habitat loss and fragmentation
what is a habitat?
a habitat is defines as the area or environment in which an organism lives, shped by:
physical factors: nutrients, temperature, light
biotic factors: food availability, competition, predators
marine habitats support marine life and include both open ocean habitats and more structured benthic coastal habitats
the open ocean is described as shifting and ephemeral compared with habitats on land
coastal marine habitats
coastal habitats extend from the shoreline to the edge of the continental shelf and include:
rocky reefs
sediment bottoms
biogenic habitats (habitats created by living organisms)
these habitats have
high productivity
complex seascapes with high biodiversity
ecosystem engineers → marine organisms that reshape the environment to create or maintain habitat for other species
ecosystem services → coastal habitats provide food, shelter, nursery areas, and many other services to ecosystems and humans
why is habitat loss so important?
slides show that marine defaunation has been slower than on land, but is now accelerating
habitat loss is highlighted as a primary emerging threat to global marine ecological function
habitat loss vs fragmentation vs degredation
habitat loss → a decrease in total habitat area when habitat is significantly compromised or eliminated
habitat fragmentation → the breaking apart of a large, continuous habitat into smaller, isolated patches
two components:
reduction in area
reconfiguration
habitat degredation → a decline in habitat quality
biological consequences
effects of habitat loss:
decline in species number and abundance
altered species interaction
fewer specialists and large bodies species
lower breeding, dispersal, and foraging success
effects of habitat fragmentation:
small patch size effects
lower habitat quality
edge effects (strong contrasts at habitat boundaries)
sometimes fragmentation can increase biodiversity
island biogeography concepts (source-sink dynamics)
reduced isoloation of population
habitat loss has stronger negative effects on biodiversity than fragmentation
drivers of habitat loss and fragmentation in coastal zones
natural events
hurricanes, typhoons
tsunamis
climate variability
direct human actvivities:
dredging
pollution and run off
destructive fishing
oil spills
tourism
climate change:
temperature rise
ocean acidification
extreme weather events
sea level rise
human pressure is especially intense in coastal zones
60% of the world’s population lives within 100 km of the coast
20% of coastal systems are highly modified
climate change and habitat shifts
slides emphasize that life is redistributing becasue of climate change
coastal change today is faster than at any time in the last 100 million years
species often shift ranges to stay within suitable environmental conditions
this leads to changes in the distribution of marine habitats
three outcomes of range shifts:
borealisation - temperate species move poleward, squeezing unique arctic habitata
tropicalisation - tropical species expand into temperate zones
novel habitats - new combinations of species appear in the tropics
case study: kelp forests
kelp = ecosystem engineers and form large marine forests
they create stucture, alter currents, waves, light, produce biomass, provide fisheries, coastal protection, and carbon recycling services
global trends:
38% of kelp firest declines in the past 50 years
58% declines for long term records
shifts to turf algae, urchin barrens, or replacement by other algae
warm southern ranges are shrinking
in the arctic, kelp may be expanding due to warming
the rise of turf algae
turfs: small dense algae with fast growth, high coverage, and sediment trapping ability
these low strucutre habitas are replacing kelp over thousands of kilometers of coastline
drivers include:
wamring, grazing pressure, nutrient run off changes, and extreme events
example: norway
we all know the dramatic loss of the kelp forests in norway
mid and north norway: urchin grazing
west coast: 40% of saccharina latissima lost
skagerrak: 80% lost
NM coast still pristine
example australia:
a 100 km contraction of kelp rang; 43% dissapeared
drivers strongly linked to wamring, storm damage, grazing, competition, and pollution
feedback systems
positive feedback loop make turf states self-reinforcing and hard to reverse
solutions and restoration
addressing the cause:
climate change mitigation
reduce fishing pressure
reduce runoff and pollution
restore habitats:
artificial reefs
reseeding
turf vacuum concept
labour intensive kelp restoration
green gravel: seeding kelp on pebbles that can be deployed at scale
ecosystem based management
management should be knowledge based, ecosystem based, adaptive
also warns about easy restirction syndroe - simplistic, ineffective restrictions
the papers - fahrig (2003) & Filbee-Dexter (2018)
habitat loss - reduction in the amount of habitat
fragmentation - breaking a continuous habitat into smaller pieces
habitat loss produces strong, consistent, negative effect on biodiversity
fragmentation per se produces weak effects, which can even be positive
in some cases, breaking habitat into smaller patches promotes coexistence by creating refuges or asynchronous dynamics
the global collapse of kelp forests and their replacement by truf algae is an exmaple of habitat loss and fragmentation
their loss represents a massive decrease in habitat quantity and quality, exactly the kind of change fahrig identifeis as most damaging
their replacement by turfs causes:
lower structural complexity, more sediment retention, less canopy and shelter, and less biodiveristy
blobal drivers of habitat loss:
ocean warming, marine heatwaves, and increased storms
local drivers of habitat loss:
overgrazing by herbivorous fish and urchins, eutrophication, sediment loading, invasive species
these processes remove habitat, leading to biodiversity declines consistent with fahrigs findings on habitat loss having string negative impact
fahrig emphasizes that fragmentation is often not harmful, and may help species coexist
in the kelp case however, the situation is different:
kelp forests dissapear over large, continuous stretches
they are replaced by turfs
the total area of high quality habitat (kelp is reduced
the remaining kelp patches are small and isolated
this is true habitat loss, not just fragemntation
once kelp patches shrink, remaining patches face more stress, turfs invade gaps, and feedback loops lock the system into a degraded state
the damaging part i the loss, not fragmentation per se
habitat loss reduces species richness, population abundance, genetic diversity, breeding success, and dsipersal success
loss of kelp forests results in collaspe of associated species, reduced fish, invertebrates, grazers, predators, lower habitat complexity, lower seocndaty production, and loss of eocsystem services such as carbon storage, nutrient filtering, coastline protection, and food resources
loss of habitat forming kelp canopy produces exactly the biodiversity declines Fahrig predicts
turf reefs trap sediment, suppress kelp recruitment, and are reinforces by warm water and grazing, making recovery difficult
these stabilizing feedbakcs refelct Fahrigs broader point that the configuration of habitat affects population dynamics, but hte main declines occur once habitat quantity drops below thresholds
both the papers highlight the need to understant drivers and mechanisms
fahrig:
manage habitat loss as the primary threat
study fragmentation separately
filbee-dexter:
reduce local stressors (nutrients, pollution, overgrazing)
improve water quality
restore kelp using active interventions (e.g. adult trasnplants)
understand thresholds for recovery