The Science

Science Conceptual Models

Ecological Conceptual Models

Ecological conceptual models simply and transparently indicate our current level of understanding of how ecological systems work (including the rainforest ecosystems at Springbrook).

We have selected the following ecological conceptual model at the broadest level to most usefully represent our understanding of ecosystem structure and function as it relates to ecological restoration. It allows us to judge if and when interventions are required, to predict responses to any interventions planned and devise the best indicators by which to measure progress in reaching objectives.

The model is based on that of Worm and Duffy (2003) and addresses drivers and response variables in ecosystem dynamics that ultimately affect ecosystem stability (resistance/resilience). It allows consideration of the impact of drivers of change on system processes and structural attributes of resistance and resilience to be considered at multiple levels including individuals, populations, species, communities, ecosystems and landscapes. The model facilitates nested models at finer scales.

1. Ecological communities are complex, self-organising, adaptive systems capable of existing in multiple alternative states depending on historical contingencies (e.g. year effects, priority effects, and disturbance regimes)
2. Community assembly can be both deterministic and historically contingent — deterministic in the general composition of trait-based functional groups, but historically contingent in the actual composition of species present:
1. stochastic forces producing variation in the sequence and timing of species arrivals (1) can cause divergence in community structure in different localities even under identical environmental conditions and the regional species pool composition (Drake 1990; Law and Morton 1993)
2. deterministic forces (the available species pool, environmental conditions such as resource supply and climate, and natural disturbance regimes) determine the types of available niches and therefore the functional groups that can fill them
3. Alternative stable states are driven and maintained by bi-directional feedback interactions between drivers (species pool, resources and disturbance regimes) and response variables (biodiversity, productivity)

The key elements of the model (and assumptions we have adopted) are described here:

THE DRIVERS

Species Pool
The regional species pool is a primary driver of successional dynamics. The size and composition of the species pool that provides the propagules to colonize restoration sites is a function of filters operating from evolutionary to historical timeframes. Life-history traits of individual species determine what species can exist within a particular resource-climate-disturbance regime. “Nestedness” within local species pools can provide clues on impacts of habitat modification, fragmentation and loss.

Resources and Climate
Resource availability and suitability (light, carbon, moisture, nutrients, space) affect what plant species (primary producers in a trophic web) can establish at a site as well as their levels of productivity (rate of carbon gain).

Resource availability is affected by climate and processes affecting soil-water infiltration, retention, and release to plant roots, indirectly affecting nutrient availability.

Disturbance
Disturbance is any event or process (physical, chemical, biological) that diminishes or destroys biomass (productivity), such as fire, frost, herbivory, disease or management interventions (historical or current) such as clearing, grazing, slashing, mowing, herbicide applications etc., and which act to initiate successional processes (dispersal, establishment, species interactions and feedbacks).

We recognise that both the external “drivers” and “response” variables can act as either external or internal drivers (proximal/distal) with feedback interactions between each of them resulting in structural, functional and interaction (feedback) shifts.

THE RESPONSE VARIABLES

Biodiversity
Biodiversity measures the extent to which a site is colonised (1) after disturbance and encompasses both compositional (taxon-based) and functional (trait-based) measures.

Taxon-based measures
Taxon-based measures include species richness, composition, and evenness. They only take into account the presence/absence and relative abundances of species (numbers and dominance), and may fail to detect functional changes in communities resulting from disturbance.
Richness - the number of species in a given area or sample (α-diversity)
Composition - the identity of species present in a given area
Evenness - various indices that reflect the relative dominance of a site by one or more species

Richness (via functional traits) influences energy and nutrient fluxes (including carbon)(4) through increased facilitation and niche complementarity at high species richness

Composition (and evenness) may affect productivity (7) through functionally dominant species that may represent a small number of species out of the total regional or local species pool.

Low taxon-based diversity may reflect species pool effects, resource-climate limitation, high disturbance or high productivity and biotic interaction (competition, predation, herbivory). Trait-based measures can help distinguish between these external and internal drivers.

Trait-based measures:
Biological traits relate to their functional attributes (morphological, physiological, behavioural, ecological characteristics). They are applicable across biogeographic boundaries and are likely to be more useful in understanding environmental change and therefore in monitoring outcomes of restoration interventions.

Morphological diversity — (e.g. size diversity) can reflect food web energy fluxes, trophic structure and niche segregation

Functional diversity — the number and dominance of life-history strategies within a community reflecting adaptation to natural disturbance

Functional diversity can indicate not only species numbers and dominance but also their functional role in an assemblage of species. Loss of functional diversity results in altered species interactions and ecosystem function, and systems resilience.

Biological traits confer resistance or resilience to stress or disturbance at either the species or assemblage level.

Trait-based approaches assume that habitat (resources-climate) selects for characteristic life history traits via natural selection (Southwood 1977) over evolutionary timeframes and biogeographic scales or as a consequence of human disturbance and habitat alteration at finer scales through a balance between colonisation-extinction processes. Consequently local species pools in isolated or fragmented environments can reveal “nestedness” within the broader regional species pool through loss of sensitive species.

Phylogenetic diversity — the taxonomic relatedness amongst species assemblages based on phylogenies reflecting long-term, conserved evolutionary adaptations potentially vulnerable to human impacts

Taxonomic distinctness — the relatedness of species within an area or sample reflecting taxonomic breadth (a simpler metric based on hierarchical Linnaean classifications)

Phylogenetic diversity (and related taxonomic distinctness index) reflects evolutionary relationships amongst species within an assemblage related to long-term evolutionary adaptations and is likely to be sensitive to human impacts rather than habitat differences. Loss of phylogenetic diversity reflects loss of evolutionary history and future options.

Ecological restoration must consider taxonomic, functional and phylogenetic diversity in order to restore and maintain species, functional and evolutionary processes.

Productivity
Productivity refers to both actual and potential biomass production (7). Potential productivity reflects gradients in resource supply (energy, water, nutrients)(2).

Increasing resource supply increases potential productivity to which local diversity adjusts (2, 7)

Stability
Stability is an emergent property of communities with respect to destabilising influences (disturbance, stress) on community biomass.

Stability encompasses the concepts of resilience and resistance to disturbance or stress.

Resilience is the capacity to re-establish structure and function after disturbance.

Resistance is the capacity to maintain integrity of structure and function under stress.

Diversity increases stability (9) because of a greater range of adaptive functional traits within the species pool that keep productivity stable under a range of resource-climate conditions.

Stability of the environment adjusts local richness within a species pool (9).

Diversity within a community can stabilize/adjust the abiotic environment by ameliorating fluctuations in resource-climate conditions (water availability, ambient temperature, light levels) to influence disturbance regimes, thence diversity. This is one important functional attribute of refugia for ancient lineages of plants or animals with highly conserved traits.

References
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