Many offspring (early maturity)
Density independent
k-selected species
stable (predictable) environments
Long-lived, large organisms
Slower development
Few offspring (late maturity)
Density dependent
Positive symbiosis
Mutualism: long-term, evolved association between two species in which both partners benefit, et benefits must exceed net costs)
Commensalism: the other partner doesn’t benefit but remains unaffected) = interspecific interactions
Negative symbiosis
Competition (– or -0)
Predation (+/-)
Parasitism (+/-) = interspecific and intraspecific interactions.
Facilitation
When one species provides another with a favourable habitat, influencing the distribution of that other species.
-In successional communities (dunes, forest growth), stressful environments (like arctic conditions, coasts).
Examples; cushion plants increase species richness of communities in Andes, marram grass stabilises sand dunes for colonisation by other species.
Competition
Interspecific- competition of shared resources between species.
Intraspecific- competition of limited resources among individuals within species.
Density dependence
Negative- individual fitness (or birth rate) declines with increasing population size. (driven by increased competition, also predation and disease).
Positive (Allee effect)- individual fitness (or birth rate increasing with population size). Organisms that live or hunt in groups.
Methods
Lab experiments: manipulate pop densities or resource availability whilst controlling for other conditions.
Fiel experiments: manipulate similar factors but with an uncontrolled background environment.
Observational studies: correlation between environment and pop change over time.
Types of niche interactions
Competitive exclusion- local exclusion of a competing species.
Niche differentiation- character displacement (evolutionary change), in sympatry (not allopatry). Coexisting competitors should show niche differentiation, potential competitors with little or no niche differentiation should be unlikely to coexist.
Coexistence- most communities contain quite similar species; evolution of niche differences promotes species coexistence.
Apparent competition- one species has a negative effect on another species, via a third species.
Community structures and assembly
Structure- attributes such as no. of species, types of species and relative abundance.
Assembly- study of processes that shape the identity and abundance of species within ecological communities.
Species diversity
Species diversity: no. and abundance of species present in a community, defined with species richness (no.), and species evenness (relative abundance of species)
Diversity measures (combine species evenness and richness. Simpson index (D or DI), Shannon-Wiener index (H or H’).
Community shaping factors
At a constant flux:
-Competition, predation/parasitism
-Environmental heterogeneity (complexity)
-Stochasticity (random events)
-Disturbance
-Mutualisms
Disturbance
Temporary change in environmental conditions that cause a pronounced change in an ecosystem
-Mechanical vs physico-chemical
-Periodic vs stochastic (random events)
-Biotic
Intermediate disturbance hypothesis (Connell 1978)
Founder-controlled disturbance
-Competition low, disturbance high
-Randomness in coloniser success (structure driven by colonisers)
-Coloniser can persist in community until death
Dominance-controlled disturbance
-Competition high, disturbance rare
-Colonisation of openings by pioneer species
-Over time other good competitors become dominant
-Sequence of species = ecological succession
Ecological succession (with disturbance)
Change in community composition + structure (disturbance). Primary succession = original substrate is destroyed, and is on newly formed geological substrate (glacial retreat, volcanic eruption). Secondary = disturbance destroys a community, but the substrate remains intact (forest fire, grazing).
Disturbed habitats often refer back to original habitat
-Pioneer (early successional) community- fast growing, lots of light + high nutrient requirements. Competitive species may be present but not dominant yet.
-Transitional community- intermediate requirements for light + nutrients, trees + other competitive species start to establish.
-Climax (late successional) community- final stable stage of succession, highly competitive, dominant species, mature mixed forest in case of Mt St Helens.
Exploitative interactions
An interaction where one member benefits and the other is exploited.
-Like host + parasite (long-term association where one benefits and the other is killed), (host + parasitoid (long-term, close association between two species in which one benefits and the other is harmed), herbivore + plant (two species in which one weeds on another living organism), predator + prey (one kills and eats the other).
Impact of exploitative interactions
-Increase the complexity of food webs in communities
-Herbivores/predators/parasites can maintain species diversity through disturbance
-Limit pop growth of highly competitive/dominant organisms
-Variable impact of enemies across landscape increases environmental habitat heterogeneity.
