As tropical forests are becoming increasingly fragmented, understanding the magnitude and timeframe of biodiversity declines is vital towards 21st century sustainability goals.Biological assemblages isolated in forest fragments typically experience a novel hyper-disturbance regime, resulting in drastic shifts in species diversity and community composition through species extinction and turnover. Such changes in species assemblages generally exhibit an ‘extinction debt' in which species experience a post-isolation relaxation period over the coming years and decades. It is therefore important to understand the time frame and extent to which species are lost following a fragmentation event. Our objectives were to quantify the rate at which native small mammal species richness and abundance changed over time, the rate at which hyper-abundance of a native generalist rodent increased over time, and identify the primary drivers impacting the trajectory of small mammal richness and abundance using path analysis. Finally, we discuss the implications of the results for the equilibrium model of island biogeography theory (ETIB) and the rates at which ‘extinction debts’ are paid. To achieve this, we reconstructed previous work conducted in 1992-94 and 2012-13 who surveyed the same small mammal communities over repeated periods on artificially created island fragments that resulted from the construction of the Chiew Larn reservoir in Thailand. In 2020, we completed a detailed timeline of the decline in species richness and abundance in response to fragmentation spanning 33 years post-isolation. Sampling effort from all time periods amounted to the total capture of 1,789 small mammal individuals from 12 species. A combination of linear modelling and piecewise Structural Equation Modeling (SEM) were used to understand the importance of Rattus tiomanicus as an indirect driver of small mammal species richness and abundance decline Our results demonstrated a complete collapse of the species-area relationship within 33 years, with no evidence of a re-colonization effect across repeatedly sampled islands. Our results further revealed a decline in species richness due to island size and isolation time, accelerated by the increasing dominance of the ubiquitous Malayan field rat, R. tiomanicus. Our results suggest that fragmented forests are highly susceptible to rapid species loss due to the competitive nature of Rattus accelerating the rate at which extinction debts are paid and represents a departure from the main tenets of island biogeography theory. To mitigate these impacts, reducing the extent of habitat disturbance and degradation, as triggered by fragmentation and exacerbated by isolation time, can sustain native biodiversity while averting Rattus hyper-abundance