Intertidal Ecology in the Gulf of Maine
Large Scale Modulation of Transition Probabilities
In rocky intertidal environments, small scale biological interactions
(e.g. predation, herbivory, facilitation and competition) interact with large-scale gradients
(temperature, productivity, and recruitment) to produce shifting
patterns in community structure. While manipulative experimentation
has shown small
scale interactions to be paramount in controlling the structure and
function of coastal communities, recent work has underscored the
importance of large
scale gradients in modulating those interactions over both space
and time. We are interested in understanding how larger-scale processes shift local interactions which is critical for forecasting the effects of climate change and managing ecosystems.
Our approach involves quantifying transition probabilities (changes in space occupancy) between ecological states (e.g. mussel replaced by barnacle). Transition probabilities are used in many fields to model system
behavior. A Markov Chain Model (MCM) can then be used to predict the path a system will follow (figure 1).
These have been used effectively in marine habitats to predict
local dynamics
in coral
reef, subtidal, and intertidal communities. Although not mechanistic,
MCMs identify the relative importance of various species interactions in
community structure
and dynamics.
Scott Morello is quantifying transition probabilities of faunal
elements for local intertidal communities at three replicate
sites in southern,
mid-coast, and northern Gulf of Maine (Cape Ann, MA; Pemaquid,
ME; Bar Harbor, ME) (figure 2). Community dynamics and structure
can
be simulated
from local transition probabilities using MCMs and compared to
observed communities to test how MCMs capture the critical ecological
processes
influencing intertidal assemblages. The transition probabilities
themselves can also be compared regionally to aid in our understanding
of how
species interactions
shift in response to large-scale gradients (figure 3).
The Role of an Ecologically Important Predator
It has long been known that predators
are major structuring forces in communities, but the
specific pathways through
which this occurs can be complicated. In the 1970s, experiments
identified the carnivorous whelk Nucella lapillus as one
of the primary species
controlling community structure in the Gulf of Maine
intertidal. Using the aforementioned
MCM framework, our research also quantifies transition probabilities
in areas where Nucella lapillus has been excluded to help understand
the specific pathways through which this species effects community structure
, and how well
MCMs predict observed changes.
Figure 1: Transition types in a Markov Chain matrix: Self-self (1) (persistence), Empty to Filled state (2) (recruitment/growth), Filled to Empty state (3) (mortality), Filled to filled state (4) (species interactions).
Figure 2: Replicate locations across the Gulf of Maine
Figure 3: PCA of local transition probabilites shows separation of locations along the first three principle componenets
Figure 4: The Atlantic Dogwhelk, Nucella lapillus, primarily feeds on mussels and barnacles
