Assessment of ecosystem function in response to fluctuations of keystone species using network analysis

Introduction

Keystone species play an important structural role in ecosystems despite often representing a low proportion of the biomass. Anthropogenic intervention and natural variability may impact keystone species which could compromise the continued normal ecosystem functioning. To be sustainable, an ecosystem must maintain its internal structure and organization in order to continue the normal metabolic activity.

Methods

Ecological network analysis is used to explore whole system interactions. Ecosystem networks of one South African estuary, Mdloti, under different seasons were used for analyses. Networks of carbon exchanges of the Mdloti estuary as described in Scharler (2012) were transferred to the ECOPATH software. Using the networks as a case study, the impact of perturbations on the food web were investigated through the study of the Mixed Trophic Impact index along with the other indices. The aims of this study were: 1) identify the keystone species in the system, 2) quantify the effects that these species have on the others which are directly or indirectly dependent on these keystone species and 3) assess the effect the keystone species have on the system as a whole. The consistencies of these effects on the networks are also assessed. We used Keystoneness Index (KSi) and Mixed Trophic Impact. Two scenarios are considered where 1) biomass of the autotrophic compartments and 2) biomass of the keystone species are increased and decreased from 10% up to 99% step-by-step in 10% intervals.

Results and conclusions

We observed that the keystone species changes over time in the different networks but they maintain their high KSi rank in the different perturbation scenarios in each network. Moreover, the effects that these species have on the different components of the system are not consistent throughout the different time periods; the effects are prominent but not consistent. Species belonging to similar trophic levels were not necessarily affected similarly. The change in behavior of the indices might help us to understand further whether the distribution of energy flows in the system change with these variations and how this change comes into effect.

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Last edited: 02 February 2016