NATF320 L13 Habitat loss fragmentation

land-use change

main driver of biodiversity loss

LULC trends across south america

decrease in tree cover, increase in cropladn, especially in argentina and brazil, increase in pastures, especially in brazil

Land use change effects

habitat loss

habitat fragmentation

habitat degradation

habitat loss

reduces the amount of area available for a species

• decline in the abundance of that species

• species can go extinct

  • decrease in the number of species

habitat loss → forest loss

the net toal forest cover in several countries in Europe/US changed little or increased in past decades, due to regenartion (secondary forests) or tree plantations

→ deforestation to tropics

habitat fragmentation

modifies the spatial configuration of the remaining area in the landscape

influences the population dynamics of a species, e.g. decreasing the dispersal among habtiat patches

indirect effects on species, e..g habitat fragmentatino increases human pressure as forest fragments become more accessible, thus increasing hunting pressure or fire risk

three main components of habitat fragmentation

edge effect

patch size

isolation

a fragmented landscape contains more, smaller and isolated patches and a larger proportion of edge effect

how to measure habitat fragmentation

patch-scale study

ladscape=scale study

patch-scale study

predictor variables are spatial attribtues of individual patches (e.g. patch size, patch isolation)

landscpae-scale study

predictor variables are spatial attributes of indivual landscapes (e.g. percentage of habitat, number of patches)

Species-area relationships (SAR)

relates the number of species to the area of habita

the result of plotting the species richness (S) of a particular sample against the area A of that sample

a linear regression estimates the relationship between S and A

species-area data are typically nonlinear → log log transformation, the steeper the slope of the line (i.e. greater Z value, the greater the difference in species richness among sampling areas

S=C+z*A

S number of taxa encountered

A area sampled

C intercept of the line

z slope

log: S=C*A^z

Species-area relationships

two main reasons to explain the increae in the number of species

1. as more area is sampled, the chance of encountering additional species increases

   1. a larger are is likely to be more environmentally heterogenous, thus containing additional species that differ in their niches

Theory of Island biogeography

the number of species on an island is related to the area of the island and the degree of isolation of the island

smaller, more isolated islands have fewer numbers of species

equilibrium thoery of island biogeography

the number of species on an island depends on the balance between immigration and extinction rates

as species richness increases the rate of immigration decreases but extinction increases due to competition and predation

the point at which the immigration and extinction curves intersect predict the equilirbium number of species on the island → the number of species that should theoretically fit on the island

habitat-fragmentation - patch size and isolation

Brazilian amazon forest

the effect of isolation on local extinction is mostly positive, but for one-third of the species, slope estimates are very close to zero, increase in isolation, increase in extinction

negative effect of patch size on local extinction, increae in patch size, decrease in extinciton

matrix

the non-native habitat between fragments of original habitat

case study-atlantic forest

how does the quality of the matrix affect habitat loss filtering

bird occurences were in general higher in high-quality matrices

low-quality matrices increased the severity of habitat loss filtering effects for forest specialist birds → higher extinction proability due to habitat loss in landscapes with loq-quality matrices

edge effects

the result of interaction between two adjacent ecosystems, when the two are separated by an abrupt transition (edge)

how do edges modify the environment

changes in microclimate (e.g. compared to a forest, crops and pastures allow more solar radiation to reach the ground during the day)

chemical fertilizers from adjacent croplands penetrate inside the forest

changes in forest structure (more light, better growing conditoins for grasses)

physiological tolerances (some forest plant species how lower densities or are absent near the edge, while others show higher densities)

changes in species interactions (e.g. highest nest predation near edges)

human disturbances (e..g higher hunting pressure along edges)

edges

nearly 20% of the worlds remaining forest is witin 100m of an edge

morre than 70% of the worlds forests are within q km of a forest edge

the largest continuous forest are in the humid tropical regions of the amazon and congo river basins

forest strips

occur in agriculatal landscapes in tropical and subtropical regoins

have been dictated by government polciies to benefit agricultura and with the assumption that these strips maintain biodiverisyt (increase connectivity)

in northern argentine environmental norms require that forest strips (at least 100 m wide) are left surrounding agricultal plots (23-37 ha of forest strips for every 100 ha of deforested land)

species do not really use the strips

fragmentation

refers specifically to an increase in the number of pathces, which often accompanies habitat loss

habitat fragmentation per se

refers to the effects of fragmentation after taking account of, or in the absence of, habitat loss

habitat degradation

reduces the quality of the habitat, while maintaining the amount of habitat area and the spatial configuration of the former landscape

forest degradation

no reduciton of forest area, but lower quality

drivers:

• selective logging, charcoal production, fires, overgrazing

  • often more difficult to characterize with remote sensing

habitat degradation - tropical rainforests

tropical forests vary in compositon, structure and function such that not all forests have similar ecological value

we often focus on forest cover and overlook anthropogenic pressures that can degrade forest quality and impact biodiversity

time delay to extinction

time taken for a community to reach a new equilibrium after landscape transformation (time taken for a specis to go extinct after landscape transformation)

extinction debt

the number of eextant speceis predicted to go extinct due to past land-use change, but which have not yet gone extinct because of time delays

extinction debt in the gran chaco

habitat amount crucial for birds and mammals → they respond immediately to habitat loss

time-delayed response to habitat fragmentation → the effects of habitat fragmentatino magnify over time

duration of the time-delayed response longer for birds than for mammals → high hunting pressure for mammals

Indirect effects of habitat loss

habitat loss can cause some extinctions directly by removing all individuals over a short period of time, but it can also be indirectly responsible for lagged extinctions by

• facilitating invasions

• improving hunter access

• eliminating prey

• altering biophysical conditions

• increasing inbreeding depression