Rasher et al. 2020 (climate change, sea otters, killer whales, kelp forests), Estes et al. 1998 = sudden sea otter decline probably due to killer whales. Rasher = sea otter pop collapse which high sea urchin abundance. No otters means larger sea urchins and higher feeding rates which cause bioerosion. Rising sea surface temps also associated with bioerosion.
Landscape level diversity and refuges from exploitation
Refuges from exploitation enemy-free space habitat heterogeneity.
-Spatial + temporal examples of refuges = physical locations, habitat quality, predator satiation, through ecology or landscape ‘of fear’.
Kohl et al. (2018) = GPS radio collar data from female elk and wolves in Yellowstone. Movement of species through a landscape. Estimated time spent by elk and wolves in different locations. There are hotspots of elk and wolf populations, there are patches where elk are more likely to be killed which turn into a ‘landscape of fear’.
Natural/human artificial disturbances
Can be natural processes to which organisms adapt
-Biotic = predation, herbivory, disease, parasitism
-Abiotic = bushfires, floods, drought, landslides
Pollution, urban environments, transport. Anthropogenic disturbance- habitat destruction/fragmentation/degradation, invasive species, climate change, pollution (light, noise, physical).
Examples: Artificial light is disrupting ecosystems and species behaviour, also in the sea (Davies et al. 2020 in Plymouth sound). Mountain paths and roads help the spread of invasive species into mountain areas, also the trampling of slow growing alpine plants. And the spread of disturbance-adapted mountain plants (positive).
Adaptive responses
-Evolutionary history of a species determines its ability to respond to disturbance, adapting to recurring disturbances have traits to survive temporary stressors.
-Problems occur when there are novel disturbances or combinations of disturbances, also when there is a new intensity or frequency of an existing disturbance (for example; wild fires at the moment, and young coral are threatened by noise pollution).
Lack of disturbances
Human actions can also reduce key disturbances in habitats reducing habitat heterogeneity.
-Farming in monocultures
-Under-grazing, loss of keystone herbivores
-Reducing structural complexity (concreting over rocky shores, straightening rivers)
Lundgren et al. 2018 = loss of keystone herbivores. Native terrestrial megafauna richness is low in many regions, consequence of late Pleistocene extinctions (human-driven). Megafauna have been introduced over the world and have replaced extinct megafauna in some places. Some were introduced for rewilding purposes, not agricultural.
How much disturbance is optimal
1.Interactions between disturbance types = disturbance increases the chance of other disturbances occurring (habitat degradation and invasion risk, deforestation and increased flood risk, extreme climatic conditions and invasion risk).
2.Ecological tipping points = situations where a disturbance results in a rapid change in community structure and composition (like algal blooms in previously clear water, dry degraded grasslands turning into dessert, macroalgae-dominated coral reefs).
3.A unifying framework for understanding future impacts = Grahams et al. 2021 identified challenged in defining disturbances, opened a call on social media. Distinguished between disturbance drivers and consequent impacts.
4.Restoring ecosystems through rewilding = balance between leaving nature alone and actively managing it by trying to induce positive changes. Rewilding aims to increase biodiversity and ecosystem resilience by using wildlife to increase habitat heterogeneity. Habitat management aims to protect a particular habitat and its species by inducing human activities to maintain habitat status.
Pest species
Any species humans consider undesirable (compete for food, transmit diseases, interfere with livestock). Like Houseflies (Musca domestica, carry pathogens/annoying).
Non-native/alien species
Grow and reproduce outside of their native range, by deliberate or accidental human activity. Like Sycamore trees (Acer pseudoplatanus, could enhance biodiversity, but could also pose a threat through colonising disturbed ground).
Invasive species
A non-native organism that economically, environmentally or ecologically adversely affect habitats they invade. Like the silver carp (Hypophthalmichthys molitrix, extremely high pops, consume 40x their body weight in a day so detrimental to local ecosystems and fish, North America).
Succesful invaders
Weeds:
-Good colonisers of disturbed ground
-Genetic variation (strong competitors in new enviros)
-Growth and flowering are highly plastic (ability to adapt to conditions)
-Highly fecund with dispersive, dormant seeds (travel fast)
-Vegetative reproduction (spread if you can’t reproduce normally)
-being too toxic to grazers
Round Goby (Apollonia melanostoma)
-Rapid pop spread through Europe, started in black and Caspian seas
-Has since spread through Europe via rivers, also found in Rhine, Scheldt rivers and Baltic Sea. Disperse via canals and ships’ ballasts. High genetic variation.
