Notes

Lecture 15. Communities and the Landscape
April 7, 2005
Principles of Landscape Ecology (FEM 565)
Organisms and Landscapes
There are obviously many implications of landscape pattern to various levels of species
organization. We will consider how landscape pattern affects organisms and populations
– especially how landscape pattern affects the spatial structure of populations and how
this translates to affects on population dynamics.
1. Landscape effects on organisms and populations
To any given organism, landscape heterogeneity implies spatial variability in the
distribution of resources (food, water, cover) and environmental conditions
(microclimates). These patterns may change over time in response to environmental
changes (e.g., global climate change) and disturbance regimes. The heterogeneity in these
factors translates to variability in the distribution of suitable habitat to an organism, with
some habitats being more suitable or of higher quality and therefore conferring greater
fitness than others. In addition, the dynamic nature of landscapes suggests that there is
spatio-temporal variation in the distribution of suitable habitat across landscapes.
Heterogeneity of suitable habitat may influence the behavior and habitat use
patterns of organisms – which will ultimately influence population dynamics and
persistence.
1.1 Individual behavior and habitat use patterns
At the individual level, heterogeneity in the distribution of resources can affect
individual behavior and spatial activity patterns. Heterogeneity may also affect an
organism’s ability to acquire resources needed to survive and reproduce. Patchy resources
may be more expensive to acquire than contiguously distributed food resources. Moving
between food patches may expose the organism to higher predation rates (see below).
Thus, patchiness can affect individual fitness.
1.2 Population structure and viability
At the population level, heterogeneity in the distribution of resources and suitable
habitat limits where individuals can exist and influences the ability of individuals to
interact. As such, heterogeneity imparts a spatial structure to the population that may
affect the population’s ability to persist.
1.3 Interspecific interactions
At the community level, heterogeneity in the distribution of resources can affect
predator-prey and competitive interactions. The general trend of these effects, however, is
not clear. Under some condition, habitat fragmentation may allow for predator-prey
coexistence, or it may positively or negatively affect the prey species. Other studies have
shown that habitat fragmentation may stabilize interspecific competition and allow the
coexistence of very similar species. David Tilman (U MN) suggested that competitors
may coexist in spatially structured habitat due to tradeoffs in colonization ability,
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competitive ability, and longevity. Others have predicted that competitive interactions
are very sensitive to landscape structure, leading to increase in competition as
fragmentation increases.
2. Metapopulation Dynamics
Spatially-structured populations are called metapopulations, which are literally
populations of populations. For example, a series of populations existing in discrete
patches formed by habitat fragmentation may be considered a metapopulation. The
classic model is that if a population is fragmented into a patchwork of subpopulations and
the probability of extinction of the subpopulation remains small, any local extinctions
would be balanced by recolonizations from neighboring subpopulations. Thus, the
process of extinction and colonization is dynamic, but may be regionally stable.
Metapopulations integrate several subdisciplines in ecology, including population
geneticists, conservation biologists, and theoretical population ecologists. For landscape
ecologists, the interest is in identifying when the landscape pattern of available habitat
patches in a landscape has implications for population dynamics or persistence. As
habitat fragmentation becomes more and more of an issue, these cases are of increasing
concern in conservation biology.
2.1 Metapopulation Definitions
Hanski and Gilpin (1991) have defined three scales at which populations work:
Local scale: Individuals interact with each other during the normal course of feeding and
breeding (living). An individual belongs to one local population for most of its life.
Metapopulation scale: A set or constellation of local populations that are linked by
dispersal.
Species’s geographic range: Encompasses all local populations and metapopulations
within it; an individual typically does not move over very much of its geographic range
during the course of its lifetime.
We will consider factors affecting the dynamics of metapopulations in discrete habitat
patches, especially the frequency of dispersal among these patches.
2.1.1 Definitions
Habitat patch - the site where a local population exists.
Local extinction - a population disappears from a given patch.
Recolonization - a local population is re-established by new immigrants.
Population turnover - how often the population is renewed, a function of the balance of
extinction and recolonization events.
Population persistence time - how long a population can be expected to maintain itself,
sometimes expressed as the inverse of its extinction rate.
