Arthropod diversity and decomposition processes in urban ecosystems: Effects of habitat complexity

Background/Question/Methods

Urban habitat patches, such as those in parks and gardens, are characterized by habitat structure and complexity with rare counterparts in natural and semi-natural ecosystems. Management and human use sustain these habitat structures over time, potentially impacting biotic communities and the underlying ecological processes. While various studies have investigated the effects of habitat complexity upon the distribution of organisms, fewer studies have tested its effects on ecological processes. The decomposition of organic matter depends on biotic and abiotic factors that can be linked to the habitat complexity, therefore representing an opportunity to explore the effects of complexity on ecosystem functioning. We hypothesized that i) decomposition and comminution rates of organic matter are slower in low-complexity habitat patches, ii) the abundance and diversity of arthropod decomposers is positively related to habitat complexity, iii) comminution rates are positively related to the abundance and diversity of arthropods. A decomposition experiment was designed to test our hypotheses. Litterbags (2 substrates (leaf litter, straw) x 2 mesh sizes x 3 replicates x 3 collection events) were placed in low-complexity park (LCP), high-complexity park (HCP) and high-complexity remnant (HCR) habitat patches (n=30) in Melbourne, Australia. Arthropods were sampled three times during the experiment using pitfall traps.

Results/Conclusions

The combined activity of arthropods and microbes resulted in the loss of 87, 84 and 44% of mass from litterbags in HCR, HCP and LCP, respectively. The effects of arthropods on decomposition ranged between 30 and 70% for leaf litter and 2 and 40% for straw. Decomposition rates of the most palatable substrate (litter, C:N ratio=41) were significantly higher than the least one (straw, C:N=57). Arthropod abundance and diversity were related to the volume of the understory vegetation (F_(1,28)=12.11, R²=0.28, p<0.01 and F_(1,28)=44.25, R²=0.60, p<0.001, respectively), and to a lesser extent to  litter cover (F_(1,28)=4.35, R²=0.11, p<0.05 and F_(1,28)=20.01, R²=0.40, p<0.001, respectively). Habitat patches with higher abundance and/or diversity of arthropods had significantly higher comminution rates for both the substrates.

The present study suggests that the simplification of the habitat structure significantly affects decomposition processes and mediated through the biotic assemblages. Microbial activity may be indirectly controlled by the habitat complexity via the microclimate of the decomposition environment. Similarly, higher habitat complexity increases arthropod abundance and diversity, leading to higher comminution rates. Further research is needed to clarify whether the effects of habitat complexity are similar in other systems characterized by different habitat complexity, biotic assemblages, climates and soils.