-Tolerate most habitats, eat others’ eggs, and reproduce successfully (highly refund).
Enemy release hypothesis
Introduced species have been moved outside of native geographic range where they evolved, so they’re freed from normal suit of pathogen/herbivore control.
-A decrease in regulation by natural enemies resulting in increased distribution + abundance in new habitat.
Carpenter & Cappuccino 2005 tested it for 39 non-native and 30 native plant species in Canada. Non-native plants suffered less herbivory than natives
-Most invasive plants suffered less herbivory than plants ranked as less invasive.
EICA hypothesis
(Evolution of increased competitive ability)
-Low defence, fast growing genotypes should be favoured by reduced herbivory.
Joshi & Tielborger 2012 compared plant size + susceptibility to herbivory in native European + introduced north American loosestrife pops. When grown in German conditions in NA the plants grew larger (cause they aren’t getting eaten), but also suffered more herbivory than European plants. Put resources into growth, none into defence.
Invasion meltdown
Process by which the negative impacts induced on native ecosystems by one non-indigenous species, are exacerbated by interactions with another.
Lach et al 2010 = Technimyrmex abipes (invasive ant species) found to guard plants secreting nectar outside the flower which they then eat. When they took the ants away, there was less plant growth and less seed production since herbivores were then stunting them. However, this exacerbated the effects of herbivory.
Habitat loss
Biggest cause of biodiversity loss. 83% of earth surface = transformed by human activity. 60% of earths ecosystems are degraded/unsustainable (MEA 2005).
-Loss of niche space particularly affects habitat specialists
-Fragmentation isolates pops (inbreeding reduces fitness + resilience)
-Lost ecosystem service provisions (reduces provisioning, cultural + regulating services)
35% of global mangrove habitat loss since 1980, vital for coastal protection, sediment capture (pollutant), nutrient cycling, endangered species, fish nursery areas. 25% loss of tropical rainforests, 30% seagrass loss, 25% sand dune loss (worth around 3.8 billion each).
Pollinator loss
Major bee decline
-Disease; colony collapse disorder
-Pesticides; neonicotinoids
-Habitat loss; shifts in agriculture
Farming methods
After world ward II, maximised farming for decades.
-Hay to silage = loss of flower rich meadows
-Larger arable fields = hedgerow loss
-76% reduction in important bumblebee forage
Habitat specialists are most disturbed by habitat change; the biggest bee loss was in niche groups of specialist bee species. Even if they survive land use change, may not survive pollinator loss.
Loss of flower meadows meant specialist flowers declined since the specialist bees weren’t there, whereas common, generalist flower species remained (lower quality pollen).
Mangrove loss
-Conversion to aquaculture + rice cultivation drives most change
-Fragmentation of mangroves reduces wave attenuation = increased erosion = failure to keep up with SLR, reduced storage.
-Distances between fragmentation affect migration corridors + patch
-Loss and fragmentation simplifies ecosystems, reducing habitat value. Density and complexity of vegetation affects refuge value of a given environment.
Relating examples
- Glazner et al 2020 artificial vegetation to create stimulated habitat (crab vs shrimp).
- Huh et al 2023 humming bird and fragmentation, higher species richness positively correlates with canopy openness and mid-sized trees.
- Mosxley et al 2021 loss of kelp habitats and risk of shark attacks on sea otters.
Elephants keystone roll
African Savannah is a mixture of open grassland and Acacia thickets, maintained by fire and grazing.
-Acacia seeds dispersed in elephant dung; saplings usually killed by fire/herbivores. Severe grazing exhausts forage/fuel for fires. Ungulates forced to migrate to look for other grazing spots.
-Without ungulate (hoofed) herbivores, acacia grows fast and seedlings emerging from dung grow quickly, soon able to withstand fire and herbivory.
-Aged thickets can develop enough to promote shade-tolerant grasses beneath.
-Elephants quickly destroy acacia trees by eating leaves and pushing plants over, this means they are destroyed and light-demanding grasses come back and further suppress regeneration.
