Research Hypotheses

MCM V is investigating ecosystem responses to amplified physical connectivity in the McMurdo Dry Valleys (MDVs) in response to changing climate.

Overarching Hypothesis

Increased ecological connectivity within the MDVs ecosystem will amplify exchange of biota, energy and matter, homogenizing ecosystem structure and functioning.

Anticipated outcomes of enhanced connectivity are changes in distribution of material (organic matter and nutrients) and biota toward greater homogeneity among and within landscape units, in effect dampening the influence of resource legacies. While the MDVs have undergone a decadal transition from summer cooling to a relatively stable summer temperature regime, the forecast for the coming decades is regional warming. Hence, our expectation is that summers of intense ice melt and permafrost thaw (as were observed in 2002, 2009 and 2011) will become more frequent and drive greater physical connectivity.

Working Hypotheses

H1. Enhanced exchange of energy and matter diminishes the influence of resource legacy, homogenizing spatial variability of ecosystem structure and functioning.

The increased exchange of energy and matter among landscape units will increase the homogeneity of ecosystem structure, thereby reducing the variability across landscape units. We expect that increased hydraulic connection between glaciers and streams will cause long streams to become more dilute in their major ion and nutrient chemistry. However, greater connectivity of streams and soils may also accelerate chemical weathering and enhanced biogeochemical transfer of some solutes. Lakes will respond to increased connectivity by continuing to increase in volume expanding and potentially diluting this habitat.

H2. Amplified connectivity increases synchrony of ecological responses among landscape units.

Synchronous ecological responses were observed during the cooling period from 1987-2000, which ended in January 2002. While the mean summer air temperatures have remained cool, the mean summer solar flux has remained high; no trend in either of these parameters has been detected for more than a decade. However, during this summer climate stasis, the landscape experienced increased connectivity, largely because of increased stream flow and increased winter katabatic winds. The MDVs ecosystem has responded with an end to and in some cases a reversal of trends previously observed during the cooling period (e.g., stream algal mat biomass has increased since 2003). Temporal aspects of ecosystem responses are best evaluated in the context of long-term studies so that potential lags in the system can be evaluated (e.g., lags in stimulation of lake productivity in response to nutrient pulses).

H3. Disturbance increases connectivity and accelerates shifts towards homogeneity in ecosystem structure and functioning in the MDVs.

The MDVs experience two general types of disturbance – direct human impacts and disturbance due to climate variation. Direct, local human impacts include emissions from local fossil fuel consumption, chemical spills, distribution of non-native species, and physical disturbance associated with trampling, vehicles or camps. Climate disturbances include for example flooding in streams (leading to scour and enhanced erosion) and soils (increased soil moisture and mobilization of salts). These disturbances can be caused by increased snow/ice melt, enhanced aeolian transport of soil/sediment, changes in albedo of ice and soil surfaces, and increased surface energy balance that results in permafrost degradation. Some of these disturbances are discrete and some are more widespread in time and space (i.e., pulse vs. press events). 

H4. Decreased heterogeneity of ecosystem structure and functioning reduces stability (resistance and resilience) of the MDVs ecosystem.

Ecosystem structure (e.g., species richness, standing biomass) and functioning (e.g., PPR, nutrient cycling) are expected to be strongly influenced by the diversity of biotic communities and the structure of food webs. As such, a negative impact to one element of the ecosystem will be compensated for, and the resultant system will become more resilient and stable. Despite low organismal diversity of upper trophic levels in the MDVs, homogenization is expected to result in even lower diversity. We propose that a simplified, homogenized community will be less resilient to changing conditions and therefore less stable. Similarly, ecosystem functions (e.g., nutrient cycling, metabolism) in a homogenized system will be conducted by fewer species. Hence, the stability of ecosystem functioning may be jeopardized by the diminished resilience of the ecosystem following species removal or populations declines, as has been observed for key species following dramatic climate events.