The eruption of a nearby supervolcano would have profound and cascading effects on the ecosystem in the valley, primarily due to the 50% reduction in sunlight over several years. Here’s a breakdown of the likely chain of events, divided into immediate, medium-term, and long-term effects:

### **Immediate Effects (First Few Months to a Year)**
1. **Plants (Primary Producers):**
   - The 50% reduction in sunlight would severely limit photosynthesis, the process by which plants convert sunlight into energy.
   - Many plants would struggle to sustain their current growth rates, leading to stunted growth, reduced leaf production, and lower overall biomass.
   - Some fast-growing or shade-tolerant species might adapt better, but most would show signs of stress (e.g., yellowing leaves, slower reproduction).

2. **Rabbits (Primary Consumers):**
   - With less plant biomass available, rabbits would face food shortages.
   - Some may starve to death, while others would shift to consuming less nutritious or less preferred plant species.
   - Reproduction rates would likely decline due to malnutrition and stress.

3. **Foxes (Secondary Consumers):**
   - As the rabbit population declines, foxes would have less food available.
   - Some foxes might starve, while others might temporarily increase their hunting efficiency or expand their territory in search of prey.
   - Pups born in this period might have lower survival rates due to insufficient resources.

### **Medium-Term Effects (1–5 Years)**
1. **Plants:**
   - Continuous reduced sunlight would lead to further decline in plant populations, with slower-growing species disappearing first.
   - Some hardy or shade-adapted plants might persist, but overall biodiversity would drop.
   - The structure of the ecosystem could shift toward dominance by a few resilient species.

2. **Rabbits:**
   - The rabbit population would likely collapse as food becomes scarcer and starvation becomes widespread.
   - Survivors might migrate out of the valley if possible, but many would perish.
   - Genetic bottlenecks (reduced genetic diversity) could occur if only a small subset survives.

3. **Foxes:**
   - With the rabbit population drastically reduced, foxes would face severe food shortages.
   - Many would die of starvation, while others might turn to scavenging or alternate prey (if available).
   - The fox population would likely shrink significantly, possibly to near extinction in the valley.

### **Long-Term Effects (5+ Years)**
1. **Plants:**
   - If the ash cloud persists for multiple years, the ecosystem could shift into a state of near-collapse.
   - Some plants might eventually adapt to low-light conditions, but full recovery would be slow.
   - Invasive or weedy species might dominate if they can tolerate the conditions better than native plants.

2. **Rabbits:**
   - If any rabbits survive, their population would remain extremely low and fragile.
   - Reintroduction from outside populations (if migration is possible) could be necessary for recovery.
   - Long-term habituation to low productivity could lead to genetic changes favoring smaller body sizes or lower energy demands.

3. **Foxes:**
   - The fox population would likely collapse or abandon the valley entirely due to lack of food.
   - If even a few foxes survive, they might switch to a different diet (e.g., insects, carrion, or even plants in extreme cases).
   - Recovery would be slow, dependent on the return of sunlight and the rebound of prey populations.

### **Causal Chain Summary:**
1. **Sunlight reduction → Reduced photosynthesis → Plant decline.**
2. **Plant decline → Rabbit food scarcity → Rabbit starvation → Rabbit population collapse.**
3. **Rabbit collapse → Fox food scarcity → Fox starvation → Fox population collapse.**
4. **Long-term ecosystem disruption → Potential shift in species dominance or local extinctions.**

The severity of these effects depends on how long the sunlight reduction lasts and the resilience of the species involved. If the ash clears within a few years, recovery might be possible, but prolonged dimming could lead to permanent changes in the ecosystem.