Elephants influence Savannah vegetation, and thus affect the abundance of light vs shade-demanding grasses. This consequently changes the way in which ungulates use savannah. They avoid thickets to reduce the risk of ambush (= less grazing within thickets).
-Despite directly influencing plant community compositions, they also influence predator-prey interactions. We know most extinct megafauna roles included moderating plant competition, but we could never be sure of other roles.
Problem: increased elephant pops in small reserves which is having a negative effect on vegetation. The higher the elephant density the more likely the reduction in landscape heterogeneity.
Wolves
Driven to extinction in Scotland in 17th century
-High deer density now prevents reforestation and reduces bird abundance, deer stalking doesn’t really control the pop
-Simulations suggest reintroducing wolves would generate conservation benefits by lowering deer densities
Wolves hunted to extinction in Yellowstone National Park in 1920’s
-Increased herbivore (elk, also known as red deer) = more pressure on native trees (Aspen), which changed habitat quality
-They returned in 1996, with immediate effect on elk pops and foraging. This increased aspen hight (most of all along rivers).
-More beaver and bison too due to reduced elk competition and more foraging availability, birds did better too due to more leaf cover and increased tree growth (good example of trophic cascades).
Rewilding
Quite controversial
-To re-establish some of now extinct megafauna
-To try and get back to the true ecosystem functions
-But habitats and evolutionary trajectories can only be established by re-introducing keystone species, but original species are available so we end up using proxi species (which can be controversial, like moving African Elefants to south America, Caro 2007).
Reintroduction (Tule Elk)
Less controversial
-Reintroduction had biggest influence on annal species of vegetation
-They reduced the abundance and biomass of an exotic grass species (Holcus lanatus)
-Some native species benefit, but some don’t. Its almost impossible to ecologically predict how re-introduction will affect the environment.
Pleistocene overkill
Huge loss of large animals, impact on ecosystem processes with significant ecological repercussions
-Megafauna still thrive in Africa, thought to be due to Hominid evolution within Africa amongst animals, this habituated the animals and made them aware of hunting earlier
-Only 2/44 sub-Saharan genera have gone extinct whereas north America has lost 33/45, south America has lost 46/58
Megafauna
Globally common until 11,000 BP due to hunting and mass extinctions. Megafauna used to generate considerable environmental heterogeneity by consuming a lot of vegetation, disturbing the soil, redistributing nutrients, affecting behaviour of other animals. Mega carnivores (species over 1000kg) and mega herbivores have reduced considerably across all continents. However, they fared much better in Africa than any other continent.
Floral systems vs less nutrients
Most diverse floral systems are associated with less nutrients (Western Australia, south Africa). High alpha diversity (species) and beta diversity (turnover) changing soils, frequent fire, long evolutionary history).
-But they are on low nutrient soil, deficient in nitrogen, phosphorus, molybdenum, copper with high porosity and leaching.
Park Grass Experiment 1856
-Divided into plots with varying nutrient levels
-Vegetation responds to increased fertility
-At higher levels of nitrogen, the biomass increases too but there is a reduction of species richness.
-Communities are characterised by species with low productivity environments, at higher nutrient, these species are lost.
-At low fertility, superior competitors are unable to dominate (coexistence)
-Under high nutrient conditions, the strongest competitors dominate and eliminate inferior species
-Plant hight and size increase, diminishing available space and light.
Nitrogen deposition
-Resource availability significantly alters the nature of the plant community
-Low nutrient = more diverse plant communities
-Increase of nitrogen has a negative effect on diversity and species richness
-Seen in Park Grass experiment: where size and hight dominate the environment leaving no room for diversity
3 ways for N to become usable
Incorporated into biological systems, otherwise unusable
-Biological N-fixation = nutrient cycling (most valuable ecosystem service)
-Mineralisation = lightening, weathering or decomposition
-Atmospheric decomposition = unimportant in natural systems
Synthetic nitrogen
-Haber-process = converts atmospheric nitrogen into ammonia, this allowed us to create synthetic fertilisers
-However, the run off from these has increased, especially into aquatic systems
-There is now an excess of manmade nitrogen sources, instead of it being processes naturally
Became very popular after industrialisation.
Cultural eutrophication
Manmade nitrogen runs off into aquatic systems, affecting trophic levels, algae and organisms.