There are a number of population processes affecting metapopulations. Metapopulation
dynamics reflect the occurrence and rates of local extinction and recolonization as
affected by interpatch movement, and factors affecting these processes.
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2.2 Movement (the Ultimate cause)
Individual movement between patches may be the most important defining feature of a
metapopulation. How far and fast organisms move imposes a scale on the landscape –
meaning a landscape that is perceived differently by a given organism.
Factors affecting movement include:
Patch characteristics: Patch size and shape influence the likelihood that an individual
will encounter the patch boundary within a given time interval.
Patch context: Whether or not an individual will cross a patch boundary upon
encountering it is a function of both the boundary itself (permeability) and the
characteristics of the adjoining patch.
2.3 Factors affecting local extinction
There are four proximate agents of local extinction:
2.3.1 Demographic stochasticity
This is important for populations that fall below a certain threshold size. Here, the
order of demographic processes becomes critical: it might make a lot of difference if an
individual dies before it reproduces or reproduces before it dies--the order of
reproduction and death matters.
2.3.2 Genetic stochasticity
This might include the loss of heterozygosity or loss of fitness due to inbreeding
in very small populations.
2.3.3 Environmental stochasticity
Factors such as weather may result in fluctuations in food supply or habitat
quality, and may act in a density-dependent manner to reduce a local population.
2.3.4 Catastrophies
Any sudden, non-recurring event where environmental stochasticity is extreme
and affects the entire metapopulation, causing local extinctions in a density-independent
(entirely independent of the population structure) manner.
In addition, there are broad-scale threats. These might include habitat loss or
fragmentation, but are not stochastic as above and are often seen as more deterministic in
their action. More discussion during the Landscape and Global Change lecture.
2.4 Factors affecting recolonization
Because recolonization depends on the successful establishment of new recruits in a
habitat patch, the factors influencing recolonization reflect the interplay between lifehistory
traits and connectivity of habitat.
2.4.1 Important recolonization factors: plants
• seed size and viability (often related to size);
• dispersal vector (wind, water, animal);
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• habitat (seedbed) requirements.
2.4.2 Important recolonization factors: animals
• distance between patches
• "resistance" of intervening habitats
• dispersal behavior (preference or avoidance of some habitats, search and
orientation during dispersal, use of stepping stones or corridors).
• mortality rates during dispersal itself, and agents of this mortality.
2.5 Metapopulations in a landscape context
We must also consider the structure and function of the intervening landscape. There are
two contrasting views of the landscape itself:
2.5.1 Island-biogeographic view
Local populations occur in a featureless matrix. Not all patches are occupied at a
given time, and local populations blink in and out as extinction and recolonization occur.
Patches may vary in size and shape, but the probability of recolonization is based mainly
on movement rates and distance between patches.
2.5.2 Landscape mosaic view
Local populations of a metapopulation occur in habitat patches that are
surrounded by a complex matrix of other habitat patches, corridors, boundaries, etc. This
landscape structure affects individual movement patterns among patches, and thus
recolonization. Distance between patches are not Euclidean, but rely on boundary
permeabilities and relative patch viscosities to moving organisms.
2.6 Approaches to (and inferences about) metapopulations
Metapopulations are logistically very difficult to study. Since a metapopulation is defined
by dispersal events that need occur only once every generation or so, these events are
unlikely to be witnessed in the field. This logistical difficulty in defining metapopulations
with field observations places an appreciable strain on the theory, which otherwise is well
developed.
Studies of metapopulations have proceeded in a variety of ways.
2.6.1 Fine-scale
Most empirical studies have dealt with short-lived species, often insects, where
short generation times and fine-scale patterns of habitat use make it easier to observe
metapopulation processes.
2.6.2 Larger-scale organisms
For larger organisms, the processes are inferred from long-term presence/absence
(or abundance) data-- but such data do not provide much information about the source of
colonists. Therefore it is difficult to demonstrate metapopulation dynamics. Recently,
there has been a push to devise ways of testing metapopulation theory empirically, using
data that actually might be collected in a reasonable field study. Some possibilities
include:
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A. Genetic similarity. In a metapopulation, we might expect to see spatial patterns in
genetic similarity of individuals collected in different patches. This requires that the rates
of genetic differentiation be appreciable relative to isolation time, and also requires
species with "well behaved" genes.
