You need to:

• Add implements Comparable<Animal> to the class declaration; and
• Implement a int compareTo( Animal a ) method to perform the comparisons.

Like this:

public class Animal implements Comparable<Animal>{
    public String name;
    public int year_discovered; 
    public String population; 

    public Animal(String name, int year_discovered, String population){
        this.name = name;
        this.year_discovered = year_discovered;
        this.population = population;
    }

    public String toString(){
     String s = "Animal name: "+ name+"\nYear Discovered: "+year_discovered+"\nPopulation: "+population;
     return s;
    }

    @Override
    public int compareTo( final Animal o) {
        return Integer.compare(this.year_discovered, o.year_discovered);
    }
}

An Employee should be comparable to other Employees, so it's Comparable<Employee>. You then need to implement the compareTo method, as the error message says:

public class Employee implements Comparable<Employee> {
    private int numOfSales;
    // other data members

    // Constructors, getters, setters, etc

    @Override
    public int compareTo(Employee e) {
        return Integer.compare(numOfSales, e.numOfSales);
    }
}