-A combination of surface fertiliser run off and sewage outfall caused mass eutrophication in the Norfolk broads, but this is also a global phenomenon (both marine and freshwater problems, and not just aquatic)
-This lead to an increase in algal growth and a reduction in freshwater vascular plant abundance and diversity
-The increased nutrient means macrophytes dominate, but epiphytes increase
-Then there are peak nutrient levels where there is also a peak in phytoplankton.
Eutrophication effects on ES
-Reduced macrophyte abundance removed refugia for the zooplankton
-Increased predations (from fish, which increased the fish pop)
-Then zooplankton populations crashed and this lead to a decrease in fish populations, less grazing of phytoplankton
5 impacts of N
Bobbink et al 2010
-Direct N gas toxicity which is locally important
-Toxic effect of NHx compounds
-Acidification and an increase of toxic metals
-Impact on plant growth and competition (more of a regional problem)
-Disruption of interactions including herbivory, pollination and mycorrhizae.
Throop and Lerdau 2004 = allocation
-Plant quality may change, increased N and amino acid content in foliage
-Change in carbon allocation or allocation to secondary metabolites
-Changes in C:N ratio may reduce allocation to C-based secondary metabolites (less defence)
-Potential for increased allocation to N-based secondary metabolites (more defence).
Community level effects
-Changes in host plant quality could ultimately knock on to selective impacts they have on other plant species
-This could cause a shift in herbivore community compositions, which could further impact species richness and ecosystem function, affecting herbivore diversity
-Increased herbivory results in faster nutrient cycling, one of the consequences of increased N deposition from atmosphere could be shifts in herbivory which just cause faster cycling of N within that same ecosystem.
Adaptive radiation
The diversification of a species into new species, to accommodate to new environments or to explore ecological niches. Important for evolution.
Intermediate disturbance hypothesis
There is an optimal level of disturbance for ecosystem function and biodiversity. Low disturbance leads to competitive exclusion and high disturbance leads to lower diversity.
Would have a large impact on communities with limited resources, and little effect on high disturbance-adapted ecosystems.
Causes of global ice ages during last 3 million years
Last 3 million years refers to the Pleistocene epoch.
- Milankovitch cycles: eccentricity, obliquity and procession (shape, tilt and wobble of orbit). This changes the intensity of the seasons.
- Greenhouse gasses (CO2, CH4): low GHG enhance cooling and ice expansion + albedo effect.
- Panama Isthmus: tectonic plates closed the Isthmus, isolating the Pacific from the Atlantic. This strengthened the Gulf Stream, and stopped warm water circulation, disrupting evaporation and precipitation. Enhancing cooling.
- Volcanic eruptions: released sulphate aerosols into the atmosphere which reflect sunlight and enhance cooling, these reinforce glacial periods. Volcanic activity also changes GHG compositions.
Ice age impacts on global patterns of biodiversity.
- Impacts and population bottlenecks: More extinctions in northern hemisphere were species had to adapt faster and to colder conditions. Mostly megafauna (woolly mammoths, giant ground sloths). Populations surviving extinctions faced bottlenecks, reducing genetic diversity.
- Refugia hypothesis and allopatric speciation: Glacial spread reduced habitats and compressed biomes. Forests shrank leaving only refugia (hospitable habitats). Isolation led to allopatric speciation (evolve to new species) like arctic fox.
- Rapoports rule: Narrow range endemics found mostly in tropics since species migrated to lower latitudes to escape glacial advantages. Post-glacial recolonisation of refugial areas and range expansions. Fewer endemics at higher latitudes.
- Marine ecosystems: Low sea levels exposed continental shelves, bridging continents opening them to species migration. Ocean circulation changed. Coral reefs came.
Life history strategies
Adaptations of an organism that influence aspects of its biology, or the pattern of allocation of resources for maintenance, growth, and reproduction throughout life.
2 major strategies: Pianka 1970
- R-selected species (short lifespan, early reproduction, high fecundity, minimal care. Unstable environments. Mice, insects.
- K-selected species (long lifespan, late reproduction, low fecundity, high parental investment. Stable, resource-limited environments. Elephants, whales.
Life history traits
- Number of offspring, time of reproduction, sex ratio
- Size at birth/maturity
- Growth rates, lifespan, mortality
Darwinian fitness
Lifetime reproductive success of an individual, selection for optimal combinations of life history traits that increase Darwinian fitness.
