If you look math the HTML for your page, you will see that your mathematics is broken up into several paragraphs. That is, the page includes
<p>$X_{\mu}=\left\lbrace\begin{array}{l}</p>
<p>\dot{x}= rx\left(1-\frac{x}{k}\right)-\left(\frac{qx^{\alpha}}{x^{\alpha}+a}\right)y-d_{1}xz,</p>
<p>\dot{y}= sy\left(1-\frac{y}{nx}\right)-d_{2}yz,</p>
<p>\dot{z}=hz\left(1-\frac{z}{c_{1}x+c_{2}y}\right),</p>
<p>\end{array}\right.$</p>
MathJax only processes $…$ when it falls entirely within a single paragraph (there can be no HTML tags within the math, other than <br/>). Your page would have to look like
<p>$X_{\mu}=\left\lbrace\begin{array}{l}
\dot{x}= rx\left(1-\frac{x}{k}\right)-\left(\frac{qx^{\alpha}}{x^{\alpha}+a}\right)y-d_{1}xz,
\dot{y}= sy\left(1-\frac{y}{nx}\right)-d_{2}yz,
\dot{z}=hz\left(1-\frac{z}{c_{1}x+c_{2}y}\right),
\end{array}\right.$</p>
in order for MathJax to process it. I suspect you are using a content-management system for editing your pages and that it is inserting the paragraph whenever you have a line break in your text. That may mean you will have to make the mathematics be all one long line, unless there is some other way to prevent the paragraphs from being formed.
Hope that helps.
Davide