B. Percent occupancy, or constancy of occupancy, in habitat patches as a function of
between-patch distance or some other index of isolation. For example, one might use
logistic regression to model patch occupancy (presence/absence) as a function of distance
to nearest other occupied patch.
C. Similarity in demographic rates, or autocorrelation in population levels.
D. Complete long-term census of clusters of local populations. This approach is
logistically difficult as well, but is probably the most reliable way to document
metapopulation dynamics.
2.7 Roles of landscape connectivity
Connectivity refers to the degree to which the landscape facilitates or impedes movement
among habitat patches and, therefore, the rate of movement among local populations in a
metapopulation. If there are abrupt changes in the connectivity of the landscape, dispersal
success may be impeded in a way that suddenly fragments widespread populations into
smaller, isolated populations – which may decrease patch occupancy and lead to
extinction of the population across the landscape. As before, two perspectives of
landscape connectivity have emerged that involve metapopulations. Connectivity can be
achieved in many ways, depending on which perspective is most appropriate:
2.7.1 Island biogeographic view
If habitat fragments are much like oceanic islands in an inhospitable sea or neutral
matrix, then connectivity is mostly a function of size and arrangement of disjunct patches
and the physical connections among habitat patches (corridors).
2.7.2 Landscape mosaic perspective
If landscapes are spatially complex, then heterogeneous assemblages of habitat
that cannot be categorized into discrete elements. Further, the landscape is viewed from
the perspective of the organism, and connectivity is assessed by the extent to which
movement is facilitated or impeded through different habtat types across the landscape.
Other important landscape patterns include:
2.7.3 Patch structure
The size, number, and distribution of habitat patches influences the physical
connectivity of habitat across the landscape (main component of island biogeographic
perspective). When habitat is abundant, connectivity is highly probable. Habitat loss
results in a reduction in the proportion of suitable habitat on the landscape, and
connectivity changes at a critical threshold (percolation theory).
2.7.4 Corridors
Physical corridors may affect connectivity. Corridors: 1) provide breeding habitat
for individuals and thus connect larger population units by maintaining gene flow; 2)
provide dispersal habitat and only facilitate movement between patches; 3) may be
barriers or filters that prevent or impede the movement of organisms across the corridor.
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2.7.5 The matrix
Under the landscape mosaic perspective, the matrix greatly affects movement
across the landscape. Organisms do not restrict their movement to suitable habitat and the
corridors between them, but move throughout the entire landscape looking for suitable
habitat. Since the matrix is a composite of patches with different permeabilities to
movement, it has great effects on connectivity.
2.8 Appropriateness of the metapopulation concept
Although most real landscapes and populations have some sort of spatial structure, when
is it appropriate to understand these as metapopulations – thereby invoking
metapopulation models?
A. When suitable habitat is neither abundant nor widespread.
B. When patch distance are far relative to the movement of individuals.
C. When movement patterns vary greatly among different elements of the landscape
(transition probabilities are unequal and low).
D. When habitat pattern is relatively stable.
Thus, conceptualization of a population as a metapopulation requires a certain type of
interaction between patch structure, individual movements, and local
extinctions/recolonization relevant to the organism of interest. Thus, the appropriateness
of metapopulation models depends on the connectivity of the landscape as perceived by
the organisms of interest. Compare three types of landscapes:
A. Highly connected landscape: Patches are abundant, widespread, and highly
connected and the environment approaches homogeneity (or the organisms treat it as
such). If this occurs at a broad population scale, then metapopulations are unlikely to
develop, and the spatial structure of the landscape might be ignored.
B. Strongly disconnected landscape: Habitat patches are rare and strongly
disconnected and each patch can be treated as separate units. Because patches in this
instance are almost completely isolated, the structure and function of the intervening
landscape might not be that important – and the classic metapopulation model might be
appropriate.
C. Near the connectivity threshold: When habitat abundance and distribution is close
to the connectivity threshold, landscape structure becomes very important, and the
organism interacts with the landscape in a meaningful way to influence movement and
extinction/recolonization rates. In this instance, the landscape mosaic view of
metapopulations may be the most appropriate.
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