Darwin-Wallace demon
Reproduces after birth, infinite number of offspring, immortal. Closest thing is probably duck weed.
Constraints: energy is finite, environmental stability.
Cost of complexity
Evolving traits to optimise function often leads to compromise. Examples: long-lifespan in k-selected species increases vulnerability to unpredictable disturbances.
Selection pressures
Pressures act opposingly, which prevents the optimisation of traits. Example: predation which suggests earlier reproduction, resource competition selects for later reproduction.
Energetic constraints
Energy is finite. Example: the metabolic cost of producing gametes, care, immune defences. Inherent limitations.
Environmental variability
Different environments favour different strategies.
- unstable = suits r-selected species (low investment, high reproduction)
- stable = suits k-selected species (high investment, low reproduction)
Trade-off studies
Walker et al. 2008: trade-off between annual fertility and body size observed in mammals, non-human apes, most primates.
Lazaró and Larringa 2018: trade-off between offspring size and offspring numbers of seeds of two Scandinavian plant communities.
Mola mola: reproductive allocation vs offspring survival. Low chance of encountering conspecifics, long life with high reproductive output. 300 million eggs.
competition
Two individual organisms use the same limited resource (water, nutrient, light, prey) and have a negative impact on one another.
Interspecific = shared resources between species.
Intraspecific = limited resources between individuals of a species.
Trewby et al. 2008 (interspecific comp)
Interspecific competition between European badgers and hedgehogs. How competition for habitat and resources affects the distribution and abundance of these species.
Active competition through direct interactions; found badgers are dominant competitors due to size and aggression. Leads to spatial displacement of hedgehogs.
Current competition is indirect for shared resources, found they share similar prey but badgers have a broader diet and forage more efficiently. Higher badger density since hedgehogs have reduced access to resources (badger presence has negative effect on hedgehog abundance/distribution). Badgers outcompete in both active and current competition.
Implication is that competition shapes interactions and influence population dynamics. Show key evidence for the role of competition in determining habitat use and coexistence.
Adler et al. 2018 (competition in plant communities)
Understand how competition interacts with other ecological processes (filtering/disturbance) to influence community composition and coexistence.
Found plants actively compete for limited resources, fast growing plants dominate in nutrient rich environments (competitive exclusion).
Found current/indirect competition comes from the depletion of resources over time.
Concluded that competitive outcomes are influenced by trade-offs, no single species was able to dominate due to the trade-offs. Competition is weaker in high stress environments with frequent disturbances. Coexistence is promoted by stress tolerance.
Implication is competition is important in shaping communities, effects are context-dependent and influenced by abiotic factors/specific traits. Competition has to be integrated into other processes.
It is fundamental in plant community assembly but interacts with other species to maintain biodiversity. Trade off’s prevent competitive exclusion.
Levine & HilleRisLambers 2009 (competition in stabilising processes)
Mechanisms of species coexistence, how competition interacts with stabilising processes to maintain biodiversity.
Found species with similar ecological niches experienced intense active/direct competition for resources. Dominant species suppress weaker species (competitive exclusion) when stabilising mechanisms are absent.
Found current/indirect competition is mediated through shared resource depletion which leads to a cascading effect on community composition. Temporal/spatial variation in resources can lead to indirect competition.
Stable environments promote coexistence and competitive exclusion is avoided. Competition will never fully explain biodiversity, environmental heterogeneity plays a large role In coexistence by modifying the outcomes of competition.
Competition is a critical force in determining the structure of plant communities.
tropics support more species than temperate regions
- lower extinction rates (no glaciation)
- larger habitats (increased opportunity)
- longer evolutionary time (climatically stable)
- warmer climate (more stable = higher speciation)
- more nutrient rich (more biotic interactions, co-evolution)
Southern temperate habitats more species rich than northern temperate equivalents
south:
- large refugia
- less glaciation (less severe/frequent)
- surrounding oceans (less seasonal variation)
- continents used to be more connected
- geographic isolation (higher rates of endemic species = reduced gene flow, more speciation)
Why are there so many species in the tropics?
