Globally, matrix intensification is an increasing threat to tropical forest remnants and their constituent biota. Matrix intensification is a relatively recent and novel landscape process and its effect on fauna occupying adjacent intact habitats may not be evident initially. In forest landscapes, matrices dominated by intensive land uses can increase the rate of mortality, lower fecundity, increase parasitic infection, and increase the vulnerability of dispersing fauna, resulting in population declines and ultimately, extinction. However, our current understanding of the responses of biota to matrix intensification is limited, particularly for high-contrast landscapes such as those where surface mining is adjacent to remnant forests.
The overall aim of this thesis is to investigate the effects of landscape transformation and matrix intensification on the distribution and condition of tropical forest birds in south-west Ghana, West Africa. It has four primary research objectives: i) to review 30 years of fragmentation and habitat change research assessing the responses of tropical forest biodiversity; ii) compare species richness among avian functional groups in remnant rainforest patches embedded in two matrix types (mining vs. agricultural) at two distances to forest edge; iii) to evaluate the effects of matrix type on remnant forest avian community composition and functional diversity; and iv) to investigate if the condition indices of forest-dependent birds differ in remnants adjacent to mining sites and farmlands with two distances from remnant edge.
The global systematic review revealed strong biases in research effort among geographical regions and taxa. The knowledge gaps identified are of particular concern because the forests most threatened in recent years – those of west and central Africa - are also regions where the effects of landscape change are not well understood. Biases in research approach among geographic regions were evident. Research examining species distribution patterns was most common (72%), whereas research focussed on ecological processes was less common in all biomes (28%), particularly in the Afrotropics. These findings suggest that increased research effort and are needed in the Afrotropics to build knowledge of threats and inform responses the region’s endemic biodiversity.
To address these knowledge gaps, I examined the influence of human-induced landscape change on West African biota using birds as case study taxon. My focal region was the fragmented upper Guinea tropical forest landscapes of south-west Ghana. I used bird survey data from a nested survey design (32 sites in 16 forest patches) to assess the relative influence of matrix type, edge proximity and site-level factors on the species richness of several avian functional guilds. Forest specialists and frugivores were negatively affected by adjacent mining, irrespective of distance to the forest edge. Forest specialist and frugivore richness were positively influenced by the density of large trees. These findings suggest that even with no additional forest loss, increased matrix intensification due to surface mining is likely to result in declines in the species richness of forest-dependent birds. Furthermore, the global trend of matrix intensification is likely to result in negative consequences for biodiversity in fragmented tropical forest landscapes worldwide.
I further investigated the relationships between landscape modification and abundance and evenness within avian functional guilds, as well as avian community structure. Matrix intensity strongly influenced the composition, abundance and evenness of several tropical forest-dependent bird functional guilds. Obligate frugivores, strict terrestrial insectivores, lower and upper strata foragers, and insect gleaners were all negatively affected by adjacent mining matrices relative to adjacent farmlands, suggesting specific ecosystem processes, such as seed dispersal, may be disrupted by landscape change in this region. These findings demonstrate that matrix intensification can affect functional group composition and related ecosystem-level processes in adjacent forest remnants.
Patterns of bird distribution among forest sites were perturbed by landscape modification in the study area. However, these patterns may not reflect an equilibrium state, but rather a point along a trajectory of faunal decline. Given the recent matrix changes, declines may extend beyond that expected due to habitat loss alone. To test the risk of future decline of forest-dependent avian species, I used condition indices, particularly residual mass (body condition) and ratios of heterophils to lymphocytes in peripheral blood (elevated chronic stress) to explore the effect of matrix intensification on two contrasting ecological categories of tropical forest-dependent passerine birds (sedentary area-sensitive habitat specialists vs. nomadic highly mobile generalists). I found that the condition of sedentary forest specialists, Alethe diademata and Cyanomitra obscura, were most negatively affected by high-intensive land-use near remnants. Individuals of the sedentary species occupying remnants near surface mining sites had lower residual mass and elevated chronic stress compared to those in remnants adjacent to agricultural lands.
The findings of this thesis contribute to our understanding of the effects of human-induced landscape changes on biodiversity. It extends the matrix effects hypothesis to include matrix intensification effects on adjacent fragments and their constituent biota in a single setting. It improves our understanding of how the intensity of the matrix land use can impact species richness and functional diversity in adjacent remnants. These intensification pressures are likely to detrimentally impact tropical forest ecosystem structure and functioning. This research used a novel eco-physiological approach, thereby improving our understanding of patterns of risk to individuals that were not detectable based on information on distribution patterns alone.