- higher metabolic rates and faster mutations (stable/warm climate)
- longer periods of evolutionary stability = increased species richness
Speciation, ecological opportunity and latitude
- temperate zones experience higher per capita species rates (due to ecological opportunities)
- higher biodiversity (due to larger areas, longer history)
- temperate regions have short bursts of speciation (topic have long-term accumulation)
Species richness in north-temperate forests
- southern temperate has a greater variety of climates
- disparity between species richness in N/S
Biological diversity in N/S hemispheres
- no difference in elevation range breadth
- climate heterogeneity in S
- N had higher species loss during glaciations
- fewer dramatic climate shifts in S
Case study: Facilitation/Competition in ecological succession
Callaway and Walker 1997 (Glacial Forelands)
- examined plant interactions in glacial forelands when primary succession occurs as glaciers retreat, leaving land for colonisation.
Facilitation = early-successional species like mosses + n-fixing plants (Dryas spp.) facilitate establishment by stabilising soil, increasing organic matter, enriching nutrient levels.
Competition = as environment becomes more hospitable, vegetation density increases so resource competition begins to dominate. Late plants like shrubs + trees (Alnus spp.) grow taller, shading out colonisers.
Bascompte et al. 2019: Facilitation/Competition
In drylands, pioneer species facilitate the establishment of other species by imposing microhabitat conditions.
Facilitation: small shrubs can reduce soil erosion and increase nutrient levels, and provide shade.
Competition: as water/nutrients become scarce, competition between plants intensifies. Later species outcompete pioneer species.
Influence biodiversity + ecosystem stability.
Facilitation
The gradual replacement of species in a community over time. When one species provides another with a favourable habitat, which influences the distribution of that species.
In successional communities; dunes = fast growth, acrtic/coast = harsh environments.
Pioneer species often modify environments for later species. More prevalent in harsh environments. Establishes competitive species and persistence in later stages.
Competition
Occurs in later stages of succession as resources become scarce. Later species outcompete pioneer species (local extinction).
Example of Facilitation
Cushion plants increase species richness in the Andes. Marram grass stabilises sand dunes and allows colonisation by other species.
Ecological succession
Change in community composition and structure over time since disturbance.
Primary = original substrate is destroyed, and is on newly formed substrate (glacial retreats, volcanic eruptions).
Secondary = disturbance destroys a community, but substrate remains intact (forest fire, grazing).
Community structure
- A set of species that co-occur at a particular place and time, normally with distinct trophic levels.
- Individual patches in a regional pool of species.
- Controlled by by predation, competition
Assemblage
A subset of organisms in a community, by taxonomic group or function.
Generalist predators
Broad diet, can consume a wide variety of prey species, rather than specialised hunting for a few.
- diet diversity
- flexibility with prey
- widespread distribution
- regulate prey populations
Like; foxes, spiders, sea otters, lion fish.
Schmitz et al. 1997
Behaviourally Mediated Trophic Cascades: Effects of Predation Risk on Food Web Interactions.
- how spiders influence grasshopper + plant communities through direct predation + behaviour mediated effects.
- generalist predators reduce herbivore populations + alter their behaviour, indirectly promoting biomass.
conc = generalist predators can regulate herbivore populations, reducing grazing pressure, to maintain plant community diversity.
Generalist predators can have cascading effects throughout a community, influencing structure and function.
Estes et al. 1995
Sea otters and kelp forests in Alaska: Generality and Variation in a community ecological paradigm.
- effect of sea otters on kelp forests by preying on urchins.
- removal of sea otters led to an explosion of sea urchins, which in turn overgrazed kelp forests. Causing a dramatic decline in biodiversity and habitat complexity.
- generalist predators play a crucial role in regulating herbivores and maintaining ecosystem function.
conc = sea otters are critical in preventing herbivore over-population and ensuring the persistence of diverse and productive ecosystems.
Crooks et al. 1999
Lag times in population explosions of invasive predators.
- the effects of invasive generalist predators like feral cats/rats on island ecosystems.
- caused a decline or extinction of native bird species, leading to significant shifts in community structure and function.
- loss of native species disrupted seed dispersal and nutrient cycling, highlighting the widespread impacts of generalist predators
conc = generalist predators dramatically reshape community structure with population declines and extinctions, altering ecosystem processes.
Emer et al. 2020
Mutualism increases diversity, stability, and function of multiplex networks of plant-pollinator interactions.
- Mutualistic interactions enhance species diversity and contribute to the stability of communities.
- improve pollinator efficiency and plant reproduction.
- plants rely on specific pollinators for reproduction, and vice versa. Mutual dependency = higher biodiversity.
- the interconnectedness of species makes a more robust and resilient community.
mutualism in conservation
Functional redundancy = plants with a diverse range of pollinators are less susceptible to the decline of a single pollinator species. They overlap, portfolio effect.
Preserving mutualism is crucial in conservation, otherwise exploitation could have a cascading effect.
Phillips and Shine 2006
Niche proportionality and the role of intraspecific rich variation in structuring a guild of generalist anurans.
- different frog species coexist by utilising different resources or habitats, reducing direct competition.
- variation is important since it can lead to niche differentiation, facilitating coexistence.
- even generalist species can make use of niches to minimise competition, by diet or habitat use.
- interspecific niche partitioning and intraspecific variation play a crucial role in allowing the frogs to coexist.
Differentiation into niches reduces competition and promotes biodiversity.
Reznick and Ghalambor 2019
Effects of rapid evolution on species coexistence
- how swift evolutionary changes can influence interactions
- interspecific competition drives rapid trait divergence, reducing niche overlap and facilitating coexistence.
Predation
Predator benefits, prey has a negative outcome.
- true predators = kill prey immediately
- grazers = consume parts of prey, but don’t kill
- parasites = feed on hosts, kill isn’t immediate
Frequency-dependent selection
Keeps prey populations roughly similar, maintains diversity since a species is not overly exploited or made extinct.
Clarke 1962
Apostatic selection = target the most common prey morphs.
- selective predation maintains genetic diversity within prey, also gives a survival advantage to rarer prey.
- frequency-dependent selection acts as a balancing force.
Janzen 1981
- seed predators favoured the most abundant seed species (frequency-dependent selection).
- predation pressures maintained seed diversity, by preventing a species from becoming overly dominant (no monopolisation).
Pfennig et al. 2001
- avian predators exert frequency-dependent selection on salamander populations with varying colour morphs.
- rare colour morphs have a survival advantage since predators form search images for more common morphs.
- this showed negative frequency-dependent selection, by promoting and enhancing colour polymorphism in salamander populations.
- but still prevented one morph from becoming overly dominant.
Effects of little disturbance (light grazing)
- uniform vegetation growth, a monotonous habitat with less variation = could reduce microhabitat availability
- allows competitive plants to dominate (no FDS), reducing plant diversity so there are fewer niches to occupy.
No disturbance (no grazing, short/long-term)
short-term
- competitive species would dominate, species diversity would decrease.
- less poo = less nutrient cycling + seed dispersal.
- reduced flowering = change in vegetation structure, microclimate change.
long-term
- woody encroachment = shrubs + trees start to dominate
- build up of dead plant material (thatch), reduces O2 (fire hazard).
- plant species could decline due to competitive exclusion.
- no turn-over/trampling, decreases soil quality, less germinating.
- some plants depend on grazing for seed dispersal or reduced competition, may become extinct.
Seaweed negative impact on coral reef diversity
- competition for space and light
- some seaweeds produce allelochemicals which could inhibit coral growth, or kill coral tissue
- could prevent coral larvae from settling
- sediment that seaweeds trap could smother coral, or reduce water quality which would impair photosynthesis
- less complex habitat structure which also reduces habitat availability and overall biodiversity.
ecological drivers for higher diversity in tropical communities
- larger geographical areas = larger habitat variation
- warmer climate = more stability
- more sunlight = higher primary productivity, more growth/nutrients
- niche specialisation = developed to exploit niches, to avoid competition
- intense biotic interactions (herbivory, predation, mutualism, competition)
- intermediate levels of disturbance, optimum for coexistence and colonisation, preventing dominance
evolutionary drivers for higher diversity in tropical communities
- higher speciation rates = stable climate, populations evolve without disturbance
- lower extinction rates = species accumulate
- longer evolutionary history (could be origin for many species -> endemic species)
- faster metabolic and mutational rates = faster generational turnover
- isolated landmasses (Pangea, then Gondwana and Laurasia), less severe/frequent glaciations
